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We studied the microstructure evolution of Mg-4Y-3Nd-2Sm-0.5Zr alloy by quasi-in-situ electron backscatter diffraction (EBSD) along with several strains under compression tests, which provided direct evidence for the influence of different twin-twin geometric structure on the twinning behavior. The results showed that the mechanical properties of the alloy were higher than traditional magnesium alloys (the maximum compressive strength reaches 402.5 MPa) due to the strengthening effect of Sm and Nd elements addition on solution strengthening, precipitation strengthening, and grain refinement. Combined with the quasi-in-situ EBSD technique, two different twin-twin geometric structures, ‘parallel structure’ and ‘cross structure’, were observed directly in the alloy. In the later stage of deformation, for ‘parallel structure’, residual stress and a large number of dislocations mainly existed in the twin boundary and tip position. For the ‘cross structure’, there was a lot of dislocation density in the interior of twins after fusion. The twin growth rate of ‘parallel structure’ was much faster than that of ‘cross structure’ because the stress of twins was mainly concentrated on the tip of twin. When the movement for the tip of twin was blocked, the growth rate of twin would be obviously decreased. Moreover, the ‘cross structure’ was easy to produce closed space. Due to the constraints of surrounding twins, the confined space was easy to stress concentration, thus inhibiting the growth of twins. At the same time, the ‘cross structure’ of twins needed a more external force to continue to deform, which also served as a strengthening structure.

Low dimensional sp3 carbon nanostructures have attracted increasing attention recently, due to their unique properties and appealing applications. Based on in silico studies, this work exploits the impacts from functional groups on the tensile properties of carbon nanothreads (NTH) – a new sp3 carbon nanostructure. It is found that functional groups will alter the local bond configuration and induce initial stress concentration, which significantly reduces the fracture strain/strength of NTH. Different functional types lead to different local bond re-configurations, and introduce different impacts on NTH. Further studies reveal that the tensile properties decreases generally when the content of functional groups increases. However, some NTHs with higher content of functional groups exhibit higher fracture strain/strength than their counterparts with lower percentage. Such observations are attributed to the synergetic effects from the sample length, self-oscillation, and distribution of functional groups. Simulations show that the tensile behaviour of NTH with the same functional percentage differs when the distribution pattern varies. Overall, ethyl groups are found to induce larger degradation on the tensile properties of NTH than methyl and phenyl groups. This study provides a comprehensive understanding of the influence from functional groups, which should be beneficial to the engineering applications of NTH.

Tectonism often plays an important role in the mineralization process, which is generally thought to be the main controlling factor in the accumulation of economic materials (e.g., gold, coal, oil and gas) through deformation. However, numerous experimental and theoretical studies have suggested that tectonic stress not only causes deformation (physical changes) in rocks and minerals but also promotes their chemical changes by acting directly on chemical bonds and causing bond scission or regeneration, called tectonic stress chemistry (TSC). In recent years, TSC actions caused by tectonic activities have provided new ideas and evidence for explaining the chemical structural evolution of coal, hydrocarbon formation, organic (coal-derived) and inorganic graphitization and hydrothermal mineralization under shear stress. These background studies have provided incentives and insights into how tectonic stress affects the chemical structures of minerals, rocks and even ore-forming fluids in the process of mineralization. In this paper, we briefly review: (1) the concept of TSC; (2) the TSC process in the formation of shear zone type gold deposits from stress concentration, brittle fracturing, sudden reduction of fluid pressure, and flash vaporization to gold precipitation; (3) mechanisms of the macromolecular structural evolution of coal and gas generation under shear stress from deformation experiments and molecular dynamic simulations; (4) coal-derived graphitization caused by preferred orientation and extension of the basic structural units (BSUs) under shear stress; and (5) some preliminary experimental explorations on inorganic graphitization in carbonate-hosted shear zones. In addition, some existing problems and possible solutions for these processes are also discussed. Finally, we propose additional potential TSC processes in extensive geological processes, e.g., the relationship between deformation and metamorphism and trigger mechanisms of slow-slip earthquakes. To further explore these processes, a combination of experiments and molecular dynamic simulations should be undertaken by researchers.

An equiatomic VNbTi medium-entropy alloy with outstanding tensile properties and unique deformation behavior is reported. The screw dislocation glide, deformation twinning, and dislocation accumulation induced kink bands are identified as three deformation mechanisms that contribute to a large elongation above 20%. The {112} twins are activated at the beginning of the yield stage accompanied by sudden stress-drop and pronounced acoustic emission. Dislocations dominate subsequent tensile deformation, and the prevalent multiplanar dislocation slip promotes the formation of complex dislocation configurations (e.g., debris, dipoles, and loops) and dense dislocation networks. The twin bands and kink bands can further impede the dislocation motion meanwhile effectively alleviate stress concentration. The synergistic activation of these deformation mechanisms provides new opportunities to design ductile refractory medium- and high-entropy alloys.

The failure mechanisms of Oxide-Oxide ceramic matrix composites AS-N610 were studied at both room temperature and high temperature using tensile and fatigue tests with and without lateral and laminar notches. The unnotched coupons had an average tensile strength of 423 MPa with elastic modulus of 97 GPa at room temperature showing a perfect elastic behaviour whereas the laminar notched samples shown similar strength of 425 MPa with elastic modulus (98 GPa) revealing pseudo-ductile behaviour. A reduction in tensile strength of the oxide ceramic matrix composites was observed at high temperatures. Thermal shock experiments revealed that the retained strength of the samples quenched from 1100 °C deteriorated by ∼10 % (395 ± 15 MPa). In all samples fracture origin was observed on the mid plane showing a higher degree of fiber pull-out, delamination and pseudo ductile behaviour. FEA confirmed higher stress concentration on the areas of failures.

Slip is one of the most critical components for the frac plug, which would lodge into the casing and lock the frac plug in place during the setting and anchoring process. However, fracture failure of slip significantly affects the hydraulic fracturing effects and has attracted tremendous attention. In this paper, a three-dimensional contact model is applied to explore the setting process of slip. The effects of key structural parameters such as apex angle, inclination angle, and wedge angle on the contact characteristics of slip are systematically investigated. Numerical results indicate that the maximum contact stress appears at the contact area between slip tooth and the casing's inner wall. Besides, the maximum contact stress generally increases with the increase of apex angle and inclination angle, while decrease linearly with the rise in the wedge angle. Experimental results show that the slip teeth get blunt and appear severe plastic deformation, which arises from stress concentration. Comparison of biting area indicates that the experimental results are about 21.3% larger, which still have a reasonable agreement with the numerical results. These obtained results can guide the parametric selection of plug slip and other similar components.

Surface structure of C/C composites has been regulated through electrochemical corrosion at room temperature to modify the residual stress and improve the joining strength when brazed to Nb. The unique crevice corrosion in C/C composites is investigated to reveal the change of corrosion depth and fiber size. The interlacing zone of carbon fiber reinforced brazing alloy replaces the reaction layer in the original joint. Joining area is increased dramatically and the continuous crack will be hindered. The interlacing zone eliminates the stress concentration and relieves the residual stress through reducing the property mismatch. All advantages contribute to the joining quality of C/C Nb. Shear strength of C/C Nb joint with 80 μm depth reached 37.7 MPa, which was 1.2 times higher than that of original joint. Surface structure design of C/C composites not only expands the application in structure component, but also exhibits the promising application in energy field.

Two types of MgO-C refractories with tight particle grading and non-tight particle grading were prepared according to Andreasen's continuous packing theory. Fracture behaviors were investigated using wedge splitting tests combined with digital image correlation method and acoustic emission techniques. The results indicated that MgO-C refractory with non-tight particle grading treated at 1400 ℃ had more in situ phases (e.g., AlN and MgAl2O4) and exhibited less brittleness than specimens with tight particle grading even though they had similar nominal tensile strengths. In contrast, specimens with non-tight particle grading had greater horizontal strain under various loading stages, reflecting their better ability to resist rupture deformation. In addition, more microcracks were initiated earlier in the pre-peak region, and more energy was consumed. The decrease in coarse particles and corresponding increase in fine powder content increased the interface between particles, benefiting for reducing the local stress concentration and improving the thermal shock resistance of refractories.

A special structure named tetrahedral twin was found in a cold-drawn CoCrFeNi high-entropy alloy. Crystallographic characterization using convergent beam electron diffraction indicated that the three-dimensional (3D) tetrahedral twins were achieved through the intersection of each couple of twin layers in four {111}FCC planes, which was caused by the low stacking fault energy and the special plane strain state during deformation. The calculation results revealed that the tetrahedron-morphological twins could result in significant stress concentration, thereby considerably deteriorating the plasticity of the high-entropy alloy.

In this study, a substrate was surface textured using a femtosecond laser to prolong the service life of thermal barrier coatings (TBCs). The selected texture showed linear and diamond shapes. The effect of the texture shape on the thermal shock resistance of the TBCs was investigated by subjecting them to thermal shock cycles. The microstructure and composition of the TBCs were investigated using various characterisation methods. The mechanical properties of the TBCs were measured using the microindentation method. Based on the theory of elastoplastic mechanics, a three-dimensional finite element numerical model was established to analyse the effect of the two textures on the stress distribution of the TBCs. The results revealed that the linear-textured TBCs showed improved stress concentration at the coating interface when the coatings expanded or shrank, which resulted in an improvement in their thermal shock resistance. Therefore, the linear-textured TBCs showed excellent oxidation resistance, fracture toughness, and heat resistance as compared to the diamond-textured TBCs.

Natural gas hydrate (NGH) reservoirs consist of the types of sediments with weak cementation, low strength, high plasticity, and high creep. Based on the kinetics and thermodynamic characteristics of NGH decomposition, herein a heat-fluid-solid coupling model was established for studying the wellbore stability in an NGH-bearing formation to analyze the effects of the creep characteristics of NGH-bearing sediments during long-term drilling. The results demonstrated that the creep characteristics of sediments resulted in larger plastic yield range, thus aggravating the plastic strain accumulation around the wellbore. Furthermore, the creep characteristics of NGH-bearing sediments could enhance the effects induced by the difference in horizontal in situ stress, as a result, the plastic strain in the formation around the wellbore increased nonlinearly with increasing difference in in situ stress. The lower the pore pressure, the greater the stress concentration effects and the higher the plastic strain at the wellbore. Moreover, the lower the initial NGH saturation, the greater the initial plastic strain and yield range and the higher the equivalent creep stress. The plastic strain at the wellbore increased nonlinearly with decreasing initial saturation.

Joining of Al2O3 ceramic to Cu has been conducted with Ag-26.7Cu-4.5Ti braze and refractory metal (W or Ta) foil. The interfacial microstructure in the joint with W foil is similar to that with Ta foil. The joining region in the joint consists of a reaction zone, braze zone I, refractory metal layer and braze zone II. The reaction zone of Cu3Ti3O with a thickness of about 5 μm develops close to Al2O3 side due to the reactions of Ti and Cu in the braze with Al2O3 substrate. The braze zones I and II are mainly composed of Ag- and Cu-based solid solutions. For the joint with W foil, the adsorption of Ti at the braze/W interfaces followed by the Ti diffusion into W foil occurs, whilst slight dissolution and diffusion of Ta into the brazes take place in the joint with Ta foil. The average shear strengths of joints with W and Ta foils are much higher than those without refractory metal foil, indicating the contribution of the refractory metal foil to the improvement of joint mechanical strength. Introduction of refractory metal foil in Al2O3/Cu joining is beneficial for the shift of joint residual stress distribution and the decrease of stress concentration in the joint since the coefficient of thermal expansion (CTE) of refractory metal layer approximately approaches that of Al2O3. Furthermore, a slight thickness increase of the Cu3Ti3O reaction zone in the joint with refractory metal foil may also give rise to the joint strength promotion.

This work aims to present a methodology to model Functionally Graded Material (FGM) in Finite Element Method (FEM) based software ANSYS and to reduce the stress concentration by applying the FGM ring around the rounded rectangular slot subjected to far field tensile loading. The effects of power law FGM parameters are analysed on stress concentration. From the analysis, it is observed that by applying the FGM ring around a rounded rectangular slot the stress concentration is reduced significantly in comparison to the homogeneous material case. The amount of reduction can be controlled by the FGM ring parameters.

Three steel beams were made-up and tested to study the effect of CFRP (Carbon Fiber Reinforced Polymer) laminates with and without end anchorage plates on the behavior of flexural strength of steel beams. Tested steel specimens are equal in clear span length, gross sectional area, and strengthened by intermediate stiffeners. Three types of specimens were made, the first specimen was considered as a reference one, while the other two were reinforced by CFRP laminates with and without end anchorage plates respectively. From the experimental outcomes, it was found that, the load deflection curve and load strain responses for specimens strengthened by CFRP strips became stiffer in comparison with the reference beam. On the other hand, it was found that, the deflection values for strengthened beams by CFRP without and with end anchorage plates were decreased to 58.33% and 79.95%, while, the strain values were decreased to 63% and 85.55% respectively, in comparison with the reference beam at yielding stage at load (Py = 20 kN) while, at failure stage at load (Pu = 45 kN) for reference beam, the deflection values for strengthened beams by CFRP without and with end anchorage plates were decreased to 81.19% and 88.13%, while, the strain values were decreased to 88.6% and 97.14% respectively, in comparison with the reference beam. Also, the percentages of increasing in yielding -load for strengthened beams by CFRP without and with end anchorage plates were increased to 100 % to 225 %, while, the percentages of increasing in ultimate-load for strengthened beams by CFRP without and with end anchorage plates were increased to 38.88 % to 111.11 %, respectively, in comparison with the reference. Finale the failure mode for reference beam was flexural failure type (plastic hinge), while, the failure mode for strengthened beams by CFRP without and with end anchorage plates was sudden; because of the de-bonding at the end of the CFRP strips and at the end of carbon fibre that due to the stress concentration at the ends of the CFRP.

The aero engine turbine rotors are under strong centrifugal load and the highest thermal load. The ventilation openings on the rotors are inevitable, because air flow need to pass through them to cool the temperatures down and keep the air pressure balanced in internal aero engine. The ventilation openings will lead to stress concentration effect. In this paper, the stress concentration factor of elliptical opening on rotating disc is deduced by superposition method. How to define the optimal location and shape of the elliptical opening on rotating disc to decrease the stress concentration effect has been investigated specifically. The reliability and accuracy of the theoretical deviation process is verified by Finite Element Method (FEM). The process of how to obtain the optimal location of the elliptical ventilation opening with particular shape on turbine sealing disc is described as an engineering application case. The investigation provides sufficient theoretical support for optimal location and shape definition of elliptical ventilation opening on aero engine rotors with stress concentration effect by pure mechanics consideration.

The application of the step lap joint has been increased due to its good characteristics such as lightweight, high stiffness, and good fatigue durability. Moreover, it has a wide application in the aerospace, automotive and marine sector because of its efficient strength and less stress concentration. The present study is concentrated on the effect of the kind of joint used for lap joint like the adhesive joint, riveted joints, and bolted joints. In addition, the length of the steps has been changed to study the behavior of adhesive lap joint in detail. Also, the hybrid joint such as a bolted-adhesive and riveted-adhesive joint is examined in the current study. The variation in the number of bolts and rivets are made for detailed analysis. The main objective of this project is to analyze the step lap joint with different length, different material for adherend and adhesive and also compare it with different mechanical fasteners such as rivets and bolts considering hybridization of it with and without the adhesive, Finite Element Analysis (FEA) is carried out in ANSYS Workbench for analysis and SolidWorks software is used for 3-D modelling.

The effect of varying abrasion pressure and nanographene on two body abrasive wear behavior of Glass – Epoxy (GE) composites under multipass condition was studied. These composites: Epoxy (EP), Glass –Epoxy (GE), GE/1 wt% nanographene (GE1) and GE/2 wt% nanographene (GE2) were developed using autoclave molding. The experimentation has been carried out by varying the abrasion load of 2.5, 5, 7.5 and 10 N under the influence of 180 Grit SiC abrasives through an abrading velocity of 0.5 m/s as per ASTM G99 method. It is observed that the wear volume loss of these composites is a linear function of abrasion load and their composition. Further, inclusion of 1 wt% nanographene showed better abrasion wear resistance than higher loading of the same nano filler. This may be due to the structural integrity of Glass Epoxy composites. Higher loading of nanographene localized the stress concentration which leads to poor tribological response. Further, hardness, good mechanical properties and improved degree of crystallanity made the GE1 composites to exhibit good abrasion wear behavior. The frictional study suggested that coefficient of friction (COF) decreases initially as an effect of adding nanographene under lower pressure whereas the reverse action has been exhibited by the composites under the influence of higher pressure. The morphology of the abrasion worn surface was studied using SEM images. It was found that microcuttng, microploughing and fatigue were some of the mechanisms responsible for the failure.

To investigate the degradation mechanism of static tensile mechanical behaviors of marble containing dynamic damage, multiple impact loading tests were performed on the disc marble samples, and then static Brazilian tests were conducted for the damaged samples. Besides, coupling modeling technology of finite difference method (FDM)—discrete element method (DEM) was used to carry out the numerical investigation. The results show that after multiple impacts, more white patches appear on the surface, and some microcracks, macro-fractures as well as pulverized grains are found by optical microscopic. The static tensile strength decreases with the increase of the dynamic damage variable characterized by the ultrasonic wave velocity of sample. The interaction between grains in the damaged sample becomes intense in the subsequent static loading process, causing a relatively large strain. The volume of the fragments falling off around the loading points becomes larger as impact number increases. As the dynamic damage increases, the absorbed energy of sample during the static loading first decreases and then tends to be stable. Both the stress concentration and the breakage of the force chains are the root causes of the degradation of the static tensile strength.

The stress concentration factor (SCF) is one of the most important parameters in the fatigue reliability analysis of tubular offshore structures. This parameter exhibits remarkable scatter which calls for more emphasis in accurately computing the SCFs for determining the fatigue life. As per the authors’ knowledge, no study has been conducted on the probability distribution of SCFs in tubular joints with FRP. Therefore, in this study, results of performing 288 FE analyses on 96 T/Y-joints with FRP have been used to propose a probability distribution model for SCFs in the FRP-reinforced joints subjected to axial, IPB, and OPB loads. According to the T/Y-connection FE models which were validated with several experimental tests, a set of SCF sample databases was generated. 11 various probability density functions (PDFs) were fitted to the relative frequency histograms of the SCF samples. For determining the parameters of every distribution, the maximum likelihood method was applied. Evaluating the goodness-of-fit according to the Chi-squared and Kolmogorov-Smirnov tests indicated that the Weibull distribution is the best one for the maximum SCF values in tubular T/Y-connections retrofitted with FRP under axial load. Also, the Gamma distribution was set as the governing probability function for T/Y-joints with FRP under IPB and OPB loads. In the final step, five fully defined PDFs were proposed for the maximum weld SCFs of in T/Y-joints with FRP under three different types of loadings.

SiC nanowires reinforced C/(PyC-SiC)n multilayered matrix composites (SM-CS for short) were prepared by combined with sol-gel and chemical vapor infiltration (CVI) method. Firstly, (PyC-SiOC)n multilayered structure was formed by cycles of impregnation and deposition. Then SiOC was transformed into SiC by heat-treatment, and (PyC-SiC)n multilayered structure would be obtained. At the same time, the PyC layer which was designed as the outmost layer could decrease gas supersaturation to form in-situ tubular SiC nanowires on the surface of multilayered structure. The results of three-point bending test showed that the maximum force of SM-CS composites was increased by the number of cycles of the preparation process, which were up to enhanced by 74.38% compared with C/C composite materials. The fracture surface showed that the improvement was due to the multiscale reinforcing system of (PyC-SiC)n multilayered structure and SiC nanowires. Multilayered structure can protect carbon fibers and release stress concentration by induction of cracks. And the mechanical interlocking effect of SiC nanowires could reinforce bonding force of the remaining matrix.

Mg–Al–Zn–Ca–Y alloys with excellent ignition and corrosion resistances—termed SEN alloys (where the letters “S,” “E,” and “N” stand for stainless, environmentally friendly, and non-flammable, respectively)—have been developed recently. In this study, the microstructure, tensile properties, and high-cycle fatigue properties of an extruded Mg–9.0Al–0.8Zn–0.1Mn–0.3Ca–0.2Y (SEN9) alloy are investigated and compared with those of a commercial Mg–9.0Al–0.8Zn–0.1Mn (AZ91) alloy extruded under the same conditions. Both the extruded SEN9 alloy and the extruded AZ91 alloy have a fully recrystallized structure comprising equiaxed grains, but the former has a smaller average grain size owing to the promoted dynamic recrystallization during extrusion. The extruded AZ91 alloy contains coarse Mg17Al12 discontinuous precipitate (DP) bands parallel to the extrusion direction, which are formed during its cool down after extrusion. In contrast, the extruded SEN9 alloy contains relatively fine undissolved Al2Ca, Al8Mn4Y, and Al2Y second-phase particles, which are formed during the solidification stage of the casting process. The tensile strength of the extruded SEN9 alloy, which has finer grains and more abundant particles, is slightly higher than that of the extruded AZ91 alloy. However, the difference in their strengths is relatively small because the stronger solid-solution hardening and precipitation hardening effects in the extruded AZ91 alloy offset the stronger grain-boundary hardening and dispersion hardening effects in the extruded SEN9 alloy to some extent. The tensile elongation of the extruded AZ91 alloy is significantly lower than that of the extruded SEN9 alloy because the large cracks formed in the DP bands in the former cause its premature fracture. Although the extruded SEN9 alloy has higher tensile properties than the extruded AZ91 alloy, the high-cycle fatigue life and fatigue strength of the former are shorter and lower, respectively, than those of the latter. The DP bands in the extruded AZ91 alloy do not act as fatigue crack initiation sites, and therefore, fatigue cracks initiate on the specimen surface at all stress amplitude levels. In contrast, in most of the fatigue-fractured specimens of the extruded SEN9 alloy, fatigue cracks initiate on the undissolved Al2Ca and Al2Y particles present on the surface or subsurface of the specimens because of the high local stress concentration on the particles during cyclic loading. This particle-initiated fatigue fracture eventually decreases the high-cycle fatigue resistance of the extruded SEN9 alloy.

Due to the complex effects of combined loadings on complete steel structures, the combined effects of high-velocity projectile impact and heat radiation due to a pool-fire on a vertical Q345 steel tank have not been generally considered. In this study, a consecutive coupling approach is established. The combined effects of projectile perforation and heat radiation on the failure modes and fire resistance of a vertical steel tank are investigated. Additionally, the effects of the projectile size, projectile shape, and separation distance are analyzed. The results indicate that compared with a fixed-roof tank without impact, the perforation results in stress concentrations in the plastic deformation zone. The stress level of the perforated tank is higher, and the fire resistance of the perforated tank is lower. Additionally, as the perforation size increases, the stress level triggering the thermal buckling of the perforated tank decreases outside of the plastic deformation zone. Additionally, the fire resistance decreases. Moreover, as the separation distance decreases, the stress level of the cylindrical shell increases. The fire resistance of the perforated tank decreases dramatically. By understanding the failure mode, this study is able to suggest fire and explosion protection measures in petrochemical industries to decrease or eliminate the domino effect risk and incident damage. Additionally, the fire resistance data can help improve guidelines and understanding for fire and rescue services, which would undoubtedly play a vital part in emergency response and rescue, and enhance the process safety level overall.

Investigating the fatigue performance of high-strength bolt is essential to prevent related accidents. An axial-tensile fatigue test under a stress ratio of 0.5 was performed on 21 specimens of high-strength bolts to investigate the constant-amplitude fatigue performance of grade 8.8 M24 high-strength bolts. Based on experimental results, S–N curves were obtained through regression analysis, and fractographic analysis was conducted on typical specimens. Moreover, the results were compared with the existing fatigue experimental results and standards. Further, a finite element model was established to analyze the stress distribution of the bolts. It was found that the fatigue strength of the high-strength bolt at 2 × 106 cycles under a stress ratio of 0.5 was 89.76 MPa, which is 1.60 to 1.87 times greater than the values specified in the current standards. Through the comparison of the fatigue tests under stress ratios of 0.3, 0.5, and 0.8, the fatigue strength decreased as the stress ratio increased. Additionally, two main fatigue failure modes were found, with most bolts fracturing at the root of the first loaded thread; this is confirmed by the stress concentration location obtained from finite element analysis. The fracture morphology of the bolts exhibited a single-point fatigue source and possessed obvious fatigue fracture characteristics. It was concluded that the area of the fatigue propagation region was basically inversely proportional to stress ratio.

The aim of this paper is to assess the stress concentration zone (SCZ) of a circular smooth specimen at an early stage. Following the detection of the stress concentration zone, the specimen residual life was calculated and compared to the experimental result. Uniaxial loading on the servo-hydraulic testing machine was used for the laboratory testing. The stress concentration tester was used to detect the SCZ on the specimen, based on spontaneous surface micro magnetic signals on the ferromagnetic surface. In addition, cyclic testing for fatigue characteristics was conducted at 70% and 75% of ultimate tensile stress (UTS). The results indicated that the estimated residual life for the 70% UTS was nearly identical to the experimental value, based on two magnetic parameters, i.e, Kmax and the contraction value. Meanwhile, the estimated residual life for the 75% UTS was equivalent to the experimental findings. The minimum and maximum error values for life estimation using both parameters were 0.47% and 24.35%, respectively, for the contraction value. The correlation value, R2, between the estimated life using MMM and experimental life was 0.95, showing good estimation. All the estimated life data using this method was distributed within the range of a 95% to 90% confidence interval. Hence, the proposed method can be used to predict the fatigue life of ferromagnetic components through magnetic flux leakage signals, especially in the stress concentration region.

In this study, a new approach is presented to determine the stress and displacement field of a shallow buried circular tunnel subject to gravity and arbitrary lateral stress based on the complex variable method. The soil mass and lining are conformally mapped onto two ring regions. A more effective method than the power series method is used to solve the undetermined coefficients of complex potential functions. The interaction mechanism between the lining and the soil mass is fully considered. The boundary conditions along the inner boundary of the lining and the ground surface and the stress and displacement continuity conditions along the excavation line are used in the analytical derivation. In the verification, the stress and displacement distributions obtained in the semi-analytical solution agree with the numerical results and satisfy the boundary and continuity conditions. Using examples, the stress distributions along the inner boundary of the lining and interface are presented to examine the influence of key parameters including the Young's modulus and buried depth. The effect of the lining in reducing stress concentration and restricting tunnel convergence is quantitatively analyzed.

Unreinforced masonry (URM) structures exhibit high vulnerability in the moderate or strong earthquakes. The door- or window-type opening would further increase the seismic vulnerability, in which there is a weak area in the structures with stress concentrations. To improve the seismic behavior of perforated URM wall, a reinforcement of engineered cementitious composites (ECC) overlay was used on the both sides of the wall. Two 1/2-scale H-shaped specimens were prepared and tested under cyclic lateral loads. The damage patterns, hysteretic curves, envelope curves, stiffness degradation and energy dissipation of specimens were analyzed and discussed. The ECC effect can be observed as an increase of lateral resistance (maximum lateral force from 87.5 kN to 255.2 kN), and energy dissipation capacity (the cumulative energy dissipation up to ultimate state from 10.6 kJ to 21.5 kJ). Moreover, ECC overlay was able to control the crack initiation and propagation. The wide cracks along mortar joints observed in the spandrel of control specimen became multiple flexural cracks in the masonry piers after ECC reinforcement. The final failure of strengthened wall was dominated by the bending of piers, which was beneficial to improving the collapse resistance of masonry structures in the strong earthquake excitation.

Roughening of the free surface in micro-forming results in the local necking and subsequent fracture, which seriously affects the formability of sheet metals. In this work, laser pre-shocking (LPS) was introduced into microscale laser dynamic flexible bulging (μLDFB) to reduce surface roughening in terms of reducing the surface roughness and refining the internal grains. In LPS experiments, when the laser energy is 447 mJ, the rebound of the copper foils can be negligible. LPS can be expected to suppress the surface roughening by reducing the initial roughness R0, increasing the N value and decreasing the rate of surface roughening. The copper foils subjected by 447 mJ LPS were used for the μLDFB experiments. The 50 μm T2 copper foils with four different grain sizes were used in μLDFB under different laser pulse energies. When laser pulse energy ranges from 325 mJ to 575 mJ, compared with μLDFB without LPS, the surface roughness on the bottom surface of the bulging sample is decreased. And the nano-hardness, elastic modulus and forming uniformity are significantly improved. In μLDFB experiments without LPS, for fine-grained samples, the cracks initiate in the bulging transition zone under 725 mJ laser energy due to surface roughening. As laser energy increases to 1050 mJ, the cracks initiate in the mold entrance zone due to the strong stress concentration. For coarse-grained samples, cracks initiate under 575 mJ laser energy, and the propagations of the cracks are difficult to predict due to non-uniform distribution of grain size and the difference of grain orientation. The experimental results suggest that LPS can reduce the possibility of the premature fracture caused by the local necking, and alleviate the uncertainty in the fracture mode.

In the present work, the influence of stress concentration and deformation temperature on the mechanical behavior of boron carbide particles reinforced aluminum matrix (B4Cp/Al) composites fabricated by powder metallurgy was investigated via the tensile test and finite element simulation based on the real microstructure. The results demonstrate that a uniform particle distribution and a good interface bonding between the particles and matrix are found in the 5 wt% B4Cp/6061Al composite. When the deformation temperature decreases from 298 to 77 K, the tensile property and notch sensitivity ratio of the notched composite increases. Combined with the microstructure observation and stress/strain evolution, the fracture mechanism of the notched composite at 77 K was revealed. Specifically, there is a difference in stress and strain distributions induced by deformation temperature in the initial stage of deformation of the notched composite. The matrix damage tends to occur from the notch and propagate to the high strain regions near the particles. Furthermore, there is a high possibility of particle fracture or interface debonding in the notched composite due to the high matrix stress at 77 K. This work provides the experimental and theoretical basis for the application of particle-reinforced aluminum matrix composites with stress concentration under cryogenic environments.

The interaction between twins and grain boundaries (GB) has an important influence on the deformation and fracture behavior of materials. In the present work, {332} twinning transfer (TT) behavior and its effect on the twin shape in a deformed β-type Ti-23.1Nb-2.0Zr-1.0O Ti alloy were investigated experimentally and with molecular dynamics simulation. The crystallographic alignment factor of the two twinning systems in the neighboring grains and the misorientation angle of the grain pairs were found to influence the occurrence of TT. This further determines the twin shape: twins present a ruler shape when TT occurs but are in a lenticular shape otherwise. Such different twin shapes are attributed to the local stress states related to TT occurring or not. Both ruler-shaped paired twinning and lenticular twinning would provide effective mechanisms to release or reduce the stress concentration in front of the twin tips in different grain pairs.

Based on a consistent couple stress elasticity theory, a two-dimensional (2D) boundary element method (BEM) is developed to solve the boundary value problems in the couple stress elastic materials. The displacement boundary integral equations (BIEs) are applied to compute the mechanical quantities on the boundary. The integral representations for the generalized strains and stresses at internal points are also presented together with the explicit expressions for the high-order fundamental solutions. The collocation method is adopted for the spatial discretization of the BIEs. Some special numerical and semi-analytical techniques are provided for the evaluation of weakly and strongly singular integrals. The sinh transformation is used to deal with the nearly singular integrals for near-boundary internal points or boundary nodes in thin structures. An error-controlled integration scheme is adopted for computing the non-singular integrals to improve the accuracy. Three representative 2D boundary value problems in the consistent couple stress elasticity theory are studied by the developed BEM. The correctness and accuracy of the present BEM results are verified by comparison with the corresponding analytical results. Then, multiple flat elliptical holes in a plate are numerically studied to show the interaction effects on the stress concentration.

A novel low-density nickel-based single-crystal superalloy was prepared and the effect of solution treatment on the creep deformation mechanism was systematically investigated. The results reveal that the creep life increases with the increase of solution treatment time, which can be ascribed to microstructural homogenization. However, when the solution treatment time exceeds a threshold limit, the relative proportion of undesirable defects significantly increases, which accelerates the failure and compromises the creep life. The homogenization of the composition leads to the precipitation of a more homogenized γ′ phase and, in turn, results in a more uniform and dense dislocation network, thus hindering the dislocation movement and improving the creep life. The undesirable defects may lead to the stress concentration and weaken the effect of the dislocation network, thus accelerating the creep process and the dislocation are more likely to shear into the γ′ phase. In general, the current study provides guidance for selecting a more reasonable solution treatment time to improve the creep strength.

Recent researchers show that the shell structure and truss structure are emerging categories of light weight structure with unique mechanical properties and high customization potential, but they are difficult to manufacture by traditional methods. Inspired by the diving bell of water spider, the reticulated shell structure designed in this work combines the characteristics of the truss structure and the shell structure, and is fabricated by laser powder bed fusion (LPBF). The effect mechanism of strut angle on the manufacturability, compressive behaviors, energy absorption property, stress distribution, and fracture morphology were studied by experiment and simulation methods. The results showed that the relative densities of reticulated shell structures increased with the increase of strut angle, which was due to the existence of a large proportion of overhang structures in the reticulated shell structures. The compressive load-displacement curves of all reticulated shell structures showed a similar trend, including three stages, which were elasticity, subsidence, and densification. The reticulated shell structure with the strut angle of 75° had the highest specific energy absorption and energy absorption capacity. Finite element simulation results revealed that, with the change of the strut angle, the stress concentration in the reticulated shell structure significantly changed and directly affected the compression behavior of structures. The structure with the strut angle of 75° exhibited the lowest stress concentration level at key locations, leading to the best compressive behavior among structures.

The flattening test, using one pair paralleled plates to flatten tube structure radially, is generally employed to examine the ductility of tube used in once through steam generator (OTSG). This study focuses on the stress condition and deformation mechanism analysis on the concentrated stress regions of Ti-2Al-2.5Zr alloy tube during the flattening test. Firstly, the finite element analysis was performed using the commercial software ABAQUS to determine the stress condition. Secondly, the Electron Back-Scattered Diffraction (EBSD) was implemented to observe the microstructure evolution of Ti-2Al-2.5Zr alloy. Finally, Schmid law was employed to analyze the activated deformation mechanism under condition of the complex stress. It was found that the condition of the complex stress in stress concentration regions, including tension and compression regions, can be simplified into two directional stresses condition. In grains whose c-axis is nearly towards TD and ND, the strain is mainly accommodated by prismatic slip, whereas in grains whose c-axis deviates about 45° from ND to TD mainly by basal slip. The {10–12} extensive twin and corresponding parent grain orientation mainly relied on the stress condition. Additionally, the intergranular twin pairs connected at common grain boundaries (GBs) with high strain compatibility were found at low angle GBs, which were activated under a combination of macro stress and strain compatibility effect.

Material mismatch between the welding bead and its surrounding matrix has been known to cause stress concentration due to incompatible deformation, and/or even crack regeneration, thus greatly affecting the performance of a welding repaired material. In this paper, a semi-analytical method (SAM) is developed to tackle problems for material mismatch in a welding repaired material with free surface under remote tensile loading. The heterogeneous welding bead is modeled by a homogeneous base material containing unknown eigenstrains through the equivalent inclusion method; after which a numerical discretization is adopted and the eigenstrains within each computational element are determined by iteratively solving a set of linear equations with the assistance of conjugate gradient method. Stress field arising from the eigenstrains can be obtained by employing previously derived influence coefficients. The SAM is then examined by a simple finite element model and utilized to analyze influences of material properties, aspect ratio, angle of differently shaped welding bead and interactions among multiple welding beads on the stress distribution. The SAM may have potential applications in dealing with problems related to residual stress in welded material due to eigenstrains.

ultruded GFRP perforated plates are widely used in composite bridge and truss. Stress concentration is an inevitable focus of the structural design and analysis. This paper investigated the stress concentration of pultruded perforated plate under both uniaxial and biaxial stress. The hole’s radius r, the width w, height h of plate and stack layer were involved. Biaxial stress parametric Finite Element (FE) model of GFRP perforated plates were created. In terms of notch stress in GFRP perforated plate, the stress concentration of 0 degreesply is significant under unidirectional tension. When height to radius ratio is greater than 0.15 (2r/h > 0.15), the stress concentration factor (SCF) is suggested to be modified based on Tan’s model. The biaxial status of tension–compression is the governing load combination of GFRP perforated plate. The analysis method of biaxial status has been proposed in the paper. By arranging the layer angle of the non-0° layer of the laminate, the relationship between SCF of perforated plate and the anisotropic factor was found.

Additive manufacturing (AM) zirconia shows excellent prospects for use in clinical applications. In this work, AM zirconia samples were fabricated in horizontal (H) and upright (U) fashion using a stereolithography appearance (SLA) system. The dimensional accuracy, density, translucency, surface quality, flexural strength and fracture toughness (KIc) of the samples were then assessed. AM zirconia fabricated in a H fashion shows excellent dimensional accuracy. Samples fabricated in a U fashion exhibit a higher density (relative density 95.4%) and translucency (4.393), but H group samples exhibit a higher KIc value (12.635 ± 1.372 MPa m1/2). The flexural strengths of the samples were measured and the values were compared according to their different build orientations, surface quality and fracture modes. Manual defects that arose in the samples as a result of their separation from the build platform in which they were made were found to lead to samples with irregular surface morphologies and increased surface roughness. However, this type of defect does not affect the flexural strength of samples fabricated in a H fashion. Polished-H samples that fracture from the stress concentration area exhibit the highest flexural strength (1151.08 ± 166.41 MPa) amongst all the samples prepared in this work. However, the flexural strength of the samples prepared in a U fashion is obviously low, even after polishing (225.44 ± 46.10 MPa). The Weibull characteristic strengths and Weibull moduli of the as-sintered samples are 920.22 MPa and 6.50 for H and 219.59 MPa and 7.99 for U, respectively. Overall, it was found that the dimensional accuracy, density, translucency, surface quality and mechanical properties of materials vary according to their different build orientations.

Stress concentration factors (SCF) in steel tubular joints have gained considerable attention since it is the crucial parameter for fatigue design of steel tubular structures. Meanwhile, among various strengthening methods, Fiber Reinforced Polymer (FRP) wrapping has shown to be an effective scheme for reinforcing steel structures. This study is dedicated to the numerical investigation of SCF values in FRP-strengthened tubular KT-joints under the action of In-Plane Bending (IPB) and Out-of-Plane Bending (OPB) moments on the central as well as outer braces with three various practical loading patterns per each moment. The Finite Element (FE) modeling methodology for joint geometry and FRP generation was validated against the available data in the literature and well-known codes. The effects of different geometrical parameters along with the FRP parameters on the SCF values were studied through post-analyzing a total of 2920 FE models. The numerical parametric studies indicated that FRP strengthening could lower the SCF values in KT-connections by about 30 and 55 percent under IPB and OPB moments, respectively. Using the driven data, practical fatigue design formulas were developed for predicting SCF in FRP-strengthened KT-joints under bending moments, and reliability of the proposed equations was examined using Fatigue Guidance Review Panel recommendations.

TiAl intermetallics was vacuum-brazed to GH3030 alloy with multi-component Ti-based amorphous filler metal (Ti36.1Zr11.1V2.8Cu38.9Ni5.6Al2.8Co2.8, at.%). The effect of brazing temperature on the microstructure evaluation and shear strength of TiAl/GH3030 brazed joint was studied. The brazed seam presented sectionalized feature with the formation of four reaction zones. Two transition zones with linear growth formed respectively adjacent to two base metals due to elemental solid-state diffusion from the filler metal to base metals. Increasing the brazing temperature accelerated the diffusion between the filler metal and base metals. And those two transition zones thickened, as well as the middle zone composed of primary phase and (Ti, Zr)(Al, Cu, Ni) matrix. Blocky primary phase Ti(Al, Cu, Ni)3 in middle brazed seam increased, thinning the Ti(Ni, Cu) reaction layer. The maximum shear strength of the brazed joint was obtained at 990 °C. During the shear test, the cracks of the brazed joint propagated along the interface of the transition zone/middle brazed seam at TiAl base metal side on account of the stress concentration and the formation of Ti(Al, Cu, Ni) and Ti(Al, Cu, Ni)3 at two sides of the fractured interface. And the failed joint exhibited layered fracture surfaces and brittle fracture feature.

In this research, a single-diamond grinding test was performed on sintered silicon carbide (SSiC) to explore the damage formation mechanism. A scanning electron microscope and a transmission electron microscope (TEM) were used to examine the surface and subsurface morphologies of the grinding groove, respectively. The characteristics of the ground surface morphologies reveal that the single-diamond grinding process of SSiC can be classified into purely ductile, primarily ductile, primarily brittle, and purely brittle stages. Based on the high-resolution TEM (HRTEM) images and the corresponding Fast Fourier transform images of the near-surface region, results reveal that the high density of dislocations and amorphization of SiC grains are responsible for the plastic deformation of SSiC. Most of the cracks congregate on the top grains of the ground surface due to the distinct obstruction of the grain boundary on the cracks propagation, and the cracks generated at the grain boundaries emit into the top grain interiors and go up toward the exposed surface for the distortedly deformed region with higher strain energy; Furthermore, stress concentration caused by the dislocation pileups at grain boundaries represents the crack initiation mechanisms for SSiC. Finally, based on the dislocations pile-up theory, a critical undeformed chip thickness model for boundary crack system nucleation is established, which considers the cutting-edge radius, grinding wheel speed, material properties, and grain size of ceramics.

The effect of surface gradient nanostructure on the fatigue life of commercial pure (CP) Zr was investigated. Four point bending fatigue tests indicated that the fatigue limit of CP Zr with surface gradient nanostructure was increased by about 28.3 % compared with the original sample (annealed state). The microstructure evolution at different fatigue loading stages was characterized. The high strength of surface gradient nanostructure could increase the crack initiation resistance. Furthermore, electron back scattered diffraction (EBSD) analysis demonstrated that the surface nanocrystals grew and rotated gradually during the fatigue loading, which was beneficial to reducing stress concentration, inhibit fatigue crack initiation, and prolong crack initiation life. The stored distortion energy of CP Zr calculated before and after fatigue indicated that the stored distortion energy decreased dramatically during cyclic loading, which provided the driving force for grain growth. Besides, the growth of nanocrystals consumed the mechanical energy produced by the applied load to a certain extent, thus, slowing down the accumulation of fatigue damage. The coarse grains at the interior could deform plastically and reduce the crack growth rate. In addition, the compressive residual stress caused by USSP treatment reduced the local effective stress and the driving force of crack growth.

A 3-dimensional finite element model was built to determine the effect of inclination angle of a corrosion defect on local mechano-electrochemical (M-E) effect in a simulated soil solution. Because of the high effect of the defect inclination angle on the M-E effect, when the inclination angle is 0° (i.e., the primary axis of the defect parallel to the longitudinal direction of the pipe), the greatest stress concentration level at the defect can be observed, which is associated with the lowest corrosion potential, the greatest anodic current density and the most serious accelerated localized corrosion. When the inclination angle is 90°, the stress concentration level reduces and the corrosion potential becomes less negative, accompanying with the decreased anodic/cathodic current densities. Besides, when the ratio (rca) of the primary axial length of the defect to its secondary axial length is 1, the defect inclination does not affect the stress and the electrochemical corrosion rate at the defect. With the increase of rca, the effect of the defect inclination angle is more apparent.

The focus of this study is to analyze hydrogen embrittlement susceptibility of a modified AISI 4130 steel by means of incremental step loading tests. Three different microstructures with a hardness of 40 HRC were analyzed: martensite with large and small prior austenite grains and dual-phase (martensite/ferrite). According to the results, the dual-phase microstructure presented the lowest hydrogen embrittlement susceptibility and martensite with large prior austenite grains, the highest. This behavior was attributed to the lower fraction of high-angle boundaries presented by the martensite with large prior austenite grains, which led to a higher diffusible hydrogen content. Moreover, the ferrite local deformation in the dual-phase microstructure enhanced its hydrogen embrittlement resistance by lowering the stress concentration. A synergic effect of decohesion and localized plasticity was identified on the hydrogen induced fracture of the tested microstructures leading to an intergranular + quasi-cleavage fracture in the martensite and quasi-cleavage in the dual-phase microstructure.

The reinforced concrete columns and steel beams (RCS) composite frames have received popularity in low to moderate seismic regions due to advantages of the system. However, the poor repairable capacity and deteriorating pinched hysteretic behaviour of the traditional RCS framed structure limit the application in practice under earthquakes. This study presents a modified RCS framed structure with replaceable members aiming to enhance the energy dissipation and resilience capability. The structural configuration character and the general predetermined levels of lateral response of the proposed RCS framed structure with replaceable members is first introduced. Seven quasi-static loading tests were subsequently conducted to evaluate the seismic behavior and replaceability of the replaceable shear links. Parameters including the link length ratio, stiffener configuration and link-to-column connection were examined. The test results depicted that the specimens exhibited the shear dominated behavior. The major failure features of the shear links included web-to-stiffener weld tear, web buckling and web fracture. All tested specimen showed a stable bearing strength, large inelastic rotation capacity and excellent energy dissipation behavior. The same link length ratio of the specimens with different specialized connections presented the similar behavior. Noted that end plate and web stiffeners connection can alleviate the stress concentration on the fillet weld of the flange-to-end plate weld and optimize the performance of the shear link. The average overstrength factor and inelastic rotation capacity of the test specimens were 1.9 and 0.087 rad significantly exceeding 1.5 and 0.08 rad provided by the AISC 341-16 provisions for the EBF links, respectively. In addition, the replaceability of the specimens was evaluated underwent the entire loading process. Compared with the T-shaped and Π-shaped plate connections, replacement of the shear link with the end plate demanded the least effort and time.

To improve the interfacial adhesion and mechanical performance of PBO fiber composites, CNTs were uniformly grafted onto them at a high-density. The grafting of CNTs with massive reactive groups can improve the surface wettability and interfacial interaction of PBO fibers with epoxy resin. The IFSS and ILSS values of the modified composites (PBO-CNT-3) increased by 103.09 and 62.73%, respectively. As CNTs can strengthen interfacial regions of the composites, the mechanical properties (hardness and modulus) of the interphase were enhanced significantly. This led to the effective transfer of interfacial load, elimination of stress concentration, and improvement in the structural stability of the composites. As a result, the impact strength of the modified composites (PBO-CNT-3) was up to 103.76 kJ/m2 (an increase of 56.24%) compared to the original composites. The surface morphology and deformation behavior of the fractured composites indicate that the interfacial failure mode of the composites grafted with CNTs changes from adhesive failure to both cohesive and substrate failure. This strategy of grafting CNTs at a high-density opens a new avenue for the interfacial regulation of structural composites, ultra-capacitor, sensor, and catalytic materials.

The tensile-tension fatigue behavior of minicomposite SiC/(PyC/SiC)2/SiC at room temperature was studied, and the residual mechanical properties of specimens were tested after 106 pre-fatigue cycles under different levels of stress. The results show that the residual strength first increases owing to the release of stress concentration and then decreases owing to excessive fiber wear. In addition, it is worth noting that the tensile curve of pre-fatigue specimens deflects twice; the first occurrence correlates with matrix crack reopening, and the second occurs when the uniaxial tensile load exceeds the pre-fatigue stress, and the degree of deflection also gradually decreases or almost disappears.

This work addresses the role of rigid inclusions in determining the elastic modulus of particle-filled colloidal networks by modifying an established fractal scaling model. The approach acknowledges the heterogeneous nature of stress distribution at length scales beyond the colloidal aggregates, while maintaining structural information at the level of individual clusters. This was achieved by introducing a scaling factor to account for system heterogeneity which contains intrinsic information about the network’s capacity to form load-bearing links. Rigid fillers bound to the network induce stress concentration, but additionally serve as junction zones which introduce additional load-bearing pathways. This gives rise to the observed increase in the modulus with filler volume fraction. The proposed relationship between the load-bearing network connectivity and scaling behavior may have additional implications on the fractal dimension determined by rheological methods. Further, this model accommodates an experimentally observed correlation between the scaling behavior of the modulus associated with the addition of fillers and that arising from increasing structurant concentration. The modified fractal model thus provides an alternative view of how fillers contribute to the small- and large-deformation mechanical behavior of filled colloidal gels in a manner consistent with experimental observations.

Grain-boundary (GB) precipitation has a significant adverse effect on plasticity of alloys, which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading. Herein, we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs. This in situ deformation-induced phase transformation at GBs introduces a well-known effect: transformation-induced plasticity (TRIP), which enables an exceptional elongation to fracture (above 38 %) at a high strength (above 1.5 GPa) in a GB precipitation-hardened high-entropy alloy (HEA). The present strategy in terms of “local stress concentration-induced phase transformations at GBs” may provide a fundamental approach by taking advantage of (rather than avoiding) the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.

This study provides an insight into the structure-property relationship of graphene network (GN) reinforced metal matrix composites (MMCs). Molecular dynamics results show that the interlocking structure improves the deformation compatibility between the GN and matrix, as well as increases the load-sharing of GN. And the mechanical interlocking can be further enhanced by the dislocation-GN interactions as the GN being straightened. The straightening of GN also enables itself to release its stress concentrations through overall deformation, which helps the GN perform better in dislocation-blocking than graphene sheets. Besides, curved portions of GN contribute more to dislocation-GN interactions while the straight portions do better in load-sharing. With the hole-like defects on whether straight or curved portions of GN, both the strength and Young's modulus of the imperfect GN reinforced composites are reduced. However, these defects enable the matrix to be connected into a continuous body, by which the ductility of imperfect GN reinforced Ni matrix composites is improved since their failures are mainly dictated by the fracture of Ni matrix, while the rupture of GN determines the failure of composites reinforced by the GN without hole-like defects.

Combination of hot isostatic pressing (HIP) and rejuvenation heat treatment (RHT) technology was used to restore creep-damaged DZ125 directional solidified superalloy, and the influence of microstructure restoration on high temperature fatigue behavior of the samples was explored. The results show that the HIP+RHT process could effectively heal internal cavities and recover the degraded γ′ phase in creep-damaged DZ125 superalloy to cubic particles similar as in as-received sample. After restoration treatment, the stress concentration areas inside the sample eradicated with the healing of the internal cavities, and the fatigue source areas were limited only to near surface than initiating from inside as in the as-received and creep-damaged samples. As a result, the restored sample had higher crack initiation life and lower crack propagation rate compared to as-received and creep damaged samples. The TEM microstructure characterization near fatigue fracture showed that the restoration of the degraded γ′ phase eliminated tangled dislocation in creep damaged sample and produced evenly distributed dislocations in the γ channel with short curved line-like morphology, like the as-received sample, which effectively hindered the dislocations movement during subsequent deformation, and strengthen the fatigue resistant of alloy. Therefore, it can be concluded that the HIP-RHT process, through the combined effect of internal cavities healing and the restoration of the degraded microstructures, renders higher high temperature fatigue life than creep-damaged and even higher than as-received DZ125 superalloy.

In this paper, two steels with different morphology of inclusions are studied. Scanning electron microscopy, finite element calculations, and electrochemical hydrogen permeation tests were conducted. Furthermore, in-situ observations of hydrogen de-trapping from the surface of steels were achieved using a hydrogen charging device and optical microscope. Results show that stripe-shaped complex inclusions exhibit a higher level of residual stress, especially at the boundary of oxide components. The hydrogen molecule is also prone to emerge at the oxide composition and intensifies the risks of cracking. In contrast, the residual stress levels around spherical inclusions are comparatively lower due to discrete sites of stress concentration. Thus, the effect of dispersed tiny dioxide sphere inclusions can help relieve the local hydrogen pressure and promote the resistance against hydrogen-induced cracking.