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Dual phase (DP) steels are amongst the most widely used structural steels for automotive applications. It is essential to understand the damage initiation and damage growth in these high strength steels and further shed light on improving mechanical properties. In this work, two DP800 dual phase grades are investigated, which exhibit identical ultimate tensile stress but significantly different elongation in the uniaxial tensile test. To explain the difference in ductility, particularly described by uniform elongation, we investigate the damage initiation and growth mechanisms by analyzing microstructural changes upon deformation, such as voids, dislocation structures and the grain morphology. Furthermore, ferrite micropillars in pre-strained samples are tested in situ to capture the strain hardening capability of ferrite. We found that the DP steel with harder martensite and softer ferrite exhibits more damage initiation sites after deforming to an identical strain. However, void growth is much slower compared to the DP steel grade with fewer initiation sites. We explain the suppressed void growth by significant strain-hardening of ferrite surrounding the voids, which is observed in the micropillar compression experiments. The improved strain hardening of ferrite originates primarily from the difference in chromium content considering the negligible influence of dispersed particles.

Bridge maintenance and repair have gained significant attention as their service life continues to extend. Numerous retrofit projects have been implemented to address the corrosion issues at steel plate girders' ends. This study focuses on evaluating the durability of the retrofitted part of the steel girder end by assessing its ability to sustain eccentric fatigue load and recognizing the possibilities of CFRP post-retrofitted damage. The novel specimen was designed to replicate corrosion on the inner side of exterior steel girder web ends and study the effect of retrofitting them with CFRP. In addition, a new bending fixture design based on the specimen design was proposed to reproduce the actual severe loading mode of a CFRP-retrofitted steel girder end. Polyurea putty (FU-Z) was used to identify the durability owing to its flexible characteristics under fatigue. The lifecycle-oriented experimental results showed that the specimens with CFRP-retrofitted steel girder ends survived for 3.5 million cycles under fatigue loading. However, specimens without polyurea putty exhibited damage characteristics, such as detachment and delamination. In addition, the shifted local stress detection of the edge joint represented the detachment of the retrofit material at 3 million cycles. The detachment occurred at the end of the corrosion web edge and delamination was observed in the cutoff section, which reduced the local bonding of 50% of the CFRP sheets. In addition, local damage resulted in durability resistance developed under elastic conditions. [ABSTRACT FROM AUTHOR]

The study aims at investigating the failure behavior of Norway spruce clear wood specimens using XFEM by modeling earlywood and latewood growth rings. The growth ring pattern in the specimens are determined using a photo-analytical processing program. For the simulations, 6 damage initiation mechanisms were considered and the subroutines were implemented in the finite element solver ABAQUS. A normal vector to the crack surface is calculated for each damage mode, taking into account the orientation of the maximum and minimum principal stresses. The XFEM model and the material properties are calibrated based on experiments, where the edge-notched specimens were subjected to 3-point bending, tensile and compression tests. The goal is to determine the ultimate load, locate structural weakpoints and obtaining a realistic crack propagation path by means of XFEM.

Mechanical responses and damage behaviors of asphalt pavement are closely associated with the mesostructure characteristics of the asphalt mixture. The aim of this research was to investigate the effect of the different mesostructure characteristics of asphalt mixtures on pavement damage behavior through a multiscale finite-element (FE) analysis method. A two-dimensional (2D) random aggregate generation algorithm was used to construct the mesostructures of asphalt mixtures with different characteristics, including aggregate distribution uniformity, shape, volume fraction, and size range. Bilinear cohesive elements were used to model damage initiation within asphalt pavement. Also, the nonuniformity coefficient (NUC) was applied to evaluate the uniformity of aggregate distribution within asphalt pavement. The results indicated that the NUC could well characterize the uniformity of coarse aggregate distribution in pavement, and the damage initiation zone became larger as the NUC increased. Compared with regular aggregates, irregular shape aggregates led to more damage initiation in asphalt pavement. For regular shape aggregates, the damage initiation zone became smaller as the aggregate shape approached the circle. In addition, the decrease of aggregate size and volume fraction could increase the possibility of "top-down" cracking of pavement. [ABSTRACT FROM AUTHOR]

A bainite dual-phase (FB) steel containing polygonal ferrite and granular bainite is thermo-mechanically rolled, followed by an accelerated cooling. Two different cooling rates are applied to obtain two different materials. The aim of the study is to explore the reasons for the differences in the mechanical response experimentally observed for these two materials which are modeled by means of high-resolution crystal plasticity simulations with a phenomenological constitutive description. First, the CP parameters of the individual constituents are determined. Second, different three-dimensional (3D) representative volume elements (RVEs)—one of which includes the substructure of bainite—are used to study the mechanical properties of both FB microstructures. It is shown that, in contrast to the macroscopic response, the microscopic response differs among the RVEs. Third, a comparison of both materials is performed by analyzing their stress–strain response. The onset of plasticity in granular bainite is found to be different for both materials in addition to the strain partitioning, although they both obeyed the iso-work assumption. Finally, a parameter study is carried out in order to investigate the correlation between different microstructures and damage initiation that can be seen experimentally in this steel. It is shown that the difference in ultimate elongation may depend on whether the first voids appear within polygonal ferrite or at the phase boundary. [ABSTRACT FROM AUTHOR]

Damage initiation and localization in uniaxial tension tests of an Al6061‐T6 alloy are evaluated and quantified using in situ X‐ray computed tomography and a neural network segmentation strategy. This segmentation strategy offers significant advantages over traditional thresholding in quantifying microstructural and damage evolution metrics. Void nucleation was observed to occur throughout the deformation process, whereas void volume did not grow significantly until later stages of deformation. Localized analysis determined that void nucleation begins in and around the necked region during the early stages of deformation, whereas limited damage nucleation occurs outside of the neck until late stages of deformation. Lastly, inhomogeneities in void and particle spacing in the initial undeformed state of the material are observed to correlate well with the location of damage localization and necking during deformation. These results provide new insights into the ductile failure of Al alloys and how preexisting defects influence damage and rupture. Highlights: Void nucleation occurs throughout the deformation process.Void volume is constant until later stages of deformation.Void nucleation is concentrated in the neck region during early stages of deformation.Void and particle spacing in the undeformed state correlate with damage localization. [ABSTRACT FROM AUTHOR]

Electron Beam Melting (EBM) is a widespread additive manufacturing technology for metallic-part fabrication; however, final products can contain microstructural defects that reduce fatigue performance. While the effects of gas and keyhole pores are well characterized, other defects, including lack of fusion and smooth facets, warrant additional investigation given their potential to significantly impact fatigue life. Therefore, such defects were intentionally induced into EBM Ti-6Al-4V, a prevalent titanium alloy, to investigate their degradation on stress-controlled fatigue life. The focus offset processing parameter was varied outside of typical manufacturing settings to generate a variety of defect types, and specimens were tested under fatigue loading, followed by surface and microstructure characterization. Fatigue damage primarily initiated at smooth facet sites or sites consisting of un-melted powder due to a lack of fusion, and an increase in both fatigue life and void content with increasing focus offset was noted. This counter-intuitive relationship is attributed to lower focus offsets producing a microstructure more prone to smooth facets, discussed in the literature as being due to lack of fusion or cleavage fracture, and this study indicates that these smooth flaws are most likely a result of lack of fusion. [ABSTRACT FROM AUTHOR]

In the last decade, fatigue damage models for fiber-reinforced polymer composites have been developed assuming the fracture energy equivalence hypothesis. These models are able to predict a fatigue life of composite laminates, but their identification requires a significant number of off-axial tests for various stress ratios. The present study proposes the stress ratio dependent model, which phenomenologically adopts a decrease in stiffness and residual strength of a unique ply according to experimental constant life diagrams. Hashin, Tsai–Hill, and the maximum stress failure criteria are utilized for damage initiation considering the residual strength of the ply. The obtained results indicate a sufficiency of using S-N curves for UD 0°, UD 45°, and UD 90° for identification of the model. The model was verified by S-N curves for UD 10°, UD 15°, and UD 30° and its applicability was demonstrated for prediction of a fatigue life of composite laminates with an arbitrary lay-up. The model is implemented into ABAQUS finite element software as a user subroutine. [ABSTRACT FROM AUTHOR]

Fatigue failure usually occurs on the subsurface in rolling bearings due to multiaxial and non-proportional fatigue loadings between rolling elements. One of the main stress components is the alternating shear stress. This paper focuses on the micromechanism of plastic accumulation and damage initiation in bearing steels under cyclic shear deformation. The distribution of subsurface shear stress in bearings was firstly investigated by finite element simulation. An atomic model containing bcc-Fe and cementite phases was built by molecular dynamics (MD). Shear stress–strain characteristics were discussed to explore the mechanical properties of the atomic model. Ten alternating shear cycles were designed to explore the mechanism of cyclic plastic accumulation and damage initiation. Shear stress responses and evolutions of dislocaitons, defect meshes and high-strain atoms were discussed. The results show that cyclic softening occurs when the model is in the plastic stage. Severe cyclic shear deformation can accelerate plastic accumulation and result in an earlier shear slip of the cementite phase than that under monotonic shear deformation, which might be the initiation of microscopic damage in bearing steels. [ABSTRACT FROM AUTHOR]

Electron Beam Melting (EBM) is a widespread additive manufacturing technology for metallic-part fabrication; however, final products can contain microstructural defects that reduce fatigue performance. While the effects of gas and keyhole pores are well characterized, other defects, including lack of fusion and smooth facets, warrant additional investigation given their potential to significantly impact fatigue life. Therefore, such defects were intentionally induced into EBM Ti-6Al-4V, a prevalent titanium alloy, to investigate their degradation on stress-controlled fatigue life. The focus offset processing parameter was varied outside of typical manufacturing settings to generate a variety of defect types, and specimens were tested under fatigue loading, followed by surface and microstructure characterization. Fatigue damage primarily initiated at smooth facet sites or sites consisting of un-melted powder due to a lack of fusion, and an increase in both fatigue life and void content with increasing focus offset was noted. This counter-intuitive relationship is attributed to lower focus offsets producing a microstructure more prone to smooth facets, discussed in the literature as being due to lack of fusion or cleavage fracture, and this study indicates that these smooth flaws are most likely a result of lack of fusion.

This study investigates the damage initiation in short glass fiber-reinforced polyamide 6.6 under fatigue loading using acoustic emission analysis. An optimized specimen geometry was developed to meet the specific requirements of this testing method, at the same time allowing further micromechanical studies. Specimens were preloaded with tensile-tensile fatigue loading, varying the maximum stress and the number of load cycles. Subsequently, the acoustic emission signals in residual strength tests were compared to those of undamaged specimens. The idea behind this approach is that only the damage that has not already occurred under fatigue load can be recorded in the residual strength tests. Using the analysis of acoustic energy, a stress threshold for damage initiation was identified. Furthermore, with tension-tension fatigue tests, the SN curve of the material was determined to estimate the lifetime for the identified stress threshold. The presented approach allows us to estimate a so-called endurance limit of short glass fiber-reinforced polyamide 6.6. [ABSTRACT FROM AUTHOR]

The developing roles of damage mechanisms in the failure response of SiC/SiC minicomposites was investigated by the characterization of microscale damage accumulation with respect to microstructure. A multi-modal approach combining spatially resolved acoustic emission (AE) with tensile testing in-SEM (scanning electron microscope) was used to simultaneously examine surface (observed in-SEM) and bulk damage (monitored via AE). Strong agreement was shown between the evolving crack density estimated by AE and in-SEM measurements. The following were observed: (i) in-plane matrix content and distribution impacted crack growth; (ii) spatially-distributed matrix cracks generated varying stress-dependent AE; and (iii) certain individual cracks became more probable failure locations due to unique combinations of damage mechanisms that drove their growth. This approach enabled characterizing potential failure determinants and suggests that early damage behavior is related to certain microstructural features (e.g. surface flaws), while subsequent damage behavior is coupled to interactions of local mechanisms evolving with stress. [ABSTRACT FROM AUTHOR]

Fatigue failure usually occurs on the subsurface in rolling bearings due to multiaxial and non-proportional fatigue loadings between rolling elements. One of the main stress components is the alternating shear stress. This paper focuses on the micromechanism of plastic accumulation and damage initiation in bearing steels under cyclic shear deformation. The distribution of subsurface shear stress in bearings was firstly investigated by finite element simulation. An atomic model containing bcc-Fe and cementite phases was built by molecular dynamics (MD). Shear stress–strain characteristics were discussed to explore the mechanical properties of the atomic model. Ten alternating shear cycles were designed to explore the mechanism of cyclic plastic accumulation and damage initiation. Shear stress responses and evolutions of dislocaitons, defect meshes and high-strain atoms were discussed. The results show that cyclic softening occurs when the model is in the plastic stage. Severe cyclic shear deformation can accelerate plastic accumulation and result in an earlier shear slip of the cementite phase than that under monotonic shear deformation, which might be the initiation of microscopic damage in bearing steels.

Curved laminates in aero-structures, such as the L-angle sections where webs and flanges meet, are prone to delamination due to high interlaminar stresses in these regions. Some efforts to investigate delamination in these structures can be found in the literature but commonly structures are limited to unidirectional layups or modelling approaches are constrained to the cohesive element based methods. In this work, multi-directional L-angle laminates were manufactured using unidirectional prepregs and tested under four-point bending load conditions to examine the interface damage. Acoustic emission technique was used to assist the capture of damage initiation and propagation. Three interface modelling strategies for predicting delamination, namely cohesive element, cohesive surface and perfectly bonded interface were used in the numerical study. The interface damage behaviour was successfully predicted by the simulation methods and differences among the strategies were compared. [ABSTRACT FROM AUTHOR]

AlSi10Mg manufactured by laser powder bed fusion (or selective laser melting) benefits from a very fine microstructure that imparts significant mechanical strength to the material compared to the cast alloy. The build platform temperature stands out as a significant processing parameter influencing the microstructure as it affects the cooling rate and thermal gradient during manufacturing. Setting the build platform temperature to 200°C yields a negligible residual stress level. However, the strength is lower compared to that obtained using a build platform temperature of 35°C, with a similar fracture strain. A detailed 3D microstructural analysis involving focused ion beam/scanning electron microscopy tomography was performed to describe the connectivity and size of the Si-rich eutectic network and link it to the strength and fracture strain. The coarser microstructure of the 200°C build platform material is more prone to damage. The α-Al cells as well as the Si-rich precipitates present a larger size in the 200°C material, the latter thus having a lower strengthening effect. The Si-rich eutectic network is also less interconnected and has a larger thickness in the 200°C material. An analytical model is developed to exploit these microstructural features and predict the strength of the two materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Bio-mechanical response of fixed fractured tibia-bone for two ranges of patient ages. • Time evolution of callus bones properties during healing period in relation to patient age. • Gap capacity to resist and allow a gradual BWP load transfer, through the callus for the fractured tibia bone. The evolutionary healing phenomenon of fractured tibia bone was investigated by comparing the bio-mechanical response of the human tibia following fracture fixation for two ranges of patient ages, when a body weight pressure (BWP) is applied. Three-dimensional finite element models have been developed by adopting the biomechanical characteristics of cortical and trabecular tibia bones, and considering the time-varying callus properties during the healing process for the two patients. The stress and strain levels generated within the fractured tibia bone by the screw tight fit during the assembly process revealed its dependence on the bone stiffness that degrades with age. They have an impact on primary stability of the implants prior to the osseointegration. The gap capacity to resist and allow a gradual BWP load transfer, through the callus for the tibia bone models, was analyzed. In fact, from 10 weeks after surgery, the callus allowed the BWP transfer for young patients, which guarantees sufficient structure stabilization of the fractured tibia. However, an insufficient load was transferred to the fracture gap for the old patient, even beyond 16 weeks, which delayed the bone consolidation [ABSTRACT FROM AUTHOR]

[Display omitted] • The dual phase steel initiating more damage sites exhibits longer uniform elongation, as the damage evolution is suppressed. • The suppression of damage growth primarily benefits from higher strain hardening capacity of ferrite in this steel grade. • The improved strain hardening of ferrite originates from alloy elements Cr and Ti. • Strain hardening capacity of ferrite is key to tailor the tradeoff of strength and ductility in dual phase steels. Dual phase (DP) steels are amongst the most widely used structural steels for automotive applications. It is essential to understand the damage initiation and damage growth in these high strength steels and further shed light on improving mechanical properties. In this work, two DP800 dual phase grades are investigated, which exhibit identical ultimate tensile stress but significantly different elongation in the uniaxial tensile test. To explain the difference in ductility, particularly described by uniform elongation, we investigate the damage initiation and growth mechanisms by analyzing microstructural changes upon deformation, such as voids, dislocation structures and the grain morphology. Furthermore, ferrite micropillars in pre-strained samples are tested in situ to capture the strain hardening capability of ferrite. We found that the DP steel with harder martensite and softer ferrite exhibits more damage initiation sites after deforming to an identical strain. However, void growth is much slower compared to the DP steel grade with fewer initiation sites. We explain the suppressed void growth by significant strain-hardening of ferrite surrounding the voids, which is observed in the micropillar compression experiments. The improved strain hardening of ferrite originates primarily from the difference in chromium content considering the negligible influence of dispersed particles. [ABSTRACT FROM AUTHOR]

The competition between ductile rupture mechanisms in high-purity Cu and other metals is sensitive to the material composition and loading conditions, and subtle changes in the metal purity can lead to failure either by void coalescence or Orowan Alternating Slip (OAS). In situ X-ray computed tomography tensile tests on 99.999% purity Cu wires have revealed that the rupture process involves a sequence of damage events including shear localization; growth of micron-sized voids; and coalescence of microvoids into a central cavity prior to the catastrophic enlargement of the coalesced void via OAS. This analysis has shown that failure occurs in a collaborative rather than strictly competitive manner. In particular, strain localization along the shear band enhanced void nucleation and drove the primary coalescence event, and the size of the resulting cavity and consumption of voids ensured a transition to the OAS mechanism rather than continued void coalescence. Additionally, the tomograms identified examples of void coalescence and OAS growth of individual voids at all stages of the failure process, suggesting that the transition between the different mechanisms was sensitive to local damage features, and could be swayed by collaboration with other damage mechanisms. The competition between the different damage mechanisms is discussed in context of the material composition, the local damage history, and collaboration between the mechanisms. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Understanding of radiation damage in a solid is important for development of advanced material technologies, namely, for space application, electron microscopy, semiconductor processing, fission and nuclear fusion. Space materials must withstand exposure to high-energy protons. Construction materials of future thermonuclear reactors must withstand exposure to high-energy neutrons. Many properties of materials including mechanical properties are governed by the presence and behavior of lattice defects. Hence, the study of the radiation-induced defects is an important task combining two fields: radiation physics and solid state physics. The paper presents new knowledge on primary defect formation in the main materials for advanced fission and nuclear fusion reactors, bcc tungsten (W) and bcc iron (Fe). The objective of this work is to compare the new experimental data of neutron- and proton-induced defects in W and Fe using well-established method of positron-annihilation lifetime-spectroscopy (PALS) in combination with the literature data with two models of radiation damage, the classical Norgett-Robinson-Torrens (NRT-dpa) model and recently developed athermal recombination corrected (arc-dpa) model. It is shown that experimental data for neutron- and proton-irradiated Fe are better described by arc-dpa model than NRT-dpa model. Whereas experimental data for neutron- and proton-irradiated W are between the NRT-dpa and arc-dpa predictions. The obtained results shed new light on the formation of the primary radiation defects in materials and indicate the need for further development of the theory of radiation damage in a solid. [ABSTRACT FROM AUTHOR]

Prediction of the fiber crack density (as one of the microstructural damages) for unidirectional fiber-reinforced polymer composite under monotonic tensile load, using strength models, has been reported in the literature. However, the microstructural damage prediction for a fiber-reinforced polymer subjected to fatigue loading is still a challenge. In this work, a progressive damage initiation model was developed to predict the fiber crack density in carbon fiber-reinforced polymer composite subjected to fatigue loading. A stochastic model was used for modeling the fiber fatigue strength. Reduction in effective life of the fiber was modeled using linear Miner's rule. Effect of fatigue strength parameters on fiber crack density was found to be considerable compared to the effect of interface shear strength. At a low number of cycles, fiber crack density obtained from the model was in good agreement with the experimentally measured fiber crack density. [ABSTRACT FROM AUTHOR]

Abstract Damage initiation is an important issue to understand the mechanical behavior of ceramic matrix composites. In the present work, a braided SiC/SiC composite tube was studied by FFT simulation tightly linked with micro-computed tomography (µCT) observations performed during an in situ uniaxial tensile test, which provide both the real microstructure, with a good description of local microstructural geometries, and location of cracks at the onset of damage. The FFT method was proven applicable to tubular structures and efficient to complete the large-scale simulation on a full resolution µCT scan (∼6.7 billion voxels) within a short time. The edge effect due to the numerical periodic boundary conditions prescribed on the real and not rigorously periodic microstructure was quantified. The obtained stress field was compared to the cracks detected by the in situ µCT observations of the same composite tube. This one-to-one comparison showed that cracks preferentially initiated at tow interfaces, where sharp edges of macropores are mostly located and generate stress concentrations. [ABSTRACT FROM AUTHOR]

Martensite/ferrite (M/F) interface damage is relevant to failure of many dual-phase (DP) steels, but the underlying microscale mechanisms remain unclear. Through an integrated experimental-numerical study, this work examines the recent hypothesis that (lath) martensite substructure boundary sliding triggers and dominates M/F interface damage initiation accompanied by apparent martensite plasticity. The mesoscale morphology and prior austenite grain reconstruction are used as modelling inputs. A multi-scale framework is adopted to predict the interface damage initiation. The M/F interface damage initiation sites predicted by the model based on a sliding-triggered interface damage mechanism adequately agree with those identified from in-situ experiments, confirming the key role of substructure boundary sliding. Moreover, the M/F interface damage initiation strongly correlates with a low M/F strain partitioning rather than the commonly accepted strong M/F strain partitioning. This fundamental understanding is instrumental for the future optimization of DP steel microstructures. [ABSTRACT FROM AUTHOR]

A coupled crystal plasticity phase field damage framework has been developed and applied to modelling damage initiation. A novel implementation of a grain misorientation angle dependent critical energy release rate has been used to determine a reduction in the local critical energy release rate resulting from the effects of intergranular carbide precipitates and grain boundary misorientation. When applied to a notched high temperature 316H austenitic stainless steel specimen, a good correlation between experimental results and void nucleation statistics for a misorientation dependent critical energy release rate was obtained. This has been evaluated through comparison with correlative electron microscopy experimental results, showing the potential of phase field models in the area of early damage formation. Additions to include plastic strain and creep deformation effects were made, and comparisons were drawn with experimental data to investigate the contributions of microstructural geometry properties such as the difference in and average values of Schmid factors across grain boundaries, as well as the loading direction stress and dislocation densities. The limitations to this approach and opportunities for further work in this area are discussed, with specific interest in the need for additional literature data characterising grain boundary carbide precipitation and cavity nucleation analysis. [Display omitted] • Grain boundary misorientation angles affect the precipitation rate of intergranular carbides in 316H stainless steel. • A novel implementation of a misorientation dependent critical energy release rate was used to model their effect on damage. • A coupled crystal plasticity phase field finite element modelling framework was developed for early damage formation. • A strong correlation in damage initiation trends with experimental data was obtained over the same microstructure. [ABSTRACT FROM AUTHOR]

Through in situ electron backscatter diffraction (EBSD) experiments, this paper uncovers dominant damage mechanisms in traditional magnesium alloys exhibiting deformation twinning. The findings emphasize the level of deleterious strain incompatibility induced by twin interaction with other deformation modes and microstructural defects. A double fiber obtained by plane-strain extrusion as a starting texture of AM30 magnesium alloy offered the opportunity to track deformation by EBSD in neighboring grains where some undergo profuse {1 0 1 2} twinning and others do not. For a tensile loading applied along extrusion transverse (ET) direction, those experiencing profuse twinning reveal a major effect of grain boundaries on non-Schmid behavior affecting twin variant selection and growth. Similarly, a neighboring grain, with its 〈c〉-axis oriented nearly perpendicular to tensile loading, showed an abnormally early nucleation of {1 0 1 1} contraction twins (2% strain) while the same {1 0 1 1} twin mode triggering under 〈c〉-axis uniaxial compression have higher value of critical resolved shear stress exceeding the values for pyramidal 〈c + a〉 dislocations. The difference in nucleation behavior of contraction vs. compression {1 0 1 1} twins is attributed to the hydrostatic stresses that promote the required atomic shuffles at the core of twinning disconnections.

Due to the extremely low permeability and the excellent creep behavior, rock salt is the optimal surrounding rock of underground energy storage. The long-term safe operation of the rock salt energy storage is closely related to the creep behavior and long-term strength of rock salt, but few researches focus on the long-term strength of rock salt. In order to more accurately predict the long-term strength of rock salt, the isochronous stress–strain curve method and the volume expansion method for determining the long-term strength were analyzed and discussed based on axial compression tests and axial creep tests. The results show that the isochronous stress–strain curve method is intuitive but will greatly increase the test cost and test time to obtain a satisfactory result. The volume expansion method is simple, but the long-term strength obtained according to the inflection point of volumetric strain is much greater than the actual long-term strength of rock salt. Therefore, a new method applicable to rock salt was proposed based on the evolution of damage in rock salt in this paper, which takes the corresponding stress value at the damage initiation point as the long-term strength. The long-term strength determined by this method is consistent with that by the isochronous stress–strain curve method. The method is more economical and convenient and aims to provide a reference for the long-term stability study of underground salt caverns.

This study focuses on exploring the initial failure of thermally degraded carbon fibre-reinforced polymers. It is the aim of this study to provide deep insight into the damage development and propagation as well as to understand failure mechanisms of thermally degraded composites. Carbon fibre-reinforced polymer panels with different fibre orientations are exposed to heat above maximum operational temperature (up to 200 ℃) for various durations (up to ca. 200 days). Thermal degradation of the material is characterized by scanning electron microscopy and infrared spectroscopy. The onset of the failure in tension is determined by acoustic emission analysis. The results show that the development and propagation of cracks depend on the level of thermal degradation and the fibre orientation relative to the applied load. With increasing thermal degradation, transverse matrix cracking, as the prevailing initiating failure mode, is replaced by crack initiation and propagation in the damaged outermost ply independent of its fibre orientation. Severe thermal matrix degradation is limited to this area, as characterized by infrared spectroscopy. With increasing thermal degradation of the polymer matrix, the onset of crack initiation and propagation is shifted to lower strains. The effects of damage initiation on fracture behaviour are discussed. [ABSTRACT FROM AUTHOR]

Abstract Multiphase bainitic steels, as a third generation of advanced high strength steels, show promising properties for automotive applications. Understanding the micro-mechanisms of damage initiation during plastic deformation is a key to further mechanical properties enhancement. The topological effect of martensite as a second phase constituent on local damage nucleation activity is significant. This effect has been studied in two different martensite banded microstructures produced by two various annealing cycles. The postmortem damage analysis by a scanning electron microscope on uniaxial loaded samples, revealed more damage nucleation along the dispersed and fragmented martensite phase within martensite banded regions. More pronounced strain partitioning was observed in coarse bainitic grains between adjacent martensite blocks. It is shown that the fracture strain is not controlled by local damage activities, implying that earlier damage initiation or an increased volume fraction of voids does not give rise to a reduced ductility. [ABSTRACT FROM AUTHOR]

A significant acceleration of degradation process is observed during occurrence of the self-heating effect, which has a character of mechanical energy dissipation and causes heating-up of structures. Following this, it is essential to evaluate a critical value of self-heating temperature at which a thermal domination of fatigue processes occurs. With a progress of degradation irreversible structural changes occur. In this study, an experimental evaluation of the degradation process was performed based on analysis of apparent thermal diffusivity of a composite structure at various self-heating temperature values. The obtained results show that the estimated apparent thermal diffusivity is a sensitive measure which allows for effective evaluation of structural changes caused by fatigue processes. [ABSTRACT FROM AUTHOR]

The effect of off-centre impact loading on damage initiation in a woven glass fibre reinforced epoxy resin was studied experimentally. Low velocity impact tests were conducted, in which the incident impact energy was increased until damage was observed in the laminates. It was shown that multiple impacts, with increasing incident energy at the same location, did not greatly influence the critical force for damage initiation, P crit . Subsequent testing on a range of panel sizes showed that the critical force is highest for central impacts, decreasing slowly as the impact location moves towards the boundary. It was also shown that, for off-centre impact loading, P crit , follows a t 3/2 (t = laminate thickness) relationship that has previously been established for central impact. The slope of the plot of P crit versus t 3/2 decreases as the impact location moves away from a central location, suggesting that the effective interlaminar shear stress also decreases with increasing offset. Tests at energies well above the damage threshold confirmed that off-centre impact is more serious than central impact loading. An energy-balance model was used to predict the off-centre impact response of the panels. Agreement between the energy-balance model and the measured impact response was good at energies that did not generate significant damage. Finally, it is suggested that the energy-balance model can also be used to predict a lower bound on the damage threshold energy in composite plates. [ABSTRACT FROM AUTHOR]

• Fracture experiments were performed on dual-phase steel at temperatures from 20 to 700 °C. • Five specimen shapes were used to generate stress triaxialities from 0 to 0.51. • Finite element analyses with a plasticity-damage model were calibrated on the experiments. • The dependency of the damage and fracture initiation strains with stress state and temperature was identified. • The calibrated material model can be used to simulate fracture of steel members in fire. Structural steels can be subjected to a range of stress and temperature conditions, and accurate characterization of their limit states is a requirement for structural design. Yet data on ductile fracture at elevated temperatures remains relatively scarce especially for high-strength steel grades. This paper describes an experimental and numerical investigation of the ductile fracture of a high-strength dual-phase steel under multiaxial stress states at temperatures up to 700 °C. Experiments were carried out on five geometries of specimens and at four temperatures to identify the temperature-dependent fracture locus across a range of stress triaxialities. Finite element analyses of the experiments provided the equivalent strain at fracture initiation. It was found that the fracture initiation strain varies markedly with stress triaxiality and increases with temperature. The effect of strain rate become significant at 500 °C. A temperature-dependent plasticity-damage model was calibrated to predict ductile fracture initiation in the dual-phase steel. The proposed model can be used in finite element analyses to support performance-based design of steel structures at elevated temperatures resulting from accidental events such as fire. [ABSTRACT FROM AUTHOR]