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Additive manufacturing (AM) techniques are widely investigated for the cost-effective use of titanium (Ti) alloys in various aerospace applications. One of the AM techniques developed for such applications is plasma transferred arc solid free-form fabrication (PTA-SFFF). Materials manufactured through AM techniques often exhibit anisotropies in mechanical properties due to the layer-by-layer material build. In this regard, the present study investigates the isothermal directional fatigue of a Ti-TiB metal matrix composite (MMC) manufactured by PTA-SFFF. This investigation includes a rotating beam fatigue test in the fully reversed condition (stress ratio, R = −1), electron microscopy, and calculations for fatigue life predictions using Paris' and modified Paris' equations. The fatigue experiments were performed at 350 °C using specimen with the test axis oriented diagonally (45°) and parallel (90°) to the AM builds directions. The fatigue values from the current experiments along with literature data find that the Ti MMC manufactured via PTA-SFFF exhibit fatigue anisotropy reporting highest strength in 90° and lowest in perpendicular (0°) AM build directions. Furthermore, calculations were performed to evaluate the optimum values of the stress intensity modification factor (λ) for fatigue life prediction in 0°, 45°, and 90° AM build directions. It was found that for the specimens with 45°, and 90° AM build directions, the computed intensity modification factors were very similar. This suggests that the initial fatigue crack characteristics such as location, shape, and size were similar in both 45°, and 90° AM build directions. However, in 0° AM build direction, the computed stress intensity modification factor was different from that of the 45°, and 90° AM build directions. This indicates that the fatigue crack initiation at 0° AM build direction is different compared to the other two directions considered in this study. Moreover, the quality of fatigue life prediction was assessed by calculating R2 values for both Paris and modified Paris predictions. Using the R2 values, it was found that the fatigue life predictions made by the modified Paris equation resulted in improved prediction accuracy for all three builds, and the percentage improvement ranged from 30% to 60%. Additionally, electron microscopy investigations of 0°, 45°, and 90° AM build specimens revealed extensive damage to the TiB particle compared to the Ti matrix as well as frequent TiB clusters in all three AM build directions. These observations suggest that the spread of these TiB clusters plays a role in the fatigue anisotropy of Ti-TiB MMCs. [ABSTRACT FROM AUTHOR]

The paper concerns an experimental study on the wind pressures over the surface of a worldwide known Gothic Cathedral: Notre Dame of Paris. The experimental tests have been conducted in the CRIACIV wind tunnel, Prato (Italy), on a model of the Cathedral at the scale 1:200, reproducing the atmospheric boundary layer. Two types of tests have been conducted: with and without the surrounding modeling the part of the city of Paris near the Cathedral. This has been done, on the one hand, for evaluating the effect of the surrounding buildings onto the wind pressure distribution on the Cathedral, and, on the other hand, to have a wind pressure distribution plausible for any other Cathedral with a similar shape. The tests have been done for all the wind directions and the mean and peak pressures have been recorded. The results emphasize that the complex geometry of this type of structures is responsible for a peculiar aerodynamic behavior that does not allow estimating correctly the wind loads on the various parts of the Cathedral based on codes and standards, which are tailored for ordinary regular buildings. [ABSTRACT FROM AUTHOR]

Selective laser melting is a form of additive manufacturing in which a high-power density laser is used to melt and fuse metallic powders to form the final specimen. By performing fatigue and tensile tests under various loading conditions, the study sought to establish the impact of internal defects on the specimens' fatigue life. Scanning electron microscopy and finite element simulation were conducted to determine the defect characteristics and the stress intensity factor of the specimens. Four different methods were used to determine the intrinsic defect length of the specimen, using data such as grain size, yield strength, and hardness value, among others. Kitagawa–Takahashi and El-Haddad diagrams were developed using the results. A correction factor hypothesis was established based on the deviation of measured data. Using Paris law, fatigue life was determined and compared to the experimental results later. The study aims to select one or more approaches that resemble experimental values and comprehend how internal defects and loading situations affect fatigue life. This study's findings shed light on how internal defects affect the fatigue life of selective laser-melted AlSi10Mg specimens and can aid in improving the fatigue life prediction method of additively manufactured components, provided an appropriate intrinsic crack criterion is selected. [ABSTRACT FROM AUTHOR]

The new emerging Wire and Arc Additive Manufacturing (WAAM) technology has significant potential to improve material design and efficiency for structural components as well as reducing manufacturing costs. Due to repeated and periodic melting, solidification and reheating of the layers, the WAAM deposition technique results in some elastic, plastic and viscous deformations that can affect material degradation and crack propagation behaviour in additively manufactured components. Therefore, it is crucial to characterise the cracking behaviour in WAAM built components for structural design and integrity assessment purposes. In this work, fatigue crack growth tests have been conducted on compact tension specimens extracted from ER70S-6 steel WAAM built components. The crack propagation behaviour of the specimens extracted with different orientations (i.e. horizontal and vertical with respect to the deposition direction) has been characterised under two different cyclic load levels. The obtained fatigue crack growth rate data have been correlated with the linear elastic fracture mechanics parameter Δ K and the results are compared with the literature data available for corresponding wrought structural steels and the recommended fatigue crack growth trends in the BS7910 standard. The obtained results have been found to fall below the recommended trends in the BS7910 standard and above the data points obtained from S355 wrought material. The obtained fatigue growth trends and Paris law constants from this study contribute to the overall understanding of the design requirements for the new optimised functionally graded structures fabricated using the WAAM technique. [ABSTRACT FROM AUTHOR]

The experimental study of fatigue crack growth (FCG) behavior in Ti6Al4V alloy manufactured by selective laser melting (SLM) was carried out on an in situ fatigue testing machine. Two specimen orientations relative to the build direction (horizontal and vertical), two temperatures (room temperature, RT, and 400°C), and two stress ratios (0.1 and 0.5) were considered, and the FCG curves with the threshold values were determined. The results showed that the FCG properties were affected by the stress ratio due to different degree of crack closure and temperature with grain softening at 400°C in threshold and Paris region. The threshold value is highly dependent on the build direction of the alloy, which was caused by the anisotropic microstructure, while in the Paris region, the effect of material direction can be negligible and FCG rate tends to be consistent. The fracture mechanism at different stages was discussed and revealed by the fracture morphology observation using scanning electron microscopy. Highlights: The FCG resistance in threshold region is highly dependent on the material orientation.The FCG behaviors were examined and compared under different stress ratios and temperatures.Fatigue crack growth mechanism was revealed ranging from 3D small crack to 2D long crack. [ABSTRACT FROM AUTHOR]

The current study presents the fatigue crack growth behaviour of titanium alloy Ti6Al4V parts manufactured by selective laser melting (SLM), obtained as standard 6 mm thick compact specimens (CT). Both the crack propagation under constant amplitude loading and the transient crack growth behaviour after the application of overloads were studied. The effect of the mean stress and the transient retardation behaviour were analysed using the crack closure parameter, obtained both by compliance and digital image correlation techniques. A reduced crack closure level for the stress ratio R = 0 was detected and for R = 0.4 no crack closure was observed. The digital image correlation technique showed better results in the Paris regime and during the transient retardation behaviour. The overload application produced crack growth retardation due to the increase of the crack closure effect. The failure surfaces showed a transgranular crack growth in β phase contouring the martensitic α phase. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) is an emerging additive manufacturing technology, capable of producing complex geometry components. The current work studied both the effect of substrate material and mean stress on the fatigue crack growth behaviour along interfaces of bi‐material specimens, substrate, and part by SLM. Fatigue tests were carried out in agreement with ASTM E647 standard, using 6‐mm‐thick compact specimens. The substrate steel has only a negligible effect both on the fatigue crack propagation rate and on the crack path. The failure occurs in the material additively manufactured by SLM, near the interface. The mean stress produced only a reduced influence on the fatigue crack propagation rate in the Paris regime. For larger values of ΔK, where Kmax approaches KIc, a significant influence of the mean stress was observed. In spite of nondetection of crack closure, the application of overloads promoted significant fatigue crack retardation, quite similar for both substrate materials, probably due to the crack bifurcation during the overload. [ABSTRACT FROM AUTHOR]

• Porosity strongly impacts LPBF IN625 ductility along the build orientation. • LPBF IN625 toughness is strongly reduced by porosity. • LPBF IN625 near-threshold fatigue crack growth is insensitive to porosity. • Porosity above ∼1% results in highly anisotropic Paris regime crack growth. Inconel 625 coupons were manufactured via laser powder bed fusion with different levels of intentionally-seeded porosity (up to 2.7%). The processing-induced porosities, morphologies and distributions were measured via Archimedes method and computed tomography. Tensile tests were performed, followed by fatigue crack propagation characterization. The static strength characteristics were not strongly affected by the levels of porosity, but the elongation at break was severely impacted. In the near threshold region of the Paris diagram, porous specimens were unaffected by porosity and build orientation. However, under higher stress intensity factors, the effect of porosity was found to be significant and build orientation-dependent. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Fatigue crack growth behavior of laser powder bed fused 17-4 PH SS was investigated. • Effect of heat treatment and microstructure on crack growth behavior was studied. • Depending on heat treatment, build orientation effect on crack growth was observed. • Fractography analysis was performed to understand the crack growth mechanism. • δ-ferrite and martensite weak interfaces caused cracking in H1025 specimens. Abstract Additive manufacturing (AM) brings more freedom to design and fabricate parts with complex geometries. However, structural integrity of additively manufactured parts must be thoroughly investigated before they can be used in critical, load bearing structural applications. This study investigates the fatigue crack growth (FCG) behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated using laser powder bed fusion (L-PBF) process, and compares it to that of wrought counterpart. The effect of different heat treatment procedures and the notch orientation relative to the build direction were also studied. FCG tests were conducted under force-controlled mode at room temperature with a load ratio of R = 0.1 and a frequency of 10 Hz up to fracture. Microstructure characterization and fractography analysis were carried out to elaborate the crack growth mechanism with respect to different heat treatment conditions and crack growth directions. Abnormal FCG behavior was observed for L-PBF 17-4 PH SS subjected to H1025 heat treatment procedure. This was associated with the presence of δ-ferrite and its weak interface with martensite boundaries. However, L-PBF 17-4 PH SS specimens subjected to CA-H900 condition were found to behave similar to the wrought counterparts in the Paris regime. [ABSTRACT FROM AUTHOR]

• Highlighting the anisotropic fatigue behaviour of additively manufactured TA6V alloy. • Investigation of the effect of pore size on the fatigue strength. • Modelling the fatigue life by using two fracture mechanics based approaches. This work is focused on the influence of porosity when dealing with the fatigue behaviour of TA6V alloys fabricated by the selective laser melting (SLM) process. The presence of porosity is one of the major issues facing additive manufacturing (AM) of metallic components subjected to fatigue loading. In order to study the effect of porosity on the fatigue behaviour, a vast experimental campaign has been undertaken. Seven specimen batches, fabricated by the SLM process with different building directions (horizontal, vertical and diagonal) were tested and a large amount of data was obtained. The link between the applied stress, the fatigue life and the pore size is highlighted by using generalized Kitagawa-Takahashi maps. It is shown that the effect of porosity on the fatigue strength is much more pronounced compared to the effect of the microstructure. In the modelling section, two approaches based on fracture mechanics are considered. The first one is based on the Paris law which is used to model long fatigue crack growth. The second approach was proposed by Caton et al. (2001) for modelling small fatigue crack growth. Finally, a simulation of the generalized Kitagawa-Takahashi is presented and good agreement with the experimental data is shown. [ABSTRACT FROM AUTHOR]

• Fatigue crack growth tests on 13 WAAM steel compact tension specimens presented. • Hardness and tensile strength of examined WAAM steels are approximately proportional. • Influence of deposition strategy on the fatigue crack growth (FCG) behaviour studied. • Test results compared against conventional steels and BS 7910 recommended FCG laws. • Fractography performed to analyse the mechanisms of crack growth. Combining welding technology with robotics, wire arc additive manufacturing (WAAM) is emerging as a viable method of construction. To facilitate its wider application, in particular with structural integrity in mind, an improved understanding of the fatigue properties of WAAM materials is needed. Hence, an experimental investigation into the fatigue crack growth (FCG) behaviour of WAAM plates made of normal- and high-strength steels has been conducted and is reported herein. Following material characterisation, FCG tests on 13 machined compact tension specimens were undertaken, extracted in different directions from WAAM plates built using a parallel deposition strategy. Fractography of the tested specimens was also performed to analyse the mechanisms of crack growth. The material testing demonstrated an approximately proportional relationship between the hardness and ultimate strength. From the FCG tests, Paris' law constants were derived for the WAAM normal- and high-strength steel specimens, revealing consistently lower material constants m for the latter. Similar FCG behaviour to that of equivalent, conventionally-produced steels, with no significant anisotropy, was found. The observed response was shown to be generally well captured by the FCG laws given in BS 7910, with the recommended curve for unwelded steels providing a close match to the experimental data and the recommended curve for welded steels providing conservative predictions. Additional FCG data on WAAM steels printed using the parallel and oscillatory deposition strategies were collected and analysed. It was shown that the two deposition strategies led to similar crack growth rates for WAAM normal-strength steels, but distinct FCG behavioural trends for WAAM high-strength steels. Finally, the fractographic analysis showed that the WAAM specimens exhibited principally brittle behaviour during FCG but somewhat ductile behaviour before fracture. [ABSTRACT FROM AUTHOR]

The present article reports the fatigue crack growth (FCG) rate and fracture toughness of 15-5 Precipitation Hardening (PH) Stainless Steel (SS) specimens fabricated by Laser powder bed fusion (L-PBF) additive manufacturing (AM) technique. The effect of notch orientation (parallel and orthogonal in reference to build direction) on FCG and fracture toughness is investigated. The FCG and fracture toughness data of L-PBF specimens in the solution annealed (SA) and SA + aged condition were compared to that of wrought 15-5 PH SS. It was found that dependence of FCG on the build orientation was insignificant up to the stress intensity factor range (ΔK) of 30 MPa√m, and beyond that, a higher FCG rate is recorded for L-PBF specimens. The SA specimens were found to have a slightly lower FCG rate at Paris region as compared to SA + aged specimens. The fractography showed predominantly transgranular mode of crack growth, with presence of fine pores and signature of plastic deformation at higher stress intensity. The crack branching and zig-zag path was observed for the L-PBF specimens with notch orientation orthogonal to build direction (horizontal notch). The fracture toughness (K 1C) of horizontally notched L-PBF fabricated 15-5 PH SS specimens in aged condition showed higher values (51–62 MPa√m) as compared to vertical (parallel) notched specimens (40–46 MPa√m). The observed fracture toughness values of L-PBF 15-5 PH SS specimens appear to be significantly lower than the reported fracture toughness values of wrought 15-5 PH SS material. • The effect of crack orientation on the FCG rate and fracture toughness of AM 15-5 PH stainless steel was investigated. • X-ray Micro CT analysis was performed to find out the defect size and orientations. • The results obtained from fracture surface and crack path analysis were correlated with the fatigue crack growth behaviour. • Fracture resistance of AM 15-5 PH stainless steel specimens were compared to the wrought specimens with valid K 1C values. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted in the recent past to assess different nickel-based superalloys fabricated using laser-based powder bed fusion (PBF-LB). Most of these works focused on the processability, microstructural evolution, post-fabrication treatments and the mechanical behaviour of components manufactured by PBF-LB under static loading. However, limited efforts have been made to investigate the fatigue crack growth (FCG) characteristics of PBF-LB built alloys, thereby imposing a barrier to their use as critical components. As such, the aim of the present study was to evaluate and understand the effect of build orientations (0° and 90°) and post-fabrication treatments (solution heat treatment and hot isostatic pressing) on the FCG behaviour of Hastelloy X (HX) fabricated by PBF-LB process. The results from the FCG tests showed that the build orientations had no discernible influence on the fatigue threshold and FCG rate within near-threshold and Paris regions. Nevertheless, the post-fabrication treatments led to an increase in the fatigue threshold and crack growth resistance within the near-threshold region because of the alleviation of detrimental tensile residual stress alongside the crack growth direction. In conjunction with FCG tests, scanning electron microscopy and electron backscattering diffraction were used to investigate the crack path and fracture surfaces within near-threshold and Paris regions. • FCG tests were performed on as-built, SHT and HIP compact tension specimens with crack growth parallel and perpendicular to build direction. • The effect of anisotropic microstructure and residual stress on fatigue crack threshold and fatigue crack growth rate is investigated. • Transgranular mode of crack propagation was observed in near-threshold and Paris regions of crack growth in as-built and post-treated PBF-LB HX. [ABSTRACT FROM AUTHOR]

In the present work, several large Ti–6Al–4V blocks were successfully fabricated by high-power laser metal deposition (HP-LMD). The high cycle fatigue (HCF) property and fatigue crack growth (FCG) behavior of the as-build Ti–6Al–4V alloy were investigated from two sampling directions (horizontal and vertical). The results showed that the fatigue limit of horizontal and vertical specimens was 357.5 MPa and 358.57 MPa, respectively. However, the HCF performance of vertical specimens was superior to horizontal specimen under a high-stress level. In addition, the HCF performance of both horizontal and vertical specimens was inferior to the wrought Ti–6Al–4V. Meanwhile, the FCG rate of vertical specimen was lower than that of horizontal specimen in both near-threshold region and Paris region. The anisotropic microstructure (prior β columnar grains with preferential orientation and α colonies) was found to be the main reason that contributed to an anisotropic HCF property and FCG behavior. For horizontal specimen, the fatigue crack interacted with only one or two columnar grains, thus leading to a lower FCG resistance. For vertical specimen, the fatigue crack interacted with multiple columnar grains, which led to a better plasticity and enhanced FCG resistance. [ABSTRACT FROM AUTHOR]

The present study analyzes the cyclic crack propagation behavior in an austenitic steel processed by electron beam powder bed fusion (PBF-EB). The threshold value of crack growth as well as the crack growth behavior in the Paris regime were studied. In contrast to other austenitic steels, the building direction during PBF-EB did not affect the crack propagation rate, i.e., the crack growth rates perpendicular and parallel to the building direction were similar due to the isotropic microstructure characterized by equiaxed grains. Furthermore, the influence of significantly different building parameters was studied and, thereby, different energy inputs causing locally varying manganese content. Crack growth behavior was not affected by these changes. Even a compositional gradation within the same specimen, i.e., crack growth through an interface of areas with high and areas with low manganese content, did not lead to a significant change of the crack growth rate. Thus, the steel studied is characterized by a quite robust cyclic crack growth behavior independent from building direction and hardly affected by typical parameter deviations in the PBF-EB process. [ABSTRACT FROM AUTHOR]

• Effects of heat treatment and crack orientation on FCG behavior of AM 17-4 PH SS. • FCG test results for AM 17-4 PH SS over a wide range of stress ratios and rates. • New methods to generate more accurate crack-growth rates in the near-threshold regime. • Microstructural characterization and failure mechanism of AM 17-4 PH SS. The effects of heat treatment and crack orientation on fatigue-crack-growth (FCG) behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated via a laser powder bed fusion (LPBF) additive manufacturing (AM) system were investigated. Accurate representation of FCG thresholds, the region defining crack growth as either very slow or nonexistent, is extremely important in adoption of AM for structural applications. In this study, new methods called compression pre-cracking constant-amplitude (CPCA) and load-reduction (CPLR) were used to generate FCG rate data in the near-threshold (low-rate) regime using non-standard or modified compact specimens. For comparison, the current ASTM load-reduction (LR) method was also used. Results indicate that FCG behavior of AM 17-4 PH SS in the Paris regime was comparable to that of wrought 17-4 PH SS in the peak-age condition. Although the post-manufacturing heat treatment was found to be necessary in order to improve tensile strength of this alloy, the results of this study indicate that this particular heat treatment process had no influence on FCG behavior of LPBF 17-4 PH SS. The effect of crack orientation with respect to the build direction however was evident. FCG rates were slightly lower for specimens with a crack parallel to the build direction (i.e. longitudinal crack) in the Paris regime as compared to that of specimens with a perpendicular crack (i.e. transverse crack). The near-threshold FCG behavior for specimens with a longitudinal crack did not show the usual stress ratio (R)-shift from low to high R. The low- R data was vastly different than the data generated using the specimens with a transverse crack. Our results also imply that both the current ASTM standard LR method and the new CPLR method induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. [ABSTRACT FROM AUTHOR]