Monotonic Mechanical Properties Research Articles

Post-fire mechanical properties of titanium-clad bimetallic steel after cold-forming process: Experiment and constitutive model

Titanium-clad bimetallic steel (TCBS), composed of a titanium alloy cladding layer and carbon steel substrate layer, has been proposed to mitigate the effects of corrosion and enhance the durability of steel structures. This study conducted tensile coupon test to explore the coupling effect of the cold-forming process and fire exposure on the monotonic mechanical properties of TCBS. Following the cold-forming process and fire exposure, there was no visible cracking observed in either the cladding or the metallurgical bonding layer. Test results revealed a notable enhancement in the strength properties and a decrease in the ductility of the TCBS specimens after cold forming. Moreover, the study elucidated the effects of the exposure temperature and cooling method on the stress–strain curves and fractures of TCBS corner specimens. Three different fracture sequences of the TCBS corner specimens were observed during monotonic tensile test. The key mechanical parameters were concerned in this study, which included the yield and ultimate strengths, yield and ultimate strains, elastic modulus, elongation after fracture, and section shrinkage. Then, the variation trends of key mechanical parameters were clarified. Predictive equations were developed for these parameters of the TCBS corner specimens after fire exposure. Additionally, two constitutive models were developed for the corner TCBS stress–strain curves with and without the yield plateau, enabling their use in numerical and theoretical investigations of the post-fire performance of cold-formed TCBS components.

Effects of pre-compression modes on mechanical properties and fatigue behaviors of rolled ZK60 magnesium alloy

The research investigated the microstructure, monotonic mechanical properties, and fatigue behaviors of ZK60 magnesium alloy treated with single-pass pre-compression and combined pre-compression. The study found that combined pre-compression significantly improved mechanical strength in both the transverse and rolling directions compared to the single-pass method. Due to the randomized texture and intricate interwoven grain structures, the combined pre-compression method significantly mitigated the anisotropy observed in samples treated with single-pass pre-compression. Both pre-compression techniques enhanced the fatigue life of the ZK60 magnesium alloy. Samples subjected to combined pre-compression exhibited fatigue performance comparable to those treated with single-pass pre-compression. Additionally, there was a clear negative correlation between the fatigue life of the three samples and the magnitude of the mean stress. The abundance of pre-existing dislocations in the combined pre-compression processed samples helped in reducing the trend towards cyclic hardening. In contrast to single-pass pre-compression, the combined pre-compression method effectively reduced twinning activities and promoted the participation of basal slip during cyclic loading, leading to a more symmetrical hysteresis loop.

Experimental study on residual monotonic mechanical properties of high-performance Q690 steel with pre-fatigue damage

In this paper, we conduct an experimental study of the fatigue performance and monotonic tensile properties with pre-fatigue damage of seismic and corrosion resistant Q690 structural steel (abbreviation high-performance Q690 steel). Based on fatigue cumulative damage theory, S-N curves and fatigue cumulative damage curves of high performance Q690 steel were obtained. According to the static mechanical properties with different degree of fatigue damage, the related curves between fatigue damage degree and residual mechanical properties were proposed. Finally, based on the Rambeger-Osgood constitutive relationship, a constitutive model for high performance Q690 steel considering fatigue damage is proposed. The results indicated that the high performance Q690 steel showed good fatigue performance. Its limit fatigue strength is 630 MPa (0.86fy) approximately, which is higher than fatigue limit values specified in steel design standards. In addition, the fatigue crack propagation stage and transient fracture stage accounted for a relatively small proportion of fatigue life. The static mechanical properties with pre-fatigue damage of high performance Q690 steel is similar to the AISI 1045 steel. As the fatigue damage degree increases, the elasticity modulus of high performance Q690 steel decreases and yield strength increases. Whereas, pre-fatigue damage has little effect on ultimate strength and elongation. Finally, a new constitutive model considering fatigue damage was proposed, which was found better simulating tensile curves with different fatigue damage.

Residual monotonic stress–strain property of Q690 high-strength steel: Experimental investigation and constitutive model

Residual monotonic stress–strain properties of Q690 high-strength steel (HSS) with pre-fatigue damage were investigated, which provided the basis for studying the residual resistance performance of HSS structures after an earthquake. Three monotonic tensile test specimens, nine low-cycle fatigue test specimens and twenty-seven pre-fatigue test specimens were included in this study. For the fatigue strain amplitude, the 0.75%, 1.0% and 1.5% were considered. From the low-cycle fatigue experimental results, the relationship between the fatigue life and fatigue strain amplitude of Q690 HSS was clarified through a predictive equation. The number of loading cycles was controlled to perform the pre-fatigue damage test, where the fatigue damage ratios were controlled to be 0.3, 0.6 and 0.8. Cracks were observed on the surfaces of the test specimens with pre-fatigue damage. A tensile coupon test was then conducted to clarify the residual mechanical properties of Q690 HSS with pre-fatigue damage. The effects of the fatigue damage ratios and fatigue strain amplitudes on the residual monotonic mechanical properties of Q690 HSS were determined. The stress–strain curve of the specimen with pre-fatigue damage was similar to that of the virgin specimen. The strength properties of Q690 HSS were reduced by pre-fatigue damage. The effects of the fatigue damage ratios on the key mechanical properties of Q690 HSS were quantified using predictive equations. A predictive method for the residual monotonic stress–strain properties of Q690 HSS with different pre-fatigue damages was proposed, which included predictive equations for key mechanical parameters and a nonlinear constitutive model. The proposed predictive method for the residual mechanical properties of Q690 HSS with pre-fatigue damage can be used in theoretical and numerical analyses as a material model.

Post-fire constitutive model on explosively welded stainless-clad bimetallic steel after cold-forming process

This study conducted an experiment on corner stainless-clad bimetallic steel (SBS) specimens to clarify the coupling effects of the cold-forming process and fire exposure on the monotonic mechanical properties of SBS, where different exposure temperatures and cooling methods were considered. The SBS produced through the explosive welding method was used to obtain cold-formed specimens. After the cold-forming process and fire exposure, no obvious cracking was observed in the stainless-steel cladding and metallurgical bonding layer. Based on the tensile coupon test results, the yield plateau in the stress–strain curve of the virgin SBS specimen disappeared because of the cold-forming process. The effects of the exposure temperature and cooling method on the stress–strain curves of the corner SBS specimen were clarified. The variation trends of the key mechanical parameters were discussed. Predictive equations were proposed for the key mechanical parameters of corner SBS specimen after fire exposure. Two constitutive models were suggested for the corner SBS stress–strain curves with and without the yield plateau, which could be used in the numerical and theoretical investigations on the post-fire-resistant performance of cold-formed SBS components. Comparing the experimental and predicted results indicated that the residual monotonic stress–strain curves of the corner SBS could be accurately described using the proposed constitutive models.

Low‐Cycle‐Fatigue Performance of Stress‐Aged EN AW‐7075 Alloy

The effect of a novel heat treatment, that is, aging under superimposed external stress, on the fatigue performance and microstructural evolution of a high‐strength aluminum alloy (EN AW‐7075) is presented. Stress aging, a combination of heat treatment and superimposed external stress, can enhance the mechanical properties of EN AW‐7075 under monotonic loading due to the acceleration of precipitation kinetics. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) reveal that a longer aging time and the presence of superimposed stress both promote the formation and growth of precipitates, that is, the precipitation of strengthening η´ precipitates. This is confirmed by differential scanning calorimetry (DSC) heating experiments of stressless and stress‐aged states. Furthermore, stress aging leads to a reduction of dimensions of precipitate‐free zones near grain boundaries. Cyclic deformation responses (CDRs) and half‐life hysteresis loops are evaluated focusing on the low‐cycle fatigue (LCF) performance of different conditions. A noticeable cyclic hardening seen in case of the specimens aged for a short time indicates the occurrence of dynamic strain aging (DSA). Eventually, stress aging allows for an enhancement of the monotonic mechanical properties of EN AW‐7075 without degrading the cyclic performance in the LCF regime.

Effect of monotonic mechanical properties on the low-cycle fatigue lifetime of carbide-free bainitic steels

The influence of monotonic mechanical properties on the low-cycle fatigue (LCF) lifetime of carbide-free bainitic steel is systematically investigated. The tensile mechanical properties, LCF, and microstructure of carbide-free bainitic steel with strength grades from 1200 MPa to 1700 MPa are tested. The results show that an increase in dislocation density and a decrease in bainitic ferrite lath size lead to an increase in tensile strength, and a decrease in retained austenite volume fraction leads to a decrease in elongation. Due to the mechanical stability of retained austenite, the influence of microstructure parameters on fatigue lifetime is not proportional. The increase of dislocation density and the decrease of bainitic ferrite lath size lead to a much larger increase of fatigue lifetime than the decrease of retained austenite volume fraction. Using a damage-hysteresis energy method, the fatigue lifetime could be modeled to show its dependency on the tensile strength and elongation. Increasing tensile strength and elongation is beneficial to increasing fatigue lifetime. The increase of fatigue lifetime caused by the increase of strength is larger than the decrease of fatigue lifetime caused by the decrease of elongation. The mathematical analysis of fatigue lifetime in terms of tensile strength, elongation and static toughness can effectively predict the variation of the LCF lifetime for different grades of bainitic steels.

Residual monotonic mechanical properties of bimetallic steel bar with fatigue damage

When a reinforced concrete structure experiences an earthquake and does not collapse, irreversible damage in the reinforcement is caused by the fatigue load, which affects the residual bearing capacity of the structure significantly. Stainless-clad bimetallic steel bars (BSBs) are a new type of coated reinforcement with effective resistance to corrosion. In this study, to accurately evaluate the residual bearing capacity of a concrete structure reinforced with BSBs following an earthquake, it is tested with different fatigue damage ratios. Thereafter, the monotonic stress–strain nonlinear performance, failure behaviour, and residual mechanical properties of BSBs with fatigue damage are explored. The evolution law of the mechanical properties of BSBs with fatigue damage is analysed. A monotonic stress–strain model of the BSB that considers the impact of fatigue damage is proposed. The test results indicate that the stainless-steel cladding of the BSB does not demonstrate apparent buckling following fatigue damage. Cracks are observed on both sides of the rib teeth. When the fatigue damage ratio is 0.6, a fracture occurs at the maximum crack caused by the fatigue damage. When the fatigue damage ratios are 0.2 and 0.4, fractures occur in the necking region of the upper or lower part of the specimen. Fatigue damage leads to the disappearance of the yield platform from the monotonic stress–strain curve of the BSB. The yield strength of the BSB decreases significantly owing to the fatigue damage ratio. The ultimate strength is barely affected by the fatigue damage ratio. Fatigue damage has an adverse effect on the deformation capacity of the BSB. With an increase in the fatigue strain amplitude and fatigue damage ratio, elongation following the fracture of the BSB gradually decreases. This study provides solid theoretical and experimental bases for the engineering application of BSBs, particularly in the post-earthquake evaluation of concrete structures reinforced with BSBs.

Enhancement of fatigue resistance by direct aging treatment in electron beam welded Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy joint

In most cases, welding processes promote the application of structural materials at the expense of strength and ductility. Post-weld treatment is proposed to regain mechanical properties of weldment, which necessitates that microstructural and the associated mechanical behavior evolutions being clearly understood. In this study, post-weld heat treatment was conducted to enhance the mechanical properties of the electron beam welded Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy joint. Microstructural evolutions and monotonic mechanical properties were characterized to optimize the post-weld heat treatment parameters. It was found that direct aging at 630 °C for 2 h can upraise the strength and ductility of the joint by ∼31% and ∼511% respectively. Metastable needle-shaped martensite α′ within rapidly solidified fusion zone had been completely decomposed, and thin recrystallized α lamella precipitated in the prior β phase without forming consecutive coarse grain boundary α after above direct aging treatment. Such microstructural characteristics enable the direct-aged joint to exhibit a higher fatigue strength from high to very high cycle region than the as-welded joint. Moreover, the fatigue crack nucleation resistance was enhanced significantly through direct aged treatment.

Effect of layer architecture on the mechanical behavior of accumulative roll bonded interstitial free steel/aluminum composites

Multi-layered interstitial free (IF) steel/aluminum (Al) composites were fabricated by the accumulative roll bonding (ARB) method. Two types of IF steel/Al6061 dissimilar layered metal composites (LMC) with varied stacking of aluminum layers were processed to examine the effect of the layer architecture. Microhardness and uniaxial tensile experiments were applied to analyze the surface and bulk monotonic mechanical properties. Besides, the cyclic mechanical response of the processed materials was investigated via high cycle fatigue (HCF) tests with positive mean stress. Microstructure and mechanical characteristics of composites with various layer architectures were analyzed up to five ARB passes. It is revealed that the monotonic and cyclic performances of all LMCs are significantly enhanced as compared to the base alloy with an aluminum layered structure. Moreover, composites with aluminum as the outer layer exhibited the highest fatigue life, due to crack branching at the interface region during propagation from the softer to the harder layer. Fracture morphology analysis of composites demonstrated that in addition to the significant impact of surface cracks on the outer layers, propagation of cracks initiating from the interface layers led to failure under cyclic loading.

A fast calibration approach of modified Chaboche hardening rule for low yield point steel, mild steel and high strength steels

Chaboche model has gained prominent utilization in numerical simulation on cyclic behaviors of metal structure. However, the calibration of its material parameter may produce considerable cost. A simplified calibration way is of great importance to ease its application. To this end, a re-evaluation on chaboche model material parameters and simulated elastic-plastic laws was conducted regarding various steel grades. Futhermore, the characterisitcs of isotropic softening was coupled with the present model hardening rule. The kinematic rule was correlated with cumulative plastic strain. The modified model was then calibrated and validated base upon the coupon test result on 41 batches of steels, whose yield stress ranges from 100 MPa to 890 MPa. The calibrated material parameter were then correlated with corresponding monotonic mechanical properties. Accordingly, a simplified calibration procedure, based upon which the material parameters of modifed Chaboche model can be fastly estimated by yield strength and yield ratio, was constructed. As for low yield point steels, mild steels and high strength steels, this calibration method can ease the calibration of modified Chaboche model considerably.

Monotonic mechanical properties of titanium grade 5 (6Al-4V) welds made by microplasma

The research concerned the possibility of using the microplasma welding technique for joining thin titanium alloy Grade 5 (6Al-4V) sheets. For this purpose, titanium reference samples without welds (Type 1) and two types of welded joints (butt joint - Type 2 and Type 3 overlap joint) were made with selected microplasma welding parameters for which strength parameters were determined and compared with the strength of the native material. The assessment of the connection quality was made on the basis of a monotonic tensile test. A metallographic analysis of the welded joints was made and the breakthroughs sample obtained during the tests was also carried out. In addition, strain distributions were determined in loaded sample types, which allowed investigating the impact of the heat affected zone size on the strength of welded joints subjected to monotonic tensile tests and Vickers hardness tests. Microplasma welded joints displayed relatively high brittleness. The failure locations are still influenced by heat affected zone despite the highly concentrated plasma beam of the microplasma arc welding.

Uniaxial mechanical properties of multi-layer thin films in use for scientific balloons

Essential and accurate mechanical parameters of multi-layer thin films for scientific balloons are indispensable for performing structural analysis. This study focuses on the determination of uniaxial mechanical properties of a new multi-layer thin film and the understanding of the effects of mechanical properties on structural behavior and altitude with three-dimensional numerical models. For uniaxial monotonic mechanical properties, a method to determine yield stress is proposed in combination of geometrical and energetic principles. It is found that average yield stresses are 20.5 MPa and 15.0 MPa for machine and transverse directions. The cyclic mechanical properties in terms of elastic modulus and ratcheting strain tend to be stable after certain cycles. Moreover, the Poisson’s ratios of 0.31 and 0.15 in machine and transverse directions are determined with digital image correlation (DIC) technique. Furthermore, these mechanical properties are utilized to analyze a 12 m diameter spherical superpressure balloons floating at the altitude of 20 km. A corresponding three-dimensional numerical model is developed to understand the effects of mechanical properties on structural behavior and altitude. The numerical results in terms of elastic moduli show that the effects of mechanical properties on structural behavior and altitude are relatively apparent and slight, respectively.

Investigation of fatigue damage in aluminum/stainless steel brazed joints

The study deals with the investigation of the fatigue fracture mechanisms in aluminum/stainless steel-brazed joints. The joints are produced by induction brazing using an AlSi10 filler at 600 °C in argon atmosphere. The filler is applied in two different ways: as a conventional filler paste or as a filler cladding on the stainless steel. The brazed joints have to endure a high number of cyclic loads during application. Therefore, in addition to the monotonic mechanical properties, the fatigue behavior must be considered. It is investigated by fatigue tests at ambient and elevated temperatures on a RUMUL resonance pulsator under load controlled condition with a load ratio of R = 0.1. At ambient temperature, a fatigue endurance limit of 107 cycles is reached by joints, brazed with a filler paste, at a stress amplitude of 7 MPa. Joints, brazed with a filler cladding, reach this number of cycles at a stress amplitude of 9 MPa. With an increase of the testing temperature, the fatigue life decreases for both combinations. At elevated temperatures, the joints endure lower stress amplitudes of 5 MPa resp. 6 MPa at 107 cycles. The formation of fatigue cracks leads to a reduction in the resonance frequency. At this point, the fatigue damage of the corresponding samples is investigated by SEM. Especially the crack initiation and crack propagation with regard to the number of cycles is observed using cross sections. The investigations of cross sections of the fracture surfaces show, that the Al7Fe2Si intermetallic layer influences the fatigue behavior of the joints, brazed with a filler paste, predominantly. For joints, brazed with a filler cladding, the Al-Fe-(Cr,Si) intermetallic layers do not have a main influence on the fatigue behavior.

Low-Cycle Fatigue Behaviour of AISI 18Ni300 Maraging Steel Produced by Selective Laser Melting

Selective laser melting has received a great deal of attention in recent years. Nevertheless, research has been mainly focused on the technical issues and their relationship with the final microstructure and monotonic properties. Fatigue behaviour has rarely been addressed, and the emphasis has been placed on high-cycle regimes. The aim of this paper is, therefore, to study, in a systematic manner, the cyclic plastic behaviour of AISI 18Ni300 maraging steel manufactured by selective laser melting. For this purpose, low-cycle fatigue tests, under fully-reversed strain-controlled conditions, with strain amplitudes ranging from 0.3% to 1.0%, were performed. After testing, fracture surfaces were examined by scanning electron microscopy to identify the main fatigue damage mechanisms. The analysis of results showed a non-Masing material, with a slight strain-softening behaviour, and non-linear response in both the elastic and plastic regimes. In addition, this steel exhibited a very low transition life of about 35 reversals, far below the values of conventional materials with equivalent monotonic mechanical properties, which can be attributed to the combination of high strength and low ductility. The total strain energy density, irrespective of strain amplitude, revealed itself to be a quite stable parameter throughout the lifetime. Finally, the SEM analysis showed for almost all the tested samples cracks initiated from the surface and inner defects which propagated through the rest of the cross section. A ductile/brittle fracture, with a predominance of brittle fracture, was observed in the samples, owing to the presence of defects which make it easier to spread the microcracks.

Stainless steel valves with enhanced performance through microstructure optimization

Compressor valves are made of hardened and tempered martensitic steels. The main design criterion for the material selection is the fatigue performance of the material under bending loads. In some cases impact loads and corrosive atmospheres additionally act on the part. For the first time, the microstructure of the most commonly used stainless steel and its influence on the properties relevant for flapper valves is presented and described in this paper. It is demonstrated how the tensile properties of a martensitic stainless steel can be enhanced by tailoring the microstructure. Electron back scatter diffraction method is carried out to explain the changes in monotonic mechanical properties. Through a modified heat treatment the martensite microstructure is refined resulting in an increase of yield and ultimate tensile strength and at the same time a significant increase of elongation.

Mechanical and Dynamic Properties of Hybrid Composite Laminates

Unsaturated polyester-based composites and reinforced with three types of fabrics, E-glass, basalt, and carbon, were fabricated by Hand-Lay Up (HLU) technique at room temperature, with various fiber configuration. Monotonic mechanical properties of hybrid composites laminate such as the tensile, flexural, inter-laminer shear strength and impact strength were investigated. The dynamic response of hybrid laminates composite under pulse load was studied theoretically and experimentally. In the theoretical part, the validity of the theoretical model for evaluating natural frequencies, mode shapes and dynamic response of hybrid composite laminates at various staking sequence has been examined by utilizing of the finite element software (ANSYS). In the experimental part, the response of hybrid composite specimens with various types of fiber configuration and four types of boundary fixations was measured by hammer test technique frequency response function (FRF). The results show that the reinforcement by adding the basalt fabric and carbon fabric based unsaturated polyester composites as a fiber configuration [2C/B/2C]S enhanced the mechanical properties of the hybrid composite laminates among other various stacking sequences. For the stacking sequence [2C/B/2C]S, it was found that the largest values of tensile, flexural strength and interlaminar shear strength (ILSS) were 128.76 MPa, 405 MPa, 20.25 MPa, respectively. The results show the good bonding adhesion at the interface between the fibers and matrix of the hybrid composite laminates. The impact properties with stacking sequences [C-C-G-C-C]s have the largest value at 3.73 Joule as compared with the other composites and stacking sequences of all hybrid composite laminates. Also, the BFRP composites specimen gives the best vibration resistance compared to the other stacking sequences of hybrid composite laminates. The comparison between experimental and numerical model shows the efficiency of the proposed mathematical model of the composite structural specimen with bonded joints.

Effects of on-axis and off-axis tension on uniaxial mechanical properties of plain woven fabrics for inflated structures

The use of enhanced plain woven fabrics for aerospace engineering needs essential and accurate mechanical parameters for analyzing structural behavior and performing numerical simulations. The uniaxial tensile tests are direct and effective approaches to investigate these mechanical properties. This study focused on uniaxial monotonic and cyclic mechanical properties of an enhanced plain woven fabrics URETEK 5893 in on-axis and off-axis tension. For uniaxial monotonic mechanical properties, an improved method to determine yield stress was proposed and digital image correlation (DIC) technique was utilized to investigate detailed breaking mechanism. It is found that yield stresses were stable in each loading direction and typical yield stresses in the warp, weft and 45° directions were 5.56MPa, 8.84MPa and 1.96MPa, respectively. Moreover, the breaking mechanism in terms of strain propagation process was identified. For cyclic mechanical properties, approximate stable ratcheting strain and elastic modulus were investigated. Nonlinear ratcheting strain was more sensitive near 45° than other directions. Moreover, Poisson ratios in three directions were obtained with DIC technique and the maximum Poisson ratio existed in the 45° direction. The comparisons between experimental and theoretical elastic moduli showed that the best agreement existed in the warp and weft directions and the accuracy of using the theoretical equation to calculate elastic modulus was dependent on material linearity.In general, this study could provide basic observations and useful values for understanding mechanical properties of plain woven fabrics in use for inflated membrane structures.

Gamma Radiation Sterilization Reduces the High-cycle Fatigue Life of Allograft Bone.

Sterilization by gamma radiation impairs the mechanical properties of bone allografts. Previous work related to radiation-induced embrittlement of bone tissue has been limited mostly to monotonic testing which does not necessarily predict the high-cycle fatigue life of allografts in vivo. We designed a custom rotating-bending fatigue device to answer the following questions: (1) Does gamma radiation sterilization affect the high-cycle fatigue behavior of cortical bone; and (2) how does the fatigue life change with cyclic stress level? The high-cycle fatigue behavior of human cortical bone specimens was examined at stress levels related to physiologic levels using a custom-designed rotating-bending fatigue device. Test specimens were distributed among two treatment groups (n = 6/group); control and irradiated. Samples were tested until failure at stress levels of 25, 35, and 45 MPa. At 25 MPa, 83% of control samples survived 30 million cycles (run-out) whereas 83% of irradiated samples survived only 0.5 million cycles. At 35 MPa, irradiated samples showed an approximately 19-fold reduction in fatigue life compared with control samples (12.2 × 10(6) ± 12.3 × 10(6) versus 6.38 × 10(5) ± 6.81 × 10(5); p = 0.046), and in the case of 45 MPa, this reduction was approximately 17.5-fold (7.31 × 10(5) ± 6.39 × 10(5) versus 4.17 × 10(4) ± 1.91 × 10(4); p = 0.025). Equations to estimate high-cycle fatigue life of irradiated and control cortical bone allograft at a certain stress level were derived. Gamma radiation sterilization severely impairs the high cycle fatigue life of structural allograft bone tissues, more so than the decline that has been reported for monotonic mechanical properties. Therefore, clinicians need to be conservative in the expectation of the fatigue life of structural allograft bone tissues. Methods to preserve the fatigue strength of nonirradiated allograft bone tissue are needed. As opposed to what monotonic tests might suggest, the cyclic fatigue life of radiation-sterilized structural allografts is likely severely compromised relative to the nonirradiated condition and therefore should be taken into consideration. Methods to reduce the effect of irradiation or to recover structural allograft bone tissue fatigue strength are important to pursue.

Monotonic mechanical properties of plasma nitrided 316L polycrystalline austenitic stainless steel: Mechanical behaviour of the nitrided layer and impact of nitriding residual stresses

The impact of plasma nitriding at 400°C on the monotonic mechanical behaviour of 316L austenitic stainless steel at room temperature has been investigated. It is shown that the residual stresses in the nitrided layer lead to a tension–compression anisotropy whose magnitude depends on the residual stresses intensity and extension (i.e. thickness of the nitrided layer). Using the stress differential technique, average residual stresses in the nitrided layer ranging from −1.5 up to −3GPa were measured. The local mechanical behaviour of the nitrided layer has also been investigated through SEM in situ tensile tests. A quasi-brittle mechanical behaviour of the nitrided layer is observed with first evidences of crack initiation for plastic strains below 1%, whatever the nitrided layer. By increasing the total applied strain up to 20%, a progressive segmentation of the layer occurs. The crack initiation location mainly depends on the local nitrided thickness.