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Multiaxial cyclic tests were performed on wrought AZ31B magnesium extrusion. Axial, torsional and multiaxial cyclic behaviours of the material are presented. Effects of phase angle on stress-strain response, cyclic hardening and fatigue life are discussed. It was found that the material exhibits additional cyclic hardening due to non-proportional loading. However, the phase angle has no pronounced effect on fatigue life. Fatigue cracking behaviour was examined and two characteristics of crack geometry, i.e., size and orientation, are presented. Multiaxial fatigue lives using the Fatemi-Socie and the Smith-Watson-Topper models were predicted. In addition, the Jahed-Varvani energy model was used for fatigue life prediction and was shown to have a potential to be evaluated at plane of maximum axial strain energy density. Reverse analysis for predicting fatigue life by pre-defined critical plane as the observed plane is discussed in details. [ABSTRACT FROM AUTHOR]

Different wrought magnesium alloys from AM, AZ, and ZK family in the form of extrusion, rolled sheet and rolled plate have been selected for this study. Monotonic and cyclic behaviours are presented and compared. In particular, multi axial behaviours under proportional and non-proportional loadings are discussed. Despite the differences between the investigated alloys, it has been found that these alloys exhibit similar monotonic and cyclic characteristics. The similarity is attributed to the limited slip system in HCP magnesium, and the dominant role of deformation twinning in causing yield and hardening asymmetry. With strain energy density merit as a suitable fatigue parameter, it is therefore hypothesized that a simple two-parameter energy-based fatigue model is capable of correlating fatigue life of wrought magnesium alloys irrespective of material process, loading conditions and loading orientations. The hypothesis is then tested over a large number of fatigue results (354 tests). It is shown that fatigue lives predicted using the energy-life model are in good agreement with experimental results. Such simple model may prove to be useful in the early design stages lightweight components out of magnesium alloys. [ABSTRACT FROM AUTHOR]

Abstract: The present study intends to examine various cyclic plasticity models in fatigue assessment of 304 and 1045 steels based on the critical plane-energy damage approach developed earlier. Cyclic plasticity models of linear hardening, nonlinear, multi-surface, and two-surface were chosen to study fatigue damage and life of materials under proportional and non-proportional loading conditions. The effect of additional hardening induced due to non-proportional loading in 1045 steel and particularly in 304 steel was further evaluated as different constitutive models were employed. In the present study, the plasticity models were calibrated by the equivalent cyclic stress–strain curves. The merits of the models were then investigated to assess materials deformation under proportional and non-proportional loading conditions. Under non-proportional loading, the cyclic plasticity models were found to be highly dependent upon the employed hardening rule as well as the materials properties/coefficients. The stress and strain components calculated through constitutive laws were then used as input parameters to evaluate fatigue damage and assess the fatigue life of materials based on the critical plane-energy approach. The calculated values of stress components based on constitutive laws resulted in a good agreement with those of experimentally obtained under various loading paths of proportional and non-proportional conditions in 1045 steels. In 304 steel, the calculated stress components were however found in good agreement when plasticity models were employed for proportional loading conditions. Under non-proportional loading, the application of the multi-surface plasticity model in conjunction with the fatigue damage approach resulted in more reasonable results as compared with other plasticity models. This can be attributed to the motion of the yield surface in deviatoric stress space in the multi-surface model encountering additional hardening effect through estimated higher stress values under non-proportional loading conditions. Predicted fatigue lives based on the critical plane-energy damage approach showed such range of agreements as ±1.05–±3.0 factors in 1045 and 304 steels as compared with experimental life data when various constitutive plasticity models were employed. [Copyright &y& Elsevier]

Abstract: A set of strain-controlled biaxial proportional and non-proportional tests were conducted on solid and tubular specimens of 30CrNiMo8HH steel. The effect of the phase angle on fatigue life was studied. This effect becomes noticeable when applying a 90° out-of-phase loading, reducing the fatigue life by a factor up to 5. It has been shown that the material has no additional hardening due to out-of-phase loading. To account for this severe path dependency, a material dependent non-proportionality modification factor is proposed. This path dependent sensitivity factor is applied to six different fatigue parameters including maximum equivalent total strain, maximum equivalent stress, Smith–Watson–Topper, Fatemi–Socie, plastic strain energy density and total strain energy density to correlate the fatigue results. The predicted fatigue lives are compared with the experiments. The cyclic plasticity models of Mroz and Chaboche were successfully employed to model the cyclic behavior of 30CrNiMo8HH steel. It has been shown that estimations based on the proposed non-proportionality modification factor agree well with the experimental results. [Copyright &y& Elsevier]

Abstract: This study investigates the cyclic plasticity of AZ31B extrusion under axial and shear loading conditions and presents a continuum-based cyclic plasticity model for the anisotropic behaviour of magnesium. An anisotropic form of the Armstrong-Frederick plasticity model is presented. It is shown that the model can independently produce hysteresis loops under pure axial and pure shear loading conditions. Two different parameters for loading and unloading were considered to address the asymmetric shape of the axial hysteresis loops. The model is evaluated by considering the cyclic stress-strain response of AZ31B extrusion in uniaxial and multiaxial loading cases. Stress amplitude, plastic energy and total energy densities are calculated using the proposed model and are compared with experimental results showing a good agreement. [Copyright &y& Elsevier]

Abstract: The present study examines the capability of a lately developed energy-based fatigue damage parameter [Jahed H, Varvani-Farahani A. Upper and lower fatigue life limits model using energy-based fatigue properties. Int J Fatigue 2006;28:467–73] to assess fatigue life of various metallic materials subjected to proportional and non-proportional loading conditions. The proposed damage is defined based on (i) shear and axial stress and strain components responsible for cracking/modes of failure dominantly Case A and Case B, (ii) energy-based fatigue coefficients analogous to Coffin–Manson’s coefficients, (iii) corresponding fatigue lives of components failed under axial and torsional loading conditions, and (iv) total elastic–plastic energy calculated from stress–strain hysteresis loops. For the latter, the modified Mroz cyclic plasticity model has been employed to calculate the hysteresis energy. Using this model in conjunction with the proposed damage parameter, fatigue lives of different materials have been predicted and then compared with reported experimental data in the literature. The predicted fatigue lives based on the proposed damage model were found in very good agreement as compared with experimental fatigue data of various metallic materials of Al 7075-T6, AISI 304, SAE 1045, 1% Cr–Mo–V steel, Inc 718 and Haynes 188 tested under both proportional and non-proportional loading conditions. [Copyright &y& Elsevier]

Abstract: Cyclic strain-life data and corresponding Coffin–Manson coefficients for both normal and shear strain-lives were first defined. Energy–fatigue life curves were then generated from strain-fatigue life properties. The upper and lower limits of life are estimated using the proposed life equations. The upper life limit is obtained by assuming that the dominant cracking mechanism is Case A and the lower life limit is obtained by assuming that the dominant cracking mechanism is Case B. The proposed method was developed based on physical evidences of crack initiation and growth as well as the amount of dissipated energy over life cycles. The fatigue life data fall between the upper and the lower limits resulting in a promising life prediction. The proposed method has been used to evaluate the fatigue life of various metallic materials of SAE 1045, AISI 304, Inc 718 and Haynes 188 reported in the literature. Results of fatigue life predictions were found in good agreement with experimental life data. [Copyright &y& Elsevier]

• The TEM in-situ observation showed the formation of intermetallic layers is diffusion controlled. • Nano indentation shows different nano-mechanical properties of the intermetallic layers. • The interfacial bonding strength increased with annealing time and temperature. • Fracture surfaces exhibits two types of intermetallic. The influence of post processing on the interface microstructure, nano and tensile properties of solid-state deposition of AA7075 aluminum alloy on AZ31B cast Mg alloy is studied in detail. Transmission electron microscopy (TEM) microstructural analysis identified a 200–300 nm layer of Mg 17 Al 12 intermetallic with a columnar grain at the interface of the samples annealed at 200 °C. The in-situ TEM observation during annealing showed that the evolution of grains and growth of the new phases were controlled by the diffusion of voids and the interfacial reaction between Al/Mg. X-ray diffraction (XRD) analysis shows that Mg 2 Al 3 at the Al side and Mg 17 Al 12 at the Mg side were formed at the interface of the sample annealed at and above 300 °C. The thickness of the intermetallic increased significantly with increasing the annealing temperature. The nano-indentation revealed that the intermetallic layers achieved considerably better nano-mechanical properties, including elastic modulus and hardness, than the AA7075 coating and AZ31B substrate. With increasing the annealing temperature from 200 °C to 400 °C, the fracture strength of the coating reached a maximum value of ~130 MPa, then decreased to ~100 MPa. The interfacial microstructure analysis shows debonding during the tensile tests with debonding strength increasing with the annealing temperature. Dimples were identified on the coating fracture surfaces of the sample annealed at 300 °C. It is believed that the formation of intermetallic due to the diffusion enhanced the mechanical properties. [ABSTRACT FROM AUTHOR]

Highlights: [•] As-extruded and stress-relived samples of AZ31B are tested on a RBM. [•] Elastic–plastic method of analysis that considers yield and hardening asymmetry of wrought magnesium alloys is proposed. [•] Stress distribution, cyclic hysteresises and strain energy density in RBT of AZ31B are obtained. [•] Using an energy-life model, fatigue life of AZ31B in RBT is predicted. [•] The model S–N results are conservatively predicting AZ31B lives in RBT. [ABSTRACT FROM AUTHOR]

• Asymmetric hysteresis loop at room temperature changed to a symmetric one at 200 °C. • Hysteresis loops at 200 °C exhibit no inflection point. • Twinning-detwinning mechanisms do not govern the plastic deformation at 200 °C. • Mean stress diminishes and cyclic softening occurs at the elevated temperatures. • Jahed-Varvani model offers the most promising predictions at all the temperatures. Fully-reversed strain-controlled tests have been performed on AZ31B magnesium alloy extrusion at room temperature, 100 °C, and 200 °C. Different fatigue life prediction models including Coffin-Manson-Basquin, Smith-Watson-Topper, and Jahed-Varvani were applied to predict the fatigue life. According to the shape of the hysteresis loops at half-life, by increasing the test temperature to 200 °C, the asymmetric behavior of the magnesium changed to a symmetric shape. No inflection point can be observed in the loops at 200 °C, the mean stress decreases dramatically, and the cyclic softening occurs. Among different models, Jahed-Varvani as an energy-based approach offers the most promising predictions at all the temperatures. [ABSTRACT FROM AUTHOR]

• Sigmoidal but symmetric hysteresis loops are observed in radial direction. • First reversal being tensile or compressive does not affect strain amplitude and life. • Ratchetting direction in radial and extrusion directions are opposite. • The extrusion direction exhibits superior stress-controlled fatigue performance. • Energy-based fatigue parameters correlate life in radial and extrusion directions. The anisotropic fatigue behavior of ZK60 is investigated through stress-control tests along two different material directions: extrusion (ED) and radial (RD) directions. The in-plane random texture along RD promotes activation of twinning/detwinning deformations in both tension and compression reversals, which brings about a sigmoidal but near-symmetric shape for hysteresis loops. The stress-strain response along ED is asymmetric, which is attributed to different deformation mechanisms in tension and compression reversals. The higher fatigue strength along ED is related to lower plastic strain energy in this direction. An energy damage parameter showed a good correlation with tests performed in RD and ED. [ABSTRACT FROM AUTHOR]

Highlights • Cyclic behavior of ZK60 Mg alloy cast prior to and after forging process is studied. • Microstructure modification after forging leads to better fatigue response. • Forged alloy asymmetric fatigue behavior is due to the induced sharp basal texture. • ZnZr 2 and MgZn 2 intermetallics are main source of crack initiation. • An energy-based fatigue life model closely estimated the cyclic life. Abstract The fatigue behavior of as-cast and cast-forged ZK60 magnesium alloy was investigated via fully-reversed strain controlled fatigue tests at different strain amplitudes. Microstructure analysis, texture measurement, and SEM fracture surface characterization were performed to discern the reason of fatigue behavior improvement via forging, and also to explain the mechanism underlying crack initiation in both cast and cast-forged conditions. It was perceived that the forged alloy contains less amount of porosities and second phase particles in its microstructure. In general, the forged alloy showed longer fatigue life for all strain amplitudes, especially when the strain amplitude is lower than 0.4%. The forging process increased the fatigue strength at 107 cycles from 0.175% to 0.22% strain amplitude. The microstructure obtained after fatigue test showed that twinning can be activated in the cast-forged alloy, once strain amplitude is higher than 0.4%. The interaction of twin bands with the grain boundaries can also adversely affect the fatigue life of the forged alloy. Also, the residual twins can develop tensile mean stress which affects the fatigue life negatively. Finally, the Coffin-Manson fatigue model and an energy-based fatigue model were employed to model the life of as-cast and cast-forged materials. While some of the predicted lives by the former were out of the ±2× boundary bounds, the latter's results were tightly clustered in ±1.5× bounds. [ABSTRACT FROM AUTHOR]

Abstract The quasi-static and strain-controlled fatigue characteristics of ZK60 extrusion have been investigated along three different directions: the extrusion direction (ED), the radial direction (RD), and 45° to the extrusion direction (45°). The quasi-static response showed symmetric behavior for the samples tested along RD and 45°, whereas the ED samples manifested completely asymmetric behavior. Although the ED samples exhibited longer fatigue lives than the RD and 45° in the high cycle fatigue, the fatigue lives in the low cycle fatigue regime were similar. The texture measurement indicated a sharp basal texture along ED, explaining its asymmetric behavior. Higher tensile mean stress and less dissipated plastic energy per cycle for the ED samples, acting as two competing factors, were the principal reasons for exhibiting fatigue responses identical to those of RD and 45° in the LCF regime. The fracture surface in the ED direction was dominated by twin lamellae and profuse twinned grains, whereas that in RD was dominated by slip bands. Finally, Smith-Watson-Topper and Jahed-Varvani models were employed to predict the fatigue lives along all directions using a single set of material parameters. Graphical abstract Unlabelled Image Highlights • In contrast to the quasi-static behavior of extrusion direction, radial and 45° directions showed symmetric responses. • Fatigue performance of as-extruded ZK60 is not sensitive to the material direction in the low cycle fatigue regime. • Extrusion direction exhibits fatigue performance superior to radial and 45° directions in the high cycle fatigue regime. • The Jahed-Varvani model predicts the fatigue life along all the directions accurately with a single set of parameters. [ABSTRACT FROM AUTHOR]

Abstract Stress-controlled uniaxial "push-pull" fatigue testing was conducted on as-received (cast and extruded) and closed-die cast-forged and extruded-forged AZ80 Mg alloy. The as-cast material possessed random texture and somewhat symmetric cyclic responses. The extruded and forged materials possessed sharp basal texture and asymmetric cyclic responses. All materials exhibited tension/compression asymmetry in their cyclic response to varying degrees, depending on the thermomechanical processing conditions. It was discovered that the style of closed-die forging being investigated had spatially varying properties with texture orientations which varied based on the local forging directions and intensities which were dependent on the starting texture as well as the thermomechanical history. Under fatigue testing, the materials all developed some form of mean strain, with the nature and magnitude of this mean strain being dependent on primarily its texture intensity and propensity to twin in either tension or compression reversals. The type of mean strain (tensile or compressive) depends upon both the orientation and intensity of the starting texture of material. The texture induced ratcheting and resulting mean strain evolution was most pronounced in the as-cast material and had a significant impact on the fatigue life. Following forging, the material exhibited an increase in fatigue life of anywhere from 2 to 15 times for the cast then forged material and more modest yet still significant 8 times longer at stress amplitudes around 140 MPa for the extruded then forged material. The extruded forged material exhibited similar fatigue lives to that of the base material at stress amplitudes which approached the yield strength. The nature of the mean stress development and degree of fatigue life improvement depended on the processing conditions and the type of base material (cast or extruded) utilized to create the forging. Two energy based models were utilized to predict the life of the forged material, and gave a reliable life prediction for a variety of material conditions that were investigated. [ABSTRACT FROM AUTHOR]

Tensile and strain-controlled fatigue tests were performed to investigate the influence of forging on the performance of cast AZ80 magnesium alloy. The obtained microstructural analysis showed that the as-cast AZ80 magnesium alloy has dendritic α-Mg phase with eutectic Mg 17 Al 12 morphology and a random texture. In contrast, the forged samples showed refined grains and a strong basal texture. During tensile testing, a maximum yield and ultimate tensile strength of 182 MPa and 312 MPa were obtained for the forged samples, representing increases of 121% and 33%, respectively, from the as-cast condition. At the same time, a significant improvement (73%) in ductility was obtained in forged samples. It was also observed that the forged samples achieved comparatively longer fatigue life under strain-controlled cyclic loading. Analysis of the fracture surfaces showed that a cleavage-type morphology was typical for the as-cast samples, while the occurrence of dimples and other evidence of plastic deformation were identified in the fracture surfaces of the forged specimens, indicating a more ductile response. Forging caused grain refinement and texture modification, both of which enhance alloy performance by improving strength and ductility, and leading to longer fatigue life. Strain and energy-based models were investigated for their suitability to predict the life of the forged material. Both the Smith-Watson Topper and the Jahed-Varvani energy-based models gave reliable life prediction. [ABSTRACT FROM AUTHOR]

Highlights: [•] Successful cold spray coating of aluminum powder on magnesium substrate. [•] Numerical modeling of local plasticity induced by coating on magnesium. [•] Sensitivity analysis of coating parameters on residual stress distribution. [•] Procedure for obtaining optimum coating parameters. [ABSTRACT FROM AUTHOR]

Abstract: The effect of cold spray coating in inducing residual stresses in the substrate and its effect on delaying crack initiation under cyclic loading have been studied on Al5052 alloy specimens. Different sets of Al5052 specimens have been coated with pure Al and Al7075 feedstock powder, using a low-pressure cold spray coating technique. Some sets of specimens were grit blasted (GB) before coating. The microstructural evolution of the substrate after coating and the fatigue behavior of the coated structure have been studied. In order to obtain the fatigue S–N diagram for each set, as-received and coated specimens with and without preceding GB treatment have been tested in a load-controlled condition. X-ray diffraction has been used to measure the residual stresses both in the deposited materials and the substrates. The results are discussed to highlight the effect of this emerging surface treatment on the characteristics of the treated material. Compressive residual stresses, which led to appreciable increase in the fatigue life, have been observed in all the coated sets. The results indicate that the fatigue strength was significantly improved up to 30% in the case of Al7075 coatings. The results show a strong dependency of the fatigue strength on the deposited material and the spray parameters. [Copyright &y& Elsevier]

• Forging of AZ31B extrusion results in significant changes in microstructure and texture. • Texture evolution at any location depends on the corresponding plastic strain induced by closed-die forging. • Forging process results in cyclic hardening and improved fatigue strength in AZ31B. • Cyclic hysteresis response is asymmetric under axial but symmetric under shear loading. • Multiple fatigue crack initiation cites observed under both axial and shear cyclic loads. Extruded AZ31B was closed-die forged at a relatively low temperature of 250 °C. Quasi-static axial and shear tests in addition to cyclic pure axial and cyclic pure shear tests were performed to characterize the uniaxial response of the forged parts. To examine the effect of forging on the mechanical behavior of the material, quasi-static and fatigue response of the starting extruded AZ31B was also studied under axial loading. The forging process refined the grain structure and modified the crystallographic texture of the extruded AZ31B. The intensity of texture and the orientation of c-axis in the forged part were found to be controlled by the extent and the direction of plastic deformation, respectively. The modified microstructure resulted in insignificant effect on yield strength, slight decrease in ultimate strength, and enhanced failure strain of the forged AZ31B under axial loading. However, the forged AZ31B exhibited superior yield strength, similar ultimate strength, and lower failure strain under shear loading. The cyclic axial strength of the forging was similar to the starting extruded material under high strain amplitudes, but slightly superior under lower strain amplitudes, due to the development of a smaller tensile mean stress. The half-life hysteresis response for the forged material was asymmetric under large axial strain amplitudes, but it was symmetric under cyclic shear loading. This difference is originated from activation of twinning/ detwinning deformation under cyclic axial loading, while dislocation slip is the dominant deformation mechanism during cyclic shear loading. Fatigue fracture surface of forged AZ31B depicts that fatigue cracks were initiated from multiple locations under both cyclic axial and shear loadings. [ABSTRACT FROM AUTHOR]