Anisotropy of mechanical properties in quaternary Al-Li-Cu-Mg alloys

Considerable anisotropy in the mechanical properties of quaternary Al-Li-Cu-Mg alloys was observed in both plate and sheet products. These alloys showed more than 100% increase in tensile ductility in the test direction oriented at 45–60° to the rolling direction as compared to that in the rolling direction (longitudinal, L). A concomitant decrease in strength was also found. These alloys exhibit superior low cycle fatigue resistance in the long-transverse (LT) direction as compared to the longitudinal (L) direction. Another observation is the occurrence of strength differential (S-D), which is seen to be directional. The trends in S-D are similar under monotonic as well as cyclic loading conditions. The fracture resistance is also highly anisotropic. An attempt is made here to correlate the observed anisotropy in the mechanical behaviour of these alloys with the microstructure and crystallographic texture.     

Initial anisotropy in demineralized bovine cortical bone in compressive cyclic loading–unloading

The mechanical properties of demineralized bovine cortical femur bone were investigated by cyclic loading–unloading compression in three anatomical directions (longitudinal, radial, transverse) within the physiological strain range. The loading responses in the radial and transverse directions were nearly linear up to 2% strain, while the response in longitudinal direction was strongly non-linear in that range. The unloading responses were non-linear for each anatomical direction, giving rise to overall loading–unloading hysteresis and cyclic dissipation of energy. The mechanical properties were observed to be anisotropic: the radial direction was found to be the most energy dissipative, while the longitudinal direction appeared to be the stiffest bone direction. The cyclic loading mostly affects the bone stiffness in the radial and transverse directions, while the longitudinal direction was found to be the least affected. These anisotropic properties can be attributed to the differences in collagen fibers alignment and different microstructural architecture in three different anatomical bone directions.     

Tensile and fatigue behaviour of wrought magnesium alloys AZ31 and AZ61

The mechanical behaviour of two hot rolled magnesium alloys, namely the AZ31 and AZ61, has been evaluated experimentally under both monotonic and cyclic loading. Both longitudinal (L) and long transverse (LT) directions were evaluated. The tensile behaviour of the L and LT directions is similar and differs only in the offset 0.2% yield strength for both materials. This difference is attributed to the angular spread of basal poles toward the rolling direction and is more pronounced for the case of AZ31. A distinct hardening response is obvious in both directions. Twinning formation was observed; it is more pronounced in the longitudinal direction while the fracture mode is intergranular and equiaxed facets are present in the fracture surfaces of the specimens. The S–N curves exhibit a smooth transition from the low to high cycle fatigue regime. AZ61 exhibits an overall better fatigue behaviour compared to AZ31. A transgranular crack initiation mode is observed in all tested specimens while the propagation of the cracks is characterized as intergranular.     

Effect of microstructure and texture on the mechanical properties of the as-extruded Mg–Zn–Y–Zr alloys

This work mainly studied the influence of the microstructure and crystallographic texture on the mechanical properties of the as-extruded Mg–Zn–Y–Zr alloys with different Y contents. The samples were machined from thick plates obtained by extrusion and the tensile tests were performed parallel to extrusion and transverse directions, respectively. Microstructure observation firmly indicated that the grain-refining effect of icosahedral quasicrystal phase (I-phase) was superior to that of the cubic W-phase. In addition, the tensile results indicated that I-phase could effectively improve the strength (yield strength and ultimate tensile strength) of alloys. However, strengthening effect of W-phase was lower. With the quantity of W-phase increasing, the strength of alloys was degraded. It also showed that the alloys were mechanically anisotropic, i.e. the longitudinal strength was higher than that of the transverse direction. However, the ductility of the transverse direction was superior. With the increase of Mg–Zn–Y phases, the anisotropy of the ultimate tensile strength (UTS) between the longitudinal and transverse directions increased remarkably. SEM fracture observations showed that the fractures of the TD samples were characterized by the typical “woody fracture”, with a large amount of cracked Mg–Zn–Y particles (I-phase and W-phase) distributed at the bottom of dimples. With Y content increasing, the average spacing of the zonal distributed Mg–Zn–Y particles on the fracture surface became narrow, which influenced the transverse mechanical properties greatly.     

Post-forming monotonic and cyclic behavior in a HSLA steel sheet after large deformations by in-plane compression

The effects of large prestrains (18–40%), produced by in-plane compression, on the asymmetry and the anisotropy of the stress response and on the fatigue life are investigated under fully reversed axial strain for a 345 MPa yield strength V–N high strength low alloy steel sheet. After prestraining, the hysteresis loops are asymmetric and the stress response is anisotropic, i.e., the response differs in directions parallel and perpendicular to that of the compressive prestrain. To understand the cyclic flow stress asymmetry, monotonic tension and compression tests were conducted in these two directions after prestraining. It is shown that the loop asymmetry is related to the Bauschinger effect after prestraining. Two cyclic stress strain curves, one corresponding to the tension side of the hysteresis loops and the other to the compression side, are defined to accurately describe the post-prestraining behavior. The amount of strengthening gained by prestraining is partially retained after cycling. Prestraining increases the fatigue life at low strain amplitudes but decreases it at high strain amplitudes.     

Texture effects under tension and torsion loading conditions in titanium alloys

The paper derives from a major research programme on texture evolution and characterisation in the titanium alloys Ti-6Al-4V and Ti550. The present publication focuses on the mechanical characterisation of the texture in rolled plate. It focuses specifically on monotonic strengths and strain control fatigue under tension and torsion loading. Dependencies of fatigue performance on specimen orientation are interpreted through EBSD evaluation of basal and prism plane intensities within the present materials. The cyclic deformation and mechanisms of failure are related to the relative magnitude of tensile and shear stress components in the tension and torsion loading modes. Stress relaxation is shown to play an important part in these processes. Its relationship to available slip systems is discussed     

Lateral behavior of concrete under uniaxial compressive cyclic loading

In reinforced concrete structures under seismic loading, concrete is subjected to compressive cyclic stress. Although cyclic stress–strain response has been described before, the cyclic behavior of strains in the direction orthogonal to loading has not been characterized yet. Such behavior can be of great importance for evaluating the efficiency of the confinement under cyclic loading. For this purpose an experimental program on cylindrical specimens of concrete strength from 35 to 80 MPa subjected to uniaxial cyclic compression was carried out. Stress versus longitudinal and lateral strains curves have been obtained both for the hardening and softening branches under monotonic and cyclic loading. Governing parameters of the lateral behavior are identified and correlated to describe the response of the lateral strain. Additionally, an analytical model to obtain the lateral deformations of concrete under cyclic uniaxial compression has been formulated and verified experimentally. Finally, some examples are presented in order to illustrate the applicability of the proposed model and its possible incorporation into a 3D constitutive cyclic model.     

Strength differential in Al---Li alloy 8090

Cyclic strength differential (SD) data have been derived from the variation of the tensile and the compressive stress amplitude with elapsed cycles during low cycle fatigue (LCF) for the quaternary Al---Li alloy 8090 in the T8E51 condition. LCF test specimens were machined out of the rolled plate such that the direction of stressing was along the longitudinal (L), L+45° and long transverse (LT) directions. The analysis of cyclic stress amplitude data corresponding to the half-life revealed an SD effect at all strain levels in the L+45° and LT directions. In the case of the L direction, at strain levels below 8.5 × 10⁻³, the SD is more than compensated by the Bauschinger effect resulting from the prestretch. The alloy exhibits a similar SD behaviour under monotonic loading conditions. The magnitude of cyclic SD at intermediate strain amplitudes in all the three test directions decreases upon cycling, which behaviour can be attributed to the relaxation of prestretch-related residual stresses leading to a decrease in σ_T only in the L direction and σ_C only in the L+45° and LT directions. As microstructural features vary in the differently heat-treated conditions, a comparison has been made of the monotonic SD data in the stretched and aged (T8E51) condition with those in the solution-treated, in the peak-aged (T6) and in the overaged (T7) conditions.     

Modification of fatigue strain‐life equation for sheet metals considering anisotropy due to crystallographic texture

The fatigue behaviour of cold rolled and annealed sheet metals are influenced by the anisotropy of mechanical properties due to crystallographic texture. However, the existing fatigue strain‐life models are primarily meant for isotropic material behaviour. In the present work, the Coffin‐Manson equation for strain‐life is modified to include the effect of anisotropy using phenomenological plasticity models. It is observed that the variation of strain hardening exponent is critical to model the strain‐life behaviour. Variation of strain hardening exponent with orientation is modelled using existing anisotropic yield criteria. The prediction of fatigue life using the proposed model correlates well with the experimental results of Al6061‐T6 along different orientations. The proposed model can be used to predict the fatigue properties along any orientation from the fatigue data along one orientation and monotonic mechanical properties along longitudinal, transverse and diagonal directions.     

Simulations of stress distributions in crankshaft sections under fillet rolling and bending fatigue tests

The residual stresses due to fillet rolling and the bending stresses near the fillets of crankshaft sections under bending fatigue tests are important driving forces to determine the bending fatigue limits of crankshafts. In this paper, the residual stresses and the bending stresses near the fillet of a crankshaft section under fillet rolling and subsequent bending fatigue tests are investigated by a two-dimensional plane strain finite element analysis based on the anisotropic hardening rule of Choi and Pan [Choi KS, Pan J. A generalized anisotropic hardening rule based on the Mroz multi-yield-surface model for pressure insensitive and sensitive materials (in preparation)]. The evolution equation for the active yield surface during the unloading/reloading process is first presented based on the anisotropic hardening rule of Choi and Pan (in preparation) and the Mises yield function. The tangent modulus procedure of Peirce et al. [Peirce D, Shih CF, Needleman A. A tangent modulus method for rate dependent solids. Comput Struct 1984;18:875–87] for rate-sensitive materials is adopted to derive the constitutive relation. A user material subroutine based on the anisotropic hardening rule and the constitutive relation was written and implemented into ABAQUS. Computations were first conducted for a simple plane strain finite element model under uniaxial monotonic and cyclic loading conditions based on the anisotropic hardening rule, the isotropic and nonlinear kinematic hardening rules of ABAQUS. The results indicate that the plastic response of the material follows the intended input stress–strain data for the anisotropic hardening rule whereas the plastic response depends upon the input strain ranges of the stress–strain data for the nonlinear kinematic hardening rule. Then, a two-dimensional plane-strain finite element analysis of a crankshaft section under fillet rolling and subsequent bending was conducted based on the anisotropic hardening rule of Choi and Pan (in preparation) and the nonlinear kinematic hardening rule of ABAQUS. In general, the trends of the stress distributions based on the two hardening rules are quite similar after the release of roller and under bending. However, the compressive hoop stress based on the anisotropic hardening rule is larger than that based on the nonlinear kinematic hardening rule within the depth of 2 mm from the fillet surface under bending with consideration of the residual stresses of fillet rolling. The critical locations for fatigue crack initiation according to the stress distributions based on the anisotropic hardening rule appear to agree with the experimental observations in bending fatigue tests of crankshaft sections.     

Effects of pressure-sensitive yielding on stress distributions in crankshaft sections under fillet rolling and bending fatigue tests

In this paper, the evolution equation for the active yield surface during the unloading/reloading process based on the Drucker–Prager yield function and a recently developed anisotropic hardening rule is first presented. A user material subroutine based on the anisotropic hardening rule and the constitutive relation was written and implemented into the commercial finite element program ABAQUS. Computations were first conducted for a simple plane strain finite element model under uniaxial monotonic and cyclic loading conditions. The results indicate that the anisotropic hardening rule with the non-associated flow rule describes well the strength-differential effect and the asymmetric closed hysteresis loops as observed in the uniaxial cyclic loading tests of cast irons. Then, a two-dimensional plane strain finite element analysis of a crankshaft section under fillet rolling and subsequent bending was conducted. For the pressure sensitivity corresponding to the cast iron crankshaft of interest, the critical locations for fatigue crack initiation according to the stress distributions for pressure-sensitive materials agree with the experimental observations in bending fatigue tests of crankshaft sections.     

Evolution of mechanical properties for a dual-phase steel subjected to different loading paths

The evolution of the mechanical properties of a dual-phase (DP590) steel sheet after being prestrained by uniaxial tension, plane strain and equal biaxial stretching was investigated. Specimens were first loaded using the three prestraining modes. Then, from the prestrained specimens, a few sub-sized samples were machined along the rolling direction and the transverse direction for further uniaxial tension testing. Six loading paths were provided. Equal biaxial stretching was performed using a cruciform specimen. The evolution of work hardening performance, elastic modulus, yield stress and tensile stress under the six loading paths were discussed in detail. The results indicate that loading paths can affect the latent work hardening performances, strain hardenability, yield stress and tensile stress evolution as well as the elastic modulus decrease during plastic deformation. The uniaxial tension–uniaxial tension path results in a cross-softening phenomenon, the largest yield stress enhancement and a mild maximum tensile stress increase. The equal biaxial stretching-uniaxial tension path leads to a cross-hardening phenomenon, the least yield stress enhancement and the largest tensile strength increase maximum tensile strength. The elastic modulus of DP590 steel not only changes with the accumulated plastic strain but also varies with the loading paths. The largest decrease of the elastic modulus equal biaxial stretching–uniaxial tension can reach 12.7% beyond 8% equivalent strain, which is 5.2% greater than that in the monotonic uniaxial tension path.     

微织构对Ti-6Al-3Nb-2Zr-1Mo合金的应力腐蚀性能的影响

采用EBSD方法对断口以及裂纹扩展路径进行观察,研究了近α型钛合金Ti-6Al-3Nb-2Zr-1Mo组织中的微织构对应力腐蚀敏感性和裂纹扩展路径的影响规律.结果表明,微织构区域的形貌为接近平行于轧面的不规则薄片层状,长度方向沿轧向扩展,微织构区的晶粒取向为c轴趋近于与板材横向平行.当加载方向与微织构区的长度方向接近平...     

Influence of loading direction on the anisotropic fatigue properties of rolled magnesium alloy

The influence of loading direction on the fatigue behavior of rolled AZ31 alloy was investigated by conducting fully reversed stress-controlled fatigue tests along the rolling direction and normal to the rolling plane. Alternating twinning and detwinning behavior during initial cycling was found to cause asymmetric hysteresis loops, resulting in a compressive strain in the rolling direction and a tensile strain normal to the rolling plane. A transition in the dominant deformation mechanism from twinning–detwinning to slip occurs at around five cycles under both conditions due to cyclic hardening, thus making their loops symmetric. The lower twinning stress in tension along the normal direction leads to an increase in fatigue damage by plastic strain, resulting in a lower fatigue resistance than along the rolling direction.     

NOTCHED FATIGUE BEHAVIOUR OF TWO COLD ROLLED STEELS

Abstract— A SAE1010 plain carbon steel and a SAE945X HSLA steel were cold rolled to various thickness reductions. Centre notched specimens were tested under stress control at a stress ratio of—1. The effect of loading direction on the fatigue strength was examined. The notched specimen fatigue strength was only slightly increased by cold rolling, since two opposing factors: the smooth specimen fatigue strength and the notch sensitivity, were increased by cold rolling. The notched specimen fatigue strength in the transverse direction was approximately the same as that in the longitudinal direction. An empirical equation and equations derived from fracture mechanics and Neuber's rule were applied to predict the fatigue notch factor for the sharp and blunt notch geometries examined. A reasonable agreement between the predictions and the experimental results was observed for the sharp notches. For the blunt notches, the predicted fatigue notch factors were conservative.     

Low and high‐cycle fatigue properties of an ultrahigh‐strength TRIP bainitic steel

The scope of this study is to characterize the mechanical properties of a novel Transformation‐Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low‐cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 10² and 10⁵ fatigue cycles. The data are used to determine the parameters of the Coffin–Manson as well as the cyclic stress–strain curve. No significant stress‐induced austenite transformation is detected. The high‐cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 10⁶ fatigue cycles at two loading ratios, namely R = ?1 and R = 0. At fatigue lives longer than 2 × 10⁵ cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions.     

HRB600钢筋屈曲受力性能试验研究

HRB600高强钢筋已纳入我国钢筋产品国家标准,并商业化生产,但关于其非弹性屈曲受力性能、考虑屈曲的低周疲劳损伤模型的研究成果很少。对于HRB600高强钢筋的原状试件,该文完成了10个单调受压屈曲试验、16个考虑屈曲的循环拉压试验,测量了屈曲钢筋的平均应力-平均应变曲线和跨中横向位移,分析了非弹性屈曲对HRB600钢筋受压强度退化的影响,以及屈曲方向对试件强度退化、疲劳损伤的影响,提出了适用于HRB600高强钢筋、可考虑屈曲效应和不同加载方式影响的修正低周疲劳损伤模型。研究结果表明:单调受压屈曲时HRB600钢筋屈曲方向稳定,随长径比增大钢筋屈曲后强度退化加快;循环加载时HRB600钢筋屈曲方向不稳定,导致局部塑性应变集中和疲劳损伤对钢筋屈曲后强度退化的不利影响减轻;传统低周疲劳损伤模型明显低估了HRB600钢筋屈曲后的疲劳损伤,修正疲劳损伤模型能合理地考虑屈曲对HRB600钢筋低周疲劳损伤和断裂的影响。     

Monotonic and fatigue behaviour of chopped-strand-mat/polyester composites with rigid and flexibilised matrix

Monotonic and tension–tension fatigue tests were carried out on E-glass chopped-strand-mat/polyester composites, varying the flexibiliser content by weight in the matrix in the range 0–30%. The flexibilising action was due to the adipic acid monomers present in the flexibiliser.In monotonic tests, the most marked effect of resin flexibility was in the transverse cracks formed during loading, whose critical density (i.e., the density at failure) was very high for the rigid matrix, resulting in a highly non-linear stress–strain curve, and in the largest apparent strain to failure. With suitably increasing the flexibiliser content, the transverse crack formation was nearly suppressed, and the overall stress–strain curve approached linearity.In fatigue, the critical crack density decreased with increasing fatigue life in the case of the rigid matrix. For the flexibilised resins, the crack density at failure was independent of the maximum applied stress, larger than observed in monotonic tests, and higher the higher was the flexibiliser content, up to about 80% of the tensile strength. Beyond this limit, it converged through the material monotonic behaviour. The evolution of the residual elastic modulus with elapsing fatigue cycles was qualitatively consistent with the number of transverse cracks observed. The more flexible the matrix, the lower was the fractional modulus loss in fatigue. However, the highest elastic modulus along all the fatigue life pertained to the composite with rigid matrix, due to the flexibiliser adversely affecting the initial rigidity.Despite the differences in monotonic response and crack formation features, all the materials tested exhibited very similar S–N curves at moderately high fatigue lives. Nevertheless, appropriately treating the experimental results in terms of fatigue sensitivity, it was found that this parameter tends to increase with increasing matrix flexibility.     

Al-5.4Zn-2.6Mg-1.4Cu合金板材的低周疲劳行为

进行Al-5.4Zn-2.6Mg-1.4Cu合金板材的室温低周疲劳实验,对比研究了轴向平行于轧制方向(RD方向)和垂直于轧制方向(TD方向)试样的低周疲劳行为。结果表明：对于0.4%~0.8%的外加总应变幅,RD和TD方向合金试样的循环应力响应行为均呈现出循环稳定;对于相同的外加总应变幅,TD方向合金的循环应力幅值高于RD方向,而RD方向合金的疲劳寿命高于TD方向。对于RD和TD方向,Al-5.4Zn-2.6Mg-1.4Cu合金的塑性应变幅、弹性应变幅与载荷反向周次均呈线性关系。在低周疲劳加载条件下,裂纹在疲劳试样的自由表面以穿晶方式萌生和扩展。     

Investigation of uniaxial low-cycle fatigue failure behavior of hot-rolled AZ91 magnesium alloy

The uniaxial low-cycle fatigue behavior of hot-rolled AZ91 magnesium alloy was investigated by asymmetric cyclic stress-controlled experiments at room temperature. The effects of the sampling direction, peak stress and stress ratio on the fatigue life were discussed. The fatigue life increases with increasing the stress ratio or decreasing the peak stress. Due to the anisotropic property, the specimen in transverse direction shows superior fatigue resistance. Considering the effects of mean stress on the fatigue strength coefficient and fatigue strength exponent, a modified Basquin model was proposed and validated to evaluate the fatigue life of AZ91 magnesium alloy.