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

Abstract

A B S T R A C TThe 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.

Fatigue of rare‐earth containing magnesium alloys: a review

“…8,9,[16][17][18][19][20] In the structural applications of Mg alloys in the transportation industry, including camshaft covers, clutch and transmission housings, intake manifolds and automobile wheels, 21 the components are inevitably subjected to cyclic stresses and strains for millions of cycles in service. 15,[22][23][24][25][26][27][28][29] It is well known that a metal subjected to repeated or fluctuating stresses will fail at a stress level much lower than that required to cause fracture on a single application of load. 30 Such a failure occurring under conditions of dynamic loading is called fatigue failure.…”

“…Despite the potential of substantial reductions in weight, most wrought Mg alloys exhibited a high degree of anisotropy and tension-compression yield asymmetry due to the presence of strong crystallographic texture owing to their hexagonal close-packed (HCP) crystal structure with limited slip systems during extrusion or rolling processes. 22,24,25,29,[32][33][34][35][36][37][38] Indeed, for the vehicle components subjected to dynamic cyclic loading, such mechanical anisotropy and tension-compression yield asymmetry could lead to irreversibility of cyclic deformation, which may have an unfavourable influence on the performance and durability of structural components. These problems could be tackled through texture modification.…”

Extension of strain–life equation for low‐cycle fatigue of sheet metals using anisotropic yield criteria and distortional hardening model

“…The major difference between the previous work and present extension is the estimation of , which is used to calculate the orientation dependence of and c . The fatigue constants using HAH model are considered to be more accurate as the strain‐hardening exponent is estimated directly from the cyclic stress–strain curve and not from an approximation from yield strength ratio.…”

“…While f ( θ ) can be determined from anisotropic yield criteria, cyclic stress–strain curves along different orientations is required to calculate g ( θ ) and . In the previous work,a special case of g ( θ ) = f ( θ ) was assumed, and isotropic assumption was employed to derive . As discussed earlier, the isotropic hardening assumption does not account for the Bauschinger effect, commonly observed in cyclic behaviour of many metallic materials.…”

“…Recently, the orientation dependence of fatigue constants in Coffin–Manson equation for strain–life relation was obtained using an anisotropic yield function . However, the modification assumed isotropic hardening and did not account for the Bauschinger effect, typical to cyclic loading.…”