A TWO-SURFACE MODEL DESCRIBING RATCHETTING BEHAVIORS AND TRANSIENT HARDENING UNDER NONPROPORTIONAL LOADING

A new superposed rule of Mroz's kinematic hardening rule and Ziegler's kinematic hardening rule based on two-surface model is proposed in the paper. Some experimental results on ratchetting of 2014-T6 aluminum alloy are predicted very well under multiaxial loading. In addition the conformability of the model is discussed for transient cyclic hardening under two kinds of nonproportional cyclic loading paths., i.e. square and rhombic path.

Fatigue Property of Additively Manufactured Ti-6Al-4V under Nonproportional Multiaxial Loading

The low cycle fatigue strength properties of the additively manufactured Ti-6Al-4V alloy are experimentally investi-gated under proportional and nonproportional multiaxial loading.The fatigue tests were conducted using hollow cylinder specimens with and without heat treatments,at room temperature in air.Two fatigue tests were conducted:one for proportional loading and one for nonproportional loading.The proportional loading was represented by a push-pull strain path(PP)and the nonproportional loading by a circle strain path(Cl).The failure lives of the additively manufactured specimens were clearly reduced drastically by internal voids and defects.However,the sizes of the defects were measured,and the defects were found not to cause a reduction in fatigue strength above a critical size.The fracture surface was observed using scanning electron microscopy to investigate the fracture mechanisms of the additively manufactured specimens under the two types of strain paths.Different fracture patterns were recognized for each strain paths;however,both showed retention of the crack propagation,despite the presence of numerous defects,probably because of the interaction of the defects.The crack propagation properties of the materials with numerous defects under nonproportional multiaxial loading were clarified to increase the reliability of the additively manufactured components.

SIMULATION OF NONPROPORTIONAL CYCLICEXPERIMENTS OF IN738LC AT 850℃

In the previous paper, in order to express steadystate ratchetting, the present s extended the cyclic plasticity model proposed by Ohno and Wang (1993), and the validity of the extended model was discussed on the basis of uniaxial ratchetting experiments of 316FR steel at room temperature. In the present paper, the validity of the extended model is discussed further on the basis of nonproportional experiments of IN738LC at 850 such as multiaxial ratchetting, multiaxial cyclic stress relaxation, circular cyclic straining with strain hold, and so on. Predictions based on the OhnoWang model as well as the ArmstrongFrederick model are also given for the sake of comparison. It is shown that the extended model is capable of simulating the nonproportional experiments accurately, and especially that the extended model can predict much less steadystate ratchetting than the ArmstrongFrederick model. It is also shown that the extended model provides almost the same predictions as the OhnoWang and th

Nonproportional Low Cycle Fatigue for 316L Stainless Steel

A series of tests are performed for 316L stainless steel under multiaxial nonproportional low cycle fatigue(LCF). The microstructures of the steel in the process of nonproportional LCF are observed with transmission electron microscopy (TEM). Based on macroscopic and microscopic experiments, the micromechanism of additional hardening and the decrease in LCF life under nonproportional cyclic loading are studied. The results of the tests indicate that 316L stainless steel obviously exhibits nonproportional cyclic additional hardening, which is mainly due to rotation of maximum shear stress plane during the LCF under nonproportional cyclic loading.

A NONPROPORTIONAL CYCLIC VISCOPLASTIC CONSTITUTIVE MODEL

A viscoplastic constitutive model is introduced,which takes into account nonproportional cyclic additional hardening and the effects of nonproportional cyclic loading history and inelastic strain amplitude history.In the model,a variable q of memory on the inelastic strain amplitude during loading is introduced,a new nonproportionality is defined,a cyclic saturated isotropic deformation resistance parameter Q,is given,and the evolution equations of the isotropic deformation resistance with the memory on the previous loading history are proposed.The present model is applied to the description of the complex cyclic deformation behavior of 1Cr18Ni9Ti stainless steel,and the predictions are in good agreement with the experimental results.This shows that the model describes the deformation behavior of nonproportional cyclic loadings and their histories very well.

AN ENDOCHRONIC CONSTITUTIVE MODEL FOR RATE-DEPENDENT AND NONPROPORTIONAL CYCLIC PLASTICITY

The total stress response of material is decomposed into a sum of an equilibrium stress response and a non-equilibrium overstress response. Correspondingly, the rate-independent intrinsic time and the rate-dependent intrinsic time are defined respectively. Additional hardening functions for describing the isotropic and anisotropic nonproportional effects are assumed to be related to the accumulation of plastic strain component along the normal of equilibrium stress trajectory, in which the effects of geometry of the loading path are included. An endochronic constitutive model for rate-dependent, nonproportional cyclic plasticity is formulated and applied to simulate the stress responses of stainless steel XCrNil8. 9 for some typical loading programs at different loading rates. A comparison between predicted results and experimental ones by Haupt and Lion shows that the former are in agreement with the latter.

ON NONPROPORTIONAL CYCLIC PLASTIC BEHAVIOR OF STEEL 40

An experimental investigation was carried out on the flow characteristic and hardening of steel 40 subjected to complex combined axial-torsional cyclic straining. For a specific cyclic strain path, the steel has mainly cyclic softening behavior when the strain amplitude is small. While with an increase of the effective strain amplitude, the softening becomes small, but there is the cyclic softening even though the steel is subjected to the cyclic loading by a square strain path. However, the steel has cyclic additional hardening by a nonproportional path, compared with the proportional cycling. Generally, the additional hardening is small and its historical effect is not obvious at small strain amplitude. The additional hardening is remarkable by a cross-triangular strain path of large strain amplitude. The memory of the history of nonproportional cyclic loading, the direction of plastic flow and the plastic modulus of the steel were also studied.

Cyclic Hardening Behavior of 1Cr18Ni9Ti Stainless Steel

The cyclic loading tests for 1Cr18NigTi stainless steel under some non-proportional strain paths and their histories are performed. Experimental resultsshow that the material has nonpropertional additional harderung and the effectsof nonproportional cycling history at the constant temper lower than700℃, but at the hisher temperattire(≥700℃) the cyclic saturated behavior isnot influenced by the prior loading history.