Sam-Son Yoon

The effect of temperature on low-cycle fatigue behavior of prior cold worked 316L stainless steel

Low-cycle fatigue tests on cold worked 316L stainless steel were carried out at various temperatures from room temperature to 650 °C and tensile tests were conducted on the cold worked and solution-treated materials. At all test temperatures, the cold worked material showed the tendency of higher strength and lower ductility. Following initial cyclic hardening for a few cycles, cyclic softening behavior was observed to dominate until failure occurred during low-cycle fatigue deformation. The softening behavior strongly depends on temperature and strain amplitude. Several life prediction models were examined and it was found that it is important to select a proper life prediction parameter since stress and strain depend strongly on temperature. A phenomenological fatigue life prediction model is proposed to account for the influence of temperature on life. The model is correlated with the experimental results.

The Influence of Temperature on Low Cycle Fatigue Behavior of Prior Cold Worked 316L Stainless Steel (I) - Monotonic and Cyclic Behavior -

Tensile and low cycle fatigue (LCF) tests on prior cold worked 316L stainless steel were carried out at various temperatures from room temperature to 650. At all test temperatures, cold worked material showed the tendency of higher strength and lower ductility compared with those of solution treated material. The embrittlement of material occurred in the temperature region from 300 to 600 due to dynamic strain aging. Following initial cyclic hardening for a few cycles, cycling softening was observed to dominate until failure occurred during LCF deformation, and the cyclic softening behavior strongly depended on temperature and strain amplitude. Non-Masing behavior was observed at all test temperatures and hysteresis energy curve method was employed to describe the stress-strain hysteresis loops at halflife. The prediction shows a good agreement with the experimental results.

Low Cycle Fatigue Behavior of 429EM Stainless Steel at Elevated Temperature

Ferritic stainless steel is recently used in high temperature structures because of its good properties of thermal fatigue resistance, corrosion resistance, and low price. Tensile and low-cycle fatigue (LCF) tests on 429EM stainless steel used in exhaust manifold were performed at several temperatures from room temperature to 80. Elastic Modulus, yield strength, and ultimate tensile strength monotonically decreased when temperature increased. Cyclic hardening occurred considerably during the most part of the fatigue life. Dynamic strain aging was observed in 200~50, which affects the cyclic hardening behavior. Among the fatigue parameters such as plastic strain amplitude, stress amplitude, and plastic strain energy density (PSED), PSED was a proper fatigue parameter since it maintained at a constant value during LCF deformation even though cyclic hardening occurs considerably. A phenomenological life prediction model using PSED was proposed considering the influence of temperature on fatigue life.

Description of Hysteresis Loops using Modified Overlay Model

Overlay model had several advantages to describe hysteretic behavior of material and showed good capability for many engineering materials. However, this model is only applicable to material obeying Masing postulate. Some materials such as 316L stainless steel do not follow Masing postulate and show cyclic hardening(or softening) and strain range dependence. Low cycle fatigue tests of 316L stainless steel at 600 were performed to investigate the characteristics of cyclic behavior of non-Masing material. From all tests cyclic softening was observed. There were differences in elastic limit of hysteresis loop according to applied strain range. To consider these features, modified overlay model was developed. Yield stresses of subelements were divided into isotropic and anisotropic part to describe the non-Masing behavior. The plastic strain range memorization was introduced to consider the strain range dependence. The prediction using modified overlay model showed a good accordance to actual hysteresis loops.

A Semi-Implicit Integration for Rate-Dependent Plasticity with Nonlinear Kinematic Hardening

The prediction of the inelastic behavior of the structure is an essential part of reliability assessment procedure, because most of the failures are induced by the inelastic deformation, such as creep and plastic deformation. During decades, there has been much progress in understanding of the inelastic behavior of the materials and a lot of inelastic constitutive equations have been developed. The complexity of these constitutive equations generally requires a stable and accurate numerical method. The radial return mapping is one of the most robust integration scheme currently used. Nonlinear kinematic hardening model of Armstrong-Fredrick type has recovery term and the direction of kinematic hardening increment is not parallel to that of plastic strain increment. In this case, The conventional radial return mapping method cannot be applied directly. In this investigation, we expanded the radial return mapping method to consider the nonlinear kinematic hardening model and implemented this integration scheme into ABAQUS by means of UMAT subroutine. The solution of the non-linear system of algebraic equations arising from time discretization with the generalized midpoint rule is determined using Newton method and bisection method. Using dynamic yield condition derived from linearization of flow rule, the integration scheme for elastoplastic and viscoplastic constitutive model was unified. Several numerical examples are considered to demonstrate the efficiency and applicability of the present method.

The Influence of Temperature on Low Cycle Fatigue Behavior of Prior Cold Worked 316L Stainless Steel (II) - Life Prediction and Failure Mechanism -

Tensile and low cycle fatigue tests on prior cold worked 3l6L stainless steel were carried out at various temperatures ftom room temperature to 650. Fatigue resistance was decreased with increasing temperature and decreasing strain rate. Cyclic plastic deformation, creep, oxidation and interactions with each other are thought to be responsible for the reduction in fatigue resistance. Currently favored life prediction models were examined and it was found that it is important to select a proper life prediction parameter since stress-strain relation strongly depends on temperature. A phenomenological life prediction model was proposed to account for the influence of temperature on fatigue life and assessed by comparing with experimental result. LCF failure mechanism was investigated by observing fracture surfaces of LCF failed specimens with SEM.