Papers by Keyword: Cyclic Hardening

Abstract: The aim of this investigation is to study the influence of temperature on the cyclic plastic deformation behavior of SA333 Gr-6 steel at two loading conditions. Strain-controlled cyclic loading experiments were carried out at ± 0.5% total strain amplitude, 1×10^-3 s^-1 strain rate, and temperature varied from RT to 400°C, whereas stress controlled ratcheting experiments were conducted at fixed mean stress (σ_m) of 50 MPa and stress amplitude (σ_a) of 400 MPa, 115 MPa s^-1 stress rate, and in the temperature range of RT to 350°C. The investigated steel shows cyclic hardening characteristic at DSA temperature regime in both the loading condition. The steel shows lower fatigue lives at 250°C and 300°C temperatures even though plastic strain amplitude is smaller. The ratcheting life of the steel increases and strain accumulation decreases with the increase in temperature up to 300°C and on further increment in temperature ratcheting life get decreased. The steel shows greater cyclic hardening at both the loading conditions at 300°C.

Abstract: Thermo-mechanical cyclic experiments on 304 stainless steel were performed at several temperature ranges which had maximum temperatures ranging from 350°C to 1000°C and a minimum temperature of 150 °C. Related isothermal cyclic experiments were also performed. Temperature-history dependent cyclic hardening significantly occurred under thermo-mechanical cyclic loading with maximum temperatures around 600°C, whereas almost no cyclic hardening was observed when the maximum temperature was 1000°C. The observed thermo-mechanical cyclic plastic behavior in the saturated state of cyclic hardening was then simulated using a cyclic viscoplastic constitutive model, leading to the following findings. It was difficult to predict the saturated thermo-mechanical cyclic behavior using only the isothermal cyclic experimental data. The saturated thermo-mechanical cyclic behavior was simulated well by introducing a cyclic hardening parameter depending on the maximum temperature. This means that the cyclic hardening parameter should not change with temperature but depend on the maximum temperature in the saturated state of cyclic hardening under thermo-mechanical cyclic loading.

Abstract: The effects of Al on the monotonic deformation behavior of Fe-Mn-C twinning-induced plasticity (TWIP) steels have been extensively investigated; however, how the addition of Al affects the fatigue properties of these steels is unknown. The present paper deals with the cyclic deformation properties of Fe-22Mn-0.6C-0Al and Fe-22Mn-0.6C-3Al steels by means of total strain-controlled low-cycle fatigue tests. The total strain amplitude ranges from 0.002 to 0.01. The evolved microstructures of fatigued samples were observed by transmission electron microscopy. Results show that the addition of Al has a significant effect on the cyclic deformation behavior, fatigue lifetime and evolved microstructure of Fe-Mn-C TWIP steel.

Abstract: This study deals with the effect of the loading history on the cyclic behavior and the fatigue life of a 304L stainless steel at room temperature. The experiments have been performed using two specimens’ categories. The first one (virgin) has been submitted to only classical fatigue tests while in the second category, prior to the fatigue test; the specimen was subjected to a pre-hardening process under either monotonic or cyclic strain control. Cyclic softening followed by cyclic hardening are observed for the virgin specimens while only cyclic softening is exhibited by the pre-hardened specimens. The obtained results show that fatigue life is strongly influenced by the pre-hardening: the latter seems beneficial under stress control but detrimental under strain control, even in the presence of a compressive mean stress. The results are discussed regarding the cyclic evolution of the elastic modulus as well as the isotropic and kinematic parts of the strain hardening in different configurations: with or without pre-hardening, stress or strain control.

Abstract: Low-pressure steam turbine blades undergo VHCF-loadings induced by inhomogenous flow behind the vanes resulting in excitation frequencies of ≈ 2 kHz for rotational speeds of 50 Hz and a typical number of stator vanes of ≈ 60. The VHCF loading is superimposed by considerable mean stresses caused by centrifugal forces. In the present study, the VHCF-behavior of the ferritic-martensitic turbine blade steel X10CrNiMoV12-2-2 is investigated using an ultrasonic fatigue testing system up to cycle numbers of 5∙10⁹ at stress ratios from R = -1 up to 0.7, i.e. up to very high mean stresses. Generally, crack initiation changes from the surface to internal inclusions at fatigue lives around 4∙10⁷. The transition between fatigue failure and run-outs is shifted to higher lifetime with increasing R, and fine grained areas (FGAs) at the crack initiation sites only occur at R < -0.1. However, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for all load ratios over 4 decades of lifetime. At R up from 0.5 considerable cyclic creep occurs, even for lifetimes above 10⁸ cycles, resulting in cyclic hardening which was proved by microhardness measurements at longitudinal sections. This effect at least partially explains the high maximum stresses close to the tensile strength of the material occurring in the VHCF regime at load ratios ≥ 0.5.

Abstract: Engineering components are often subjected to cyclic load excursions beyond elastic limit and hence cyclic plastic deformation of engineering materials becomes inevitable. Since the resultant elastic-plastic stress-strain response of the material plays a pivotal role in analysis, design and failure of the component, it becomes important to understand the cyclic plastic deformation behaviour of engineering materials. Also, cyclic hardening parameters are required in the design of structural components subjected to large plastic deformation. Constitutive equations were proposed by Prager, Armstrong and Frederick, Chaboche, and Ohno-Wang based on the stabilized strain-controlled hysteresis curve to evaluate the hardening parameters. In the present study, cyclic hardening parameters for SA 312 Type 304LN stainless steel have been determined based on the results of constant amplitude strain-controlled fatigue tests carried out earlier at CSIR-SERC under five different strain amplitude values, viz, 0.20%, 0.35%, 0.65%, 0.80% and 0.95%. It is observed that in isotropic hardening, the values of Q decreased with increase in strain amplitude. In kinematic hardening, the values of C₁ and γ₁ are constant for all values of strain amplitude.

Abstract: Life of an aero-engine is limited by the life of the turbine blade in particular and that of hot end components in general. Design of aero-engine is always conservative in nature considering the flight safety as paramount important. Earlier engines have been assigned life in hours but the life of the component is limited by LCF cycles particularly during the start-stop cycles. In this paper LCF behaviour of a typical Russian origin nickel base wrought super alloy AP220BD used for turbine blade has been studied at room temperature (RT), 400 °C and 700 °C that corresponds to idle rating and cruise rating of a typical aero-engine. Low cycle fatigue (LCF) tests have been carried out at RT, 400°C and 700 °C at three strain amplitudes of ±0.3%, ±0.5% and ±0.8%. Hysteresis loop have been developed at each strain and temperature. It has been observed that LCF life of the nickel base wrought alloy AP220BD is not influenced significantly at strain amplitude of ±0.3% till it reaches 400° C. Reduction in LCF life with increase in strain amplitude from ±0.3% to ±0.8% is much significant compared to that of increase in temperature up to 700°c.The higher life at intermediate strain of ±0.5% may be due to DSA(dynamic strain aging) of the material. Transgranular fracture has been observed at RT & 400° C while intergranular fracture at 700° C.

Abstract: The strain-controlled fatigue tests on extruded AZ31B magnesium alloy were conducted under the uniaxial loading with strain ratio Rε=-∞, frequency of 0.1 Hz and strain amplitude of 2% at room temperature. The cyclic hardening behavior was investigated. It was found that, during the low cycle fatigue (LCF) process, as the number of cycles increases, the stress amplitude increases corresponding to the decrease of the plastic strain amplitude. The development of dislocation density can be described as the function of the number of fatigue cycles, and the behavior can be explained well based on the dislocation density development model.

Abstract: Based on a simplified equivalent 2-D plane strain finite element model, the bending fretting fatigue process of 316L stainless steel is simulated numerically by ABAQUS code. In this simulation, the effect of ratchetting on the fretting fatigue process is discussed by implementing an advanced cyclic elasto-plastic constitutive model for cyclic hardening materials into ABAQUS code as a user material subroutine (UMAT). From the numerical simulation, the effect of bending loads on the bending fretting fatigue of 316L stainless steel is addressed, and then the failure lives are predicted by using Smith-Watson-Topper critical plane criteria. Comparison with the corresponding experiments shows that the predicted results are in good agreement with the experimental ones.

Abstract: There are situations where taking normative specimens is impossible due to the dimensions of the objects investigated (e.g. extruded sections) and one of the solutions is to use mini specimens. As for non-standard specimen testing, it is essential to define the effect of size on fatigue strength. The research methodology facilitates the determination of fatigue characteristics (S-N) for EN AW-6063 aluminum alloy. The material is used to manufacture the extruded section in the process of extrusion of the material through the extruding die. The methodology assumes the geometry of the mini specimen and the normative specimen. As for the material strength identification, a static tensile test for the specimens made directly from finished elements and preliminarily strained in cycles was carried out. As a result of the cyclic material reinforcement, an increase in yield strength Re was observed, which, in turn, rejects Re as the upper criterion of the high-cycle fatigue range. The essential fatigue tests were performed based on unilateral cyclic tension (R = 0.1). The effect of size on fatigue strength was defined. Theoretically aluminum alloy non-sensitive to changes in the size of the cross-section showed a different strength in mini and normative specimens.