Increasing Mean Strain Research Articles

Theoretical Prediction of Forming Limit Diagrams for B1500HS Steel at Elevated Temperature Based on Modified Arrhenius and M–K Models

Abstract Boron steel B1500HS deforms at elevated temperature under the complete austenite state, and it accords with the 0-angle necking type, irrespective of the strain state on the left- or right-hand side of the forming limit diagram (FLD), which is in accordance with the M–K simplified model. However, the constitutive equation significantly affects the prediction of the FLD of the sheet metal. In this study, the modified Arrhenius model, including the deformation activation energy and the deformation temperature, describing the hot deformation of the sample with austenitic microstructure combined with the Logan–Hosford yield criterion and the Von Mises yield criterion, was used to predict the forming limits of boron steel B1500HS. The theoretical calculation of the forming limits for B1500HS steel at elevated temperature and under different conditions was carried out, and the FLD predicted by the theoretical calculation was found to be in good agreement with the experimental results. It was concluded that with decrease in the initial inhomogeneity of the material, the hot forming limit curves for B1500HS steel at elevated temperature decreased, and the initial inhomogeneity coefficient f0 = 0.997 was better for the accurate prediction of the hot forming limits of boron steel. With increasing forming temperature, the forming limit curves calculated for B1500HS steel at elevated temperature increased. With increasing mean strain rate, the forming limit curves of B1500HS steel at elevated temperature increased slightly.

Effect of relative humidity on uniaxial cyclic softening/hardening and intrinsic heat generation of polyamide-6 polymer

Strain-controlled uniaxial cyclic tests on a polyamide-6 (PA6) polymer with various relative humidity (RH) levels were performed in a room environment. The effects of RH on the cyclic softening/hardening and intrinsic heat generation of the PA6 are discussed by using the experimental data obtained in the cyclic tests with various applied strain amplitudes, mean strains and strain rates. It is observed that the cyclic softening/hardening of PA6 depends greatly on the applied strain amplitude and strain rate, and the cyclic softening observed in the cyclic tests with a larger strain amplitude and at a higher strain rate is mainly caused by the remarkable temperature rise due to the intrinsic heat generation. The variation of relative humidity does not change the cyclic softening/hardening feature of PA6, but does influence the extent of cyclic softening/hardening; an apparent mean stress relaxation occurs in the asymmetrical strain-controlled cyclic tests, and its extent increases with increasing mean strain but decreases with increasing relative humidity.

Uniaxial cyclic deformation and internal heat production of ultra-high molecular weight polyethylene

The cyclic deformation and corresponding internal heat production of ultra-high molecular weight polyethylene (UHMWPE) polymer were investigated under the uniaxial strain-controlled and stress-controlled cyclic loading conditions. It is seen that the UHMWPE behaves basically a cyclic stabilizing feature since the responding stress amplitude does not remarkably change during the cyclic loading, except for that at high strain rate (where an obvious cyclic softening is caused partially by the thermal softening); an apparent mean stress relaxation occurs in the asymmetrical strain-controlled cyclic tests, and the degree of mean stress relaxation increases with the increasing mean strain; an obvious ratchetting takes place in the asymmetrical stress-controlled cyclic tests, and the ratchetting strain depends greatly upon the applied mean stress and stress amplitude, as well as the prescribed stress rate. Moreover, it is found that the temperature on the surface of specimen increases apparently in the uniaxial strain-controlled cyclic tests and the temperature variation becomes more remarkable when the prescribed strain rate is higher. However, the temperature variation is not so apparent in the uniaxial stress-controlled cyclic tests due to much smaller responding strain amplitude.

Crack-closure based method for modelling the fatigue behavior of superelastic NiTi

One remarkable trait reportedly exhibited by NiTi is the improvement of its fatigue performance with increasing mean strain; this is in stark contrast to typical engineering materials where a decrease in fatigue behavior is expected. Clarification into this phenomenon, however, still remains incomplete in literature. In this study, the computational methodology developed by Bruzzi et al. (2002) is adapted to predict the complex fatigue behavior of superelastic NiTi. This defect tolerant approach correlates local crack-tip driving force conditions of an initial small crack with an experimental long crack growth rate curve, using crack closure. Computationally derived predictions of fatigue life are attained for stable austenitic, stable martensitic and superelastic NiTi specimens for a multitude of loading conditions and initial defect sizes. Predicted stress-life curves (S–N curves) are compiled and converted to the more relevant strain-based constant life diagram. Further insight is therefore achieved into the individual contribution of the stable austenitic and stable martensitic phases on the unique fatigue behavior of superelastic NiTi. In this manner, this study offers a possible quantitative explanation toward the experimentally exhibited increased fatigue performance of superelastic NiTi.

Influence of Microstructure on the Performance of Nitinol: A Computational Analysis

One material that has found particular favor for use in biomedical endovascular stents is the near equi-atomic NiTi alloy, Nitinol. One remarkable trait exhibited by this superelastic material is the improvement of its fatigue performance with increasing mean strain (Ref 1, 2). Clarification into this phenomenon still remains incomplete in the literature. This study proposes a microstructural explanation for this unique macroscopic behavior; it is hypothesized stress-induced martensite (SIM) will stabilize with increasing strain which, in turn, leads to the observed increase in fatigue life. Finite element analysis (FEA) is employed to investigate the behavior of a “v-strut” stent subcomponent under various strain levels. The volume fraction of SIM is analyzed to identify its potential influence on macroscopic response and, ultimately, fatigue behavior. In addition, a computational investigation is performed on the effect of crystallographic texture on macroscopic response. Granular transformational behavior is analyzed using FEA models with realistic and idealized grain structures, specifically evaluating the effect of individual grain orientations on the stress-induced martensite transformation.

Microscopic analysis of the influence of ratcheting on the evolution of dislocation structures observed in AISI 316L stainless steel during low cycle fatigue

When subjected to controlled cyclic deformation, the response of austenitic stainless steel typically involves primary hardening followed by softening, and eventually cyclic stabilization with or without secondary hardening. If a continuously drifting mean strain is superposed to an alternating strain path (i.e. strain controlled ratcheting), the response in terms of mean stress and strain amplitude is significantly different. A series of low cycle fatigue and ratcheting experiments are performed at room temperature on round specimens extracted from a batch of AISI 316L hot rolled plate. The experiments are interrupted at cycle numbers selected to correspond with the different strain controlled cycle response stages. The as-received material and the fatigued specimens are analyzed by means of transmission electron microscopy to characterize the microstructure and its evolution with cyclic loading. The low cycle fatigue experiments, performed to establish a reference point for the zero mean strain loading condition, are in line with observations reported for AISI 316L stainless steel by other authors. The continuously increasing mean strain is found to induce higher dislocation densities in the channels of the evolving microstructure, being responsible for the macroscopically observed additional hardening. The observed polarized dislocation walls at least partially accommodate the continuously drifting mean strain and play a role in the non-zero mean stress response.

Fatigue behaviour of the superalloy IN 713C under LCF-, HCF- and superimposed LCF/HCF-loading

Low cycle fatigue (LCF), high cycle fatigue (HCF) and superimposed LCF/HCF tests were carried out on IN 713C at 600 °C in air environment under total strain control. Under HCF loading, the strain fatigue limit is reduced with increasing mean strain up to a mean strain from which no significantly higher mean stresses are induced. Superimposed HCF loadings decrease the lifetime compared to the pure LCF loading if the amplitude of the superimposed loading is high enough. Under pure LCF loading cracks initiated below the surface at casting defects. Under LCF/HCF loading, cracks initiated at casting defects or due to slip bands.

Fatigue and durability of Nitinol stents

Nitinol self-expanding stents are effective in treating peripheral artery disease, including the superficial femoral, carotid, and renal arteries. However, fracture occurrences of up to 50% have been reported in some stents after one year. These stent fractures are likely due to in vivo cyclic displacements. As such, the cyclic fatigue and durability properties of Nitinol-based endovascular stents are discussed in terms of an engineering-based experimental testing program. In this paper, the combined effects of cardiac pulsatile fatigue and stent-vessel oversizing are evaluated for application to both stents and stent subcomponents. In particular, displacement-controlled fatigue tests were performed on stent-like specimens processed from Nitinol microtubing. Fatigue data were collected with combinations of simulated oversizing conditions and pulsatile cycles that were identified by computer modeling of the stent that mimic in vivo deformation conditions. These data are analyzed with non-linear finite element computations and are illustrated with strain-life and strain-based constant-life diagrams. The utility of this approach is demonstrated in conjunction with 10 million cycle pulsatile fatigue tests of Cordis SMART Control ® Nitinol self-expanding stents to calculate fatigue safety factors and thereby predict in vivo fatigue resistance. These results demonstrate the non-linear constant fatigue-life response of Nitinol stents, whereby, contrary to conventional engineering materials, the fatigue life of Nitinol is observed to increase with increasing mean strain.

Martensite Transformations and Fatigue Behavior of Nitinol

Abstract In addition to excellent biocompatibility and corrosion resistance, the unique flow behavior of NiTi pseudo-elastic alloys renders them ideally suited for several medical device applications utilizing minimally invasive technologies. For example, self-expanding stents are indicated chronic therapies for treatment of a variety of vascular diseases. Long-term device integrity is thus critical with the FDA recommending a minimum 10-year life. Historically, the majority of published fatigue data on NiTi alloys have been generated under strain-controlled conditions using wire or single-diamond cell samples designed to replicate the behavior of the repetitive unit within a stent. Resulting data are then presented on modified Goodman or Soderberg diagrams in order to define regions of acceptable device life. Consistent with their unusual monotonic flow characteristics these alloys are found to exhibit unique cyclic behavior in fatigue: published data clearly demonstrate that increasing mean strain often increases high-cycle fatigue life. This anomalous behavior is generally attributed to any of a number of possible microstructural or mechanical effects associated with the reversible stress-assisted austenite-to-martensite phase transformation. Although the majority of published work has focused on these macroscopic properties in order to characterize and adequately define device life, various efforts are currently underway to better understand the more fundamental metallurgical and mechanical aspects controlling fatigue life in this alloy. Such studies include crack initiation and propagation rates, determining the roles of absolute and relative inherent flow behavior of the transforming structures, mechanical instabilities resulting from the heterogeneous nature of the transformation, latent heat effects during cyclic transformation, as well as the effects of testing frequency, crystallographic texture, grain-shape anisotropy, and stress-state on fatigue life. A review of existing literature delineating these behaviors and current efforts is presented in light of known beneficial effects of transformation plasticity and toughening in enhancing fatigue and fracture properties of a variety of metallic and nonmetallic materials.

Effect of mean strain on the cyclic deformation and stress relaxation in polypropylene

AbstractThe influence of mean strain Changes in the range from 0 to 4% on the cyclic deformation and stress‐relaxation properties is investigated using rod specimens of polypropylene. An extensometer measures and controls the axial strain in a closed‐loop, electrohydraulic, servocontrolled mechanical testing machine. The hysteresis loop at different numbers of cycles N of N = 35, 45, and 50 are irregular in shape, and the tensile portion decreases in size as the number of cycles was increased. The stress value of the tensile portion for three mean strains of ϵm = 1.0%, 2.0%, and 4.0% remains constant, but the value of the compressive portion decreases as the mean strain was increased. The stress level at a strain width of 3% changes little with mean strain. For the strain widths of 5% and 7%, however, the minimum stress levels at N = 35, 45, and 50 increase with increasing mean strain, in contrast to the behavior of maximum stress level. The stressrelaxation tests show that the drop of stress decreases with an increase in number of cycles. The discrepancy in the results of relaxation tests is due to the effect of the difference in strain rates.

An Investigation of Low Cycle Fatigue Life of Wide Plate with a Notch

In the previous paper, a pulsating tension test was carried out using wide plate specimens of mild steel with a central notch of stress concentration factor Kt to be 5 and 10. By simulating the cyclic behaviors of the strain at the tip of a notch to hour-glass shaped small specimens, the low-cycle fatigue crack initiation life of a plate specimen was compared with that of a small bar specimen. The results showed good correlation between the wide plate and the small bar specimen, but the life of the plate specimen with a notch of 5 in Kt was slightly longer than that of small bar specimen, while that for notch of 10 in Kt was slightly shorter. This fact shows that the crack initiation life depends on not only the cyclic behavior of strain but also the configulation of a notch.In this paper, a same test was carried out using wide plate specimens and small bar specimens of mild steel and 50 kg/mm2 high tensile steel, and the crack initiation life was investigated more precisely based on the present and previous results. It was revealed that dominant factors affecting the crack initiation at the tip of notch in the pulsating tension test are maximum strain at the first loading, strain amplitude, increasing mean strain and restraint of deformation at the tip of a notch. Since the influence of the restraint of deformation at the tip of notch can not be realized in the strain controlled test simulating the cyclic behavior of strain at the tip of a notch, the larger Kt of a notch is, the shorter the crack initiation life of the plate specimen is than that of simulating test on the small bar specimen. By comparing with the results of completely reversed strain-cycling test of small bar specimens, it was found that mean strain and its increase show supper posed effect on the crack initiation life when the life of the plate specimen is shorter than about 700 cycles. As for the effect of the increasing mean strain, an effect of the increasing mean strain was found to be appeared only when the increasing rate of mean strain is larger than about 0.1%/cycle, from the results of comparing test of linearly increasing mean strain with that of constant mean strain.In this paper, two cumulative damage rules in low-cycle fatigue are proposed taking into consideration of four factors affecting the crack initiation. One is based on the cyclic behavior of strain, and the other on the cyclic behavior of notch opening displacement. These two rules are able to estimate the fatigue life of the plate specimen within a small extent of deviation.

Initiation and Propagation of a Crack and Cyclic Behavior of Strain in Low-Cycle Fatigue

The cyclic behaviors of strain in mild steel plate specimens with a central notch were investigated in static tensile test and zero-to-tension cyclic loading test. In the cyclic test a cumulative damage law was applied to the crack initiation life by using the strain behavior at the tip of the notch, and satisfactory results were obtained. The strain controlled fatigue test simulating the strain behavior at the tip of notch was carried out by using small bar specimens of the same material, and a good correlation was found between the crack initiation life of the plate specimen and that of the bar specimen. By comparing the fatigue test of a linearly increasing mean strain and that of a constant mean strain, the crack initiation life in the increasing mean strain test was shorter than that in the constant mean strain test.The fatigue crack propagation in the plate specimen was observed under the condition of a constant, stepwise increased or stepwise decreased net section stress range. The crack propagation rate was correlated to the maximum strain at the tip of a crack or the stress intensity factor range. The characteristics of the crack propagation and the strain behavior after the stepwise decrease of net section stress range were very different from the other condition of net section stress range.