Papers by Keyword: Pre-Strain

Abstract: For high-accuracy finite element (FE) simulation of automobile crashing behavior, a work hardening curve that involves pre-strain from press forming is required. Here, the plastic strains exceeding the uniform deformation region are generally introduced through processes such as bending, but such large pre-strain effect have not been reported. Therefore, in this study, for DP590 steel, the work hardening curve for second-stage tension under pre-strain exceeding the uniform deformation region was identified. This identification was enabled by the diameter measurement tensile test developed by the authors. As a result, in the second-stage tension in the same direction as the first-stage tension, the initial yield stress showed a tendency to overshoot relative to the original work hardening curve, revealing that strain aging occurred. The overshoot portion formed a stress plateau that continued up to an equivalent plastic strain of 0.18. Such a tendency has not been observed in DP590 steel, making this a phenomenon revealed for the first time. When the tensile direction in the second stage was orthogonal to the first stage, the cross-hardening effect (reduction in initial yielding due to the Bauschinger effect and overshoot from the original work hardening curve) was observed. The stress plateau region due to overshoot continued up to an equivalent plastic strain as large as 0.6. These large plateaus concluded that work hardening presents perfect plasticity at large deformed press parts.

Abstract: This study discusses the performance and durability of manganese steel wire ropes under different loading conditions and environmental exposures. Steel wire ropes are critical elements in several industries that require high tensile strength and elasticity. It is driven by the fact that these ropes fail prematurely within two years of service despite them being designed for five years during the process of fabrication. The nature and cause of these premature failures can be attributed to residual stresses, which increase the wear, fatigue, and corrosion. These tests included tensile testing at various strain rates in both the aged and unaged states, and fatigue testing performed under a 0.7 strain rate in a purely non-pre-strained state, and pre-strained under the three conditions of dry pre- strain, corrosion pre-strain, and aging pre-strain. An SEM analysis was performed to determine the failure mechanism. An increase in the strain rate reduced the lifespan but increased the yield and ultimate tensile strength; the 0.7 strain rate represented the highest energy density compared to the strain rates of 0, 0.43, 1.0 and 1.35 strain rate. This study highlights the critical role of residual stress in steel wire ropes in terms of their performance and lifetime. The residual stress increased with the strain rate. Corrosive conditions showed a drastically reduced fatigue life, and the non-pre-strain condition had the longest cycles. Understanding the mechanical effects of steel wire ropes and optimizing the testing conditions will increase the durability and reliability of steel wire ropes with reduced maintenance costs and increased safety in industrial applications.

Abstract: The aim of this work is to evaluate the characteristics of continuous precipitates (CP) developed within the grain and grain boundary precipitates through statistical analysis of the number density and size (i.e., length and width) at varying aging conditions of AZ80 Mg alloy. Scanning electron microscopy illustrates the characteristics and features of precipitates, distinctively. The results reveal an increment of number density, whereas the reduction in the size of precipitates with decrease in the aging temperature for the varying aging times. The variation in hardness values at different aging conditions has been ascribed to this.

Abstract: Effects of pre-strain and tempering on mechanical properties in high-strength martensitic steels were investigated. In this study, strain tempering (ST) and quenching and tempering (QT) martensitic steels were prepared, and their mechanical properties were studied. In the tensile tests at the deformation temperatures between 296 and 573 K, the ST sample increased both of tensile strength (TS) and uniform elongation (U.El) from 473 to 523 K whereas the QT sample increased U.El with little change of TS. From the in situ neutron diffraction experiments, stress partitioning to the bcc phase increased with an increase in the deformation temperature from 296 to 523 K. The difference of phase stress between the bcc and cementite phases decreased with increasing the temperature because of a decrease in the cementite strength. In the ST sample, Pre-straining of 0.5% increased YS at 296 K with slight work hardening. The initial value of dislocation density (ρ) decreased at 523 K but ρ increased significantly after yielding, leading to better combination of TS and U.El. The combinations of pre-strain, tempering, and deformation temperatures have changed ρ before deformation and the increase of ρ after yielding of the martensitic steels.

Abstract: Pre-strain (PS) and pre-aging (PA) treatments are often applied during the preparation of Al-Mg-Si automotive aluminum alloy. In this study, the effect of combined PS and PA on the precipitation behavior and age hardening response for Al-Mg-Si alloys was investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile test, Vickers hardness test, and differential scanning calorimetry (DSC). It was found that the dislocations introduced by PS treatment and the cluster (2) formed during PA treatment effectively inhibited the cluster (1), which further strengthened the inhibition of natural aging hardening at room temperature (RT). The strength increment of the alloys was kept below 10.0 MPa during two weeks RT storage. The dislocations provided heterogeneous nucleation for the precipitates forming and the cluster (2) transformed into β″ strengthening phase during bake hardening treatment. With the acceleration response of the dislocations and the cluster (2), the age hardening response of Al-Mg-Si alloys obviously improved with the denser and larger β″ strengthening phase formed.

Abstract: Creep does not only appear at high temperature, but also appears at low temperature for 316L stainless steel that threatens the safety of equipment. In this work, the creep behavior of as-received and pre-strained 316L stainless steel at 373K was investigated by uniaxial creep (UC) tests and small punch creep (SPC) tests. The parameters of power-law creep model were determined from stress dependence of UC tests. Then, the creep behavior of SPC test was analyzed by finite element (FE) simulation combined with power-law creep model. Comparing with experimental creep deflection, the results of FE simulation can reasonably reflect the creep behavior of as-received and pre-strained small punch specimens. Based on the comparison of as-received specimen and pre-strained specimen from UC test, SPC test and FE simulation, pre-strain significantly restrains creep behavior of 316L austenitic steel at 373K.

Abstract: Bending and hemming process are used in automotive industries for assembling the car body panel.The main failure mechanism under bending loads is the intercrystalline fracture. This is due to the fact that the Forming Limit Curve (FLC) describes first occurrence of membrane instability and no material failure in consequence of an intercrystalline fracture at bending.The FLC fails to predict the formability in hemming processes since difference in failure mechanism. A new failure criterion, the so-called Bending Limit Curve (BLC) has been developed. In this work, the left hand side BLCs are experimentally determined for Advanced High Strength Steel grade DP1000, Stainless Steel grade SUS430 and Deep Drawing Steel grade SPCC having a thickness of 1.0 mm. The influence of various bending radii and level of pre-strain on the bending strains are investigated and discussed by using the Three Point Bending Test. Bendability of investigated materials are evaluated by using optical strain measurement system GOM-Aramis to determine maximal achievable bending strain on the specimens. The developed left hand side BLCs were found to be higher level than conventional FLCs. The bigger bending radius established lower bending limit strain. The higher bending strain was obtained from the higher pre strain level.

Abstract: Uniaxial ratcheting behaviors of Z2CN18.10 austenitic stainless steel under both tensile pre-strain (TP) and compressive pre-strain (CP) were experimentally studied at room temperature. The experimental results show that: TP restrains ratcheting strain accumulation of subsequent cycling with positive mean stress; lower level of CP is found to accelerate ratcheting strain accumulation while higher level of CP retards the accumulation. Based on the Ohno-Wang II kinematic hardening rule, rate-independent model, viscoplastic model, isotropic hardening model and a modified model were constructed to describe the ratcheting behaviors under various pre-strain conditions. All the four models gave fairly good prediction on ratcheting strains for various TP. The isotropic hardening model and modified model predicted acceptable ratcheting strain though still showed slight tendency of over prediction.

Abstract: Several different DP590 high-strength steel sheet cruciform specimens were tested by biaxial tensile experiment system. According to the experimental results, an appropriate cruciform specimen could be used for biaxial tensile pre-strain was selected to be optimized. The effects of the number and location of slits were discussed through ANSYS software numerical analysis. The biaxial tensile properties of DP590 high-strength steel sheet were obtained by the biaxial tensile test system. The results show that the uniform deformation area of the cruciform specimen center is about 80%, and the maximum biaxial tensile pre-strain is nearly 3.14%.

Abstract: Stiffened sheet metal structure where sheet metal is reinforced by frame has been claimed as the most effective structure because it has low volume and weight. It is generally applied to large car body structure such as bus and train body. Frame and sheet are commonly joined by welding process. Due to the local heating of welding, distortion or deformation will occur in this structure. To mitigate this distortion, new method called stretching technology was proposed in this work. In this method, sheet was stretched to certain pre-strain, kept in this condition and then welded to frame. Special equipment powered by hidroulic system was designed to support this method. Low carbon steel SPAC specimens with dimension of 400mm, 1824mm and 3mm in width, length and thick respectively were prepared to evaluate the method. Hidroulic power was controlled to meet the sheet pre-strain variations of 0.00%, 0.05%, 0.10% and 0.15%. The distortion of the specimen was measured by dial indicator with mesh point of 50mm and shown in 2D contour chart. The study results revealed that the welding process on the sheet without pre-strain had the highest distortion of 8.34mm while that with pre-strain of 0.05% provided the lowest distortion of 3,3mm or 60% lower than without pre-strain specimen. The pre-strain of 0.10% and 0.15% produced the sheet distortion of 7.05mm and 7.9mm respectively. The excessive pre-strain was an ineffective method to mitigate the welding distortion because the reverse tension force of sheet would destroy the weld joint when the hydraulic force was released.