Anisotropic Hardening of Sheet Metals at Elevated Temperature: Tension-Compressions Test Development and Validation

New test equipment has been developed to measure the in-plane cyclic behavior of sheet metals at elevated temperatures. The tester has clamping dies with adjustable side force to prevent the sheet specimens from buckling during compressive loading. In addition to the room temperature experiment, cartridge type heaters are inserted in the clamping dies so that the specimen can be heated up to 400 °C during the cyclic tests. For the strain measurement, a non-contact type laser extensometer is used. In order to validate the newly developed test device, the tension-compression (and compression-tension) tests under pre-strains and various temperatures have been performed. As model materials, the aluminum alloy sheet which exhibits a large Bauschinger effect and the magnesium alloy sheet which exhibits different amounts of asymmetry under cyclic loading are used. The developed device can be well-suited to measure the cyclic material behavior, especially the anisotropic and asymmetric hardening of light-weight materials.     

Influence of Thermally Induced Chemorheological Changes on the Inflation of Spherical Elastomeric Membranes

When an elastomeric material is deformed and subjected to temperatures above some chemorheological value T _cr (near 100°C for natural rubber), its macromolecular structure undergoes time and temperature dependent chemical changes. The process continues until the temperature decreases below T _cr. Compared to the virgin material, the new material system has modified properties (often a reduced stiffness) and permanent set on removal of the applied load. A recently proposed constitutive theory is used to study the influence of chemorheological changes on the inflation of an initially isotropic spherical rubber membrane. The membrane is inflated while at a temperature below T _cr. We then look at the pressure response assuming the sphere's radius is held fixed while the temperature is increased above T _cr for a period of time and then returned to its original value. The inflation pressure during this process is expressed in terms of the temperature, representing entropic stiffening of the elastomer, and a time dependent property that represents the kinetics of the chemorheological change in the elastomer. When the membrane has been returned to its original temperature, it is shown to have a permanent set and a modified pressure-inflated radius relation. Their dependence on the initial inflated radius, material properties and kinetics of chemorheological change is studied when the underlying elastomeric networks are neo-Hookean or Mooney–Rivlin.     

Influence of thermally induced chemorheological changes on the torsion of elastomeric circular cylinders

When an elastomeric material is deformed and subjected to temperatures above some characteristic value T _cr (near 100^∘C for natural rubber), its macromolecular structure undergoes time and temperature-dependent chemical changes. The process continues until the temperature decreases below T _cr. Compared to the virgin material, the new material system has modified properties (reduced stiffness) and permanent set on removal of the applied load.A new constitutive theory is used to study the influence of the changes of macromolecular structure on the torsion of an initially homogenous elastomeric cylinder. The cylinder is held at its initial length and given a fixed twist while at a temperature below T _cr. The twist is then held fixed and the temperature of the outer radial surface is increased above T _cr for a period of time and then returned to its original value. Assuming radial heat conduction, each material element undergoes a different chemical change. After enough time has elapsed such that the temperature field is again uniform and at its initial value, the cylinder properties are now inhomogeneous. Expressions for the time variation of the twisting moment and axial force are determined, and related to assumptions about material properties. Assuming the elastomeric networks to act as Mooney-Rivlin materials, expressions are developed for the permanent twist on release of torque, residual stress, and the new torsional stiffness in terms of the kinetics of the chemical changes.     

The effect of microstructure on stress-induced martensitic transformation under cyclic loading in the SMA Nickel-Titanium

A combined experimental and analytical study to determine the configurations of transforming martensite during ambient temperature cyclic deformation of superelastic Nickel-Titanium has been conducted. Full-field, sub-grain-size microscale strain measurements were made in situ during cycling using distortion-corrected Digital Image Correlation combined with Scanning Electron Microscopy (SEM-DIC). Using grain orientation maps from Electron Backscatter Diffraction analysis, possible configurations of martensite formed during cyclic deformation were identified by matching the calculated and measured strain fields. This analysis showed that the inclusion of Correspondence Variants (CVs) in addition to Habit Plane Variants (HPVs) of transformed martensite was necessary to provide a robust fit between calculated and measured strain fields. The approach also provided evidence that there was a more rapid accumulation of residual strain in CV regions and that a correlation existed between residual strain accumulation and the loss of actively transforming martensite in later cycles. It was also found that regions of CVs could coexist with untransformed austenite and Habit Plane Variants (HPVs) in individual grains throughout the microstructure, and that these regions of CVs formed before the end of the macroscopic stress plateau. The CV structure that forms during the initial superelastic deformation of Nickel-Titanium plays a critical role in shaping and stabilizing subsequent martensite recovery during cyclic loading.     

Scission induced by circular shear and heat conduction in an elastomeric cylinder

Constitutive equations for the thermo-mechanics of elastomeric materials generally assume that they do not undergo microstructural change. A constitutive theory is discussed here which accounts for such changes arising from continuous scission of macromolecular junctions of elastomeric networks due to deformation and high temperatures and the subsequent cross-linking of molecules into new networks with new reference states. The total stress is the superposition of the stresses in the remainder of the original network and in each subsequently formed network. Each network acts as a temperature-dependent non-linear elastic material. The interaction of this material response with inhomogeneous deformation and temperature fields is studied for finite circular shear of a cylinder. Numerical results illustrate how the mechanical response of the cylinder depends on the temperature dependence of both the scission–cross-linking process and the properties of the elastic networks.     

Ratchetting process in the stainless steel AISI 316L at 300 K: an experimental investigation

The cyclic creep behaviour of austenitic stainless steel (316L) was studied at room temperature. Particular attention has been paid to the effect of peak stress (Σ_max) and mean stress (Σ_m) on the ratchet strain rate. Threshold stress (230 MPa) for the transition from retardation to acceleration in the cyclic creep has been observed and interpreted as a transition between planar slip to wavy slip. The effect of stacking fault energy on this threshold stress is also studied. The cyclic creep rate increases as a function of Σ_max. The influence of maximum stress has been considered as an effect of tensile plastic strain history. This has been previously characterised in terms of back stress and effective stress. In particular, three domains have been identified—R₀, R_I and R_II—where the cyclic mechanisms are different. The slip mode and long-range internal stress state is at the origin of the three cyclic creep domains. Moreover, the evolution of internal stress states during cyclic creep test has been studied.     

Experimental and numerical investigation of combined isotropic-kinematic hardening behavior of sheet metals

To prevent a sheet specimen from buckling subjected to a tension-compression cyclic loading, a new fixture has been developed to use with a regular tensile-compression machine. The novelty of this device lies in 4-block wedge design with pre-loaded springs. This design allows blocks to freely move in the vertical direction while providing the normal support to the entire length of the specimen during the tension-compression cycle. The entire test is easy to setup, which is another advantage of this design. In order to measure the strain accurately, the transmission type laser extensometer was utilized together with the implementation of double-side fins in the specimen. Experimental results of tension-compression tests are presented followed by a review of existing testing methods. In order to describe the accurate cyclic tension-compression behavior, the combined isotropic-kinematic hardening law based on the modified Chaboche model and the practical two-surface model based on Dafalias-Popov and Krieg models have been modified in this work, considering the permanent softening behavior during reverse loading and the non-symmetric behavior during reloading. Through tension-compression tests, the material characterization has been performed for three base materials, BH180, DP600 steels and AA6111-T4 sheets.     

Time dependent scission and cross-linking in an elastomeric cylinder undergoing circular shear and heat conduction

When an elastomeric material is at a sufficiently high temperature, there can be time-dependent scission of macromolecular network cross-links. The affected molecules can recoil and cross-link to form a new network in a new reference configuration. The material then consists of several molecular networks. This microstructural change affects the mechanical response and leads to permanent set. A constitutive equation is presented, based on the experimental work of Tobolsky (Properties and Structures of Polymers, Wiley, New York, 1960, pp. 223-265), which can account for the influence of this temperature-dependent microstructural change on the mechanical response. It is used to study an elastomeric cylinder undergoing circular shear and transient heat conduction.     

Multiaxial creep and cyclic plasticity in nickel-base superalloy C263

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ^′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ^′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.     

Experimental Evidence of Thermal Effects in Multiphase Ceramic Specimens Subjected to Cyclic Loading

The aim of this work is to observe and to analyze various phenomena that exist in a multiphase ceramic material subjected to cyclic compressive loads. An infrared camera is used for this purpose. The material under study is an andalusite-based low-cement castable, which exhibits a pre-existing diffused damage (microcracks and debonded interfaces) before mechanical testing. The temperature variation in the specimen during the tests is investigated both at a macroscopic scale and a mesoscopic scale. In the first case, the material compaction, the thermoelastic coupling and the temperature increase due to mechanical dissipation are clearly evidenced. In the second case, local temperature variations related to microcracks are observed. The technique used and the results obtained are described and discussed in the paper.     

304不锈钢室温和高温单轴循环塑性的实验研究

对304不锈钢进行了室温和高温单轴应变控制和应力控制下的系统循环试验。揭示和分析了循环应变幅值、平均应变及其历史和温度历史对材料应变循环特性的影响以及应力幅值、平均应力及其历史以及温度对循环棘轮行为的影响。也讨论了应变循环和应力循环间交互作用对材料循环塑性行为的影响。研究表明,无益单轴应变循环特性还是非对称单轴应力循环下的棘轮效应不仅取决于当前温度和加载状态,而且强烈依赖于其加载历史。研究得到了一些有助于304不锈钢室温和高温单轴循环行为本构描述的结果。     

Plastic ratcheting induced cracks in thin film structures

In the microelectronic and photonic industries, temperature cycling has long been used as a reliability test to qualify integrated materials structures of small feature sizes. The test is time consuming, and is a bottleneck for innovation. Tremendous needs exist to understand various failure modes in the integrated structures caused by cyclic temperatures. This paper presents a systematic study of a failure mechanism recently discovered by the authors. In a thin film structure comprising both ductile and brittle materials, the thermal expansion mismatch can cause the ductile material to plastically yield in every temperature cycle. Under certain circumstances, the plastic deformation ratchets, namely, accumulates in the same direction as the temperature cycles. The ratcheting deformation in the ductile material may build up stress in the brittle materials, leading to cracking. The paper introduces an analogy between ratcheting and viscous flow. An analytical model is developed, which explains the experimental observations, and allows one to design the structure to avert this failure mode. Design rules with increasing levels of sophistication are described. Concepts presented here are generic to related phenomena in thin film structures.     

饱和砂土的循环边界面本构模

饱和砂土在循环加卸载过程中,土体发生显著的组构变化和塑性变形的累积。试验现象分析表明,循环过程中,砂土的首次与再循环塑性模量既有区别又有联系。因此,论文引入考虑组构变化的剪胀内变量,并提出循环塑性模量的关系式,真实描述上述两种现象;进而基于已有本构模型的基础上,建立饱和砂土的循环边界面本构模型。最后,将饱和砂土的模型预测结果与三轴试验结果进行验证对比,得到较好地吻合,这表明该模型能够合理反映饱和砂土循环加卸载的变形行为。     

Nonlinear deformation of elastomeric foams

The nonlinear deformation of a porous foam-type elastomeric material is studied, both theoretically and experimentally. The elastomer is modeled by the neo-Hookean material. The one-dimensional compressive behavior of the foam is analysed by using certain kinematic assumptions. The stress required to compress the foam is predicted by the model in terms of the porosity of the foam and the single constant in the neo-Hookean stress-strain form. A particular silicone foam is used as a test of the theory. The neo-Hookean constant is evaluated from a test of the homogeneous elastomer. Hence the behavior of the corresponding foam is predicted theoretically and compared with experimental results. The general results are applicable to closed-cells foams of intermediate density.     

Using the scanning infrared camera in experimental fatigue studies

Materials under cyclic loading dissipate energy in the form of heat due to hysteresis effects in the material. At locations of high stress levels, more heat is released than elsewhere, resulting in a local temperature rise in those areas. The scanning infrared camera has been used in this study to visualize the surface-temperature field on steel and fiberglass-epoxy composite samples during fatigue tests. The information achieved in this manner allows one to predict the probable location of the greatest fatigue damage well before such damage becomes visible in the form of a crack. The use of the scanning infrared camera for monitoring traveling cracks and mapping the temperature fields resulting from stress concentrations in cyclically loaded materials is also demonstrated. The results indicate that this instrument is of value in both nondestructive testing and crack-propagation studies.     

Development of a Novel Experimental Device to Investigate Swelling of Elastomers in Biodiesel Undergoing Multiaxial Large Deformation

The lifetime of an elastomeric product depends on the nature of mechanical loading and the environmental condition during the service. In this context, at least two important aspects contribute to the degradation of the elastomeric parts in service: diffusion of aggressive liquids leading to swelling and fluctuating multiaxial mechanical loading leading to fatigue failure. Moreover, the amount of swelling of elastomers in solvent is affected by the presence of mechanical loading. Hence, it is essential to understand the interactions between the two phenomena for durability analysis of the component. The present study investigates the swelling of elastomers due to diffusion of palm biodiesel in the presence of static multiaxial large deformation. For this purpose, new experimental device and specimen are developed. The device consists of a hollow diabolo elastomeric specimen attached to specially-designed circular metallic grips and plates such that immersion tests can be conducted while the specimens are simultaneously subjected to various mechanical loadings: simple tension, simple torsion and combined tension-torsion. Thus, diffusion of liquids takes place in the material which concurrently undergoes multiaxial large deformation. Two types of elastomers are investigated: Nitrile Rubber (NBR) and Polychloroprene Rubber (CR). The particular features of the device and specimen are discussed and perspectives for further improvement are drawn.     

疲劳过程中生热机理的实验探讨

传统的疲劳试验方法确定材料的疲劳极限时试验周期长、需要试件多,故高试验成本成为疲劳试验中一个难以解决的问题.文中利用具有准确、快速、便捷、低成本等优点的热像法测定了多种载荷工况下Q235钢的疲劳极限,并对不同的黏或/和塑性效应主导的生热机制进行了探讨.材料疲劳过程中,疲劳极限之下的载荷引起的温度波动来源于热弹性效应,温升来源于材料的非弹、塑性效应(如黏性效应);而疲劳极限之上的载荷引起塑性功累计,导致疲劳损伤产生,使得温升机制出现转折.通过对试验数据的分析,求出了材料的黏性系数,给出了利用塑性能耗的起点确定材料疲劳极限的方法.     

Effect of γ − α′ phase transformation on plastic adaptation to cyclic loads at cryogenic temperatures

Metastable, type FCC metals and alloys are often applied at extremely low temperatures because of their excellent ductility over the whole temperature range practically down to the absolute zero. These materials (like stainless steels) are frequently characterised by the low stacking fault energy and undergo at low temperatures the plastic strain induced transformation from the parent phase “γ” to the secondary phase “α′”. The phase transformation process consists in creation of two-phase continuum, where the parent phase coexists with the inclusions of secondary phase in thermodynamic equilibrium. The evolution of material micro-structure induces strain hardening related to interaction of dislocations with the inclusions and to increase of equivalent tangent stiffness as a result of evolving proportions of both phases, each characterised by different stiffness. The corresponding hardening model is based on micromechanics and on the Hill concept (1965) supplemented by Mori and Tanaka (1973) homogenisation scheme. Identification of parameters of the constitutive model has been carried out for 304L and 316L stainless steels, based on the available experimental data. The model has been used to describe phase transformation in rectangular beams, circular rods and thin-walled shells subjected to cyclic loads at cryogenic temperatures. Moreover, non-proportional loading paths were studied. A new feature of structures made of metastable materials has been observed. As soon as the γ ? α′ phase transformation begins, the evolution of material micro-structure accelerates the process of adaptation of structural member to cyclic loads and enhances therefore its fatigue life when compared to classical elastic–plastic structures.