Fracture Characterization of Steels by Means of the Small Punch Test

A hot rolled API X-70 steel plate and its heat-affected zone (the region with the maximum hardness and lowest toughness of the welded joint made using this steel) were employed to obtain the material’s room temperature elasto-plastic fracture toughness, J_Ic, by means of small punch tests (SPTs) using both conventional un-notched samples and longitudinally-notched SPT specimens. In the latter case, the notches were manufactured by micromachining different notch depth-to-thickness ratios (a/t?=?0.3 and 0.4). The representative toughness parameter used with the conventional SPT tests was the maximum strain measured directly in the failed region, while in the case of the notched samples, the consumed energy until the initiation of a crack from the tip of the notch was considered the most useful parameter of choice. The onset of crack initiation was determined directly from the load-displacement plot of each test with the aid of scanning electron microscope observations performed on different samples over which interrupted tests had been conducted. These tests were interrupted at different percentages of the maximum registered load. A simple correlation between the energy consumed until the initiation of crack growth in the notched SPT sample and the critical J value obtained using standard tests (J-R curves) was determined, defining an easy and promising way to derive fracture toughness from miniature SPT tests.     

Understanding the need of the compression branch to characterize hyperelastic materials

Soft biological tissues are frequently modeled as hyperelastic materials. Hyperelastic behavior is typically ensured by the assumption of a stored energy function with a pre-determined shape. This function depends on some material parameters which are obtained through an optimization algorithm in order to fit experimental data from different tests. For example, when obtaining the material parameters of isotropic, incompressible models, only the extension part of a uniaxial test is frequently taken into consideration. In contrast, spline-based models do not require material parameters to exactly fit the experimental data, but need the compression branch of the curve. This is not a disadvantage because as we explain herein, to properly characterize hyperelastic materials, the compression branch of the uniaxial tests (or valid alternative tests) is also needed, in general. Then, unless we know beforehand the tendency of the compression branch, a material model should not be characterized only with tensile tests. For simplicity, here we address isotropic, incompressible materials which use the Valanis-Landel decomposition. However, the concepts are also applicable to compressible isotropic materials and are specially relevant to compressible and incompressible anisotropic materials, because in biomechanics, materials are frequently characterized only by tensile tests.     

On the determination of material parameters for internal variable thermoelastic–viscoplastic constitutive models

The work presented in this paper deals with the determination of material parameters used in internal variable constitutive models. In order to determine the best suited material parameter set, in the less computationally expensive way, two optimization approaches are used: (i) a gradient-based method and (ii) a continuous evolutionary algorithm (EA) method. The first approach uses a combination of the steepest descent gradient and the Levenberg–Marquardt techniques. The performance of this method is known to be highly dependent on the starting set of parameters and its results are often inconsistent. The EA-based technique provides a better way to determine an optimized set of parameters (the overall minimum). Thus, the difficulty of choosing a starting set of parameters for this process is minor. The main application in this work is a 16 parameter thermoelastic–viscoplastic constitutive model. Experimental data was obtained from tensile and shear tests at different temperatures and used to compare with numerical results and to determine the correct set of material parameters. Numerical constraints were introduced to enforce physical requirements on the material parameters. Both methods are used to determine the 12 material parameters needed for an AA1050-O aluminium alloy. Although the EA-based method achieved a slightly better result, it proved to be computationally more expensive than the gradient-based method.     

Numerical simulation of Taylor impact tests

The present paper discusses some aspects in the numerical simulation of Taylor impact tests. A phenomenological internal variable theory is presented and restricted by the second law of thermodynamics. The constitutive model consists of a thermo-hyperelastic equation for the stresses and an evolution equation for the plastic internal variable. Specific thermo-plastic constitutive functions are proposed and corresponding material parameters are identified. Taylor impact tests are numerically simulated using the explicit finite element program LS-DYNA augmented by an user-defined material subroutine and the effect of variation of selected model parameters is discussed. Numerical results allow new interpretations of experimental observations and test data and gives advice on identification of material parameters in rate-dependent inelastic constitutive models.     

Nonlinear constitutive models from nanoindentation tests using artificial neural networks

This paper presents a new approach using Artificial Neural Networks (ANNs) models to simulate the response during nanohardness tests of a variety of materials with nonlinear behavior. The ANNs continuous input and output variables usually include material parameters, indentation deflection, and resisting force. Different ANN models, including dimensionless input/output variables, are generated and trained with discrete finite-element (FE) simulations with different geometries and nonlinear material parameters. Only the monotonic loading part of the load–displacement indentation response is used to generate the trained ANN models. This is a departure from classical indentation simulations or tests where typically the unloading portion is used to determine the stiffness and hardness. The experimental part of this study includes nanoindentation tests performed on a silicon (Si) substrate with and without a nanocrystalline copper (Cu) film. The new ANN models are used to back-calculate (inverse problem) the in situ nonlinear material parameters for different copper material systems. The results are compared with available data in the literature. The proposed FE–ANN modeling approach is very effective and can be used in calibrating and predicting the in situ inelastic material properties using the monotonic part of the indentation response and for depths above 50 nm where the overall resisting force represents a continuum response.     

Identification of material parameters for inelastic constitutive models: Stochastic simulations for the analysis of deviations

For parameter identification a distance function between the measured and the simulated data has to be minimized. Therefore, the influence of three different norms used in the definition of such a distance function is investigated. The nonlinear optimization problem is solved using a modified random search algorithm originally proposed by Price (1978). Next a stochastic model for the generation of artificial test data is presented. This model is used for a stochastic simulation of test data (constant strain rate tension with relaxation and creep). From these artificial data the material parameters of the model of Chan, Bodner and Lindholm are identified. To measure the quality of the identified material parameters their mean values and empirical standard deviations are computed. Furthermore, the coefficients of the empirical correlation matrix for the material parameters are computed. The model responses for tensile tests with the parameter vector generated from all tests and with the estimated parameters (from stochastic simulations) differ not considerably. However, for the creep tests the different parameter estimations lead to quite different model responses. Received October 22, 1999     

On quasi-non-destructive strength and toughness testing of elastic–plastic materials

Non-destructive evaluation of mechanical material properties, like strength and fracture toughness, is impossible for principal reasons. However, there are possibilities of quasi-non-destructive estimation methods, which can be quite useful in practice. Instrumented indentation tests are often suitable to get information about the elastic–plastic behaviour, where the indentation depth is measured as a function of indentation force. By approximate analytical methods, key parameters like ultimate tensile strength, work-hardening exponent or even yield stress can be derived from these measurements. A mobile indenter is presented here and its use in ambulant testing is described. To obtain the uniaxial stress–strain curve more directly and more exactly, the same instrument can be used for a miniature compression test, where a small pin is machined out from the surface of the material. Furthermore, to get information about the toughness of materials, a carving instrument has been developed, which allows the energy required to introduce a defined furrow to be measured and correlated with toughness parameters.     

Influence of the Flame Straightening Process on Microstructural, Mechanical and Fracture Properties of S235 JR, S460 ML and S690 QL Structural Steels

The flame straightening of steel components is based on heating a local region of the part by means of a torch in order to induce a permanent deformation through a field of residual stresses. Although this is a very common practice, it is not devoid of serious drawbacks. In this paper, the influence of the flame bending procedure on the microstructure of three very different structural steels (S235 JR, S460 ML and S690 QL, respectively), widely used for the construction of metallic structures, is analysed. The consequences of the heat treatment on the mechanical and fracture properties were characterised through micro-hardness Vickers and Charpy impact tests; in addition, some elastic-plastic fracture tests were performed on precracked Charpy specimens manufactured with the S235 JR steel. The relationship between the microstructural features and the material mechanical and fracture behaviour was studied in depth in all cases, correlating the changes induced by the flame heat treatment on the microstructure with the macroscopic mechanical and fracture response. For a proper understanding of the microstructural consequences of this straightening heat treatment, it was necessary to develop a Finite Element numerical model. Based on the experimental results, this study has revealed that the consequences of the flame straightening on the microstructure, mechanical or fracture behaviour strongly depend on the nature of the material; for this reason, it is not possible to establish general recommendations. Nevertheless, the paper proposes a series of guidelines for good practice for steels similar to those characterised here.     

Identification of Material Damage Model Parameters: an Inverse Approach Using Digital Image Processing

A reliable prediction of ductile failure in metals is still a wide-open matter of research. Several models are available in the literature, ranging from empirical criteria, porosity-based models and continuum damage mechanics (CDM). One major issue is the accurate identification of parameters which describe material behavior. For some damage models, parameter identification is more or less straightforward, being possible to perform experiments for their evaluation. For the others, direct calibration from laboratory tests is not possible, so that the approach of inverse methods is required for a proper identification. In material model calibration, the inverse approach consists in a non-linear iterative fitting of a parameter-dependent load–displacement curve (coming from a FEM simulation) on the experimental specimen response. The test is usually a tensile test on a round-notched cylindrical bar. The present paper shows a novel inverse procedure aimed to estimate the material parameters of the Gurson–Tvergaard–Needleman (GTN) porosity-based plastic damage model by means of experimental data collected using image analysis. The use of digital image processing allows to substitute the load–displacement curve with other global quantities resulting from the measuring of specimen profile during loading. The advantage of this analysis is that more data are available for calibration thus allowing a greater level of confidence and accuracy in model parameter evaluation.     

An experimental and numerical investigation of the behaviour of AA5083 aluminium alloy in presence of the Portevin–Le Chatelier effect

An experimental investigation of the Portevin–Le Chatelier (PLC) effect in the aluminium alloy AA5083-H116 is undertaken in this study through five different tests involving round, prismatic and flat notched specimen geometries. Measurements based on strain gages and digital image correlation (DIC) are used to capture and characterize the spatio-temporal features of the PLC behaviour. Inhomogeneous deformation with various localization bands caused by the PLC effect is observed in all tests, and the band characteristics are measured. The McCormick elastic–viscoplastic constitutive relation, developed for metals exhibiting this type of dynamic strain aging, is then described in detail, before the various parameters required by the model are determined based on available material tests. The model is finally used in full-scale 3D numerical simulations of the physical tests using the explicit solver of the non-linear finite element code LS-DYNA. It will be shown that the numerical results are able to reproduce most of the experimentally observed phenomena with reasonable accuracy. However, if the model is used to study the micromechanical mechanisms controlling the macromechanical behaviour of materials exhibiting PLC effects (such as the band morphology), more advanced constitutive relations may be required.     

小冲杆试验确定材料拉伸断后伸长率的研究

小冲杆试验技术可以近似无损地测试材料的力学性能.本文对几十种材料进行了小冲杆试验研究,对小冲杆试样的抽向挠度δ与拉伸断后伸长率A进行了经验关联.研究表明,对于试验中涉及的几十种材料,二者之间没有统一的线性相关公式,但可以按材料的杭拉强度和屈强比将材料分类,然后分别对δ和A进行线性关联.对于抗拉强度小于600MPa的材料...     

金属材料三维断裂韧度厚度效应的有限元模拟

随着金属材料大壁厚结构件在工程中的广泛应用,对其断裂韧度的厚度效应研究具有重要的科学意义和工程价值。本研究基于有限元和实验相结合的方法,对金属材料断裂韧度的厚度效应进行预测。首先,通过一组薄壁厚金属材料标准三点弯曲试验得到试样失效时的临界载荷值,并利用内聚力模型与基于虚拟裂纹闭合技术的裂纹扩展模拟方法得到裂纹扩展时的单元临界能量释放率。随后,以此临界能量释放率作为裂纹扩展的启裂准则门槛值,通过有限元计算得到不同试样厚度下裂纹启裂时的裂尖断裂参数随着厚度的变化规律。最后,为了验证有限元模拟结果的准确性,本研究进行了另外两组不同厚度下三点弯曲试样的断裂韧度试验,并将试验结果与有限元结果进行了对比,验证了有限元所模拟的断裂韧度厚度效应的准确性。本研究旨在,通过薄壁厚三点弯曲试样的实验结果结合有限元模拟工作,即可实现金属材料断裂韧度的整个厚度效应曲线,为任意厚度下金属材料断裂韧度预测提供一种可靠的研究方法,有益于缩减试验成本,为大壁厚工程结构件的失效预测提供依据。     

A constitutive model in linear thermoviscoelasticity of polymers based on the concept of cooperative relaxation

New constitutive relations are derived for amorphous glassy polymers based on the concept of cooperative relaxation. A polymer is treated as a system of rearranging regions (flow units) embedded into a homogeneous elastic matrix. The viscoelastic (time-dependent) response of a medium is explained by rearrangements of segments of long chains in relaxing regions which occur at random instants. The kinetics of rearrangement is described in the framework of the Eyring concept of thermally activated processes, whereas the energy of any flow unit is assumed to randomly change at the instant of its reformation. Based on experimental data, phenomenological formulas are proposed for material functions. Adjustable parameters are found by fitting observations for mixtures of nylon with lithium halides in isothermal tensile relaxation tests. The thermoviscoelastic response in other tests is studied numerically. It is demonstrated that the material behavior predicted by the constitutive model in non-isothermal tests substantially differs from that predicted by conventional models whose adjustable parameters are determined by using the same experimental data. Received September 30, 1998     

On Spatial Effects of Modelling in Linear Viscoelasticity

The parameters of a linear model of a viscoelastic material are determined by testing the material in homogeneous (i.e. spatially constant) states. Some of the qualitative properties of the behaviour of the material observed in the tests may be unexpectedly lost if the material is confined, so that the behaviour varies in space and is thus not homogeneous. One such property is the (Lyapunov) stability of the deformation. To ensure that the material possesses these properties it is necessary to impose some additional restrictions on the model parameters. These restrictions are found by analysing the boundary value problems for viscoelastic bodies of various shapes and subjected to various boundary conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Direct and Inverse Problems in Elastic Multilayers with Reflection Data

The paper deals with the reflection-transmission process generated by a multilayer with homogeneous, isotropic elastic slabs. The direct problem, namely the determination of reflected and transmitted (and internal) waves in terms of the incident one, is investigated and the solution is proved to exist and be unique. The proof is based on the validity of a first integral. Next the inverse problem is considered to determine the material parameters of the slabs through the measure of the (frequency-dependent) reflection coefficient at the boundary of the multilayer. As a result, the material parameters are found in closed form, though for two slabs only.     

哈达山水利枢纽坝基砂层震动液化研究

砂层震动液化问题是哈达山水利枢纽工程首要的工程地质问题。根据工程勘察取得的大量试验资料,通过标贯、GJS原状取砂、OYO— 3000系列密度测井及瑞利波测试等综合方法,对砂层震动液化问题进行了深入的研究,为坝基加固处理提供了重要依据。其中瑞利波测试技术用于砂土震动液化是一个新的尝试。     

复合材料层合板的率模可靠度研究

传统的结构可靠性分析是基于二值状态假设的,即结构要么处于失效状态,要么处于安全状态,但在许多工程实际中,在失效和安全之间有一系列中间状态,因此判断结构是否失效,引用模糊状态的概念更为合理．该文将失效（或安全）的模糊状态假设引入到复合材料层合板的可靠度分析和计算中,建立了复合材料层合板的率模可靠度模型和相应的计算模型．在该模型中,考虑了单层复合材料强度指标的随机性,以及极限状态的模糊性．由层合板结构可靠性分析的具体算例,说明了该文模型和计算方法的合理性及有效性．同时,对参数的影响也进行了分析和讨论．结果表明,强度参数的均值以及失效准则的选取对可靠度有较大的影响．     

NiTi形状记忆合金丝力学性能试验研究

形状记忆合金(SMA)是一种具有多种特性的新兴功能材料,其力学性能与材料本身的元素组成比例、应力状态以及周围的环境温度等有着复杂的关系．本文选用某金属开发有限公司生产的镍钛SMA为研究对象,对12组48根奥氏体状态下形状记忆合金丝试件进行力学性能试验,通过改变电流、加载幅值、加载速率、循环次数和直径等主要试验参数,研究了形状记忆合金材料的力学特性．结果表明：加载幅值和加载速率是影响奥氏体SMA材料力学性能的主要参数．     

Bending analysis of five-layer curved functionally graded sandwich panel in magnetic field: closed-form solution

In this paper, an exact closed-form solution for a curved sandwich panel with two piezoelectric layers as actuator and sensor that are inserted in the top and bottom facings is presented. The core is made from functionally graded(FG) material that has heterogeneous power-law distribution through the radial coordinate. It is assumed that the core is subjected to a magnetic field whereas the core is covered by two insulated composite layers. To determine the exact solution, first characteristic eq...     

A stochastic model for parameter identification of adhesive materials

In practice, there are only a very limited number of experimental data available. Therefore, the prediction of material behaviour is difficult and a statistical analysis with a stochastic-based method is nearly impossible. In order to increase the number of tests based on experimental data, we apply the method of stochastic simulation based on time series analysis. The generated artificial data have the same stochastic behaviour as the experimental data. Advantages of artificial data are the arbitrary number of data available, and as a conclusion, the process of parameter identification can be statistically analysed. Here, we especially have experiments for adhesive materials for substantial tension tests performed at two different strain rates. Artificial data provide a stochastic proved analysis of the parameter identification concerning distribution and deviations. The analysis shows the possible range of the different material parameters and, therefore, gives a detailed view of the identification process.