New Inverse Method for Determining Uniaxial Flow Properties by Spherical Indentation Test

The spherical indentation test has been successfully applied to inversely derive the tensile properties of small regions in a non-destructive way.Current inverse methods mainly rely on extensive iterative calculations,which yield a considerable computational costs.In this paper,a database method is proposed to determine tensile flow properties from a single indentation force-depth curves to avoid iterative simulations.Firstly,a database that contain numerous indentation force-depth curves is established by inputting varied Ludwic material parameters into the indentation finite elements model.Secondly,for a given experimental indentation curve,a mean square error(MSE)is designated to evaluate the deviation between the experimental curve and each curve in the database.Finally,the true stresses at a series of plastic strain can be acquired by analyzing these deviations.To validate this new method,three different steels,i.e.A508,2.25Cr1 Mo and 316L are selected.Both simulated indentation curves and experimental indentation curves are used as inputs of the database to inversely acquire the flow properties.The result indicates that the pro-posed approach provides impressive accuracy when simulated indentation curves are used,but is less accurate when experimental curves are used.This new method can derive tensile properties in a much higher efficiency compared with traditional inverse method and are therefore more adaptive to engineering application.

Physical Macrohardness of the Kinetic Indentation of the Material: Function and Universal Unit of Measure (Part 1)

Three directions of development of kinetic indentation methods.Physical-energetic analysis of the indentation force diagram according to ISO 14577.Physical theory and universal criterion for the macrohardness of a material.Model of the physical process,thermomechanical potential,function of the state of the kinetic macroindentation process.Method for determining the physical function and unit of measurement of the kinetic macrohardness of a material.The ratio of the values of the empirical(standard)and physical macrohardness of the material.Physical reason for the appearance of the size effect in empirical indentation methods.The principle of determining the standard value of physical macrohardness.

Evaluation of Residual Stresses Induced by High Speed Milling Using an Indentation Method

In this work, a recently developed method based on the change of distance between collinear indents is used to evaluate different states of residual stress, which were generated in samples of AA 6082-T6 and AA 7075-T6 aluminium alloys milled at high speed. One of the advantages of this method, which needs a universal measuring machine, is not requiring neither the use of specific equipment nor highly skilled operators. Also, by integrating an indentation device to the mentioned machine, the absolute error of measurement can be reduced. In results obtained in samples subjected to different cutting conditions it is observed a correlation between the stress values and the depth of cut, showing the AA 6082-T6 alloy higher susceptibility to be stressed. Furthermore, the high sensitivity of the method allowed detecting very small differences in the values reached by different normal components in the zones corresponding to climb and conventional cutting. It is important to note that these differences were similar for both evaluated alloys. Finally, the directions associated with the principal components of residual stress, where maximum local plastic stretching occurs, were found to be strongly dependent on the rolling direction prior to machining.

PIEZOSPECTROSCOPIC STUDY OF RESIDUAL STRESSES AROUND INDENTATIONS IN SiC/Al_O_3 NANOCOMPOSITE

A new experimental measurement of residual stresses around Vickers′ indentations on the surface of the SiC/Al 2O 3 nanocomposites is proposed with the aid of a Raman microprobe. Results s how that the shifts of R lines in the fluorescence spectra va ry with the distance from the centre of indentation. The magnitude of load appli ed on the surface of the materials through the indenter influences the shifts of R lines to great extent. The luminescence of R lines of the materials before indenting is used to determine the residual stresses around the indentation in the materials, assuming that the stress tensor is transversely isotropic. Final ly, the term of hydrostatic stress is adopted to explain and compare different residual stresses around indentations with the increase of the indenting load an d the distance from the centre of indentations. <

PIEZOSPECTROSCOPIC STUDY OF RESIDUAL STRESSES AROUND INDENTATIONS IN SiC/AlO3 NANOCOMPOSITE

A new experimental measurement of residual stresses around Vickers′ indentations on the surface of the SiC/Al 2O 3 nanocomposites is proposed with the aid of a Raman microprobe. Results s how that the shifts of R lines in the fluorescence spectra va ry with the distance from the centre of indentation. The magnitude of load appli ed on the surface of the materials through the indenter influences the shifts of R lines to great extent. The luminescence of R lines of the materials before indenting is used to determine the residual stresses around the indentation in the materials, assuming that the stress tensor is transversely isotropic. Final ly, the term of hydrostatic stress is adopted to explain and compare different residual stresses around indentations with the increase of the indenting load an d the distance from the centre of indentations. 【

Novel method for measuring surface residual stress using flat-ended cylindrical indentation

Instrumented indentation is a promising technique for estimating surface residual stresses and mechanical properties in engineering components.The relative difference between the indentation loads for unstressed and stressed specimens was selected as the key parameter for measuring surface residual stresses in flat-ended cylindrical indentations.Based on the equivalent material method and finite element simulations,a dimensionless mapping model with six constants was established between the relative load difference,constitutive model parameters,and normalized residual stress.A novel method for measuring the surface residual stress and constitutive model parameters of metallic material through flat-ended cylindrical indentations was proposed using this model and a mechanical properties determination method.Numerical simulations were conducted using numerous elastoplastic materials with different residual stresses to verify the proposed model;good agreements were observed between the predicted residual stresses and those previously applied in finite element analysis.Flat-ended cylindrical indentation tests were performed on four metallic materials using cruciform specimens subjected to various equibiaxial stresses.The results exhibited good conformance between the stress–strain curves obtained using the proposed method and those from traditional tensile tests,and the absolute differences between the predicted residual stresses and applied stresses were within 40 MPa in most cases.

Compressional Deformation in Indentation Process for Microlens Array Mold

The structure of a microlens array（ MLA） can be formed on copper by an indentation process which is a new manufacture approach we applied here instead of a traditional method to test the material property,thereby work time can be saved. Single-indentation and multi-indentation are both conducted to generate a single dimple and dimples array,namely micro lens and MLA. Based on finite element simulation method,factors affecting the form accuracy,such as springback at the compressed area of one single dimple and compressional deformation at the adjacent area of dimples arrays,are determined,and the results are verified by experiments under the same conditions. Meanwhile,indenter compensation method is proposed to improve form accuracy of single dimple,and the relationship between pitch and compressional deformation is investigated by modelling seven sets of multi-indentations at different pitches to identify the critical pitch for the MLA＇s indentation processing. Loads and cross-sectional profiles are measured and analyzed to reveal the compressional deformation mechanism. Finally,it is found that MLA at pitches higher than 1. 47 times of its diameter can be manufactured precisely by indentation using a compensated indenter.

ANALYSIS ON PSEUDO-STEADY INDENTATION CREEP

Theoretical analysis and finite element （FE） simulation have been carried out for a constant specific load rate （CSLR） indentation creep test. Analytical results indicate that both the representative stress and the indentation strain rate become constant after a transient period. Moreover, the FE simulation reveals that both the contours of equivalent stress and equivalent plastic strain rate underneath the indenter evolve with geometrical self-similarity. This suggests that pseudo-steady indentation creep occurs in the region beneath the indenter. The representative points in the region are defined as the ones with the equivalent stress equal to the representative stress. In addition, it is revealed that the proportionality between indentation strain rate and equivalent plastic strain rate holds at the representative points during the pseudo-steady indentation creep of a power law material. A control volume （CV） beneath the indenter, which governs the indenter velocity, is identified. The size of the CV at the indented surface is approximately 2.5 times the size of the impression. The stress exponent for creep can be obtained from the pseudosteady indentation creep data. These results demonstrate that the CSLR testing technique can be used to evaluate creep parameters with the same accuracy as conventional uniaxial creep tests.

Influence of Residual Stress on the Elastic-plastic Response to Indentation

The indentation method is useful in determining the residual stress according to the elastic-plastic properties of materials.So the effect of the residual stress on the elastic-plastic indentation properties of materials was studied by using the finite element method to find better indentation parameters which are strongly induced by the residual stress.The results show that load-depth curve,plastic pile-up,indentation shape,indentation contact stress and indentation residual stress are affected by different residual stress,and these parameters can be used to deduce the residual stress.Also,a special indentation equipment was developed to analyze the elastic-plastic properties of materials with different residual stress,and the experimental results show a good agreement with the FEM results.For practical application,the elastic-plastic indentation properties of materials with unknown residual stress could be obtained by the developed equipment to deduce the residual stress comprehensively.

Numerical Crack Analysis of Blunt Rock Indenters by an Indirect Boundary Element Method

Linear elastic fracture mechanics principles are widely applied for the analysis of crack problems in rock fracture mechanics. Rock indentation is an important and complicated problem among rock engineering issues. In this paper, in addition to the fracture criterion of maximum tangential stress adjacent to crack tip, the higher order displacement discontinuity method (which is a version of the indirect boundary element method) has been used for modeling the crack propagation mechanism under blunt indenters. In order to achieve more accurate results, higher order boundary elements i.e. quadratic elements, has been used to calculate displacement discontinuities and also to reduce the singularities of stress and displacement fields near the crack tip, the special crack tip elements has been used to calculate the stress intensity factors (SIF) at the crack tips. In this modeling, the effect of crack angle on stress intensity factors has been investigated. The numerical results of stress intensity factors obtained from some example problems were compared to the theoretical and experimental results cited in the literature which always show a percentage error less than one percent. The simulated results may pave the way for increasing the efficiency of mining and drilling by improving the design of tools and indentation equipments.

Finite Element Simulation of Residual Stress Determination Based on Spherical Indentation Accumulation Morphology

The introduction of residual stress during the processing of materials has an important impact on the properties of the materials, so it is important to accurately measure the residual stress of the material. This paper established a finite element model of spherical indentation under the action of non-equivalent biaxial residual stress. Then we extracted the full-field accumulation state near the indentation under different stress states from the simulation results and summarized the pile height distribution near the indentation under different stress states. From the simulation, we found that the maximum pile-up height near the indentation point presented a regular trend.

Quasicontinuum simulation of indentation on FCC metals

Quasicontinuum simulations were performed to study the processes of incipient plastic deformation on three FCC metals (Ag,Ni and Pd) under the action of a rigid indenter.Four widths (9.3,18.6,27.9 and 37.2 ) of the indenters were modelled for each metal specimen.A series of load-displacement responses and the strain energy versus displacement responses of the indenter were presented.It is shown that the abrupt drop of the load in the load-displacement response is triggered by the nucleation of dislocations in the metals.The critical load of each metal specimen increases with the increase of the indenter width,while the hardness of the metal specimen decreases as the indenter width increases.Furthermore,the microscopic mechanism of deformation in the films was analyzed.Two dislocations are nucleated respectively beneath the right and left sides of the indenter.Each dislocation successively decomposes into two Shockley partial dislocations after the first nucleation process.The distances between the two partial dislocations are equal for both sides of the indenter,which are in agreement with the theoretical values.

球压痕法获取不同变形率下316L不锈钢的力学性能

核电焊接结构由于在加工过程中受到冷作硬化导致局部区域力学性能发生改变且难以准确获取,利用ABAQUS有限元分析软件建立了弹塑性材料下的压痕实验三维模型进行数值模拟,通过识别载荷-深度曲线中的相关参数,结合量纲分析法理论计算316L奥氏体不锈钢材料在不同变形率下的力学性能参量。结果表明,随着变形率的不断增大,材料的塑性降低,硬度增大,且力学性能参数如屈服强度、抗拉强度及硬化指数均相应增大。

基于响应面法的螺杆转子注射成型工艺研究

针对传统金属型螺杆转子制造工艺复杂、加工精度难以控制等问题,提出一种新型螺杆转子注射成型方法,尝试用塑料材质取代金属型螺杆转子。使用Moldflow模流软件对玻纤增强聚酰胺材料的螺杆转子齿面注射成型进行注塑模拟,将缩痕深度和翘曲变形作为质量标准,对影响塑件成型质量的7个工艺参数进行筛选分析,确定了影响2个质量标准的3个工艺参数。采用BBD响应面实验设计方法,研究了熔体温度、保压时间、保压压力等注塑工艺参数对缩痕深度和翘曲变形的影响,获得了最小缩痕深度和最小翘曲变形的多项式回归响应模型。利用回归模型优化的工艺参数组合为熔体温度281.0℃,保压时间18.7 s,保压压力115.7 MPa。数值模拟了最优参数组合,结果表明数值模拟与模型预测值的误差分别为3.75%和0.53%。

基于压痕加载曲线的豌豆籽粒硬度测定方法优化

为确定基于压痕加载曲线测定豌豆籽粒硬度的最佳作业参数组合,以豌豆为研究对象,以压入深度、压头类型、待测表面粗糙度为自变量,豌豆硬度测定值为响应值,根据正交设计原理,设计三因素三水平正交试验,采用Design-Exporter数据处理软件进行极差、方差分析,对各因素进行探究,并利用有限元法对试验研究结果进行验证。结果表明,测量过程中最佳的作业参数为压入深度0.5 mm,压头类型为45°三棱锥,待测籽粒表面粗糙度为100目。有限元法与实际试验测定结果表现一致,即45°三棱锥压头平均误差最小。

A Reverse Numerical Approach to Determine Elastic-plastic Properties of Multi-layer Material Systems with Flat Cylindrical Indenters

In the present study, the indentation testing with a flat cylindrical indenter on typical multi-layer material systems was simulated successfully by finite element method. The emphasis was put on the methods of extracting the yield stresses and strain-hardening modulus of upper and middle-layers of three-layer material systems from the indentation testing. The slope of the indentation depth to the applied indentation stress curve was found to have a turning point, which can be used to determine the yield stress of the upper-layer. Then, a different method was also presented to determine the yield stress of the middle-layer. This method was based on a set of assumed applied indentation stresses which were to be intersected by the experimental results in order to meet the requirement of having the experimental indentation depth. At last, a reverse numerical algorithm was explored to determine the yield stresses of upper and middle-layers simultaneously by using the indentation testing with two different size indenters. This method assumed two ranges of yield stresses to simulate the indentation behavior. The experimental depth behavior was used to intersect the simulated indentation behavior. And the intersection corresponded to the values of yield stresses of upper and middle-layers. This method was also used further to determine the strain-hardening modulus of upper and middle-layers simultaneously.

Analytical Solution for an Elastic Sphere Indenting a Layered Half-Space

The contact problem for the elastic sphere indenting a layered half-space is considered. Analytical methods for solving this problem have been developed on the basis of the 3-D fundamental solution of a half space with a single coating layer under a normal concentrated force on the surface. The normal pressure distribution within the contact zone is assumed as Hertzian type. The solutions are constructed using superposition principle in the form of infinite series. Through comparing with the numerical results of FEM,it can be verified that the exact solutions have a rapid convergence rate and the stresses and displacements are mainly determined by the first term,which is corresponding to the solution of homogeneous half-space under Hertzian loading. The contact radius can be predicted applying the method.

自洽法和Mori Tanaka法预测水泥浆体徐变的比较

为了预测水泥浆体的徐变性能,并对比不同预测模型的精度。使用微米压痕测试确定了水泥浆体的徐变模量,使用纳米压痕点阵测试获得微观物相的接触徐变模量。随后基于一系列微观测试获得物相的体积分数。将水泥浆体划分为2个尺度,分别基于自洽(self-consistent,SC)法和Mori Tanaka法,采用4种组合方式进行徐变均匀化预测。结果表明,在凝胶尺度采用Mori Tanaka法,在浆体尺度采用SC法时,水泥浆体的长期徐变性能的预测精度较高。该研究为进行水泥基材料徐变均匀化时选择合适的计算方法提供了依据。

X80管道环焊缝强度的仪器化压痕测试应用

对接环焊缝的强度在高钢级管线中起重要作用。本文利用仪器化压痕法、常规拉伸试验法和维氏硬度法对X80的环焊缝、热影响区以及母材的材料性能进行测试,并比较仪器化压痕法和常规拉伸试验方法获得的强度结果。测试结果表明,仪器化压痕法可以给出对接环焊缝的屈服强度、抗拉强度,仪器化压痕法可以正确反映管线的焊缝与母材的匹配关系,还可以获得焊缝热影响区等更加精细的强度分布。通过对环焊缝进行常规拉伸与仪器化压痕法测得的强度对比发现,上游母材、焊缝中心及下游母材的屈服强度误差分别为6%、6.5%、7.5%,抗拉强度误差分别为16%、5.5%、3%。仪器化压痕法测得屈服强度基本低于常规拉伸的屈服强度,测得抗拉强度高于常规拉伸的抗拉强度。但仪器化压痕法测试结果与常规拉伸试验结果的规律一致,即母材强度大于环焊缝中心强度且上游母材强度低于下游母材。

Theoretical and numerical studies of rock breaking mechanism by double disc cutters

A plane mechanical model of rock breaking process by double disc cutter at the center of the cutterhead is established based on contact mechanics to analyze the stress evolution in the rock broken by cutters with different spacings. A continuous-discontinuous coupling numerical method based on zero-thickness cohesive elements is developed to simulate rock breaking using double cutters. The process, mechanism,and characteristics of rock breaking are comprehensively analyzed from five aspects: peak force, breaking form, breaking efficiency, crack mode, and breaking degree. The results show that under the penetrating action of cutters, dense cores are formed due to shear failure under respective cutters. The tensile cracks propagate in the rock, and then rock chips form with increasing penetration depth. When the cutter spacing is increased from 10 to 80 mm, the peak force gradually increases, the rock breaking range increases first and then decreases, the specific energy decreases first and then rises, and the breaking coefficient of intermediate rock decreases from 0.955 to 0.788. The area of rock breaking is positively correlated with the length of the tensile crack. Furthermore, the length of the tensile crack accounts for 14.4%–33.6% of the total crack length.