ISO TR 29381-2008 Metallic materials - Measurement of mechanical properties by an instrumented indentation test - Indentation tensile properties《金属材料 用仪器压痕试验测量机.pdf

1、 Reference number ISO/TR 29381:2008(E) ISO 2008TECHNICAL REPORT ISO/TR 29381 First edition 2008-10-15 Metallic materials Measurement of mechanical properties by an instrumented indentation test Indentation tensile properties Matriaux mtalliques Mesure des caractristiques mcaniques par un essai de pn

2、tration instrument Caractristiques de traction par indentation ISO/TR 29381:2008(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed

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7、 ii ISO 2008 All rights reservedISO/TR 29381:2008(E) ISO 2008 All rights reserved iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 1 3 Terms and definitions. 1 4 Symbols and designations 2 5 Descriptions of the different methods 3 5.1 Method 1: Representative stress

8、and strain 3 5.2 Method 2: Inverse analysis by FEA. 10 5.3 Method 3: Neural networks 18 6 Summary 23 Annex A (informative) Measurement of residual stress by instrumented indentation test 25 Bibliography . 29 ISO/TR 29381:2008(E) iv ISO 2008 All rights reservedForeword ISO (the International Organiza

9、tion for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established

10、has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. In

11、ternational Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publi

12、cation as an International Standard requires approval by at least 75 % of the member bodies casting a vote. In exceptional circumstances, when a technical committee has collected data of a different kind from that which is normally published as an International Standard (“state of the art”, for exam

13、ple), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have to be reviewed until the data it provides are considered to be no longer valid or useful. Attention is drawn to the possibi

14、lity that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO/TR 29381 was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals, Subcommittee SC 3, Hardness testing. ISO/TR

15、 29381:2008(E) ISO 2008 All rights reserved v Introduction 0.1 General information for tensile properties For centuries the elastic properties of materials have been described by Hookes Law (ca. 1660) and the practical parameter of Youngs modulus. This simple ratio of stress/strain is a practical, u

16、seful measure and, combined with a value for Poissons ratio of a material (a measure of the dimensional change of a material in directions other than the principal axis in which it is being strained), it is possible to determine the stresses introduced by loading even quite complex structures. When

17、the applied force is removed from an elastically deformed structure, it will recover completely. If, however, the stress in a material exceeds its yield point, then it will deform plastically and will retain a permanent deformation after the applied force is removed. The simplest description of the

18、mechanical properties of the material is, therefore, a plot of stress vs. strain, from zero to the strain at which the material fails completely. y p y yy 1 n E E Key E is Youngs modulus y , y are the yield point coordinates p is the nonlinear part of the total accumulated strain beyond y r is the e

19、lasto-plastic strain induced by r , the stress above the yield point Figure 1 Schematic of a typical true stress-strain curve for a work-hardening metal Figure 1 shows just such a curve. From this curve, the key tensile properties of the material can be obtained. Youngs modulus E is the gradient of

20、the initial portion of the curve. It is also the gradient of the straight line along which elastic recovery occurs from any point along the curve. The deviation of the curve from a straight line marks the yield point, often described as the yield stress. A straight-line recovery, of gradient E, from

21、 any point at higher stress or strain than this point would no longer pass through the origin, i.e. plastic deformation will have occurred. ISO/TR 29381:2008(E) vi ISO 2008 All rights reserved The gradient of the curve after yielding is a measure of the work hardening of the material, i.e. elastic r

22、ecovery occurs along a straight line, gradient E, and re-stressing the material also follows the same line such that further plastic deformation only begins once the previous maximum stress has been exceeded. The point at which the material fails completely marks two parameters of interest, one bein

23、g the ultimate tensile stress (UTS); the other being the strain at failure. These parameters form the key material specifications for any structural or functional design. It can be seen that the stress-strain curve is an essential “fingerprint” of the type of material. An elastic then perfectly plas

24、tic material will deform elastically up to the yield stress, and then it will continue to strain at constant stress until failure occurs at the strain-to-failure point. The yield stress is therefore also the UTS. A perfectly elastic, brittle material does not have a yield point, but exhibits a strai

25、ght line (gradient of the Youngs modulus) until it fails by fracture. A work-hardening material yields but is able to support increasing stresses as it strains to its UTS and maximum strain at failure point. The toughness of the material is often related to the area under the curve up to the failure

26、 point. This is a measure of the energy absorbed by the material before it fails. The tougher a material is, the more energy it absorbs before failure. Beyond extraction of the key tensile properties described above, the whole stress-strain curve is highly desirable input for the design of structure

27、s and components, to ensure that they do not yield or fail in service. Computing power has become more available and so the use of software such as Finite Element Analysis (FEA) programs, which determine the stress and strain throughout structures by considering them as an array of connected small v

28、olumes of material, is increasingly common. For a purely elastic calculation, the input parameters of Youngs modulus and Poissons ratio are exactly the same as for an analytical stress analysis. However, if plasticity is to be considered, then a yield stress is required plus a description of the amo

29、unt of plastic deformation that will occur at each stress above the yield point. This in effect requires input of the entire stress-strain curve. Measurement of the tensile properties of a material is most commonly performed using a uniaxial tensile testing machine. A sample of material is clamped i

30、n the machine and the strain is induced by the application of an ever-increasing stress (stress and strain being measured by suitable means). The exact method has improved and evolved over time, but the general principle has remained the same for centuries. It is possible to obtain the Youngs modulu

31、s of a material by other means, e.g. by using acoustic wave propagation 1 , and materials property reference sources often quote elasticity values obtained by just this method 2 , but tensile testing is the traditional method of choice for obtaining the yield stress and the plasticity part of the st

32、ress- strain curve. The uniaxial tensile test has the benefit of making a measurement that is very similar to the final application in an easily understood way. However, it has a number of significant drawbacks. It has proved surprisingly difficult to reduce the test uncertainty below the 10 % level

33、, although recent European projects have improved the identification and control of key uncertainties (EU project TENSTAND). Alignment in the instrument and the methods used to measure strain are key sources of uncertainty, as is the wide variety of algorithms used to obtain the tensile properties f

34、rom the measured data. The material must be available in volumes large enough to be tested. Small-scale testing and micro- tensile testing are becoming possible but have additional uncertainties. It must be possible to machine the materials to a controlled geometry without damaging them or changing

35、their properties (in particular their work-hardened state). The test is destructive and averaging includes uncertainties due to sample-to-sample inhomogeneity. 0.2 General information for indentation and tensile properties The widespread use of FEA to simulate indentation force vs. displacement curv

36、es is ample evidence that there is a direct forward link from a stress-strain curve to the indentation response of a material. However, the increasing use of modelling and the attendant requirement to obtain the stress-strain curve as input to the models raises the question of whether it is possible

37、 to solve the inverse problem, i.e. obtain a stress-strain ISO/TR 29381:2008(E) ISO 2008 All rights reserved vii curve from the indentation response of a material. If this were possible, it would remove many of the drawbacks of tensile testing and revolutionize the availability of tensile property i

38、nformation. Nano-indentation is able to measure microscopic volumes of material, thus the tensile properties of materials that exist only as small particles or as surface treatments or coatings would become obtainable. Indentation testing can be made portable and thus non-destructive, in situ, on-si

39、te testing would become available, with relatively little (or no) sample preparation. Lifetime monitoring of real structures would become cheaper and easier without the need for witness specimens. In 1951, Tabor 3demonstrated empirically that there was clearly a relationship of some form between the

40、 hardness response and the relative strain imposed by indentation, since plots of mean indentation pressure vs. relative indentation size (the ratio of indent radius to indenter radius, a/R, see Figure 2) appeared to map onto stress-strain curves for many metals. NOTE For a sphere, the strain induce

41、d by the indenter is proportional to a/R and is therefore a function of depth. Figure 2 Spherical indentation The availability of instrumented indentation has made the collection of such information a simple matter. Indeed, there is a common instrumented indentation testing cycle, often called the p

42、artial unloading method 4 , which applies a progressively increasing force but stops at a series of steps where the force is partially removed to obtain the top part of the force-removal curve necessary to obtain the contact stiffness and contact depth (hence the contact radius, a) at that force. Pr

43、ogressively increasing and partially removing the force on an indenter in this way allows a wide range of indentation sizes to be applied in the same place. This makes it possible to make a truly local measurement of material response over a wide range of strains, which might then be repeated with r

44、elative ease to form a map of the mechanical properties of a material. This Technical Report is intended to be a summary of the state of the art in deriving tensile properties from the indentation response of a material. Three approaches are described, and the key requirements, advantages and drawba

45、cks are summarized in table form. The three methods are: a) representative stress and strain, b) inverse FEA methods, and c) neural networks. All three methods have been shown to “work”, in that they are able to obtain from indentation data a stress vs. strain relation that can be validated against

46、tensile testing. However, more extensive intercomparison and sensitivity analyses are necessary to establish the robustness of each methods ability to identify the unique, best solution to the problem. ISO/TR 29381:2008(E) viii ISO 2008 All rights reservedThe three methods described all start from t

47、he assumption that input of the correct stress-strain curve into a suitable FEA package will enable exact simulation of the observed indentation response. Therefore, in principle, the inverse method is a brute force simulation, using all possible combinations of input parameters until the best fit t

48、o the measured indentation response is found. Such an inverse method is the benchmark method, as it can unambiguously identify the globally best solution and, if convergence is not possible, can identify that fact and demonstrate where the lack of convergence lies. Surprisingly, the increasing avail

49、ability of distributed computing networks makes this less unlikely than it might at first seem. It is clear, however, that any method that can economise on this amount of effort and obtain equivalent results (perhaps validated against selected distributed computing solutions) would be preferable. All of the methods described here are, in effect, different strategies for reducing the computing required by the user. The representative stress vs. strain approach uses FEA to