Residual Stress Measurements Using Elasto Plastic Indentation And Espi

Material hardness is independent of residual stresses at predominantly plastic deformation in the region of contact between an indenter and the material. This is an important fact, as such a situation is relevant for most metals and alloys. It has, however, been established that when elastic and plastic deformations are of equal magnitude, this independence is lost. This fact complicates residual stress determination in such a situation, pertinent to materials such as ceramics and polymers, but it also provides additional important information for performing such a task in an accurate manner. Presently, a relevant formula for taking advantage of this information is presented. The formula is based on previous results pertinent to indentation analyses of stress-free materials. The predictions are compared to finite element results from previous studies describing cone indentation of materials with residual stresses. The analysis is restricted to classical elastoplasticity.

This book provides complete coverage of the slitting method. It details new results in analysis, computation, and estimation and discusses different roles of residual stresses from the fracture mechanics perspective. It provides detailed formulations and examples of compliance functions, weighted least squares fit and convergence test in stress estimation, and computer programs to facilitate the implementation of the slitting method.

With contributions from 24 authorities from around the world, this handbook provides the most authoritative reference resource available on the impact of residual stresses on mechanical properties of materials and structures. You'll find detailed descriptions of a full range of measuring techniques, including hole drilling, layer removal, sectioning, X-ray diffraction, neutron diffraction, and ultrasonic methods. A variety of case studies which illustrate use of specific techniques are included to facilitate your understanding. Design and structural engineers, metallurgists, and material scientists will find a wealth of valuable information covering recent developments in residual stress measuring techniques, with guidelines provided for selecting the right measuring strategy for each specific application, and many helpful tips for improving quality control.

Residual stresses (RS) are stresses present inside materials even in the absence of any applied load. They are of capital importance because they can impact greatly on the mechanical strength of the material, on its dimensional correspondence to design specifications as well as on the fatigue life of the part. RS measurement and evaluation is currently an important research topic where a lot of challenges still need to be addressed. This book aims to provide the reader with an overview of the principal novelties in this field including current limitations and potential future developments. Both radically new experimental approaches as well as recent evolutions of consolidated ones will be presented, along with the latest novelties in the area of numerical residual stress evaluation.

Residual stresses are "locked-in" within a material, and exist without any external loads. Such stresses are developed during most common manufacturing processes, for example welding, cold working and grinding. These "hidden" stresses can be quite large, and can have profound effects on engineering properties, notably fatigue life and dimensional stability. To obtain reliable and accurate residual stress measurements for uniform and non-uniform stress states, a novel and practical method using crossing-slitting and ESPI is presented here. Cross-slitting releases all three in-plane stress components and leaves nearby deformation areas intact. The ESPI (Electronic Speckle Pattern Interferometry) technique gives an attractive tool for practical use, because measurements provide a large quantity of useful data, require little initial setup and can be completed rapidly and at low per-measurement cost. A new ESPI setup consisting of shutter and double-mirror device is designed to achieve dual-axis measurements to balance the measurement sensitivities of all in-plane stress components. To evaluate data quality, a pixel quality control and correction procedure is also applied. This helps to locate bad data pixels and provides opportunities to correct them. The measurement results show that this procedure plays an important role for the success of residual stress evaluation. Based on the observed displacement data and finite element calculated calibration data, an inverse computation method is developed to recover the residual stresses in a material for both uniform and non-uniform cases. By combining cross-slitting and ESPI, more reliable results for the three in-plane residual stress components can be obtained.

A proper knowledge of the mechanical properties and residual stresses of materials has a significant role in the prediction of engineering failures. Indentation is a simple, nondestructive test that is capable of estimating both residual stresses and mechanical properties. In frequent studies, the response of materials during the indentation process has been used as a key parameter to distinguish different substances. Here, a state-of-the-art method with no undesirable restriction is suggested to attain the work hardening exponent, yield strength, and planar nonequibiaxial residual stresses. In the current work, an extensive series of Knoop indentation simulations were performed using two indenter angles. Subsequently, a precise observation was made in order to find existent relationships. A local method was employed based on characteristics of similar materials to obtain stress-free sample parameters through a genetic algorithm, and then another error function was defined in order to measure the yield strength and work hardening exponent. After the determination of the mechanical properties and the stress-free sample's parameters, a particular and precise categorization was made. Then, neural network analysis was employed to derive planar residual stresses. Experimental validation was conducted using six types of aluminum and steel specimens. The results confirmed a good agreement between the test data and those predicted using the suggested procedure.

This manual has been assembled as a ready reference source for the engineer who must measure residual stresses. Although the literature has abundant information on the individual methods of residual stress measurement, this manual is unique in that most of the useful methods have been selected, described, classified, and compared.Methods of measuring residual stress may be classified as mechanical, physical, or chemical. Mechanical methods that are most widely used involve the machining of the part to release the residual stresses. These methods are considered in detail and occupy the bulk of this manual.Of the physical methods, x-ray diffraction has many advantages in solving some residual stress problems. One of the major advantages is that it is completely nondestructive if only surface residual stresses are desired. This method is briefly described and its limitations discussed so that the investigator may choose this method if it fulfills his needs. The method and the necessary techniques have been covered in detail in Ref. 1.Of the chemical methods, the stress corrosion cracking technique seems to be the most useful. This cracking is known to be dependent upon the surface tensile stress level. At present it is only a qualitative measure of the stress, but perhaps at some future date, quantitative chemical methods may be developed.To make residual stress measurements, the engineer must: 1Select the most appropriate method. 2Completely familiarize himself with the theory behind the method chosen. 3Learn the techniques of measurement used by previous investigators.Note that the word "method" is reserved to mean the individual category of stress analysis, and that "technique" applies to the experimental procedure.The choice of the method depends to a large extent upon the geometry of the specimen, the precision desired, the type and location of the stress it is desired to measure, and the equipment available. The numerous mechanical methods available are classified into four groups. These are: the parting-out method, the layer removal method, the boring and turning method, and the hole drilling method. The nonmechanical x-ray method is briefly discussed here for comparison.Once the method has been selected, measurements should not be taken and their values blindly substituted into equations. Each term in the expressions relating the deformation to the residual stress should be completely understood and the sign convention used for these terms should be well known. For this reason, a derivation for each method has been included with the necessary assumptions and limitations. Following a study of these equations, it may be decided whether some automatic or computer technique should be used. In some instances in which a large number of specimens are to be analyzed, the computer technique offers obvious advantages. At this point, it is also necessary to determine the precision with which the stresses are to be determined. This, of course, can be related directly to the precision used in the measurement of the deformation changes and the original dimensions of the specimen itself.The correct choice of the many and varied techniques for measuring dimensional changes used in the determination of residual stress is extremely important if accurate and reproducible data are to be obtained. Machining stress introduced in dissecting must not be larger or more significant than the stress that is to be measured. The techniques used to remove material are numerous, and some of the better currently used techniques are considered. The specific techniques associated with any particular method are included and special fixtures and procedures are described and sample calculations are given.