September/October 2005 EXPERIMENTAL TECHNIQUES 17 N TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TECHNIQUES by K. Kesavan, K. Ravisankar, S. Parivallal, and P. Sreeshylam NON DESTRUCTIVE EVALUATION OF RESIDUAL STRESSES IN WELDED PLATES USING THE BARKHAUSEN NOISE TECHNIQUE R esidual stresses have an influence on the behaviour of structural components during their service life. Depending on their nature, magnitude and distri- bution they can contribute to increasing the ex- pected life of a component or to its premature failure. Gen- erally the manufacturing processes of structural components introduce residual stresses in the component. The use of non-destructive methods for detection and measurement of residual stresses has been increasing in the last few years, because they do not promote changes in the material char- acteristics under examination and do not interfere in its later use. The non-destructive test method based on Bark- hausen emissions from ferromagnetic materials is sensitive to changes in micro structural characteristics and the stress state of the material. Even though, other non-destructive techniques like X-ray, neutron diffraction and ultrasonic methods are available, the Barkhausen noise method is more useful in determining the residual stresses in ferro- magnetic materials. X-ray diffraction can be used for quan- titative analysis of macro and micro level residual stresses separately. But it is very difficult to use it for continuous monitoring of stresses in industrial processes. Another im- portant method is the ultrasonic method which is based on changes in the velocity of ultrasonic waves due to stress. Here, the time resolution should be in the order of 0.1 na- nosec for accurate measurement. The non-destructive test- ing tool which is finding wide application in stress analysis is the magneto elasticity method, based on the Barkhausen noise principle. This paper describes the use of the Barkhausen noise tech- nique for the evaluation of residual stress due to the welding process in a ferromagnetic plate specimen. The advantages of the Barkhausen noise technique for stress measurements are that it is fast, reliable, and requires no surface prepa- ration. The main advantage is the requirement of proper cal- ibration to obtain absolute value of stress. RESIDUAL STRESS Residual stresses are present in a material or component even when there are no external forces acting on it. Residual stresses may be generated or modified at every stage in the component life cycle, from original material to final disposal. The residual stresses are classified into three types known as first, second and third kinds. Residual stresses of the 1 st kind (Macro stresses) are nearly homogeneous across large areas (several grains) of a material and are in equilibrium over the bulk of the material. Residual stresses of the 2 nd kind (Micro stresses) are nearly homogeneous across micro- scopic areas (one grain, or part of a grain) of material and are equilibrated across a sufficient number of grains. Resid- K. Kesavan, K. Ravisankar, S. Parivallal, and P. Sreeshylam are Scientists inthe Experimental Mechanics Laboratory of the Structural Engineering Research Cen- ter, CSIR Campus, Chennai, India. ual stresses of the 3 rd kind (sub-micro stresses) are homo- geneous across submicroscopic areas of a material, say by some atomic distances within a grain and are equilibrated across small parts of a grain. Usually, superposition of resid- ual stresses of the 1 st ,2 nd , and 3 rd kind determines the total residual stress acting at a particular point of a material or component. Sources of Residual Stress In the manufacturing process, residual stresses are gener- ated by a combination of some or all of the following sources. • Inhomogeneous plastic deformation in different por- tions of the components due to mechanical loads or con- straints. • Inhomogeneous plastic deformation due to thermal loads. • Volumetric changes and transformations plastically during solid state phase transformations. • A mismatch in the coefficient of thermal expansion. Characteristics of Residual Stress Residual stresses are elastic in nature even through they are produced by inelastic strains. Residual stresses are self- equilibrating types of stresses. Any disturbance like removal of material, introduction of thermal or mechanical loads will alter the equilibrium and thereby result in stress re- distribution. They get distributed across the section thick- ness and cross section. Many a time, they are inevitable and can not be eliminated fully but are only minimized. MEASUREMENT OF RESIDUAL STRESS Residual stress measurement methods are broadly classified into the following: • Destructive methods • Non-Destructive methods Destructive Methods Destructive methods are based on the removal of part of an internally stressed body. Important methods presently avail- able are: 1. Blind-hole drilling, 2. Layer removal, and 3. Sectioning technique The most widely used practical technique for determining residual stresses is the blind-hole drilling strain gage method. 1 In this method, a specially configured strain gage rosette is bonded to the surface of the test object, and a small shallow hole (around 1.6 mm diameter and 1.6 mm deep) is introduced into the structure, through the centre of the strain gage rosette using a precision drilling apparatus. Strains in the immediate vicinity of the hole are measured and the relaxed residual stresses are computed from these measurements. There is yet another method called layering and sectioning technique, where the residual stresses are