NON-DESTRUCTIVE TESTING, EVALUATION OF STAINLESS STEEL MATERIALS PROJECT REPORT

INTRODUCTION

1.1 WHAT IS NDT?     

      NDT stands for non-destructive testing. In other words it is a way of testing without destroying. This means that the component- the casting, weld or forging, can continue to be used and that the non -destructive testing method has done no harm [3]. In today's world where new materials are being developed, older materials and bonding methods are being subjected to higher pressures and loads, NDT ensures that materials can continue to operate to their highest capacity with the assurance that they will not fail within predetermined time limits. NDT can be used to ensure the quality right from raw material stage through fabrication and processing to pre-service and in-service inspection. Apart from ensuring the structural integrity [2], quality and reliability of components and plants, today NDT finds extensive applications for condition monitoring, residual life assessment, energy audit, etc.  

        Non Destructive Evaluation (NDE) comprises many terms used to describe various activities within the field. Some of these terms are nondestructive testing (NDT), nondestructive inspection (NDI), and nondestructive examination (which have been called NDE,). These activities include testing, inspection, and examination, which are similar in that they primarily involve looking at (or through) or measuring something about an object to determine some characteristic of the object or to determine whether the object contains irregularities, discontinuities, or flaws. 

      The terms irregularity, discontinuity, and flaw can be used interchangeably to mean something that is questionable in the part or assembly, but specifications, codes, and local usage can result in different definitions for these terms. Because these terms all describe what is being sought through testing, inspection, or examination, the term NDE (non-destructive evaluation) has come to include all the activities of NDT, NDI, and NDE used to find, locate, size, or determine something about the object or flaws and allow the investigator to decide whether the object or flaws are acceptable. A flaw that has been evaluated as rejectable is usually termed a defect.

1.2 NDT TECHNIQUE AND ITS APPLICTIONS

Now a day’s everyone going for higher reliability and stringent quality checks, the NDT techniques have gained even more importance. Whether one talked about quality assurance or quality control or reliability, one has to understand the science and engineering of various NDT techniques to firm a firm base. Also in the view of the newer materials being developed at a very rapid rate to meet the diverse requirement of modern engineering industries. Using NDT technique one may select a relatively “defect-free” material and better understand the behaviour of newly developed material. NDT techniques assumes greater significance in high reliability sectors such as nuclear, space, aircraft, defence, automobile ,chemical and fertiliser industries. In fact practically every medium and large-scale engineering industry uses NDT technique in one form to the other. 

       Besides being used for defects evaluation and location, NDT techniques are used for assessing the severity or otherwise of the defects too. Whereas certain defects may be very harmless in view of their small size or innocuous position (e.g. Micro voids in a low stressed region of composite), others may be quite harmful in view of their large size, type and position. These harmful defects grow to dangerous proportion in service but the harmless defects, as their name suggests may be ignored. In addition to harmful and harmless categories, there is a third category of defects too. They are classified as beneficial defects. These defects are not severe and they are beneficially located, certain beneficial defects arrest or impede the progress of a propagating crack and certain other beneficial defects divert the path of crack progress by providing easier path but in a different direction which is relatively safer hence the NDT engineer should not reject a material/component merely because of the presence of defects. The NDT engineer should find out whether the defects are harmless one or a beneficial one or otherwise and thereafter categorically state the rejection criteria.

1.3 NEW DEVELOPMENTS IN NDT

With the advent of modern advances in the field of electronics, computers and data processing, greater use of electronic gadgets, computers interfaces and data processors are being made to improve the existing NDT techniques, this has enabled development of instruments and software which quickly detect the defects and identifies their nature, shape, size and criticality. Another current trend in the field of Non-destructive inspection is to continuously monitor high-risk structures subjected to service loads (e.g. Acoustic emission monitoring of pressure vessels and storage tank). This continuous monitoring results in higher reliability of structures/components in service and permit prolonged service life of the structure/components, in addition to this continuous in service monitoring, NDT techniques are also being used for on-line monitoring during manufacturing too.(e.g. on-line monitoring for welding defects during welding operation using Acoustic emission testing) As mentioned in the previous section, NDT techniques are used for the evaluation of material behaviour too. One may study in detail various stages leading to ductile failure of different metals or different stages leading to failure of fibre reinforced plastic composite etc, using various NDT techniques one may use NDT techniques for predicting fatigue behaviour of different materials, for evaluation of different mechanical properties (modulus of elasticity, tensile strength, burst strength etc.) for evaluation of in-service embrittlement, for evaluation of grain size in various alloys, to quantify composition of alloys, to differentiate between different metals and alloys (i.e. identification of metals and alloys), for measurement of fibre volume fraction in fibre reinforced plastics composites etc. Yet another emerging usage of NDT technique is in the field of engineering “post-mortem” i.e. for analyzing the test data after a simulated failure or after proof loading, fractured surfaces are also studied for probing the reasons for failure. Material composition at the fracture site, texture of fractured surfaces, fibre pull-outs, presence of defects (cracks, voids, inclusions etc.) on the fracture surfaces, all help in proper investigation of the cause of failure.