Defects found during NDT

Defects: Definition, Types and Origin

Often, in NDT and Quality control anomaly, discontinuity, defect, flaw, imperfection, non-conformance are the terms used when the material/component tested deviates from requirement/ideality. Though all of them look similar, there exists a vast difference in their meaning and interpretation. The term ‘flaw’ means a detectable lack of continuity or a detectable imperfection in a physical or dimensional attribute of a part. The term ‘nonconforming’ means only that a part is deficient in one or more specified characteristics. In many instances, a non-conforming part is entirely capable of performing its intended function, even in its non-conforming form. In other instances, a non-conforming part can be reworked to make it conform to specifications. Hence, it should not be automatically assumed that a non-conforming part is unfit for use.

The types of defects that NDT is called upon to find, can be classified into three major groups:

1. Inherent defects - introduced during the initial production of the base or raw material.

2. Processing defects - introduced during processing of the material or part.

3. Service defects - introduced during the operating cycle of the material or part.Some kind of defects or structural variations which may exist in these three groups are, cracks, surface and subsurface, arising from a large number of cases; porosity; tears; machining, rolling and plating defects; laminations; lack of bond; inclusions; segregation; lack of penetration in welds; pipe; fatigue defects; seams; blow holes, dross shrinkage etc.The origin of defects in a material can take place during manufacturing stage, or during assembly, installation, commissioning or during in-service (7).

We can broadly categorise these steps into two viz. pre-service and in-service. In the pre-service scenario, the defects may be present in the raw material stage or may be introduced during machining, fabrication, heat treatment, assembling.

The pre-service quality can be achieved essentially by good engineering practice i.e. by way of selecting suitable quality raw materials and by ensuring that harmful defects are not produced during the subsequent stages of fabrication and assembly, prior to putting the part/component into service. However, even with the highest quality of materials and workmanship, the occurrence of some form of imperfections during manufacture is inevitable and there will be a typical distribution of imperfection sizes associated with a particular manufacturing process and quality.

The ideal situation is where the inherent distribution of initial imperfection sizes is well separated from the distribution of critical defect sizes which may cause failure. Hence, the role of NDT is not only to detect the defects but also to give information about the distribution.There are little benefits derived out of repairing the parts/components with defects for their delivery to the customer. Here, the industry should aim at produce parts / components without defects. In the subsequent section, it is shown how to achieve this objective. On the other hand, in the in-service scenario, defects will be generated due to deterioration of the component/structure as a result of one or combination of the operating conditions like elevated temperature, pressure, stress, hostile chemical environment and irradiation leading to creep, fatigue, stress corrosion, embattlement, residual stresses, microstructural degradation etc. which, in turn, result in deterioration of mechanical properties, crack initiation and propagation, leaks in pressurised components and catastrophic failures

(8).NDT techniques are increasingly applied to components/systems for the detection and characterisation of defects, stresses and microstructural degradation to ensure the continued safety and performance reliability of components in industry. NDT techniques improve the performance reliability of components through periodic in-service inspections, by way of preventing premature and catastrophic failures.

(9,10) NDT also provide valuable inputs to plant specification and design i.e. to determine which components are the most likely to fail and then to ensure that those have easy maintenance access for repair or replacement. In in-service scenario, it is rather difficult to stop the formation of defects and the growth of defects already formed. Role of Fracture MechanicsFrom the above sections, it is clear that, in spite of utmost care by ensuring pre-service quality, optimum operating conditions and in-service inspection programme, the degradation of components/structures does take place and is unavoidable just as the ageing of human beings.

Sooner or later the inspection of any large engineering structure is likely to result in the identification of a possible defect. It is essential to know whether the detected defect is likely to impair the life or performance of the structure. A common approach is to estimate the fatigue life of the structure in the presence of the defect. This is the number of fatigue cycles the structure can withstand before the defect grows to a critical size and rapid fracture ensues (Paris Law). Further, the application of fracture mechanics helps identifying whether the defect is harmful or not

(11). Fracture mechanics is the applied mechanics of crack growth. It helps to quantify the rather elusive concept of a material's toughness which is the resistance to crack growth under a static load. This is measured in terms of a critical value of the stress intensity factor, a material property. According to fracture mechanics, defects present in materials lead to failure by growing to a critical, self propagating size. The fracture mechanics concepts allow one to calculate the critical sizes of defects as a function of their depth, length, active stress system and stress intensity and such properties of the material as its elastic modules, yield strength and fracture toughness.

Therefore, by knowing the dimensions of defects present in a component, it is possible to estimate both remaining life of the component and extent of degradation using the fracture mechanics concepts.In particular, the size of the defect, its nature, its location, the stress to which it is subjected to and the local properties of the material in which it is embedded will play a major role in determining its rate of growth. It is common to place a limit on the acceptable height or depth of the defects to be accepted in a structure. It is the task of the NDT operator to determine the size of the defect that is used in future behaviour.