Page 245 - CITS - Welder - Trade Theory

WELDER - CITS

MODULE 9 : Inspection & Testing

Lesson 83 - 97 : Non Destructive testing of Metal - Visual

inspection

Objectives

At the end of this lesson you shall be able to
• describe the non destructive testing
• explain the visual inspection
• explain the visual inspection after welding.

INTRODUCTION
Non destructive testing is a branch of the materials sciences that is concerned with all aspects of the uniformity,
quality and serviceability of materials and structures. The science of nondestructive testing incorporates all the
technology for detection and measurement of significant properties, including discontinuities, in items ranging
from research specimens to finished hardware and products. By definition, nondestructive techniques are the
means by which materials and structures may be inspected without disruption or impairment of serviceability.
Nondestructive testing (NDT) has been defined as comprising those test methods used to examine an object,
material or system without impairing its future usefulness.
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture
and during use is often essential. During the process of casting a metal object, for example, the metal may shrink
as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do
not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion
of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a
pipeline to rupture.
During their service lives, many industrial components need regular nondestructive tests to detect damage that
may be difficult or expensive to find by everyday methods. For example:
• aircraft skins need regular checking to detect cracks;
• underground pipelines are subject to corrosion and stress corrosion cracking;
• pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
• concrete structures may be weakened if the inner reinforcing steel is corroded;

• pressure vessels may develop cracks in welds;
the wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires
and other damage is important.

An industrial product is designed to perform a certain function. The user buys a product with expectation that it
performs the assigned function well and gives trouble – free service for a stipulated period of time. Trouble free
service given by any product may be termed as “reliability”. Reliability comes through improving the quality level
of the component. The quality of products, components or parts depends upon many factors. Quality is related
to the presence of defects and imperfections in the finished product which impair the performance level. In the
correct sense, a defect is a rejectable discontinuity or flaw of rejectable nature. Certain flaws acceptable in one
type of product need not be of acceptable nature in another product. A defect is definitely a discontinuity but a
discontinuity need not necessarily be a defect. Acceptance standards dictate the type of inspection and testing
the weld is subjected to.
Nondestructive testing (NDT), also called nondestructive examination (NDE) and nondestructive inspection (NDI),
is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components
and structures. To detect different defects such as cracking and corrosion, there are different methods of testing
available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo
blip on the screen). This article is aimed mainly at industrial NDT, but many of the methods described here can be

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