Nondestructive testing defects detected

Magnetic particles is one of the most used nondestructive testing techniques in industry. It allows detection and localization of surfacic and subsurfacic defects of ferromagnetic pieces by making conspicuous leakage fields by a magnetic developer. 

Most defects can be detected using one or more appropriate nondestructive testing techniques. However, in the absence of routine nondestructive testing inspections, identification of defects in installed equipment is generally limited to those that can be observed visually. Defects such as high residual stresses, microstructural defects such as sensitized welds in stainless steel, and laminations will normally remain undetected. Defects that can be detected visually have the following features ... 

Identification. If accessible, defects from burnthrough may be visually identified as fused holes in the tube wall. Various nondestructive testing techniques, such as radiography and ultrasonics, may also detect this defect. The defect generally causes leakage soon after affected equipment is placed in service. 

Thermal imaging is sensitive to infrared radiation that detects temperature changes over the surface of a part when heat has been applied. Thermal diffusion in a solid is affected by variation in composition or by the presence of cracks, voids, delaminations, etc the effects are detected by surface temperature changes. Defects cannot be detected if their depth below the surface is more than two to three times their diameter. Nondestructive testing has been primarily used for composites and analysis of adhesive bonds or welds. Several studies are documented in the literature (322—327). 

Nondestructive testing (NDT) is far more economical, and every assembly can be tested if desired. However, there is no single nondestructive test or technique that will provide the user with a quantitative estimate of bond strength. There are several ultrasonic test methods that provide qualitative values. However, a trained eye can detect a surprising number of faulty joints by close inspection of the adhesive around the bonded area. Table 7.16 lists the characteristics of faulty joints that can be detected visually. The most difficult defects to be found by any method are those related to improper curing and surface treatments. Therefore, great care and control must be given to surface-preparation procedures and shop cleanliness. 

Detection of defects is easier after thermal processing. Pressure and temperature must be monitored closely, usually by self-correcting mechanisms and without manual adjustment. After thermal processing, the part may be subjected to appropriate nondestructive testing, as described later. 

Nondestructive testing (NDT) is the development and application of technical methods for the detection, location, measurement, and evaluation of discontinuities, defects and other imperfections, the assessment of integrity, the assessment of properties and composition, or the measurement of geometrical characters without impairing the intended use or application of the test object (31). 

Perhaps the most serious need at the present time is for a method to assess the strength of an adhesive bond without actually breaking it. Methods exist for detecting defects at the interface, where the materials are not in contact, and this is an important part of nondestructive testing. But, as far as the authors are aware, there is no currently available method for assessing by nondestructive means whether a given bond is weaker than it should be. 

A number of nondestructive test (NDT) techniques have been developed that permit detection and measiu-ement of both internal and surface flaws. Such techniques are used to examine stmctmal components that are in service for defects and flaws that could lead to premature faUme in addition, NDTs are used as a means of quahty control for manufacturing processes. As the name implies, these techniques do not destroy the material/structure being examined. Furthermore, some testing methods must be conducted in a laboratory setting others may be adapted for use in the field. Several commonly employed NDT techniques and their characteristics are listed in Table 8.2. One important example of the use of NDT is for the detection of cracks and leaks in the walls of oil pipelines in remote areas such as Alaska. Ultrasonic analysis is utilized in conjunction with a robotic analyzer that can travel relatively long distances within a pipeline. 

The nondestructive temperature differential test by infrared is used. In this method, heat is applied to a product and the surface is scanned to determine the amount of infrared radiation is emitted. Heat may be applied continuously from a controlled source, or the product may be heated prior to inspection. The rate at which radiant energy is diffused or transmitted to the surface reveals defects within the product. Delaminations, unbonds, and voids are detected in this manner. This test is particularly useful with RPs. 

With nondestructive ultrasonic test back and forth scanning of a specimen is accomplished with ultrasonics. This NDT can be used to find voids, delaminations, defects in fiber distribution, etc. In ultrasonic testing the sound waves from a high frequency ultrasonic transducer are beamed into a material. Discontinuities in the material interrupt the sound beam and reflect the energy back to the transducer, providing data that can be used to detect and characterize flaws. It can locate internal flaws or structural discontinuities by the use of high frequency reflection or attenuation (ultrasonic beam). 

Bond Inspection. After the adhesive or sealant is cured, the joint area can be inspected to detect gross flaws or defects. This inspection procedure can be either destructive or nondestructive. The nondestructive type of tests can be visual or use advanced analytical tests. These types of bond inspections are described below. 

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