Material defects locations

Ultrasonic examination is currently the most commonly used NDT method for inspection of composites. It presents desirable features such as providing information about defects situated deeply inside a material, but equally this method has several limitations. Flaws modify ultrasonic parameters such as wave velocity, refraction, reflection, scattering, and intensity, thus affecting the efficiency of ultrasound in defect location. In order to fully understand the concept of ultrasonic testing, it is necessary to use some mathematics, which will be kept to a minimum. The principle, advantages, and limitations of ultrasonic NDT techniques for composite inspection are described next. 

From an A-scan. material thickness T or defect location DL can be determined using the equations... 

Figure 15 Diagrams illustrating the calculation of (a) material thickness T. and (b) defect location I.

X-ray microscopy n. This instrument and technique is similar to an optical microscope except that X-rays are utilized to magnify and study an object instead of visual light. Images beneath a surface can be studied because X-rays penetrate materials including metals. Defects located within the interior of an object (plastic pipe) can be observed without disturbing the structure or cutting a cross-section. The technique is useful for studying the... 

To date, penetrant diffusion throu polymers has been studied by MD only in amorphous polymers. This is mainly due to the commonly held belief that only very few penetrants can enter the tightly packed crystallites and that those which do, get trapped at defect locations and hardly move afterwards. Therefore, diftusivities in the crystallites should be orders of magnitude smaller than those in amorphous regions and diffusion in a senu-crystalline material should be dominated by its amorphous part, even if it is small. In addition, if diffusion is excessively slow, one is unlikely to observe it on the time scale accessible to MD (i.e., ns). 

Possible nondestructive testing techniques for adhesively bonded structures and composite materials will be introduced along with a literature survey of successes and applications to date. Emphasis on ultrasonic inspection will also be highlighted, including such topics as ultrasonic wave generation, wave velocity, dispersion, reflection factor, wave refraction, attenuation, ultrasonic field analysis, resolution, thickness and defect location measurement, and C-scan testing. 

The most popular ultrasonic nondestructive testing application has been associated with thickness measurement of a test object and defect location within the particular test object. Most of the applications to date have been associated with the testing of homogeneous isotropic materials. Recent work has extended the basic ultrasonic test philosophy to the field of composite materials and adhesive bonding inspection. Unfortunately, many difficulties occur because of the inhomogeneous and anisotropic characteristic of a composite material. This section includes a review of the physical principles associated with ultrasonic testing and the particular items that must receive special attention when inspecting composite materials or adhesively bonded sections of a structure. 

The majority of all work carried out to date makes use of arrival time analysis, and subsequently a wave velocity value for thickness measurement and for defect location. The wave velocity in a composite material is difficult to use precisely because of the anisotropic characteristics of a composite material. A typical wave or phase velocity profile in polar coordinates is illustrated in Figure 8, showing the nonspherical nature of the wave velocity distribution. The wave velocity distribution can even be more complex for a realistic composite material structure. Despite a possible weakness in the understanding of basic physics and wave propagation in a composite material, thickness measurement can still be carried out by using carefully selected calibration specimens and procedures. Calibration of a test instrument can therefore be carried out for subsequent thickness measurement and defect location analysis in a structure. 

FIGURE 16. Composite material thickness measurement and defect location (A mode display), (a) Typical A mode display for a homogeneous material (b) typical A mode display for an inhomogeneous material. 

In the sections that follow, illustrations of homogeneous and nonho-mogeneous dislocation nucleation are presented. The former case implies dislocation formation in a material system that is otherwise spatially uniform nucleation is equally likely at all locations. Nonhomogeneous nucleation, on the other hand, implies that spatial nonuniformity arising through configuration, material structure, or material defects renders certain sites in the structure far more susceptible to dislocation nucleation than other sites. 

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

Weld inspection duties of personnel responsible for judging the quahty of welding with regard to specifications have been treated (15). Some of these duties involve the visual inspection of welds to determine if they are of the proper size, location, and type and are free of defects. Specifications of materials used must be checked, as must equipment and procedures. 

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). 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

Nickel oxide is a classical nonstoichiometric oxide that has been studied intensively over the last 30-40 years. Despite this, there is still uncertainty about the electronic nature of the defects present. It is well accepted that the material is an oxygen-excess phase, and the structural defects present are vacancies on cation sites. Although it is certain that the electronic conductivity is by way of holes, there is still hesitancy about the best description of the location of these charge carriers. 

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