Nondestructive testing methods, discussion

Handbook of Polymer Testing provides in one volume that comprehensive coverage of physical test methods for polymers. The properties considered cover the whole range of physical parameters, including mechanical, optical, electrical, and thermal as well as resistance to degradation, nondestructive testing, and tests for processability. All the main polymer classes are included rubbers, plastics, foams, textiles, coated fabrics, and composites. For each property, the fundamental principles and approaches arc discussed and particular requirements and the relevant international and national standards for the different polymer classes considered, together with the most-up-to-date techniques. 

The impedance test for anodized aluminum (ASTM B 457) is used to study the seal performance of anodized aluminum. In this sense, the test is similar to the FACT test, except that this method uses a 1 V root mean square 1 kHz signal source from an impedance bridge to determine the sealed anodized aluminum impedance. The test area is again defined with a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5% NaCl. The impedance is determined in ohms X 10. In contrast to the methods discussed previously, this test is essentially nondestructive and does not accelerate the corrosion process. 

As discussed in the previous chapter a preparation procedure leads to several sets of samples, often produced in batches. It is necessary to assess that no difference exists within each of the sets, between sets and between batches. Consequently, homogeneity testing will try to measure differences between sub-samples within or between vials of materials. As it is not possible to measure all samples produced (unless a nondestructive technique is available) a strategy for the selection of representative samples is necessary. To demonstrate the absence or the existence of differences between samples, it is necessary that the analytical procedure is fully reproducible. If differences between measurements are too large due to the measurement method, inhomogeneity cannot be detected. In order to reveal presence of spot contamination, the measurement must be done on the substance of interest or any other substance known to present exactly the same properties and showing the same behaviour or distribution pattern (tracer). 

The nature of the specimen and the question that has been raised about it determine the course of action and the avenue of the analysis. After consideration of all known facts, and after nondestructive observations are completed, a particular test is selected in an attempt to answer a specific question. The results of this test determine the next step chosen. Some of the methods that may be employed for specific groups of polymers are discussed in the following sections. Microscopic and spectroscopic methods yield a great amount of information in the least amoimt of time, and, subsequently, are most frequently used by forensic trace examiners. Therefore, microscopy and spectroscopy are discussed in the greatest depth. 

The literature on NDT of polymers and PMC comprises the classes shown in Table 1. References illustrate the available range of literature or a specific topic. Omission or inclusion of a reference does not imply a rating by the authors. The amount of information available in electronic format only is steadily increasing, eg, References 2-8, and many journals feature electronic on-line editions. Electronic database searches are sensitive to spelling, eg, non-destructive yields 27, and nondestructive 384, active standards of the American Society for Testing and Materials International. Most NDT methods apply to various material classes. Those documents that explicitly mention polymers or PMC deal with effects specific to certain materials, eg, the Felicity effect in PMC (9), or with polymer products used in large quantities (10-14). Standards tend to lag behind technical developments, as discussed for acoustic emission (15), but similar arguments hold for other NDT methods. 

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