Testing

When discussing tesdng they range from material to product testing. With no prior history or no related data available on a material or product, the usual approach is to conduct tests on the material and finished fabricated product. Choosing and tesdng a plasdc when only a few existed that could be used for specific products would prove relatively simple, but the variety of plastics has proliferated (35,000 worldwide). Today's plastics are also more complex, complicating not only the choice but also the necessary tests. Fillers and additives can drastically change the plastic's basic characteristics, blurring the line between commodity and engineering plastics (Chapter 1). Entirely new plastics have been introduced with esoteric molecular structures. Therefore, plastic suppliers now have many more sophisticated tests to determine which plastic best suits a product design or fabricating process.

For the product designer, however, a few basic tests, such as a tensile test, will help determine which plastic is best to meet the performance requirements of a product. At times, the complex test may be required. The test or tests to be used will depend on the product's performance requirements.

To ensure quality control, material suppliers and developers routinely measure such complex properties as molecular weight and its distribution, stereochemistry, crystallinity and crystalline lattice geometry, and detailed fracture characteristics. They use complex, specialized tests such as gel permeation chromatography, wide- and narrow-angle X-ray diffraction, scanning electron microscopy, and high-temperature pressurized solvent reaction tests to develop new polymers and plastics applications.

Understanding and proper applications of the many different tests is rather an endless project. There are destructive and nondestructive tests (NDTs). Most important, they are essential for determining the performance of plastic materials to be processed and of the finished fabricated products. Testing refers to the determination by technical means properties and performances to meet product performance requirements. This action, when possible, should involve application of established scientific principles and procedures. It requires specifying what requirements are to be met. There are many different tests (thousands) that can be conducted that relate to practically any product or material requirement. Usually only a few will be applicable to meet your specific application.

A different type of evaluation is the potential of a material that comes in contact with a medical patient to cause or incite the growth of malignant cells (that is, its carcinogenicity). It is among the issues addressed in the set of biocompatibility standards and tests developed as part 3 of ISO-10993 standard that pertain to genotoxicity, carcinogenicity, and reproductive toxicity. It describes carcinogenicity testing as a means to determine the tumorigenic potential of devices, materials, and/or extracts to either a single or multiple exposures over a period of the total life span of the test animal. The circumstance under which such an investigation may be required is given in part 1 of IS0 10993.

There is usually more dTan one test method to determine a performance because each test has its own behavior and meaning. As an example there are different tests used to determine the abrasion resistance of materials. There is the popular ASTM Taber abrasion test. It determines the weight loss of a plastic or other material after it is subjected to abrasion for a prescribed number of the abrader disk rotations (usually 1000). The abrader consists of an idling abrasive speed controlled rotating wheel with the load applied to the wheel. The abrasive action on the circular specimen is subjected to a rotary motion.

Other abrasion tests have other types of action such as back and forth motion, one direction, etc. These different tests provide different results that can have certain relations to the performance of a product that will be subjected to abrasion in service.

With the more popular destructive testing, the original configuration of a test specimen and/or product is changed, distorted, or usually destroyed. The test provides information such as the amount of force that the material can withstand before it exceeds its elastic limit and permanendy distorts (yield strength) or the amount of force needed to break it. These data are quantitative and can be used to design structural products that would withstand a certain static load, heavy traffic usage, etc.

The primary purposes of testing related to shock and vibration are to verify and characterize the dynamic response of the equipment and components thereof to a dynamic environment and to demonstrate that the final design will withstand the test environment specified for the product under evaluation (Chapter 2). Basic characterization testing is usually performed on an electrodynamic vibration machine with the unit under test hard-mounted to a vibration fixture that has no resonance in the pass band of the excitation spectrum. The test input is a low-displacement-level sinusoid that is slowly varied in frequency (swept) over the frequency range of interest. Since sweep testing produces a history of the response (displacement or acceleradon) at selected points on the equipment to sinusoidal excitadon over the tested excitadon frequencies and displacements.

Caudon is advised when using a hard-mount vibradon fixture, as the fixture is very stiff and capable of injecting more energy into a test specimen at specimen resonance than would be experienced in service. For this reason, the test-input signal should be of low amplitude. In service, the reaction of a less stiff mounting structure to the specimen at specimen resonance would significantly reduce the energy injected into the specimen. If a specimen response history is known prior to testing, the test system may be set to control input levels to reproduce the response history as measured by a control accelerometer placed at the location on the test specimen where the field vibration history was measured.

Vibration-test information is used to aid in adjusting the design to avoid unfavorable responses to service excitation, such as the occurrence of coupled resonance. It is a component having a resonance frequency coincident with the resonance frequency of its supporting structure, or structure having a significant resonance which coincides with the frequency of an input shock spectrum. Individual components are often tested to determine and document the excitation levels and frequencies at which they do not perform. This type of testing is fundamental to both shock and vibration design.

For more complex vibration-service input spectra, such as multiple sinusoidal or random vibration spectra, additional testing is performed, using the more complex input waveform on product elements to gain assurance that the responses thereof are predictable. The final test exposes the equipment to specified vibration frequencies, levels, and duration, which may vary by axis of excitation and may be combined with other variables such as temperature, humidity, and altitude environments,

NDT examines material without impairing its ultimate usefulness. It does not distort the specimen and provides useful data. NDT allows suppositions about the shape, severity, extent, distribution, and location of such internal and subsurface residual stresses; defects such as voids, shrinkage, cracks, etc. Test methods include acoustic emission, radiography, IR spectroscopy, x-ray spectroscopy, magnetic resonance spectroscopy, ultrasonic, liquid penetrant, photoelastic stress analysis, vision system, holography, electrical analysis, magnetic flux field, manual tapping, microwave, and birefringence (Table 7.1).

To determine the strength and endurance of a material under stress, it is necessary to characterize its mechanical behavior. Moduli, strain, strength, toughness, etc. can be measured microscopically in addition to conventional testing methods. These parameters are useful for design and material selection. They have to be understood as to applying their mechanisms of deformation and fracture because of the viscoelastic behavior of plastics (Chapters 1 and 2). The fracture behavior of materials, especially microscopically brittle materials, is governed by the microscopic mechanisms operating in a heterogeneous zone at the crack tip or stress raising flow.

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs (thermoplastics), craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented microfibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction techniques, scanning electron microscope, and transmission electron microscopy.

Conditioning procedures of test specimens and products are important in order to obtain reliable, comparable, and repeatable data within the same or different testing laboratories. Procedures are described in various specifications or standards such as having a standard laboratory atmosphere [50 ± 2% relative humidity, 73.4 ± I.8F (23 ± 1C)] with adequate air circulation around all specimens. The reason for this type or other conditioning is due to the fact that the temperature and moisture content of plastics affects different properties.

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