Tensile testing, plastics standard mechanical tests

Turner, S., Mechanical Testing of Plastics, The Butterworth Group, London (1973). ASTM D 638-95 Standard Test Method for Tensile Properties of Plastics, Philadelphia (1996). 

The most important mechanical property of a plastic is its tensile stress-strain curve (Figs. 10-1 and 7-4). This curve is obtained by stretching a sample in a testing machine and measuring its extension and the load required to reach this extension. Plastics show viscoelastic behavior (as reviewed in Chapter 1) that is highly sensitive to temperature and, in some materials, to relative humidity variations so it is important to use samples of standard shapes, preconditioned at constant and standard temperature and relative humidity before testing. Requirements are explained in the ASTM specifications. 

Evaluation of chemical resistance may establish the potential for extraction of plasticizer incorporated in the material as well as an effect of plasticizer on durability of tested material. The standard contains information on testing chemical resistance with 50 test liquids using two methods immersion test and test under mechanical stress. The list of test liquids includes white oil which may be regarded as the only example of plasticizer among test liquids. Samples of known dimensions and weights are immersed in selected liquids for 168 h at room temperature. Containers are stirred every 24 h. Changes in appearance are recorded and samples can be subjected to mechanical property testing. Tensile properties of immersed samples are most frequently compared with control samples but other mechanical tests may also be used. 

Manufacturers handbooks give data on tensile and flexural yield or fracture stress, tensile elongation at break, notched Charpy or Izod impact strength, and fatigue life. Whilst these data are useful for materials selection, they are not always sufficient for quantitative design. To date, fracture mechanics has been used to only a limited extent in design with plastics, and data are not provided in data books. The lack of approved standards for testing of polymers is currently a major obstacle to the wider use of fracture mechanics. 

Fracture tests on adhesive joints are always more complex than tests on bulk adhesive specimens and a G rather than a K approach is invariably followed for their analysis. The adhesive is present as a thin layer, it may be constrained by the presence of nearby substrates and the failure paths may be influenced by the poor adhesion with the substrate. Fracture tests on adhesive joints are commonly conducted in mode I (tensile opening mode), mode II (inplane shear mode), and mixed-mode I/n (combinations of mode I and n). The key to success in all LEFM fi"acture mechanic tests is to ensure that the substrates do not deform plastically during loading. Plastic deformation of the substrates would violate the assumptions of LEFM and invahdate the results. In mode I, the DCB and TDCB test specimens are almost universally employed and these tests have been standardized. Either test maybe used for the determination of Gic- The choice of which to use is likely to depend upon factors such as the toughness of the adhesive, the properties of the substrate material employed, and whether or not crack length... 

---