Laboratory tests ductility

Breakage of a component or article is frequently a limitation on its use this subject has, therefore, received considerable attention. Failure may be brittle or ductile, the former occurring at low elongation, the latter with considerable deformation, and as the culmination of creep. Brittle failure in plastics materials is sometimes inherent and sometimes unexpected. Many laboratory tests indicate that most plastics are either tough or brittle, which is very convenient, since there are rules for dealing with ductile materials, and different rules for dealing with brittle ones. However, some plastics break at unexpectedly low stresses and, even more seriously, products made from plastics which are expected to be ductile from laboratory tests and general experience can also fail in a brittle manner. 

Considerations based on a fracture mechanics analysis are necessary if the designer is to progress beyond the primary division of plastics into ductile or brittle , based on simple laboratory tests (see Chapter 2). 

CHARPY KEYHOLE IMPACT TESTS. The results of the Charpy keyhole impact tests are shown in Fig. 8. This test is also a severe test, combining all three of the embrittling service conditions. It is one of the laboratory tests most likely to produce a ductile—brittle transition in a material that is susceptible to brittle fracture. 

Recently published laboratory creep studies22 have shown that the rupture strength and rupture ductility of 2.25Cr-lMo steel are diminished when tested in hydrogen as compared to their values in air. These tests were a continuation of previously-reported tests23, 25 that showed somewhat conflicting results in shorter term tests. 

Impact measurement. The impact tests were conducted in the usual manner on single sheets with a Gardner Laboratory Impact Tester (Model IG-1120) with a 0.625" diameter punch hammer. At least ten drops were performed in the center 50% of the sheets and with points of impact at least 1" apart. The failure criterion was a brittle (not ductile or tear) break. The values reported in Table I are 50% probabilities (energy at which 50% of failures are brittle). The standard deviation of the values is about 10%. 

Most of the properties given in Table 14.1 are relatively standard physical property measurements with standard methods referenced. The notched Izod ductile-brittle transition temperatiure is an approximate method to estimate the midpoint temperatiure at which notched Izod behavior shifts from fully ductile to fully brittle. It may be very sensitive to a variety of differences among samples. It is used here only for iUus-trative purposes to show the low-temperature impact capabihty hmits as measured by this notched Izod test. Each user should determine the relevance of this small-scale laboratory impact test on small laboratory-molded parts to full-scale molded parts at end-use conditions for a given application. 

The falling weight or dart drop test method simulates actual day-to-day abuse and can be carried out either on standard laboratory specimens or on the articles themselves. Failure may occur in various ways ranging from brittle to ductile failure (Figure 14). Particular care must be taken to avoid the brittle failure by proper selection of grade. At temperatures below -20 °C, elastomer-modified PP is more impact resistant than PP copolymer and homopolymer. 

As the results of the research began to filter into practice, a number of new design technologies became popular. For example, for the successful design of a T-stub coimection (Fig. 21a), it is necessary to first determine all the possible ductile and brittle failure modes and prioritize them from most brittle to most ductile. One approach to this task is the component approach, in which each deformation mechanism in a joint is identified and individually quantified through a series of small laboratory component tests and associated analytical studies. These tests are carefully designed to measure one deformation component at a time. Each of these components is then represented by a spring with either linear or nonlinear characteristics (Fig. 21b). These... 

Prior to the advent of fracture mechanics as a scientific discipline, impact testing techniques were estabhshed to ascertain the fracture characteristics of materials at high loading rates. It was realized that the results of laboratory tensile tests (at low loading rates) could not be extrapolated to predict fracture behavior. For example, under some circumstances, normally ductile metals fracture abruptly and with very little plastic deformation imder high loading rates. Impact test conditions were chosen to represent those most severe relative to the potential for fracture —namely, (1) deformation at a relatively low temperature, (2) a high strain rate (i.e., rate of deformation), and (3) a triaxial stress state (which may be introduced by the presence of a notch). 

Test specimens loaded under laboratory conditions, but also regular engineering components fracturing in service, provide many different data which lend themselves to evaluation of the fracture process. These data are, for instance, the time to onset and completion of fracture, details of the fracture pattern (ductile or brittle fracture, appearance of the breaking specimen and of the fracture surface), crack dynamics, and change in physical or chemical properties. Naturally the most straightforward evaluation of a test or a set of data is the direct correlation of the property of interest (e.g., time under stress) to the environmental parameter(s) of interest (e.g., stress and temperature). In Figure 1.4 a set of data with just these variables has been plotted (PVC pipes under internal pressure). If one uses such a plot a number of questions arise ... 

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