Falling-weight impact tests

The normal day-to-day abuse experienced by an article is more closely simulated by the falling weight impact-type test (see BS 2782) [32]. It is also much easier to vary the type and thickness of specimens in this test. Furthermore, any directional weakness existing in the plane of the specimens is easily detected. 

ATS FAAR supplies the Fall-O-Scope Universal apparatus for free-falling dart impact resistance tests. The apparatus can be set to obtain the rate of energy absorption dnring the impact. The apparatus can be used to conduct tests to different specifications with computerized operation in the range 70°C-200°C. 

Besides the average level at which failures occur in the falling weight impact test, the type of failure can also be of importance in judging the relative impact performance of a material in practice. There are generally three types of failure  

in a given test, failures are all of the tough type, a reliable and reproducible measure of the impact strength of the sample can be obtained. If, however, differing types of failures are observed, a much wider variation in impact values can be obtained from repeat tests. 

In practice the lower level at which failure is likely to occur is important as the level at which 50% of the samples fail, i.e., Fjq, which is the value reported in the falling weight impact test. This can be obtained by repeating the falling weight test using the probit rather than the staircase procedure. 

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Falling Weight (Impact) Test. Ten tests with 0.05 to 0.1 Og samples, using an apparatus having an anvil 1.27cm(0.5") in diameter, should show that the explosive is less sensitive than Picric Acid (PA)... 

Figure 8.13 Dependence of two estimates on the impact strength on the amount of PDMS in PDMS-modified Si02 glasses.144 The impact strengths were obtained from (O) the Charpy pendulum impact test (A) the falling-weight impact test.

Mechanical Properties. The mechanical properties of the selected blends studied, including the results of tensile, Izod impact, and falling-weight impact tests, are summarized in Table I. Generally, lower-than-expected toughening efficiency (especially for Izod impact) and data scattering were obtained because it is hard to maintain consistent moisture content of the specimens (especially for blends in which PA is the major component). The presence of the compatibilizer in all the blends resulted in consistent and substantial improvement in properties (Nos. 2, 3, and 4 vs No. 1, and No. 20 vs... 

The simplest way to approach detection of brittleness point is quickly to bend the material while holding it at the low temperature, and a considerable number of tests of this type have been devised and some standardized. The degree of strain is often not precisely defined, and these tests tend to be operator-dependent, so that reproducibility can be poor. A more satisfactory approach is to impact a test piece at low temperatures. Adaptations of falling weight impact tests are tests are commonly used for rigid plastics, but for flexible materials a procedure in which a strip test piece held as a simple cantilever and impacted by a striker is very widely standardized. 

A new part to ISO 179 is under preparation that will cover the instrumentation of the Charpy pendulum so that force-time (and by integration, force-dcflection) curves can be obtained. This allows for a fuller characterization of the impact behavior of the plastic than can be derived from only the energy to break of the typical test. There has been an instrumented version of the falling weight impact test (see later in this section) for several years and the same principles apply to both. 

Fig. 8.41 Video images during the falling weight impact test for neat PA above) and PA/EGMA alloy below)...

Instrumented falling weight impact test were conducted on a home build machine. Details of which have already been given elsewhere (6). Test panels were circular clamped between metal plates with a circular opening of 80 mm. The mass of the falling weight was 0.9 kg. A hemispherical dart with a diameter of 10 mm was used in all cases. 

The cause of failure of sheet material in practice is more likely to be revealed by a falling-weight impact test than by an Izod test (14). Falling-weight tests thus... 

Falling Weight Impact Tests Further Discussion... 

In the falling-weight impact test a circular disc of material (typically 6 cm diameter and 2 mm thickness, freely supported on an annulus of 4 cm diameter) is impacted by a metal dart with a hemispherical tip (t3q5ically of radius 1 cm). The tests are carried out either under conditions where the impact energy is far in excess of that required to break the specimen or at low levels of impact energy so that damage tolerance and the possible initiation of a crack can be observed. 

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