High-rate impact test

HE high frequency HRIT high-rate impact test... 

High-speed impact testers have been proven very useful in material evaluation. At low strain rates, some polymers fail in a ductile manner. The same polymers appear to show brittle failure at high strain rates. The point at which this ductile to brittle transition takes place is of particular importance. A high-rate impact test can provide such information in a graphical form. Tests can also be carried out at different temperatures to find ductile-brittle transition points at various temperatures (46). Other useful applications of the high-speed impact tester include the process quality control, design evaluation, and assembly evaluation. 

Table 3. Impact Resistance of Polypropylene (MTS High Rate Impact Test) ...

First, the role of rubber modification in high rate impact is to suppress spallation by inducing the material to yield in the presence of dynamic tensile stresses arising from impact. Second, this yield-spall transition occurs at different strain rates for different rubber contents and may be predictable using quasistatic, low temperature tests of this type. These tests can also provide information concerning the basic nature of the yield process in these materials through the activation parameters which are obtained. Third, the Bauwens-Crowet equation seems to be a good model for the rate and temperature sensitive behavior of the American Cyanamid materials and is therefore a likely candidate for a yield criterion to use in the analytical code work on these materials which we hope to perform as a continuation of this work. 

Two types of mechanical tests have been used the low rate of deformation tensile, compressive or bending tests and the high speed impact tests. Immiscibility of polymers is reflected in both. For example, in tensile tests the maximum strain at break (or the maximum elongation), and the yield stress (or the maximum strength) can be... 

The rate at which the impact occurs also affects the impact strength of a viscoelastic material. Often, it is very difficult to achieve very high rates of impact with conventional laboratory testing. In certain applications (military and aerospace) high-speed impact tests have been developed. In order to attain the high speeds required chemical explosives or electromagnetic energy are employed [59]. 

Many other important impact design parameters such as yield stress, energy to yield, initial modulus, and deformation at break can be measured with high-speed tension tests (38). In spite of the capability of high-rate tension tests to provide stress at strain rates that simulate actual service, the tests have not been popular because the delivered stress is uniaxial. Normally, the real-life impact stress is multiaxial. 

The biggest advantage of the falling-weight impact test over the pendulum impact test or high-rate tension test is its ability to duplicate the multidirectional impact stresses that a part would be subjected to in actual service. The other obvious advantage is the flexibility to use specimens of different sizes and shapes, including an actual part. Unlike the Izod impact test, which measures the notch... 

Most versatile high-speed impact testing machinery is capable of testing everything from the thin film which may require as low an impact rate as 30in./min to the plastic automotive bumper which may require an impact rate up to 30,000in./min. The specimen or product can be tested under a controlled environment of tempera-... 

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). 

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

Data was collected over a two-year period on the effect of water on DuPont s Zytel 101. In an Arrhenius plot of this data the failure point was the time when the elongation and impact strength started to decrease. This is not a chemical degradation, but rather a permeation or diffusion rate phenomenon. It shows that high temperature water tests can be used to predict normal temperature exposure results. 

The disruption and fragmentation of the reactor core as a result of the excursion hurled all the capsules from both locations at least 500 feet away. The Pb styphnate sample which had been mounted on the pressure vessel showed evidenoe of a high-order detonation, which was assumed to be the result of the high rate of energy input from the KIWI—TNT test. This assumption was based upon several premises. Primarily, the possibility of the material detonating as a result of impact rather than by radiation did not seem reasonable, since the other sample... 

The main characteristic of an impact test is the sudden application of a load at a high speed (typically, strain rates are in the range 10 1-102s 1. 

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