Toughness instrumented impact tests

Yamamoto, I., Miyata, H., Kobayashi, T. (1990). Instrumented Impact Test and Evaluation of Fracture Toughness in Polymers. Proceedings Benibana Intern. Symposium October 8-11,1990. Yamagata, Japan, 184-189. 

Several experimental procedures are employed in order to give an estimation of the fracture toughness of nanocomposites, which makes it sometimes difficult to assess this feature. The simplest ones are those that employ either analogical or instrumented impact tests (Charpy, Izod, and falling weight), and the impact strength is... 

While instrumented impact testing can be quite simple (as in the instrumented Izod test described earlier), the real value of instrumented testing lies in the ability to isolate and control the various test parameters and to precisely record the output data. When applied properly, instrumented testing can provide valuable data on almost aspect of material toughness. The primary disadvantage of instrumented testing is that the equipment can be quite complex (and expensive). 

Instrumented Charpy impact tests on unnotched samples are conducted to determine the energy absorbing capability and dynamic fracture behaviour of the C/SiC composites. The dynamic fracture toughness (ak) is calculated using the following equation ... 

The failure load of a fracture toughness specimen depends on the rate of load application. If a cracked compact tension specimen of 5 mm thick polycarbonate is loaded in a tensile testing machine, there is time for a neck to develop from the crack tip, so plane stress fracture occurs at a crack velocity of 5ms . However, if the load is applied in 1ms by impact loading, plane strain fracture occurs at a low Xjc value, and the crack velocity exceeds 200 ms . Special instrumentation, having a quartz crystal force gauge that responds in 0.1ms, and a grid of resistance lines on the surface to monitor the crack velocity, are needed to measure the Kic value. 

Figure 1.73 illustrates the instrumented CIT system in a block diagram and the impact-response curve method, as applied to the above tests. In addition to the aforementioned partially stabilized zirconia (PSZ ZrOi-S mol%Y203), samples of S3N4 were also investigated by Kobayashi et al. [32], Typical hammer load-times and strain-gage signal-time curves of PSZ and Si3N4 are found in Fig. 1.74. The impact-response curves of PSZ and S3N4, at several impact velocities are shown in Fig. 1.75. The impact curves of these specimens are impact-velocity-dependent. This technique enables the determination of the dynamic fracture toughness of these ceramics by means of the impact-response curve method. Impact-response curves quantitatively relate the response of the specimen to the impact, which depends solely on the elastic reaction between the specimen and the actual impact.

Since the impact toughness of a material, as it is commonly called, depends on the technique of measurement, the testing instruments and the technique must be considered first. 

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