13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Fig. 2 shows different strain gage instrumentations which were compared with respect to their force measurement capability. All strain gages were statically calibrated before the tests. Figure 2. Strain gage instrumentations for direct force measurement on SE(B)25-specimens, blue: ASTM full bridge at specimen quarter points, red: 2 BS half bridges at W/2. Basic principle of low blow tests is that a defined amount of energy is transferred to the specimen by a single hit of the striker causing deformation and stable crack growth. Therefore, it has either to be realized that second and following hits of the rebounded striker are prevented or that they are of such a magnitude that they will not cause crack growth in the specimen. Catching of the rebounded striker is practically impossible due to the low rebound height and the corresponding short time. Fig. 3 and Fig. 4 show force-time records of a low blow test on a DCI SE(B)25-specimen with a0/W = 0.5 at -40 °C. The first hit of the striker causes the first force peak (low blow test) and the peaks (impulses) 2 to 4 are caused by the successively rebounding striker. After the test a stable crack growth of 0.62 mm was measured on the fracture surface (aE/W = 0.52). As can be seen from comparison of the force signal height with the yield load of the specimen after the first hit (aE/W = 0.52) or even assuming a maximum crack length of amax/W = 0.55, the magnitude of the second and following impulses is clearly below the yield loads. This holds for BS- as well as ASTM-strain gage force measurement. The differences between ASTM- and BS-records will be discussed below. For the nonce, it can be concluded that second and following impulses cause only elastic deformation and do not contribute to crack growth so that they can be ignored. Figure 3. Force-time records of a low blow test, DCI, SE(B)25-specimen, a0/W = 0.5, -40 °C.
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