9.3.2.3 Identification of Strain Rates The strain history of Fig. 9.3d is given in Fig. 9.5a. It was obtained from a strain gauge mounted on the specimen. The nearly constant strain rate can be identified as 2/s by taking the derivative of the largely linear portion of the strain history as shown in Fig. 9.5a. The corresponding stress–strain relation for the nearly constant strain rate of 2/s is given in Fig. 9.5b. The Young’s modulus and the yielding stress can then be identified as 30 GPa and 165 MPa, respectively. Tests for higher strain rates of up to 63/s were also performed and the resulting Young’s moduli and yielding stresses at several strain rates are summarized in Table 9.1. 9.3.2.4 Higher Strain Rates As far as the concern of force balance, thin specimens held with metallic angles were useful for strain rate testing up to 8/s. For higher strain rates, thick specimens that are constructed by bonding four thin specimens together are required. Figure 9.6a shows the result obtained from a thick specimen at a strain rate of 18/s. It was also found that thick specimen was useful for strain rates up to 63/s. As shown in Fig. 9.6b, however, the top force and the bottom force deviate when the strain rate reaches 125/s. A softer shaper will be required to achieve the balance of the forces between the two load cells although it is also recognized that the strain rate will decrease as the softer shaper is employed. Possible Young’s modulus and yielding stress based on Fig. 9.6b also presented in Table 9.1. a b stress-strain relation strain history and strain rate Fig. 9.5 DWIT based compressive testing results Table 9.1 Young’s modulus and yielding stress at various strain rates Strain rate (1/s) 0 2 3 8 18 30 63 125 200 250 Young’s modulus (GPa) 28 30 30 34 31 29 30 31 26.5 23.4 Yielding stress (MPa) 160 165 185 250 275 160 270 150 160 265 strain rate: 18/s strain rate: 125/s a b Fig. 9.6 Force histories measured by load cells at different compressive strain rates 9 A Testing Technique for Characterizing Composite at Strain Rates up to 100/s 77
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