7 Evaluation of the Influence of Water Absorptivity on the Properties of CFRP Cylinder Materials by SHPB Impact Compression Test 63 Unit :mm Specimen Strain gauge 2000 1000 1000 1000 2500 1300 Fig. 7.3 Hopkinson bar method Fig. 7.4 Input and output bar side stress after calibration (0.1~0.4 MPa) strength and strain rate can be measured. Stress σ, strain rate, and strain can be expressed by using an incident, reflected, and transmitted strain waves εi, εr, and εt. σ = EA 2AS (εi +εr +εt ) ˙ε = E ρC0lS (εi −εr −εt ) ε = t 0˙ εdt where E, ρ, C0, A, As, and ls are Young’s modulus of the input and output bars, the density of bars, the velocity of elastic wave, area of bars, area of the specimen, and gauge length of the specimen. The accuracy of the SHPB testing machine used in this study was verified. The testing machine was calibrated under air pressure 0.1~0.4 MPa. The input rod and output rod of the testing machine were directly fitted together without the specimen. The stress wave after striker collision was measured from strain gauges bonded to input and output bars. Based on the one-dimensional stress wave theory, the axial stress at the joint parts between the input and output ends was calculated. Figure 7.4 is shown the axial stresses. Furthermore, since noise occurred due to the deflection of the outer end of the input and output bars, the deflection of the end was eliminated by using a jig.
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