Estimating Method of Dynamic Fracture Toughness in C/C Composites

Abstract

Due to low impact strength of C/C composites, it is important to evaluate impact damage properties and impact strength. In this study, the relationship between the impact load and displacement on a drop weight impact test at a low impact speed is used to evaluate the estimating method for dynamic mode II and mixed-mode fracture toughness of C/C composites. The measured impact load is decomposed in approximation components and detail components, and is reduced higher mode of vibration by multiple resolution analysis in wavelet transform. As a basis developing experimental identification techniques for boundary conditions, a technique for end notched flexure (ENF) and mixed-mode flexure (MMF) specimen that a beam of C/C composites is proposed. The proposed technique enables one to determine, using data of load variations and displacement variations, the boundary conditions in terms of spring constants of elastic support, damping coefficients, impact velocity and equivalent masses. The results are as follows : (1) The Daubechies' wavelet is useful for smoothing the impact load signals by multiple resolution analysis in wavelet transform. (2) This boundary condition identification method by the numerical analysis of a load variation and a displacement variation is effective, and it can use for calculating the dynamic fracture toughness value of C/C composites.