Dynamic fracture initiation in brittle materials under combined mode I/II loading

A new test method has been developed to measure the resistance of dynamic fracture initiation in brittle materials under combined mode I/I loadings. The Brazilian disks with center-cracks have been fractured under oblique impact loadings in diarnetral-compression. The dynamic stress intensity factors of mode I and H are evaluated from the superposition integrals of the step response functions for the cracked disk. The experimental results are presented to elucidate the influence of loading rate on the combined mode fracture toughness for ceramics and glasses. 1. I N T R O D U C T I O N In an ordinary case that an impact load acts on a structure, stress waves obliquely impinge to various cracklike flaws in the components. The stress waves produce combined mode stress states around the crack-tips. Thus the dynamic fracture initiation criterion under combined mode loadings has to elucidate for assessments of structural integrity in relation to the impact fracture. Accurate evaluations of the dynamic fracture toughness can be achieved by development of fully dynamic test procedures including precise analyses of dynamic stress intensity factors for all modes. In order to assess pure mode I, pure mode I and combined mode fracture toughness of brittle materials, center-cracked Brazilian disk specimens have been used under quasi-static loading [1,2]. Combined mode stress states in the disk specimens can be controlled by selecting the inclination angle 0 of the center-precrack relative to the line of loading. Pure mode I is obtained a t the angle BI = 0" by loading in compression along a diameter through the precrack. Pure mode B is achieved at On 21 23' for a /R 21 0.5, where a is the half length of the center-crack and R the radius of the disk [2,3]. This quasi-static fracture test technique in diametral-compression is particularly attractive because of the simple loading and specimen geometry. Thus we have developed a new test method for combined mode impact fracture using the center-cracked disk specimen. The impact fracture tests can be performed under arbitrary combinations of mode I and 1 by varying the inclination Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19948106 C8-696 JOURNAL DE PHYSIQUE IV