Unlike many other mechanical tests that measure the stiffness and strength properties of a material, fracture mechanics testing addresses the resistance to growth of cracks in materials. A variety of specialized test methods, requiring specialized test fixtures, have been developed for different materials and for different loading modes, denoted as mode I (opening), mode II (shearing) and mode III (twisting), as well as mixed-mode conditions. Specialized fixtures and accessories used for commonly performed fracture toughness tests are listed.