strength-and-danger-of-crack-lengthThe greater the utilized strength of a material, thesmaller the flaws must be. This means that testingprocedures play a decisive role.KIC ˜ ac Kth ˜ ath Life spanload changesCrack lengthFracture ac1ac2 ath1ath2Harder material "1"under correspondinglyhigh loadsLess strong material "2"under correspondinglylower loadsL1L2 Higher strengths are utilized through higher loads. This leads to considerablesafety problems:With the same threshold (Kth) of fracture toughness and higher stress, the cracks size (ath) above which crack growth occurs becomes smaller. Due tothe higher loads the life span, i.e. number of load changes (L), decreases until failure. The crack growth rate increases and the critical crack length (ac), at which spontaneous failure occurs, becomes smaller.