| In this investigation, The crack opening stresses of a crack emanating from an edge notched specimens in three different materials a 2024-T351 aluminum alloy, a 1045 as-received steel and a 1045 quenched and tempered steel were measured under three different Society of Automotive Engineers (SAE) standard service load histories for a number of different average mean stress levels.; The fatigue notch model calculations were based on elastic and plastic notch strain calculations based on Neuber's formula, a crack opening stress calculation using the crack opening stress model, and a crack growth rate curve obtained during closure-free crack growth. The average measured crack opening stresses were within between 0.6 and 4.7 percent of the average calculated crack opening stresses.; In the interest of simplifying the use of crack-opening stress in design, the average crack-opening stress was correlated with the accumulative number of occurrence of the cycle that reduced the crack-opening stress to the average level. The use of a crack-opening stress level corresponding to the cycle that caused a reduction to a crack opening stress level reached once per 200 cycles gave a conservative estimate of fatigue life for the soft metals 2024-T351 aluminum alloy and 1045 as received steel, while for the 1045 quenched and tempered steel use of the cycle that caused a reduction to a crack opening stress level reached once per 10 cycles gave a conservative life estimate.; The results showed that at a constant strain amplitude of near the fatigue limit only one shear crack initiated and propagated in both the length and depth direction maintaining a geometry shape factor of about 0.75 for blunt and sharp notches. On the other hand, at a constant strain amplitude of twice the fatigue limit multiple shear cracks initiated at numerous locations on the specimen surface of both materials, and grew in both the length and depth directions. They linked up after overlapping and maintained a geometry factor of about 1.1. |
