Fatigue

When reviewing fatigue one studies their behaviors of having materials under cyclic loads at levels of stress below their static yield strength. Fatigue test, analogous to static creep tests, provides information on the failure of materials under repeated stresses. The more conventional short-term tests give little indication about the lifetime of an object subjected to vibrations or repeated deformations. When sizing products so that they can be modeled on a computer, the designer needs a starting point until feedback is received from the modeling. The stress level to be obtained should be less than the yield strength. A starting point is to estimate the static load to be carried, to find the level of vibration testing in G levels, to assume that the part vibrates with a magnification of 10, and to multiply these together to get an equivalent static load. The computer design model will permit making design changes within the required limits.

If the loading were applied only once the magnitude of the stresses and strains induced would be so low that they would not be expected to cause failure. With repeated constant load amplitude tests, maximum material stress is fixed, regardless of any decay in the modulus of elasticity of the material. Constant deflection amplitude fatigue testing is less demanding, because any decay in the modulus of elasticity of the material due to hysteretic heating would lead to lower material stress at the fixed maximum specimen deflection.

Material fatigue data are normally presented in constant stress (S) amplitude or constant (s) strain amplitude plotted vs. the number of cycles (N) to specimen failure to produce a fatigue endurance S-N

Figure 3. I l Typical S-N curve

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