Fatigue tests have been completed using a weldable aluminium alloy NS 8 over a range of alternating and mean stress levels. Specimens have been cut from thick plate material containing either a typical production joint or one of a range of notches. Results have enabled a comparison to be made between the fatigue strengths of welded joints with those for specimen containing stress concentrations. To describe fatigue curves analytically a general empirical fatigue equation has been developed and found to fit results with good accuracy. This equation exhibits the characteristic shape of the S-N diagram and also distributes mean stress curves according to a Goodman (linear) law relationship with the alternating stress at all fatigue lives. An analytical study of random signals has resulted in the development of a general probability density function which is suitable for describing complex random load spectra on a fatigue diagram. This function, together with the empirical fatigue equation and expression obtained from notch specimen results, have been used in a general design computer programme that enables the fatigue life of a component section, of known stress concentration factor, to be predicted for a given complex random load environment. The design programme requires a limited knowledge of the material constant amplitude fatigue behaviour which is obtainable from a small number of fatigue tests. This technique has been confirmed by random load fatigue tests performed on similar specimens.
|Date of Award||1980|