Experimental And Numerical Study On Fatigue Life Enhancing Effect Of 300M Lugs

Fastener hole is a typical stress concentration in aircraft structure.It threatens the safety of the aircraft and significantly reduces the fatigue life.To improve the fatigue and stress corrosion resistance of the fasten hole,compressive residual stress around the fastener hole was uniformly introduced by using the traditional fatigue strengthening techniques,for examples,cold expansion,interference fit and shot peening.In this thesis,the stress concentrations introduced by different processing and the residual stress distribution induced by cold expansion,interference fit and shot peening are widely studied by using finite element method.Furthermore,fatigue test on the lugs strengthened by different techniques are carried to quantitatively obtain the fatigue strengthening effect.This research is useful for anti-fatigue design of a lug made of 300 M steel.The stress concentration factors of lugs without and with chamfered edge and round fillet are obtained by using the commercial finite element software ABAQUS.Compared to the lug without further manufacture,round fillet can reduce the hole edge stress concentration,and chamfered edge will increase the hole edge stress concentration factor.By using elastic-plastic finite element method,the residual stress distribution of a lug strengthened by cold expansion,interference fit and shot peening are obtained.The finite element results show that lug sustain tensile stresses,with the distance from the hole edge increasing,tensile stress decreases first and then increases.It is also observed that the tensile stress of hole edge chamfering is larger than that of the hole edge round fillet.The finite element results show the tangential residual stress at the beginning of theextrusion is lower than that in the middle and end of the extrusion.Experimental fatigue test is designed and tested to quantitatively study the fatigue life improvement by different strengthening methods.Furthermore,the fatigue lives of lugs strengthened by different methods are predicted by using the classical fatigue model and the residual stress determined from the finite element analysis.The predicted fatigue life agrees well with the experiment result.From the fractography analysis of the strengthened lugs,the fatigue crack is initiated in the subsurface instead of the surface of the un-strengthened lugs.