A Study on Fatigue Crack Propagation in Copper under Superimposed Sinusoidal Loading

Abstract

The notched copper specimens of rectangular cross section were fatigued by the cyclic bending tests using superimposed sinusoidal and square stress patterns. One of the superimposed sinusoidal stress patterns was the sinusoidal stress superimposed only on the compression side of square stress wave, and the other the sinusoidal stress wave superimposed both on the tension and compression sides of square wave. All sinusoidal stress waves were superimposed in the pulsating tensile or compressive condition. Based on these test results and the fractographic observations, the crack propagation behavior under the superimposed sinusoidal loading was discussed.The crack propagation rate under sinusoidal stress wave superimposed only on the compression side of square stress wave is slightly larger than that in the simple square stress wave having the maximum tensile and compressive stresses equal to those of the superimposed stress. On the other hand, the crack propagation rate remarkably increases as the amplitude of the superimposed sinusoidal stress wave increases. Thus it is concluded that the increase in the crack propagation rate under superimposed loading only on compression side is mainly due to the increase in the amplitude between the maximum tensile stress and the maximum compressive stress.Fractographic observations show that the sinusoidal stress wave superimposed on the compression side of square stress wave does not form the striation. The slightly larger crack propagation rate in the sinusoidal stress wave only on the compression side of square wave than that in the simple square stress wave equivalent to this might be due to the effect that the superimposed sinusoidal stress in pulsating compression develops the substructure at the crack tip. The striations formed under the sinusoidal stress superimposed both on tension and compression sides of square wave are composed of those corresponding to the pulsating tensile stress and the clear grooves corresponding to the pulsating compressive stress.