アルミニウム合金の疲労強度およびき裂進展特性に及ぼす圧縮平均応力の影響

抄録

The present study investigated the effect of a compressive mean stress on the fatigue strength and crack growth behavior using smooth and notched specimens, respectively, for aluminum alloys of 6061, 6066 and 7075 in dry and humid air environments by an ultrasonic fatigue testing machine. Very high cycle fatigue strength of the smooth specimen of 6061 and 6066 alloys were insensitive to the relative humidity of the air, while it was much reduced in humid air than in dry air for 7075 alloy. The influence of the compressive mean stress on the fatigue strength was small for all the alloys regardless of the relative humidity. Crack growth characteristics tested by the notched specimen with precrack revealed that the crack began to grow under stress intensity factor range, ΔK, constant condition when the tensile component of ΔK, i.e. maximum stress intensity factor, K_max, exceeded the threshold value of effective stress intensity factor range, ΔK_eff,th, obtained by crack growth tests at high stress ratio. Crack growth life was predicted by the fatigue crack growth characteristics obtained by the latter tests. Also, it was compared with the fatigue strength characteristics and the small crack initiation and growth behavior obtained by the former tests. The results suggested that the crack closure point reduced to be compression side by the compressive mean stress. Therefore, a fatigue life under compressive mean stress could be evaluated conservatively by a full load range including compressive component.