To study the effect of multiimpact damage and highcycle shear fatigue on the damage evolution

buckling behavior and failure modes of stiffened composite panels

nine stiffened composite panels were manufactured with similar configurations

and the impact test

highcycle shear fatigue test and residual shear strength test were designed. During the multiimpact and highcycle shear fatigue tests

the damaged zones were detected by the ultrasonic Cscan system. Cscan images indicated that the length and width of damaged zones increased with the increasing number of cycles. Compared with initial undamaged specimen

the average failure load of specimens with multiimpact damage and highcycle shear fatigue declined about 50%. Furthermore

the failure modes of the specimens were also affected by the initial damage caused by impact or fatigue. For the impact and fatigue specimens

the deformation of local buckling skin appeared

and the failure cracks were much near the impact damaged zones.