Fatigue Life Reduction Caused by Surface Roughness inside Bolt Holes

摘要

The difference in fatigue life between perfectly reamed bolt holes, i.e., those with maximum roughness height (R max) 4 microns, and worst acceptable bolt hole (R max = 12 microns) was studied. The influence of scratches (R max = 30 to 60 microns) was also studied. Aluminum joint specimens were subjected to loading, using the FALSTAFF flight load spectrum at a gross area stress = 234 MPa. Holes with R max 4 microns and R max 12 microns do not show any significant difference in fatigue life. Surface irregularities of this order of magnitude do not act as crack starters. For reamed holes, cracks are initatied at inclusions in the bore of the hole. Axial scratches with R max = 30 to 60 microns reduce fatigue life. Expanded holes are less sensitive to surface roughness than reamed holes. The detrimental effect of scratches is most pronounced for reamed holes at large load transfer. For double shear large load transfer specimens, scratches reduce fatigue life by a factor of 7.