Non-Propagating Cracks in Vee- Notched Specimens Subject to Fatigue Loading

Abstract

SummaryReversed direct stress and rotating bending fatigue tests have been carried out on Vee-notched specimens of aluminium alloy, nickel chromium steel and mild steel.Diagrams are presented showing the relationship between the geometric stress concentration factor Kt and the strength reduction factor Kt. It was found that non-propagating cracks formed in the roots of the sharper notches. These cracks formed at or above some critical value of Kt, the value depending on the specimen material. Below the critical value of Kt, cracks did not form unless the applied nominal stress exceeded that at the fatigue limit, and a crack, once formed, continued to propagate until the specimen failed. Above the critical value of Kt, non-propagating cracks formed at nominal stresses less than the fatigue limit, the stress having to be increased to the latter value in order to propagate the crack. This critical value of Kt coincided with the maximum Kf value realised. It would appear that Kf equals Kt up to a certain value of Kt; there is then a transition where Kt reaches a maximum at the critical value of Kt. Increasing the value of Kt above the critical value causes no further increase and may tend to decrease the value of Kt.The conclusions drawn apply only when the fatigue stresses are completely reversed, i.e. the mean load is zero.