Fatigue crack growth thresholds - the influence of Young's modulus and fracture surface roughness

In the present investigation it is shown that the effective fatigue threshold is uniquely correlated to the Young's modulus for a wide range of metallic and composite materials (Delta K-th,K-eff = 1.64.10(-2).E). It is also demonstrated that the crack closure level K-cl increases with increased roughness of the fracture surface (H) over bar. K-cl and (H) over bar are quantitatively related via the equation K-cl = 1.2.(H) over bar(1/3) for steels with widely different mechanical properties and grain sizes (120 MPa < R-p < 1100 MPa, 1 mu m < mu < 100 mu m). This relation can be extended to materials other than steels (e.g. aluminium and WC-Co alloys) by normalising against Young's modulus. The roughness value (H) over bar represents the standard deviation of height of the fracture surface and is shown to be simply related to the length and angle distributions of the linear length elements constituting the fracture profile. (C) 1998 Elsevier Science Ltd. All rights reserved.