Yield strength and fracture toughness belong to the most important mechanical properties of materials. The yield strength plays also an important role in fatigue resistance. However, the microstructural features leading to high yield strength often have the opposite effect on the crack growth threshold. Moreover, the yield strength or other required material properties are often deteriorated under cyclic loading due to changes in microstructure. Therefore, this should become a subject of consideration when designing new materials. The crack growth threshold becomes very important for estimation of the residual fatigue lifetime, since it cuts off the majority of the loading amplitudes. A change of the threshold by 1 MPam^0.5 may result in a great difference in the residual fatigue lifetime.
The significance, quantification and mechanism of each of the components of the crack growth threshold will be discussed in this contribution. The resistance is divided into the intrinsic (effective threshold) and the extrinsic (crack tip shielding) components. In many cases the crack tip shielding, mostly represented by crack closure, is more significant than the effective threshold.
For metallic materials, the effective threshold is a well predictable parameter independent of the stress ratio and the yield strength (for conventional grain size materials). It has relatively low and nearly equal values for all alloys based on a particular metal matrix, e.g., for steels it is approximately 3 MPam^0.5 (for Ti alloys ≈ 2 MPam^0.5, for Al alloys ≈ 1 MPam^0.5). The same trend was also found for the mode II and mode III effective thresholds, where the crack tip shielding component is even more significant due to friction between fracture surfaces. Therefore, there are not many options to influence it and many material investigations focus on the crack tip shielding component of the threshold. Experimental methods, quantitative models and factors influencing the components of crack closure will be discussed. The up-to-date results of investigations of crack closure in metallic materials and understanding of the mechanisms can provide a good basis for studying of these phenomena in new and advanced materials.