Crack growth: Does microstructure play a role?

Abstract

The experimental data presented in this paper reveals that even if the growth of long cracks in two materials, with different microstructures, have different ⁄ versus ∆ curves the corresponding small crack curves can be similar. We also see that long cracks in a large range of steels with different microstructures, chemical compositions, and yield stresses can have similar crack growth rates. The materials science community is challenged to explain these observations. The experimental data also suggests that the threshold term ∆ thr in the Hartman-Schijve variant of the NASGRO crack growth equation appears to have the potential to quantify the way in which small cracks interact with the local microstructure. In this context it is also noted that the variability in the life of operational aircraft is controlled by the probability distribution associated with the size and nature of the material discontinuities in the airframe rather than the probability distribution associated with the scatter in the growth of small cracks with a fixed initial size.

Effect of corrosion and fatigue on the remaining life of structures and its implication to additive manufacturing

“…This hypothesis is supported by the fact that the da/dN versus ΔK curves associated with the growth of small cracks in a 350 MPa mild steel [43] is similar to the growth of fatigue cracks in both conventionally manufactured Ti-6Al-4V and in in Additively Manufactured LENS (Laser engineered net surface) Ti-6Al-4V. To illustrate this Figure 13 presents the da/dN versus ΔK curves associated with the growth of small cracks in a 350 MPa mild steel [43], which were tested at a range of R ratios, together with: i) the R = 0.1 short crack curve for Mill Annealed Ti-6Al-4V [44], ii) the small crack da/dN versus ΔK curve for the growth of small cracks in Additively Manufactured LENS (Laser engineered net surface) Ti-6Al-4V [45]. T iii) the small crack da/dN versus ΔK curve for the growth of small cracks in the aerospace quality titanium alloys Ti-6246 [46] and Ti-17 [47].…”

“…The present paper therefore suggests that the methodology outlined above has the potential to study the growth of small naturally occurring cracks that arise and grow form rough surfaces in Additively Manufactured Ti-6Al-4V. This hypothesis is supported by the fact that the da/dN versus ΔK curves associated with the growth of small cracks in a 350 MPa mild steel [43] is similar to the growth of fatigue cracks in both conventionally manufactured Ti-6Al-4V and in in Additively Manufactured LENS (Laser engineered net surface) Ti-6Al-4V. To illustrate this Figure 13 presents the da/dN versus ΔK curves associated with the growth of small cracks in a 350 MPa mild steel [43], which were tested at a range of R ratios, together with: i) the R = 0.1 short crack curve for Mill Annealed Ti-6Al-4V [44], ii) the small crack da/dN versus ΔK curve for the growth of small cracks in Additively Manufactured LENS (Laser engineered net surface) Ti-6Al-4V [45].…”

Local Monte Carlo Method for Fatigue Analysis of Coarse-Grained Metals with a Nanograined Surface Layer

“…For small fatigue cracks, both fine-grained metals and coarse-grained (CG) metals have a small fatigue threshold and thus a similar crack growth rate [3,4]. Equivalently, microstructural effects often vanish for small, naturally-occurring cracks [3,4]. For long fatigue cracks, compared with fine-grained metals, CG metals possess a larger fatigue threshold and thus a lower crack growth rate [3,4].…”

“…Equivalently, microstructural effects often vanish for small, naturally-occurring cracks [3,4]. For long fatigue cracks, compared with fine-grained metals, CG metals possess a larger fatigue threshold and thus a lower crack growth rate [3,4]. Therefore, metals with a nanograined surface layer and coarse-grained (CG) interior hold promise for applications against fatigue failure.…”

“…The way that microstructures affect the growth of both small and long fatigue cracks can be captured by changing the fatigue threshold term in the Hartman-Schijve variant of the NASGRO crack growth equation [2]. For small fatigue cracks, both fine-grained metals and coarse-grained (CG) metals have a small fatigue threshold and thus a similar crack growth rate [3,4]. Equivalently, microstructural effects often vanish for small, naturally-occurring cracks [3,4].…”