Crack Growth Testing
Crack growth testing is a fracture-mechanics test that measures how fast a pre-existing crack propagates under repeated loading, typically reported as da/dN versus ΔK. In metals, it follows ASTM E647 or ISO 12108 using precracked CT, SENB, SENT, or CCT specimens. Crack length is tracked by compliance or DC potential drop, and the resulting Paris-law constants support fatigue-life prediction and inspection-interval setting.
In manufacturing qualification, crack growth testing predicts fatigue life and sets inspection intervals for weldments, forged parts, aerospace brackets, shafts, and additively manufactured metal components. CNC shops use these results to qualify surface finish and machining processes. A lab machined coupon is precracked so the flaw resembles a natural defect, then cycled under controlled amplitude loading while crack extension is recorded against cycle count. The resulting Paris-law behavior da/dN = A(ΔK)^m is used to fit material constants and model service behavior under repeated stress. This matters when residual stress, machining marks, notch radii, heat-affected zones, or surface defects dominate fatigue performance, because it quantifies stable crack extension instead of only final break strength. On the shop floor, it validates heat treatment, weld procedure, or surface finish and supports damage-tolerance maintenance planning.
What does da/dN mean in fatigue crack growth testing?
da/dN is the incremental crack growth per load cycle, typically expressed as crack length per cycle. It is the dependent variable in Paris-law analysis and is plotted against ΔK to characterize a material's resistance to stable crack extension under cyclic loading.
What does ΔK represent in crack growth testing?
ΔK is the range of stress intensity factor at the crack tip during cyclic loading. It is the primary fracture-mechanics variable controlling the growth rate in standard FCGR testing, and it accounts for applied stress and crack geometry.
Why are precracked specimens used instead of notched-only coupons?
A fatigue precrack better represents a real flaw and minimizes the influence of notch-root plasticity and machining damage. This gives more defensible material constants for fracture-mechanics life prediction and damage-tolerance assessments.