According to the Paris Law for stable fatigue crack propagation in aerospace alloys, $\frac{da}{dN} = C (\Delta K)^m$, the crack extension per load cycle $\frac{da}{dN}$ is directly governed by:

(A) The range of the stress intensity factor $\Delta K = K_{\max} - K_{\min}$

(B) The static ultimate tensile strength $\sigma_{uts}$ independent of stress range

(C) The density of the alloy divided by Poisson's ratio

(D) The cyclic load frequency independent of crack geometry

Answer Key:

A

### 5-Stage Derivation & Analytical Solution: 1. **Stage 1: Paris-Erdogan Formulation:** $\frac{da}{dN} = C (\Delta K)^m$. 2. **Stage 2: Driving Parameter:** $\Delta K$ is the stress intensity factor range during the cyclic loading. 3. **Stage 3: Selection:** Option A is correct.

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