GATE 2023 MT – Question 64
A specimen has maximum initial surface-crack size 1.5 mm and cyclic stresses $\sigma_{max}=300$ MPa, $\sigma_{min}=0$ MPa. For specimen geometric factor 1, use the graph to find the crack-growth rate in μm/cycle (nearest integer). Here $N$ is cycle count, $a$ crack length, $R$ stress ratio, and $\Delta K$ stress-intensity range.
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Correct answer: 7 to 15
Explanation
The stress ratio is $R=\sigma_{min}/\sigma_{max}=0$, selecting the R = 0 curve. The stress-intensity range is $\Delta K=300\sqrt{\pi(0.0015)}=20.59$ MPa$\sqrt{\mathrm m}$. Reading that curve at this abscissa gives approximately **10 μm/cycle**. The accepted interval accommodates reading the logarithmic graph.