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4. (25 pts.) Analyze the residual strength curves for the stiffener, panel and rivet shown below. Indicate, on the graph, the critical crack length, residual strength and identify regions of unstable and stable crack growth. Under what conditions (if any) will an unstable crack arrest? Stiffened Panel Residual Strength 80 Panel Stiffener Rivet 70 60 50 oksi) 40 - 30 20 10 10 15 20 a (in.) 4. (25 pts.) Analyze the residual strength curves for the stiffener, panel and rivet shown below. Indicate, on the graph, the critical crack length, residual strength and identify regions of unstable and stable crack growth. Under what conditions (if any) will an unstable crack arrest? Stiffened Panel Residual Strength 80 Panel Stiffener Rivet 70 60 50 oksi) 40 - 30 20 10 10 15 20 a (in.)