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Fracture Mechanics
 
Problem 10
This question introduces the concept of leak-before-break which is widely used in structural integrity design of pressurised components and vessels. The basic concepts behind the leak-before-break philosophy are given in the theory card. The question requires some thought about how crack length (a) is defined for through-thickness and semi-elliptic cracks, but is otherwise straightforward and should take about 15 minutes to complete. a) The stress intensity solution for a semi-elliptic flaw in tension is given below. The K1C value for a Ti-6Al-4V titanium alloy with a yield strength of 910 MPa, is 115.4 MPam½. Determine the size of the largest stable surface flaw (a/c = 0.4) in a 40 mm thick plate of this alloy, for a design stress in the plate of 75% of the yield strength. This requires assuming an initial value of a/B and iterative calculations of stress intensity, if necessary. b) For the same alloy and design stress, calculate the maximum wall thickness of a pressure vessel which could be designed on a leak-before-break criterion. You may assume that the aspect ratio (a/c) of the surface flaw remains constant at 0.4, and that for the through-thickness crack: 
What thickness of plate would you order for the vessel?
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Failure Analysis - Fracture Mechanics - Failure As A Design Criterion |