Fracture toughness converter
Convert fracture toughness between metric and US units, and see what crack size it tolerates.
That's 1.67× as tough as aluminum alloy, while structural steel is 2× tougher. At 200 MPa of stress, a crack about 19.89 mm long would become critical.
What it means
How tough is that?
This is plane-strain toughness, K_IC, which is the worst case for a thick section. Thin sheet tolerates more.
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Learn
Three things worth knowing
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Critical crack = (K ÷ σ)² ÷ π
At 200 MPa of working stress, a 100 MPa·√m steel tolerates a crack 80 mm long. Glass at 0.7 tolerates four micrometers.
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Griffith and the glass fibers
Griffith found in 1921 that thin glass fibers were far stronger than thick ones, and worked out that the difference was the size of the flaws they contained, not the material.
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Tough is not strong
Hardened steel is stronger than mild steel and much less tough. Making something harder usually makes it crack more readily, which is the central trade of metallurgy.
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Background
About fracture toughness
Strength says when a flawless sample yields. Fracture toughness says how large a crack a real part can carry before it runs. Glass is strong in compression and has a toughness of 0.7; structural steel reaches 100.
The odd unit — stress times the square root of a length — falls straight out of the crack-tip mathematics.