TABLE OF CONTENTS Among mechanical engineers there is a rule-of-thumb: avoid materials with plane-strain fracture toughnesses K 1 C less than 15 MPa.m 1/2. Almost all metals pass: they have values of K 1 C in the range of 20-100 in these units. White cast iron and some powder-metallurgy products fail; they have values as low as 10 MPa.m 1/2. Ordinary engineering ceramics have values in the range 1 6 MPa.m 1/2; mechanical engineers view them with deep suspicion. But engineering polymers are even less tough, with K 1 C in the range 0.5 3 MPa.m 1/2 and yet engineers use them all the time. What is going on here?
When a brittle material is deformed, it deflects elastically until it fractures. The stress at which this happens is
| (6.33) |  |
where K c is an appropriate fracture toughness, a c is the length of the largest crack contained in the material and C is a constant that depends on geometry, but is usually about 1. In a load-limited design a tension member of a bridge, say the part will fail in a brittle way if the stress exceeds that given by equation (6.33). Here, obviously, we want materials with high values of K c.
But not all designs are load-limited; some are energy-limited, others are deflection limited. Then the criterion for selection changes. Consider, then, the three scenarios created by the three alternative constraints...
