TWI Frequently asked questions
by C S Wiesner
Fracture toughness is influenced by many factors, including microstructure, thickness and constraint, strain rate, environment and temperature. For ferritic steels, temperature is of particular importance, as a transition in fracture behaviour occurs from the ductile upper-shelf (high fracture toughness) at higher temperatures to brittle lower-shelf behaviour (low fracture toughness).
Another important factor is the thickness of the material in question. Figure 1 shows the generic effect of specimen dimensions on the fracture toughness transition curve. For identical material microstructure, increasing the dimensions (i.e. thickness and width of the specimen) shifts the transition from brittle to ductile fracture behaviour to higher temperatures and also increases the upper shelf toughness. For a given temperature, e.g. T 1 in Fig.1, thick sections of identical steel can therefore exhibit predominantly brittle behaviour whilst thinner components and specimens behave in a ductile fashion.
At very low temperatures, where the fracture behaviour is fully brittle, i.e. plane strain conditions apply, there is no effect of thickness.
If the fracture toughness at a given thickness, B 1, is known, the effect of thickness on fracture toughness in the lower transition region can be predicted using the following relation.
Where B 1 is the thickness at which the fracture toughness,
, is to be determined. The validity of this relationship is limited to K=[(E b R e)/30] 0.5 where E is Young's modulus, b is specimen uncracked ligament width and R e the yield strength. Higher limits apply therefore for larger specimens (increasing b) made from stronger steel (increasing R e).
Fig.1. Generic effect of specimen dimensions on fracture toughness
| 