FFS Forum: The Master Curve Approach to Estimating Fracture Toughness

Transition Temperature Behavior

First, a little background. Estimating the toughness of ferritic steels by the master curve or other methods is driven by the fact that toughness is a function of temperature.

Ferritic materials undergo a change in toughness as a function of temperature, with higher toughness at higher temperatures and lower toughness at lower temperatures. But the change is non-linear, and it happens at different temperatures for different materials. Figure 1 shows example curves for three ferritic materials. They all show a characteristic drop in toughness in the so-called “transition” region. The higher and lower toughness regions are referred to as the “upper shelf” and “lower shelf,” respectively. Notice the large difference between upper and lower shelf toughness in Figure 1: upper shelf > 45-ft-lbs; lower shelf < 10-ft-lbs.

It's worth noting that the shapes of the curves are very similar, and they can be thought of as “shifting” left or right for different materials and even different heats of materials. Also notice that the beginning of the upper shelf region can vary widely in temperature.

Generally, materials that are warm enough to be on the upper shelf have sufficient toughness for typical pressure systems applications. Brittle fracture problems typically arise when the temperature is lower and the material toughness precipitously drops through the transition region toward the lower shelf value.