Page 25 - Mechanical Behavior of Materials

24 Chapter 1 Introduction

Figure 1.5 Fracture toughness test. K is a measure of the severity of the combination of
crack size, geometry, and load. K Ic is the particular value, called the fracture toughness,
where the material fails.

200

K , Fracture Toughness, MPa m Ic 150 Low alloy Maraging steels
TRIP steels

Q and T
steels

100

P-H stain-
less steels
50
1000 1500 2000
σ , Yield Strength, MPa
o
Figure 1.6 Decreased fracture toughness, as yield strength is increased by heat treatment,
for various classes of high-strength steel. (Adapted from [Knott 79]; used with permission.)

strength generally have low fracture toughness, and vice versa. This trend is illustrated for several
classes of high-strength steel in Fig. 1.6.
Ductile fracture can also occur. This type of fracture is accompanied by significant plastic
deformation and is sometimes a gradual tearing process. Fracture mechanics and brittle or ductile
fracture are especially important in the design of pressure vessels and large welded structures,
such as bridges and ships. Fracture may occur as a result of a combination of stress and chemical