Page 373 - Mechanical Behavior of Materials

374 Chapter 8 Fracture of Cracked Members

Figure 8.30 Fracture surfaces from a K Ic test on A533B steel that used a compact specimen
of dimensions t = 25, b = 51 cm. (Photo courtesy of E. T. Wessel, Haines City, FL; used with
permission of Westinghouse Electric Corp.)

Values of K Q meeting this requirement are denoted as K Ic to distinguish them as worst-case values.
In engineering design that employs material of thickness such that K Q is somewhat greater than K Ic ,
values of K Ic can be used, while recognizing that this provides some extra conservatism. Such an
approach is often necessary, since only K Ic values are widely available, as in Tables 8.1 and 8.2.
A later section of this chapter will consider in more detail plastic zone size effects and other
aspects of fracture toughness testing. We will now proceed to discuss the trends in K Ic with material,
temperature, loading rate, and other influences.

8.6.1 Trends in K Ic with Material
√
Values of K Ic for engineering metals are generally in the range 20 to 200 MPa m. For increasing
strength within a given class of engineering metal, it has already been noted that fracture toughness
decreases along with tensile ductility. See Figs. 1.6 and 8.7. As a further example, the effect on K Ic
of heat treating the alloy steel AISI 4340 to various strength levels is shown in Fig. 8.32.