Page 342 - Mechanical Behavior of Materials

Section 8.2 Preliminary Discussion 343

500
AISI 1045 steel 3000
400
Stress, ksi 200 σ f σ o σ 2000 MPa
300

100 u 1000
0 0
1.0
True Fracture Strain 0.6 ε f
0.8

0.4
0.2
0 80
80
Fracture Toughness, ksi in 60 K Ic 60 MPa m

40
40
20
20
0
200 400 600 800 0
Brinell Hardness, HB

Figure 8.7 Comparison of properties from tension tests and fracture toughness tests for
AISI 1045 steel, all plotted as functions of hardness, which is varied by heat treatment.
(Illustration courtesy of R. W. Landgraf, Howell, MI.)

The relative sensitivity to flaws associated with different a t values for different materials helps
to explain a number of sudden engineering failures that occurred in the 1950s and 1960s. New high-
strength materials, such as steels and aluminum alloys developed for the aerospace industry, had
sufficiently low fracture toughness that they were sensitive to rather small cracks. One example was
the British-made Comet passenger airliner, two of which failed at high altitude in the 1950s, with
considerable loss of life in the resulting crashes. Other examples are the late 1950s failures of rocket
motor cases for the Polaris missile, and the F-111 aircraft crash in 1969. Such failures accelerated
the development of fracture mechanics and led to its adoption by the U.S. Air Force as the basis of
their damage tolerant design requirements.
Also, some apparently mysterious brittle failures in normally ductile steels occurred in the
1940s and earlier. These were finally understood years later to be due to cracks that were sufficiently
large to exceed even the relatively large a t value of the ductile steel. One example of this is the
failure in Boston in 1919 of a large tank, about 90 feet in diameter and 50 feet high, that contained
2 million gallons of molasses. Other examples include welded Liberty Ships and tankers that broke
completely in two during and shortly after World War II, and other ship and bridge failures.