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1656_C007.fm  Page 304  Monday, May 23, 2005  5:54 PM





                       304                                 Fracture Mechanics: Fundamentals and Applications



























                       FIGURE 7.6  ASTM notation for specimens extracted from disks and hollow cylinders. Taken from E 1823-
                       96, ‘‘Standard Terminology Relating to Fatigue Fracture Testing.’’ American Society for Testing and Materials,
                       Philadelphia, PA, 1996 (Reapproved 2002).

                          Figure 7.7 illustrates the precracking procedure in a typical specimen, where a fatigue crack
                       initiates at the tip of a machined notch and grows to the desired size through careful control of the
                       cyclic loads. Modern servo-hydraulic test machines can be programmed to produce sinusoidal
                       loading, as well as a variety of other wave forms. Dedicated fatigue precracking machines that
                       cycle at a high frequency are also available.
                          The fatigue crack must be introduced in such a way as not to adversely influence the toughness
                       value that is to be measured. Cyclic loading produces a crack of finite radius with a small plastic
                       zone at the tip, which contains strain-hardened material and a complicated residual stress distribution
                       (see Chapter 10). In order for a fracture toughness measurement to reflect the true material
                       properties, the fatigue crack must satisfy the following conditions:
                           • The crack-tip radius at failure must be much larger than the initial radius of the fatigue
                             crack.
                           • The plastic zone produced during fatigue cracking must be small compared to the plastic
                             zone at fracture.




















                       FIGURE 7.7  Fatigue precracking a fracture mechanics specimen. A fatigue crack is introduced at the tip of
                       a machined notch by means of cyclic loading.
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