Page 349 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)
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1656_C007.fm  Page 329  Monday, May 23, 2005  5:54 PM





                       Fracture Toughness Testing of Metals                                        329


                       initiates at P . A test on a fully ductile material, such as steel on the upper shelf, produces a load-
                                 i
                       displacement curve like Figure 7.29(c); a maximum load plateau occurs at P . The specimen is
                                                                                      m
                       still stable after maximum load if the test is performed in displacement control. Three types of
                       CTOD result, d , d , and d , are mutually exclusive, i.e., they cannot occur in the same test.
                                   c
                                            m
                                      u
                          As Figure 7.29 illustrates, there is usually no detectable change in the load-displacement
                       curve at the onset of ductile crack extension.  The only deviation in the load-displacement
                       behavior is the reduced rate of increase in load as the crack grows. The maximum load plateau
                       (Figure 7.29(c)) occurs when the rate of strain hardening is exactly balanced by the rate of
                       decrease in the cross section. However, the initiation of crack growth cannot be detected from the
                       load-displacement curve because the loss of cross section is gradual. Thus d  must be determined
                                                                                    Ic
                       from an R curve.


                       7.6 DYNAMIC AND CRACK-ARREST TOUGHNESS

                       When a material is subject to a rapidly applied load or a rapidly propagating crack, the response
                       of that material may be drastically different from the quasistatic case.  When rapid loading or
                       unstable crack propagation are likely to occur in practice, it is important to duplicate these conditions
                       when measuring material properties in the laboratory.
                          The dynamic fracture toughness and the crack-arrest toughness are two important material prop-
                       erties for many applications. The dynamic fracture toughness is a measure of the resistance of a material
                       to crack propagation under rapid loading, while the crack-arrest toughness quantifies the ability of a
                       material to stop a rapidly propagating crack. In the latter case, the crack may initiate under either
                       dynamic or quasistatic conditions, but unstable propagation is generally a dynamic phenomenon.
                          Dynamic fracture problems are often complicated by inertia effects, material rate dependence,
                       and reflected stress waves. One or more of these effects can be neglected in some cases, however.
                       Refer to Chapter 4 for an additional discussion on this subject.


                       7.6.1 RAPID LOADING IN FRACTURE TESTING
                       Some testing standards, including ASTM E 399 [8] and E 1820 [4] include annexes for fracture
                       toughness testing at high loading rates. This type of testing is more difficult than conventional
                       fracture toughness measurements, and requires considerably more instrumentation.
                          High loading rates can be achieved in the laboratory by a number of means, including a drop tower,
                       a high-rate testing machine, and explosive loading. With a drop tower, the load is imparted to the
                       specimen through the force of gravity; a crosshead with a known weight is dropped onto the specimen
                       from a specific height. A pendulum device such as a Charpy-testing machine is a variation of this
                       principle. Some servo-hydraulic machines are capable of high displacement rates. While conventional
                       testing machines are closed loop, where the hydraulic fluid circulates through the system, high-rate
                       machines are open loop, where a single burst of hydraulic pressure is released over a short time interval.
                       For moderately high displacement rates, a closed-loop machine may be adequate. Explosive loading
                       involves setting off a controlled charge that sends stress waves through the specimen [21].
                          The dynamic loads resulting from impact are often inferred from an instrumented tup. Alter-
                       natively, strain gages can be mounted directly on the specimen; the output can be calibrated for
                       load measurements, provided the gages are placed in a region of the specimen that remains elastic
                       during the test. Crosshead displacements can be measured directly through an optical device
                       mounted to the cross head. If this instrumentation is not available, a load-time curve can be converted
                       to a load-displacement curve through momentum transfer relationships.
                          Certain applications require more advanced optical techniques, such as photoelasticity [22, 23] and
                       the method of caustics [24]. These procedures provide more detailed information about the deformation
                       of the specimen, but are also more complicated than global measurements of load and displacement.
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