Crack Arrest Toughness of Two High Strength Steels (AISI 4140 and AISI 4340)
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INTRODUCTION
C R A C K arrest toughness is used as a fracture criterion in Section XI of the ASME Boiler and Pressure Vessel Code, and it is presently being considered as the critical fracture parameter in developing fracture control plans for cryogenic storage tanks and bridges as well. In each of these applications, there is a need for a fracture property that evaluates the ability of a material to arrest a fast running crack, because in some loca ! regions of the structure, the toughness may be abnormally low, or the stresses abnormally high. Preventing cracks from initiating at these locations may not be possible, but it may be possible to arrest the crack, even if it is running rapidly, when it encounters material having more normal toughness and/or material which is acted on by stresses closer to their design values. Crack arrest toughness may also be a useful fracture parameter for designing against crack initiation in structures that are rapidly loaded. This is especially useful for nonaustenitic steel structures, since their toughness decreases with loading rate. The toughness is not expected to decrease to zero, however, but rather to approach some lower limit, and it has been suggested that this limit value is the crack arrest toughness, Ka. There are two types of experimental evidence that support this view. First, tests run on SA533B-1 steel over a van~ety of temperatures at increasing loading rates extrapolate to the values of arrest toughness (Figure 1). Second, studies of the dependence of crack velocity, ci, on driving force, K, show that cracks cannot propagate below some value of K, and this value is thought to be Ka. The dependence of ci on K for'fast cracking for all materials and temperatures is thought to be represented by an inverted "L" shape curve, as shown for a low strength steel in Figure 2(a). 2 A run-arrest segment of crack extenE. J. RIPLING is President, Materials Research Laboratory, Inc., Glenwood, IL 60425, J. H. MULHERIN is Metallurgist, United States Army, Armament Research and Development Command, Dover, NJ 07801, and P.B. CROSLEY is Associate Professor, University of Illinois at Chicago Circle, Chicago, IL. Manuscript submitted April 8, 1981. METALLURGICALTRANSACTIONS A
A533 GR. B CLASS f PLATE STEEL PLATE 12 ~ THICK (HSST PLATE 0 2 )
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WESTINGHOUSE 9 ITCT SPECIMEN 0 2TCT A 4TCT. MRL
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Fig. 1--Initiation toughness
vs K
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sion is started when a sufficiently large driving force, KQ, is applied to the steel to cause crack propagation to occur. Shortly after the crack initiates, it attains the velocity associated with KQ (Figure 2(b)). If the driving force is decreased, the crack velocity decreases; oscillating about the ~t vs K curve, and arresting at Ka. Increasing K is thought to cause the crack to initiate at lower values of both K and a , again arresting
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