Early stage of crack extension in impact loaded high strength steels
- SRI International, Menlo Park, CA (United States)
Measuring accurate dynamic initiation fracture toughness values requires the understanding of the early stage of crack extension to interpret experimental records and determine the initiation point correctly. The early stage of dynamic crack extension in high strength steels was investigated by impacting fatigue-precracked specimens of R{sub C} 50 AISI 4340 and 300M steel in a range of sizes using the one-point-bend test method. By controlling load amplitude (by varying the impact velocity) and load duration (by varying the specimen geometry and dimensions), we were able to control dynamic crack extension to achieve complete fracture of the specimen, or crack arrest after a few tenths of millimeter to several millimeters of propagation. Crack speeds in these experiments did not exceed a few hundred meters per second. A two-dimensional analysis of the experimental results show that the resistance to dynamic crack extension in terms of the stress intensity factor K, increases substantially (30-50% above the value at initiation) during the first millimeters of propagation and that this increase is not directly dependent on crack velocity. Fractographic observations of arrested cracks reveal that even when only small shear lips developed during fracture, the crack front shape is significantly curved, with the front at the center of the specimen leading the front at the surfaces by several millimeters. The good correlation between the distance over which the propagation toughness increases and the depth of the curved crack front suggests that the rise in toughness is associated with the development of the curved Crack front. This study demonstrates that three-dimensional effects are important in dynamic fracture of even such model materials as macroscopically brittle high strength steels. These effects should be carefully considered when analyzing fracture under short duration impulsive loads or interpreting initiation data.
- OSTI ID:
- 175191
- Report Number(s):
- CONF-950686-; TRN: 95:006111-0111
- Resource Relation:
- Conference: Joint applied mechanics and materials summer meeting, Los Angeles, CA (United States), 28-30 Jun 1995; Other Information: PBD: 1995; Related Information: Is Part Of AMD - MD `95: Summer conference; PB: 520 p.
- Country of Publication:
- United States
- Language:
- English
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