Electron Fractographic Studies of Mechanical Fracture Processes in Metals

Abstract

Electron microscope studies of metal fracture surfaces have shown the existence of finescale plastic tearing during the early stages of crack growth for a large number of specimens. "Running" fractures in many structural metals (steels, aluminum alloys, and titanium alloys) propagate by the satellite nucleation of one or more different fracture mechanisms with the resultant submerged cracks linking with one another and with the major fracture to form incremental advances of the fracture front.The transition of macroscopically flat fracture to shear lips may occur over distances of about 10 to 100 microns, with the fine-scale fracture mode often changing from that of the fast fracture mode to tearing, normal rupture, and shear rupture. Shear lips always exhibit shear rupture dimples at high magnifications.Fatigue cracks are usually propagated by extremely fine-scale plastic rupture processes.

Measurement and interpretation of threshold stress intensity factors for steels in high-pressure hydrogen gas.

“…It can be speculated that PRHIC involves some combination of stresscontrolled micro-cracking events, such as martensite lath interface decohesion or cleavage within martensite laths, combined with strain-controlled ductile rupture of the remaining material. This general mechanism remains consistent with the original intent of the term quasicleavage [44], i.e., some combination of stress-and strain-controlled fracture modes, as well as the original explanation of PRHIC , e.g. [35].…”

“…In a following publication, Beachem identified the quasi-cleavage fracture plane to be, in fact, consistent with (001) ferrite cleavage planes and thus determined that the modifier "quasi… appears to be unnecessary for quenched and tempered steels" [47]. The primary difference in appearance between classical cleavage fracture and those cases initially described as quasi-cleavage in appearance was related to the nucleation sites of the cleavage cracks; for those surfaces previously identified as quasi-cleavage, the cracks nucleated at dispersed sites leading to many small, isolated cleavage facets connected by tear ridges [44].…”

“…The term quasi-cleavage was originally derived by Beachem to describe cleavage-like fracture of martensitic steels in which the complex fracture surfaces did not allow the fracture plane to be positively identified as a cleavage plane [44]. In a following publication, Beachem identified the quasi-cleavage fracture plane to be, in fact, consistent with (001) ferrite cleavage planes and thus determined that the modifier "quasi… appears to be unnecessary for quenched and tempered steels" [47].…”

“…[41,42,43]. For those attempting to use fractography to understand fracture mechanisms, however, quasi-cleavage is a rather unsatisfying term due to the fact that it is generally invoked only to describe a fracture surface appearance [44,45,46] whereas the other common fracture mode descriptions (i.e., intergranular, microvoid coalescence, and cleavage) pertain to specific fracture mechanisms that have unequivocal fracture surface appearances [30].…”

Review of Concepts and Status of Procedures for Fracture-Safe Design of Complex Welded Structures Involving Metals of Low to Ultrahigh Strength Levels

“…We do not propose to extensively document these studies in this report. Excellent references as to procedural techniques and interpretive procedures have been issued by Beachem (14,15,16), based on NRL studies.…”