September_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 9 2 3 *Member of ASM International F our principle fracture modes include dimple rupture, cleavage, fatigue, anddecohesive rupture. Dimple rup- ture is caused by a process known as mi- crovoid coalescence; as a component is being pulled apart, ductile material creates tiny bubbles (microvoid coales- cence) in thematerial. The bubbles break when the part fractures leaving cuplike depressions, or dimples, and fractures with this appearance are associated with ductile fractures. A mild-steel bolt frac- tured at room temperature used to show ductile fracture is shown in Fig. 1. SEM im- ages of the fracture surfaces in Figs. 2 and 3 clearly show the dimpled surfaces asso- ciated with ductile fracture. Cleavage is a low-energy type of fracture that displays matched faces; flat and featureless surfaces associat- ed with brittle fractures. A brittle frac- ture surface was examined to illustrate the fracture mode identified as cleav- age. To produce a brittle fracture, a bolt made of the same material used in the ductile fracture example was fractured after cooling in liquid nitrogen at - 321 ° F (Fig. 4). SEM images of a cleavage frac- ture are shown in Figs. 5 and 6. Fatigue fractures are associat- ed with repetitive and/or cyclic load- ing. The material is subjected to three stages: crack initiation (Stage 1), crack propagation (Stage 2), and catastrophic fracture (Stage 3). Decohesive rupture is a fracture associated with a reactive environment (e.g., hydrogen, sulfur, phosphorus, or chloride) and the fracture propa- gates along/across grain boundaries. The part breaks due to the corrosive environment in which it is used; frac- ture occurs along the grain boundaries of the material. SEM: AN INVALUABLE TOOL IN FAILURE ANALYSIS Many broken parts look similar at a glance, but examination of the failed part using advanced equipment and methods such as scanning electron microscopy (SEM) reveals much more about the origins of the failure. Richard Ellsworth* and Chris Spies,* EDT Forensic Engineering & Consulting, Kansas City, Missouri Fig. 1 — Mild-steel bolt fractured at room temperature to produce ductile fracture. Fig. 2 — Fracture surface of bolt in Fig. 1 shows dimpled surface typical of ductile fracture. Magnification: 1000x. Fig. 3 — Same fracture surface as in Fig. 2 at a higher magnification of 5000x. Fig. 4 — Mild-steel bolt fractured at -321°F to produce brittle fracture. Fig. 5 — Fracture surface of bolt in Fig. 4 shows cleavage surface typical of brittle fracture. Magnification: 1000x. Fig. 6 — Same fracture surface as in Fig. 5 at a higher magnification of 5000x.