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 4 CASE STUDY: FATIGUE FRACTURE A fatigue fracture surface was ex- amined to illustrate the characteris- tics associated with the fatigue fracture mode. Fatigue fractures resemble brit- tle fractures, in that they occur with little deformation of material. These types of fractures often occur as the result of thousands of load cycles and have a number of distinct features that enable their identification. The part examined is from the tongue of a failed trailer hitch, near the location where the trailer ball is at- tached (Fig. 7). A part like this experi- ences thousands of load cycles from loads generated by the weight of the trailer as it traveled down the road, tak- ing off and stopping, and traveling over the bumps and undulations in the road. While some of our sample’s fea- tures are identifiable without magni- fication, others are only observable under high magnification. Examination by the naked eye shows the part con- tains a series of arcs starting at the cor- ner (Fig. 7, upper left corner of part, and Fig. 8, lower right corner), increasing in size as they advance across the fracture surface. The area formed by these arcs (referred to as “beach” marks) indicate sites where cracking came to a tempo- rary stop when loading dropped below a damaging level then resumed when the damaging load was reapplied. The smooth area of the fracture containing these arcs forms the fatigue fracture surface. Beyond the smooth fatigue sur- face, a more textured area (resembling the overloaded bolt in Figs. 5 and 6) is observable. The rough area indicates where the final overload failure of the part occurred. Another visual indication of fatigue fracture is visible at the lower right corner where a dark, rust-colored arc appears (see Fig. 8). Rusting is typi- cal of fatigue fractures of iron and steel parts because the unprotected material of the fracture surfaces is often exposed to the environment for an extended pe- riod of time before final failure of a part occurs. The shinier spots on the frac- ture surface also offer a clue that this is a fatigue fracture as they show where raised portions of the mating fracture surfaces were flattened by repeated contact after the initial fracture event. Examining the shiny spots via SEM at 50x magnification (Fig. 9) does not tell the entire story, but increas- ing the magnification to 250x confirms the flattening of the contacting areas (Fig. 10). Examination of an unflat- tened, unaltered portion of the fracture surface at higher magnification (1000x) shows aligned striations, faintly observ- able at even this level of magnification (Fig. 11). The aligned microscopic stri- ations are characteristic of fatigue frac- tures. A variation in the spacing of the striations can be an indication that changes in loading occurred during the progression of the fatigue failure, with an increase in spacing implying an in- crease in load, and vice versa. ~AM&P This article is adapted from an article appearing on the EDT Blog “ The Stress Point .” Formore information: RichardEllsworth and Chris Spies are consulting engi- neers at EDT Forensic Engineering & Consulting in the Kansas City District Office, 8405 Melrose Drive, Kansas City, KS 66214, 913.859.9580, rellsworth@ edtkc.com , cspies@edtkc.com, www. edtengineers.com. Fig. 7 — Failed tongue of steel trailer hitch showing location of fatigue fracture (upper left). Fig. 8 — Location of same fatigue fracture (lower right) at lowmagnification (30x) showing beach marks within a smooth area of the fracture and a more textured area beyond the smooth area, typical of fatigue failure. Also visible is an arc of rust-colored material. The shinier spots are a result of raised portions of the mating surfaces flattening due to repeated contact after the initial fracture. Fig. 9 — SEM view of shiny spots in the fatigue fracture. Magnification: 50x. Fig. 10 — SEM view of shiny spots in the fatigue fracture at higher magnification (250x) showing the flattening of mating surfaces. Fig. 11 — SEM view of unflattened, unaltered portion of the fatigue fracture surface at higher magnification (1000x) shows aligned striations characteristic of fatigue fractures.