Search Results for Fractures

The cross bars of conveyor belt links that served to transport glass containers through a stress relief furnace fractured in many cases. They consisted of wires of 5 mm diam made of low-carbon Siemens-Martin steel, while the interwoven longitudinal bars were made of strip steel of 4 x 2 sq mm. The furnace temperature was said to be 500 deg C. In addition to the fractures they also showed many more or less advanced cracks. These occurred in the circumferential grooves that recurred at regular intervals. The fractures were abraded and oxidized. They could have been fatigue fractures. The fracture probably was induced by the pressing-in or abrading of the sharp steel band edges into the surface of the cross bars. Torsion fatigue fractures may have started from these notches. Relaxation then contributed positively through recovery and recrystallization. Such damage occurs less frequently in round wire conveyor belt links because the round wire neither impresses so sharply nor abrades against the cross bars, and it also exerts less torsion than the flat wire.

Galvanized A36 steel unsleeved shear-type anchor bolts failed during installation. The galvanized steel bolts were approximately 18 mm (0.7 in.) in diameter with a 90 deg bend between the long and short legs. As-fractured, sawcut, and unfractured specimens were examined. Failure analysis revealed dark thumbnail regions at the fracture origins and a very narrow and uniform shear lip. The thumbnail region exhibited zinc deposits with no apparent fracture detail, indicating preexisting cracks that had occurred before galvanizing. The balance of the fracture exhibited a transgranular mode with cleavage and ductile, dimpled shear. Hardness values as high as 35 HRC were measured in the bend area. The as-galvanized bolts fractured in a brittle manner. Failure was attributed to improper bending of the bolts, which provided a severely cold-worked bend area susceptible to strain-age embrittlement.

Sudden and unexplained bearing cap bolt fractures were experienced with reduced-shank design bolts fabricated from 42 CrMo 4 steel, quenched and tempered to a nominal hardness of 38 to 40 HRC. Fractographic analysis provided evidence favoring stress-corrosion cracking as the operating transgranular fracture failure mechanism. Water containing H7S was subsequently identified as the aggressive environment that precipitated the fractures in the presence of high tensile stress. This environment was generated by the chemical breakdown of the engine oil additive and moisture ingress into the normally sealed bearing cap chamber surrounding the bolt shank. A complete absence of fractures in bolts from one of the two vendors was attributed primarily to surface residual compressive stresses produced on the bolt shank by a finish machining operation after heat treatment. Shot cleaning, with fine cast shot, produced a surface residual compressive stress, which eliminated stress-corrosion fractures under severe laboratory conditions.

During construction of a river bridge with 80 twisted cables, one or more fractures were found in each of 21 wires of 18 cables before assembly. All were located at the outside wrapping whose Z-profile wires were galvanically zinc-coated. It was suspected that hydrogen played a role during crack formation, and that it penetrated during pickling or galvanizing. This supposition was confirmed also by the fact that the wire fractures were not observed during cable winding, but only subsequently to it, and therefore seemed to have appeared only after a certain delay.

A crankshaft was overloaded on a test stand and suffered an incipient crack in the crank pin. The crack run generally parallel to the longitudinal axis and branched off at the entrance into the two fillets at the transition to the crank arm. It consisted of many small cracks, all of which propagated at an angle of approximately 45 deg to the longitudinal axis, and therefore were caused by torsion stresses. Neither macroscopic nor microscopic examination determined any material or processing faults. Experience has shown that torsion vibration fractures of this kind usually appear in comparatively short journal pins at high stresses. This crankshaft fracture was an example of the damage that is caused or promoted neither by material nor heat treatment mistakes nor by defects of design or machining, but solely by overstressing.

Cracks, with no other damage, were observed in a niobium alloy (Nb-106) part when it was pulled from several months of protective storage for assembly into a rocket nozzle. SEM views showed the cracks to be intergranular, with contaminant particles on a large number of the grain facets. EDX analysis showed they consisted of niobium and fluorine. Plastic replicas, prepared by standard TEM techniques, were analyzed with selected-area electron diffraction, showing a pattern match for niobium tetrafluoride. Auger analyses showed electron spectra containing peaks representing carbon, oxygen, nitrogen, fluorine, and chlorine. Investigation into the processing history of the part showed the tenacious oxide film formed by the affinity of niobium for oxygen - even when heat treated in a vacuum – was removed with a combination of strong acids: nitric, hydrochloric, hydrofluoric, and lactic, resulting in the contaminants found on the surface. Thus, residues of the cleaning acid on the part had caused SCC during storage, with the tensile stresses necessary to generate SCC assumed to have been residual stresses from the heat treatment. Recommendation was made that more stringent cleaning procedures to remove any trace of the cleaning acids be used.

Eight cylinderhead screws cracked after a short running time in motors. They were made of Fe-0.45C-1Cr steel, had rolled threads, were heat treated to 110 kg/sq mm tensile strength, and were electrolytically galvanized. All fractured at the root of the thread. The surfaces of fracture were fine-grained and had not spread by rubbing. Because the screws were electrolytically galvanized, failure resulted from “delayed fracture.” Experience has shown that this type of fracture is seen on production parts made of high-strength steels, which absorbed hydrogen during pickling or during a galvanic surface treatment. Such parts will rupture below the elastic limit during continuous stressing. This often occurs only after the expiration of a certain time period, and preferably at locations of stress concentrations such as changes in cross section or threads. As a rule, the hydrogen cannot be verified analytically because most of it escapes again after prolonged storage at room temperature or short heating at 100 to 200 deg C.

A bolt breaks along a change in cross section well below its rated capacity. An anchoring screw spins freely in place, having snapped at its first supporting thread. A motor unexpectedly disengages its load, its driveshaft having fractured near a keyway. Such failures – involving axles, leaf springs, engine rods, wing struts, bearings, gears, and more – can occur, seemingly without cause, due to vibrational fracture. Vibrational fractures begin as cracks that form under cyclic loading at nominal stresses which may be considerably lower than the yield point of the material. The fracture is proceeded by local gliding and the development of cracks along lattice planes favorably orientated with respect to the principal stress. This non-reversible process is often misleadingly called “fatigue” and presents significant challenges to engineering teams that ill-advisedly take to searching for material faults. Several examples of notch-induced vibrational fractures are presented along with guidelines for investigating their cause.

A working roll of 210 mm diam and 500 mm face length was examined because of shell-shaped fractures. The roll consisted of Fe-0.83C-1.6Cr steel. The chromium content was low for a roll of this diam. The crack origin was located about 10 mm under the roil face. Surface hardness (HV1) of 900 kp/sq mm was exceptionally high corresponding to the martensitic peripheral structure. An untempered piece with such a thick cross section and a hardened peripheral zone with such high hardness must have high residual stresses that culminate in the transition zone. Therefore it must be very sensitive against additional stresses, be these of a mechanical or thermal nature. This contributed to the fragmenting of the roll face.

An electric transport vehicle, similar to an electric trolley or subway rail car, experienced frequent breakdowns due to in-service fractures of torsion springs that support the weight of an overhead electric pickup assembly. Scanning electron microscopy and metallographic examinations determined that the fractures stemmed from electric arc damage. Intergranular quench cracks in the transformed untempered martensite on the surface of the spring provided crack initiations that propagated during operation causing fatigue fracture.

Valve seats fractured during testing and during service. The seats were machined from grade 11L17 steel and were surface hardened by carburization. Investigation (visual inspection, hardness testing, 59x SEM images, and 2% nital etched 15x cross sections) supported the conclusion that the fracture occurred via brittle overload, which was predominantly intergranular. The amount of bending evidence and the directionality of the core overload fracture features suggest that the applied stresses were not purely axial, as would be anticipated in this application. The level of retained austenite in the hardened case layer likely contributed to the failure.

The conical helical spring sealed, within each switch enclosure, fractured to lead to the failure of several electrical toggle switches. The spring was fabricated from 0.43 mm diam AISI type 302 stainless steel wires. Appreciable amount of scale was observed on the fracture surface and tool marks were revealed on the inner surface of the broken spring. A typical fatigue fracture that originated at a tool mark on the wire surface was revealed by inspection of a fracture surface of the broken springs. Regions which displayed beach marks around the fracture origin and parallel striations within the beach-mark regions were revealed by scanning electron microscopy. As a corrective measure, the spring-winding operation was altered to eliminate the tool marks.