Search Results for concentric grooves

The head on a golf club driver developed multiple cracks during normal use. The head was a hollow shell construction made from a titanium alloy. Analysis and additional investigation revealed a progressive failure that initiated on the interior surface of the face plate along a deep, concentric groove created during a press forming operation. It was also determined that atmospheric contamination occurred during the welding of the head, causing embrittlement, which may have also contributed to the failure. Recommendations were made addressing the problems that were observed.

Milling machine arbors were inserted with satellite spindles having a maximum speed of 1500 rpm, and broke out between the groove and the flange. The appearance of the fracture surface was the same on both arbors. The pronounced scan lines characterized the fractures as fatigue fractures. The appearance of the fracture in the arbors indicated ductile fatigue fracture which had its origin in the radii between groove and flange. These radii of 0.15 and 0.2 mm were too small for the load on the milling machine. In addition there were grooves at the base of the radii which had an unfavorable effect on the life of the component by acting as notches with their resulting stress concentration. Considering the great hardness of the case, the small radii would have been critical even without grooves. Measures were taken so that the critical radius of the milling machine was increased and the surface roughness measured more precisely.

The main shaft in a locomotive turbocharger fractured along with an associated bearing sleeve. Visual and fractographic examination revealed that the shaft fractured at a sharp-edged groove between two journals of different cross-sectional area. The dominant failure mechanism was low-cycle rotation-bending fatigue. The bearing sleeve failed as a result of abrasive and adhesive wear. Detailed metallurgical analysis indicated that the sleeve and its respective journal had been subjected to abnormally high temperatures, increasing the amount of friction between the sleeve, bearing bush, and journal surface. The excessive heat also softened the induction-hardened case on the journal surface, decreasing its fatigue strength. Fatigue crack initiation occurred at the root fillet of the groove because of stress concentration.

A stainless steel screw securing an orthopedic implant fractured and was analyzed to determine the cause. Investigators used optical and scanning electron microscopy to examine the fracture surfaces and the microstructure of the austenitic stainless steel from which the screw was made. The results of the study indicated that the screw failed due to fatigue fracture stemming from surface cracks generated by stress concentration likely caused by grooves left by improper machining.

Two 38 mm (1.5 in.) diam threaded stud bolts that were part of a steel mold die assembly from a plastics molding operation were examined to determine their serviceability. Chemical analysis showed the material to be a plain carbon steel that approximated 1045. Visual examination revealed evidence of severe hammer blows to the clevis and boss areas and a gap between the die and the underside of the boss. Magnetic particle inspection showed cracks at the thread roots that, when examined metallographically, were found to contain MnS stringers. The cracking of the threads was attributed to a poor stud bolt design, which allowed a high stress concentration to occur at the base of the threads upon application of a lateral load. It was recommended that bolts of a new design that incorporated a stress-relieving groove be used. Threading of the bolt to eliminate the gap between the lower face of the boss and the die and an improved method of inserting or removing the bolt to avoid hammering (use of a wrench on a square or hexagonal boss) were also recommended.

A stainless tool steel bone drill broke during an operation on a patient and was examined. It showed two fatigue fractures, one of which had started from a sharp-edged, coarsely milled slot (fracture A1), and the other from a point on the outer sheath surface which was not subjected to particularly high stresses (fracture A2). Fatigue fracture A1 resulted from the stress concentration built up at this point as a result of the sharp edges and the coarse machining grooves. The remains of a number, which had been inscribed with an electrical engraving tool for identification purposes, were found at the point of origin of fracture A2. The material had been heated to the melting point during the engraving of the number, and multiple cracking occurred during cooling. One of these cracks led to the development of fatigue fracture A2.

An AISI 4340 threaded steel connecting rod that was part of a connecting linkage used between a parachute and an instrumented drop test assembly fractured under high dynamic loading when the assembly was dropped from an airplane. A large flaw that originated from the root of a machined thread groove was visible on the fracture surface. Heavy oxidation at elevated temperatures was indicated as most of the surface of the flaw was black. Fine secondary cracks aligned transverse to the growth direction was revealed by scanning electron microscopy. It was established that intergranular cracking observed in this alloy was caused during heat treating as the thread root served as an effective stress concentration and induced quench cracking. It was found that fracture in the overload region occurred by a ductile void growth and coalescence process. Premature failure of the threaded rod was thus attributed to the presence of the quench crack flaw caused by an improper machining sequence and heat treatment practice.

The crane hook (rated for 13000 kg) failed in the threaded shank while lifting a load of 9072 kg. The metal in the hook was revealed by chemical analysis to be killed 1020 steel. It was disclosed by visual examination that the fracture had at the last thread on the shank and rough machining and chatter marks were evident on the threads. Beach marks that emanated from the thread-root locations on opposite sides of the fracture surface identified these locations to be the origins of the fracture. A medium-coarse slightly acicular structure was revealed by metallographic examination which indicated that the material was in the as-forged condition (which meant lower fatigue strength). The fracture was concluded to have occurred due to stress concentration in the root of the last thread. Normalizing of the crane hook after forging was suggested as a corrective measure. A stress-relief groove with a diam slightly smaller than the root diam was placed at the end of the thread and a large-radius fillet was machined at the change in diameter of the shank.

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 heavy duty facing lathe failed when the tool post caught one of the jaws on the rotating chuck, causing the spline shaft that drives the main spindle to fracture. A detailed analysis of the fracture surfaces (including fractography, metallography, and analytical stress calculations) revealed areas of damage due to rubbing with evidence of cleavage fracture on the unaffected surfaces. The results of stress analysis indicated that repeated reversals of the spindle produced stresses exceeding the fatigue limit of the shaft material. These stresses led to the formation of microcracks in a retaining ring groove that were accelerated to sudden failure when the tool post and chuck collided.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

While undergoing vibration testing, a type 347 stainless steel inlet header for a fuel-to-air heat exchanger cracked in the header tube adjacent to the weld bead between the tube and header duct. Investigation (visual inspection and liquid penetrant inspection) supported the conclusion that the crack in the header tube was the result of a stress concentration at the toe of the weld joining a doubler collar to the tube. The stress concentration was caused by undercutting from poor welding technique and an unfavorable joint design that did not permit a good fit-up. Recommendations included manufacturing the doubler collar so that it could be placed in intimate contact with the header duct, and a revised weld procedure was recommended to result in a smaller, controlled, homogeneous weld joint with less distortion.

On 21 April 1995, the contents of a large blender (6 cu m) reacted and caused an explosion that killed and injured a number of workers at a plant in Lodi, NJ. A mixture of sodium hydrosulfite and aluminum powder was being mixed at the time of the accident. This report focuses on evaluations of the blender to determine if material or mechanical failures were the cause of the accident. The results indicate that the mixing vessel was metallurgically sound and did not contribute to the initiation of the failure. However, the vessel was not designed for mixing chemicals that must be isolated from water and excessive heat. Water leaking into the vessel through a graphite seal may have initiated the reactions that caused the accident.