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.

Plunger shafts machined from 4150 steel bar stock were involved in a series of fatigue failures. The fractures consistently occurred at two locations on the shafts: the shaft fillet and either side of a machined notch. The material specification for the shafts required 41xx series steel with a carbon content of 0.38 to 0.53%, a hardness of 35 to 40 HRC for the shaft, and a hardness of 50 to 55 HRC for the notch (which was case hardened). Analysis (visual inspection, chemical analysis, hardness testing, and magnetic particle inspection) supported the conclusions that all the fractures were fatigue-induced failures due to sharp radii in the fillets. The stress-concentrating effects of the fillets caused fatigue cracks to initiate and grow under cyclic loading until the crack depth was critical, causing the shaft to fail and rendering the assembly inoperative. Recommendations included increasing the radii of the notch and shaft fillets. If fatigue cracking had continued to be a problem with this component, shot peening of the subject radii would be appropriate. This process produces residual compressive stresses in the surface of the part, thereby retarding initiation of fatigue cracks.

After the mixing machines were introduced into service, excessive assembly stresses and inappropriate detail design caused the premature failures of ice cream drink mixer blades shortly. The mixer blade is slightly deformed by the contact between the wavy washer at the bottom of the assembly and the bends at the bottom shoulders of the two mixer arms. Multiple fatigue crack origins on the inside radii of the bends at the bottom shoulders was revealed by analysis of the failed mixer blades. It was revealed by metallographic examination that the shoulders on the arms were cold bent, introducing tensile residual stresses on the inside radii of the shoulders and creating a localized area of fatigue susceptibility due to the inherent notch sensitivity of cold-formed 300 series stainless steel. It was established that the physical root cause was the design of the mixer blade. The addition of a stand-off washer between the wavy washer and the bottom shoulders of the blade or modification of the shape of the wavy washer to prevent contact with the blade shoulders was recommended.

Several deburring drums that fractured were filled with abrasive, water, and small parts, such as roller bearing rollers, and rotated on their axis at 36 rpm. Cracks were discovered very early in the service lives of these high-chromium white iron cast structures. All of the fractures were through bolt holes in the mounting flange. The holes had a sharp edge and exhibited uneven wear on the inside diameter. In operation, the mounting bolts were frequently found to be loose and in at least one case broken off. A 25x scanning electron microscopy (SEM) fractograph from near this fracture-initiation area showed fatigue striations. No casting or metallurgical structural defects were found that could explain the failures. This evidence supports the conclusion that cracking was a result of the stress-concentration site at the bolt holes where a fatigue-initiated fracture occurred. Recommendations included that the radii be increased at the sharp corners and that lock-wiring be used to secure against bolt loosening.

On 16 July 1999, a Boeing 737-800 on final approach for landing sustained a major lightning strike. Damage to the fuselage structure primarily was in the form of melting or partial melting of widely-separated rivets and adjacent Alclad 2024-T3 fuselage skin. The damage was confined to a 0.25-in. (6.4-mm) radii around the affected rivets. The repair process involved removal of the locally-affected material and addition of a skin doubler to restore the aircraft structure to the originally designed condition. Damage features are described briefly.

Drive cables from a rubber processing machine were failing in less than 8 h of operation, the expected service life being much greater than 100 h. Comparison cables were tested to failure under known stress conditions, including tensile overload, torsional loading, reversed bending alternating stress, and buckling (compressive) cyclic loading. The mode of failure was found to be reversed bending fatigue caused by drive cables moving over guide pulleys of small radii. Modifications of the machinery and drive cable system were suggested.

A drum pinion shaft (1030 steel) which was part of the hoisting gear of a crane (capacity 18,140-kg) operating in a blooming mill failed while lifting a 9070 kg load. Chatter marks, rough-machining marks, and sharp corner radii were revealed in the keyway which extended into a shoulder at a change in diam. A circular recess below the keyway surface was revealed at each end of the keyway. A sharp corner at the end of the keyway was revealed by examination to be the origin of fracture. Beach marks were found radiating from the origin over a large portion of the fracture surface which confirmed failure of the shaft by fatigue fracture. As a corrective measure the shaft was replaced with one made of 4140 steel, quenched and tempered to a hardness of 286 to 319 HRB. The keyway was moved away from the change in section and was machined with a 1.6-mm radius in the bottom corners and a larger-radius fillet was machined at the change in section.

There was a fracture of an anchor link that rides on a post with a spherical surface. No bevel or radius was called for, leaving an extremely sharp edge. A fatigue crack grew from the top sharp edge. The casting had good toughness and strength (1448 MPa, or 210 ksi, tensile strength). This fatigue failure was the result of poor design, which incorporated a notch that acted as a stress raiser, leading to fatigue-crack growth. A generous radius was recommended to avoid similar failures.

A 13 mm diam 18 x 7 fiber-core improved plow steel nonrotating wire rope, brought into service as a replacement for 6 x 37 improved plow steel ropes, failed after 14 months of service on a stacker crane. The change was reported to have been caused by difficulties twisting of the 6 x 37 rope. The hoist arrangement for this crane was found to consist of one rope with each end attached to a separate drum and the rope was wound around two 30-cm diam sheaves in the block and back up around an equalizer sheave. The rope section that had been in contact with the sheaves was deduced by measurement checks. The presence of broken wire ends, which indicated that the rope failed by fatigue, was revealed by reverse bending of the section of the rope which was normally subjected to this flexing. It was found that minimum sheave diam for a 13-mm 18 x 7 wire rope was 43 cm and hence the currently used smaller sheaves caused excessive bending stresses in the rope. The 18 x 7 rope was replaced by two 6 x 37 side-by-side counter-stranded steel-core ropes as a corrective measure.