In addition to failures in shafts, this article discusses failures in connecting rods, which translate rotary motion to linear motion (and conversely), and in piston rods, which translate the action of fluid power to linear motion. It begins by discussing the origins of fracture. Next, the article describes the background information about the shaft used for examination. Then, it focuses on various failures in shafts, namely bending fatigue, torsional fatigue, axial fatigue, contact fatigue, wear, brittle fracture, and ductile fracture. Further, the article discusses the effects of distortion and corrosion on shafts. Finally, it discusses the types of stress raisers and the influence of changes in shaft diameter.

One of the connecting rods of a vertical, four-cylinder engine with a cylinder diameter of 5 in. failed by fatigue cracking just below the gudgeon-pin boss. Failure took place in line with the lower edge of a deposit of weld metal. The fracture surface was smooth, conchoidal, and characteristic of that resulting from fatigue. The origin of the major crack was associated with a crescent-shaped area immediately below the weld deposit. This showed brittle fracture characteristics and appeared to be an initial crack that occurred at the time of welding and from which the fatigue crack subsequently developed. The rod was made from a medium carbon or low-alloy steel in the hardened and fully tempered condition. Evidence indicated that, following modification to the oil feed system, the rod that broke was returned to service with fine cracks present immediately below the weld deposit, which served as the starting points of the fatigue cracks. Following this accident, the remaining three rods (which had been modified in a similar manner) were replaced as a precautionary measure.

A connecting rod (forged from 15B41 steel and heat treated to a hardness of 29 to 35 HRC) from a truck engine failed after 73,000 Km (45,300 mi) of service. A piece of the I-beam sidewall of the rod, about 6.4 cm (2 in.) long, was missing when the connecting rod arrived at a laboratory for testing. Analysis (visual inspection, 100x nital-etched micrograph, fluorescent magnetic-particle testing, and metallographic examination) supported the conclusion that the rod failed in fatigue with the origin along the lap and located approximately 4.7 mm below the forged surface. The presence of oxides may have been a partial cause for the defect. Recommendations included better inspection of the forgings by fluorescent magnetic-particle testing before machining.

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.

Examination of a connecting-rod shell bearing from a six-cylinder gasoline engine was done after it was returned to the factory. Copper-lead alloy SAE 485 bonded to a low-carbon steel backing was used to make the bearing and the oil used in the engine was the recommended one. Measurable material loss was visible over most of the bearing halves particularly in a wide region at the centerline. A brittle waxlike substance identified to be a mixture of copper and lead sulfides covered the visible shallow pits and the darkened region. Change of oil with greater frequency to prevent the buildup of sulfur compounds or bearing halves that have corrosion-resistant overlay materials were recommended as best solutions.

A motorboat engine connecting rod forged from carbon steel fractured in two places and cracked at the small end during service. The analysis (visual inspection, 50x micrographs of sections etched with 2% nital, magnetic-particle inspection, and metallographic examination) supported the conclusion that the connecting rods were rendered susceptible to fatigue-crack initiation and propagation by the notch effect of coarse folds formed during the forging operation. One fracture was caused by fatigue resulting from operating stresses, and the other was a secondary tensile fracture. No recommendations were made.

A connecting rod from a motor boat was broken in two places at the small end. At position I there was a fatigue fracture brought about by operational stress, whereas the fibrous fracture surface II was a secondary tensile fracture. Furthermore the transition on the other side of the rod was cracked symmetrically to the fatigue fracture (position III). Magnetic inspection showed indications of cracking at the transition between the rod and small end in six other connecting rods from the same batch. Metallographic investigation showed the connecting rods were rendered susceptible to fatigue by the notch effect of coarse scale-filled folds formed during forging.

The master connecting rod of a reciprocating aircraft engine revealed cracks during routine inspection. The rods were forged from 4337 (AMS 6412) steel and heat treated to a specified hardness of 36 to 40 HRC. H-shaped cracks in the wall between the knuckle-pin flanges were revealed by visual examination. The cracks were originated as circumferential cracks and then propagated transversely into the bearing-bore wall. No inclusions in the master rod were detected by magnetic-particle and x-ray inspection. Three large inclusions lying approximately parallel to the grain direction and fatigue beach marks around two of the inclusions were revealed by macroscopic examination of the fracture surface. Large nonmetallic inclusions that consisted of heavy concentrations of aluminum oxide (Al2O3) were revealed by microscopic examination of a section through the fracture origin. The forging vendors were notified about the excess size of the nonmetallic inclusions in the master connecting rods and a nondestructive-testing procedure for detection of large nonmetallic inclusions was established.

In a helicopter engine connecting rod, high-cycle, low-stress fatigue fractures in bolts and arms progressed about 75% across the section before the final rupture. Factors involved were insufficient specified preload, inadequate tightening during assembly, and engine overspeed. The assigned main causes were design deficiency, improper maintenance during overhaul, and abnormal service operation. The problem can be solved by proper overhauling that ensures bolted assemblies are tightened evenly and accurately, in accordance with recommended torque values. Also, the manufacturer made various modifications, such as a thicker rod, fatigue resistant bolts, and more accurate preload measurements. The configuration of these rods were changed to a tongue-and-groove design to increase service life.

A connecting rod from a failed engine ruptured in fatigue without evidence of excessive stresses, detonation, overheating, or oil starvation. The origin of the fatigue failure was completely mutilated but decarburization was observed. Significant amounts of decarburization (0.010 to 0.015 in.) were found also in other forgings, such as exhaust rocker arms, main rotor drag brace clevises, bolts of carriage diagonal struts, and spring legs of main landing gears. The failure mode was low-stress, high-cycle fatigue involving tension and bending loads. The main cause was a manufacturing deficiency. The usual way to eliminate decarburization is to machine off the soft skin or employ better quality control when making them. Many aircraft manufacturers employ forged parts with machined surfaces or with shot-peened as-forged surfaces without excessive decarburization.

A connecting cap from a truck engine fractured after 65,200 km (40,500 mi) of normal service. The cap was made from a 15B41 steel forging and was hardened to 29 to 35 HRC. Visual examination of the fracture surface disclosed an open forging defect across one of the outer corners of the cap. The defect extended approximately 9.5 mm (3/8 in.) along the side of the cap. The fracture surface exhibited beach marks typical of fatigue. The surface of the defect was stained, indicating that oxidation occurred either in heat treatment or in heating during forging. Deep etching of the fracture surface revealed grain flow normal for this type of forging, but no visible defects. 400x metallographic examination of a section through the fracture surface showed that the microstructure was an acceptable tempered martensite. However, oxide inclusions were present at the fracture surface. This evidence supported the conclusion that fatigue fracture initiated at a corner of the cap from a forging defect that extended to the surface. Fatigue cracking was propagated by cyclic loading inherent in the part. Recommendations included more careful fluorescent magnetic-particle inspection of the forged surfaces before machining and before putting the part into service.

This article discusses failures in shafts such as connecting rods, which translate rotary motion to linear motion, and in piston rods, which translate the action of fluid power to linear motion. It describes the process of examining a failed shaft to guide the direction of failure investigation and corrective action. Fatigue failures in shafts, such as bending fatigue, torsional fatigue, contact fatigue, and axial fatigue, are reviewed. The article provides information on the brittle fracture, ductile fracture, distortion, and corrosion of shafts. Abrasive wear and adhesive wear of metal parts are also discussed. The article concludes with a discussion on the influence of metallurgical factors and fabrication practices on the fatigue properties of materials, as well as the effects of surface coatings.