This article provides details on several of the classifications of polymer wear mechanisms, using wear data and micrographs from published works. The primary goals are to present the mechanisms of polymer wear and to quantify wear in terms of wear rate. The discussion begins by providing information on the processes involved in interfacial and cohesive wear. This is followed by sections describing the wear process and applications of elastomers, thermosets, glassy thermoplastics, and semicrystalline thermoplastics. The effects of environmental and lubricant on the wear failures of polymers are then discussed. The article further includes a case study describing the tribological performance of nylon. It ends by presenting some examples of wear failures of plastics.

During a routine preflight inspection of a piston aircraft engine, part of a cooling fin was found that had broken off the cylinder. The piece, made of aluminum-silicon alloy, was cleaned and examined. Based on the fracture characteristics revealed by an electron microscope, it was concluded that the fin failed in a brittle manner by overload.

The nose bullet of an aircraft engine developed a crack and was removed and examined to determine the cause. The bullet was made from a titanium alloy and was of welded construction. Close-up views show that the crack originated in an area of rework along the weld seam and propagated on either side. The case is a reminder that rework in a welded region requires careful planning and thought.

This chapter explains how investigators determined that a stabilizer link rod fractured due to overload, possibly by a combination of tension and bending forces that occurred during an accident. It includes images comparing the fractured rod with its undamaged counterpart recovered from the starboard side of the aircraft. A close-up view of the threads near the fracture surface provides evidence of bending, while the presence of dimples in an SEM fractograph supports the theory that the link rod failed as a result of overload.

This chapter discusses the failure of an aileron control cable in an aircraft and explains how investigators determined the cause. Based on their observations and the results of SEM fractography, investigators concluded that the cable had been damaged by a shearing tool, leading to its failure.

An adaptor and a bolt were overloaded during a flight causing them to fracture. This chapter recounts the circumstances that led to the failure and the investigation that followed. It includes images of the fracture surfaces which show that both components failed quickly due to overload conditions. It also recommends the use of twin suspension hooks to make attachment points more stable under difficult flight conditions.

This chapter discusses the failure of a control cable on an aircraft and the findings of an investigation that followed. The cable was made of stranded steel wire that was visibly worn. All seven strands had snapped and bore evidence of corrosion, pitting, nicks, and rubbing. Based on their observations and the results of SEM fractography, investigators concluded that tensile overload was the predominate cause of failure.

This chapter discusses the investigation of a helicopter tail rotor blade that fractured during a test flight. It includes images of the damaged blade along with close-ups of both sides of the blade tip showing that the tip tore off at the rivets. Based on their observations, investigators concluded that the rotor blade encountered a foreign object in flight causing the tip to shear off.

A first-stage compressor blade failed prematurely in an aircraft engine, fracturing at the midpoint of the root transition region. An examination of the fracture surface revealed beach marks, striations, and pitting, indicating that the blade failed by fatigue due to a crack initiated by corrosion pits in the root transition region. The chapter recommends further investigations to determine the cause of pitting, which appears to be confined to the dovetail region.

A cardon shaft operating in an aircraft engine failed and was taken out and analyzed to determine the cause. A photograph of the broken shaft in the as-received condition shows the location and orientation of the fracture. The fracture surface appeared smooth, indicating that a considerable amount of rubbing occurred after the shaft broke. SEM fractography revealed deformation marks and elongated dimples, typical of shear overloads, along with other details. Based on their analysis, investigators concluded that the cardan shaft failed under torsional overload. They also cited a need for a more detailed examination of the driven end of the shaft.

Several hydraulic pumps that failed in service on a particular type of aircraft were received for analysis. Hydraulic testing was not an option, so the pumps were disassembled and their plungers and cylinders were cleaned and examined. Based on their observations, investigators concluded that cavitation erosion damaged the plungers, causing them to seize.

Fastening screws used in fuel-injection pumps failed during assembly and were examined to determine the cause. Based on observations and the result SEM fractography and hardness measurements, the screws failed by brittle intergranular fracture due to hydrogen embrittlement associated with plating procedures. The report includes recommendations for improving the quality of the screws.

This chapter discusses the failure of an aircraft control cable and the investigation that followed. It explains how visual examination showed that the failure occurred in a heavily worn section that runs over a pulley. Further examination under a stereoscope and in a SEM found that 40 out of 49 wires had thinned and broke apart, and that the remaining nine wires failed in tension evidenced by cup and cone fracture and the presence of dimples on the fracture surface. Based on their findings, investigators concluded that cable snapped because of excessive thinning due to contact wear.

The quill shaft in an aircraft engine was found in two pieces following a flameout. One piece was short, straight, and otherwise undamaged; the other piece was bent in several places as was the sleeve that covered it. The facture surface, as viewed under optical and scanning electron microscopes, was flat and shiny with deformation marks and dimples, typical of torsional overload, and signs of severe rubbing on the periphery. Based on their observations, investigators concluded that the quill shaft failed by torsional overload, the source of which could not be determined.

A high-pressure turbine blade in an aircraft engine failed prematurely, fracturing close to the root. Visual examination revealed significant plastic deformation on the leading edge of the blade, blocky cleavage on the trailing edge, and a region covered with fissures in between. Based on their observations and the results of SEM imaging described in the chapter, investigators concluded that the blade failed by low-cycle fatigue, acting on a preexisting crack.

A compressor blade made of titanium alloy fractured during an engine test. The material and processing conditions of the blade were found to be satisfactory, turning the focus of the investigation to operating anomalies and human error. A photograph of the failed blade shows well-defined chevron marks along the fracture surface that end in a shear lip on the convex side. Further examination using a SEM shows that the failure was due to overload. Based on these observations and the results of tensile testing and microstructural analysis, investigators concluded that a sudden impact load on the concave side of the blade caused it to fracture.

This chapter describes an investigation that was conducted to determine why quill shafts were failing prematurely in gear boxes on aircraft engines. The investigation focused on the splines in a splined bore. Visual examination showed that the splines were heavily worn and covered with red powder on one end. Investigators also observed blueing, an effect of overheating. Based on these observations and the results of SEM imaging, it was concluded that the splines in the spline bore were wearing out for lack of lubrication.