Fracture Analysis of Compressor Crankshaft

Analysis of Fracture of Compressor Crank

Abstract

This paper is concerned with the analysis of a fracture on the crank of a compressor, which caused the operation of the compressor to be stopped. The crank was observed to have a clear brittle fracture towards one end only and is separated into two parts. The fractographic examination was used to help in understanding the mechanism of the fracture. Furthermore, metallographic examination of the fractured surface, applicable mechanical tests and chemical analysis of the material were conducted to get an accurate picture of the material properties of the material. The results of this study disclose that the brittle fracture of the crank was caused by low-cycle fatigue which changed the properties of the microstructure of the material due to high-temperature exposure. The microstructure and the material properties of the crank were found to be non-uniform, probably due to metallurgical process defects. Recommendations for the redesign of the crank and an increase in the monitoring of the temperature in the device are proposed.

1. Introduction

A compressor plays an essential role in air conditioning and refrigeration systems in order to provide and maintain the cooling effect. The French company XYZ purchased a compressor from a Korean company ABC International and fitted it in their refrigeration system. After a few months of operation, the compressor stopped and it was observed that the crank had suffered a brittle fracture. An extensive study of the crank and its fracture was conducted in order to come up with a non-destructive solution for this and similar occurrences in the future.

2. Materials and Methods

The highly-stressed cylindrical shaft was constructed of the alloy AISI 1020. The tear bar test was conducted at room temperature, followed by hardness test, tensile test and a micro hardness test. A fractographic examination of the fractured surface was conducted in order to evaluate the type of fracture. Metallographic examination was also performed to observe the microstructure of the crank. Chemical analysis of the material was also conducted by optical emission spectrometry.

3. Results

The macroscopic observation of the crank revealed a clear brittle fracture located on one end only, and the crank was separated into two parts. The fractographic examination revealed that is was a brittle fracture due to fatigue induced by cyclic loads. The tensile test showed an increase in the ductility of the material towards the fracture end, which showed that the limits of the critical fatigue were crossed. The results of the metallographic examination and the chemical analysis showed that the microstructure and the material properties of the crank were non-uniform and probably due to metallurgical process defects.

4. Discussion

The brittle fracture of the crank was probably due to a fatigue failure as revealed by the fractographic examination. The fatigue mechanism is initiated from the local stress concentrations like free standing fins or areas of interrupted structure which raised the cyclic stress at the location of the fracture. The fatigue strength of the material was lower than the static strength due to the high-temperature exposure of the crank which changed its microstructure and the material properties. The microstructural non-uniformity observed in the tested specimens and the chemical composition altered by severe thermal cycles may explain why the cyclic strength of the material is much lower than its static strength.

5. Conclusion

The brittle fracture of the crank was caused by a low-cycle fatigue as indicated by the fractographic examination and the mechanical tests. The high temperature exposure changed the properties of the material and lowered its fatigue strength. The microstructure and the material properties of the crank were found to be non-uniform, possibly due to metallurgical process defects. Repair of the fracture was carried out and the recommenced actions for redesign of the crank to increase its fatigue strength and monitoring of the temperature in the device in order to prevent similar occurrences in the future are proposed.

6. References

Husain, M. I., Masui, T., Kodaira, S., & Kimura, K. (1999). Failure analysis of cracked compressor crankshaft in a car engine. Steel Research International, 71(2), 67–73.

Timoney, J. (2017). Introduction to notch toughness. Metals and Alloys in the Unified Numbering System, 6th Edition.

Sen, S. (2012). Failure Analysis and Prevention of Crankshafts. Journal of Failure Analysis and Prevention, 12(2), 72–80.

Wang, Y., Zhou, J., & Wang, T. (2006). Methodology for assessment of crankshaft. International Journal of Fatigue, 28(9), 1213–1224.

Compressor crankshaft breakage analysis

A crankshaft is an essential part of a compressor, which is a type of mechanical device that is used to compress air and other gases. Crankshafts are connected to machined parts and are responsible for transferring the rotational force from the engine to the compressors parts. As the crankshaft rotates, it also absorbs loads and accelerations in order to control the compressors speed.

When a crankshaft breaks, it is usually due to excessive loading or fatigue caused over time. In the case of compressor crankshafts, the most common cause is poor lubrication or contamination of the lubricant with foreign particles, which can damage the bearing surfaces and lead to failure. Additionally, an overly tight seal between the holes of the crankshaft and the surrounding casing can put too much strain on the crankshaft, leading to fracture.

In order to analyse a broken compressor crankshaft, there are several tests that can be performed. Firstly, the surface of the broken crankshaft can be examined for signs of wear, such as scoring, pitting or surface discoloration. Secondly, the crack itself can be inspected for features such as crack shape and size, number of cracks, and distribution pattern. This information can help determine the type of failure that has occurred and the contributing factors such as loading and lubrication.

The next step is to investigate the lubricating system. This involves the examination of the oil filter, oil pump, and other components to assess the condition and quality of the lubricant. This examination can help determine whether the lubricant has been contaminated or its viscosity was too low to protect against excessive wear.

Finally, it is important to examine the compressors operation characteristics, such as operating speed and loading cycles, to determine if there have been any abnormal conditions. This analysis can help identify the root cause of failure, which may include improper maintenance or assembly.

By performing the aforementioned tests, the cause of a broken compressor crankshaft can be identified and the appropriate steps can be taken to prevent similar failures from occurring in the future.