The story
A merchant vessel was sailing along the Western Canadian Coast when the drive shaft of its propulsion system broke. Following the failure of the drive shaft, the vessel started to drift, and a towing boat was required to bring it back to port.
Inspection of other components of the boat propulsion system, carried out after failure, showed that, except for the drive shaft, no other failures occurred. The drive shaft was submitted to a full metallurgical investigation to determine the probable cause(s) of its failure.
The investigation was conducted by the undersigned and the results were originally published on February 9th, 2012, in the online edition of the Journal of Failure Analysis and Prevention. The following is a summary of the investigation and our findings.
The drive shaft had been in service for about six (6) years at the time of its failure. A general image of the fracture surface of the broken drive shaft is illustrated in Figure 1. The drive shaft was manufactured from low-alloy steel grade 4340, which was heat-treated to improve its mechanical properties (quenched and tempered). The fracture occurred at the section change between a gear seat and a bearing seat. A fillet radius was present at the section change.
Figure 1 – General view of the fracture surface
Source: From the original article published in the Journal of Failure Analysis and Prevention, ISSN 1547-7029, DOI 10.1007/s11668-012-9551-7.
The investigation
Visual examination of the fracture surface showed that drive shaft failed in fatigue, under low rotating-bending variable stress. The fracture initiation occurred on the drive shaft’s periphery, in a fillet radius machined at the section change between a gear seat and a bearing seat. The fracture then propagated progressively on about 95% of the total shaft’s cross-section, under both low-cycle and high-cycle fatigue mechanisms, as illustrated in Figure 2.
Figure 2 – Illustration of the initiation and propagation of the fatigue fracture
Source: From the original article published in the Journal of Failure Analysis and Prevention, ISSN 1547-7029, DOI 10.1007/s11668-012-9551-7.
Several destructive tests, such as chemical analysis, tensile testing, hardness measurements and optical microscopic examination confirmed the alloy grade and its general compliance with the design requirements.
Visual examination and fillet radius measurements showed that the fillet radius present at the fracture’s origin location was smaller than the one provisioned by design and inscribed on the drawing (1.58 mm compared to 2.5 mm required by design). A smaller than required fillet radius promoted a stress concentration at this location and the initiation of fatigue cracks. Hence, it played the role of a stress raiser (also known as stress concentrator).
The analysis
The findings of the investigation showed that the drive shaft broke in rotating-bending fatigue. The fracture initiated at the shaft periphery, at a fillet radius, and then progressed through about 95% of the shaft cross-section before the final fracture. The fillet radius was smaller than the one provisioned by design (1.58 mm compared to 2.5 mm required by design).
It is generally accepted that, for a fatigue crack to be initiated, two conditions must be simultaneously met:
- presence of variable stress; and
- presence of a stress raiser (stress concentrator).
Variable stress comes from the normal and/or abnormal operation load, vibrations, start/stop sequences, etc. In this case, every point on the periphery of the shaft sustained a tensile stress, then a compressive stress once every revolution. These conditions are usually present in rotating shafts and are consistent with the normal functioning of a drive shaft.
Stress raisers are areas where the load acting on a part, which could be at normal operation level or even lower, multiplies and concentrates at values exceeding the material strength. Thus, locally, the load perceived by the material is higher than its strength and small fatigue cracks will occur.
Stress raisers can have metallurgical origins (microstructure alterations, metallurgical anomalies, material defects) or can be geometrical details (grooves, fillet radii or thread roots). Improper surface finish, tool marks and sudden changes in shape are also cited among the most usual stress raisers.
In the case under study, the stress raiser was a too small fillet radius for the part’s diameter and section change. Indeed, fatigue cracks originated at a cross-section transition marked by a fillet radius of about 1.58 mm. According to the design requirements, this fillet radius should have been 2.5 mm.
This situation had a significant impact on the fatigue life of the shaft. Indeed, a smaller fillet radius at this location produced higher stress concentration than the one associated to a larger fillet radius (the one required by design) and, ultimately, promoted fatigue initiation and propagation which resulted in the fracture of the drive shaft.
The conclusion
Our investigation revealed that the drive shaft broke in fatigue, under rotating-bending stress. The fracture occurred at the shaft surface, at a location containing a fillet radius, which is a strong geometrical stress raiser and a location generally prone to fatigue cracking. The actual fillet radius value, which was lower than the one required by design and inscribed on the drawing, further increased this effect, weakened the area and promoted fatigue cracking followed by the complete fracture of the part.
We concluded that the cause of the drive shaft’s failure was a manufacturing defect, i.e. noncompliance with the design requirements and drawing, by the machinist. Our conclusion was validated by a finite element analysis (FEA) performed by the designer upon reception of our investigation report.
Questions on the subject? Contact the author of the article!
By Marina Banuta, P. Eng., Ph. D., Head of Department – Electrical, Mechanical and Materials
