Why Failure Analysis Is Important to Safety and Consistency
April 13, 2022
In any manufacturing facility, equipment reliability is essential to maintaining a steady production schedule. When instruments fail, especially a system feature that is crucial to safety, the subsequent operational shutdown can have far-reaching effects on a company’s bottom line.
This is why failure analysis services are crucial to pinpointing problem areas in system workflow. By investigating, discovering and analyzing the root cause of failure, corrective steps can be taken to enhance reliability, heighten safety and ensure an uninterrupted workflow.
Learn Why Failure Analysis Is Important to Safety and Consistency
Recently, Constellation PowerLabs assisted an industrial manufacturer in pinpointing the root cause of a failed air flow component. The piece of hardware in question was a length of stainless-steel tubing which broke at a ⅜” compression fitting. In this case, the tubing supplied air to the primary containment instruments of a main steam line isolation valve. The break caused the valve to close which, in turn, created a safety issue. Operations were suspended and the broken piece was extracted and presented for laboratory analysis.
A Thorough Investigation Protocol
Initial inspection of the as-received tubing revealed a complete radial fracture on the end closest to the expansion loop while the fitting at the opposite end appeared to be intact. Further examination of the cross-section of the broken end of the tubing displayed two fracture initiation regions centered at approximately 135° and 315°. It was noted by engineers that the two ferrules (small nut-like rings used to seal and support the tubing) on the broken end did not exhibit the standard gap between the ferrules — a feature that is consistent with a compression fitting that has been excessively tightened.
An Issue of Varying Degrees
It was then observed that the planar, smooth, brittle appearance of the fracture surface was characteristic of fatigue. Next, two cracks on either side of the tubing at the failed end were detected which were highlighted by two regions of distinctly varied discoloration. This observation determined that there were two fracture initiation regions on either side of the tube which suggested the crack was propagating from both sides.
Yet, further analysis concluded that the 315° side was where the crack initiated and propagated due to the size differences in the final overload ligaments, as the final overload ligament on the 135° side indicated a low load at the time of failure.
The broken end of the tube which was left in the fitting after failure was then examined once removed from the front ferrule. The standard thickness of the tubing wall, which should have been at 0.70” thickness, measured 0.65” at 315° and 0.067″ at 135°. This discrepancy was consistent with an overtightened fitting.
Failure Analysis Digs Deeper
In order to confirm these initial suspicions, the failure analysis team elected to document the fracture path by arranging the fracture mating surfaces side by side, lining up the tube in its initial position at the time of failure and analyzing the break patterns from a lateral viewpoint. It was then noted that the notch formation left by the back ferrule was the primary site of the break in the tubing and fretting damage was noted at the 135° and 315° initiation regions. Also, fretting on the inner dimension of the back ferrule was consistent with that on the outer dimension of the tubing at the back ferrule position.
Finally, the failure analysis services team used a scanning electron microscope (SEM) to inspect the molecular condition of the metal on the ferrule side of the tubing at both the 135° and 315° locations of the break. The latter site displayed trans granular and non-branched characteristics which are typical of fatigue.
Finally, the small, non-elongated dimples in the metal at the overload area were consistent with a tensile (bending) load rather than being subjected to torsion or shearing. It was also noted that vibration could cause fractures of this nature, yet that possibility was ruled out as the location of the crack would have moved away from the ferrule notch and the number of cycles required to initiate and propagate failure would have increased. Also, it was noted that signs of over tightening were also present in the ferrule fittings at the opposite end of the tubing run.
Failure analysis determined that the ferrules at the broken end were overtightened, causing the failure overload to occur, which created a safety issue and system shutdown. Failure analysis services noted that a special tool made by the manufacturer of the component is currently available so that this situation can be avoided. This instrument is known as a “gap inspection tool” and it verifies that fittings of this type are not overly tightened. The team concluded that continued, consistent use of the gap inspection tool could help eliminate over tightening as a root cause of failure and thus, decreasing production shutdown due to safety concerns.