Failure investigation of fuel tubes of a jet engine augmenter assembly

• Failure modes and failure mechanism of the fuel tube are fully analyzed.
• Premature cracking of the fuel tubes was the assembly stress due to force fitting.
• Stress analysis of the tube is conducted using FEM and residual stress measurement.
• 3D scanning is conducted to measure dimensional mismatch of the failed tube.

Failure of the fuel tube in a jet engine augmenter assembly has been investigated. A routine inspection detected that the fuel tube connected to the fuel augmenter was completely fractured into two pieces in two separate jet engines. Both fractures of the tubes occurred adjacent to a mounting area in the engine augmenter assembly. Fractographic observation revealed that fatigue cracking was initiated from multiple origins on the outer surface of the tube and progressed over 95% due to fatigue along the circumference, ultimately resulting in fracture. Features of the fractured surfaces in both failed tubes showed almost identical morphologies. X-ray observations showed no evidence of defects in the joint area. 3D scanning technology was employed to precisely measure the tubes, which revealed that both failed tubes had significant dimensional mismatches with the design drawings. Stress analysis including both finite element analysis and X-ray diffraction was used to identify the stress concentration level of the failed tube. Through detailed investigation, it was revealed that the primary reason for premature fatigue cracking of the fuel tube was unintended assembly stress due to the force of mounting the tube during installation.