A Position Paper on Failure Assessment and Establishing Root Cause in Failure Analysis

A. K. Koul

Introduction

Engineering failure analysis is an enormously important branch of engineering science. Notwithstanding the considerable ingenuity of scientists and engineers to design, build and operate sophisticated instruments, equipment, machinery and structures, sometimes their performance does not follow expectations. We see this in all types of man-made articles from transportation systems to energy delivery equipment. Gas turbine engines are no exception. There are many basic reasons:

• A part may be designed to requirements that do not fully represent the service conditions
• The operator of, say an aircraft or engine, may substantially change the way the equipment is used so that the original design assumptions are no longer valid, and damage may accumulate during service at a much faster rate than expected.
• A design may be faulty, involving inadequate safety margins of stress or operating temperatures. The choice of materials used in manufacture may be wrong.
• A defect, introduced during manufacture or assembly may remain undetected in the part during service.
• Damage introduced during service may miss detection during scheduled inspection and overhaul, and remain to cause premature failure.

The purpose of failure analysis is to determine how a component failed and what caused the failure to occur. Failure analysis is therefore one of the most important tasks carried out by the engineer because if correctly performed it should resolve any uncertainties about the validity of the original design, about the materials and manufacturing methods employed, and about the way the equipment has been used. If the basic design, materials, manufacturing or maintenance methods are found to be defective, then failure analysis provides a basis for sensible corrective action. This is a process often referred to as Retroactive Design.

However, the examination of the fracture surfaces or metallographic examination of the failed parts may not always be sufficient to get at the root cause of the failure of a component that may have been subjected to complex thermal-mechanical loading histories in highly corrosive or oxidizing operating environments. Examples of failures of a high strength steel bolt in an aircraft structure and a Ni-Fe base superalloy turbine disc in a land based turbine engine are presented in Figure 1 (a) and Figure 1 (b) respectively. A typical fracture surface attributed to stress corrosion cracking in stainless steel material is also included for comparison, Figure 1 (c). All fracture surfaces look very similar. Only upon examining the material manufacturing processes, part operating environments and loading conditions in detail, was the root cause of the steel bolt failure established to be hydrogen embrittlement and prolonged low temperature creep loading at 0.45 to 0.53 T/Tm of the alloy in the case of the turbine disc. In the case of the Ni-Fe base superalloy turbine disc, the client was convinced that stress corrosion cracking as opposed to creep mechanism was responsible for the turbine disc failures because the OEM including several other consultants that they had engaged, prior to involving LPTi, had reached this conclusion.

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