Understanding the “blue spot”

Abstract

During hot component fatigue tests there have been two cases of low life crack initiation of gas turbine rotating parts manufactured from the Titanium alloy Ti-6246. Both exhibited a small (~0,1mm) elliptical "blue spot" at the origin. Through validated striation count work and fracture mechanics it was established that fatigue had propagated with a near-nil initiation life. Early investigation suggested that the "blue spot" was possibly a region of stage 1 fatigue growth, and was therefore a material behaviour concern with potential implications for service. During an investigation of a later cracking incident in this alloy, subsequently shown to have resulted from Stress-corrosion cracking (SCC), near-identical fractographic characteristics to that seen in the "blue spot" were found that subtly differentiated it from stage 1 fatigue. Also, similar "blue spots" have since been identified on Ti6246 Laboratory hot LCF test specimens and found to have been due to contamination by NaCl, through the application of focussed long-term EDX examination and other novel chemical analyses techniques. By the application of those techniques, fractography, and comparison against these specimens, Rolls-Royce and Imperial College London have collaborated to show that the original two component "blue spots" were subtle examples of NaCl-related Hot Salt Stress-Corrosion Cracking (HSSCC). Such cracking has not been found to occur in service components, due to air pressure within the engine, and the effect is therefore confined to Laboratory and component tests at near-atmospheric pressure or below.

Failure Analysis of a Humeral Shaft Locking Compression Plate—Surface Investigation and Simulation by Finite Element Method

“…The morphology, although most features obscured, still resembles the one for a fatigue failure. The crack origin location can be inferred in the top right corner where there is a feature similar to a “blue spot” [44], a fracture aspect which was observed in stress corrosion cracking in a NaCl environment. Given this aspect, an NE–SV crack growth direction was considered.…”

Hydrogen in Ti and Zr alloys: industrial perspective, failure modes and mechanistic understanding

“…Rolls-Royce failure investigation experts also employed validated striation counting and fracture mechanics to establish that fatigue had propagated with a near nil initiation life. The reader is referred to Saunders et al [78] for further details. It should be noted that in all cases (rig test components and laboratory LCF specimens) the blue colour was uniform across the feature and did not show a gradual colour change with crack length.…”

“…Rolls-Royce failure investigation experts also employed validated striation counting and fracture mechanics to establish that fatigue had propagated with a near nil initiation life. The reader is referred to Saunders et al [78] for further details.…”

Effect of plasma electrolytic oxidation on the hot salt corrosion fatigue behavior of the TC17 titanium alloy

“…[7,8] Studies on the coupling of salt deposition, high temperature, and stress mainly focus on hot salt stress corrosion cracking, and the applied loads include constant loads [9] and slow strain rate tensile loads. [10][11][12] In terms of the mechanism causing the hot salt stress corrosion of titanium alloy, it is generally believed [9,[13][14][15][16] that solid NaCl deposited on the surface of a titanium alloy reacts with TiO 2 on the surface at high temperatures and with a certain amount of humidity to generate Na 2 TiO 3 and HCl; the HCl then reacts with metal elements in the exposed matrix to generate metal halides and hydrogen. [17] Hydrogen adsorption and diffusion into the lattice of a titanium alloy substrate may lead to hydrogen embrittlement.…”