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Engineering and Metallographic Aspects of Gas Turbine Engine Failure Investigation: Identifying the Causes by Robert E. Dundas, Principal Engineer Factory Mutual Engineering & Research Technicians are seldom assigned the provide knowledge of how such responsibility of analyzing the cause failures can be avoided in the future. of a gas turbine engine failure, but it Knowing the features of cracks and is not uncommon for them to be in- fractures in materials used in gas tur- volved in the investigation of a pre- bine engines is important in associat- mature gas turbine engine ing them with failures. malfunction or failure. Table 1 (page 2) lists the most likely As a participant in the tear-down in- initiating failure mechanisms for the spection of the engine and review of major components of gas turbine en- the findings, the technician must un- gines. The list is derived from en- derstand the modes of metallic com- gine case histories and an ponent failures and how each understanding of the design and can be identified during the function of each component. The investigation. mechanisms in boldface represent the most common failures. The purpose of investigating a gas turbine engine failure is to identify a Cracks and fractures in gas turbine definitive cause of the failure to engine parts, both rotating and FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JANUARY/FEBRUARY 1994 1 Table 1 Failure Mechanisms of Gas Turbine Components SectionComponents Problem Compressors Disks High-cycle fatigue Low-cycle fatigue Blades High-frequency fatigue because of resonant vibration High-frequency fatigue because of flutter Clashing and clanging because of surge* Stator vanes Clashing because of surge High-frequency fatigue because of resonant vibration or flutter Combustors Liner Thermal fatigue Overheating Turbines Disks Low-cycle fatigue, possibly aggravated by thermal gradients Fracture because of embrittlement Blades High-frequency fatigue because of resonant vibration Creep-rupture Nozzle vanes Thermal fatigue Overheating *Problems in boldface represent the most common failures. Source: Robert E. Dundas stationary, occur as a result of four • Creep; and, mechanisms: • Embrittlement. • Short-term loading; In nearly all cases of short-term • Fatigue; loading, cracks progress almost in- stantly to complete fracture. 2 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JANUARY/FEBRUARY 1994 Fatigue, creep and embrittlement are is by rotor overspeed, and a rotating time-dependent phenomena, so cracks disk is usually the part with the least sometimes become evident before margin in overspeed limitations. complete fractures occur. Overspeed damage is usually easily identified without metallography Almost all materials of importance in analysis. gas turbine engine failure investiga- tions are ductile, which means that the Ductile material fractures as a result materials can be stretched, drawn or of shearing flow of the material at hammer-forged without breaking. an angle approximately 45 degrees Nickel-based alloys used in turbine to the applied stress. This creates blades have very low ductility at room the characteristic appearance of the temperature, but the alloys become fracture surface, typified by a lip at more ductile at their operating tem- the edge, formed by shearing forces peratures. as the two parts of the component separate. The face of the fracture The features of cracks and of fracture surface tends to be rough and angu- surfaces provide indications about lar because of the shearing flow. which mechanism produced the crack Ductile fractures are usually or fracture. These can be studied by transgranular, in contrast with brittle fractography (macroscopic or micro- fractures, which are usually scopic) and metallography. intergranular. Fractography is the examination of fracture surfaces by macroscopic (use Fractured Blades Become of the naked eye or up to 10x magnifi- Missiles cation) or microscopic means. Micro- scopic examination can be up to 60x magnification. Scanning electron mi- When a rotating blade fractures in a croscopy (SEM) also permits magni- gas turbine engine, it becomes a high- fications of 5,000x or greater. energy missile that can do extensive impact damage to other blades in Metallography is the study of the struc- the same stage and downstream tures and physical properties of met- stages of the engine. If a gas turbine als and alloys. failure involves the fractures of many blades, it is necessary to determine Short-time fracture occurs when a which blade failed first. Sometimes component is overloaded, as in a this is obvious (as in the case of tensile-testing machine. The only way high-cycle fatigue), but it is usually this can occur in a gas turbine engine less obvious (as in the case of creep FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JANUARY/FEBRUARY 1994 3 rupture). Thus, it is important to rec- vibration. This is usually resonant vi- ognize the characteristics of a short- bration, but, increasingly, there are ex- term impact fracture. amples of self-excited vibration (flutter). This type of failure can oc- Impact fracture of a rotating blade cur very early in the life of a gas involves a combination of bending turbine engine. and transverse shear. In the bending mode, the fibers on the side of Another category includes low-cycle impact are in tension, and the tension fatigue, thermal fatigue and corrosion field quickly moves across the blade fatigue. This type of failure occurs as the fracture progresses to comple- only after the gas turbine engine has tion. The macroscopic fracture sur- been in service for a long period. face is rough and angular, but there may be a distinct pattern of ridges The stress/cycle curve in Figure 2 roughly parallel to the direction of (page 5) shows in broad terms the fracture progression. cycle regimes of the various types of fatigue. Figure 1 shows the features of an im- pact fracture involving a thin section. The regime of low-cycle fatigue and This is typical of the impact fracture thermal fatigue extends up to 10,000 of a thin compressor blade or of a cycles of operation. A cycle includes thin-walled turbine vane or blade. startup, followed by operation and shutdown. The upper end of this range Fatigue failures might be or might not may never be realized, although air- be life-related. One category includes craft engines are typically designed high-frequency fatigue that is caused for fatigue-free periods of 20,000 by blade, stationary vane and disk cycles or more. Graphic not available Figure 1. Matching surfaces of a thin plate of medium-carbon alloy steel, frac- tured by impact. Origin is at top of upper face and at bottom of lower face.1 4 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JANUARY/FEBRUARY 1994 Low-Cycle Fatigue High-Cycle Fatigue Alternating Stress 10 2 10 3 10 4 10 5 10 6 10 7 Source: Robert E. Dundas. Cycles Figure 2. Stress/cycle (S/N) curve shows in broad terms the cycle regimes of various types of fatigue. The high-cycle or high-frequency fa- until failure, the crack might then be tigue regime is where the S/N (stress initiated at one of these minute flaws. against number of alternating load The cause of the failure is often incor- cycles to failure) curve is a horizontal rectly attributed to the flaw, when the line marking the fatigue strength or actual cause might be resonant vibra- endurance limit. For most materials tion, flutter, shaft misalignment or an used in gas turbine engines the S/N excessive number of operating cycles. curve “runs out,” and the fatigue life should be indefinite. The high-cycle regime is usually con- sidered to start at 106 cycles. This re- The curve in Figure 2 represents the gime involves stresses that are usually number of cycles at which the first just above the fatigue strength after discernible (by usual nondestructive stress concentrations are included. testing) fatigue crack appears on the Such stress ranges are usually not surface. This is generally considered experienced unless the component to be a crack 0.03-0.06 inches (0.76- suffers damage that would increase 1.52 millimeters) long, which is no the stress concentration at the point of larger than a material flaw that can be maximum stress, or the fatigue reliably detected by nondestructive strength is reduced by corrosion. testing. Thus, a new part that has passed inspection for flaws can actu- If the misalignment of a gas turbine ally contain minute flaws. If the part engine’s bearings is sufficient to pro- is then subjected to alternating stress duce alternating stresses in the shaft FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JANUARY/FEBRUARY 1994 5 above the fatigue strength, the shaft in an elliptical pattern from an origin will eventually fail in high-cycle fa- on the blade surface until it covered tigue. Cycles of alternating stress build 40 percent or more of the blade cross at the rate of one per revolution. Thus, section. The crack surface is smooth, 107 cycles could be developed in ap- while the short-term final fracture sur- proximately 50 hours and, after initia- face is rough and characteristic of a tion of a fatigue crack, final fracture ductile material. The crack surface could occur in a few hours. contains distinctive lines or patterns concentric with the origin. These lines, High-frequency fatigue falls into the known as beachmarks, are associated category of high-cycle fatigue. How- with progression of the crack at dif- ever, the process of failure in high- ferent rates and stress levels as it ex- frequency fatigue is brief, after the tends through the blade cross section. excessive alternating vibratory stresses are established. For example, the ini- In all investigations of turbine blade tial crack can appear in 14 hours at a failure, microscopic examination of vibratory frequency of 200 Hz (hertz), 5,000x or greater helps identify stria- a typically low value, with final frac- tions whenever the crack surface is ture occurring no more than a few smooth and does not extend over hours later.

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