4.4 Environmentally Induced Failure
4.4.7 Metal Embrittlement
When evaluating material durability and predicting the part life cycle, one of the most challenging tasks is to minimize the unexpected abrupt failure.
This type of failure often occurs without noticeable precursors because of the subtle crack initiation process and the rapid crack propagation rate, such as the failures resulting from hydrogen embrittlement or cryogenic embrittle- ment. When a part is expected to serve under the conditions that have poten- tial to cause embrittlement, the evaluation of these embrittlement effects on part performance is essential in reverse engineering.
The absorption of hydrogen into an alloy lattice can result in brittle fail- ure for some alloys, for example, ferritic and martensitic steels, when they are under stress. The hydrogen lowers the bonding force of the metal lat- tice at the crack tip and locally embrittles the metal; consequently, the metal fails before yielding occurs. This phenomenon is referred to as hydrogen embrittlement. It often occurs in a humid environment, or an environment with the presence of sulfide, for example, oil well operations, which induces the evolution of hydrogen atoms. In contrast to stress corrosion cracking that usually results from anodic dissolution, hydrogen embrittlement is caused by cathodic polarization that introduces hydrogen atoms, and is reversible when the absorbed hydrogen is released.
The failures associated with hydrogen embrittlement observed in ferrous materials, particularly high-strength steels, often occur without warning and cracks can propagate rapidly. As a result, they can be catastrophic. The cracks of hydrogen embrittlement are usually intergranular and initiate at the sites with the highest tensile stress. It is highly advisable in reverse engi- neering to conduct a precautionary analysis on hydrogen embrittlement on a part operating in an environment susceptible to hydrogen embrittlement because of its abruptness and unpredictability.
Liquid metal embrittlement is another catastrophic brittle failure mode and deserves proper consideration in reverse engineering. A normally duc- tile metal can fail rapidly when it is coated with a thin film of liquid metal such as cadmium. The necessary conditions required for liquid cadmium embrittlement of steel are pure, unalloyed cadmium in contact with steel under tensile stress at temperatures in excess of 320°C. This temperature condition limits the cadmium embrittlement only to the parts exposed to relatively high temperatures, such as the steel aircraft engine compressor disk. The time to failure is generally a function of the temperature and the stress of the exposed part. To prevent cadmium embrittlement, the steel part is first coated with a layer of nickel, followed by a cadmium outer coating.
The nickel and cadmium react to form an alloy with higher melting tempera- ture than pure cadmium, thus immobilizing the cadmium and preventing cadmium embrittlement.
The mechanical behaviors of materials at cryogenic temperatures are com- plex and vary from alloy to alloy. Certain alloys show excellent durability at cryogenic temperatures and are referred to as cryogenic alloys. The yield and tensile strengths of these structural cryogenic alloys will increase as the temperature decreases. For example, plastic deformation on stainless steels such as 301 and 304 at cryogenic temperatures causes partial transformation to martensite, which strengthens these alloys. The effects of low-temperature exposure on ductility and toughness of cryogenic alloys usually depend on alloy composition and structure. Most face-centered cubic (FCC) metals, such as 2024 and 7075 aluminum alloys and IN718 nickel-base superalloy show bet- ter tensile and yield strengths and fracture toughness with comparable duc- tility at cryogenic temperatures; the fatigue crack growth rate is either equal to or lower than the rate at room temperature for IN718. Significant increases in yield and tensile strengths are observed for Ti–6% Al–4% V as the tempera- ture is reduced from room temperature to cryogenic temperatures. However, in contrast to IN718, the fatigue strength of Ti–6% Al–4% V is significantly weaker when the test temperature is reduced from room temperature to cryo- genic temperatures. Cryogenic embrittlement is noticeably observed in some metals, for example, carbon steels, at temperatures below –150°C, which space vehicles can be exposed to at high altitude and in outer space. When reverse engineering any parts for a cryogenic service, the effects of temperature and cryogenic embrittlement in particular have to be carefully evaluated.
The environmental effects on mechanical properties once again demon- strate that the interrelationships between various mechanical properties are material specific and rely on many factors, from temperature to humidity.
A higher yield strength for one alloy under one environmental condition might imply better fatigue resistance; however, specific supporting data are required to draw any inference to any other alloy in different circumstances.
In summary, the demonstration of equivalent material durability and part life limitation in reverse engineering requires part-specific substantiation data.
References
Colavita, M., and De Paolis, F. 2001. Corrosion management of Italian Air Force fleet. In Life management techniques for ageing air vehicles conference proceed- ings. RTO-MP-079 (II). Neuilly-sur-Seine: NATO Research and Technology Organization.
Dieter, G. E. 1986. Mechanical metallurgy. New York: McGraw-Hill.
Kuruvilla, A. K. 1999. Life prediction and performance assurance of structural materials in corrosive environments. A state of the art report in AMPT-15. Rome: AMPTIAC.
Kvernes, I. A., and Kofstad, P. 1972. Met. Trans. 3:1518.
NTSB. 2005. http://www.ntsb.gov/Pressrel/2005/051222a.htm (accessed December 24, 2009).
Rice, R. C. 1988. Fatigue design handbook. Warrendale, PA: SAE.
Sawai, T., Matsuoka, S., Abe, T., et al. 2003. Method of evaluating high fatigue strength material in high tensile strength steel and creation of high fatigue strength mate- rial. U.S. Patent 6546808.
Walker, K. 1970. Effects of environment and complex load history on fatigue life. ASTM STP 462. West Conshohocken, PA: ASTM International.
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5
Material Identification and Process Verification
Material identification and process verification are essential to reverse engi- neering. This chapter will discuss the techniques used to analyze chemi- cal composition, microstructural characteristics, grain morphology, heat treatment, and fabrication processes. The chemical composition of a mate- rial determines its inherent properties. The microstructural characteristics are closely related to a material’s mechanical properties. Grain morphology reveals the grain size, shape, texture, and their configuration in a material.
These material characteristics are often analyzed simultaneously. For exam- ple, during an electron probe microanalysis, elemental chemistry is ana- lyzed to identify alloy composition; at the same time, a micrographic image will also be taken to understand the phase transformation that leads to veri- fication of heat treatment and the manufacturing process. The evolution of constituent phases in an alloy is a direct consequence of the prior manufac- turing process this alloy has experienced. The identification of these phases by their compositions and quantifying their amounts in an alloy will help engineers verify the manufacturing process used to produce the part.
The end product of material identification and process verification is usu- ally the confirmation of a material specification that is called out by the OEM in its production. Theoretically speaking, all the characteristics listed in a material specification should be tested and verified before it can be called equal to the specification of an OEM design. However, in real-life reverse engineering practice, usually only select characteristics are tested and com- pared. The characteristics that are tested are determined by their criticalities to the part functionality. The data that are specified in a typical engineering material specification will be reviewed in the next section to establish a foun- dation and create guidelines for future discussions.