#failurefriday

Creep

Also known sometimes as cold flow, creep is the slow plastic, or permanent, deformation of solid materials that occurs over time when said materials are subjected to constant stress. Creep is most significant in solid materials that are operated at high temperatures, such as jet engine turbines, as the rate to failure is typically faster the higher the temperature. Several internal mechanisms contribute to creep, dependent on materials' microstructures, properties, composition, etc., including dislocation movement, grain boundary sliding, and diffusion. Creep is not just limited to polycrystalline metallic solids; plastic, wood, concrete, and glass materials, among others, also experience creep.

Travel ban failed. Trumpcare failed. Tweeting policy has failed. The wall is failing. Endless leaks from within his administration. 100s of positions yet to be filled. Hiding from the press. Failure Friday. Sick of winning yet?

Fouling

Fouling can be defined as "the settlement, and sometimes the growth, of undesired materials on solid surfaces in a way that reduces the efficiency of the affected part." Though it rarely leads to catastrophic failure of materials, fouling nevertheless impacts a part's function, leading to a failure of its intended purpose. Like corrosion, fouling has considerable economic impact. There are numerous types of fouling, including biofouling (accumulation of living organisms), precipitation fouling (accumulation of crystals precipitating from solution), and particulate fouling (accumulation of particulates), among others.

Though similar, with overlap in their definitions, fouling and corrosion are not necessarily the same thing. (Though corrosion fouling is one type of fouling where the corrosion deposits build up, and fouling corrosion is another thing entirely, where corrosion is caused by fouling.) Corrosion is caused in-situ, as a result of reactions with the material/part. Fouling is caused by build up of ex-situ material, i.e., material that is not a component of the part that is being fouled.

The effect of fouling on heat exchanges is of particular interest to the industries involved, as is membrane fouling.

Environmental Stress Cracking

One of the most common causes of brittle failure in thermoplastics is known as environmental stress cracking (beyond thermoplastics, it is more common in amorphous plastics than crystalline or semi-crystalline plastics). Environmental stress cracking refers to the fact that the failure is caused by both chemical exposure (the environment the plastic is exposed to) and stress - it is not the same as chemical attack and requires both components. This type of failure is also often referred to as an 'unexpected' failure, because the failure of a part can occur long after the chemical exposure that led to the issue. As such, testing for resistance to environmental stress cracking can take months.

Fracture Toughness

The fracture toughness of a material is typically defined as the critical stress intensity, above which cracks in a material will grow and lead to failure. This definition relies on preexisting cracks or flaws in a material, however while these flaws can be the initial stages of a crack they can also be pores in a material, inclusions, weld defects, etc. Fracture toughness for a given material is usually given as a range, as it can depend on environmental conditions as well as the respective stress and flaw orientations and the size/thickness of the part.

Case Study: Liberty Ships Failure

In the 1940s, the US built over 2700 vessels referred to as the Liberty Ships. While it was far from the majority (most of the sunken Liberty ships were a result of German attacks), several of the Liberty ships suffered sudden, catastrophic failures that would go on to change the field of failure analysis. Examples include that of the Schenectady, pictured above cracked in half. These ship failures are perhaps some of the best well known examples of materials failure, often taught (or at least mentioned) in introductory classes.

While there were a number of mechanisms that led to the failure of the ships, one of the most cited causes is the ductile to brittle transition temperature (DBTT). Once the ships entered colder waters the structure of the steel changed and fracture occurred. The DBTT was actually discovered as a result of these failures by metallurgist Constance Tipper, of Cambridge. As expected, the thousands of Liberty ships had a significant impact during World War II, but, more unexpected, they had a significant impact on physical metallurgy.

Case Study: Air France Flight 066

In September 2017, Air France Flight 066 suffered an uncontained engine failure during its scheduled flight from Paris to Los Angeles. The aircraft was a Airbus A380-861, a four engine craft, and only one of the four engines failed. As such, the crew was able to land safely nearly two hours later in Goose Bay Airport in Canada (the engine failed while the aircraft was over Greenland). No injuries or fatalities resulted from the incident.

Search and recovery for the parts, stated as crucial to the investigation, ultimately took two years to complete. The ultimate cause of the failure was determined to be cold dwell fatigue. This was critical because the scientific community at the time believed the alloy used in the engine parts that failed (Ti-6Al-4V, or Ti64) did not suffer from this particular failure mode. The ultimate recommendations to prevent similar failures in the future went not to aircraft operators or maintenance, but to the manufacturers of the engines, asking for further inspections of critical parts.

Fatigue Strength, Fatigue Limits, and S-N Curves

With fatigue a result of failure due to cyclic loading, materials do not have singular fatigue strength values. Instead, fatigue strength is based on the cycles to failure - the fatigue strength of a material at one thousand cycles will be different from the fatigue strength of a material at one million cycles. These values can be seen on what are known as S-N curves, or plots of stress (S) versus number of cycles to failure (N). The more cycles to failure, typically speaking, the lower the required stress (or fatigue strength). Another value that can be determined from some S-N curves is the fatigue limit. This is the stress value (theoretically - the real world is always more complex) below which a material will not fail, no matter how many cycles are endured. Not all materials have a fatigue limit.