The Paris – Versailles Express, hurtling down the tracks at the then astounding speed of 50 miles per hour, exploded in flames when the drive axle on the lead locomotive broke, digging its front end into the railbed. The second locomotive in the tandem drive set smashed into the firebox of the lead engine along with the first three cars, killing 57 passengers outright and injuring over a hundred more. It was the 1800’s equivalent of a jumbo jet crash, and the great scientific minds of the day focused their collective wisdom on perhaps the first major failure analysis in history. The result of their decade long investigation produced the beginnings of our understanding of fatigue.
Fatigue is the most common type of fracture in engineered components. Fatigue fractures are also particularly dangerous because they can occur under normal service conditions, with no warning that a progressively growing crack is developing until the final catastrophic failure. The component, whether it’s the outer aluminum skin of a commercial jet or a simple tubular chair leg, often appears to be perfectly sound with no visible distortion to warn of impending failure.
A technical understanding of fatigue requires a comprehensive knowledge of metallurgy, physics, and phenomena like plastic deformation, slip planes and dislocation theory. In fact, there are several competing theories on exactly what happens at a microscopic level when a fatigue crack initiates. But a practical understanding of the process is extremely beneficial and has direct application to its prevention in service and the manufacturing environment.
To the non-technically inclined, the term “fatigue” suggests this type of failure is related to the age of a component; that the material is “tired”. In fact, fatigue fracture can occur within hours of a component going into service or, conversely, even highly stressed components can operate for decades with no fatigue cracking or failure.
Fatigue fractures result from repeated, or cyclic, stresses. These stresses can take a variety of forms, such as bending (in one direction), reverse bending (back and forth in two directions), torsion (twisting in one or more axis) and rotation. Regardless of this variation in form, the stress on the component at the initiation point of a fatigue fracture is always tensile stress. In other words, the point of origin at which the fracture begins is being “stretched apart”, or pulled in opposite directions. To illustrate this, visualize a tube which is being repeatedly bent in one direction. The side of the tube that is concave when it is bent is being compressed. The side of the tube which is convex is being “stretched”, or subjected to a tensile stress. This is the side on which a fatigue crack will initiate.
Fatigue cracks initiate at stresses below the tensile strength of the material. Tensile strength is the stress, or load, at which a material breaks when pulled in opposite directions. Each metal alloy has a specific tensile strength, expressed as a numerical value, varying somewhat depending on heat treatment and other processing operations. These values are widely available in engineering reference manuals, typically expressed as pounds per square inch in American references. The fact that fatigue cracks can initiate at stress levels below the tensile strength of a material is difficult to explain. Theories on why this occurs suggest physical and structural changes at the microscopic (0.0001” or less) area of crack initiation.
Fatigue is a progressive fracture mechanism. Once a fatigue crack initiates, it advances further into the component with each stress cycle. This crack growth process continues as long as the component is subjected to cyclic stress. Depending on the magnitude and frequency of the stresses, the crack may grow over time frames ranging from hours to years. Eventually, the crack advances to a point where the remaining intact cross section of the component cannot sustain the next cyclic stress load – “the straw that breaks the camel’s back” – and complete fracture of the component occurs.
In Failure Analysis of Fatigue – Part 2 we will discuss strategies to prevent fatigue initiation that can be implemented at the design and manufacturing stages of a components life.