What is Fracture and Fatigue?
Fracture and fatigue, more specifically Fracture Mechanics and Fatigue Anaylsis, are two areas of study within the field of Mechanics of Materials that hope to find how and why materials fracture under stress, and how materials fail after cyclical loading. Fracture Mechanics studies how cracks and other defects behave under stress, how they propagate, and can result in material failure. Fatigue Analysis studies how materials behave and can fail under cyclical loading, such as
Why Study Fracture and Fatigue?
As with any discipline within engineering, the desired outcome is to solve problems and improve human lives through the application of scientific and mathematical principles. Fracture Mechanics and Fatigue Analysis are especially important as they touch many areas of our lives. Below are a few specific examples of how their impacts are felt across different industries.
AEROSPACE
Ever wonder why airplanes have rounded passenger windows? In the 1950s, the world's first commercial jet airliner, the de Havilland DH-106 Comet, experienced a series of catastrophic failures only a year after it was introduced due to metal fatigue of the airframe - something that was not fully understood at the time. Extensive testing after the final crash showed that the square windows created higher than anticipated stress concentrations, resulting in a much shorter fatigue life, which ultimately led to the catastrophic failures that occurred.
AUTOMOTIVE
Since its inception in the late 19th century, the automobile has gone through significant changes over the last century, where a key evolution has been the transition to aluminum parts. Aluminum yields many advantages such as being lightweight, resistant to corrosion, and highly recyclable, but comes at the consequence of lower yield strength than steel, which is what was primarily used for most automotive parts in the past. The transition to a new material meant using different techniques to predict fatigue life and crack growth, to prevent catastrophic failures on the road as a result of a part fracturing.
INFRASTRUCTURE
An extremely tragic event, and unfortunate example in engineering principles, was the collapse of the Silver Bridge on December 15, 1967 resulting in the deaths of 46 people. It was constructed using an eyebar chain suspension system, where pairs of massive steel bars were joined by pins to support the weight of itself and the traffic. There was no built-in redundancy, so the failure of one eyebar or pin would cause the failure of the entire bridge. The mechanism that caused the failure was a tiny crack that began at a material defect, growing over time due to fatigue from traffic and temperature, eventually fracturing. Originally built in 1928, it was designed and tested to the standards at the time, where unfortunately fatigue, crack growth, and non-desctructive testing were not widely understood at the time.
MEDICAL IMPLANTS
Implanted devices, especially orthopedic implants such as artificial joints, bone plates, screws, rods, and pins, have to be strong and safe, as to not just to avoid replacement surgeries for as long as possible, but to most importantly protect the patient. Fracture is one potential failure mode, where repeated stresses from daily activities could cause microscopic cracks that grow until reaching a critical size and breaking. Another way implants can fail is fretting damage, where damage to the implant is caused by sliding between surfaces in contact. This can be especially dangerous as netal ions and debris can be released into the body and can lead to poisoning. Protecting patients' lives and ensuring they maintain a good quality of life after surgery is paramount, and understanding and being able to prevent and mitigate the different mechanical failure modes and mechanisms of implants is the first step from an engineering perspective.