By: Mitch Carswell
It has long been recognized that materials experience failure through either a large single force applied to the material OR repeated applications of a lesser load.
The Fatigue Failure Theory (FFT) has been around since the 1800’s and has gone by many different names, but the focus has always been how metallic or plastic materials become damaged when they experience repetitive stresses. A common example would be stretching a rubber band, when a force is applied to the end of the rubber band repeatedly and under low force the it would return to its normal shape. However, if the frequency and/or the force increases the rubber band becomes less elastic, more likely to stay deformed or potentially snap in half.
In 2014, Sean Gallagher & Richard Sesek completed research utilizing the same fatigue failure theory and applied it to the musculoskeletal system of humans to develop several ergonomic assessment tools. Each of these assessment tools have the same purpose as those that have been utilized for decades within ergonomics, to identify the risk of developing a musculoskeletal disorder (MSD). However, the use of a new model could be useful for something larger than simple lifting assessments. All ergonomists understand the difficulty and level of detail required to complete a thorough assessment that captures all elements of a worker’s job, it may not even have been possible to input all of the lifting and lower conditions into one tool and assess all aspects of the job. The time and effort that goes into an accurate cumulative assessment is one that requires new tools and new theories that can be applied in ways that biomechanical and physiological models cannot or potentially do the same assessments yet be much more efficient.
The research that was by started by Gallagher and Sesek has continued at Auburn University for several years now and has indicated that all materials including biomaterials share these basic material properties. This discovery as well as several others pertaining to recovery seem to fill a gap within the ergonomic assessment tool realm and that is an easy to use assessment tool that can be utilized to understand the cumulative effects of work and the damage incurred over the process of fatigue failure. These findings have led to several new ergonomic tools that aim to better understand and assess the effects of cumulative damage.
- LIFFT- lifting Fatigue Failure Tool
- Used to assess cumulative lifting tasks using a practitioner friendly tool based on the fatigue failure theory.
- DUET Model Logic.
- A new risk assessment tool used to assess Hand and Arm intensive activities based on fatigue failure theory.
- The Shoulder Tool.
- A new risk assessment tool for shoulder intensive occupational tasks based on fatigue failure theory.
The Fatigue Failure Theory (FFT) is an alternative to the previous biomechanical assessment tools that have been utilized by ergonomists for years. The ideal scenario would be to design tasks with low forces, low repetitions, neutral postures, and avoid vibration for prolonged periods of time. However, under the understanding that, realistically, that is extremely difficult, the goal would be to design jobs with low levels of risk for developing an MSD and measure it with ergonomic assessment tools to effectively detect the risk of injuries and make improvements.
If you or your team are interested in learning more about the FFT and the assessment tools that accompany it, Sandalwood is here to help with the process to reduce injury risk factors.
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- Sean Gallagher & Mark C. Schall Jr. (2016): Musculoskeletal disorders as a fatigue failure process: evidence, implications and research needs, Ergonomics, DOI: 10.1080/00140139.2016.1208848