In-service fatigue failure of engineered products and structures – Case study review

Service failure of engineered products and structures can occur by cyclic application of stresses (or strains), the magnitude of which would be insufficient to cause failure when applied singly. As a failure mechanism, fatigue involves initiation and gradual growth of cracks until the remaining section of material can no longer support the applied service load. Typically, both metallic and polymeric materials are susceptible to fatigue failure, whereas ceramics tend to be more resistant. There are a range of mechanisms found in practice that include high-cycle; low-cycle; thermal; surface; impact; corrosion and fretting fatigue. Fatigue as a failure mechanism has been attributed to playing a significant role in the demise of many engineered products and is considered one of the most common causes of structural failure found in-service, even though laboratory fatigue behaviour of most metals and alloys is well understood. There are a plethora of variables that influence the mechanism of fatigue, many of which are taken into account when attempting to designing for safe life of a component. However fatigue failures still regularly occur, demonstrating the complex nature of this problem. Instruction for the failure or forensic engineer will frequently demand focus on both mode and root cause of failure. When considering fatigue as a failure mode, there are common recurring design issues, along with exposure to unusual service conditions, which play a repeated role. Consequently, a brief review of salient points relating to fatigue as a failure mechanism is presented and illustrated by a range of studies from the authors’ forensic casebooks that attempt to exemplify the recurring nature of fatigue initiators experienced in practice.