One of the most common features of machinery, consumer products, and assemblies of any type is the bolted joint. Sometimes the joint fails, with results ranging from inconsequential to catastrophic. The design of a joint is in the purview of an engineer, who must consider the material to be joined, the geometry of the joint, the loads imposed on the joint, the strength of the connectors (i.e., bolts, screws) environmental effects (i.e., temperatures, corrosion) and perhaps other factors.
We have investigated a number of bolted joint failures at Warren. Often, it is helpful to perform a metallurgical examination of fractured bolts or screws to determine the mode of failure, for example, failure due to overload or failure due to material fatigue.
If a bolt fails due to overload, the cause of the overload must be ascertained- was it due to an unexpected external load on the joint, or was it an inadequate design? Both tensile and shear loading must be considered.
If a bolt failed due to material fatigue (such as the classic example of bending a paper clip back and forth until it easily breaks), was fatigue loading considered in the design? Fatigue failure occurs only when there is a cyclic component of loading. A bolt that is preloaded by tightening properly is much more capable of handling cyclic loads. Consequently, fatigue failure of a bolt is not necessarily due to inadequate design- it may be due to improper tightening or failure to maintain the proper preload over time.
A joint designed for high strength fasteners, such as Grade 8, should always be reassembled with an equivalent or higher grade fastener. The grade of a bolt can be determined by the markings on the head of the bolt. Failure to attend to this kind of detail may result is serious injury or death when a component fails.
Roger Davis, a senior consulting engineer at The Warren Group, is a licensed professional engineer in South Carolina, North Carolina, Alabama, Georgia and Mississippi. He holds a Bachelor of Science in Mechanical Engineering and a Master of Business Administration from the University of South Carolina. He’s also achieved a Certificate in Crane Safety from the Georgia Institute of Technology, Distance Learning and Professional Education Center, Atlanta, Georgia. Roger is a certified fire and explosion investigator and certified vehicle fire investigator. He is experienced with municipal water, sanitary sewer, and storm water system design, construction, and operations. His expertise also includes property damage and personal injury investigations involving municipal utilities. He is an accomplished gas and diesel engine mechanic and has more than 30 years of experience with hydraulic plumbing and piping issues. Roger has investigated claims and injuries ranging from pressure piping system failures and material and personnel handling equipment to large engine failures and fires involving machinery, generators and vehicles.