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A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 69: Spring 2018
Bolted Connection Failures
By John L. Ryan, P.E.
© 2018 M.A.S.E. LLC
Connection failures have devastating results if the failure is part of a structural member or releases stored energy unintentionally. Connection failures on guards can result in exposed, unguarded hazards that result in accidents. Bolt failures can occur anywhere a bolt is used to connect components. There are two factors to correctly connecting components—design of the connection, and construction/implementation of the connection. Later damage or corrosion weakens the connection.
If a bolted connection becomes loose, the forces acting on it can place bending forces on the bolt. If the forces acting on a loose bolt act at an angle, it will cause the connected members to tilt and place levering forces on the bolt. The loose bolt allows this deflection and loading of the bolt in bending which drastically reduces the strength of the connection.
Bolt failure with bending
The simplest failure mode is tensile failure of the entire bolt itself. Tensile failure is identified by a necking down of the bolt near the fracture location. This is where the material yielded and stretched out lengthwise, resulting in a smaller cross section, prior to failure. These types of failures can be due to an under-designed connection such as using too small of a diameter of bolt, inadequate material used in the bolt, and/or too few bolts to complete the connection. Tensile failure can also occur if other bolts have come loose, which eliminates the functionality of these bolts, at least until the bolt is again placed in tension once the bolt starts to stretch and yield. Corrosion, creep, or embrittlement of the bolt can reduce the strength of the bolt.
Bolt loaded in tension
The material that the bolt is holding together can also fail, in shear, prying, or tensile pull-out. This can occur in concrete anchors if the force applied is greater than the concrete can withstand. A bearing failure occurs when the material around the bolt fails. We have seen material failure at bolted connections occur on ladder rails at the stabilizer bar connection point, in concrete and stone anchors, wood screw connections on chairs and attic ladders, and on machine guarding barriers.
Ladder rail shear failures
Other Failure Modes
Torsional failure occurs if a twisting force is applied to a bolt that exceeds its strength.
Another potential failure mode is failure of the threads themselves. In this type of failure, the bolt itself remains intact, but the threads are sheared off when the bolt is placed in tension. This can occur on the threads of the bolt, or of the nut. The cause of thread failure can be incorrectly sized female thread diameter which places greater stress on the male threads of the connection. Another cause can be due to varying materials used in the male and female threads. A higher strength male thread may be able to withstand the forces applied to it while a female threaded piece of a softer, weaker material can result in failure of these softer threads.
Shear failure occurs when forces act perpendicular to the axis of the bolt that exceed the strength of the bolt.
Bolt loaded in shear
Fatigue failure can occur at bolted connections if there is repeated, cyclic loading of the bolted connection. Engineers consider fatigue failure as a part of the design process, and aim to ensure that the fatigue failure occurs well beyond the upper limit of the life expectancy of the product, or not at all. For fatigue failure to occur, there must be cyclic loading, levels of stress greater than the endurance/fatigue limit of the material (which can be low relative to the ultimate strength of a material), and often a stress concentration point such as a sharp edge, a scratch, hole, or steep angle.
How We Can Help
At MASE, we can determine the cause of a bolted connection failure. We have multiple ways to investigate bolt failures:
Engineering Calculations : Bolted connection design is a fundamental aspect of mechanical design, and a historical part of mechanical engineering knowledge and educational curriculum.
Destructive Testing: Destructive testing can prove the failure strength of bolted connections to help identify the cause of failure.
Hardness Testing: Hardness testing can establish the strength of materials in a nondestructive manner.
Microscopic Examination: Microscopic examination can provide additional data to pinpoint the failure mode.
Finite Element Analysis: Finite Element Analysis uses powerful software to determine stresses in complicated shapes and multiple loading directions to determine the effect of forces on bolted connections, which can provide more accurate data than traditional engineering calculations.
Finite Element Analysis Examples
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