Detailed measurements of bearing failure in composite bolted joints

To design a bolted joint, the transition bearing stress and maximum bearing stress have to be regarded. Transition bearing stress is the bearing stress where bearing failure begins and maximum bearing stress where the joint goes to total failure. A method for studying rotation in a bolted joint has been developed using an optical instrument called Digital Speckle Photography, DSP, which is a non-contact extensometer using digital cameras, together with a load frame. The data from DSP system is transformed and analyzed to get rotations for different parts of the joint. Thinner joints give higher values on transition bearing stress, higher maximum bearing stress and higher maximum bolt rotation, than thicker joints. Bolt torque has a big impact and higher torque gives the same positive results as thinner joints. Increased secondary bending makes the joint more resistant to bearing failure, which increases the transition bearing failure stress, maximum bearing failure stress and maximum bolt rotation. There is a relationship between the curve slope in a bearing stress versus bolt rotation figure and the transition bearing stress. If the joint has a lower value on bolt rotation, in comparison with other joints, at the same bearing stress, this will lead to a higher transition bearing stress.