Meaning
Loss of compressive force in a bolted thermoplastic assembly over time is the metric of interest. In fastened polymer joints, clamp load decay occurs as the compressed material relaxes under the constant strain imposed by the fastener. This behavior can lead to loose joints and failure of the seal or mechanical attachment.
Viscoelastic Behavior
Continuous stress relaxation in the polymer under the bolt head is the primary mechanism of force reduction. Thermoplastics are viscoelastic, meaning they behave both like elastic solids and viscous liquids, which causes them to flow away from areas of high localized pressure. Elevated temperatures accelerate this molecular rearrangement, causing the compressive force to drop rapidly in the first twenty-four hours after assembly.
Moulding Influence
Moulding conditions and resin choice directly influence the long-term tightness of the joint. Semicrystalline resins hold their tightening force better than amorphous materials.
Mitigation Strategy
Mechanical solutions can compensate for this drop in holding force. Designers often specify compression limiters, which are metal inserts that carry the compressive load and prevent the plastic from squeezing out. Alternatively, using wider washers distributes the force over a larger area, lowering the local stress and keeping the loss within acceptable limits.
