Failure Analysis of Plastic Fasteners
2011-02-23 Author:Ni-Yuan Ma Class:CFW
Plastic failure comes in several modes and can have more than one failure occurred in a single design. General failures occurred include boss failure, low pullout, cracking, material and environmental factors, low drive and unloading ratio and joint relaxation.
Plastic series include more than hundreds of formulas, ranging from soft, rubber material to strengthening material with strength close to mild steel. Many types of plastics are regarded as slow flow liquid or cold flow (moving at stress and time). Plastics are divided into thermoplastic and thermosetting, while thermoplastic takes up the majority. Furthermore thermoplastic can be re-melted and injected into mold in the melting status to form individual component. Thermosetting is originally in the form of powder and changes from powder into solid hard substance due to pressure and heat quantity after being injected into mold. Additionally thermosetting cannot be re-melted and cannot be re-used. Except resin, thermosetting can add some additives, such as glass fiber, bead, anti-oxidant, bactericide, pigment and ultraviolet ray stabilizer, yet designer depends on purpose to decide which additive should be added. Plastic component is usually plugged into boss or positioning hole as its adhesive method. Boss comes in a cylinder type and includes a positioning hole for screw to plug in. General failure is boss radial cracking, cracking close to or far away from boss, screw pullout, plastic cracking and actual axial removal by using torque or screw.