Failure relating to material selection

Failure in polymeric products & materials can have many root causes. Some failures are a combination of causes such as poor processing coupled with poor selection of material for the end use. Some failures are caused by a lack of awareness of key issues, such as environmental, chemical or mechanical sensitivity of a particular material.

Smithers Rapra have classified the basis of primary failure mode for polymers in general as shown in Figure 1.0. A further breakdown of polymer product failure due to human causes is given in Figure 2.0 of which 45% are due to material mis-selection & poor specification.

Figure 1.0 Material / phenomenological causes of failure

Figure 2.0 Human causes of plastic product failure

Additional information concerning incorrect material selection can be found in Dr Chris O’Connor's article on ESC.

The majority of polymeric product / component failures occur in the mid to long term. Premature failure of ductile, tough & resilient polymers due to brittle fracture resulting in short service life is a common problem. Such brittle failures are most commonly caused by:

Material misselection

Failures due to poor material selection are very common in & appear to be related to a lack of awareness & understanding of polymeric properties. When, considering the design & development of a plastic component it is imperative that designers & engineers fully understand the fundamental nature of plastics. They should be aware & appreciate that:

• They are non-linear, visco-elastic materials
• They have temperature & time dependant properties
• They are materials that physically & chemically age
• They are susceptible to chemical attack & environmental stress cracking (ESC)
• They are susceptible to weathering
• They will, under the action of a tensile stress, eventually fail
• The time to failure will diminish:
  • as the stress increases
  • as the temperature increases
  • in the presence of certain environments
  • under the action of cyclic loading
• The moulding process can result in significant levels of moulded-in (residual) stress
• Weld lines due to converging melt fronts results in planes of weakness, particularly in fibre reinforced materials
• They exhibit notch sensitivity
• The addition of any form of filler will always have some form of detrimental effect on a material
• They can show mechanical anisotropy due to the alignment of fibre reinforcement