Plastics failure can cause economic and legal problems, as well as contributing to personal injury and death. The ‘owners’ of plastic products that have failed are, for obvious reasons, generally reluctant to publicise the fact. Failure diagnosticians tend to be restricted from doing so by confidentiality agreements and for this reason the activity is predominately covert. As a consequence the potential benefits such as learning from the mistakes and misfortunes of others, and identifying priorities for research and critical issues in product development are far from being fully exploited.
It is clear from the extent of plastic failures received by Rapra that this limited dissemination of plastic failure knowledge within the public domain has resulted in a continual cycle of plastic failure incidents from all industrial sectors. The lessons of good plastic product design are not being learnt even in light of the enormous growth in product liability cases that have imposed an entirely new dimension on the consumer product environment. It is now well established in law that manufacturers are liable for injuries resulting from defective product; for injuries from a hazard associated with a product against which the user should have been warned; or for damages caused by misapplication of a product which could have been foreseen by the manufacturer.
It is a practical necessity to understand why plastics fail in order to minimise the failure scenario. Rapra Technology has acquired this knowledge due to 80 years dealing with a diverse clientele providing technical services aimed at problem solving and in particular failure diagnosis.
Failure is a practical problem with a product and implies that the component no longer fulfils its function. Frequently, the ability to withstand mechanical stress or strain (and thereby store or absorb mechanical energy) is the most important criterion in service and consequently mechanical failure is usually a primary concern. However failure may be attributed to loss of attractive appearance or shrinkage etc.
The two main forms of mechanical failure are ductile and brittle failure. Ductile failure is, by definition, failure at high strain. It is relatively straightforward to design plastic components to avoid ductile failure. However, in practice, ductile materials often fail in a brittle manner, which becomes much more difficult to predict from a theoretical standpoint. Brittle fracture is a low energy process characterised by failure at low strain, with little or no deformation. Components contain small, crack like defects which can act as stress concentration features; these micro-cracks grow under load and may eventually lead to rapid catastrophic failure.
When considering the design and development of a plastic product it is imperative that a designer fully understands the fundamental limitations of plastics. A designer must be aware that plastics are:-
· Non-linear, visco-elastic materials
· Temperature dependant
· Materials that physically age
· Susceptible to chemical attack and environmental stress cracking
· They will, under the action of a tensile stress, eventually fail
· The time to fail will diminish as the stress increases
· The time to fail will diminish as the temperature increases
· The time to fail will diminish in the presence of certain environments
· The time to fail will diminish under the action of cyclic loading
· The moulding process can result in significant levels of residual stress in components
· Weld lines are planes of weakness, particularly in fibre filled materials
· Most plastics are highly notch sensitive.
· Mechanical anisotropy due to the alignment of fibre reinforcement
· Moulded articles rarely achieve theoretical material properties
Rapra’s experience has shown that many designers do not consider and / or are aware of these issues when considering the use of plastic materials. We have designers who can design but have no real appreciation for the material they are proposing to use. Rapra has found that Poor product design is endemic to all plastic sectors including the medical, pharmaceutical, automotive, rail, aerospace, packaging, oil / gas, energy, engineering and construction industries.
More than 5,000 failures have been the subject of study at Rapra. Interestingly the number of failures evaluated has increased significantly during the past five years.
Rapra Technology Plastic’s consultancy provides a range of services which allow engineers to prove designs at an early stage, ensuring that their product will be right first time inclusive of finite elemental analysis (FEA - ABAQUS), material selection (Plascams), evaluation of design aspects, injection mould simulation (3D Sigma), long term durability studies (creep, creep rupture, fatigue and actual product endurance testing.
A key to good plastic product validation is the generation of durability long term performance data. It is essential that designers and manufacturers of plastic products understand that short-term data provided by material manufacturers is useful only as a comparative guide between generic and sub-generic groups and cannot be used to gauge material performance in the long term.
In order to provide confidence that a plastic component will perform in the long term a prediction of failure stress in time under simulated in-service conditions i.e. temperature, environment is strongly recommended. Predicted long term data can then be correlated with 3D Sigma / FEA calculated residual and operational in-service stresses to determine a safe working life time for the product. Ideally testing of actual components is preferred so that the effects of possible moulding defects, moulded-in stresses etc can be assessed. However, due to complex component geometries and sizes this is not always feasible and subsequently material test specimens are tested.
Typical long term mechanical failure mechanisms resulting in catastrophic brittle cracking include creep rupture, fatigue i.e. cyclic stressing and environmental stress cracking which are discussed as follows:
Creep Rupture
Over a long period of time at constant load, most polymers will creep, causing failure. An aggressive environment accelerates failure. Creep rupture analysis generates a time to failure data for different constant stress levels. This data can be used to predict the life of a component and can be used in design calculations.
This method generates a time to failure curve for static creep at different stress. The data can be used to predict the effective life of a component where it is continually loaded under static conditions. The test can be carried out in aggressive environments to simulate operating conditions. Each test at an individual stress level is run for a maximum of 1000hrs.
For longer-term predictions, tests are carried out at elevated temperature. Then, data is predicted using time-temperature superposition techniques. Time temperature superposition is a well-established technique that is used extensively in the assessment of the long term (50 year) design stress of plastic pipes ISO 1167, BS EN ISO 9080.
These curves are then shifted to fit “by eye”, T5 to T4 and T5 + T4 to T3, etc. and a common shift factor found that can be applied to all of the data generated to produce a long term master curve at the required temperature.
The master curve can then be used to establish the failure stress (sf) of the material in the environment at the service temperature and at the desired life of the component.
Dynamic Fatigue Testing
If the component is subjected to any form of cyclic loading, then fatigue failure will be prominent, especially when there is an aggressive environment. Most plastics undergo a ductile to brittle transition during fatigue. Therefore, after a number of cycles, the fatigue strength dramatically drops. An ESC agent can make the transition more dramatic or occur after less cycles.
Fatigue testing is carried out at a relatively slow cycle rate (typically 0.5-1Hz). The number of cycles to failure (to a maximum of 106 cycles) is determined for different stress levels. The resulting curve can be used to predict the life of a component if the cyclic stress can be measured or calculated. In many cases the maximum permitted stress in a plastic component is significantly lower than expected from FEA calculations. High frequency cyclic loading measurements are erroneous since heat is generated causing the material to be plasticised.
Fatigue testing can be carried out at elevated temperatures and in aggressive environments. For longer-term predictions, tests are carried out at elevated temperature. Then, data is predicted using time-temperature superposition techniques. Samples can be tested in tension or compression and also with a cyclic load on top of a baseline load or a cycle through compression and tension.
Summary
The causation of plastics failure has many forms, most of which would be pre-empted by undertaking a thorough plastic feasibility study to ensure attainment of at least adequate product quality.
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