At a time when UK plastics processing faces unprecedented global competition, speed to market is an important consideration. In consumer markets such as electronics, product lifecycles are getting shorter as companies vie to sell us the next ‘must-have’ item. Designing components for such a fast-paced environment is becoming increasingly challenging, and accurately predicting how a component will perform during its service life is an important part of the design process, particularly in safety-critical applications.
Reducing the Need for Prototyping
A project currently under way at the National Physical Laboratory (NPL) in Teddington, UK, aims to reduce or eliminate the need for the costly and time consuming prototyping stage by developing accurate computer modelling of plastic parts. The findings could have far reaching implications, both in terms of human safety and the competitiveness of key sectors of UK industry. One such UK manufacturer is already closely involved in the work. Land Rover has supplied the test piece, an interior door trim.
Using Finite Element Analysis to Predict Behaviour
‘The principal aim [of the project] is to show how finite element analysis can be successfully used to predict the behaviour of plastics under impact loading,’ says NPL project leader Greg Dean. ‘Designers using FEA packages such as Abaqus, Oasys and others will be immediately interested in the project outputs. Experimental results and the work on testing protocols will also impact upon the materials suppliers.’
Computer Simulations to Predict the Impact Behaviour of Plastics
The three year, DTI funded project will specifically evaluate the use of computer simulation to predict the behaviour of a plastic component experiencing an accidental impact loading. This may be a safety critical impact, such as that experienced by a car in a collision, or a construction worker’s hard hat, hit by a falling object. Accidents also occur in transportation and handling, e.g. of chemical tanks or other packaged liquids. Another commercially important area is the accidental dropping of devices such as mobile phones.
Many modern grades of plastics are tough materials that can sustain large strains before failure. Consequently, they are particularly suited to applications where accidental impacts are possible, and the material must withstand this without failure, or must limit the force level sustained by adjacent components.
Capability of Computer Modelling
Computer methods based on FEA are already available and widely used in some industry sectors for calculating forces, deformations, stresses and strains in components experiencing an impact. Using these methods, it is possible to explore the influence of different materials and component geometries on the forces and deformations experienced in the impact event. Testing trials can be reduced or eliminated, and the influence of design variables can be explored relatively quickly and easily. As the capabilities of computers and software continue to increase, this approach will continue to develop. However, in the case of plastic components, there are several factors that limit confidence in the accuracy of the predicted results. The key issues relate to the nature of plastic materials, and the ability to measure and model their behaviour in relation to impact performance.
FEA Modelling with Polymers
Materials models used within finite element systems have largely been developed for metals, and their value in predicting plastics behaviour is uncertain. To accurately predict the performance of a polymeric material, a model must take account of its non-linear and rate-dependent properties. In a previous project, the NPL team has developed such a model, based on a theory of cavitation, i.e. the growth of cavities or voids within a polymeric material under strain. In the current project they will evaluate the predictive capabilities of this model, and others, in order to provide a clearer guide to industry as to their appropriateness and accuracy. Studies will compare measurements and performance predictions for a vehicle interior part supplied by Land Rover and moulded by Intier Automotive.
There is a further problem in evaluating plastic materials behaviour, namely premature failure under specific circumstances. Materials may exhibit adequate toughness in simple standard tests yet fail in service. Temperature may be a factor, as well as complex stress states and structural features introduced in processing. There is evidence that standard test specimens do not provide an accurate indication of component performance. In previous work at NPL, failure criteria for ductile materials were explored, and this work will be continued in the current project.
Finally, there is a problem around the acquisition of the data required to accurately model the behaviour of plastic materials. Current standard test methods describe procedures for measuring plastic properties that are several orders of magnitude lower than those experienced in impact conditions, and there are experimental problems associated with property measurement at high strain rates. These must be resolved, and new test procedures validated and standardised. Again, NPL researchers have started to approach the problem, and the current project will take the work forward.
Comparing Test Samples with Component Samples
The current project, which is part of a broader research programme entitled ‘Measurements for Processability and Performance of Materials’, involves a number of other partners. Rapra has been involved in comparing component samples with standard test pieces in order to evaluate the influence of processing, and also to investigate the applicability of data generated from test specimens for design purposes. The cavitation model described above is being developed with the assistance of software experts at Oxford University, who are developing the code that will enable it to be used within both Abaqus and Oasys finite element software.
Significance of the Impact Strength Study
Although an automotive component has been selected for study, the results will have significance in many other sectors. With legislative issues such as the ELV (End of Life Vehicles) and WEEE (Waste Electrical and Electronic Equipment) directives forcing change upon component manufacturers, the results of the three-year project will not only validate the performance of plastic materials under impact loading, it will also Inevitably have an impact on how materials are designed for an increasing number of industry sectors.