Impact Testing

The properties and failure modes of reinforced and unreinforced plastics are significantly rate dependent. For many engineering applications 'impact' data are required to design for adequate energy absorption (e.g. crash helmets, car facias, car bumpers) and against failure under use or abuse conditions (e.g. consumer goods, storage drums). For the more critical and cost conscious applications currently under consideration, it is necessary to have improved engineering data1, 2, 3 rather than the energy data which can be obtained from notched or unnotched Izod or Charpy impact tests.

NPL is developing two complementary and mutually self-supporting techniques in high rate (servo-hydraulic) testing and instrumented falling weight impact testing. In both cases the aim is to produce data on the stiffness and strength properties equivalent to that obtained in slow loading rate tests (i.e. failure in 30 to 90 seconds).

1. High rate servo-hydraulic tests

A servo-hydraulic test machine with a high flow rate valve is used to conduct tests at constant rates from 10-5 mm/s to 5 m/s. Normally, decade steps in loading or displacement rates are sufficient to follow rate dependence in the absence of a brittle-ductile failure mode transition. The large working area afforded by the servo-hydraulic frame allows both standard tests (e.g. tension, compression, shear) and component or sub-component tests to be conducted. The in-built instrumentation provides load-displacement data throughout the test, so that the standard data points of initial stiffness (modulus), ultimate strength, yield point etc. can be obtained for a range of displacement rates, from normal static tests to laboratory impact tests. The tests can also be conducted in an environmental cell with -200 oC to +200 oC capability. Reduced temperatures are useful as they have a similar effect to high test rates, increasing the specimen stiffness and strength for reinforced materials. It should be noted that under similar conditions unreinforced ductile plastics will eventually embrittle with a corresponding fall in ultimate strength and failure strain.

2. Instrumented falling weight impact tests

Instrumentation of the falling dart test has increased the cost effectiveness of this test compared to the previous 'staircase' method of testing and is already providing more valuable data.

A quartz load cell built into the indentor produces a load-time trace of the "impact" event. This can be converted by a computer program to a load-displacement trace which is equivalent in form to the trace obtained from a general static or slow rate test. Provided that the impact trace can be correctly interpreted, mechanical property data may be obtained using similar methods to those used for the slow rate tests.

The preferred impact geometry consists of a centrally loaded disc and analyses for this loading case are being developed for both strength and stiffness properties4,5. If this approach is successful then "impact rate" data will be obtained for impact velocities ranging from 0.5 m/s to 7 m/s.

NPL has prepared an addendum to the ISO standard for this test method (ISO 6603-2) to incorporate a larger diameter support (100mm compared to the 40 mm current standard). This provides a better flexural impact condition, rather than a puncture shear failure, and reduces the effect of the support jig on the extent of damage. It is proposed that this test could provide the impact phase of the "compression-after-impact" test for assessing residual properties of an impacted panel.

Current research is aimed at understanding the effect of the test conditions themselves on the recorded data (e.g. load cell type and position, filtering, load cell and software calibration) and of the interaction between material properties, specimen size and test conditions (e.g. support size, span-to-depth, indentor diameter etc.). A procedure has been developed6 for calibration of the load cell which has been adopted by at least one manufacturer and has been used by NAMAS for accreditation purposes. Work is continuing on a frequency calibration procedure.

References

SIMS G D, "A study of the impact performance of fibre reinforced plastics", PhD Thesis, Surrey University, 1976.
SIMS G D, "Understanding Charpy impact testing of composite materials", ICCM-6/ECCM-2, Imperial College, 1987.
LOCKETT F J, COUSINS R R and DAWSON D, "Engineering design basis for selection and use of crash padding materials", Plastics and Rubber: Processing and Applications 1, p 25, 1981.
SIMS G D and JOHNSON A F, "High rate materials properties from analysis of instrumented drop weight impact tests", International Conference on Impact Testing and Performance of Polymeric Materials (Paper 4), Plastics and Rubber Institute, University of Surrey, 1985.
MONEY M W, "Investigation of the instrumented falling weight test method", PhD Thesis, Queen Mary College, London University, 1987.
MONEY M W and SIMS G D, "Calibration of quartz load cells: An in-situ procedure for instrumented falling weight impact machines", Polymer Testing 8, pp 429-442, 1989.

For further information: click here