National Physical Laboratory

Impact and High Rate Testing

The properties and failure modes of reinforced and unreinforced plastics can be 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 more critical and cost dependent applications, it is necessary to have improved engineering data rather than the energy data that can be obtained from notched or unnotched Izod or Charpy impact tests.

NPL has capability in two complementary and mutually self-supporting techniques
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).

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 0.01  mm/s to 3.5vm/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 instrumentation provides load-displacement data throughout the test, so that 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 -55 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.

Instrumented falling weight impact tests

An instrumented drop-weight tower is used to conduct impact tests at velocities of between 0.5 and 7.5 m/s.

A quartz load cell built into the indentor produces a force-time trace of the "impact" event. This can be converted by computer software 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.

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.

NPL has prepared an addendum to the ISO standard for this test method (ISO 6603-2) to incorporate a larger diameter support (100 mm 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 developed 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.

Evaluation of the repeatability under static loading of a compression-after-impact test method proposed for ISO standardisation (Adobe Acrobat PDF file 1.85 MB)

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For more information, please contact Mike Gower

Last Updated: 5 May 2016
Created: 24 Jul 2007

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