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Making the Most of Impact Testing

Falling weight impact tests can yield much more relevant information than single-point impact testing; however, they come with their own challenges. 

In my previous blog, I indicated that ‘single point’ impact testing (Izod and Charpy) at best gives some sort of ranking of materials but has less relevance to product design, materials selection and product quality control. Falling weight impact tests, either from dropping a product onto a hard surface or dropping a weight onto a product, can yield much more relevant information. However, it may take a large number of tests to generate statistically significant averages.  Instrumented falling weight tests can identify and quantify contributions from crack initiation and crack propagation. They can also measure peak force and deflection.  Variables such as impact speed, impact geometry and temperature are also easier to explore.

Many years ago, while working in the PVC flooring industry, I encountered a problem of cracking in 60m rolls during transit in cold weather.   To evaluate the reformulation of the PVC, we ended up cooling the rolls in a cold store. Then we tossed them off a loading bay.  This provided my technicians with an adrenaline rush. However, what appeared to be an expensive test turned out to be more cost-effective than the months of laboratory testing on a modified low-temperature impact test and solved the cracking problem.

So how to evaluate the impact resistance of a plastics product thoroughly? Tests would have to be carried out corresponding to a range of service conditions. These include temperature, humidity, speed of impact, the geometry of impact and stress concentration factors.

 

This article was written by Dr.Charlie Geddes for Hardie Polymers.

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