Rubber, without fail - longer life and failure resistance through modelling
Rachel Lawler reports on Extending the Life of Rubber Products, a talk hosted by the Rubber in Engineering Society at IOM3 in London, UK.
‘Rubber is one of the least understood engineering materials, particularly when compared to our knowledge of metals, such as iron and steel.’ Gary Crutchley, Lead Consultant at Smithers Rapra, based in Shropshire, UK, was clear that improving our knowledge is key in extending the life of rubber products. Crutchley also pointed to the various reasons behind rubber component failure, which include poor selection of materials, chemical attack and UV degradation.
But one of the biggest problems facing many rubber products is misuse. ‘Product abuse is something we always have to consider,’ Crutchley continued. A simple example demonstrated just how big a problem this is for manufacturers. ‘The hot water bottle is one of the most commonly abused products.’ While most households own at least one hot water bottle, few people use them correctly. ‘Natural rubber has a working temperature of around 80°C, so when filled with water that is 95°C or more the material’s physical properties are considerably changed. Hot water bottles also don’t have a sell-by-date so they tend be used over a very long period,’ he said. ‘One of the most common failures we see is the bottle’s neck tearing after being filled with water that is much too hot,’ said Crutchley.
David Ensor, Senior Consultant Engineer in the Vehicle Development Engineering Department at MIRA, based in the UK, explained some of the testing processes used in the automotive industry – another area where the performance of rubber components can be paramount to user safety. Ensor explained the evolution of automotive testing methods from three laps of a short test track in the 1920s through to today’s extensive process. ‘We continuously survey the usage of vehicles,’ said Ensor. He explained how MIRA uses this data to track the emergence of failure in vehicle components, although he warned that this was only part of the story. ‘The fact that you’ve found the failure does not mean you have found the failure mode. Correlation is not causation.’ Instead, vehicles testers must analyse all the collected data to discover exactly what conditions and circumstances led to the component’s failure. This is where detailed knowledge of materials is useful. ‘We rely on materials experts to let us know what the real parameters are for rubber,’ he said.
Another area where knowledge of rubber composites is crucial in ensuring safety is in the offshore oil and gas industry. Salim Mirza, FEA Manager at Element Materials Technology, talked about his work with elastomeric bonded hoses for the oil industry. ‘We would like to know beforehand when the failure may occur,’ said Mirza. But these hoses are massive structures, usually a couple of metres in diameter and often many kilometres long, so testing their performance is not a feasible option. In order to predict failure, his firm performs tests on samples.
Mirza explained, ‘By embedding fibres within the elastomer layers, a composite material is produced that exhibits higher resistance to failure.’ The team then used a variety of cyclical and time-dependent tests to track the likelihood of failure in these materials using an intermediate model of the hose. They tested various different levels of pressure and other variables that the hose would encounter in its life span. With many of these products using 10 layers of fabric reinforcement, this was no easy process. ‘Modelling an elastomer is a simple thing, but modelling fabric reinforcement is much more complicated,’ said Mirza. But with failure occurring soon after cracks start to grow in the rubber hoses, predicting this occurrence is essential.
Regardless of applications, the performance of rubber components is often critical. Even in cases where rubber components do not affect safety, their failure can be a huge problem. As Crutchley noted, ‘There is always a cost associated with failure,’ and as all the speakers seemed to agree, better knowledge of rubber composites and their properties is the best way to extend their working life.