Composite materials have revolutionized many fields of engineering by providing lightweight strong components whose internal structure can be tailored to optimise their load-bearing capabilities. Engineering composites consist of high-strength fibres embedded in a lightweight matrix that keeps the fibres in position and provides the shape of the component. While many composite materials have an impressive structural performance, some also exhibit spectacular failure modes with noises like guitar strings snapping when fibres start to fail and with jagged eruptions of material appearing on the surface, as shown in the image. A year ago, I reported on our work in the DIMES project, to test the capabilities of our integrated measurement system to detect and track damage in real-time in a metallic section from an aircraft wing [see ‘Condition monitoring using infrared imaging‘ on June 17th, 2020]. Last month, we completed a further round of tests at Empa to demonstrate the system’s capabilities on composite structures which have been tested almost to destruction. One of the advantages of composite structures is their capability to function and bear load despite quite high levels of damage, which meant we were able to record the progressive rupture of one of our test panels during cyclic fatigue loading. Watch and listen to this short video to see and hear the material being torn apart – ignore the loud creaking and groaning from the test rig, it’s the quieter sound like dead leaves being swept up.
The moral of this story is don’t travel with me. Last week, I wrote about my train being delayed by someone pulling the emergency handle before we got to the end of the platform in Liverpool [see ‘Stopped in Lime Street’ on June 26th, 2019]. Four days later, I was once again on a late afternoon train to London waiting for it to leave Lime Street station. This time we didn’t even get started before the train manager announced that a road vehicle had hit a bridge between Crewe and Liverpool; and, so we were being held in Liverpool for an unknown period of time. I sent a message to my family telling them about the delay and one, an engineer, replied that I was ‘hitting the low frequency failure modes on the service quality pareto’. The Pareto principle is also known as the 80/20 principle. I first encountered it when I was working at the University of Sheffield and the Vice-Chancellor, Professor Gareth Roberts, used it to describe the distribution of research output in academic departments, i.e., 80% of research was produced by 20% of the professors. In service maintenance, it is assumed that 80% of service interruptions are caused by 20% of the possible failure modes. Hence, if you can address the correct 20% of failure modes then you will prevent 80% of the service interruptions, which is an efficient use of your resources. The remaining, unaddressed failure modes are likely to occur infrequently and, hence, can be described as low frequency modes; including passengers pulling emergency handles or people driving vehicles into bridges.
How do you drive into a bridge and block the main railway lines between London and the north-west of England? Perhaps the driver was using their smart phone which was not smart enough to warn them of the impending collision with the bridge. So, there’s a new product for someone to develop: a smartphone app that connects to dashboard camera in your vehicle and warns you of impending collisions, or better still just drives the vehicle for you. Yes, I know some vehicles come with all of this installed but not everyone is driving the latest model; so, a retro-fit system should sell well and protect train passengers from unexpected delays caused by road vehicles damaging rail infrastructure.
By the way, the 14:47 to London magically became the 15:47 to London and left on time!