Food research could lead to low cost crack detection

Recent research into the ‘crunchiness’ of biscuits using a simple mechanical force in combination with an acoustic envelope detector, could lead to the development of similar but lower cost ultrasonic inspection and crack detection equipment suitable for general engineering and materials testing. Food physicist at the University of Leeds’ Department of Food Science Professor Malcolm Povey, told Eureka that his recent discoveries during the last 18 months, in which he has tested for the optimum ‘crunchiness’ of biscuits on behalf of leading food manufacturers such as Proctor & Gamble, McVities and Unilever, could pave the way towards low cost versions of his ultrasonics equipment for crack detection applications within general engineering. Using a microphone, an acoustic microscope and some software, Povey realised that the energy produced by the very first crack of a biscuit breaking in the mouth is released as distinct pulses of ultrasound – sound waves beyond the range of human hearing. Slowed down and plotted onto a graph, these pulses can be seen as a series of tall peaks, but actually last for only milliseconds and are generated at frequency levels more usually associated with bats, whales and dolphins. Povey told Eureka: “The discovery of recordable ultrasound pulses is expected to be of great interest to the food manufacturers, who in the pursuit of the perfect crispy/crunchy texture for their products, employ an army of trained tasting panels. These people form the crux of manufacturers’ efforts at product consistency and quality control in terms of creating the optimum texture for a product.” Povey is convinced that the same ultrasound measuring techniques could potentially be applied to other textures in food manufacturing as well as having major applications outside the food industry. He continued: “Essentially, our methods measure what happens when a material fails. So this technique could easily be transferred to industry to detect failures in materials used in engineering or the aerospace industry, for example. “Materials testing usually requires expensive equipment, but we’ve proved that recording, measuring and comparing sound pulses is rigorous and accurate. In the same way engineers used to tap wheels on railway engines to listen for faults, we can use our highly-sensitive microphones to record a much wider frequency range to pick up tiny defects. Its potential is enormous,” he added. Povey’s research over the last 18 months centred around the use of an innovative piece of ultrasound equipment supplied by Stable Micro Systems, a UK-based company that specialises in developing unique measuring systems for measuring and analysing the texture and consistency of foods. The research was a joint collaboration effort between Stable Micro Systems and the University. The actual system Povey used for the biscuit research comprises a texture analyser and an acoustic envelope detector (AED). The AED enabled Povey to calibrate the sounds detected from the breaking of the biscuits. The AED can be placed near to the mouth as the biscuit is crunched. The data collected from the AED can then be compared with what happens in the texture analyser. The texture analyser uses a probe which exerts a downward force on the biscuit and measures the force. The probe is moved by the machine at a constant speed through the biscuit so that both the force and the distance moved can be recorded. At the same time, the sound output from the biscuit is measured with a high quality, calibrated microphone. In very, crispy, crunchy materials, the sound pulses arrive so quickly that what appears to be one pulse is actually made up of multiple (typically four or five) pulses. In one graph shown to Eureka, a sound pulse was highlighted by the measurement software. When it was expanded, there were five separate sound pulses associated with each drop in the force of the texture analyser probe. “We tried to capture cracks being formed with photography,” explained Povey, “but the cracks were being formed too quickly. From initiation of the crack to propagation was around one microsecond. I therefore knew that at these speeds, ultrasound must be generated. I happened to have a very sensitive ultrasonic microphone in the lab so I then used it in combination with the Stable Micro Systems equipment, to do further tests. “There is an amazing future for ultrasound,” said Povey. “Especially as it is combined with other techniques such as light lasers. But it is not just ultrasound. The understanding of materials that we’ve developed using ultrasound techniques, combined with our expertise in creating, manipulating and characterising nanoparticles, means that in a sense we’re only just at the beginning. We now collaborate extensively with the Institute of Particle Science and Engineering at Leeds University and have many joint projects on the go.” Aside from the biscuit research, the University has also developed similar, unique ultrasonic apparatus for other tests, including a ‘Cygnus UVM’ ultrasound velocity meter; an ‘Acoustiscan’ ultrasound scanner; an ‘FSUPER’ miniature ultrasound spectrometer; and a very high pressure acoustic cell. Eureka was shown an acoustic microscope which can show water droplets suspended in oil and ultrasound transducers were being used to monitor the level of liquid in a tank and characterise the liquid too. Other systems can determine the solid content of fat in fatty foods Povey cited potential applications of the latest ultrasonic technology in the remote sensing of mechanical properties, medical applications (imaging human tissue and photoacoustics), nanotechnology and the detection of cracks in aircraft structures. In the latter application, Povey suggested that a lower cost version of the technology could be used to detect hairline cracks in aircraft wings or engine components. “You would only need to simulate the mechanical force or stress on the wing to the level you think that wing would receive under normal working conditions, then listen to the ultrasound pulses emitted. You could inspect railway tracks using a similar system. At the very least, my software here would help these manufacturers refine their existing crack detection software that they use. “The texture analyser, acoustic envelope detector and microphone, you could correlate the sound to the elasticity of the material or composite. Our research has shown that there is a definite statistical correlation [greater than 90%] between the mechanical forces applied, the acoustic output the way the material fails,” stated Povey. Our equipment used here for the research costs around £11,000, but the system could be re-engineered or adapted for general engineering use without too much difficulty and at lower cost. Existing ultrasonic crack detection systems are relatively expensive and so I see this as a possible direction for the technology in the future.” The future direction of ultrasonic inspection Looking to the future, those in the field of non-destructive testing see exciting opportunities. Defence and nuclear power industries have played a major role in the emergence of the discipline. At the same time, ageing infrastructures, from roads to buildings and aircraft, present a new set of measurement and monitoring challenges for engineers and technicians. Ultrasonics will find its way into the manufacturing process, to help improve productivity. Non-destructive ultrasonic inspection would increase the amount of information about failure modes and the speed with which information can be obtained and facilitates the development of in-line measurements for process control. What is ultrasound? * Ultrasound are vibrations which occur too quickly for us to hear them * For humans, our ears are most sensitive around 2kHz * Most of us hear very little above 16kHz * The AED and ultra sensitive microphone pick up sound pulses to around 200kHz * If we could hear ultrasound, we would find that it sounds differently according to the material through which it is passing