A way of predicting the remaining lifetime of stressed steel has been demonstrated for the first time by physicists in Japan. Hirotsugu Ogi and colleagues of Osaka University have shown that structural faults in carbon steel make it absorb sound very effectively at a certain point in its life. The non-contact technique is based on the effect of this absorption on a magnetic field, and could be used to monitor the condition of axles in trains and motor vehicles (H Ogi et al 2002 J. Appl. Phys. 91 1849).
Metal components that are under repeated strain can break without warning. Until now, scientists have found it difficult to predict when such damage will occur because it does not depend on the age of the metal or the applied stress. Some existing tests are also unsuitable for real-life systems because a sample of the component needs to be removed for analysis.
But the technique devised by Ogi and colleagues – known as electromagnetic acoustic resonance – allows the lifetime of components to be tested in situ. The team placed coils around a carbon steel rod 14 millimetres in diameter to generate an oscillating magnetic field around it. Bending stresses of between 140 and 490 MPa were then applied to the rod as sound waves were sent through it.
Ogi’s team found that these waves made the oscillations in the magnetic field interfere constructively with each other, and these disturbances induced an electrical signal in the coils. This allowed the sound absorption to be measured without touching the rod, which would otherwise lead to loss of acoustic energy.
When the steel rod had been stressed for a certain length of time, the researchers noticed a dip in this signal, which showed that the rod was absorbing more acoustic energy. Scanning tunnelling micrographs of an identically stressed rod showed that this drop coincided with the appearance of a large number of dislocations. The replica rod also returned to its original state after heat treatment, which is a signature of dislocation damage in a metal.
After the dip was observed, Ogi and colleagues continued to bend the rod until it broke. They found that – depending on the experimental set-up – the dip occurred at either 85% or 72% of the total lifetime of the rod. These fractions remained the same regardless of the carbon content of the rod or the stress applied to it.
Dislocations reduce the energy of an acoustic wave as it propagates because they vibrate ‘anharmonically’. Since these defects are a common feature of metals, Ogi and colleagues believe that their technique will be suitable for a wide range of materials.
