Tearing down testing

Deb Hopkins and the Lawrence Berkeley National Laboratory’s Engineering Division are investigating the non-destructive evaluation (NDE) techniques that may help vehicle manufacturers.

Deb Hopkins and the Lawrence Berkeley National Laboratory’s Engineering Division are investigating the non-destructive evaluation (NDE) techniques that may help vehicle manufacturers realise a quality-performance internal combustion engine that would get about 80 miles to a gallon of petrol.

‘Lightweight composite materials must be adhesively bonded which means the tear-down safety inspections of spot-welded steel structures can’t be used on them,’ said Hopkins. Even for conventional welding, the tear-down method of testing, in which the structural components of a vehicle are literally stripped off to assess the integrity of their welds, could stand improvement, Hopkins stated.

‘On-line non-destructive evaluation techniques offer the promise of testing 100 percent of all production components in real-time with the ability to spot problems immediately,’ said Hopkins. ‘It is also the only way to ensure the integrity of adhesive-bonded joints in lightweight composites.’

Hopkins and her colleagues have been working to develop NDE sensors and data processing and modelling tools. They have currently experimented with a wide range of NDE methods including acoustics and infrared thermography.

The acoustic NDE techniques being examined entail a component with a transducer, which converts electrical pulses into mechanical stress waves, then analysing the elastic waves that are generated.

‘It is well established that these waves are sensitive to the presence of flaws, voids, cracks, heterogeneities, or some other change in material properties,’ said Hopkins. ‘For example, a crack will reflect some of the wave energy, slow and attenuate the wave, and change the resonance properties of the component being tested.’

The success of any NDE technique in the automobile industry is said to depend upon its ability to quickly and accurately distinguish between critical defects that affect the structural integrity of a component and defects in low stress areas that would not significantly affect the component’s stiffness or strength.

‘Cycle times in the auto industry typically run about 15 to 20 seconds and the data acquisition and analysis of an on-line real-time NDE must be fast enough to render a go/no-go decision within this timeframe,’ continued Hopkins.

With acoustic-based NDE techniques, the compromise, said Hopkins, is between speed and resolution.

Infrared thermography is said to offer speed and high resolution as well as the advantage of full-field imaging. Measurements for this NDE technique are then derived from changes in thermal resistance that arise in the flow of heat through the components.

To utilise IR thermography, a heat flow must first be established through the components to be tested. This is done by heating one side of the component whilst cooling the other. The resolution obtained through IR thermography depends upon the thickness and thermal conductivity of the material being tested but Hopkins said the technique is applicable to a wide range of lightweight composites.

‘We’ve proven that acoustic and IR thermography NDE techniques work well in a laboratory setting,’ said Hopkins.’ The next step is to show that they can be made to work equally well in a factory setting.’

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