In most people’s minds, mass car production involves lines of robots armed with welding guns, rapidly assembling a car body. Spotwelding has been the main method of sheet metal joining in automotive manufacturing for more than 80 years, but in the past decade manufacturers have been looking for alternative methods of joining. Adhesive bonding technology is rapidly gaining acceptance as an alternative to spotwelding, as it offers an attractive range of benefits to the design engineer. These include increased fatigue resistance and structural stiffness, the ability to join dissimilar materials - for example, metals to plastics or composites - and greater flexibility in manufacturing.
Use of Adhesives in Automotive Construction
Today, adhesives are widely used in combination with spotwelds (weldbonding) and for panel stiffening and vibration damping. Most modern cars have adhesive materials somewhere in their structures, but there are few instances in which true structural adhesive bonding has been used in safety-critical or load-bearing joints.
Reasons for Slow Uptake in Adhesively Bonded Structural Components in Automobiles
The reason for this lies with the problem of ensuring joint integrity on a high volume production line. Currently, good process control and an expensive regime of destructive tests are used to check that adhesive joints are of sufficiently high quality. However, new and more economic techniques are needed for process control and quality assurance before manufacturers will consider more extensive use of adhesive bonding technology.
Ultrasonic Testing of Adhesive Joints
Adhesive bonding is widespread in the aerospace industry, in which ultrasonic nondestructive testing techniques are used to assess joint quality. Ultrasound is sensitive to the mechanical properties of the structure under test and can be used to detect hidden defects such as areas of porosity and disbonds.
Ultrasonic Testing and the Automotive Industry
Many ultrasonic inspection systems require calibration of the equipment to the structure and a skilled operator to interpret the measurement data. These inspection systems are difficult to use in the automotive industry, in which there are many different types of structure with different coatings, adhesive materials and joint geometries. More importantly, automotive manufacturers prefer a ‘black box’ instrument that can be used by a non-specialist operator and will give an automatic assessment of the adhesive bond condition.
Advances in Ultrasonic Testing of Adhesive Joints
Researchers at the Ultrasonics and Digital Signal Processing Laboratory, based at Nottingham University, have spent several years developing signal processing routines that can analyse ultrasound signals from a range of structures and make intelligent decisions on the joint quality. These include an analysis of adhesive placement, material thickness and detection of defects such as disbonding. The techniques employ methods such as adaptive digital signal processing and neural network analysis to allow computer characterisation of the ultrasound test results.
Development of an Ultrasonic Testing Device for Adhesive Bonding in Automobiles
Recently these software techniques have been combined with advanced ultrasonic instrumentation that has embedded computing power for automated data analysis. NDT Solutions, a spin-off company based at Keele University, in collaboration with engineers at Ford Motor Company and Jaguar Cars, has developed a prototype system that can be used by non-specialist personnel.
How the System Works
The system guides the operator to enable correct placement of an ultrasound probe and then excites, receives and analyses ultrasound signals from the structure. The system makes rapid measurements and can therefore be used in conjunction with scanning rigs. The results of the test are given on a pass/fail display and can be logged for later analysis, for example as part of statistical process control procedures. The system is currently undergoing pre-production field trials at Jaguar Cars and recently won a European Technical Achievement Award as part of the Henry Ford Technology Award Program.
The use of an automated NDT inspection system enables the manufacturer to greatly reduce costs and wastage by minimising the need for destructive testing. The system is also a key factor to achieving wider implementation of adhesive bonding technology and the benefits that this would bring to the consumer.