Spring 2005: Society for Adhesion and Adhesives One Day meeting on 'Fatigue and Durability of Adhesive Joints'
One day meeting on Fatigue and Durability of Adhesive Joints, held at the Society of Chemical Industry, 15 Belgrave Square, London on 20 April, 2005
This one-day meeting drew a good attendance and some excellent papers. Clearly considerable progress has been made in the last few years. These papers gave a timely review of some useful work.
The first paper was given by Prof. Tony Kinloch of Imperial College London and was entitled ‘The Fatigue Behaviour of Structural Adhesive Joints’.
He started by saying that fatigue during exposure to aqueous environments was the major concern. Aircraft and many bonded structures are exposed to the weather for most of their lives. The interface between the adhesive and the structure is the critical area. Fatigue loading has shown much lower resistance to crack propagation than monotonic loading. Tony reported that the cyclic-fatigue behaviour of bonded joints was studied under dry, humid and wet conditions. Various surface preparations were studied. These were grit-blasted and degreased, chromic acid etched and phosphoric acid anodised. Double cantilever beam fracture mechanics fatigue tests were used to obtain the relationship between and the rate of crack growth per cycle, dA/dN, and the maximum strain energy release rate, Gmax, applied during the fatigue cycle. The grit blasted surfaces failed at the interface during wet cyclic loading and the chromic/sulphuric acid etched and phosphoric acid anodised joints failed in the oxide layer.
This is an excellent paper, well worth reading in full.
The second paper: ‘Development of The RDCB Test For Assessing the Durability of Bonded Joints’, was given by John Harris of MERL.
This was another excellent paper and made us realise that although great efforts are being made to standardise test methods and to reduce their cost, there seems to be a continuing need to develop specific test methods to suit new problems. The Reinforced Double Cantilever Beam test (RDCB) is one of these. In this test the reinforcing beams are standard but the sheet material to be tested has to be bonded to them.
Bonding of all three gluelines is carried out in one operation. This adds to time and cost for each test but does allow the sheet metal being studied to be reinforced so that the fracture energy of the bond or the adhesive can be measured. It is also used as a fatigue test. This test is mainly used by the automotive industry at the present time. It is used in a similar way to the tests in the first paper and after testing at RT [dry] and at 40oC dry further tests were made in an environmental chamber. This too was a very interesting and detailed paper that needs to be read in full.
Paper three was given by Gregory Schueneman of Henkel Corporation and was entitled, ‘Ultrasonic Calibration of Accelerated Ageing of Silicone Sealant and Elucidation of Degradation Processes’.
This paper described the testing of gaskets and oil pan seals from engines of widely different ages. Lap shear tests were found to be much weaker after a long time in service and degradation was thermally and chemically accelerated. It was found that a loss of silicone polymer occurred leaving a material consisting mainly of filler and oil. Ultrasound was an effective method of detecting increasing hardness of the compound. Aerated oil at 140°C was found to cause the most damage. On engines in service no significant changes occurred below 250,000 miles of operation. Oil pan conditions were found to be more severe than gaskets; an interesting application of ultrasonics.
Paper 4 was entitled, ‘The Corrosion Characteristics and Corrosion Protection of Automotive Hybrid Hem Flange Joints- A Review’ was given by Lisa Young of Ford Motors.
This was a good paper showing the effort that goes into making durable cars. A typical hem flange joint was shown and a similar one but with aluminium for the outer skin and steel for the inner skin. This is being developed and it is hoped that with suitable design and choice of sealants and adhesives it will have good durability.
Much thought was given to the problems of crevice corrosion, galvanic corrosion, general corrosion, paint defects and drainage. The pH, electrical conductivity and water uptake of adhesives and sealants chosen could be important.
Paper 5, entitled ‘On the Need for Durability Testing of Bonded Joints for Marine Structures’, was given by Jan Weitzenbock of Das Norske Veritas, Norway.
He told us of the growing interest in adhesive bonding in shipbuilding and mentioned various International Safety Codes for Shipping, which have to be met. Joints that contribute to the structural strength of essential parts of ship's hulls have to comply with rules on ship design. Jan told us that they have a set of failure criteria (limit state equations) that have to be satisfied. He reviewed the testing required to meet fatigue and durability requirements and anyone entering this field would do well to read his paper and the regulations involved and set up the quality assurance methods needed to meet them.
Paper 6, given by Dr Andy Crocombe of the University of Surrey, was entitled, ‘Investigating the Evolution of Fatigue Damage in Adhesively Bonded Joints’.
He began with the usual S/N curve and then discussed load/life and stress/life laws. Variable amplitude fatigue loading was covered next and then fatigue crack propagation under constant amplitude, variable amplitude and variable frequency. He went on to discuss back face strain measurement as a sensitive detector of the evolution of fatigue damage in a non-destructive manner. Good correlation has been found between damage predicted in this way and damage revealed by microscopy on the sectioned joint. He concludes by saying that the best understanding of the fatigue response of bonded joints is obtained by studying the crack initiation process. This is a complex an interesting paper worth studying in full.
Paper 7, the last paper of this meeting, was given by Prof. Bob Adams of the University of Bristol. His paper was entitled, ‘The Effects of Moisture on the Durability and Strength Retention of Single Lap Joints’.
Bob discussed the effects of water, temperature and time. Some discussion took place on whether water entered down the bond line, through the adhesive or both. His work used only grit blasted surfaces and was compared with other treatments used by Critchlow. These showed that the quality of surface treatment was important and that further work was required. We look forward to the next instalment in this investigation.
This was a good day with useful papers showing significant progress in the development of test methods and techniques. The work on fatigue of bonded joints in particular has made great progress.