Dielectric studies of ageing in aluminium epoxy adhesively bonded structures: Design implications

Proceedings of the Institution of Mechanical Engineers, Part L:

Armstrong

Banks³

et al. 2004

Self Cite

Dielectric measurements and mechanical tests on adhesively bonded joints are reported for bonds having been aged by complete immersion in water at 50 and 65 C and by changing between these temperatures. Such joints have direct application to aircraft structures and other important engineering areas. The dielectric frequency domain measurements at 3 MHz are used to monitor moisture ingress into the adhesive bond. Initially the rate of change in the permittivity follows pseudo-Fickian behaviour consistent with simple moisture diffusion. In all the cases studied, acceleration in the rate of increase in the permittivity was observed consistent with the moisture converting the interfacial oxide layer to hydroxide. Time domain reflectometry (TDR) measurements indicate that physical changes occur in the bond line. Shear and cleavage measurements were performed at regular intervals and indicate a loss of mechanical properties of the bond. Comparison of the TDR measurements with ultrasonic and optical images indicated that the dramatic loss of mechanical properties is coincident with the observation of defects having been created in the interfacial layer. A correlation between certain of the mechanical properties and the change in the dielectric permittivity is observed.

Section: Dielectric Measurementsmentioning

confidence: 99%

Dielectric and mechanical studies of the durability of adhesively bonded aluminium structures subjected to temperature cycling. Part 1: Examination of moisture absorption

Proceedings of the Institution of Mechanical Engineers, Part L:

Armstrong

Banks³

et al. 2004

Self Cite

“…92 X-ray diffraction measurements indicate that the hydroxide formed was predominantly of the boehmite (γ-AlO(OH)) form, but also with some of the diaspore (α-AlO(OH)) form. 95,96 Comparison of the TDR measurements with ultrasonic and optical images indicated that the dramatic loss of shear and cleavage strength is coincident with the observation of defects having been created in the interfacial layer. It has been observed that provided the ageing has not crossed the plasticisation to hydroxide line drying of the joint will allow almost complete recovery of the initial strength, however once moisture has changed the interfacial layer, recovery becomes more difficult.…”

Section: The Nature Of Water Diffusion Into Jointsmentioning

confidence: 89%

“…This pattern of changes has been observed in investigations of a number of other systems. 90–96 The frequency domain dielectric data can be used to monitor the rate at which water is absorbed and by tracking the loss at ∼10 6 Hz it is possible to determine the extent to which corrosion of the oxide layer has occurred. The point at which the T g matches the ageing temperature and swelling of the adhesive occurs can be easily detected as a change in the TDR traces.…”

Section: The Nature Of Water Diffusion Into Jointsmentioning

confidence: 99%

“…Measurements of the adhesive bond strength have been carried out on joints which have been studied with dielectric methods and the data for a typical bond set are shown in Figure 23. 95 The shear strength of the joint falls over the first 45 days when the joint is aged at 70 ℃ in water. This drop in strength is associated with the lowering of the T g as a consequence of the plasticising action of water on the epoxy resin.…”

Section: The Nature Of Water Diffusion Into Jointsmentioning

confidence: 99%

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Design and ageing of adhesives for structural adhesive bonding – A review

Proceedings of the Institution of Mechanical Engineers, Part L:

2014

This review outlines some of the issues which have to be addressed when selecting an adhesive for a particular structural adhesive bonding application. The designer may find that a number of adhesives provide the required load bearing properties and the selection may require a more detailed consideration of the processes involved in the fabrication of the joint, the temperature over which it is to work and the environment in which it will have to operate. The article attempts to provide the reader with an insight into the effects of factors such as time and temperature used in the cure process, details of the components in the mixture and the effects of various additives on the physical properties of the adhesives which are created. The performance of an adhesive may vary with time as a consequence of stress-induced changes which are triggered by thermal effects or by hygrothermal factors. The review considers joints created using metal substrates and carbon fibre composites. The performance and durability of an adhesive bond is critically dependent on the stability of the interface between the adhesive and adherend and is sensitive to the pre-treatment processes used in the creation of the bond. The review outlines how broadband dielectric measurements can be used to non-destructively monitor the cure of adhesive bonds and the processes involved in ageing of joint. The dielectric method gives an insight into the fundamental processes which are giving rise to the changes in the mechanical properties of the joint as they age.

“…In this paper, a study of the aging characteristics of CFRP/epoxy/CFRP bonded structures in water and a range of other solvents systems is reported. The application of the dielectric method to the study of aging in aluminium adhesive bonds has been investigated previously [11][12][13][14][15] and it has been shown [1,2] that a correlation can be made between the change in the mechanical properties of the bond and the variation in its dielectric properties. This study attempts to explore whether a similar correlation might exist in the case of CFRP bonded structures.…”

Section: Introductionmentioning

confidence: 99%

Dielectric and mechanical studies of the durability of adhesively bonded CFRP structures subjected to aging in various solvents

Banks

Proceedings of the Institution of Mechanical Engineers, Part L:

Armstrong

et al. 2004

Self Cite

Dielectric and mechanical tests of adhesively bonded carbon fibre reinforced plastic (CFRP) joints aged in a series of solvents are reported. The solvent systems include; deicing fluids (ethylene glycol), aviation fuel, moisture, brine and paint strippers and solvents (dichloromethane and butanone). These measurements indicated that the ingress of the solvent leads to swelling of the joints and plasticization of the adhesive with consequential changes in the mechanical properties. A study of the drying process after the joints have been aged indicated that in certain cases it was possible for the mechanical properties of the joints to be recovered to almost their original values. In other cases the recovery was limited and reflected changes occurring within the adhesive layer. It was difficult to establish an unambiguous correlation between the dielectric constant of the bond and the change in the mechanical strength although in the case of water and ethylene glycol a correlation does appear to exist. The paper discusses the usefulness of the dielectric method for monitoring the ageing of CFRP adhesively bonded joints.