The electrical and mechanical properties of structural adhesives

Abstract

The developing use of carbon fibre composite(CFC) material in aerospace applications, due to its high specific strength and
stiffness has brought an urgent need to revise engineering techniques that are based predominantly on metals. High performance structural adhesives are used to bond CFC panels together, though the performance characteristics of bonded joints in CFC panels differ considerably from those of bonded joints in metal and alloy panels, particularly with regard to the electrical characteristics.
Poorly conducting bonded joints would seriously compromise the electromagnetic integrity of an airframe. The aim of this research project was to measure the electrical and mechanical properties of one such adhesive, Redux 319A, used in the
aircraft industry. A value of 11 ± 7 .ftm was found for the resistivity of the adhesive. This would cause serious electrical
discontinuities in an airframe built of CFC which has a resistivity of 3.1 10 5 St m in comparison. A study was also made of the microscopic structure of the adhesive in order to identify the electrical conduction mechanism within the adhesive and a
one-dimensional computer nodel was developed simulating charge percolating through the adhesive.
A study of published work was made into the effect of the microscopic structure of adhesives on their mechanical performance and a series of tensile stress/strain tests were carried out on cast Redux 319A adhesive specimens. From these, a value of 3.4 * 0.7 C Nnr 2 was determined for the Young'i Modulus of the adhesive. The Redux suffered less than 3% strain before failing in tension, compared with up to 40% in shear found by other workers. A value of 0.1 nn was calculated for the Griffith's critical crack length. Any crack in the adhesive that developed beyond this value would have continued to propagate without needing any increase in load.