Лако-красочные материалы — производство

Whilst the properties and behaviour of adhesives in bulk form are linkable to their composition, the behaviour of joints constructed with adhesives is less predictable. This is to be expected from a consideration of the factors affecting joint strength outlined earlier in the chapter. In particular, joint behaviour will be determined largely by the joint’s geometrical configuration and by the way in which it is loaded, since these factors will determine the nature and magnitude of the resulting bondline stresses and stress concentrations. To a large extent, and provided that a reasonable amount of care in the design of the joint has been taken, the adhesive’s stiffness will determine the general behaviour. However, because the stiffness of adhesives changes with loading and environmental conditions (e. g. time, temperature and moisture), consideration must be given to the effect of these conditions on joint behaviour.

Creep

The time-dependent component of polymer response is of extreme importance to the use of structural adhesives which are required to sustain either permanent or transient loads. At temperatures well below the adhesive’s Tg, overloading is far more likely to lead to stress rupture than to creep(5). However, at temperatures close to or at Tg, some creep of loaded joints is to be expected. Recalling Chapter 2, highly cross-linked epoxies which are cured at elevated temperature possess the best resistance to creep.

Adhesives belong to the class of solids whose stress-strain behaviour may be described at visco-elastic. That is, in addition to having some of the characteristics of viscous liquids, they also possess some of the characteristics of elastic solids. Unlike an elastic material the strain lags behind the stress, so that it is necessary to describe the variation of stress and strain with time independently. The actual shear strain, measured as the increase in length of a standard lap-shear joint as it creeps, is minute. Allen and Shanahan(71, 72) studied the tensile creep of lap-shear joints at temperatures around that of the adhesive’s Tg. They found that the creep under load was preceded by a delay or induction period which was temperature and load dependent. The steady state creep which took place was logarithmic with time, giving way eventually to an accelerated creep terminating in stress rupture. This behaviour was undoubtedly linkable both to the particular joint geometry and to the adherend and adhesive materials employed. One reason for the delay in the onset of creep may well be because of changes in the adhesive due to temperature and humidity, enabling some bondline stress redistribution — particularly if that results in a higher level of stress having to be borne by the central regions of a joint. Clearly in joint configurations comprising large bonded areas sandwiched between impermeable adherends, this delay will be very large indeed. Althof and Brockmann(73) advocated the measurement of bondline deformation in real joints, whilst subjected simultaneously to sustained loading and environmental exposure, to give limit values for design.

It is clear that there are load, and therefore stress, levels below which creep will not occur, but it must be recognised that changes in the adhesive’s stiffness due to environmental conditions may well give rise to creep after a delay or induction period. This could, of course, be reversible such that steady state creep is not a necessary

outcome. A better understanding of creep mechanisms provides a useful topic for further research. The practical approach to both delaying the onset of creep and to reducing its rate is by providing for large bonded areas — for example, long-overlap shear joints(19).