Лако-красочные материалы — производство

4.1 Introduction

The truly structural adhesive joint is relatively new. The evolution of the various design approaches follows the empirical development of appropriate joint configurations(l-7) — themselves following on from the long historical development of load-bearing joints in, and between, engineering materials. It must however be emphasized that structural bonded joints existing in engineering disciplines other than those involving civil engineering tend to be formed with thin bondlines, often with relatively high modulus adhesives, whereas the general concern in the construction industry is with thick bondlines — often with lower modulus materials. This is an important difference, since the nature of the resultant bondline stress distributions of loaded joints may be significantly different.

The training normally given to an engineer in the various means of joining materials leaves him at a disadvantage when it comes to using adhesives, with essential choices between the many types available and with the design approach appropriate to structures assembled with these. Naturally the basis for design must stem from the intended function and service environment of the joint, and from a consideration of the loads and stresses which are likely to be encountered in service. As with any fastening method, it follows that the design must be dependent upon the nature of the materials to be joined as well as on the method of joining. It is, for instance, not sufficient simply to substitute adhesive bonding for welding, bolting or riveting.

The properties of the composite made when two adherends are united by adhesive are a function of the bonding, the materials involved and their interaction by stress patterns. Potential problems implied by the latter stem from the inherent mismatch between adhesives and the materials commonly employed in construction (Table 4.1). For instance, concrete adherends would benefit from being united with flexible and relatively low modulus products in

Introduction

order to reduce interfacial stress concentrations which might initiate fracture of the concrete. On the other hand, sustained loading may lead to excessive deformation and creep unless the adhesive used is relatively unmodified and therefore highly cross-linked, while also possessing a high glass transition temperature. However, thin sheet metal, metal alloys and composite materials demand either a

toughened product or a flexibilised adhesive with a large strain to failure in order to accommodate gross adherend strain under load.

It is apparent that in order to realise optimum performance from a load-bearing joint, balance between the requirements of the different materials to be joined can only be achieved by rational design which requires, inter alia, quantitative data on the properties of structural adhesives. Currently there is an acute lack, both of adhesives performance data and of appropriate test procedures for determining relevant structural properties. Indeed, because so many factors affect joint strengths, extreme care must be taken when interpreting published performance data. For instance, details such as joint configuration, testing conditions or surface treatments may be insufficiently described to make comparisons of results collected from different sources useful. Appropriate test procedures, with a view to the long-term performance as well as to the short, are therefore discussed in this chapter.

It has been emphasised already that a successful adhesive bonded joint depends upon several factors:

(1) appropriate design of the joint

(2) selection of a suitable adhesive

(3) adequate preparation of the adherend surface

(4) controlled fabrication of the joint

(5) protection of the joint itself from unacceptably hostile con­ditions in service

(6) post-bonding quality assurance.

The significance of some of these factors, particularly surface pretreatment, tends to become more apparent with regard to durability and to long-term performance.

Kinloch(4) observed that the selection of appropriate failure criteria for the prediction of joint strength by conventional analysis is fraught with difficulty. The problem is in understanding the mechanisms of failure of bonded joints, and in assigning the relevant adhesive mechanical properties. Current practice is to use the maximum shear-strain or maximum shear-strain energy as the appropriate failure criterion. However, the failure of ‘practical’ joints occurs by modes including, or other than, shear failure of the adhesive. This difficulty has led to the application of fracture mechanics to joint failure.

Factors affecting joint strength