Performance bonded joint fabrication

Proof of durability and safe performance are, rightly, onerous requirements for any innovations in the construction industry. The parameters affecting environmental durability have been summarised, and water has been identified as the most hostile environment for bonded joints that is commonly encountered. Identification of the general failure mechanisms is useful because it highlights the procedures necessary for the satisfactory fabrication of reliable and durable bonded joints. It also enables the development and adoption of appropriate test methods, since real joint configurations are of limited use in assessing experimentally environmental effects (e.g. bonded areas must be minimised in order to allow environmental access within a reasonable time-scale). Fracture mechanics methods. 

The tensile strength and the elongation-at-break of the polymer films were measured according to EN ISO 527 [22], Mechanical tests were performed on the adhesive joints as described in the standard [8] by adhesively bonding two beech wood specimens using a pneumatic press. In particular, density, moisture content, dimensions, thickness and fabrication conditions are specified in the specific standards. 

Fig. 7.10. Stress versus strain characteristics of three lap joints and an unbonded test coupon. Joint A, 25 mm X 12 5 mm overlap B, 25 mm X 190 mm overlap C, 25 mm X 25 0 mm overlap. Adherend mild steel, 16 gauge. Adhesive heat cured toughened epoxide (Permabond ESP105). Test coupon 25 X 75 mm, 16 gauge mild steel of the type used to fabricate the joints represented by continuous line. The classical form of the elastic/plastic deformation of the unbonded test coupon is clearly seen. (A) This specimen does not fail until after the test coupon has become plastic. (B) Although possessing 50% more bond area, the load required to fail specimen B is not very much greater than that needed to fail A however, the toughness of the adhesive and the spare capacity of the initially unloaded central area are clearly illustrated by the ability of the joint to sustain a load even though it has cracked. (C) This example emphasises the point made with specimen B. A performance such as this makes it difficult to say whether the adhesive has failed the steel or the steel has failed the adhesive.

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