Time - and Temperature - Dependant Properties of Structural Timber Adhesives used for Onsite Bonding

Abstract

Bonded-in technology for connections in timber structures relies on the effective bonding of steel or reinforced plastic rods to timber using room temperature cure epoxy adhesives. Thixotrophic adhesive are the best option for timber structural joints which require in-situ assembly. The only drawback with room temperature cured adhesive is their partially cure and possess relatively low glass transition temperature. There are concerns associated with the stability of these adhesive especially in service temperature which may reach up to 50°C.This research investigated the creep properties of four commercially available epoxy-based adhesive. Prior to investigating the creep properties of the adhesives, their microstructure was analysed and the mechanical and thermal properties of the adhesives and bonded joints were evaluated. The adhesives investigated were partially cross-linked adhesive with a Tg value of 35.4°C to 53.4°C.Creep testing of adhesive was conducted within a Dynamic Mechanical Thermal Analyser with a step-wise increase of temperature. The tests were conducted in tension loading in static mode at various stresses level. Under Tg and Tg + 15ºC all of the adhesives experienced creep to a limit conforming with the classic viscoelastic behaviour of polymers. Above Tg + 15ºC the adhesives behaved like rubbers. Therefore room temperature cured epoxy adhesives are still able to function above Tg under creep load. The adhesives tested in creep failed by rupture at high stresses and temperatures.The performance of bonded-in connections under various combinations of temperature and relative humidity was also investigated. Creep experiment on specially designed model of bonded-in joint were conducted at temperatures of 20, 30, 40 and 50°C with corresponding levels of relative humidity (RH) of 65, 75, 85, and 95%. Under intermediate condition, secondary creep to a limit was observed. Tertiary creep to failure occurred on samples tested at maximum temperature and humidity.

Date of Award	31 Dec 2013
Original language	English
Awarding Institution	University of Bath
Supervisor	Martin Ansell (Supervisor) & Richard Harris (Supervisor)