Problem Statement
Despite the fact CFRP strengthened steel structures have become more common in recent years, there are still aspects of the performance that are not fully understood.
One such issue is the effects of warm temperatures (40-80°C). These temperatures could occur during the normal service of the strengthened structure which could lead to unexpected problems. An example of this in the UK is a steel bridge which could reach temperatures of around 50°C during summer (Highways Agency 2001). Another problematic situation can arise in factory buildings. Processes on site may produce large amounts of heat and this could affect the performance of a CFRP-strengthened beam. Well insulated CFRP strengthening could suffer from similar effects during a fire. Steel and CFRP have very different coefficients of thermal expansion and subsequently, the steel expands when heated while the CFRP does not. The differential heat expansion results in increased stresses within the adhesive joint. Moreover, glass transition of the epoxy adhesive starts at temperatures of approximately 40°C (Stratford & Bisby 2012), depending on material type and curing conditions (Othman et al. 2013). During this process the adhesive starts to soften and transforms from a glassy to rubbery state. This leads to a serious reduction in mechanical properties (e.g. strength and stiffness), however this process also increases the deformation capacity of the joint. At elevated temperatures a combination of these effects leads to premature failure of the adhesive at a lower load than would be expected at ambient temperature. Experimental work has shown that while at ambient temperatures strengthened beams tend to fail due to failure mechanisms related to the steel beam but at elevated temperatures most failures are caused by debonding of the adhesive (Stratford & Bisby, 2012). |
This emphasises the significance of understanding this behaviour and the need for more research into the effects of temperature on the bonding adhesive in CFRP strengthened steel structures.
Research on this topic to date has included information on how the curing temperature of the adhesive affects the glass transition temperature (Othman et al. 2013). Another significant publication investigates the glass transition of the adhesive in terms of its reduction in stiffness. This can be seen in the figure below: However there has been no significant research on the effect elevated temperature has on the strength or creep properties of the adhesive. This research project will focus on obtaining data on the strength and creep properties of the adhesive at warm temperatures and, combined with previous research, look to enhance the understanding of the overall behaviour of a CFRP strengthened steel beam at such temperatures. |
Aims & Objectives
Primary Aim
To examine the effects of warm temperatures on the strength of epoxy resin and evaluate how the altered properties of the adhesive affect the behaviour of the CFRP strengthened beam. Secondary Aim Contribute to the overall understanding of CFRP strengthened beams by studying and describing creep behaviour of epoxy resins through experimental work. |
Objectives
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