Comments for Section
Controlling the thickness of the adhesive is important to ensure good bonding between the reinforcement and the substrate. Also the adhesive thickness is an important parameter in the design analysis of the adhesive layer and an accurate knowledge of its value is required. A lack of control of the adhesive layer thickness during application therefore results in a large partial factor.
The strength of the adhesive decreases as the length of time the load acts continuously. Therefore, if the design requires the adhesive to be loaded continuously then allowance for load duration is required. Generally, for most thermosetting adhesives the strength reduction from short-term to long-term is approximately 2. This value is deduced by extrapolation from creep measurements performed for up to 1 year duration.
The influence of the environment is not split into the effects of temperature and humidity but is considered only in terms of whether the application is within or outside test data bounds. If only the effects of temperature on the adhesive are required then rather than the simplistic treatment given in the Table above, the following formula may be used:
Values for these partial factors are presented in the Table below. They are taken from TR-55 (2004) with the exception of the partial factor for environmental effects, which is taken from ASME PCC-2 (2004). Note in this definition of partial factors, the partial factor for strength reduction is given by:
i.e. strength equals modulus multiplied by ultimate strain. This is the value quoted in CIRIA C595 (2004).
The partial factor for manufacturing method takes account of material variability. Ideally a testing strategy, including number of tests, sample preparation and test standards should be developed by the Material Suppliers to remove the need for this partial factor.
Value of material partial factors for composite reinforcement
The strength reduction factor is actually based on the combined reductions of the laminate modulus and the long-term (ultimate) strain to failure of the reinforcement. The partial factors quoted above assume that the reinforcement is primarily constructed from uni-directional plies, implying that reinforcement performance is dominated by fibre type.
modulus of the reinforcement of uni-directional FRPs does not decrease
significantly with applied load duration irrespective of the fibre type,
therefore a default value independent of fibre type of 1.1 is recommended.
As the response of the reinforcement is essentially linear elastic then
the reduction in ultimate strain as a function of load duration follows
that of the strength, as the modulus is independent of load duration.
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