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Environmental durability of bond behaviour between steel and carbon fibre reinforced polymer
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posted on 06.02.2017, 03:20by Nguyen, Tien Cuong
Since the last three decades, many studies have proven the effective use of CFRP
composites in strengthening and retrofitting steel structures. However, one of
the main restrictions to more widespread use of CFRP in strengthening and rehabilitating
steel structures has been the uncertainty in the durability of steel
/CFRP adhesively-bonded systems. This is especially true when such systems
are exposed to outdoor environments. The data reporting this issue in the literature
is currently limited and thus a systematic investigation of the effects of
outdoor environments are required.
The outdoor environments mainly include temperature, moisture and ultraviolet.
Temperature is a detrimental factor to the adhesive layer between steel
and CFRP because it may reach the glass transition temperature (Tg) of the adhesive,
leading to a rapid reduction in the bond strength. Moisture, as another
aspect of the durability, by diffusion through the adhesive or transport along
the oxide/adhesive interface or absorption through the porous adherend, can
penetrate to the interfaces between the adhesive and the steel adherend. Once
moisture has penetrated into the steel /CFRP systems, degradation can occur
due to two types of degradation: the degradation of the adhesive itself, and
the degradation of the adhesive/adherend interface. In terms of ultraviolet, although
the Earth’s atmosphere filters out most of the total solar radiation, 6% of
the total solar radiant flux is ultraviolet radiation and is able to reach the Earth’s
surface. Ultraviolet radiation has a wavelength between 290 and 400 nm, and
the energy created by these wavelengths can dissociate the molecule bonds in
most polymers, leading to the degradation of polymeric materials. The degradation
may range from mere surface discoloration to extensive loss of mechanical
properties. It should be also mentioned that combining these degrading
factors together with loading may cause a significant degradation in mechanical
properties of steel /CFRP systems.
Double strap joints were chosen to investigate the performance of steel/CFRP
composite systems subjected to environmental effects. When a double strap
joint is tensioned, the strain in the carbon fibres must develop directly from the
bond between the metal and the fibre. In this way, the fundamental behaviour
of the metal-fibre bond, including the maximum strength and the effective bond
length, can be investigated under static load.
In the present research, the first aim was to investigate the effects of elevated
temperatures on the mechanical characteristics of steel /CFRP double
strap joints. It was found that due to elevated temperatures, the joint failure
mode changed from cohesion to CFRP and adhesive interface debonding. Effective
bond length was found to rapidly increase. Finally, joint strength and
stiffness significantly decreased with temperature especially when temperature
reached above the glass transition temperature (Tg) of the adhesive. Based on
the ultimate load prediction model developed by Hart-Smith for double lap
joints and kinetic modeling of the mechanical degradation of the adhesive, a
mechanism-based model is proposed to describe the change of effective bond
length, stiffness and strength degradation for steel/CFRP double strap joints at
In the next part of the research, time factor was taken into account. By applying
different load levels, the time-to-failure of the joints was monitored and
analyzed. It was found that when temperature was close to or higher than Tg,
the joint failure was time dependent. Based on the proposed temperature and
time-dependent material property models, the time dependence failure time of
steel /CFRP double strap joints was well described.
Next, the effects of long term exposure to UV and associated temperatures
were investigated. It was shown that UV exposure does not influence the tensile
strength of CFRP composites. The tensile strength of the adhesive reduced
slightly while modulus showed significant increase after long term exposure.
Although the tensile modulus of adhesive exposed to only thermal environment
also increased, it is considerably less than that induced by UV exposure.
The UV exposure also led to a decrease in joint strength but an increase in stiffness,
caused by the temperature effect rather than the UV rays.
The next stage of the research investigated the effects of sea water and moisture.
It was shown that sea water appeared to reduce the joint strength and
stiffness both by 20% after 1 year of exposure, and the joints exposed to combinations
of temperature and humidity in the environmental chamber showed
little decrease in joint strength and stiffness after 1000 h of exposure. A model
was developed to predict the strength and stiffness degradation of the joints in
sea-water and the results compared well with the experimental results.
Finally, the effects of sustained load and curing were examined. Itwas found
that the effects of sustained load were significant which could catastrophically
cause joint failure when it was combined with cyclic temperature and humidity.
Elevated curing of the steel /CFRP bonded joint helped to improve the mechanical
performance of the joints at elevated temperatures and increased the
time-to-failure of the joints. Partial safety factors proposed in design guidelines
for FRP composites under environmental effects were used to evaluate the experimental
results obtained, suggesting a very critical degradation of joint mechanical
properties due to the environmental effects investigated in this study.