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Durability of adhesive joints between concrete and FRP reinforcement in aggressive environments
textThe durability of the bondline between concrete and its fiber-reinforced
polymer reinforcement was characterized at various temperature and humidity
levels. The bondline consisted of an epoxy primer, an epoxy putty and an
epoxy saturant. In principle, fracture could occur anywhere in this bondline, but
attention was focused on the concrete/primer interface in this study because
preliminary experiments indicated that this was the dominant failure mechanism.
The initial part of the constitutive modeling of the epoxy primer was
conducted using linear viscoelastic experiments. Confined compression
experiments determined two linear material functions simultaneously. Because
this was a relatively new experiment, the results were validated by conducting
bulk compliance experiments. The viscoelastic region of the bulk modulus was
as wide as that of the tensile and shear relaxation moduli. This result
contradicts previous conceptions but is agreement with some other recent
observations. Thermal and hygral expansions were also measured and used in a
hybrid nonlinear viscoelastic constitutive model.
The hybrid model captured the hygrothermal nonlinear viscoelastic
deformation of the epoxy primer. This model is a combination of Schapery’s
model and Popelar’s shear modified free volume model. Torsion tests were
conducted and used to calibrate the distortional parameters in the free volume
model. Tension experiments were performed at four different temperature and
humidity levels and were used to calibrate the dilatational, thermal and hygral
parameters in the hybrid model. The linear and hygrothermal nonlinear
viscoelastic constitutive models were used in the analysis of time-dependent
interfacial fracture between concrete and epoxy primer.
A generalized time-dependent J integral was used as a fracture parameter for
characterizing the time-dependent interfacial fracture. This was used instead of
the strain energy release rate and the stress intensity factor because of the
nonlinear viscoelastic deformation of the primer. Schapery’s pseudo stress
model was calibrated using tension data at various temperature and humidity
levels because it is required for the generalized J integral. An instrumented
wedge test was conducted in order to determine the interfacial fracture energy at
several loading rates and various temperature and humidity levels. The crack
length was measured as a function of wedge speeds during steady state crack
growth. The generalized J integral and cohesive zone size or failure zone size
were computed using finite element analyses that incorporated the pseudo stress
model. The pseudo stress model, cohesive zone size and the generalized J
integral were all used to compute the work input into the failure zone, which
was then equated to the fracture energy. The loading rate, temperature and
humidity level all affected the fracture energy, which decreased with increasing
temperature and humidity levels.Aerospace Engineering and Engineering MechanicsEngineering Mechanic
Characterization and modeling of strain assisted diffusion in an epoxy adhesive layer
AbstractIn this paper a coupled model for strain-assisted diffusion is derived from the basic principles of continuum mechanics and thermodynamics, and material properties characterized using diffusion experiments. The proposed methodology constitutes a significant step toward modeling the synergistic bond degradation mechanism at the bonded interface between a Fiber Reinforced Polymer (FRP) and a substrate, and for predicting debond initiation and propagation along the interface in the presence of a diffusing penetrant at the crack tip and at elevated temperatures. It is now well-known that Fick’s law is frequently inadequate for describing moisture diffusion in polymers and polymer composites. Non-Fickian or anomalous diffusion is likely to occur when a polymer is subjected to external stresses and strains, as well as elevated temperatures and humidity. In this paper, a modeling methodology based on the basic principles of continuum mechanics and thermodynamics is developed which allows characterization of the combined effects of temperature, humidity, and strain on diffusion coefficients as well as on moisture saturation level, from moisture weight gain data. For tractability, the diffusion governing equations are simplified for the special case of 1-D diffusion subjected to uniaxial strain and a uniform strain gradient. A novel test specimen that introduces a uniform strain gradient is developed, and diffusion test data for an epoxy-based primer/adhesive are presented and employed for complete characterization of material constants used in the model