Moslem Shahverdi
Fiber-reinforced polymer (FRP) composites are increasingly used in many load-bearing
structures. Joints are the most critical structural elements as they often become the weak links
in engineering structures. Bonding and bolting are two of the main joining techniques used for
composite elements but, although bolted and bonded joints have their advantages and
disadvantages, adhesively-bonded joints are often preferred. This is thanks to their better
performance in terms of cost-effective structures with uniformly distributed stresses without
the need for cutting fastener holes that result in high stress concentrations in the FRP
adherends at the edges of the holes. In addition, due to the absence of stress concentrations,
adhesively-bonded joints exhibit better fatigue behavior. However, the efficient and reliable
use of adhesively-bonded joints requires a thorough understanding of their failure
mechanisms under mixed-mode quasi-static and fatigue loading conditions. In real structural
applications, failure of adhesively-bonded joints occurs due to crack initiation and
propagation under varying mixed-mode ratios. Since it is difficult to describe this progressive
fracture in a structural joint, fracture joints with constant mode-mixity ratios are used to
determine the strain energy release rates for crack initiation and propagation. Subsequently,
mixed-mode failure criteria based on the determined strain energy release rates can be
established and used to model/predict the fatigue and fracture behaviors of structural joints.
The main objectives of this thesis were to understand the fracture and fatigue behaviors of
adhesively-bonded FRP fracture joints under Mode I, Mode II, and mixed-Mode I/II loading
conditions and, consequently, develop mixed-mode static and fatigue failure criteria
applicable to structural joints. To fulfill these objectives a combined experimental-analytical-numerical study was undertaken. Double cantilever beam (DCB-Mode I), end-load split
(ELS-Mode II), and mixed-mode bending (MMB-Mode I/II) specimens were examined under
quasi-static and constant amplitude fatigue loading. Analytical approaches were used to
determine the strain energy release rates under quasi-static and fatigue loading, whereas
phenomenological models were developed to characterize the fatigue and fracture behaviors
of the joints. A new approach designated the “extended global method” was established and
applied for the analysis of the quasi-static and fatigue experimental data and the fracture mode
partitioning in the mixed-mode experiments.
In parallel, non-linear finite element models were developed to simulate the quasi-static
fracture behavior and investigate the effects of both the existing asymmetry and the observed
fiber bridging on the fracture behavior of the examined joints. The bridging zone was modeled by zero-thickness cohesive elements. An exponential traction-separation description of the cohesive zone model was shown to appropriately model the fiber bridging and calculate its
contribution to the fracture energy under quasi-static loading. The FE models were
successfully employed for the separation of the fracture parameters, i.e. strain energy released
at the crack tip (Gtip) and the amount of energy released along the crack-bridging zone (Gbr),
in the quasi-static mixed-mode failure criterion for crack propagation.
In addition to the studies on the quasi-static fracture behavior of the joints, a new
phenomenological fatigue crack growth formulation for the modeling and prediction of Rratio
effects on the Mode I fatigue behavior of adhesively-bonded joints was also derived. The
model was used for prediction of the fatigue behavior of the examined joints under different
R-ratios. The results proved that the model could accurately simulate the fatigue behavior of
the examined joints and was also capable of predicting behavior exhibited under unseen
loading conditions, i.e. the R-ratios used for estimating the model parameters were different
from those used for validating its prediction capability.
The experimental results and numerical analyses combined with the phenomenological
models were used to establish mixed-mode static and fatigue failure criteria for crack
initiation and crack propagation for the adhesively-bonded joints. The derived mixed-mode
failure criteria can be used for simulating/predicting the crack initiation and progressive crack
propagation in structural joints comprising the same adhesive and adherends.
EPFL2013
