Modelling residual stresses and environmental degradation in adhesively bonded joints

The aim of this research was to develop predictive models for residual stresses and environmental degradation in adhesively bonded joints exposed to hot/wet environments. Different single lap joint configurations and a hybrid double lap joint with dissimilar adherends (CFRP/AIIFM73 double lap joint), were exposed to different ageing environments in order to determine the durability of the joints and the effects of ageing on the failure load. Thermal residual stresses in bonded joints were investigated with analytical solutions and finite element modelling, first with a bimaterial curved beam to validate the modelling process and determine the most suitable method for calculating thermal stresses in bonded joints. It was found that none of the analytical solutions and 2D geometric approximations was fully able to describe the 3D stress state in the strip. The incorporation of geometric and material non-linearity into the models was necessary to obtain accurate results. The validated methods were then used predict the thermal residual stresses in bonded lap joints. The thermal stresses were found to be highest in joints with dissimilar adherends. Moisture uptake in bonded joints was investigated using Fickian diffusion modelling. Gravimetric experiments were used to determine the Fickian diffusion parameters for the bulk adhesive and composite adherends. Transient diffusion modelling was used to predict the uptake in bonded joints. It was seen that moisture diffusion is a fully three dimensional process, and the effects of moisture absorption can only be adequately studied using 3D FEA. The effects of swelling from moisture absorption in bonded joints were investigated using coupled stress-diffusion FEA models. Coupled stress-diffusion 3D FEA was used to predict the transient and residual hygroscopic stresses that develop in bonded lap joints as a function of exposure time in accelerated ageing environments, taking into account the effects of moisture on the expansion and mechanical properties of the adhesive and CFRP substrate. It was seen that moisture absorption induces significant stresses in the joints and markedly different behaviour was seen in the cases of absorbent and non-absorbent adherends. Hygro-thermo-mechanical stresses arising from the exposure of single and double lap joints with thermal residual stresses to hot/wet environments were investigated. In the single lap joints, a reduction in the stresses present in the adhesive was predicted, owing to swelling of the adhesive from moisture absorption. In the double lap joint with dissimilar adherends, exposure to hot/wet environments initially reduced the stresses in the joint when dry, followed by an increase in the magnitude of some stress components and reductions in others with increasing levels of moisture absorption. This led to a higher equivalent stress state in the adhesive than when dry. Thermal residual and mechanical strains predictions were validated with internal strains measured by neutron diffraction and surface strains measured by moire interferometry. Comparisons of predicted and measured thermal residual strains showed low levels of strain in joints with similar adherends. The magnitude of strains in the CFRP/AI double lap joint was significant, with the same spatial distribution and magnitude in both measured and predicted strains. The comparison of mechanical strains predicted by FEA and measured strains by moire interferometry showed good agreement. High magnification moire interferometry also confirmed the location of strain concentrations predicted by FEA. A path independent cohesive zone model (CZM) and a coupled continuum damage model were used to predict and characterise damage and failure initiation in bonded joints. Progressive failure prediction was calibrated in the cohesive zone model using the moisture dependent cohesive fracture energy of FM73. There was a reasonably good agreement with the experimental failure loads. This implementation of the cohesive zone model is limited by the ability of the interface elements used, thereby creating mesh dependency. The Gurson-Tvergaard-Needleman (GTN) coupled damage model was used to predict the effects of residual stresses on failure loads. However, this method is difficult to implement, given the numerous parameters required. The failure loads predicted by the GTN model were comparable with the experimental data when the joints were dry or wet. The damage models were capable of predicting the sudden crack growth and propagation seen experimentally

ANALYSIS OF LOAD TRANSFER INTO COMPOSITE STRUCTURE

The paper presents advantages and disadvantages of metal foils insertion between composite layers. Composites are complex materials of anisotropic structure leading to various failure mechanisms. Mechanism of compressive load transfer into composite laminates by shear of the matrix is analysed. The method of improvement compressive strength of laminates is presented according to literature and analysed for a selected case. Simplified models of a laminate structure modified using various metal foils configurations are analysed with MSC.Marc code. Axial stress in prepreg layers and shear stress in adhesive layers are studied

Experimental And Numerical Investigations On Bond Durability Of Cfrp Strengthened Concrete Members Subjected To Environmental Exposure

Fiber reinforced polymer (FRP) composites have become an attractive alternative to conventional methods for external-strengthening of civil infrastructure, particularly as applied to flexural strengthening of reinforced concrete (RC) members. However, durability of the bond between FRP composite and concrete has shown degradation under some aggressive environments. Although numerous studies have been conducted on concrete members strengthened with FRP composites, most of those studies have focused on the degradation of FRP material itself, relatively few on bond behavior under repeated mechanical and environmental loading. This thesis investigates bond durability under accelerated environmental conditioning of two FRP systems commonly employed in civil infrastructure strengthening: epoxy and polyurethane systems. Five environments were considered under three different conditioning durations (3 months, 6 months, and 1 year). For each conditioning environment and duration (including controls), the following were laboratory tested: concrete cylinders, FRP tensile coupons, and FRP-strengthened concrete flexural members. Numerical investigations were performed using MSC MARC finite element software package to support the outcomes of durability experimental tests. Precise numerical studies need an accurate model for the bond between FRP and concrete, a linear brittle model is proposed in this work that is calibrated based on nonlinear regression of existing experimental lap shear data. Results of tensile tests on FRP coupons indicate that both epoxy and polyurethane FRP systems do not degrade significantly under environmental exposure. However, flexural tests on the FRP strengthened concrete beams indicate that bond between FRP and concrete shows significant degradation, especially for aqueous exposure. Moreover, a protective coating suppresses the measured degradation. Also, experimental load-displacement curves for control beams show excellent agreement with numerical load-displacement curves obtained using the proposed bond iii model. Finally, a bond-slip model is predicted for concrete leachate conditioned beams by matching load-displacement curves for those beams with numerical load-displacement curves

Polyurethane Fiber Reinforced Polymer Strengthening of Shear Deficient Reinforced Concrete Beams

The use of externally-bonded fiber-reinforced polymer (FRP) composites has been established as an effective means for the strengthening of shear-deficient reinforced concrete (RC) flexural members. Epoxy-based wet layup systems were predominantly employed in previous studies. In this study, carbon FRP pre-impregnated with polyurethane resin is utilized in strengthening shear-deficient RC beams and compared to an epoxy resin. Fourteen small-scale (96 in span, 6 in width, and 12 in height) and five large-scale (132 in span, 12 in width, and 17 in height) flexural specimens were tested, considering FRP system type (polyurethane versus epoxy), size effect, shear span-to-depth ratio, FRP configuration (U-wraps versus side bonding), and FRP scheme (sheets versus strips with 45 degrees or 90 degrees). Experimental strength testing under four-point loading demonstrated similar or enhanced shear capacity when strengthening by the polyurethane compared to the epoxy composite systems. The shear behavior of polyurethane-based FRP composite system is investigated in this research using analytical and numerical approaches. A closed-form mechanics-based analytical model, utilizing the principle of effective FRP stress and upper-bound theorem, illustrated that the shear behavior and debonding mechanism were dependent on both FRP composite and bond characteristics. The analytical model is expressed in terms of shear crack opening crossed by the FRP laminate and gives good agreement with experimental results. The finite element analysis (FEA) model shows that the stresses in the FRP are not in single direction as in the coupon tests, and the biaxial stress states should be taken into consideration. The structural behavior of RC members strengthened with externally-bonded FRP composites is mobilized through the composite action technique. Bond stress can be defined as the shear stress acting in the interface between FRP and concrete. It is of crucial importance to evaluate the failure mode behavior. Debonding (loss of adhesion) failure is one of the most common modes of failure encountered in shear strengthening RC members in practice. Numerous constitutive bond-slip models have been proposed and derived numerically and mathematically based on experimental data with an assumption that the FRP width bp is taken as a variable and all stresses or strains at the same longitudinal coordinate (L direction) are uniform. No attention has been given to study the bond states of stress which are mainly altered by the inclined shear cracks in concrete. A new bond-slip law was proposed to address the biaxial two-dimensional (2D) states of stress problem. Numerical solution by finite difference (FD) was conducted to solve four partial differential equations per node (2 for FRP and 2 for concrete in each direction) with appropriate boundary conditions to obtain the stresses, slips, and strains based on the proposed bond-slip model. A new experimental setup was proposed to represent the 2D bond-slip model by lap shear tests in both directions by laminating two perpendicular strips on concrete blocks with the proposed strain profile. Experimental calibration has been carried out by using nonlinear least-squares regression (fitting) of the experimental strain data with the numerical FD equations to obtain the bond-slip parameters for the 2D FRP-to-concrete polyurethane interface system

An in-situ experimental-numerical approach for interface delamination characterization

Interfacial delamination is a key reliability challenge in composites and microelectronic systems due to (high density) integration of dissimilar materials. Delamination occurs due to significant stresses generated at the interfaces, for instance, caused by thermal cycling due to the mismatch in thermal expansion coefficient and Poisson’s ratio of the adherent layers. Predictive finite element models are generally used to minimize delamination failures during the design and optimization of these materials and systems. Successful prediction, however, requires a relevant interface model that can capture the observed (irreversible) crack initiation and propagation behavior in experiments. To this end, dedicated delamination experiments with in-situ microscopic visualization are needed to identify the relevant delamination mechanism(s) and to accurately measure the interface properties, such as the interface toughness, as a function of mode mixity (i.e. loading angle). Hence, the goal of this research is to develop experimental-numerical tools required for accurate characterization and prediction of interface delamination. As a first step to reach this goal, a novel Miniature Mixed Mode Bending (MMMB) delamination setup, which enables in-situ characterization of interface delamination in miniature multi-layer structures, was designed and realized. This setup employs an inventive loading configuration to sensitively measure global load-displacement delamination curves for the full range of mode mixities from which the interface toughness or Critical Energy Release Rate (CERR) can be determined, while it was designed with sufficiently small dimensions to fit in the chamber of a scanning electron microscope or under an optical microscope for detailed real-time fracture analysis during delamination. The performance of the setup was assessed using dedicated test samples, supported by finite element analyses. The measurement concept was successfully validated on homogeneous bilayer sampleswith a glue interface system. The validation experiments also revealed roomfor improvement of themeasurement accuracy, robustness, and applicability. Therefore, further optimization in the design was performed and an improved version of the MMMB setup was developed. This setup can access a considerably larger range of interface systems, shows significantly higher accuracy and reproducibility in load-displacement measurements, and is more robust. The potential of the new in-situ experimental technique for interface parameter identification was also illustrated. For instance, high resolution in-situ SEM imaging during delamination allows for measurement of the strain maps and crack opening displacement (COD) fields using digital image correlation in addition to the identification of the delamination failure mechanism. In-situ SEM observation of delamination in different interface structures reveals failure mechanisms ranging from interface damage to interface plasticity. Hence, an irreversible model description of the interface behavior that can capture the observed unloading-reloading responses is needed for accurate prediction of, for instance, crack branching and crack propagation at multiple interfaces using predictive finite element models. Therefore, a combined damage and plasticity formulation was presented that is suitable for modeling of the unloading response of an interface ranging from full damage to full plasticity, while it introduces a minimum number of model parameters that can be experimentally determined. The unloading model can be used with the existing mixed-mode cohesive zone laws that describe the interface loading behavior. The relevance and applicability of the unloading model was demonstrated, in combination with the existing improved Xu-Needlemanmixed mode cohesive law, by modeling the observed combined damage-plasticity unloading response of the above-mentioned glue interface system. In addition, a procedure to identify the model parameters has been presented. Permanent deformation of the sample structure often occurs during delamination tests, particularly, if the layers forming the interface are ductile and the interface is strong. Therefore, accurate determination of the interface fracture toughness requires identification and separation of the contribution of structural plasticity to the total energy dissipation, taking into account the presence of plasticity mechanisms within the fracture process zone at the interface that contribute to the interface fracture toughness. To this end, a semi-analytical approach accounting for the structural plasticity in the sample layers was developed, in order to obtain an accurate value of the interface fracture toughness in a mode I experiment. The approach was numerically verified by employing a finite element model with cohesive zone elements (at the interface). The proposed approach was experimentally assessed by characterizing the interface fracture toughness of industrially relevant copper lead framemolding compound epoxy (CuLF-MCE) structures with different layer thicknesses. In summary, the combined application of in-situ MMMB experiments, the analytical procedure to determine the CERR, and the cohesive zone model with the parameter identification procedure allows for accurate characterization of the delamination mechanism(s) and prediction of the interface mechanics. As a demonstration, industrially relevant coated CuLF-MCE and uncoated CuLF-white molding compound (WMC) interface systems have been characterized in detail using the developed experimental tools

Degradation models for the collapse analysis of composite aerospace structures

For the next generation of aircraft, the use of fibre-reinforced polymer composites and the design of "postbuckling" structures to withstand immense loads after buckling are key technologies for considerable weight and cost savings. However, the application of postbuckling composite structures has been limited, as today's analysis tools are not capable of accurately predicting the collapse of these structures under compression. The major objective of this PhD work was the development of an analysis methodology and complementary software package for composite postbuckling structures, which included the degrading effects of the critical damage mechanisms. From a comprehensive literature review and extensive benchmark study, a methodology was developed for analysing postbuckling composite structures that was capable of representing the critical damage mechanisms. One aspect of this was a degradation model to represent the growth of interlaminar damage such as delaminations and skin-stiffener debonds. In this degradation model, layers of shell elements were tied with user-defined multi-point constraints (MPCs), and fracture mechanics calculations using the Virtual Crack Closure Technique were applied to control the release of these MPCs to model crack growth. Another aspect of the methodology was an approach to predict the initiation of interlaminar damage in intact structures. This was implemented using strength-based criteria, and demonstrated on cross-section models of skin-stiffener interfaces, on which the input of deformations from a global postbuckling analysis of an entire panel was also shown. Separately, a degradation model was developed to capture pl y damage such as matrix cracking and fibre fracture, which was based on the Hashin criteria for damage prediction and the Chang-Chang approach for material softening. The analysis methodology was implemented into the finite element (FE) code MSC.Marc, using a combination of nine user subroutines and several external data files. The methodology was incorporated into a user-friendly software tool in MSC.Patran, which provided a suite of functions for including the damage representations into nonlinear FE analysis. Extensive validation was performed with experimental results from the European Union project COCOMAT, which included fracture mechanics characterisation coupons, single-stiffener flat panels, and large, multi-stiffener curved panels representative of composite fuselage designs. This demonstrated the applicability of the methodology for the design and analysis of both intact and pre-damaged postbuckling composite structures, and the capacity for accurate and detailed analysis of the critical damage mechanisms