Debond Crack Growth Research Articles

Debonding detection in FRP-strengthened concrete structures utilising nonlinear Rayleigh wave mixing

This paper reports an investigation on the use of Rayleigh wave mixing method for the debonding detection in concrete structures strengthened with externally bonded fibre-reinforced polymer (FRP) composite plates. A three-dimensional (3D) finite element model is developed to simulate the propagation of Rayleigh waves in FRP-strengthened concrete structures. In this model, the frequency domain of the transmitted waves shows the generation of second harmonics and combinational harmonics on account of debonding. The numerically simulated results are then experimentally validated with carbon fibre-reinforced polymer (CFRP)-plated concrete blocks containing two different sizes of debonding. Based on the experimentally verified numerical model, parametric studies are then conducted, where a nonlinear debonding crack growth parameter is proposed. The nonlinear Rayleigh wave mixing method is proven to be practical, reliable, and sensitive for detecting debonding at bonded concrete-FRP interfaces in FRP-strengthened reinforced concrete (RC) structures.

Matrix and interface cracking in cross-ply composite structural battery under combined electrochemical and mechanical loading

In this paper propagation of matrix cracks and debonds at the coating/matrix interface in the 90°-layer of a cross-ply structural composite battery are studied numerically. The structural composite battery consists of micro-battery units, made of a solid electrolyte coated carbon fiber embedded in an electrochemically active polymer matrix. During charging the fiber swells and the matrix shrinks leading to high stresses on the fiber/matrix scale and to anisotropic free expansion of the composite ply. Two load cases are considered, pure electrochemical load (intercalation) and combined electrochemical and thermomechanical load. Energy release rates (ERR) of radial matrix cracks along two potential propagation paths are calculated using 2-D finite element models of the transverse plane in a cross-ply laminate with a square packing of fibers in the 90°-ply and using homogenized 0°-ply. Results show that the matrix crack growth towards the nearest fiber is unstable, and that the debond crack growth is in mixed mode. For a cross-ply structural battery composite the sequence of macro-scale crack forming events differs from a conventional cross-ply composite, as well as for a UD composite battery laminate. The most likely course of failure events in a cross-ply laminate are: 1) vertical radial matrix crack initiation and unstable growth; 2) debond is initiated at certain length of the matrix crack.

Matrix and interface microcracking in carbon fiber/polymer structural micro-battery

In this paper, the propagation of radial matrix cracks and debond cracks at the coating/matrix interface in unidirectional carbon fiber structural micro-battery composite are studied numerically. The micro battery consists of a solid electrolyte-coated carbon fiber embedded in an electrochemically active polymer matrix. Stress analysis shows that high hoop stress in the matrix during charging may initiate radial matrix cracks at the coating/matrix interface. Several 2-D finite element models of the transverse plane with different arrangements of fibers and other matrix cracks were used to analyze the radial matrix crack growth from the coating/matrix interface of the central fiber in a composite with a square packing of fibers. Energy release rates of radial cracks along two potential propagation paths are calculated under pure electrochemical loading. The presence of a radial matrix crack imposes changes in the stress distribution along the coating/matrix interface, making debonding relevant for consideration. Results for energy release rates show that the debond crack growth is governed by mode II.

Micromechanical model for prediction of the fatigue limit for unidirectional fibre composites

A theoretical model is developed for the prediction of a fatigue limit of unidirectional fibre composites subjected to cyclically varying tensile loads in the fibre direction. The fatigue limit is defined from the condition that failure of one fibre does not lead to progressive failure of the neighbouring fibres. The model describes the fibre/matrix interface in terms of a fracture energy and a frictional sliding shear stress. Fatigue damage in the form of cyclic debond crack growth and a decrease in the frictional sliding shear stress along the fibre matrix interface is considered. Describing the fibre strength variation in terms of a Weibull distribution, an equation for the fatigue limit is developed. The model is applied for the prediction of the fatigue limit for glass fibre composites. Effects on the fatigue limit of R-ratio, fibre volume fraction, residual stress, interfacial fracture energy as well as the interfacial frictional sliding stress and its decrease over long time forward-reverse slip are investigated. A good agreement is found between model predictions of a fatigue limit and experimental data from the literature (run-out at 106–1010 cycles).

Effects of inter-fiber spacing on fiber-matrix debond crack growth in unidirectional composites under transverse loading

The energy release rate (ERR) of a fiber–matrix debond crack in a unidirectional composite subjected to transverse tension is studied numerically. The focus of the study is the effect of the proximity of the neighboring fibers on the ERR. For this, a hexagonal pattern of fibers in the composite cross-section is considered. Assuming one fiber to be debonded at certain initial debond arc-length, the effect of the closeness of the surrounding six fibers on the ERR of the crack is studied with the inter-fiber distance as a parameter. Using an embedded cell consisting of discrete fibers in a matrix surrounded by the homogenized composite, a finite element model and the virtual crack closure technique are used to calculate the ERR. Results show that the presence of the local fiber cluster accelerates the crack growth up to a certain initial crack angle, beyond which the opposite effect occurs. It is also found that the residual stress due to thermal cooldown reduces the ERR. However, the thermal cooldown is found to enhance the debond growth in plies within a cross-ply laminate.

Energy Release Rate Based Fiber/Matrix Debond Growth in Fatigue. Part II: Debond Growth Analysis Using Paris Law

The strain energy release rate related to debond crack growth along the fiber/matrix interface in a unidirectional composite with a broken and partially debonded fiber is analyzed. The focus of this article (Part II) in contrast to the self-similar crack growth analysis in Part I [1] is on the growth of short debonds near the fiber break. Since the self-similarity condition is not valid for interactive cracks, numerical FEM simulations were used to calculate magnification of previously described coefficients in the strain energy release rate expression. The findings from these studies are used in simulation of the debond growth in tension-tension fatigue using Paris law.

Energy Release Rate Based Fiber/Matrix Debond Growth in Fatigue. Part I: Self-Similar Crack Growth

The strain energy release rate related to debond crack growth along the fiber/matrix interface in a unidirectional (UD) composite with a broken fiber is analyzed. The UD composite is represented by a model with axial symmetry consisting of three concentric cylinders: broken and partially debonded fiber in the middle surrounded by matrix, which is embedded in a large block of effective composite. Analytical solution for Mode II energy release rate is found and parametric analysis performed in the self-similar debond crack propagation region. It is shown that many anisotropic elastic constants of the fiber, which are usually not known, have a small effect on .

Investigation of sub-critical fatigue crack growth in FRP/concrete cohesive interface using digital image analysis

The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.

Interface debond crack growth in tension–tension cyclic loading of single fiber polymer composites

Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension–tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.

Design of composite stiffener run-outs for damage tolerance

A major concern in stiffener run-out regions, where the stiffener is terminated due to a cut-out, intersecting rib, or some other structural feature which interrupts the load path, is the relatively weak skin–stiffener interface in the absence of mechanical fasteners. More damage tolerant stiffener run-outs are clearly required and these are investigated in this paper. Using a parametric finite element analysis, the run-out region was optimised for stable debonding crack growth. The modified run-out, as well as a baseline configuration, were manufactured and tested. Damage initiation and propagation was investigated in detail using state-of-the-art monitoring equipment including Acoustic Emission and Digital Image Correlation. As expected, the baseline configuration failed catastrophically. The modified run-out showed improved crack-growth stability, but subsequent delamination failure in the stiffener promptly led to catastrophic failure.

Unidirectional composite in mechanical fatigue: modelling debond growth from fibre breaks

The aim of this paper is to analyse fibre/matrix debond crack growth during high stress cyclic tension–tension loading of unidirectional composites. The debond crack evolution analysis is based on fracture mechanics concepts that mode II energy release rate calculations are performed analytically for long debonds, where crack growth is self-similar, and numerically for short debonds by finite element method in combination with virtual crack closure technique. From the calculation results simple expressions are derived for an arbitrary mechanical and thermal loading case. Finally, the obtained expressions are applied in Paris law for debond growth simulations in cyclic tension–tension loading.

Modeling mechanical fatigue of UD composite: Multiple fiber breaks and debond growth

The objective of this paper is to analyze fiber/matrix debond crack growth in unidirectional (UD) composites during high stress cyclic tension-tension loading. High stress loading means that fiber breaks and consecutive fiber/matrix interface debond growth are expected. Fracture mechanics concepts are applied to analyze damage evolution

Characterization of Fatigue Behavior of Bonded Composite Repairs

Composite patches are bonded to a cracked metallic surface either symmetrically (double sided ) or unsymmetrically (single sided ) to extend service life. The stresses in the metallic panel are greatly affected by the repair symmetry. Unsymmetric repairs present the greatest challenge because of the presence of out-of-plane bending. Thermal residual stresses are present because of the thermal coefe cient mismatch of the patch and the aluminum plate. Debonding along an adhesive ‐adherend interface can reduce the patch effectiveness. A simple analysis with Mindlin plate theory is investigated to model the host and the repair plate. The two plates are connected by an adhesive layer modeled by effective springs. Large dee ection theory is used in the case of unsymmetric repairs. The springs are ineffective in the debond zone and are removed. Both the aluminum and the debond cracks are characterizedbyfracturemechanicsby useofthestressintensity factorand strain-energyreleaserate,respectively. Experiments on aluminum 2024-T3 plate, AS4/3501-6 carbon/epoxy composite patch and FM73 adhesive include determining the thermal residual stresses in the aluminum plate and observation of debond development by use of an ultrasonic C-scan. Tests are conducted to examine the metallic and debond crack growth interaction on unsymmetric repairs.

Analytical and experimental methods for a fracture mechanics interpretation of the microbond test including the effects of friction and thermal stresses

An energy release rate model based on a generalized fracture mechanics of composites was developed for analyzing the microbond test. This model, which extended a previous model, includes both friction at the fiber/matrix interface and residual thermal stresses. A series of microbond tests on macroscopic specimens were carried out for evaluating the model. In some specimens we could observe debond crack growth. These results could be interpreted with a fracture mechanics R-curve which led to a measured interfacial fracture toughness. In many specimens, debond crack growth could not be observed. We developed an approximate method for determining interfacial fracture toughness even without knowledge of debond crack size. The macroscopic specimens were designed for studying the optimal approach to analysis of microbond specimens. The geometry of the macroscopic specimens, however, could also be used to measure the mode II toughness of adhesive bonds.

Stress transfer in the fibre fragmentation test

An improved micromechanics model has been developed of the stress transfer for a single fibre embedded in a matrix subjected to uniaxial loading. Debond crack growth is analysed based on the shear strength criterion such that when the interfacial shear stress reaches the shear bond strength, debonding occurs; and the average strength concept based on Weibull statistics is considered for fibre fragmentation. The influences of the interfacial shear bond strength and the fibre strength on the stress distributions in the composite constituents are evaluated (...)