Debond Length Research Articles

An Investigation of Interfacial Fatigue in Fiber Reinforced Composites

Based on the shear-lag model and the modified degradation formula for coefficient of friction, the interfacial fatigue and debonding for fiber reinforced composites under cyclic loading are studied. The loading condition is chosen as the kind that is the most frequently used in fiber-pull-out experiments. The stress components in the debonded and bonded regions are obtained according to the maximum and minimum applied loading. By the aid of theory of fracture mechanics and Paris formula, the governing equation is solved numerically and the interfacial debonding is simulated. The relationships between the parameters (such as the debond rate, debond length, debond force) and the number of cycles are obtained.

Elastic foundation analysis of local face buckling in debonded sandwich columns

An elastic foundation model is developed for analysis of the local buckling behavior of foam core sandwich columns containing a through-width face/core debond. Parametric studies are performed to evaluate the effect of debond length, core thickness, core density, face sheet thickness, face sheet modulus, and boundary conditions, on the local buckling load. Model predictions are compared to experimental results obtained for sandwich specimens made from glass/vinylester face sheets over various PVC foam cores containing a face/core debond. The predictions are in reasonable agreement with measured buckling loads.

Micromechanical phenomena during hygrothermal ageing of model composites investigated by Raman spectroscopy. Part I: Twaron fibres with different surface treatments

Abstract Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal ageing in model composites based on an epoxy resin containing a single filament of a Twaron PPTA fibre. Three Twaron fibres have been studied: untreated fibre (HM), fibre with a surface finish (HMF) and fibre with an activated surface (HMA) using two specimen configurations: the diffusion slab (DS) and the double pullout geometry. Time evolutions of the Raman strain profiles e(x,t) and the water uptake M(t) have been determined for specimens immerse in liquid water and exposed to water vapour. Experimental data are modelled using relationships that describe Fickian diffusion into a solid parallelepiped. Simple micromechanical models have been used to rationalise the Raman strain profiles in terms of the mid-fibre strain e(x=0,t), the dimensionless parameter, n (controlling the stress transfer along interface) and the fibre strain energy G(t). For some fibres and at longer exposure times debonding was observed and the time dependence of the debond length Ld was quantified. Eliminating the exposure time, the swelling behaviour of the matrix is assessed and a concept for determination of the interface fracture energy is proposed. Time dependencies of n, Ld and G(t) are used to rank surface treatments of Twaron fibres as to durability of their interfaces during hygrothermal ageing.

Micromechanical phenomena during hygrothermal ageing of model composites investigated by Raman spectroscopy. Part II: comparison of the behaviour of PBO and M5 fibres compared with Twaron

Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal aging in model composites based on an epoxy resin containing a single filament of a polymeric fibre. Two fibres have been studied—PBO and M5—using two specimen configurations: The diffusion slab (DS) and the double fibre pull-out (DFPO). Time evolutions of the Raman strain profiles e(x,t) and the water uptake M(t) have been determined for specimens immersed in liquid water. Simple micromechanical models developed for rationalising data obtained for Twaron are used to process measurements for PBO and M5 (PIPD). In this manner, a direct comparison of the time evolutions of the moisture uptake M(t), mid-fibre strain e(x=0,t), the dimensionless Raman shear lag-parameter n and a debond length Ld allows conclusions to be drawn on the aspects of hygrothermal ageing such as durability of a fibre/matrix interface and fibre swelling.

Is there any contradiction between the stress and energy failure criteria in micromechanical tests? Part III. Experimental observation of crack propagation in the microbond test

A direct observation of crack propagation in the microbond test was carried out for five different fiber/polymer matrix systems. This technique appeared to be a very effective tool for interface characterization. Experimental plots of the force required for further crack propagation as a function of debond length were analyzed using both energy-based and stress-based models of debonding. The fracture mechanics analysis was used to construct families of crack resistance or R-curves which showed the variation of energy release rate, G, with the debond length, and included the effect of interfacial friction in debonded regions. For the first time, analogs of the R-curves were created within the scope of the stress-based model to present the local shear stress near the crack tip, τ, as a function of crack length. In both models, the behavior of the interfacial parameter (G or τ) strongly depends on the assumed value of the interfacial frictional stress (τf). However, for each matrix/fiber system there exists such a τf value for which the investigated parameter is nearly constant over the whole region of stable crack propagation (70–90% of the embedded length). Moreover, these best-fit τf values for each specimen appeared to be practically the same for both energy-based and stress-based approaches. Thus, both interfacial toughness, G ic, and local interfacial shear strength, τd, adequately characterize the strength of a fiber/matrix interface. Extrapolation of R-curves and their analogs to zero crack length allows measurement of the interfacial parameters with good accuracy.

Effect of Water Absorption on Interaction between Matrix Cracking and Fiber Bridging for Stress Corrosion Cracking of PMC

The effect of water on fiber bridging of polymer matrix composites (PMCs) has been studied for stress-corrosion cracking (SCC). The fiber bridging during crack propagation in the PMC was observed directly using a transparent polymer matrix. Specimens were immersed and weighed just before conducting the crack propagation test. The contribution of mechanical degradation caused by the water to the bridging was measured as a function of weight gain due to water absorption. The critical energy release rate of the matrix decreased with increase of the water absorption. The energy release rate of a bridged crack varied with the water absorption. To investigate the variation in the bridging mechanism under the water environment, we calculated the bridging stress the σt and the bridging contribution to crack propagation resistance ΔG based on a single-fiber bridging mechanism with the interfacial debonding length and the interfacial debonding energy. The debonding length was measured directly, and the debonding energy was obtained in a fragmentation test. The calculation showed that the increase in the interfacial debonding length influenced the fiber bridging mechanism.

20107 フラグメンテーション試験による繊維強度の評価 : 試験速度を考慮した評価方法の提案(界面の強度,OS10 先進材料の力学と強度)

Single fiber fragmentation test is one of the methods to evaluate strength of a fiber embedded in matrix resin. This method has two advantages; the Weibull parameter can be obtained from a few tests and an actual strength of the embedded fiber in practice can be obtained. This test evaluates relationship between the fiber strength and the fiber length. However, the fiber length is defined by the length of the gage length divided by the number of fragment points, and the fiber length is overestimated. Therefore, in the conventional evaluation, the fiber strength is not independent to interfacial strength and test speed. To assume the exact fiber length, an interfacial debonding length and a stress recovery length must be considered. In this study, the new corrected evaluation to generalize the fiber strength is suggested. To discuss the validity of the method, several fragmentation tests are performed under the condition of the various interfacial strength and the various test speed.

Analysis of the fracture behaviour of a stitched single-lap joint

The effect of stitching on the fracture response of single-lap composite joints was studied by a combined experimental and numerical analysis. Unstitched and Kevlar stitched joints were tested under static and fatigue loading to characterize damage progression and failure modes; a three-dimensional finite element analysis was carried out to evaluate the influence of stitches on strain energy release rates as a function of damage and to identify the role of various stitching parameters on the fracture behaviour of joints. It was observed that the failure of the joints occurs as a consequence of the propagation of delamination at the interface between the adherends; the propagation is stable under fatigue loads and unstable under static loads. Stitching does not improve the static strength of joints but significantly prolongs the duration of the crack propagation phase under fatigue loading. The results of finite element modelling indicate that the incorporation of stitches reduce GI to zero after the delamination front passes the stitch line, but it is not effective in reducing mode II energy release rate. They also show that strain energy release rates are not greatly affected by the length of stitch-laminate debonding, which, conversely, does influence stitch tensioning. Moreover, 3D analysis reveals that stitches become less efficient in reducing the crack driving force with increasing stitching steps.

Prediction of creep rupture in unidirectional composite: Creep rupture model with interfacial debonding and its propagation

This paper describes a creep rupture model that takes into account the interfacial debonding and its propagation around broken fibers in unidirectional fiber-reinforced polymers. The interfacial debonding is accompanied by fiber breaks under a longitudinal tensile load, and the interfacial debonding length depends on both the fiber stress at the moment of the fiber breaks and the properties of the interfacial adhesion. In this paper, the interfacial debonding length, its propagation as a function of time, the probability of fiber breaks and an interfacial shear stress are investigated by fragmentation tests with a single glass fiber embedded in each vinylester resin specimen. Subsequently, it was found that a relationship between the fiber strength and the fiber effective length, the length of the debonding length and stress recovery length subtracted from one fiber length, was log linear, and that the larger the fiber stress, the longer was the interfacial debonding length. Next, creep tests were performed to investigate the creep constants of the vinylester resin, and interfacial debonding propagation tests were also performed to identify the interfacial debonding-propagation behavior as a function of time using single fiber composites subjected to constant strains. We modified the creep rupture model by taking into account the interfacial debonding length, which has a time-dependent characteristic, into a conventional Curtin–McLean creep rupture model. It was found that the predicted creep rupture time of the modified model was shorter than that of the conventional model and that there was no threshold creep stress in the modified model unlike in the conventional model because the interfacial debonding propagation makes the prediction creep rupture time lower as time progresses.

Static Debonding Initiation Stress of Fiber Glass Composite

Mesoscale debonding initiation stress of a bundle of glass fibers and epoxy matrix under static loading is investigated. A special cylindrical bar specimen was designed for the experiment. The specimen contains a bundle of glass fibers in the center and the bundle diameters were 1 and 2mm. The fiber diameter is 7µm. A static tension test was performed under the displacement control. The applied load-displacement curve was recorded for each specimen and then the debonding initiation load was defined as the deviation point from the linear curve. In addition, the debonding length was measured directly from the tested specimen. Using the debonding load measured from experiment, an axisymmetric finite element analysis is performed and debonding parameters K1 and K2 were computed. The results show the debonding initiation is fully dominated by an opening mode rather than a sliding mode and the stress intensity factor K1 for the debonding initiation is 2.0E+05Pa√m. The mixed mode fracture takes place in the debonding propagation with constant mode mixity around 41.0 degree and the averaged stress intensity factors are 1.67E+05Pa√m and 1.41E+05Pa√m for the opening and the sliding mode, respectively.

212 単繊維モデル複合材料における界面き裂の時間依存特性 : はく離部の界面摩擦応力の緩和を考慮した界面はく離進展(OS 高分子/高分子基複合材料(1))

Interfacial debonding propagation is very important problem for long-term creep in unidirectional composites because material's properties of the unidirectional composite decrease with increase of the interfacial debonding length. In this study, time-dependent interfacial failure was investigated by using the Raman Micro Spectroscopy. The specimen was single fiber conposite consisting one carbon fiber embedded in the vinylester resin and it was subjected to constant strain condition to assume the long-term creep in the unidirectional composites. The interlace between the carbon fiber and the vinylester resin is relatively weak; therefore interfacial frictional stresiss is one of the most important factors, which dominated the interfacial debonding propagation. In this paper, the compression stress applied to the interface in the direction of fiber rudius direction is calculated as a function of time on the consideration of Poisson compression and thermal residual stress. The interfacial frictional stress is assumed to be dominated by the Coulomb's friction and compared with the Experimental results. The interfacial debonding propagation is discussed by using the relaxation of the frictional stress. Long-term behavior of the interfacial debonding propagation is predicted in the single fiba composites.

Failure phenomena in fibre-reinforced composites. Part 6: a finite element study of stress concentrations in unidirectional carbon fibre-reinforced epoxy composites

A three-dimensional (3-D) finite element (FE) analysis of the stress situation around a fibre break in a unidirectional carbon fibre-reinforced epoxy composite has been performed. Two cases were considered: (i) good fibre/matrix adhesion and (ii) fibre/matrix debonding. In the case of good adhesion, three matrix systems differing in yield stress were investigated. For all three matrix systems stress concentration factors (SCFs) of approximately 1.07 are found for a fibre volume fraction ranging from 25 to 70%. According to a fibre interaction criterion reported earlier (Composites Science and Technology 1997;57:899), this is large enough to cause coordinated fibre failure. In the case of poor fibre/matrix adhesion, it was found that in addition to the stress concentration at the plane of fracture, a second stress concentration is located at the end of the debonded zone. For a 60 fibre vol.% composite with a debond length of ten fibre diameters, these SCFs are approximately 1.03 and 1.05, respectively. Furthermore, in order to experimentally obtain an indication of the SCFs in a full composite, Raman experiments were performed on 2.5-D carbon/aramid/epoxy hybrid model composites in which a single carbon fibre was placed on top of a bundle of ten aramid fibres. The results indicated that in such a hybrid model composite the SCF is slightly larger than in a full (3-D) unidirectional composite. Furthermore, these experiments confirmed the existence of the second SCF in the case of debonding.

Investigation of hygrothermal aging phenomena in model composites by Raman spectroscopy

Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal aging in model composites based on an epoxy resin containing a single filament of a reinforcing fibre. Three fibre types, namely PBO, M5 and Twaron, were investigated using two specimen configurations - the diffusion slab (DS) and the double fibre pullout (DFPO) geometry. Variables such as the fibre type, surface treatment in the case of Twaron fibres, and the penetrant type (water vapour and liquid phase), were screened by establishing development of the Raman strain profiles ϵ(x, t) and the water uptake M(t) with time. Experimental data were successfully modelled using relationships that describe Fickian diffusion into a solid parallelepiped. Relative water uptakes determined gravimetrically were reduced to time dependencies controlled by the specimen geometry, diffusion coefficient of the matrix D and the equilibrium uptake of water, M(∞). Using simple micromechanical models, the Raman strain profiles were rationalised in terms of the midfibre strain ϵ(x=0, t) and the dimensionless parameter n, which controls the stress transfer along the interface. The fibre strain energy G(t) was also obtained from the area under the Raman profile. At longer exposure times and for some fibres, debonding was observed and the time dependence of the propagation of the debond length Ld was quantified. Generic conclusions on composite behaviour were drawn by eliminating the exposure time. The swelling coefficient β was derived from the linear dependence of midfibre strain on the water uptake and a change in the fibre strain energy was related to the debond propagation.

一方向繊維強化プラスチックのクリープ特性と寿命予測―繊維破断部近傍の界面はく離を考慮したクリープ破断モデル―

This paper describes a creep rupture model considering the interfacial debonding in broken fibers in unidirectional polymer matrix composites reinforced by glass fibers. Interfacial debonding was accompanied with fiber breaks under the tensile load, depended on the fiber stress at the simultaneous fiber breaks. The interfacial debonding length, fiber strength and an interfacial shear stress were investigated by the fragmentation test. A vinylester/glass fiber interface's properties were investigated by the fragmentation test. Subsequently, the debonding length and a stress recovery length were quantitatively studied and it was found out that the relation between the strength and the length of fiber was linear. The debonding length was calculated by a measure of the distance between the fiber breaking faces. The interfacial debonding length was taken into the Curtin's fracture model. Moreover, the creep test of the unidirectional composites, it was vinylester resin matrix reinforced by glass fibers, was performed to investigate the creep constants. Then, the prediction in the use above parameters of the creep rupture time was discussed.

941 一方向 FRP のクリープ寿命予測 : 繊維破断部近傍の界面はく離を考慮したクリープ破断モデル

This paper describes the creep rupture model considering the interfacial debonding and its propagation from the end of broken fibers in unidirectional polymer matrix composites reinforced by glass fibers. Interfacial debonding is accompanied with fiber breaks under the tensile load, which develops longer. In the present study, the interfacial initial debonding length, the fiber strength and the interfacial shear stress are investigated by the fragmentation test. The interfacial debonding propagation is also investigated by the creep test or the constant strain test with the use of a single fiber within a polymer matrix specimen. The interfacial debonding length is taken into the modified Curtin-McLean's creep rupture model. This modified creep model does not have the threshold stress, because the failure stress in this model decreases according to the interfacial debonding propagation as time progresses. The predicted life of a creep rupture is shorter than that of the conventional model and there is no threshold stress in the long-term creep.

Influence of surface morphology on the adhesion strength of epoxy–aluminum interfaces

Adhesively bonded aluminum joints have been increasingly used in the automotive industry because of their structural and functional advantages. Interfacial debonding in these joints has become a major concern limiting their performance. The present work is focused on experimental investigation of the influence of surface morphology on the interfacial fracture behavior of the epoxy- aluminum interface. The specimens used in this experimental study were made of an epoxy- aluminum bimaterial strip in the form of a layered double cantilever beam (LDCB). The LDCB specimens were debonded by peeling off the epoxy layer from the aluminum substrate using a steel wedge. Interfacial fracture energy was extracted from the debonding length using a solution for the specimen geometry based on a model of a beam on an elastic foundation. This model was validated by direct finite element analysis. The experimental results establish a direct correlation between the surface roughness of aluminum substrate and the fracture resistance of the epoxy-aluminum interface. The results emphasize the importance of choosing surface features at an appropriate length scale in studying their effects on interfacial fracture resistance.

An Efficient Analysis for the Debonding of Stitched Skin-Stiffened Structures

An analysis based on plate finite elements, nonlinear spring elements and the virtual crack closure technique has been implemented to study the effect of stitching on strain energy release rates for debond configurations. The stitches were modeled as discrete nonlinear spring elements with a compliance determined by experiment. The axial and shear behavior of the stitches was considered, however, the two compliances and failure loads were assumed to be independent. Both a double cantilever beam (mode I) and a mixed mode skin-stiffener debond configuration were studied. In the double cantilever beam configuration, G I began to decrease once the debond had grown beyond the first row of stitches and was reduced to zero for long debonds. In the mixed-mode skin-stiffener configuration, G I showed a similar behavior as in the double cantilever beam configurations; however, G II remained nonzero over the range of debond lengths considered.

Analytical and Experimental Studies of the Debonding of Stitched and Unstitched Composite Joints

The effect of stitches on the failure of a single lap joint configuration was determined in a combined experimental and finite element study. The experimental program was conducted to determine debond growth under static monotonic loading. The stitches were shown to delay the initiation of the debond and provide load transfer after total debonding of the lap joint. The experimentally determined debond length versus applied load was used as an input parameter in the finite element analysis of both stitched and unstitched configurations. The strain energy release rates at the debond front were calculated using plate elements. Discrete nonlinear spring elements were used to model the stitches and multipoint constraints were used to model the contact problem. Models of the unstitched configuration showed significant values of mode I and II strain energy release rates across the width of the joint and showed that mode III is zero at the centerline but increases near the free edge. Models of the stitched configuration showed that the stitches were effective in reducing the mode I component to zero, but had less effect on modes II and III.

Interfacial bond strength and fracture energy at room and elevated temperature in titanium matrix composites (SCS-6/Timetal 834)

Push-out experiments in the temperature range from room temperature to 530 °C have been performed. On the basis of the measured maximum load to break the fibre/matrix bond and the load to overcome frictional shear stresses finite element analyses are performed to derive an interfacial shear strength and interfacial fracture energy for the different test temperatures. The thus derived material properties decrease at increasing test temperature. In the analyses the in-homogeneity of the (shear) stress distribution plays an important role for the thermal as well as for the mechanical stress distribution. Although only the energy concept requires principally an initial debond length, it is shown that without an initial defect the shear strength concept leads to unrealistic results. The results presented are valid if it can be accepted that during specimen preparation an initial debonded zone of 12–18 μm at the surface is introduced.

Microstructure of Silicon Nitride Containing β‐Phase Seeds: II

The microstructure of silicon nitride containing different percentages of β‐seeds was investigated. The average grain size and volume fraction of large grains increased with the incorporation of β‐seeds. The length and aspect ratio of large grains in sintered Si3N4 initially increased by incorporating β‐seeds, and then decreased. Similar trends were observed in the fracture toughness. The toughening mechanisms and fracture behaviors were correlated with the grain morphology measured from image analysis. Plots of measured toughness versus volume fraction of large grains fit very well with the equation derived by Becher and Budiansky, which strongly suggested the essential role of debonding length in the toughness of silicon nitride. Further improvement of fracture toughness in silicon nitride may be possible by tailoring the amount of incorporated β‐phase seeds and leading to optimize the volume fraction, grain size, and aspect ratio of the elongated grains.