Fracture Toughness Of Adhesive Joints Research Articles

Experimental and numerical analysis on mode II fracture toughness of CFRP adhesive joints using a nonlinear cohesive/friction coupled model

In this work, a nonlinear cohesive/friction coupled model was proposed by combining experiments and numerical simulations for mode II fracture analysis of hybrid carbon nanofillers toughened carbon fiber composite bonded joints. First, end-notched flexure tests were performed to investigate the toughening effects of multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) hybrids modified adhesive on mode II fracture toughness of adhesive joints. Testing results show that 0.75 wt% MWCNTs/GNPs hybrid has an improvement of 45% compared to pure adhesive. Secondly, a phenomenological progressive rising friction model was introduced into an exponential cohesive zone model for delamination analysis of adhesive layer under shear and normal compression loadings. Finally, the proposed model was implemented in ABAQUS/Explicit via user subroutine VUMAT and model parameters were determined by fitting experimental results of end-notched flexure tests for adhesive joints.

Effect of infill ratio on fracture toughness of Fused Filament Fabrication (FFF) polymeric adhesive joints

ABSTRACT In order to optimize the stiffness/weight ratio and to reduce the manufacturing time, Additive Manufacturing (AM) components are built up by a solid skin and a partially filled core. The filled/total volume ratio is called infill ratio and it is one of the printing parameters to be selected. Since the infill ratio affects the global as well as the local adherend stiffness, i.e. the stiffness close to the joint interface, this paper aims at exploring the effect of this parameter on the Mode I fracture toughness of adhesive joints of AM materials, for a given infill pattern. Therefore, PLA (PolyLactic Acid) AM adherends have been manufactured with an FFF (Fused Filament Fabrication) printer at infill ratios ranging between 20% and 100% and bonded with a structural epoxy adhesive to form a DCB (Double Cantilever Beam) joint for Mode I fracture toughness testing. Finite Element (FE) simulation of a stationary crack in the adhesive layer has been done to evaluate the strain energy release rate along the crack front at different infill ratios in order to highlight local changes in its distribution that can correlate the variation of fracture toughness with infill ratio recorded in the experiments.

Exploitation of rubbery electrospun nanofibrous mat for fracture toughness improvement of structural epoxy adhesive bonded joints

The improvement of the fracture toughness of adhesive joints is a key factor in many structural applications. The ability of nylon electrospun nanofibrous mat to act as an adhesive carrier and reinforcing web in adhesive bonding has been demonstrated by the Authors in previous works. It has been shown that the impregnation method developed and refined during the previous studies allow generating high-quality pre-preg nanomats out of a 2k unfilled epoxy resin. By applying this methodology, in the present work, rubbery nanofibrous mats have been adopted for the first time to reinforce and increase the fracture toughness of adhesive joints. Rubbery nanofibers were produced by electrospinning of nitrile butadiene rubber (NBR) and poly(ε-caprolactone) (PCL). The addition of the semi-crystalline polymer (PCL) is exploited to maintain the nanofibrous morphology, which the rubber alone (NBR) would not be able to ensure due to its low glass transition temperature (Tg). The nanofibers thus obtained have been integrated into a two-component high strength epoxy resin for structural applications. S235 steel adherends for Double Cantilever Beam (DCB) tests have been manufactured and sandblasted to improve adhesion. An optimization of the sandblasting parameters (distance, pressure, angle and time) has been carried out evaluating the shear strength and the fracture surfaces on S235 steel Single Lap Joints (SLJ). Finally, DCB tests have been performed to compare the mode I fracture toughness with and without the rubbery electrospun nanomats.

Fracture energy assessment of adhesives Part II: Is GIIc an adhesive material property? (A neural network analysis)

Different joint configurations were considered to analyze the fracture toughness of adhesives. However, based on the literature, fracture toughness depends on various factors including geometrical parameters, material properties, and testing conditions. In Part I of this study, the tensile fracture toughness (GIc) of adhesives was assessed. The present Part II of this research deals with the influencing factors in shear fracture energy of adhesive materials. The main goal of this study is to find the role of the effective parameters on mode II fracture toughness (GIIc) of adhesives. The rational connection between these parameters and the shear fracture energy is also obtained. The interaction between the effective variables is also investigated. To attain this goal, the artificial neural network (ANN) technique was conducted on over 45 values of GIIc already obtained and reported by different authors using different joint geometries and adhesive materials. The influence of the variables was obtained, and the least important variables were detected and omitted from the model. The effect of different parameters and their interaction on the obtained GIIc were also analyzed. The results indicated that GIIc is remarkably sensitive to the alteration of the adherend Young's modulus and its thickness, the loading rate, and the crack length. It was found that GIIc is less influenced by the adhesive stiffness. In addition, the interaction results indicated that the loading rate has the highest interaction with the other studied variables. This study enables to estimate the mode II fracture toughness of adhesive joints and also to advise a configuration to improve the shear fracture energy of a specific adhesive. Designing a test procedure to minimize the effects of substrate parameters on the obtained GIIc is also possible using the obtained results.

Influence of surface micropatterns on the mode I fracture toughness of adhesively bonded joints

Surface patterning has been used to enhance the fracture toughness of adhesive joints. In this study, the effect of the variable bondline thickness introduced by the patterns and the effect of pattern geometry on the fracture behaviour of adhesive joints were analysed. Surface patterns including longitudinal grooves, transverse grooves, dimples and grids were fabricated by means of laser texturing. The patterned surfaces were bonded using a tough structural adhesive and the mode I fracture toughness was measured using the J-integral method. The toughness of the patterned joints was compared with the results from bonding with control surfaces and with grit blasted in combination with chromic acid etched (GB-CAE) surfaces. It was shown that both longitudinal and transverse grooves led to the highest value of toughness. Grid patterns yielded a toughness close to the GB-CAE treatment, both of which were higher than the dimple patterns. It was also shown that the variable bondline thickness due to the existence of surface patterns, which influences the size of the plastic zone, reduced rather than increased the fracture toughness.

Real-time damage monitoring in trailing edge bondlines of wind turbine blades with triboluminescent sensors

Adhesively bonded joints in wind turbine blades, particularly trailing edge joints, are prone to damage due to high-intensity stress concentrations, missing adhesive, or deficient bonds at the bonding surfaces. If not detected early, cracks or disbonds in the trailing edge might propagate rapidly resulting in catastrophic failure of the whole blade. No dedicated in situ sensor system is, as of now, available for real-time damage monitoring in the critical adhesively bonded joints. This article discusses the use of in situ triboluminescent optical fiber sensors for damage monitoring in adhesively bonded composite joints. Double cantilever beam and three-point bend end-notched flexure samples were fabricated from a thick glass fiber/vinyl ester composite and glued with a two-part methacrylate-based structural adhesive. The in situ triboluminescent optical fiber sensors were embedded into the adhesive layer. Double cantilever beam and three-point bend end-notched flexure tests were performed to determine the mode I and mode II fracture toughness of the adhesive joints, respectively. The results show that the triboluminescent emissions occur at the onset of damage when performing double cantilever beam and three-point bend end-notched flexure fracture tests on adhesively bonded composite panels of different adhesive thicknesses. The triboluminescent response increases with the fracture toughness of adhesive joints. The double cantilever beam and three-point bend end-notched flexure specimens undergo cohesive failure in the adherend thickness. Optical microscopy was utilized to obtain better insights of fracture lines. Results indicate that in situ triboluminescent optical fiber sensors can monitor damage in adhesive in real-time without generating false alarms.

Moisture and temperature degradation of double cantilever beam adhesive joints

In this work, the double cantilever beam (DCB) test is analysed in order to evaluate the combined effect of temperature and moisture on the mode I fracture toughness of adhesives used in the automotive industry. Very few studies focus on the combined effect of temperature and moisture on the mechanical behaviour of adhesive joints. To the authors’ knowledge, the simultaneous effect of these conditions on the fracture toughness of adhesive joints has never been determined. Specimens using two different adhesives for the automotive industry were subjected to two different ageing environments (immersion in distilled water and under 75% of relative humidity). Once they were fully degraded, they were tested at three different temperatures (−40, 23 and 80 °C), which covers the range of temperature an adhesive for the automotive industry is required to withstand. The aim is to improve the long term mechanical behaviour prediction of adhesive joints. The DCB substrates were made of a high strength aluminium alloy to avoid plastic deformation during test. The substrates received a phosphoric acid anodisation to improve their long term adhesion to the adhesive. Results show that even though a phosphoric acid anodization was applied to the adherends, when the aged specimens were tested at room temperature and at 80 °C, they suffered interfacial rupture. At −40 °C, however, cohesive rupture was observed and the fracture toughness of the aged specimens was higher.

A Precise Calculation for Fracture Toughness of Adhesive Joint by Dynamic DCB Test

A Precise Calculation for Fracture Toughness of Adhesive Joint by Dynamic DCB Test

Comparative evaluation of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for estimation of the tensile fracture toughness of adhesive joints

The continuous development observed in bonded joints, along with the improvements of the adhesives’ properties, are resulting in an increase of the bonded joint applications, as well as the variety of applications. Regarding the strength prediction of adhesive joints, two highly relevant methods are Fracture Mechanics and Cohesive Zone Models (CZM). By Fracture Mechanics, this is usually carried out by an energetic analysis. CZM enable the simulation of damage initiation and propagation. The tensile critical strain energy release rate (GIc) of adhesives is one of the most important parameters for predicting the joint strength. Two of the most commonly used tests are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB). This work aims to assess the capability of the DCB and TDCB test to estimate the value of GIc of adhesive joints. Three types of adhesives with different levels of ductility are used, to study the accuracy of the typical data reduction methods under conditions that are not always consistent with Linear Elastic Fracture Mechanics (LEFM) principles. For both test protocols, methods that do not require measurement of the crack length (a) during the test are evaluated. In the DCB test, these are the Compliance Calibration Method (CCM), Corrected Beam Theory (CBT) and Compliance-Based Beam Method (CBBM). The methods used in the TDCB test are the Simple Beam Theory (SBT), CCM and CBT. With few exceptions, the results were consistent between the different methods considered for each test. The discrepancy of results is higher when comparing the two types of tests, except for the brittle adhesive. It was concluded that the data reduction methods for the TDCB test are too conservative to measure GIc of ductile adhesives.

On the validity of linear elastic fracture mechanics methods to measure the fracture toughness of adhesive joints

The analysis of large-scale fracture processes, such as those involved in the fracture of adhesive joints, falls outside the scope of Linear Elastic Fracture Mechanics (LEFM). However, experimental data produced in testing adhesive joints are usually reduced with LEFM methods. The consequent error has not yet been evaluated. In this work, an experimental characterization under pure-mode loading of an FM-300 epoxy film adhesive is presented for different adhesive and adherend thicknesses. The experimental data is analyzed using both LEFM-based and J -integral-based data reduction methods in order to study their suitability to analyze adhesive joints. LEFM-based data reduction methods are shown to entail a relevant deviation in the fracture toughness results that heavily depends on the size of the fracture process zone. It is concluded that LEFM methods are not suitable to characterize adhesive joints and that their use is restricted, at best, to the measurement of initiation values. The effect that the adhesive and the adherend thicknesses have on the fracture toughness and the R-curve of the material is studied. Adhesive and adherend thicknesses are shown to have a significant influence on the bond fracture toughness and the source of such influence is discussed.

On the effect of pulsed laser ablation on shear strength and mode I fracture toughness of Al/epoxy adhesive joints

The present work describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with substrates pre-treated by pulsed laser ablation. An ytterbium-doped pulsed fiber laser was employed to perform laser irradiation on AA6082-T4 alloy. Morphological and chemical modifications were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thick adherend shear tests were carried out in order to assess the shear strength while the mode I fracture toughness was determined using the double cantilever beam. For comparison, control samples were prepared using classical surface degreasing. The results indicated that laser ablation has a favorable effect on the mechanical behavior of epoxy bonded joints; however, while a + 20% increase was recorded for shear strength, a remarkable threefold enhancement of fracture toughness was observed with respect to control samples. XPS analyses of treated substrates and SEM observations of the fracture surfaces indicated that laser pre-treatment promoted chemical and morphological modifications able to sustain energy dissipation through mechanical interlocking. As a result cohesive failure within the adhesive bond-line was enabled under predominant peel loading.

필라멘트 와인딩된 카본/에폭시 복합재의 층간파괴인성에 미치는 온도 영향

본 논문은 필라멘트 와인딩 공법으로 제작된 구배 단면을 갖는 돔 분리형 복합재 압력용기 접착체결부의 온도영향에 따른 층간파괴인성 평가를 위해 모드 I, II 그리고 혼합모드의 시험적 평가를 수행하였다. 모드 I과 혼합모드 층간파괴인성은 DCB 시편을 사용하였으며, 모드 II 층간파괴인성은 ENF 시편을 사용하여 평가하였다. 와인딩 각도는 [±10°]SUB6/SUB, [±27°]SUB6/SUB 그리고 ([±10°]SUB3/SUB/FM73/[±10°]SUB3/SUB )이며 곡면 단면을 고려하였다. 시험 평가에 적용된 온도 환경은 환경 챔버와 전기로를 이용하여 -30℃ 에서 60℃로 조성하였다. 층간파괴인성에 온도가 미치는 영향을 평가한 결과, 층간파괴인성은 저온에서 높게 나타났으며, 온도가 증가함에 따라 감소하는 경향을 확인하였다. 시편 종류별 결과에서는, 돔부와 헬리컬부가 접착 체결된 [±10°]SUB3/SUB/FM73/[±10°]SUB3/SUB 와인딩 시편이 가장 높은 층간파괴인성을 가짐을 확인하였다.

Advanced techniques for estimation of the tensile fracture toughness of adhesive joints

Adhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. As a result, adhesives are being increasingly used in industries such as the automotive, aerospace and construction. Thus, it is highly important to predict the strength of bonded joints to assess the feasibility of joining during the fabrication process of components (e.g. due to complex geometries) or for repairing purposes. When using the Finite Element Method with advanced propagation laws, the tensile (Gnc) and shear (Gsc) fracture toughness of adhesive joints must be determined with accuracy. Several conventional methods to obtain Gnc and Gsc exist in the literature, mainly based on Linear Elastic Fracture Mechanics (LEFM). The J-integral technique is accurate to measure these parameters for adhesives with high ductility. In this work, the J-integral is used to obtain Gnc by the Double-Cantilever Beam (DCB) test. An optical measurement method is developed for the evaluation of the crack tip opening and adherends rotation at the crack tip during the test, supported by a Matlab® sub-routine for the automated extraction of these quantities. As output of this work, an optical method that allows an easier and quicker extraction of the parameters to obtain Gnc than the available methods is proposed (by the J-integral technique) and some results are presented regarding joints with different geometry and adherend material.

Advanced techniques for estimation of the tensile fracture toughness of adhesive joints

Adhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. As a result, adhesives are being increasingly used in industries such as the automotive, aerospace and construction. Thus, it is highly important to predict the strength of bonded joints to assess the feasibility of joining during the fabrication process of components (e.g. due to complex geometries) or for repairing purposes. When using the Finite Element Method with advanced propagation laws, the tensile (Gnc) and shear (Gsc) fracture toughness of adhesive joints must be determined with accuracy. Several conventional methods to obtain Gnc and Gsc exist in the literature, mainly based on Linear Elastic Fracture Mechanics (LEFM). The J-integral technique is accurate to measure these parameters for adhesives with high ductility. In this work, the J-integral is used to obtain Gnc by the Double-Cantilever Beam (DCB) test. An optical measurement method is developed for the evaluation of the crack tip opening and adherends rotation at the crack tip during the test, supported by a Matlab® sub-routine for the automated extraction of these quantities. As output of this work, an optical method that allows an easier and quicker extraction of the parameters to obtain Gnc than the available methods is proposed (by the J-integral technique) and some results are presented regarding joints with different geometry and adherend material.

Effects of aramid-fibre toughening on interfacial fracture toughness of epoxy adhesive joint between carbon-fibre face sheet and aluminium substrate

Brittle epoxy adhesive joints, between the carbon-fibre/epoxy face sheets and aluminium substrate, were toughened using randomly-distributed short aramid fibres. In this study, effects of the epoxy adhesive thickness on interfacial fracture toughness of the adhesive joints, with and without short aramid-fibre toughening, were investigated. Short aramid fibres of 6 or 14mm in length with an area density of 12g/m2 were inserted between the carbon-fibre face sheet and aluminium substrate during the laminating process. Two and six layers of aluminium foils were inserted at the interface to form the controlled thin and thick adhesive joints, which are around 20 and 70μm in thickness. The two “composite adhesive joints”, with different volume densities of short aramid fibres, reversed the adhesive-thickness influence on the interfacial toughness in comparison to that of the plain epoxy adhesive joints. However, both “composite adhesive joints” with low and high aramid-fibre densities resulted in significant improvement in the interfacial toughness. Analytical models, together with optical and scanning electron microscopy observations, were used to explain the experimental findings, and relevant toughening mechanisms.

Fracture Toughness Measurement of Adhesively Bonded Joints at Cryogenic Temperatures

A sharp initial crack tip is required for measuring the fracture toughness or energy release rate of adhesive joints, especially at cryogenic temperatures because a blunt initial crack tip in the adhesive may lead to a higher value of fracture toughness than a sharp initial crack tip. Although ASTM D3433 standard specifies how to measure the fracture toughness of adhesive layers in bonded joints, it does not state the way to generate crack tips. In this work, the fracture toughness of adhesive joints was measured at the cryogenic temperature of −150°C with respect to the crack generation method and location of the initial crack, and compared to that at 25°C. From the experimental results, it was concluded that an initial sharp crack tip should be generated in the adhesive region to measure accurately the energy release rate of the adhesively bonded joint. Also, the fracture toughness of the adhesive joint at the cryogenic temperature of −150°C should be measured with respect to crack tip location.

Improvement of Fracture Toughness of Stainless Steel Adhesive Joints at Cryogenic Temperature

Film-type epoxy adhesive was reinforced with E-glass and polyester fiber mats to improve the fracture toughness of adhesive joints composed of austenite stainless steel adherends at the cryogenic temperature of −150°C. To evaluate the fracture toughness and crack resistance of the stainless steel–epoxy adhesive joints, cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed with respect to crack length and fiber volume fraction at the cryogenic temperature. From the experiments, it was found that the crack propagated in stable mode with significantly increased fracture toughness when the film-type epoxy adhesive was reinforced with the E-glass and polyester fiber mats. Also, optimum volume fractions for the E-glass and polyester fiber mats were suggested for the film-type epoxy adhesive in the adhesive joint subjected to the cryogenic temperature.

Improvement of the fracture toughness of adhesively bonded stainless steel joints with aramid fibers at cryogenic temperatures

Adhesives should be reinforced with reinforcing fibers for the bonding of adherends at cryogenic temperatures because all the adhesives become quite brittle at cryogenic temperatures. In this work, the film-type epoxy adhesive was reinforced with randomly oriented aramid fiber mats to decrease the CTE (Coefficient of Thermal Expansion) of the adhesive and to improve the fracture toughness of adhesive joints composed of stainless steel adherends at the cryogenic temperature of −150°C. The cleavage tests of the DCB (Double Cantilever Beam) adhesive joints were performed to evaluate the fracture toughness and crack resistance of the adhesive joints. Also, the thermal and mechanical properties of the fiber reinforced adhesive layer were measured to investigate the relationship between the fracture toughness of adhesive joints and fiber volume fraction of aramid fibers. From the experiments, it was found that the crack propagated in the adhesive with the stable mode of significantly increased fracture toughness when the film-type epoxy adhesive was reinforced with aramid fiber mats. The optimum volume fraction of aramid fibers was suggested for the film-type epoxy adhesive in the adhesive joint at the cryogenic temperature of −150°C.

PS45 表面微細構造を有する複合材料の接着界面特性評価(フェロー賞対象ポスターセッション)

In order to improve interfacial properties of adhesively bonded composites, surface modification using Nanoimprint lithography (NIL) is investigated. Since the interfacial propeilies such as bonded strength or fracture toughness are highly depend on a surface morphology, these properties can be improved by designing and fabricating microstntures appropriately in the interface. Considering this fact we introduced NIL technique to the curing process of composites and fabricated microstnictures on the surface of composites, witich gives us stiff interface without any surface preparation such as sand blasting or chemical etching. We simulated the behavior of crack propagation along the interface with Finite element method using cohesive zone model and estimated model apparent fracture toughness of adhesive joints.

固体状ゴム粒子変成エポキシ樹脂接着剤層中のき裂の破壊靱性値とき裂先端ひずみ分布に対する接着剤層厚さの影響

The fracture behavior of a crack in an adhesive joint is important to investigate the integrity of adhesive structures. It is well known that the fracture toughness of a crack in an adhesive joint using ductile adhesive depends on the bond thickness. However, the mechanism of the dependence has not yet been elucidated. In the present study, we measured the fracture toughness of adhesive joints consisted of aluminum and rubber modified epoxy resin under mode I loading. The distributions of strain around a crack tip were measured using a micro-video-scope and the digital image correlation method (DICM). The measured distributions of strains were compared with that estimated using the finite element method (FEM) in conjunction with Gurson's model. The fracture toughness of adhesive joints used in this study decreased with the decrease of the bond thickness. According to the measurement and analyses, it is estimated that the stress and damage around a crack tip in a thinner adhesive layer were more increased by the higher constraint effect of adherends.