The Fracture Mechanics of the Pin and Collar Test for High Temperature Anaerobic Adhesives

A mechanical test method for the studies of high-temperature anaerobic adhesives has been established, based on fracture mechanics, by modifying the standard test method of collar and pin test. Linear Elastic Fracture Mechanics approach was applied to the establishment of the relationship between adhesive fracture surface energy “R”, fracture load and crack length. Hence, from the joints containing a given artificial flaw the adhesive fracture surface energy can be determined; alternatively, from the strength of the joints without artificial flaws the inherent flaw size “a_i” can be calculated to account for the decrease of joint strength.

The experimental techniques were applied to examine the mechanical behaviour of the joint system based on high temperature anaerobic adhesives. It was found that the joints cured at room-temperature had higher adhesive fracture surface energy but lower joint strength than the joints postcured at high temperatures. The “a_i” data explained this interesting phenomenon. The joints cured at room-temperature had extraordinarily large “a_i”, which was found to be formed by the uncured adhesive near the edges of the joints and the adhesive further cured in the postcure processes to reduce the “a_i”. Also, the growth of intrinsic flaw was found to be responsible for the deterioration of the joints in a short-term, high-temperature ageing process.     

Butt joint tensile strength: interface corner stress intensity factor prediction

The asymptotic form of the interface corner stress field in a butt joint is discussed, and a failure analysis based on the stress intensity factor defining the magnitude of this asymptotic stress field is validated. A stress singularity of type Kr^δ(δ < 0) exists at an interface corner in a butt joint (i.e. where an interface intersects a stress-free edge). A simple relation defines the stress intensity factor K for an idealized butt joint composed of a thin elastic adhesive layer bonded between rigid adherends and subjected to transverse tension and uniform adhesive shrinkage. This stress intensity factor, referred to here as the free-edge stress intensity factor K_f, is applicable to both plane strain and axisymmetric geometries. The way that uniform adhesive shrinkage (thermal contraction) during cure alters interface corner stress fields is also discussed. When adhesive shrinkage is present, both constant and singular terms must be included in the asymptotic solution to attain good agreement with full field finite element results over a reasonably large interface corner region. Experiments have been carried out to investigate the applicability of a K_f-based failure criterion to butt joints. Butt joints were fabricated by bonding two stainless steel rods together with an epoxy adhesive (Epon 828/T-403). The measured joint strength increased by a factor of 2 as the bond thickness was reduced from 2.0 to 0.25 mm. The observed bond thickness effect is accurately predicted when failure is presumed to occur at a critical K_f value. This fracture criterion suggests that the butt joint tensile strength varies roughly as the reciprocal of the cube root of bond thickness when the adhesive's Poisson's ratio is between 0.3 and 0.4, residual stress levels at the interface corner are negligible, the adherends are essentially rigid relative to the adhesive, and small-scale yielding conditions hold at the interface corner.     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

Strength and interface failure mechanism of adhesive joints

Adhesive joints have a wide range of applications in the civil engineering, automotive and aircraft industries. In the present research, we use the finite element method to systematically study the overall strength and interface failure mechanism of single lap joints, which are subjected to tensile loading, focusing on the effects of various system parameters including fracture energy of the adhesive layer, overlap length and adhesive layer thickness on the load-bearing capability of the joints. The results show that the overlap length and the adhesive fracture energy have combined influences on the load-bearing capability. On the other hand, a preliminary damage analysis of the adhesive layer is carried out, considering the situations when the loads arrive to the peak values. Furthermore, the interface behavior is investigated, including the interface stress analysis and interface slip. The rotation of the joint during loading and its influence factors are studied as well. Obtained results suggest that the interface stress distributions are related to the slip and the rotation angle.     

Elastoplastic Finite Element Analysis and Strength Evaluation of Adhesive Butt Joints of Similar and Dissimilar Hollow Shafts Subjected to External Bending Moments

Stress distributions and deformation of adhesive butt joints are analyzed by an elastoplastic finite element method when the joints of similar and dissimilar shafts are subjected to external bending moments. The effects of the ratio of Young's modulus for the adherends to that for an adhesive and the effects of the adhesive thickness on the interface stress distribution are investigated. Joint strength is predicted by using the elastoplastic interface stress distributions. It is found that the singular stress at the edge of the interfaces increases with an increase of the ratio of Young's modulus. Measurement of strains in joints and experiments on joint strength were conducted. The numerical results are in fairly good agreement with the experimental results. It is observed that the joint strength for dissimilar shafts are smaller than those for similar shafts. A fracture of dissimilar adhesive up-bonded shafts occurred from the interface of the adherend with smaller Young's modulus. It is seen that joint strength increases as the adhesive thickness increases.     

A generalized stress singularity approach for material failure prediction and its application to adhesive joint strength analysis

The concept of stress is very useful to describe the effect of external loads on structures. However, as a basis for the prediction of failure the concept of stress becomes meaningless when the structure encompasses singularities as a result of discrete stiffness steps or geometric anomalies such as cracks. In this article it is argued that the concept of failure stress is incorrect and should be replaced by a generalized concept based on stress intensity factors and singularity orders. It appears that material failure stress is the critical stress intensity factor for a zero-order singularity stress field. By plotting the critical stress intensity factor as a function of singularity order, the strength of a material can be characterized in a general fashion that integrates tensile strength, fracture toughness and critical singularities in adhesive joints. It is also shown that plasticity does not eliminate the stress singularity in an adhesive joint but changes the order of the singularity due to the induced change in interface corner angle between the dissimilar materials in the joint.     

Prediction of crack propagation and fracture in residually stressed glass as a function of the stress profile and flaw size distribution

Engineered stress profile (ESP) glasses are noted for narrow strength distributions and the potential for stable growth of multiple surface cracks under applied tensile stress. This behavior depends on the interaction of the surface flaw size distribution with the residual stress profile in the material. In this work, several surface preparation methods were used to produce a range of flaw size distributions in soda lime silica glass specimens. Two ion exchange processes were then performed on these specimens to produce ESP glass. For each condition, crack growth behavior and fracture strength were experimentally observed. Residual stress profiles resulting from each ion exchange process were measured with an optical technique. These stress profiles were used to calculate stress intensity factors as a function of crack geometry, using a weight function method. Crack growth and fracture strength predictions based on these stress intensity factors were compared to experimental data, resulting in good agreement in most cases.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

The Strength of Adhesive Joints

The main rules pertaining to the strength of adhesive joints are: (1) This strength is a mechanical (or rheological) property. The local stress which causes the extension of a pre-existing crack can be determined only if the stress pattern in the whole adhint is known and the intensification of stress at flaws is taken into account. (2) The rupture occurs in a material, not between two materials. Consequently, the molecular forces across the adhesive-adherend interface are irrelevant, and the “adhesion tension” does not determine the adhint strength.     

Thermo-fracture analysis of composite-aluminum bonded joints at low temperatures: Experimental and numerical analyses

Adhesive joints have found extensive applications in aerospace structures because of important advantages such as uniform stress distribution, thermal, acoustic and electrical insulation as well as capability of joining dissimilar materials. These joints in aerospace structures frequently experience severe low temperatures. Lack of experimental data in this field motivated the study of the fracture of adhesive joint at low temperatures in this paper. Fracture parameters of carbon-fibre-reinforced polymer-based composites (CFRP) and aluminum bonded joints were investigated in a temperature range of −80 to +22 °C. In order to understand the mechanical behavior of different components of the bonded joint, firstly, the components (adhesive, composite, and aluminum) were characterized by conducting tensile tests. Subsequently, specimens of cracked bonded joint were tested at low temperatures in different loading modes (mode I, mode II, and mixed mode I/II). The finite element model of the bonded joint was developed in order to obtain the dimensionless functions of stress intensity factors at lower temperatures. The results showed that a reduction in temperature down to a particular value contributes to improved critical stress intensity factors, while any further reduction in the temperature tends to lower the critical stress intensity factors, eventually leading to decreased fracture energy absorption capacity of the structure. In the final section of this paper, a study on fracture surfaces and fracture mechanisms was performed via macroscopic and scanning electron microscopic (SEM) analyses.     

Strength of Elastomeric Adhesive Films in the Triaxial Stress State

Abstract

In model experiments, the triaxial stress state is realized in thin polymer films placed between two solid surfaces and subjected to extension under the action of a force oriented perpendicular to the interface. In this case, the ultimate (fracture) stresses in adhesive joints of uncured flexible-chain polymers of narrow and wide molecular-mass distributions with solids of various nature have been studied as a function of the rate of loading. The results are in good agreement with the data obtained by investigating the durability of the same materials under the conditions of the triaxial stress state. In the region of cohesive fracture the strength of adhesives of narrow MMD is substantially dependent on molecular mass and temperature. In adhesive tear-off, the strength is noticeably affected by the nature of the support, but it is not practically influenced by the molecular mass of the adhesive. The results of the investigation of the strength properties of thin polymer films in the triaxial stress state have been compared with the data obtained for the same polymers subjected to uniaxial extension and shear flow.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

Taguchi analysis of bonded composite single-lap joints using a combined interface–adhesive damage model

The mechanical behaviour of bonded composite joints depends on several factors, such as the strength of the composite–adhesive interface, the strength of the adhesive and the strength of the composite itself. In this regard, a finite element model was developed using a combined interface–adhesive damage approach. A cohesive zone model is used to represent the composite–adhesive interface and a continuum damage model for the adhesive bondline. The influence of the composite–adhesive interfacial adhesion and the strength of the adhesive on the performance of a bonded composite single-lap joint was investigated numerically. A Taguchi analysis was conducted to rank the influence of material parameters on the static behaviour of the joint. It was found that the composite–adhesive interfacial fracture energy and the mechanical properties of the adhesive predominantly govern the static performance of the joints. A parametric study was performed by varying the most important material parameters, and a response surface equation is proposed to predict the joint strength. It is shown that the influence of experimental parameter variations, e.g. variation in adhesive curing and surface preparation conditions, can be numerically accommodated to investigate the static behaviour of bonded composite joints by combining finite element and statistical techniques. The methods presented could be used by practicing engineers to describe the failure envelope of adhesively bonded composite joints.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

Failure analysis of AA8011-pultruded GFRP adhesively bonded similar and dissimilar joints

In this work, a comparative failure analysis of aluminum (AA8011/AA8011) and glass fiber reinforced polyester (GFRP/GFRP) based similar and dissimilar joints is presented. The GFRP is prepared using pultrusion technique. Single lap joints are prepared by using Araldite R2011 epoxy as an adhesive. The lap joints are then tested under tension to estimate the average shear strength of the assembly. It is observed that the average bond strength of AA8011/AA8011 is lesser than that of the GFRP/GFRP joint. The failure of similar joints occurred by fracture within the adhesive. The dissimilar joint is failed predominantly by interface debonding. Further, a detailed three dimensional stress analysis of the joints is carried out using finite element method (FEM). The damage analysis of adhesive layer is carried out by coupling FEM with cohesive zone model (CZM). The stress, damage distributions and failure mechanisms are compared for similar joints in detail. A failure mechanism is proposed for AA8011/AA8011 type joint that favours a rapid crack growth in the adhesive after crack initiation, which is responsible for lesser bond strength. The increase in overlap length has positive effect that the peak load increases proportionally with overlap length.     

Characterization of fracture in adhesively bonded double-lap joints

A broad finite element study was carried out to understand the stress fields and stress intensity factors behavior of cracks in adhesively bonded double-lap joints, which are representative of loading in real aerospace structures. The interaction integral method and fundamental relationships in fracture mechanics were used to determine the mixed-mode stress intensity factors and associated strain energy release rates for various cases of interest. The numerical analyses of bonded joints were also studied for various kinds of adhesives and adherends materials, joint configurations, and thickness of adhesive and different crack lengths. The finite element results obtained show that the patch materials of low stiffness, low adhesive moduli and low tapering angles are desirable for a strong double-lap joint. In the double-lap joint, the shearing-mode stress intensity factor is always larger than that of the opening-mode and both shearing and opening mode stress intensity factors increase as the crack length increases, but their amplitudes are not sensitive to adhesive thickness. Results are discussed in terms of their relationship to adhesively bonded joints design and can be used in the development of approaches aimed at using adhesive bonding and extending the lives of adhesively bonded repairs for aerospace structures.     

Analysis of adhesive tensile test methods

A common bond strength evaluation method is the tensile butt test. However, it is the opinion of the authors that the interpretation of the experimental results obtained from such a test often leaves much to be desired. The results of finite element analyses of butt tensile tests are presented, The stresses in the joints are not uniform as P/A would suggest. It can be argued that failure is more closely related to local extremes in stress than it is to the average stress generally reported. As added support for adhesive fracture mechanics, the analysis of butt tensile joints indicates that the maximum stress occurs at the edge of the joint. The location of the maximum energy release rate, G, varies with the ratio of thickness to diameter (h/D) of the adhesive. For a given “inherent flaw” size, G has a maximum value in the interior of the joint for small h/d and at the bond edge for large h/D. Studies of a transparent adhesive-adherent system demonstrated that failure initiated at the points of maximum, energy release rate. The value of the failure load was also consistent with the trend predicted by fracture mechanics.     

Three-parameter,elastic foundation model for analysis of adhesively bonded joints

A novel three-parameter, elastic foundation model is proposed in this study to analyze interface stresses of adhesively bonded joints. The classical two-parameter, elastic foundation model of adhesive joints models the adhesive layer as a layer of normal and a layer of shear springs. This model does not satisfy the zero-shear-stress boundary conditions at the free edges of the adhesive layer due to the inherent flaw of the two-parameter, elastic foundation model, which violates the equilibrium condition of the adhesive layer. To eliminate this flaw, this study models the adhesive layer as two normal spring layers interconnected by a shear layer. This new three-parameter, elastic foundation model allows the peel stresses along the two adherend/adhesive interfaces of the joint to be different, and therefore, satisfies the equilibrium condition of the adhesive layer. This model regains the missing “degree of freedom” in the two-parameter, elastic foundation model of the adhesive layer by introducing the transverse displacement of the adhesive layer as a new independent parameter. Explicit closed-form expressions of interface stresses and beam forces are obtained. The new model not only satisfies all boundary conditions, but also predicts correctly which interface has the strongest stress concentration. The new model is verified by continuum models existing in the literature and finite element analysis. The new three-parameter, elastic foundation model provides an effective and efficient tool for analysis and design of general adhesive joints.     

The Interphase in Adhesion

The role of weak boundary layers (WBL) in determining the breaking stress of adhesive joints has been proposed to be that of a discrete surface layer of material with strength properties inferior to the bulk material from which it originated. The breaking strength behavior of various polyethylene-epoxy adhesive lap joints has been used as prima facie evidence for the presence or absence of WBL. So-called WBL in PE appear not to be removable by extraction, by abrasion or by fractionation but they seem to disappear when one uses PE as a hot-metal adhesive. We shall attempt to place the WBL matter in better perspective by discussing boundary layers in a general way; that is, evidence for their existence, what they might be where we can isolate and identify them, how they develop and how we can control their properties and, finally, how they influence joint behavior. We conclude that the WBL postulated to explain the behavior of unmodified PE is probably a fiction and that the mechanics of the composite system alone, independent of any material property change in the vicinity of the interface, can determine that the joint shall fail in a thin layer of PE (with properties no different from the bulk) near the interface. Modification of PE mechanical properties in a thin layer near the interface, as by crosslinking or the presence of a transcrystalline structure, can markedly change the mode and locus of failure. The basis for these changes may be as “simple” as a decrease in stress concentration at the joint edges associated with the rise in toughness of the PE produced by cross-linking or transcrystallinity in the surface region.     

A stress singularity approach to failure initiation in a bonded joint with varying bondline thickness

The stress singularity at the theoretical point of maximum stress in an uncracked single lap joint is analysed by a finite element method. By treating the interface corner of a bonded joint (between adherend and adhesive) as a perfectly bonded wedge and using a fracture mechanics method, considerable advantages over other continuum mechanics approaches for investigating the bondline thickness effect on joint strength are shown. This study has essentially two aims: (i) determination of the strength of the singularity by finite element analysis and comparison with the analytical prediction of Bogy for varying bondline thickness; and (ii) determination of stress intensity factors for varying bondline thickness. Good agreement is shown between the numerically-calculated strength of the singularity with the analytical value obtained from Bogy. The calculated stress intensity, after an initial decrease in the low bondline thickness range, is found to increase with increasing bondline thickness. This agrees well with the trends predicted by experiments.