Single Lap Joint Tests Research Articles

Delamination analysis of woven fabrication laminates using cohesive zone model

A new test method was proposed to evaluate the cohesive strength of composite laminates. Cohesive strength and the critical strain energy for Mode-II interlamiar fracture of E-glass/epoxy woven fabrication were determined from the single lap joint (SLJ) and end notch flexure (ENF) test, respectively. In order to verify their adequacy, a cohesive zone model simulation based on interface finite elements was performed. A closed form solution for determination of the penalty stiffness parameter was proposed. Modified form of Park-Paulino-Roesler traction-separation law was provided and conducted altogether with trapezoidal and bilinear mixed-mode damage models to simulate damage using Abaqus cohesive elements. It was observed that accurate damage prediction and numerical convergence were obtained using the proposed penalty stiffness. Comparison between three damage models reveals that good simulation of fracture process zone and delamination prediction were obtained using the modified PPR model as damage model. Cohesive zone length as a material property was determined. To ensure the sufficient dissipation of energy, it was recommended that at least 4 elements should span cohesive zone length.

Polyelectrolytes to promote adhesive bonds of laser-structured aluminium

Aluminium is one of the most popular construction materials in machine and equipment manufacture as well as vehicle and aircraft construction. Particularly, in automotive and aircraft industries, the adhesive bonding of aluminium requires the pre-treatment of the adhesive surfaces. In this study laser pre-treatments were used to laterally control the surface roughness and clean the substrate surfaces by forming fresh aluminium oxide layers. In order to keep the adhesive properties stable over time, the laser pre-treated aluminium surfaces were subsequently coated with weak polyelectrolytes. The applied polyelectrolytes lower the driving forces for the adsorption of unwanted surface contaminations and provide reactive amino groups for the subsequent coupling of reactive adhesives. The surface topographies of the laser-treated aluminium surfaces were investigated in relation to the applied laser parameters (such as pulse frequency, and laser power) by means of scanning electron microscopy (SEM) and light-microscopic techniques (confocal microscopy). The adsorption of the polyelectrolytes was studied by X-ray photoelectron spectroscopy (XPS). Inverse water contact angle measurements using captive air bubbles were carried out to study the wettability (hydrophilicity/hydrophobicity) of the modified aluminium surfaces. Single lap joint tests carried out on joined AlMg3 sheets showed that the shear strengths can be significantly increased by pre-treatment with laser and coating of the alloy surfaces with weak polyelectrolytes. Furthermore, the application of polyelectrolytes improved the stability against corrosion. The article shows the increase of tensile shear strengths at adhesively bonded single lap shear samples after laser pre-treatment and also an increase in long-term stability due to of the combination of laser pre-treatment and coating with polyelectrolytes. Adhesive bonds of laser treated samples with and without polyelectrolyte coating have a higher stability against corrosion compared to untreated samples.

Macroscopic strength and failure properties of flow-drill screw connections

Force–displacement responses and failure behaviour of connections using flow-drill screws to join aluminium sheets were investigated under various quasi-static loading conditions. This included single connector tests under tensile, shear and combined tensile and shear loadings, using cross test coupons in a new test set-up, and peeling and single lap-joint tests. The strength of the connection increased with the amount of shear loading, while the ductility decreased. No effect of the anisotropy of the sheets on the behaviour in the single connector tests was found. Axial crushing tests of aluminium single-hat sections joined with flow-drill screws were also performed. Two connection failure modes not observed during the single connector test were found in these tests. For comparison, equivalent single connector and component tests were carried out for self-piercing rivet connections. Similar trends with respect to the ductility, maximum force and shape of force–displacement curves were observed for the two connections, but the local failure modes were different.

Adhesion and Tensile Properties of a Novel Fiber-Metal Laminate Based on Polypropylene Reinforced with Aramid Fibers.

ABSTRACTThe aim of the present study is to analyze interfacial adhesion and characterize the tensile properties of a FML elaborated from thin layers of an aluminum alloy and layers of maleic anhydride modified polypropylene (MAHPP) reinforced with an aramid woven fabric. For the analysis of interfacial adhesion, a microbond test is carried out on the MAHPP-aramid fiber system and a single lap joint test is performed on FML constituent materials, as well as the tensile characterization of the FML and its constituents is conducted accordingly. Microbond testing revealed an improvement in interfacial shear strength for the MAHPP-aramid fiber system in comparison with that of polypropylene-aramid fiber systems reported in the literature. The apparent shear strength between the FML constituent materials is comparable to that for bonding of aluminum with MAHPP. Tensile characterization of the FML and its constituents showed that the FML presented greater tensile strength than the aluminum alloy; and a more ductile behavior in comparison with its individual components due to the degree of adhesion between the constituents which allows the material to deform in unison.

The Influence of Alloying Elements on Adhesive Properties of Epoxy-Quasicrystal Composites

Quasicrystals (QC) are materials that exhibit good surface properties such as high hardness, low surface energy, low friction coefficient, and good resistance to oxidation and corrosion. Despite these properties, QC alloys are highly brittle, discouraging their uses in applications requiring mechanical stresses. Hence, the development of composites using quasicrystalline materials as reinforcement to ductile matrices is an interesting alternative. In this work, Al-Cu-Fe quasicrystals were used as a reinforcement to epoxy resin in order improve adhesion with metallic substrates. Adhesive properties were evaluated by single lap joint (SLJ) tests and the QC/epoxy interface was analyzed by scanning electron microscopy (SEM). Aluminum, copper, and iron, individual elements, were used to investigate the contribution of each element to the adhesive. Results show that it was possible to significantly enhance adhesion properties when 25% QC was added to epoxy and that the effect of QC amount on SLJ bond strength did not follow a linear trend. Moreover, the contribution of each alloying element was important to explain the influence of QC addition on adhesive properties of the epoxy-based composites.

Mechanical characterization of a high elongation and high toughness epoxy adhesive

In this paper, a new epoxy adhesive has been mechanically characterized. The adhesive combines the properties of an epoxy adhesive and typical polyurethane (PU) adhesive, such as high elongation and high toughness. Experimental tests were performed to measure the tensile properties, shear properties, thermal properties and fracture properties. The tensile test shows high tensile strength and high elongation. The single lap joint (SLJ) test shows that the failure load is proportional to the overlap length for hard steel adherends. For the SLJs with mild steel adherends, the failure occurred due to adherend yielding. Impact tests were conducted using SLJ specimens and the results are consistent with the SLJ tested under static conditions. The Tg was obtained using a Dynamic Mechanical Analysis (DMA) type of test. The toughness in mode I was determined using the Double Cantilever Beam (DCB) test and the toughness in mode II using End Notched Flexure (ENF) test.

On the mechanical behavior of BFRP to aluminum AA6086 mixed joints

The aim of this work is to analyze the possibility to join aluminum alloy AA6086 and composite laminates reinforced with basalt fibers, an innovative material which use is growing in several applications as an alternative to glass fibers. To this goal, three joining techniques were investigated: mechanical by Self Piercing Riveting (in the next called SPR), adhesive by co-curing technique and mixed in which the joining techniques (i.e. adhesive and mechanical) were combined. Two manufacturing technologies (i.e. hand lay-up and vacuum bagging) were used both to produce composite substrates and to realize co-curing adhesion between the substrates to be joined. Mixed joints were realized by inserting the rivets in co-cured joints after 48h than the initial phase of the curing process (i.e. the phase of mixing the resin with own hardener). Overall, six lots of joints were realized (two for each joining technique). All joints were characterized by performing single lap joint tests. The mechanical results were analyzed through a two way analysis of variance.The experimental results show that adhesive joints, realized by vacuum bagging method, show average failure load 22.9% higher and standard deviation 70.6% lower than those realized by hand lay-up, respectively. This means that the vacuum bagging technology allows to increase the adhesion strength of the interface between metal sheet and Basalt Fiber Reinforced Polymer (in the next BFRP), allowing the above failure load growth. Furthermore the failure mechanisms change from adhesive mode to partially cohesive one for the adhesive joints realized by hand lay-up and vacuum bagging, respectively. By comparing mixed joints, different results are obtained: i.e. the hand lay-up joints show both higher average failure load (+42.9%) and standard deviation (+208.3%) than those realized by vacuum bagging. The poor performances of the mixed joints realized by vacuum bagging can be considered due to the excessive value of the chosen riveting load. Statistically, two variables were investigated: i.e. joining technology (i.e. mechanical, adhesive and mixed) and manufacturing process (i.e. hand lay-up and vacuum bagging). These can influence the properties of the joints. In particular, the joining technology results a significant factor. Moreover, an interaction between the two variables exists.

Characterization of Aluminium Single-Lap Joints for High Temperature Applications

In this study, an experimental investigation into the shear strength behaviour of aluminium alloy single-lap adhesive joints was carried out in order to understand the effect of temperature on the strength of adhesively bonding joints. Single lap joints (SLJs) were fabricated and tested at RT and high temperatures (100°C, 125°C, 150°C, 175°C and 200°C). Results showed that the failure loads of the single-lap joint test specimens vary with temperature and this needs to be considered in any design procedure. It is shown that, although the tensile stress decreased with temperature, the lap-shear strength of the adhesive increased with increasing of temperature up to the glass transition of the adhesive (Tg) and decreased for tests above the Tg.

A study on diffusion bonding of steel and copper alloy

AbstractIn combustion chamber of liquid propellant launch vehicle, the inner shell of the chamber is copper alloy with cooling channels for regenerative cooling and outer shell is steel to maintain high pressure inside the chamber. The purpose of this study is to find the optimum condition for diffusion bonding of copper and steel and the experimental conditions were 3 different pressures at temperatures from 800°C to 950°C. In order to characterize the flow strength of materials at high temperatures, several tensile tests were performed at several temperatures from 800°C to 950°C. This information is used to estimate the test condition for diffusion bonding and superplastic forming. Mechanical properties of bonded specimen were evaluated with single lap joint tests and shear tests. Microstructure of bonded layer has been also observed with SEM with EDX. It is shown that the optimum condition of diffusion bonding is 7 MPa at 890°C, for one hour. Pressurization test of bonded specimen with cooling channels was performed with hydraulic pressure of 87 MPa without failure.

Comparative study on the behavior of woven-ply reinforced thermoplastic or thermosetting laminates under severe environmental conditions

This work aims at determining whether thermoplastic-based composites can be used in secondary aircraft structures to replace thermosetting-based composites or not. In order to answer this question, the mechanical behaviors of carbon fiber fabric reinforced thermoplastic (PPS or PEEK) and thermosetting (epoxy) laminates subjected to different stress states under severe environmental conditions (120°C after hygrothermal aging) have been compared. In addition to usual mechanical tests (tensile, open hole tensile), single-bolt double lap joint and single-bolt single lap joint tests were also performed. Severe conditions help enhance the ductile behavior of the epoxy matrix, but degrade the fiber/matrix interface, resulting in lower stiffness and strength of laminates with a quasi-isotropic lay-up. In thermoplastic-based laminates, the degree of retention of mechanical properties is quite high even for PPS-based laminates when T>Tg. In laminates with a [45]7 lay-up, severe conditions adversely affect the mechanical properties of the three composite systems. However, the combination of matrix ductile behavior, and the strain gradient near the hole, lead to an extensive plastic deformation along the ±45° oriented fibers bundles in notched A-P laminates. It results in decreasing significantly the hole-sensitivity of C/PPS and C/Epoxy under severe conditions. In bolted joints, a severe environment has a limited impact on the bearing strength of epoxy-based laminates. In the case of thermoplastic-based laminates, it increases the strength of double lap joints, but is detrimental to the strength of single lap joints.

Experiments and Cohesive Zone Model Parameters for T650/AFR-PE-4/FM680-1 at High Temperatures

This paper reports an experimental program to establish a cohesive zone model for the T650/AFR-PE-4 (laminate) and FM680-1 (adhesive) system. The cohesive zone model is based on a four parameter characterization: in each mode, a range of values for the critical energy release rate and cohesive strength are computed from a set of experimental results. Values of each parameter are determined over the temperature range of 20–350°C. Owing to experimental limitations, two methods for determining the Mode I critical energy release rate are reported from the double cantilever beam test: the area method and the inverse method. The Mode I strength is determined from a button peel stress test. The values of the Mode II parameters are determined by using a mapping procedure that accounts for multiparameter dependence in models of the end notch flexure and single lap joint tests.

Carbon fiber fabric reinforced PPS laminates: Influence of temperature on mechanical properties and behavior

AbstractThis paper is aimed at investigating the influence of temperature on mechanical properties and behavior of carbon fiber fabric reinforced polyphenylenesulfide (PPS) laminates subjected to different loadings: tensile, open hole tensile, in‐plane shear, compressive, flexural, interlaminar shear, single‐bolt double lap joint, and single‐bolt single lap joint tests. The resulting experimental database is of the utmost importance for generating design allowables for structures integrating such thermoplastic‐based composites. Depending on the test and the laminate's staking sequence, a temperature increase higher than the glass transition temperature (Tg = 98°C of fiber fabric reinforced PPS laminates softens the matrix behavior and significantly degrades the quality of the adhesion at the fiber/matrix interface, resulting in reduced mechanical properties (in‐plane shear, compressive, flexural and interlaminar shear). However, it does not seem to be detrimental for the tensile and the double lap joint's properties in quasi‐isotropic laminates. In angle‐ply laminates, the ductile behavior of the PPS matrix is exacerbated at high temperature resulting in larger plastic deformations and a lesser hole sensitivity of this material. © 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 80–95, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20239

Comparisons of processing and strength properties of two adhesive systems for composite joints

In the present study rheological, static and impact tests are carried out on two adhesive resins usually employed in marine applications; then single lap joint tests are conducted on composites joints evidencing the effect of the curing time of both resins on the mechanical properties of the joint. The applicability and workability conditions of the adhesive resins are determined evaluating the curing evolution by a preliminary rheological analysis; then the relation of curing properties with the mechanical performances of the resins themselves and with ones of the composite joints is estimated. Static flexural and Izod impact tests are performed on the resins at increasing time after the specimen production (curing time); also the resistance properties of adhesives joints are studied as well the stabilisation of the mechanical properties after long times. Moreover a relationship between the different failure mechanisms, observed for both resins joints, and the curing reaction is observed. A numerical model of the single lap joint test is developed: a simple and versatile numerical analysis (FEA with software ANSYS) is carried out in elastic regime. Such model should be suitable for designing and/or verifying the mechanical performances of composites joints, evaluating the shear, axial and peel stress trends on the joint overlap length.

Mode-I adhesive fracture energy measurement with modified single-lap joint test geometry

The adhesive fracture energy or fracture toughness of adhesively-bonded joints comprising carbon-fiber-reinforced polymer composite substrates and three different types of adhesives was detemined using a modified single-lap joint (MSLJ). This joint was made by implanting end pre-cracks in the adhesive layer at the center of the bondline of a conventional single-lap joint (SLJ). This modification ensured that the crack propagated from a sharp starter crack from both ends of the overlap during testing, reducing the effect of spew fillets on the measured adhesive fracture toughness scatter band. The MSLJ specimens were tested to failure and the adhesive fracture energy was calculated using the Kinloch–Osiyemi model. The values of the adhesive fracture energy obtained from the MSLJ tests were compared with those from SLJ and the double-cantilever beam (DCB) test geometries. The fracture energy values obtained from the MSLJ specimens were significantly lower than those from SLJ specimens and agreed well with those from DCB specimens. The three differenent types of aerospace grade film adhesives tested were Redux 322, Redux 335K and Redux 319A.

Liquid crystalline vinyl ester resins for structural adhesives

Rigid-rod vinyl ester resins represent an interesting family of materials. They can be synthesized starting from the corresponding epoxy resin. Due to their composition, they exhibit outstanding properties, such as heat and chemical resistance, high impact, and corrosion resistance. These features, combined with the ease of curing, make vinyl ester resins good candidates for several industrial applications, such as glass fiber composites, adhesives, corrosion-resistant coatings, electrical encapsulation, and radiation-curable inks. In the first part of this study, a new liquid crystalline methacrylated monomer was prepared and tested as an adhesive. The synthesis was carried out by reacting methacrylic acid with a rigid-rod epoxy monomer derived from the reaction between p-(2,3-epoxypropoxy)-α-methylstilbene (DOMS, diglycidyloxy α-methylstilbene) and aniline in a molar ratio of 3 to 2. The monomer was named md3a2 (methacrylated derivative of DOMS-aniline epoxy compound). Md3a2 is liquid crystalline in a wide range of temperatures, as shown by differential scanning calorimetry (DSC) and optical microscopy. In the second part of the work, the adhesion properties of the novel liquid crystalline resin were tested and compared with those exhibited by a conventional (non-liquid crystalline) adhesive, the methacrylated derivative of an epoxy compound synthesized from a 3 to 2 molar ratio of Epon 825® (which is a commercial epoxy) and aniline. This monomer was named mep3a2 (methacrylated derivative of Epon 825-aniline epoxy compound). The adhesion of the two resins on a pretreated aluminum substrate was evaluated using mechanical tests (single-lap joint test, according to ASTM standard D1002-72) and scanning electron microscopy (SEM).

Surface modification of superdrawn polyoxymethylene fibres

The pull-out fracture of surface-modified superdrawn polyoxymethylene fibres embedded in rubber is discussed from a fractographical viewpoint. The morphologies of the pull-out fracture plane were very similar to those of the fracture surface in single lap-joint tests and the true pull-out stress coincided with the shear strength of a single lap-joint, indicating that the pull-out failure is strongly related to single lap-joint shear fracture

Increase of adhesive bond strength through the mechanochemical creation of free radicals: I

AbstractA new technique for improving the strength of bonded joints between various materials and different adhesives is described. Increased strength was obtained by mechanical Surface Activation Beneath Reactive Adhesives (SABRA). This technique results in higher bond energies due to the creation of free radicals. The surfaces were activated by abrasion with emery paper in the presence of the adhesive itself (epoxy), or of some suitable primer such as methyl methacrylate, acrylonitrile (AN) or hydrogen sulfide gas. The effects of this technique were demonstrated by four different methods: 1) Improved wetting of standard epoxy or water drops on the treated surface. 2) Achievement of bond energies between primers and polymeric surfaces exceeding the energy of solution in solvents, which were revealed by attenuated total reflectance infrared. 3) The detection of free radicals created by bond scission during abrasion by means of diphenylpicryl hydrazyl, (DPPH), a free radical scavenger, monitored by spec‐trocolorimetry, 4) Destructive testing of single lap joint test Pieces, scarfed tube joints and pee) test samples. These experiments will be described in a second paper. The theoretical base of the new approach is supported by a tentative estimate of energies set free by mechanochemical creation of free radicals and the excess heat released during abrasion.