Process optimization of solvent based polybenzimidazole adhesive for aerospace applications

The use of adhesive bonding for high temperature applications is becoming more challenging because of low thermal and mechanical properties of commercially available adhesives. However, the development of high performance polymers can overcome the problem of using adhesive bonding at high temperature. Polybenzimidazole (PBI) is one such recently emerged high performance polymer with excellent thermal and mechanical properties. It has a tensile strength of 160 MPa and a glass transition of 425 °C. Currently, PBI is available in solution form with only 26% concentration in Dimethyl-acetamide solvent. Due to high solvent contents, the process optimization required lot of efforts to form PBI adhesive bonded joints with considerable lap shear strength. Therefore, in present work, efforts are devoted to optimize the adhesive bonding process of PBI in order to make its application possible as an adhesive for high temperature applications. Bonding process was optimized using different curing time and temperatures. Epoxy based carbon fiber composite bonded joints were successfully formed with single lap shear strength of 21 Mpa. PBI adhesive bonded joints were also formed after performing the atmospheric pressure plasma treatment of composite substrate. Plasma treatment has further improved the lap shear strength of bonded joints from 21 MPa to 30 MPa. Atmospheric pressure plasma treatment has also changed the mode of failure of composite bonded joints.     

Experimental study of adhesively bonded CFRP joints subjected to tensile loads

The tensile performance of adhesively bonded CFRP joints has been investigated experimentally. In this study, overlap length, adherend thickness, adherend width and scarf angle were chosen as design parameters. All load–displacement curves are linear, except that the thicker single-lap joints behave slightly nonlinearity due to the bending effect caused by eccentric loading. The lap shear strength is not directly proportional to overlap length, adherend thickness, adherend width and scarf angle for the brittle adhesive studied in the paper. The major failure mode includes adhesive shear failure and adherend delamination failure, sometimes accompanying with some fiber pull-out. Finally, the lap shear strength of three different lap types with similar bonding area (W=25 mm, L=10 mm, θ=5.71°) and adherend thickness (0.96 mm) was analyzed. It is found that the double-lap joint has the highest ultimate failure load. However, when considering the lap region weight, the scarf-lap joint is the most efficient.     

Moisture absorption–desorption effects in adhesive joints

This paper presents a study of moisture absorption–desorption effects in single lap adhesive joints. Experiments were carried out to characterise the moisture uptake of the single part epoxide adhesive, FM73. Tensile testing of single lap joints manufactured from aluminium alloy 2024 T3 and O and FM73 adhesive was carried out after the joints were exposed to different conditioning environments. The experimental results revealed that the failure strength of the single lap joints with 2024 T3 adherends progressively degraded with time when conditioned at 50 °C, immersed in water. However, the joint strength almost completely recovered after moisture was desorbed. The single lap joints with 2024 O adherends showed decreased strength for 28 days of conditioning, after which strength recovered, reaching a plateau after 56 days. Again, strength almost completely recovered on desorption of moisture. The strength recovery of the joints, after desorption of moisture, showed that the degradation of the adhesive was largely reversible. Analysis of the failure surfaces revealed that the dry joints failed cohesively in the adhesive layer and that the failure path moved towards the interface after conditioning. The failure mode then reverted back to cohesive failure after moisture desorption.     

Mechanical behaviour of adhesively bonded polyethylene tapping tees

The mechanical properties of adhesively bonded MDPE joints were studied. The lap-shear joints were prepared using PE80 polyethylene gas pipe and four adhesive types; two acrylic and two epoxy resins. The key mechanical properties of lap shear strength and impact resistance were investigated as a function of adhesive type and surface preparation technique. Mechanical abrasion of the PE80 surface increased the strength of the bonds from 40 to 460% for the four adhesives, with the best performing acrylic adhesive having a lap-shear strength of 1.76 MPa and impact strength of 2.5 kJ/m². When used to bond PE80 tapping tees to PE80 gas pipe, the acrylic adhesive produced a gas tight seal at both the standard test pressure of 0.4 MPa and at an increased pressure of 0.8 MPa, and outperformed the PE80 tapping tee during shear testing and withstood a maximum of 10 cycles of 175 J during impact testing. These results highlight the potential of adhesive bonding as a method of joining PE80 tapping tees to PE80 gas pipe.     

Low temperature tensile lap-shear testing of adhesively bonded polyethylene pipe

This work studies the lap-shear strength performance of polyethylene pipeline bonded with acrylic adhesive in the temperature range -10 to +20 °C. Single lap shear test samples were firstly prepared at 20 °C under various clamping pressures and curing times to determine suitable conditions under which to prepare and test further samples at temperatures of -10, -5, 0, +5 and +20 °C. It was found that a decrease in curing/testing temperature to zero degrees resulted in a steady reduction in the lap-shear strength performance of the bonded joints from a mean value of 2.72 MPa at +20 °C to 1.15 MPa at 0 °C. Below zero degrees the strength of the bonded substrates was significantly reduced; no samples bonded at -5 °C had sufficient strength to test and only one sample bonded -10 °C was tested, which had very low strength of 0.105 MPa.     

Effect of thermal cycling on tensile properties of degraded FML to metal hybrid joints exposed to sea water

In the present paper, the mechanical properties of hybrid bonded bolted joints between Fiber metal laminate (FML) and stainless steel adherends are investigated using experimental tensile tests. Three and five layered FMLs were fabricated using 430 stainless steel sheets and fiberglass prepreg layers. The adherends were bonded by AD-314 resin mixed with HA-34 hardener as adhesive and steel bolt was used for the mechanical fastening. The specimens were immersed into the sea water for 30 days and degradation of the mechanical strength of the joints was studied. Thermal cycles including heating (40 °C to100 °C) and cryogenic (−100 °C to −40 °C) cycles were applied in order to study their effects on the strength of the degraded joints. The failure mode for the adhesive bond was mixed failure and that of the bolted joint was the net-tension failure. The results showed 52% strength recovery in hybrid joints subjected to heating cycles. Cryogenic cycles also caused a 50% improvement in the tensile strength of the hybrid joints. In addition, the joint stiffness and absorbed energy of the specimens were improved significantly for both heating and cryogenic cycles. Moreover, the effect of FML stacking sequence on the results was also investigated. The results revealed that the mechanical fastening failure load for 5 layered FML joint is more affected by thermal cycles in comparison with 3 layered FML joint.     

Static and fatigue behavior of adhesive joints in SMC-SMC composites

This paper discusses the static and fatigue behavior of adhesively bonded single lap joints in SMC-SMC composites. Effects of lap length and adhesive thickness on the static and fatigue strength of SMC-SMC adhesive joints are studied. Effects of SMC surface preparation and test speed on the joint performance are evaluated. Finally, the effect of water exposure on the joint durability is also investigated. Results show that the static behavior of adhesive joints in SMC-SMC composites is significantly influenced by the lap length and adhesive thickness. With an increase in lap length from 12.7 mm to 38.1 mm, the joint failure load increases by 37%. The joint failure load also increases with the adhesive thickness, but it reaches a maximum at an adhesive thickness of 0.33 mm and then decreases. However, lap length and adhesive thickness have negligible effect on the ratio of fatigue strength to static strength. The fatigue strength at 10⁶ cycles is approximately 50% to 54% of the static strength for various adhesive thicknesses and lap lengths investigated in this study. Adhesive failure, fiber tear or combination of these two failure modes are observed during both static and fatigue tests.     

Effect of proanthocyanidin incorporation into dental adhesive on durability of resin–dentin bond

BackgroundProanthocyanidin has shown to have beneficial effects on dentin bonding via its collagen cross-linking and protease inhibitory effects.ObjectiveThis study evaluated the effect of incorporation of 1–3% PA into a dental adhesive on durability of resin–dentin bond.Materials and methodsThe experimental adhesive was first formulated by combining 50 wt% comonomer mixtures with 50 wt% ethanol. PA was then added to the ethanol-solvated adhesive to yield three groups of adhesives at concentrations of 1.0 wt%, 2.0 wt% and 3.0 wt%. The PA-free adhesive served as control. Flat dentin surfaces from forty extracted third molars were etched with 32% phosphoric acid and the specimens were randomly assigned to one of the four adhesive groups. Two layers of experimental adhesives were applied to etched dentin and light-cured for 20 s after solvent evaporation. Composite build-ups were performed using Filtek Z250 (3M ESPE). The bonded teeth were divided into three subgroups for different methods of storage: (1) 24 h indirect water exposure (IE), (2) 6 M IE and (3) 6 M direct water exposure (DE). After the designated period of water storage, the bonded teeth were sectioned into 0.9 mm×0.9 mm beams for bond strength testing. Bond strength data were evaluated by two-way ANOVA and Tukey׳s tests (α=0.05). Interfacial nanoleakage was examined using a field-emission scanning electron microscopy. Two-way ANOVA and Tukey׳s tests were used to examine the effects of PA concentration and water exposure on bond strength and percentage of nanoleakage (α=0.05).ResultsTwo-way ANOVA showed that the factors, water exposure and PA concentration had a significant effect on bond strength (p<0.001). Interaction between the two factors was also significant (p<0.001). Bond strength of all four adhesives decreased with PA concentrations and ageing. Type of water exposure had no effect on the bond strength of PA-incorporated adhesive; while direct water exposure significantly reduced the bond strength of PA-free adhesive. Conversely, the factors, water exposure and PA concentration showed a significant effect on nanoleakage percentage (p<0.001). Interaction between the two factors was not significant (p>0.05).ConclusionIncorporation of proanthocyanidin into a dental adhesive did not prevent resin–dentin bond degradation over time.     

Study on the fatigue strength of AA 6082-T6 adhesive lap joints

A research study on the fatigue behaviour of aluminium alloy adhesive lap joints was carried out to understand the effect of surface pre-treatment and adherends thickness on the fatigue strength of adhesive joints. The adherend material used for the experimental tests was an aluminium alloy 6082-T6 in the form of thin sheets, and the adhesive used was a high strength epoxy (Araldite 420 A/B). The surface preparation included an abrasive preparation (AP joints) and sodium dichromate–sulphuric acid etch (CSA joints).A maximum fatigue strength was obtained for the CSA surface treatment with a 1.0 mm adherends’ thickness. The fastest fatigue damage was related with a high surface roughness and a high stress perpendicular to adhesive surface, which helps to promote the adhesive failure. A numerical analysis was also performed to understand the effect of the adherends thickness on the stress level. Results showed an increase of the out-of-plane peak stresses with the increase of adherends thickness.     

Hygrothermal ageing of adhesive joints with nanoreinforced adhesives and different surface treatments of carbon fibre/epoxy substrates

The effect of water absorption on the strength of single lap adhesive joints subjected to accelerated hygrothermal ageing (55 °C, 95% relative humidity, 800 h) was analysed. Two different variables were studied: the surface treatment of the carbon fibre/epoxy laminates (peel ply, grit blasting and atmospheric pressure plasma) and the addition of carbon nanofillers (0.5 wt% nanofibres and 0.25 wt% nanotubes) to the epoxy adhesive. The joint strength and the failure mode of the joints were investigated. Furthermore, the amount of water absorbed by the adhesive was determined.Adhesive joints with peel ply-treated laminates exhibit an increase in their strength, which is attributed to a relaxation of stresses in the adhesive/laminate interface; with grit blasting, this property remains almost constant. Plasma treatment provides the worst ageing behaviour because this treatment results in a surface with a higher surface free energy, which is more susceptible to environmental attack. The nanoreinforcement of the adhesive has a beneficial effect: it decreases the amount of absorbed water.     

Effect of the temperature on the strength of adhesively bonded single lap and T joints for the automotive industry

Adhesively bonded lap and T joints are used extensively in the manufacture of automotive structures. In order to determine the effect of using a structural adhesive instead of spot-welding, a detailed series of tests, supported by finite element analyses, was conducted using a range of loadings. The adhesive was a toughened epoxy and the adherend was a grade of mild steel typical of that used in the manufacture of car bodyshells. The lap joints were tested in tension (which creates shear across the bondline) and three point bending. Previous studies at room temperature have shown that joint failure is dictated by adherend yielding and adhesive strain to failure. In the present study, to asses the effect of temperature that an automotive joint might experience in service, tests were carried out at ?40 and +90 °C. It is shown that the failure criterion proposed at room temperature is still valid at low and high temperatures, the failure envelope moving up and down as the temperature increases or decreases, respectively.     

Characterising moisture ingress in adhesively bonded joints using nuclear reaction analysis

The moisture ingress in bonded structures is usually characterised by gravimetric experiments on bulk adhesive samples. This is a relatively economic and convenient method. However, this approach poses a problem as only the total amount of moisture in the specimen can be determined and not the moisture concentration distribution throughout the adhesive layer. In this study, the moisture profile at two different ageing times (43 and 96 h) in a bonded joint has been determined by use of nuclear reaction analysis (NRA). The moisture ingress profile, which was found to be Fickian in the bulk adhesive sample, was dominated by apparent Case II kinetics in the bonded joint specimens. The ingress in the laminates was seen to be much faster than if the moisture transport had been governed by Fickian diffusion.     

Determination of the impact tensile strength of structural adhesive butt joints with a modified split Hopkinson pressure bar

The impact tensile strength of structural adhesive butt joints was determined with a modified split Hopkinson pressure bar using hat-shaped specimens. A typical two-part structural epoxy adhesive (Scotch weld^® DP-460) and two different adherend materials (Al alloy 7075-T6 and commercially pure titanium) were used in the adhesion tests. The impact tensile strength of adhesive butt joints with similar adherends was evaluated from the peak value of the applied tensile stress history. The corresponding static tensile strengths were measured on an Instron testing machine using joint specimens of the same geometry as those used in the impact tests. An axisymmetric finite element analysis was performed to investigate the static elastic stress distributions in the adhesive layer of the joint specimens. The effects of loading rate, adherend material and adhesive thickness on the joint tensile strength were examined. The joint tensile strength was clearly observed to increase with the loading rate up to an order of 10⁶ MPa/s, and decrease gradually with the adhesive thickness up to nearly 180 μm, depending on the adherend materials used. The loading rate dependence of the tensile strength was herein discussed in terms of the dominant failure modes in the joint specimens after static and impact testing.     

Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics

Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m^1/2, which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness.     

High-performance nanoadhesive bonding of titanium for aerospace and space applications

In this investigation, attempts are made to prepare high-performance nanoadhesive bonding of titanium for its essential applications to aviation and space. The high-performance nanoadhesive is prepared by dispersing silicate nanoparticles into the ultra-high-temperature-resistant epoxy adhesive at 10 wt% ratio with the matrix adhesive followed by modification of the nanoadhesive after curing under high-energy radiation for 6 h in the pool of SLOWPOKE-2 nuclear reactor with a dose rate of 37 kGy/h to promote crosslink into the adhesive. Prior to bonding, the surfaces of the titanium sheets are mechanically polished by wire brushing, ultrasonically cleaned by acetone and thereafter the titanium sheets are modified by plasma ion implantation using plasma nitriding. The titanium surface is characterized by X-ray photoelectron spectroscopy (XPS). The thermal characteristics of the epoxy adhesive and the high-performance nanoadhesive are carried out by thermal gravimetric analysis (TGA). The TGA studies clearly shows that for the basic adhesive there is a weight loss of the adhesive, however, in the case of epoxy–silicate nanoadhesive, there is almost 100% retention of weight of the adhesive, when the adhesive is heated up to 350 °C. Lap shear tensile strength of the joint increases considerably, when the titanium surface is modified by plasma-nitriding implantation. There is a further massive increase in joint strength, when the plasma-nitriding implanted titanium joint is prepared by nanosilicate–epoxy adhesive and further modification of the adhesive joint under high-energy radiation results a further significant increase in joint strength. In order to simulate with aviation and space climatic conditions, the joints are separately exposed to cryogenic (?196 °C) and elevated temperature (+300 °C) for 100 h and thermal fatigue tests of the joints are carried out under 10 cycles by exposing the joint for 2 h under the above temperatures. When the joint completely kept at ambient condition and the joint strength compared with those joints exposed to aviation and space climatic conditions, it is observed that the joint could retain 95% of the joint strength. Finally, to understand the behavior of the high-performance silicate–epoxy nanoadhesive bonding of titanium, the fractured surfaces of the joints are examined by scanning electron microscope.     

Tensile shear strength of wood bonded with urea–formaldehyde with different amounts of microfibrillated cellulose

Urea–formaldehyde (UF) adhesive mixtures with a 5% suspension of microfibrillated cellulose (MFC) at 0.5, 1, 3, and 5 wt% loading levels based on the solid weight (62.4%) of the UF adhesive were prepared. Beech lamellas with dimensions of 5 mm×20 mm×150 mm were prepared from beech lumbers using a planer saw. The UF adhesive (E0 class) was mixed with the MFC using a magnetic stirrer to achieve a proper distribution of the MFC in the UF adhesive. The tensile shear strength of single lap-joint specimens bonded with UF adhesive containing MFC was determined in accordance with EN 205 (2003). The specimens bonded with UF adhesive containing the MFC showed better tensile shear strengths as compared to the control. As compared to the control specimens, the tensile shear strength of the specimens increased by 5.7% as 3 wt% of the MFC was incorporated into the UF adhesive. However, a further increment in the MFC content up to 5 wt% decreased the tensile shear strength of the specimens (−14.3% of control specimen). The MFCs were well dispersed in the UF resin and were cross-linked to form a network to reinforce the bond line, improving bonding performance.     

Strengths of additions to composite or resin-modified glass-ionomer

ObjectivesAdding a new layer of material to cured resin-based composite (RBC) or resin-modified glass ionomer (RMGI) restorations is necessary in dental practice. This study investigated strengths of additions to the two materials.Material and methodsBeam-shaped specimens were made from monolithic RBC or RMGI, or additions of RBC and RMGI onto RBC or RMGI half-bar substrates. For the additions, the substrates were left undisturbed or were ground with silicon carbide paper followed by the application of a self-etch adhesive. Sample size was 10. Flexural strengths were determined by a 4-point bending test in a universal testing machine. Results were statistically analyzed with one-way ANOVA followed by Student–Newman–Keuls post-hoc test (α=0.05).ResultsFlexural strength of the monolithic RBC (86.7±21.8 MPa) was significantly higher than RMGI (52.6±13.1 MPa). Addition of RBC to cured RBC significantly reduced flexural strength regardless of the substrate surface conditions (34.1±11.5–45.7±21.1 MPa). Addition of RMGI to cured RMGI did not significantly reduce flexural strength (36.2±8.4–52.7±25.2 MPa). Flexural strength of RBC added onto cured RMGI that was ground and bonded was the lowest (21.5±10.0 MPa). Most specimens from this group exhibited adhesive failure.ConclusionsRBC/RBC additions reduced flexural strength whereas flexural strength of RMGI/RMGI additions was not significantly lower than its cohesive strength. RBC added onto RMGI in the sandwich restorative configuration had lowest failure strength.     

High-frequency welding of polypropylene using dielectric ceramic compounds in composite adhesive layers

In this study, high-frequency welding of polypropylene by melting composite adhesive layers containing dielectric ceramics was investigated. Various dielectric ceramics were mixed in a fixed ratio with polypropylene to make the composite adhesive layers, and the resulting dielectric properties were measured using an impedance analyser. The highest loss factor in the composite adhesive layer was found when 40 vol% silicon carbide (SiC) was used in the mixture. Dynamic viscoelasticity measurements showed that all composite adhesive layers softened at approximately 170 °C and melted (fluidised) at approximately 190 °C. Each composite adhesive layer was inserted between two polypropylene plates, and irradiated at a frequency of 40 MHz. The composite adhesive layers that included 20 vol% anatase-titanium oxide, 20 vol% or 40 vol% zinc oxide and 20 vol% or 40 vol% SiC melted in 40–70 s. The bond strength of the welded material obtained was high, and the adherend failure occurred by a tensile lap shear test. The heating efficiencies of the composite adhesive layers by high-frequency radiation were related to the tanδ/ε′ value of the composites.     

Rapid diffusion bonding of WC-Co cemented carbide to 40Cr steel with Ni interlayer: Effect of surface roughness and interlayer thickness

WC-Co cemented carbides were rapidly diffusion bonded to 40Cr steels with pure Ni as interlayers by utilizing plasma activated sintering (PAS). The bonding was carried out at 750 °C for 13 min under a pressure of 40 MPa. It was found that the roughness of the initial surfaces still plays an important effect on the microstructure and mechanical behavior of the joints diffusion bonded by PAS irrespective of the electric current applied during bonding. The adoption of smoother original surfaces was significantly favorable to eliminate the interfacial interstices and microvoids. Correspondingly, the shear strength of the diffusion bonded joints increased with decreasing surface roughness. Additionally, the effect of interlayer thickness on the shear strength of the joints was also evaluated, and the results showed that the strength decreased sharply when thicker interlayer was employed. A maximum value of shear strength, 293.07 MPa, was obtained when the original surfaces was ground with P1200 grit SiC paper and at the same time 50 µm thick interlayer was used. In this case, the fracture initiated and run predominantly along the bonding interfaces instead of in the WC-Co substrate.     

Strength Prediction of Adhesively Bonded Single Lap Joints Under Salt Spray Environment Using a Cohesive Zone Model

The aim of this research was to develop an experimental–numerical approach to characterize the effect of salt spray environment on adhesively bonded joints and predict the degradation in joint strength. Experiments were conducted on bulk adhesive specimens and single lap joints (SLJs) under salt spray condition and the corresponding experimental results were reported. The environment degradation factor, Deg, was incorporated into a bilinear cohesive zone model (CZM) to simulate the degradation process of the joints. The degraded CZM parameters, determined from static tests on bulk adhesive, were imported into the CZM using an approximate moisture concentration gradient approach. The reduction in residual strength of SLJ under salt spray environment was successfully predicted through comparing the experimental and numerical results.