Comparison of Tensile Strength and Fracture Toughness of Co-Bonded and Cold-Bonded Carbon Fiber Laminate-Aluminum Adhesive Joints

The purpose of this work is to compare the co-bonding vs. cold-bonding route on the adhesive joint performance of a CFRP (Carbon Fiber Reinforced Polymer) laminate–aluminum connection. In particular, the overlap shear, tensile strength and Mode I and Mode II fracture toughness will be evaluated. The adhesives for co-bonding and cold-bonding are, respectively, a thermosetting modified epoxy, unsupported structural film and a two-component epoxy adhesive, chosen as representative of applications in the high-performance/race car field. The emerging trend is that, in tensile e Mode I fracture tests, the failure path is predominantly in the composite. Mode II fracture tests instead resulted in a cohesive fracture, meaning that, under pure shear loading, the weakest link may not be the composite. The lap-shear tests are placed midway (cohesive failure for co-bonding and composite delamination for cold-bonding, respectively), probably due to the different peel stress values related to the different adhesive Young’s modulus. The exploitation of the full capacity of the adhesive joint, hence the possibility of highlighting better, different performances of co-bonding vs. cold-bonding, would require consistent improvement of the out-of-plane strength of the CFRP laminate and/or to someway redistribute the peel stress on the bondline.     

Novel Photoreactive Pressure-Sensitive Adhesives (PSA) Based on Acrylics Containing Additionable Photoinitiators

A new class of additionable ultraviolet photoinitiators that can be used, through addition, for modification of the acrylic polymer chain and their influence of main properties of acrylic pressure-sensitive adhesives (PSAs) is described here. The photoinitiators studied are based on benzophenone, dibenzofuran and anthraquinone chromophores. The propyleneimine carbonyl is the reactive additionable group incorporated in the photoinitiator structure. First, the solvent-borne acrylic pressure-sensitive adhesive was synthesized and characterized. Then, a photoinitiator suitable for addition to the acrylic polymer chain possessing a carboxyl group was added before UV-irradiation. A mechanism of UV-initiated cross-linking reaction of acrylic PSA with additionable photoinitiators was done as well. The influence of the concentration and type of photoinitiator, UV-crosslinking time and UV-dose on peel adhesion, shear strength and tack of solvent-borne acrylic pressure-sensitive adhesives cross-linked by UV light was studied and presented here. It was found that the tack depends on the UV-dose and photoinitiator concentration. An increase of UV dose results in an increase of shear strength of acrylic pressure-sensitive adhesive (PSA) formulations.     

Experimental Study of Mechanical Properties of Epoxy Compounds Modified with Calcium Carbonate and Carbon after Hygrothermal Exposure

The objective of this paper is to analyze the effects of hygrothermal exposure on the mechanical properties of epoxy compounds modified with calcium carbonate or carbon fillers. In addition, comparative tests were carried out with the same parameters as hygrothermal exposure, but the epoxy compounds were additionally exposed to thermal shocks. The analysis used cylindrical specimens produced from two different epoxy compounds. The specimens were fabricated from compounds of epoxy resins, based on Bisphenol A (one mixture modified, one unmodified) and a polyamide curing agent. Some of the epoxy compounds were modified with calcium carbonate (CaCO₃). The remainder were modified with activated carbon (C). Each modifying agent, or filler, was added at a rate of 1 g, 2 g, or 3 g per 100 g of epoxy resin. The effect of the hygrothermal exposure (82 °C temperature and 95% RH humidity) was examined. The effects of thermal shocks, achieved by cycling between 82 °C and −40 °C, on selected mechanical properties of the filler-modified epoxy compounds were investigated. Strength tests were carried out on the cured epoxy compound specimens to determine the shear strength, compression modulus, and compressive strain. The analysis of the results led to the conclusion that the type of tested epoxy compounds and the quantity and type of filler determine the effects of climate chamber aging and thermal shock chamber processing on the compressive strength for the tested epoxy compounds. The different filler quantities, 1–3 g of calcium carbonate (CaCO₃) or activated carbon (C), determined the strength parameters, with results varying from the reference compounds and the compounds exposure in the climate chamber and thermal shock chamber. The epoxy compounds which contained unmodified epoxy resin achieved a higher strength performance than the epoxy compounds made with modified epoxy resin. In most instances, the epoxy compounds modified with CaCO₃ had a higher compressive strength than the epoxy compounds modified with C (activated carbon).     

Analysis of Changes in the Tensile Bond Strenght of Soft Relining Material with Acrylic Denture Material

Abrasions and pressure ulcers on the oral mucosa are most often caused by excessive pressure or incorrect fitting of the denture. The use of soft relining materials can eliminate pain sensations and improve patient comfort. The main functional feature of soft elastomeric materials is the ability to discharge loads from the tissues of the mucosa. (1) Background: The aim of the work was a comparative laboratory study of ten materials used for the soft lining of acrylic dentures. (2) Methods: There were materials based on acrylates (Vertex Soft, Villacryl Soft, Flexacryl Soft) and silicones (Sofreliner Tough Medium, Sofreliner Tough Medium, Ufi Gel SC, GC Reline Soft, Elite Soft Relining, Molloplast). Laboratory tests include the analysis of the tensile bond strength between the relining material and the acrylic plate of the prosthesis. The tests were conducted taking into account 90-day term aging in the distilled water environment based on the methodology presented in the European Standard ISO 10139-2. (3) Results: After three months of observation, the highest strength of the joint was characterized by Flexacryl Soft acrylic, for which the average value was 2.5 MPa. The lowest average value of 0.89 MPa was recorded for the GC Reline Soft silicone material. Over time, an increase in the value of the strength of the combination of acrylic materials and a decrease in these values in the case of silicone materials was observed. (4) Conclusion: Each of the tested silicone materials showed all three types of damage, from adhesive to mixed to cohesive. All acrylic-based materials showed an adhesive type of failure. Time did not affect the type of destruction.     

The Thickness Effect of PSF Nanofibrous Mat on Fracture Toughness of Carbon/Epoxy Laminates

The geometrical features of nanofibers, such as nanomat thickness and the diameter of nanofibers, have a significant influence on the toughening behavior of composite laminates. In this study, carbon/epoxy laminates were interleaved with polysulfone (PSF) nanofibrous mats and the effect of the PSF nanomat thickness on the fracture toughness was considered for the first time. For this goal, the nanofibers were first produced by the electrospinning method. Then, double cantilever beam (DCB) specimens were manufactured, and mode-I fracture tests were conducted. The results showed that enhancing the mat thickness could increase the fracture toughness considerably (to about 87% with the maximum thickness). The toughening mechanism was also considered by presenting a schematic picture. Micrographs were taken using a scanning electron microscope (SEM).     

An Experimental and Numerical Study of the Influence of Temperature on Mode II Fracture of a T800/Epoxy Unidirectional Laminate

Studies on mode II fracture have promoted the establishment of the delamination theory for unidirectional composite laminates at room temperature. However, under thermal conditions, the fracture behavior of composite laminates will exhibit certain differences. The delamination theory should be extended to consider the temperature effect. To achieve this goal, in this study, the mode II static delamination growth behavior of an aerospace-grade T800/epoxy composite is investigated at 23 °C, 80 °C and 130 °C. The mode II fracture resistance curve (R-curve) is experimentally determined. A fractographic study on the fracture surface is performed using a scanning electron microscope (SEM), in order to reveal the failure mechanism. In addition, a numerical framework based on the cohesive zone model with a bilinear constitutive law is established for simulating the mode II delamination growth behavior at the thermal condition. The effects of the interfacial parameters on the simulations are investigated and a suitable value set for the interfacial parameters is determined. Good agreements between the experimental and numerical load–displacement responses illustrate the applicability of the numerical model. The research results provide helpful guidance for the design of composite laminates and an effective numerical method for the simulation of mode II delamination growth behavior.     

Effect of Nanofiller on the Mechanical Properties of Carbon Fiber/Epoxy Composites under Different Aging Conditions

In this study, carbon fiber-reinforced epoxy composites (CFRPs) containing multi-walled carbon nanotube (MWCNT) and halloysite nanoclay were fabricated. The effects of these nanofillers (MWCNT and nanoclay) on the tensile and flexural properties of the CFRPs under different aging conditions were studied. These aging conditions included water soaking, acid soaking, alkali soaking, and thermal shock cycling. The experimental results showed that, after accelerated aging, the mechanical performance of the CFRPs decreased. The performance degradation in the soaking environment depends on the immersion temperature and immersion medium. High-temperature accelerated the aging behavior of the CFRPs, resulting in low strength and modulus. The CFRPs were more vulnerable to acid soaking and alkali soaking than water soaking. The MWCNT and halloysite nanoclay are beneficial to improve the immersion aging resistance of the CFRPs, and the additions of nanofillers delayed the performance degradation under immersion aging conditions. However, nanofillers hardly improve the aging resistance of the CFRPs under thermal shock cycling condition. The fracture morphologies were observed by scanning electron microscopy (SEM) to reflect the failure modes of the CFRPs under various aging conditions. Differential scanning calorimeter (DSC) and fourier transform infrared (FTIR) spectroscopy tests were used to estimate the changes in the chemical structures and properties of epoxy resin and its composites under different conditions.     

Tensile Behaviour of Double- and Triple-Adhesive Single Lap Joints Made with Spot Epoxy and Double-Sided Adhesive Tape

Dual adhesives are mainly used to increase the strength of single lap joints (SLJs) by reducing the stress concentration at its ends. However, they can also be used to design the characteristics of the joint so that its operation and failure occur in several stages. This paper presents the results of uniaxial tensile tests for dual-adhesive and triple-adhesive SLJs. The adherends were made of aluminum and glass fiber-reinforced polymer (GFRP) composite. For dual-adhesive SLJs, 10 epoxies and 1.6 mm thick double-sided adhesive tape were used. The stiffest (Epidian 53 (100 g) + “PAC” hardener (80 g)) and most elastic (Scotch-Weld 2216 B/A Translucent) joints were determined, which were then used in a triple-adhesive joint with the same double-sided adhesive tape. Circular holes of different diameters from 8 mm to 20 mm were made in the double-sided adhesive tape, which were filled with liquid epoxy adhesive by injection after the adherends were joined. By using the double-sided adhesive tape, the geometry of the epoxy joints was perfect, free of spews, and had a constant thickness. The effect of the spot epoxy joint diameters and the arrangement of stiff and elastic joints in the SLJs were analyzed using digital image correlation (DIC).     

Mechanical Properties of Polyurethane Adhesive Bonds in a Mineral Wool-Based External Thermal Insulation Composite System for Timber Frame Buildings

This paper aims to provide a preliminary assessment of polyurethane adhesive applicability as an alternative to conventional cement-based adhesives used to fix thermal insulation materials to substrates concerning mineral wool-based external thermal insulation composite systems. Currently, polyurethane adhesives are only used in expanded polystyrene-based ETICS. This study discusses the suitability of polyurethane adhesive for ETICS with lamella mineral-wool for timber frame buildings. Bond strength, shear strength and shear modulus tests were conducted. In addition, microstructure and apparent density were analysed. Mechanical properties were analysed in terms of the influence of substrate type and thermal and moisture conditions, taking into account solutions typical for sheathing on timber frame (oriented strand boards (OSB), fibre-reinforced gypsum boards (FGB) and cement-bonded particleboards (CPB)), as well as limit conditions for adhesive application. It was found that PU adhesive can achieve adhesion, both to MW and OSB, and FGB and CPB at ≥80 kPa, which is considered satisfactory for PU adhesives for EPS-based ETICS. Favourable shear properties were also obtained. There was no significant effect of sheathing type on the properties considered, but the influence of temperature and relative humidity, in which the bonds were made, was spotted. The results obtained can be considered promising in further assessing the usefulness of PU adhesives for MW-based ETICS.     

Influence of Physical Modification of the Adhesive Composition on the Strength Properties of Aerospace Aluminum Alloy Sheet Adhesive Joints

One of the most important design factors in the constitution of adhesive joints is the correct choice of adhesive. Currently, there is a full range of options on the commercial market in this regard, but there is increasing research into modifying adhesives for specific engineering applications. The aim of this study was to analyze the effect of physical modification with fillers on the properties of the adhesive composition and the adhesive joints. The adhesives used in the study were a composition of Epidian 5 epoxy resin and PAC curing agent modified with 1% montmorillonite, 5% calcium carbonate and 20% activated carbon. The adhesive compositions in the cured state were subjected to strength tests and SEM and DSC analyses. Using these compositions, adhesive joints of EN AW 2024 T3 aluminum alloy sheets were also made. The tests carried out showed that, due to the use of different fillers, their effects on certain properties of the adhesive compositions are different types. It was shown that physical modification of the adhesive composition does not always result in positive effects. The study also attempted to determine the correlation between the properties of the adhesive compositions in the cured state and the strength of the adhesive joints.     

Effect of Bond-Line Thickness on Fatigue Crack Growth of Structural Acrylic Adhesive Joints

With an increasing demand for adhesives, the durability of joints has become highly important. The fatigue resistance of adhesives has been investigated mainly for epoxies, but in recent years many other resins have been adopted for structural adhesives. Therefore, understanding the fatigue characteristics of these resins is also important. In this study, the cyclic fatigue behavior of a two-part acrylic-based adhesive used for structural bonding was investigated using a fracture-mechanics approach. Fatigue tests for mode I loading were conducted under displacement control using double cantilever beam specimens with varying bond-line thicknesses. When the fatigue crack growth rate per cycle, da/dN, reached 10⁻⁵ mm/cycle, the fatigue toughness reduced to 1/10 of the critical fracture energy. In addition, significant changes in the characteristics of fatigue crack growth were observed varying the bond-line thickness and loading conditions. However, the predominance of the adhesive thickness on the fatigue crack growth resistance was confirmed regardless of the initial loading conditions. The thicker the adhesive bond line, the greater the fatigue toughness.     

Investigation of Adhesive Joining Strategies for the Application of a Multi-Material Light Rail Vehicle

To meet the high demand for lightweight energy-efficient and safe structures for transport applications, a current state-of-the-art light rail vehicle structure is under development that adopts a multi-material design strategy. This strategy creates the need for advanced multi-material joining technologies. The compatibility of the adhesive with a wide range of material types and the possibility of joining multi-material structures is also a key advantage to its success. In this paper, the feasibility of using either epoxy or polyurethane adhesive joining techniques applied to the multi-material vehicle structure is investigated. Importantly, consideration is given to the effect of variation in bond thickness for both families of structural adhesives. Multi-material adhesively bonded single lap joints with different adhesives of controlled bond thicknesses were manufactured and tested in order to experimentally assess the shear strength and stiffness. The torsional stiffness and natural frequency of the vehicle were modelled using a global two-dimensional finite element model (FEM) with different adhesive properties, and the obtained vehicle performances were further explained by the coupon-level experimental tests. The results showed that the vehicle using polyurethane adhesive with a target bond thickness of 1.0 mm allowed for optimal modal frequency and weight reduction.     

Simplified Testing of the Bond Strength of Adhesives Used for Bonded Anchors

The analysis presented in this paper is focused on problems of bond strength as an overall bond quality parameter of industrial adhesives for structural anchoring. In the first part, the problem of bond strength as the most important parameter influencing the final anchor resistance to tension load is described. Further in the text, a new methodology of simplified testing of the strength parameters of adhesives is described. Special test specimens made from steel are repeatedly used in this methodology. Additionally, results of these tests on some new recipes for adhesive are presented. Especially, epoxy resins with special fillers, such as carbon fibres, carbon nanotubes or graphene, were tested. The use of these adhesives in temperatures close to zero degrees Celsius was also tested.     

Evaluation of Bonding Gap Control Methods for an Epoxy Adhesive Joint of Carbon Fiber Tubes and Aluminum Alloy Inserts

The aim of this paper is to compare two methods of epoxy adhesive bond gap control: one with a geometrical (mechanical) solution and the other with glass beads, which have the diameter of the desired bond gap and are mixed with an epoxy adhesive. The adhered materials were carbon fiber composite tubes and aluminum alloy inserts, which were used as wishbones in a suspension system of a motorsport vehicle. It was assumed that the gap thickness would be equal to 0.2 mm and the length of a bond would be 30 mm. The internal diameter of the tubes was 14 mm and 18 mm, whereas the inserts’ external diameter was 13.6 mm and 17.6 mm. Their surface has been subjected to mechanical treatment with sand paper starting from 240 grit up to 400. The adhesives used were EA 3425 and EA 9466 cured at 80 °C for 2 h. The results showed that the glass beads method provides more consistent and better results as compared to the geometrical (mechanical) method. Further study in the area of fatigue and interfacial failure modes could be useful.     

Rheologically Assisted Design of Conductive Adhesives for Stencil Printing on PCB

Driven by the need to deliver new, lead-free, eco-friendly solder pastes for soldering electronic components to Printed Circuit Boards (PCB), electrically conductive adhesives (ECAs) based on epoxy, carbon nanotubes (CNT), and exfoliated graphite (EG) were designed. The rheology of the adhesives prepared is paramount for the success of the deposition process, which is based on stencil printing. Thus, a rheological analysis of the process was first performed. Then, an experimental protocol was defined to assess the relevant viscoelastic characteristics of the adhesives for stencil printing application. Different composite formulations of epoxy/CNT/EG were produced. Their rheological characteristics were established following the designed protocol and benchmarked with a commercial solder paste. The thermal and electrical properties of the composite formulations were also characterized. As a result, a new, electrically conductive adhesive was delivered with potential to be an eco-friendly alternative to the solder paste currently used in stencil printing of PCB.     

Prediction of the Delamination at the Steel and CFRP Interface of Hybrid Composite Part

The purpose of this study was to predict the adhesive behavior of steel and carbon-fiber-reinforced plastic (CFRP) hybrid parts based on the cohesive zone model (CZM). In this study, the steel sheet and CFRP were joined by epoxy resin in the CFRP prepreg during the curing process, which could generate delamination at their interface because of the springback of steel or the thermal contraction of the CFRP. First, double cantilever beam (DCB) and end-notched flexure (ENF) tests were performed to obtain various adhesion properties such as the critical energy release rate of mode I, mode II (G_I, G_II), and critical stress (σ_max). A finite element (FE) simulation was performed to predict delamination using CZM, which was also used to describe the interfacial behavior between the steel sheet and the CFRP. Finally, a U-shape drawing test was performed for the steel/CFRP hybrid parts, and these results were compared with analytical results.     

Cold-Cured Bisphenolic Epoxy Adhesive Filled with Low Amounts of CaCO3: Effect of the Filler on the Durability to Aqueous Environments

The effects of aging exposures to three non-saline aqueous environments on the compressive mechanical properties of a calcium carbonate-filled bisphenolic epoxy adhesive, cold-cured with the addition of two curing agents suitable for the cure at ambient temperature (i.e., Mannich base and triethylenetetramine), were assessed. The amount of the added filler (CaCO₃) varied from 1 to 3 g per 100 g of resin; the immersion times in each of the selected medium varied from 1 to 10 months. It was found that the mechanical properties measured in compression mode on cylindrical specimens of unfilled and CaCO₃-loaded epoxy were scarcely influenced by the kind of curing agent employed; only the compressive modulus was limitedly affected by this parameter. Referring to the behavior when aged in water, the CaCO₃-filled epoxies displayed noticeable growths in modulus, small reductions in strength, and limited variations in strain, with a certain influence of the exposure time, especially when comparing the properties at the lowest time with those at medium–long times. On the basis of the results of statistical MANOVA analysis, it can be concluded that among the compositional factors (i.e., the type of curing agent employed to cure the epoxy compounds and the micro-filler content), only the amount of CaCO₃ filler significantly affects the compressive modulus.     

Influence of Surface Treatment on Steel Adhesive Joints Strength—Varnish Coats

The purpose of the paper is to determine the impact of surface treatment on the strength of adhesive joints, made from various steel sheets. Two variants of the surface treatment steel adherends were used: without the varnish coat and with the varnish coat, using three polymer-based varnishes (a simple, a hybrid, and a gel). Two types of the adhesives were used to prepare the adhesive joints: a single-component cyanoacrylate adhesive and a two-component epoxy adhesive. A strength test of the adhesive joints (EN DIN 1465 standard), a coating adhesion test (ASTM D3359-B standard), and surface topography, as well as surface roughness, parameters (PN-EN ISO 11562, PN-EN ISO 4287, and PN-EN ISO 25178 standards) were used. Based on the strength tests, it was observed that the adhesive joints, with the hybrid varnish onto the adherend’s surface, achieved markedly lower shear strength. The best results, in terms of the adhesive joint strength, made using the cyanoacrylate adhesive were achieved for the joints where the adherends were coated with a simple varnish, while in the joints made using the epoxy adhesive, the highest shear strength was achieved by the joints of sheets whose surfaces were coated with the gel varnish.     

Effect of Mechanical Pretreatments on Damage Mechanisms and Fracture Toughness in CFRP/Epoxy Joints

Adhesive bonding of carbon-fiber-reinforced polymers (CFRPs) is a key enabling technology for the assembly of lightweight structures. Surface pretreatment is necessary to remove contaminants related to material manufacturing and ensure bond reliability. The present experimental study focuses on the effect of mechanical abrasion on the damage mechanisms and fracture toughness of CFRP/epoxy joints. The analyzed CFRP plates were provided with a thin layer of surface epoxy matrix and featured enhanced sensitivity to surface preparation. Various degrees of morphological modification and fairly controllable carbon fiber exposure were obtained using sanding with emery paper and grit-blasting with glass particles. In the sanding process, different grit sizes of SiC paper were used, while the grit blasting treatment was carried by varying the sample-to-gun distance and the number of passes. Detailed surveys of surface topography and wettability were carried out using various methods, including scanning electron microscopy (SEM), contact profilometry, and wettability measurements. Mechanical tests were performed using double cantilever beam (DCB) adhesive joints. Two surface conditions were selected for the experiments: sanded interfaces mostly made of a polymer matrix and grit-blasted interfaces featuring a significant degree of exposed carbon fibers. Despite the different topographies, the selected surfaces displayed similar wettability. Besides, the adhesive joints with sanded interfaces had a smooth fracture response (steady-state crack growth). In contrast, the exposed fibers at grit-blasted interfaces enabled large-scale bridging and a significant R-curve behavior. While it is often predicated that quality composite joints require surfaces with a high percentage of the polymer matrix, our mechanical tests show that the exposure of carbon fibers can facilitate a remarkable toughening effect. These results open up for additional interesting prospects for future works concerning toughening of composite joints in automotive and aerospace applications.     

Physico-Mechanical and Rheological Properties of Epoxy Adhesives Modified by Microsilica and Sonication Process

Industrial waste from the production of metallic silicon and silicon–iron alloys, which includes silica fumes (microsilica), is subject to numerous applications aiming at its reuse in concrete and polymeric composites. Recycling solves the problem of their storage and adverse environmental impact. Six different formulas of epoxy resins were tested, differing in the type of polymer, the mixing process (sonication or not) and the presence of microsilica. The study showed that microsilica added to the epoxy resin changes its viscosity and free surface energy, and these are the parameters that determine the adhesion of the polymer to the concrete surface. Strength tests and SEM analysis have determined how microsilica molecules can penetrate the structure of polymer macromolecules by filling and forming temporary chemical bonds. Mixing the fillers with the adhesive was achieved by using a sonication process. The analysis of the obtained results showed that, depending on the initial composition of the polymer, the addition of microsilica can change the chemical, physical and mechanical properties of the hardened adhesive to varying degrees. In the case of adhesives used in the construction industry to strengthen and glue structural elements, these changes significantly affect the durability of the adhesive joints.