Industrial Applications of Adhesives

The long curing times of the adhesives are an essential deficit of the bonding technology. UV-curing adhesives enable complete curing within seconds, but require at least one light-transparent joining partner. In order to make use of the advantages of UV-curing adhesives also on non-transparent joining parts, the approach of light transfer into the adhesive joint is pursued. A polymer optical fibre (POF) is embedded in the adhesive as a waveguide. With a lateral activation of the fibre surface, a radial curing is realized. In this paper, the effects of side light activation by thread cuts and gradual micro cuts on a PMMA POF with a diameter of 1.0 mm are characterized and validated in corresponding adhesive tests. An experimental setup is established to characterize the activated fibre with regard to sidelight intensity and homogeneity. The wavelengths of the UV light used are 365 and 400 nm. While lateral homogeneous adhesive layer depths of up to 2.5 mm are achieved at 400 nm, the use of UV light at 365 nm proves to be unsuitable for the selected tests.

The aim of this paper is to assess the potential of accelerating the curing of epoxy adhesives by means of induction and ferromagnetic curie particles in the electronic and the automotive industries. Commercially available Araldite 2011 and Jowat 690 epoxy adhesives were considered, while Mn-Zn-Ferrite particles were considered for the modification of the adhesives. Induction experiments were performed at frequencies ranging from 188 to 262 kHz, and with particle weight content of 16.7 and 30%. The effects of the induction frequency and the particle content on the temperature of the adhesive were investigated. In addition, the effect of the particle content on the lap shear strength of bonded joints was also studied. Moreover, a comparison between the lap shear strength of inductively cured joints and reference oven cured joints was performed. Results showed that the temperature of the adhesive increased with increasing both the induction frequency and the particle content. However, the lap shear strength of joints with Araldite 2011 was reduced by 17% when increasing the particle content to 30%. No impact on the strength was observed in the case of the Jowat 690 at 16.7 and 30% particle contents. This suggests that increasing the induction frequency is more efficient in increasing the adhesive temperature without compromising the lap shear strength of the brittle adhesive. Furthermore, the inductively cured samples of the Araldite 2011 with 30% particles achieved a substantial 120% of the reference lap shear strength of oven cured samples in 8% of the curing time. For samples with Jowat 690 with 30% particles, the inductively cured samples achieved 43% of the reference lap shear strength of oven cured samples in 8% of the curing time. This shows a high potential of accelerating the curing time for industrial applications by induction and ferromagnetic curie particles, to reach either high strength as in case of Araldite 2011 or the handling strength as in case of Jowat 690.

Adhesive joints are increasingly being used in structural applications due to their improved mechanical performance when compared to other classical mechanical joining methods. In order to predict the mechanical performance of adhesive joints, it is first necessary to determine the mechanical properties of the materials being used (e.g. adhesives and adherends). These properties can be characterized under three basic loading modes: tensile, compression and shear. Tensile and shear tests are commonly used to determine the Young’s modulus, tensile strength, and the strain to failure of materials. However, shear tests under torsion loading have been shown to provide significantly higher accuracy in the measurement of the strain to failure since no stress concentrations occur in the specimen. To explore the advantages associated with shear testing, a novel torsional testing machine was developed. It allows for accurate determination of the mechanical properties of structural adhesives by ensuring correct alignment of the specimens and avoiding any spurious bending moments, compression or tension loads during the tests. Different structural adhesives were selected and characterised under shear loads using this novel machine and the results obtained were compared with those obtained with thick adherend shear tests (TAST) results. This work showed that by performing torsion tests in the newly developed testing machine, the adhesive is properly characterised, especially for the determination of the strain to failure.

Adhesive joints are widely used in the production of goods, mainly in the transport industry. However, their industrial applications often have non-standard complex shapes. Computer simulation, like the finite element method (FEM), is widely used for their analysis but limitations still exist. Meshless methods have been in development and offer an option to overcome some limitations of the FEM; however, these are still in development. In this work, an efficient meshless method, the ‘natural neighbour radial point interpolation method’ (NNRPIM), has been applied to the analysis of adhesive joints including an elasto-plastic formulation for the adhesive. First, experimental data corresponding to four overlap lengths (\(L_O\)) and two different adhesives were measured, as a benchmark. Afterwards, joint strength (\(P_{\mathrm{max}}\)) was analytically obtained as a second benchmark. Then, all the joint geometries were simulated utilising the FEM and NNRPIM methodologies, and \(P_{max}\) were calculated from those simulations. Finally, the results were compared against the first and second benchmarks. The meshless method proved to be a good alternative to the FEM, providing similar strength prediction. Moreover, the stress distribution curves were compared. In conclusion, the NNRPIM provides accurate results and could be utilised for further study of adhesively-bonded joints.

The automotive industry is currently faced with increasingly strict requirements with regards to crash testing of its structures, which necessitates the development of powerful numerical tools that can reproduce high speed impacts and predict the behaviour of bonded structures under these conditions. The design of adhesive joints for quasi-static loadings is currently well understood, enabling the manufacture of strong, multi-material structures, essential for lightweight and efficient new vehicles. However, the design of impact resistant structures requires the determination of accurate models of the strain rate dependent behaviour of the materials used in vehicle construction. The work recently performed at the Advanced Joining Processes Unit (AJPU) at the Faculty of Engineering of the University of Porto (FEUP) and the Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI) has focused extensively on processes for high strain rate characterization of the mechanical behaviour of modern crash testing adhesives for the automotive industry. Special testing procedures and experimental equipment have been developed for the determination of strain rate dependent tensile and shear properties as well as fracture mechanics properties such as the fracture toughness. Special attention has been devoted to the study of mixed mode behaviour under impact conditions, with the design of a mixed mode apparatus for impact conditions. To enable the validation of cohesive numerical models based on the experimental data and allow for design optimization procedures, full scale testing of components has also been performed.

The supply of materials on the construction market is very broad, almost inexhaustible, in an effort to satisfy the financial and social requirements of all investors. Steel, aluminium and composites are currently the most commonly used materials in construction applications. The joining of various parts and components is usually ensured by traditional mechanical or welded joints, the efficiency and durability of which has been already proven many times. On the other hand, adhesive joints proved their efficacy too and moreover, they allow the implementation of flexible joints that provide a solution with uniform stress distribution. This article presents the results of the stress/strain behaviour of flexible adhesive joints with steel and composite substrates that are very often brittle; drilling or welding could cause their weakening. High strength 1-K polyurethane and 1-K modified polymer adhesive systems were tested, four in total, in combination with different representatives of the substrate. Joint design similar to butt joint was used, therefore each test sample was composed of two components: (1) EN AW-6060 (AlMgSi) T66 aluminium alloy disc; and (2) the tested substrate: (a) COR-TEN^® steel, (b) aluminium composite panel (Alu-Bond), and (c) high-pressure laminate (HPL). Moreover, all test samples were exposed to artificial weathering, as a result the effect of different temperatures (ranging from −20 up to +70 °C) and environments (from 0 up to 100% humidity) on bond strength and failure mode was monitored. The relation between bond failure energy density and bond tensile strength was investigated. For all the test samples a consistent relation was observed, however, the strain energy density that corresponds to the bond failure energy was entirely different for each of the adhesives tested. The dependency of adhesive stiffness on strain energy density was manifested. On the contrary, the effect of different weathering methods was not dominant, only slight deviation were observed in the combination with HPL substrate.

The understanding of ageing processes in the field of adhesion technology is of utmost importance for long-term durable bonded joints. Environmental factors such as moisture, heat and UV radiation negatively affect the characteristics of adherends, adhesives and their interfaces and thus the overall bond performance over time. However, the evaluation of the precise ageing condition of adhesive joints remains challenging by means of well-established non-destructive testing (NDT) methods like shearography or thermography. Recently, it has been shown that terahertz (THz) time-domain spectroscopy is well suited as an NDT approach to study adhesive bonds. Specialized data extraction schemes enable the contact-free determination of the refractive index and the absorption coefficient from such measurements. The presented work wants to give an overview on the developments in the application of THz spectroscopy for the ageing condition determination of bonded joints, including the correlation between THz refractive index/absorption and the physical, chemical and mechanical property changes of adherends and adhesives due to ageing, the determination of THz specific values of adhesive joints by means of a reflection setup, the suitability of peak-to-peak amplitude evaluation, and THz imaging.