Adhesive stresses in axially-loaded tubular bonded joints - Part II: development of an explicit closed-form solution for the Lubkin and Reissner model

The literature presents several analytical models and solutions for single- and double-lap bonded joints, whilst the joint between circular tubes is less common. For this geometry the pioneering model is that of Lubkin and Reissner (Trans. ASME 78, 1956), in which the tubes are treated as cylindrical thin shells subjected to membrane and bending loading, whilst the adhesive transmits shear and peel stresses which are a function of the axial coordinate only. Such assumptions are consistent with those usually adopted for the flat joints. A former investigation has shown that the L-R model agrees with FE results for many geometries and gives far better results than other models appeared later in the literature. The aim of the present work is to obtain and present an explicit closed-form solution, not reported by Lubkin and Reissner, which is achieved by solving the governing equations by means of the Laplace transform. The correctness of the findings, assessed by the comparison with the tabular results of Lubkin and Reissner, and the features of this solution are commente

Moisture degradation of open-faced single lap joints

To obtain experimental data in short time on the degradation of adhesives exposed to moisture, a valuable technique is represented using the open-face configuration. With this technique, a layer of adhesive is first applied on one adherend and exposed to the humid environment; then, the second adherend is bonded and the joint can now undergo mechanical testing. Apart from the acceleration of moisture uptake which is obtained due to the larger area exposed, a further advantage is the uniformity of degradation. A further acceleration can be obtained by adding a hygroscopic contaminant at the adhesive/adherend interface, which speeds up moisture uptake and accentuates the interfacial nature of the failure. The main aim of this work was to evaluate the decay of the mechanical strength in the absence or presence of a contaminating agent. The specimens studied were single lap joints, tested under static shear loading. Two sets of specimens were considered; in the first set, the adhesive was applied in standard way and in the second set, the adhesive/adherend interface was contaminated with droplets of CaCl2 aqueous solution. Both sets were subjected to humid and warm environment (100% relative humidity, 50 °C). After the desired exposure times in the range 1–5 weeks, groups of specimens were dried and bonding of the second adherend was carried out. Then, mechanical testing was performed; the fractured surfaces were examined by scanning electron microscopy. The results show that before degradation the failure type is cohesive, but it changes to interfacial failure as the degradation proceeds. Uncontaminated specimens exhibit gradual degradation during the exposure time; contaminated specimens achieve almost half of the degradation in less than one week; after that, the process continues at lower speed and at the end of the observed period both methods show similar values of failure loads. Additional tests were carried out to assess the moisture absorption in the adhesive layer and relate it to the exposure time

An experimental-numerical methodology for the nondestructive assessment of the dynamic elastic properties of adhesives

In the last years, lightweight design has become a priority in many industrial sectors, like as the aerospace and the automotive industry, mainly due to the strict regulations in terms of gas emission and pollution. Together with lightweight materials, the use of adhesives to join different parts permits to significantly reduce the weight of mechanical assemblies. For a proper design of the joints, the mechanical properties of adhesives should be correctly experimentally assessed. However, the experimental assessment of the adhesive mechanical properties can be complex, since they can be hardly estimated from traditional experimental tests on lap joint or butt-joint specimens. The development of an experimental procedure for the assessment of the adhesive properties is therefore of interest. In the present paper, a methodology for the assessment of the dynamic elastic properties of adhesives, i.e., Young's modulus and the loss factor, is proposed. The procedure is based on the Impulse Excitation Technique and Finite Element Analyses (FEA). An automated routine has been written to assess the elastic properties by minimizing the difference between the frequency response obtained experimentally and through FEA. The proposed methodology has been experimentally validated to estimate the mechanical properties of an epoxy adhesive for automotive applications

experimental investigation on adhesively bonded u shaped metallic joints using the arcan test

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Dynamic behaviour of polyolefin thermoplastic hot melt adhesive under impact loading conditions

Dynamic behavior of polyolefin thermoplastic hot melt adhesive under impact loading conditions R. Ciardiello1, A. Tridello1, G. Belingardi1, L. Goglio1. 1 Politecnico di Torino, Department of Mechanical and Aerospace Engineering, Torino, 10129, IT. The mechanical behaviour of adhesive joints under impact loadings is an active area of research due to significant industrial interests. Furthermore, the absence of a unique adopted standard for the study of bonded joints under impact loading increases the academic interests for this topic [1]. In this work, the static and the dynamic response of adhesive joints, bonded with a polyolefin hot-melt adhesive (HMA), were investigated by means of Single Lap Joint (SLJ) tests. The adhesive studied in this work is used in automotive application for bonding plastic internal and external plastic components [2], such as plastic bumpers that can be subjected to impacts during its life. The mechanical and thermal properties of this adhesive are presented in [3]. The main aim of this study is to test standard specimens, SLJ, under dynamic impacts with the use of a modified Charpy pendulum in order to compare the differences between static and dynamic behaviour. The substrate used in this activity are made of a polypropylene copolymer with 10% in weight of talc. Figure 1 shows the testing machine with the clamping system of the specimen. These special fixtures were designed by Goglio et al. [4] with the aim to apply a dynamic load on the tested SLJ. The specimen is fixed to the hammer at the front end, as shown in the right part of Figure 1; the back end is connected to a transverse tail, which hits the two stoppers fixed on the pendulum base, shown in the red circle of Figure 1. The fixtures hold the specimen during the fall of the hammer and transmit the load. A tail in aluminium alloy with T cross section was used, in order to guarantee a high stiffness during the impact, without adding excessive inertia to the system. The system is able to perform dynamic tests for SLJ specimens up to 3.75 m/s. Figure 1: Charpy pendulum used for the experimental tests. Mechanical tests show that there is a clear influence of the load rate on force-displacement diagram and on the maximum force for the tested adhesive. Figure 2 illustrates the differences between a representative curve of quasi-static test and dynamic tests with two different velocities. Figure 2: Force vs linear displacement: comparison between quasi-static and dynamic tests. Figure 3 shows the average values of the peak force and absorbed energies. This Figure illustrates that the velocity increase leads to an increase of the maximum force while the adsorbed energy significantly decreases by comparing quasi-static and dynamic tests. Figure 3: Peak loads and absorbed energy of the quasi-static and dynamic tests. Finally, the fracture surfaces of the SLJ specimens were assessed by means of visual inspection. This analysis showed that the joint separation in the quasi-static tests is mostly cohesive, whereas it becomes completely adhesive in dynamic tests. [1] J.J.M. Machado, E.A.S. Marques and L.F.M. da Silva, J. Adhes., (2017). https://doi.org/10.1080/00218464.2017.1282349. [2] G. Belingardi, V. Brunella, B. Martorana and R. Ciardiello, in Adhesives applications and properties, Cap.13, p.341, A. Rudawska Ed. (INTECH, Rijeca, 2016). [3] E. Koricho, E. Verna, G. Belingardi, B. Martorana, and V. Brunella, Int. J. Adhes. Adhes. 68, 169–181 (2016). [4] L. Goglio and M. Rossetto, in Proceedings of ESDA2006 8th Biennial ASME Conference on Engineering Systems Design and Analysis, 637-643 (2006)

Single-lap joints of similar and dissimilar adherends bonded with a polyurethane adhesive used in the automotive industry

The mechanical performances of single-lap joints between similar and dissimilar adherends bonded with a bi-component polyurethane adhesive have been studied in the present work. The substrate materials include both carbon fibre reinforced composite material (CRFP) and painted metal substrates (PMS). The following substrate combinations were tested: CFRP/CFRP, PMS/PMS, and CFRP/PMS. Two adhesive overlaps, 12 mm and 24 mm, with a fixed thickness were studied to assess the mechanical behaviour of the adhesive joints. The experimental results have been used to construct a finite element model of the single lap joint tests. The objective is to determine the material cohesive properties, in particular the maximum shear stress and the corresponding energy release rate, of the adhesive layer for each retained combination of substrates. An optimization scheme based on transient nonlinear finite element analysis has been here considered, where cohesive parameters of the adhesive layer are handled as design variables. Material parameters are firstly identified for the 12 mm overlap, minimizing the discrepancy between the experimental and numerical force-displacement curves. Then, to validate the obtained properties, results of the 24 mm overlap single lap joint tests are used. The comparison between the experimental and numerical results shows a very good agreemen

Bioelectrochemical Nitrogen fixation (e-BNF): Electro-stimulation of enriched biofilm communities drives autotrophic nitrogen and carbon fixation

A new approach to microbial electrosynthesis is proposed, aimed at producing whole biomass from N2 and inorganic carbon, by electrostimulation of complex microbial communities. On a carbon-based conductor under constant polarization ( 120.7\u202fV vs SHE), an electroactive biofilm was enriched with autotrophic nitrogen fixing microorganims and led to biomass synthesis at higher amounts (up to 18 fold), as compared to controls kept at open circuit (OC). After 110\u202fdays, the electron transfer had increased by 30-fold, as compared to abiotic conditions. Metagenomics evidenced Nif genes associated with autotrophs (both Archaea and Bacteria) only in polarized biofilms, but not in OC. With this first proof of concept experiment, we propose to call this promising field \u2018bioelectrochemical nitrogen fixation\u2019 (e-BNF): a possible way to \u2018power\u2019 biological nitrogen fixation, organic carbon storage and soil fertility against desertification, and possibly a new tool to study the development of early prokaryotic life in extreme environments

A study of microbial communities on terracotta separator and on biocathode of air breathing microbial fuel cells

Recently, terracotta has attracted interest as low-cost and biocompatible material to build separators in microbial fuel cells (MFCs). However, the influence of a non-conductive material like terracotta on electroactive microbiological communities remains substantially unexplored. This study aims at describing the microbial pools developed from two different seed inocula (bovine and swine sewage) in terracotta-based air-breathing MFC. A statistical approach on microbiological data confirmed different community enrichment in the MFCs, depending mainly on the inoculum. Terracotta separators impeded the growth of electroactive communities in contact with cathodes (biocathodes), while a thick biofilm was observed on the surface (anolyte-side) of the terracotta separator. Terracotta-free MFCs, set as control experiments, showed a well-developed biocathode, Biocathode-MFCs resulted in 4 to 6-fold higher power densities. All biofilms were analyzed by high-throughput Illumina sequencing applied to 16S rRNA gene. The results showed more abundant (3- to 5-fold) electroactive genera (mainly Geobacter, Pseudomonas, Desulfuromonas and Clostridia MBA03) in terracotta-free biocathodes. Nevertheless, terracotta separators induced only slight changes in anodic microbial communities

Development of Crop.LCA, an adaptable screening life cycle assessment tool for agricultural systems: a Canadian scenario assessment

There is an increasing demand for sustainable agricultural production as part of the transition towards a globally sustainable economy. To quantify impacts of agricultural systems on the environment, life cycle assessment (LCA) is ideal because of its holistic approach. Many tools have been developed to conduct LCAs in agriculture, but they are not publicly available, not open-source, and have a limited scope. Here, a new adaptable open-source tool (Crop.LCA) for carrying out LCA of cropping systems is presented and tested in an evaluation study with a scenario assessment of 4 cropping systems using an agroecosystem model (DNDC) to predict soil GHG emissions. The functional units used are hectares (ha) of land and gigajoules (GJ) of harvested energy output, and 4 impact categories were evaluated: cumulative energy demand (CED), 100-year global warming potential (GWP), eutrophication and acidification potential. DNDC was used to simulate 28 years of cropping system dynamics, and the results were used as input in Crop.LCA. Data were aggregated for each 4-year rotation and statistically analyzed. Introduction of legumes into the cropping system reduced CED by 6%, GWP by 23%, and acidification by 19% per ha. These results highlight the ability of Crop.LCA to capture cropping system characteristics in LCA, and the tool constitutes a step forward in increasing the accuracy of LCA of cropping systems as required for bio-economy system assessments. Furthermore, the tool is open-source, highly transparent and has the necessary flexibility to assess agricultural systems