Polyisobutylene (PIB) or butyl rubber has been used widely in applications such as construction materials, adhesives and sealants, agricultural chemicals, medical devices, personal care products, and fuel additives. Due to the unique low gas permeability, flexibility, and excellent weathering resistance, PIB or PIB based materials are frequently employed in photovoltaic (PV) industry as sealant to protect the electrical assembly in the package as well as moisture sensitive PV cells from aggressive environments. Long term behavior of the PIB sealant within the operating temperature range of the PV devices thus becomes a critical factor to the reliability of the device. In this paper, an experimental study of the temperature dependent fatigue behavior of a PIB based joint is presented. A finite element model capturing the joint region geometry is developed and an approach to estimate lifetime is proposed.
Experimental results for the fracture behavior under mixed-mode in-plane loading conditions of adhesively bonded wood specimens are reported. The material systems considered involved yellow-poplar (Liriodendron tulipifera), a hardwood of the Magnoliaceae family, as adherends bonded with two different adhesives, a moisture-cure polyurethane (PU) and a phenol/resorcinol/formaldehyde (PRF) resin. A dual actuator test frame permitted fine scanning of fracture behavior over a full range of mixed-mode I/II levels for double cantilever beam (DCB) geometry specimens. These tests showed that, in the considered material systems, the critical strain energy release rate, c, tends to increase as the mode-mixity of the loading increases. In particular, the increase is steeper in proximity to pure mode II loading for the PRF bonded specimens. The experimental values of c obtained were fairly scattered, as is common when testing wood systems. This variability is due in part to the natural variability of wood but also to other factors such as the orientation of the grain in the bonded beams and variations of bondline thickness. In particular, measurements of adhesive layer thickness were performed. This analysis was implemented with microscopic examination of samples cut from untested DCB specimens, where the bondline had not been disrupted by the test. Although the wood parts were power planed prior to bonding, rather large variations of the adhesive layer thickness were observed: on the order of 1–100 μm for specimens bonded with the PU resin and 10–50 μm for specimens bonded with the PRF resin, which showed somewhat more consistent fracture behavior.
When wood beams are bonded to form double cantilever beam (DCB) specimens, the resulting fracture properties are often quite scattered. Random variations of properties of wood are usually considered as the reason for the data scatter, but there are also morphological aspects that can possibly be accounted for. The present paper focuses on these morphological aspects and, in particular, an analytical model has been developed for evaluating how the orientation and stiffness of the layers of beams of constant cross section infl uences the stiffness variation of the beam along its length. Wood in DCBs is a common example of a material with oriented layers resulting from the alternating earlywood (EW) and latewood (LW). Part of the paper is dedicated to Douglas fi r ( Pseudotsuga menziesii ), where the variability of equivalent elastic stiffness is found to be on the order of ± 6 -8 % in bonded DCBs, depending on grain orientation. Other representative cases of bonded wood beams are also presented, where the stiffness variability is on the order of ± 15 -20 % . Part 2 of this paper will evaluate how these levels of elastic stiffness variations infl uence the measured critical strain energy release rate, G Ic .
Insulated joints (IJs) are often required every few kilometres along railway tracks for signal blocks and rail break detection; practical experience has shown that their life is often a fraction of the life of other track elements on some rail lines subjected to high tonnage freight. This article reports findings from a project conducted to study different bond systems consisting of various combinations of adhesives, fibrous insulators, and rail surface treatments that were of potential interest for increasing the service life of IJs for rail applications. The study was performed in parallel with a finite-element analysis and did not focus on testing real IJs but rather on common adhesion test specimens such as the single lap joint and double cantilever beam configurations. The aim of using these specimens was to simulate potential load and environmental conditions on standard test specimens that were less expensive and easier to construct, test, and analyse. The main goal of the project was to compare a number of combinations of potential IJ components through an extensive test programme. The results highlighted several possible combinations that may warrant further study as actual IJ prototypes. In particular, several material combinations involving materials not currently used by IJ vendors had higher overall performances when compared to currently used combinations, although the extension of improved test specimen performance to actual IJ configurations and service conditions may not follow.
Part 1 of the paper evaluated how the orientation and stiffness of the layers in layered beams of constant cross section used in double cantilever beams (DCB) specimens infl uence the stiffness variation of individual adherends along their length. The behavior of bonded DCB specimens made with such materials, of which wood is a common example, are analyzed in this part 2 of the paper to determine errors associated with common data analysis methods for mode I fracture testing: simple beam theory (SBT), corrected beam theory (CBT), experimental compliance method (ECM or Berry method), and area method (AM). In particular, the fi rst three methods are described in British Standards (BS) 7991 (1991) or the American Society for Testing and Materials (ASTM) D 3433 (1999/2005), while the AM is not suggested by the mentioned standards. SBT, CBT, and ECM, although initially developed for characterizing bonds between uniform and isotropic adherends, have also been commonly applied to other materials including wood. Nevertheless, these three standardized methods may lack precision for determining the critical strain energy release rate, Ic , when applied to bonded wood adherends, due to the elastic stiffness variability that can occur along the length of the bonded beams. The AM yields more coherent results in the developed analytical procedure, although practical issues can limit its reliability with experimental results. Another physical problem that arises with the adherend stiffness variation is the onset of a slight mode II loading component that is not anticipated, nor accounted for, in the traditional data analysis methods.
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