Rdsg Campilho

Effect of humidity on the mechanical properties of adhesively bonded aluminium joints

The presented work focuses on the effects of water degradation on the long-term behaviour of adhesive joints. The objective of this study is to measure the evolution of various mechanical properties such as tensile stress and fracture toughness as a function of humidity for two distinct adhesives, using bulk adhesive and double cantilever beam specimens in unaged and aged conditions in order to understand the influence of humidity on the adhesive properties. A mathematical equation that allows the prediction of each property degradation as a function of water is proposed and validated, which takes into account various parameters such as the diffusion coefficient, resulting in a general equation for mechanical property degradation prediction of potentially any adhesive. It was also found that the distinct adhesive properties such as strength, stiffness and fracture toughness all decreased due to water degradation with the exception of the strain that increased, concluding that water reduces the joint strength and lifespan of the studied adhesives, although in different ways.

Overview of different strength prediction techniques for single-lap bonded joints

Adhesive joints have been used in several fields of engineering, and their applications are vast. Due to their easy and quick fabrication process, single-lap joints are a common configuration. The increase of strength, weight reduction and resistance to corrosion are some of the advantages of this kind of joint over traditional joining methods. However, stress concentrations at the overlap edges are one of the main disadvantages. There are very few accurate design techniques for the diversity of bonded joints that can be found in real applications, which constitutes an obstacle to the use of this bonding method in structural applications. This work aims at comparing different analytical and numerical methods in the strength prediction of single-lap joints with different overlap lengths (LO). The main objective is to evaluate which predictive method is the best. Adhesive joints were produced between aluminium adherends using a brittle epoxy adhesive (Araldite® AV138), a moderately ductile epoxy adhesive (Araldite® 2015) and a ductile polyurethane adhesive (Sikaforce® 7888). Different analytical methods were considered, together with two numerical techniques: cohesive zone models (CZM) and the extended finite element method (XFEM), allowing the comparative analysis. The analytical methods showed that they only give relatively accurate results in very specific conditions. The CZM analysis with the triangular law revealed to be a very accurate method, with the exception of joints with very ductile adhesives. On the other hand, the XFEM analysis was not adequate, especially for crack growth in mixed mode.

Effect of the size reduction on the bulk tensile and double cantilever beam specimens used in cohesive zone models

Cohesive zone elements used in finite element analysis are a reliable way to design and predict the behaviour of the joint. The characterisation of the traction separation law used in these models is done using tensile and fracture tests, and the parameters of such laws depend on humidity and temperature. Water diffusion tests are therefore necessary, which are dependent on specimen geometry, meaning a bigger specimen takes longer to fully saturate. To solve this problem and increase the efficiency of the ageing process, smaller tensile bulk and double cantilever beam (DCB) specimens are necessary. Another advantage of smaller DCB specimens is that they can be tested in smaller high-temperature chambers, where normal DCB specimens do not fit. Smaller geometries of the bulk tensile and DCB tests are analysed, and a proposed geometry for each test is shown to produce very satisfactory results, validating the use of these specimens.

Mode I fracture toughness of CFRP as a function of temperature and strain rate

Composite structures currently used in the automotive industry must meet strict requirements for safety reasons. They need to maintain strength under varied temperatures and strain rates, including impact. It is therefore critical to fully understand the impact behaviour of composites. This work presents experimental results regarding the influence of a range of temperature and strain rates on the fracture energy in mode I, GIC, of carbon fibre reinforced plastic plates. To determine GIC as a function of temperature and strain rate, double cantilever beam specimens were tested at 20, 80 and −30℃, with strain rates of 0.2 and 11 s−1. A complementary numerical study was performed with the aim of predicting strength using the measured values. This work has demonstrated a significant influence of the strain rate and temperature on GIC of the composite materials, with higher strain rates and lower temperatures causing a decrease in the GIC values.

Fracture toughness in Mode I (GIC) for ductile adhesives

Works carried out in this publication belong to a project that seeks the replacement of welded joints by adhesive joints at stress concentration nodes in bus structures. Fracture toughness in Mode I (GIC) has been measured for two different ductile adhesives, SikaTack Drive and SikaForce 7720. SikaTack Drive is a single-component polyurethane adhesive with high viscoelasticity (more than 100%), whose main use is the car-glass joining and SikaForce 7720 is double-component structural polyurethane adhesive. Experimental works have been carried out from the test called Double Cantilever Beam (DCB), using two steel beams as adherents and an adhesive thickness according to the problem posed in the Project, of 2 and 3 mm for SikaForce 7720 and SikaTack Drive, respectively. Three different methods have been used for measuring the fracture toughness in mode I (GIC) from the values obtained in the experimental DCB procedure for each adhesive: Corrected Beam Theory (CBT), Compliance Calibration Method (CCM) and Compliance Based Beam Method (CBBM). Four DCB specimens have been tested for each adhesive. Dispersion of each GIC calculation method for each adhesive has been studied. Likewise variations between the three different methods have been also studied for each adhesive.