Resumen de Análisis de la degradación de adhesivos en ambiente marino: evaluación del comportamiento mecánico de las uniones en modo mixto para su aplicación en materiales híbridos estructurales
Innovative products to be competitive in the market, have a tendency to use new types of material and unusual geometries. The advantage of adhesive joints against other joint methods to assemble different materials and irregular geometries in a profitable way and at the same time in a light and imperceptible way, make this type of joints to be used more and more often in the industry.
Adhesive joints can work under unfavourable environmental conditions such as temperature, humidity, oxidation, etc., during the process. The joint action of these environmental factors causes a degradation in the adhesives which is increased when the adhesive joint is subjected to a heavy load for a long time. Therefore, becoming aware of its efficiency in the long term is crucial, because these adhesive joints not only must support external loads, but also thermal ageing and/or humidity. Due to this, it is crucial to understand how these environmental factors and their synergic effect in the adhesive joint affect its mechanical behaviour.
Apart from the inconvenience such as the synergy effect produced by the environmental conditions, we must take into account that the mechanical features of the adhesives change throughout the time, which explains the importance of developing test in order to accelerate the process to determine the efficiency of the same adhesives in different environmental conditions (temperature, humidity, etc.) throughout the time.
Due to this, in this research, we are going to make fluency tests at constant load, taking note of the data about the loss of the shape throughout the time. There are very few works that study the viscoelastic behaviour combined with humidity and temperature, because they are joined effects and give way to complex characteristics. Due to the lack of a model-theory capable of predicting the behaviour to structural adhesive fluency in a precise way and under different environmental conditions, in this study we will use the viscoelastic model put forward by Ngay in order to predict the behaviour in the long term in different test conditions. This model allows to extrapolate the behaviour of the joint in a long term, from experimental data taken in a short term, in the rank of viscosity.
From the experimental short-term information and taking a reference temperature, the ¿Master Curve¿ can be made under the Time Temperature Superposition Principle in order to estimate the long term efficiency of polymers.
Nevertheless, the more and more spread usage of adhesive joints makes the prediction of its mechanical behaviour to be very important in the design process. However, the joints rarely work in a pure mode (mode I or II) but on average they are required in a mixed mode (a combination of peel and shear). Therefore, a basic problem consists of the experimental determination of the fracture characteristics of the structural adhesives and under mixed-mode conditions are even more difficult to be measured. In order to solve this problem, literature shows the development of a great number of test tubes and different test techniques. It has been tested that the mode mixities depends on the external loads, the adherent characteristics and often, on the flexibility of the adhesive layer, this last factor is not always taken into account by many authors when choosing the type of test tube to be used.
Due to the problems concerning the use of standard test tubes, in this work we will use a new configuration designed by Högberg et al. known as Mixed Cantilever Beam (MCB). This new configuration is based on the geometry of a symmetric semi-infinite test tube, Double Cantilever Beam (DCB).
The MCB test tube offers exceptional flexibility, variety and stability. Moreover, it has suitable geometry to assess the cohesive behaviour law. In this work, we will test different mixed modes with tools designed to subject the adhesive joint to seven independent loads of stress (seven angles between 0o and 90o).
In order to test this type of test tubes under different environmental conditions, a new specific test unit has been made so as to get experimental data from the evolution of the deformations in the bond line in the short-term. Then, by means of the Principle of Superposition, there is a monitoring of the adhesive flexibility and the behaviour is estimated in the long term.
To study the equivalence of the effects of temperature and humidity over the evolution of the mechanical behaviour of the adhesive, adhesive test tubes have been introduced into sea water, immersion, for different periods of time to measure the amount of absorbed humidity. By means of dissemination models in water, it is possible to make predictions in the long term about the entrance of humidity in the adhesive joint.
This way, the water concentration profiles have been determined in the adhesive joint for both adhesives from the coefficients of dissemination, calculated by gravimetric methods. Thus, the necessary time so that the water concentration exceeds in the core area has been determined, over this critical value, irreversible damage in the polymer is produced.
On one hand, microstructures changes, due to the action of water, have been observed by scanning electron microscopy (SEM) and on the other hand, the polymer chemical degradation has been studied by infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The results from the different characterization techniques show that due to the sea water absorption, degradation phenomena have taken place.
After water immersion, the test tubes have been tested in the autonomous station under different modes and temperatures, getting the curves of the fracture energy against the elongation and the laws of cohesive behaviour. It has been observed that the fracture energy goes down when humidity and temperature increase and this energy variation is greater in the modes where the shear component influence is higher than in the modes where the peel component influence is higher.
As the polyurethane adhesive is highly deformable, it is very sensitive to be tested in one or other load mode. However, the vinilester resin which is a very rigid polymer is less sensitive to the working methods thus; there is not a broad spread between the fracture energy values in each mode. Dealing with the vinilester resin case, it has been observed that adding distilled water to the joint for short periods of time favours its fracture resistance. This is due to the plasticising effect caused by the distilled water in the polymer.
This methodology can serve as a guideline for the selection of adhesives. In this case, the critical loading mode that draws a distinction using polyurethane or vinilester resin is 30o because the fracture energy of both polymers practically coincide. Under 30o it is more effective to use polyurethane adhesive and over such temperature, working with vinilester resin is better.
The cohesive behaviour laws for pure modes I and II have been made under different test conditions. These laws allow us to determine the cohesion characteristic parameters of the adhesive layer. Such information is very useful to properly model the adhesive joints behaviour in models of finite elements. Through the cohesive behaviour laws, the results obtained from the mixed mode tests have been substantiated.
Also, a new methodology has been developed to determine the master curves for both tested adhesives in Mode I and in Mode II. Different viscoelastic curves have been obtained at different test temperatures. The tests have been accelerated and the long term behaviour of these adhesive joints have been predicted. The polyurethane adhesive is more sensitive to the temperature variable, than the vinilester resin because we work with the vinilester resin well below its glass transition temperature, whereas we work with the polyurethane adhesive close to its glass transition temperature.
Finally and in order to validate numerically the experimental results obtained, we have simulated the mechanical behaviour of the joint through a model by finite elements. First of all, we have made a model of water dissemination depending upon the time for the polyurethane adhesive from the dissemination coefficients experimentally determined and for MCB-geometry test tubes. Then, the viscoelastic test modelling has been done in order to assess the failure of cohesive elements and study how the crack spreads in the adhesive joint.
In short, in this work suitable tools have been designed in order to require the adhesive up to seven cases in mixed mode and also, a specific experimental station has been made and set up which allows to measure the evolution of the deformations in the bond line using a specific test tube geometry (MCB) and under different temperature conditions. In addition, a new methodology has been developed to determine the cohesive behaviour laws of adhesives under different test conditions. Besides, the master curves have been determined to predict the long term behaviour of the adhesive joints in real conditions of service. Lastly, we have simulated the behaviour of the adhesive joint through a model by finite elements, which has allowed us to substantiate the experimental results obtained.
