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Authors
Advisor(s)
Abstract(s)
O uso de ligações adesivas tem vindo progressivamente a aumentar, graças às suas
vantagens comparativamente a outros métodos de ligação. O seu uso encontra-se
bastante difundido pelas mais diversas áreas, que vão desde a construção, embalagem,
indústria automóvel, aeronáutica até às aplicações médicas. Em virtude disso, e devido
às exigências das respetivas aplicações, torna-se cada vez mais necessário estimar de
forma fiável o seu comportamento mecânico. Experimentalmente existem vários
métodos normalizados para esse efeito, todavia estes requerem sempre equipamentos
e materiais que nem sempre estão ao alcance de todos. Recentemente têm vindo a usarse
modelos de dano coesivo (MDC) que, em conjunto com software de método de
elementos finitos (MEF), permitem de uma forma segura calcular a resistência das juntas
adesivas. No âmbito dos MDC a taxa de libertação de energia ao corte (GIIc) é um dos
parâmetros que mais influência tem na previsão de resistência de juntas adesivas. Como
tal, é de grande relevância estudar em detalhe os vários ensaios e respetivos métodos
de redução disponíveis para a obtenção deste parâmetro.
Com esta dissertação pretende-se efetuar uma avaliação numérica aos ensaios End
Notched Flexure (ENF) e Four-Point End Notched Flexure (4ENF) com o intuito de
determinar GIIc. Para o efeito foram utilizados dados experimentais de trabalhos
anteriores [1, 2] relativos a três adesivos, um frágil, um intermédio e um dúctil,
respetivamente, Araldite®AV138, Araldite®2015 e SikaForce®7752. A estes dados foram
aplicados os métodos de redução Compliance Calibration Method (CCM), Direct Beam
Theory (DBT), Corrected Beam Theory (CBT) e Compliance-Based Beam Method (CCBM)
nos ensaios ENF, e CCM e CBT nos ensaios 4ENF, de forma a obter as respetivas curvas
resistentes (curvas-R). Os valores de GIIc daí obtidos foram a posteriori introduzidos em
modelos realizados no código comercial ABAQUS®, que através do MEF e de um MDC
triangular, permitiu estimar a lei coesiva em corte para cada um dos adesivos.
Seguidamente, foi realizado um teste de sensibilidade à variação de GIIc e à resistência
coesiva ao corte (ts
0). Por fim, foi efetuada uma validação dos métodos de redução
utilizados na parte experimental através da sua aplicação aos dados dos ensaios
numéricos e respetiva comparação dos valores de GIIc resultantes com os introduzidos
inicialmente nos modelos numéricos. Do trabalho apresentado, conclui-se que os
valores de GIIc para cada adesivo obtidos pelos ensaios ENF e 4ENF apresentam uma boa
concordância entre si. As leis coesivas obtidas numericamente pelos dois ensaios
também apresentam parâmetros característicos similares. A validação numérica dos
métodos de redução também permitiu a validação dos mesmos para a obtenção de GIIc.
The use of adhesive bonding has been steadily increasing, thanks to its advantages compared to other joining methods. Its use is widespread in several fields, ranging from construction, packaging, automotive industry, aeronautics and medical applications. As a result, and due to the requirements of their application, it becomes increasingly necessary to reliably estimate their mechanical behaviour. Experimentally, there are several standard methods for that purpose, but these always require equipment and materials that are not always available. Recently, the use of Cohesive Zone Modelling (CZM), together with finite element method (FEM) software, provides an accurate technique to calculate the strength of adhesive joints. In the context of CZM, the shear fracture toughness (GIIc) is one of the parameters that has higher influence on the strength prediction of adhesive joints. As a result, it is of great importance to study in detail the various tests and respective reduction methods available to obtain this parameter. This thesis aims to make a numerical evaluation of the End Notched Flexure (ENF) and Four-Point End Notched Flexure (4ENF) tests to determine GIIc. For this purpose, experimental data from previous studies [1, 2] were used regarding three adhesives, a brittle, an intermediate and a ductile, respectively the Araldite®AV138, Araldite®2015 and SikaForce®7752. To this data, the Compliance Calibration Method (CCM), Direct Beam Theory (DBT), Corrected Beam Theory (CBT) and Compliance-Based Beam Method (CCBM) data reduction methods were applied for the ENF test and the CCM and CBT reduction methods were applied for the 4ENF test, to obtain the respective (Resistence- Curves) R-curves. The obtained values of GIIc were subsequently introduced in ABAQUS® which, by the FEM and a triangular CZM law, allowed to estimate the cohesive CZM parameters for each one of the adhesives. Next, a parameter influence study was carried out on the three adhesives, varying GIIc and the cohesive shear strength ts 0. Finally, a validation of the data reduction methods used in the experimental part was carried through to the data of the numerical tests, by comparing the resulting GIIc values with the respective values initially introduced in the numerical models. It was concluded that the GIIc values for each adhesive obtained by the ENF and 4ENF tests are in good agreement. The cohesive laws obtained by the two tests also exhibit similar characteristic parameters. The validation of the numerical data reduction methods also enabled their validation of to obtain GIIc.
The use of adhesive bonding has been steadily increasing, thanks to its advantages compared to other joining methods. Its use is widespread in several fields, ranging from construction, packaging, automotive industry, aeronautics and medical applications. As a result, and due to the requirements of their application, it becomes increasingly necessary to reliably estimate their mechanical behaviour. Experimentally, there are several standard methods for that purpose, but these always require equipment and materials that are not always available. Recently, the use of Cohesive Zone Modelling (CZM), together with finite element method (FEM) software, provides an accurate technique to calculate the strength of adhesive joints. In the context of CZM, the shear fracture toughness (GIIc) is one of the parameters that has higher influence on the strength prediction of adhesive joints. As a result, it is of great importance to study in detail the various tests and respective reduction methods available to obtain this parameter. This thesis aims to make a numerical evaluation of the End Notched Flexure (ENF) and Four-Point End Notched Flexure (4ENF) tests to determine GIIc. For this purpose, experimental data from previous studies [1, 2] were used regarding three adhesives, a brittle, an intermediate and a ductile, respectively the Araldite®AV138, Araldite®2015 and SikaForce®7752. To this data, the Compliance Calibration Method (CCM), Direct Beam Theory (DBT), Corrected Beam Theory (CBT) and Compliance-Based Beam Method (CCBM) data reduction methods were applied for the ENF test and the CCM and CBT reduction methods were applied for the 4ENF test, to obtain the respective (Resistence- Curves) R-curves. The obtained values of GIIc were subsequently introduced in ABAQUS® which, by the FEM and a triangular CZM law, allowed to estimate the cohesive CZM parameters for each one of the adhesives. Next, a parameter influence study was carried out on the three adhesives, varying GIIc and the cohesive shear strength ts 0. Finally, a validation of the data reduction methods used in the experimental part was carried through to the data of the numerical tests, by comparing the resulting GIIc values with the respective values initially introduced in the numerical models. It was concluded that the GIIc values for each adhesive obtained by the ENF and 4ENF tests are in good agreement. The cohesive laws obtained by the two tests also exhibit similar characteristic parameters. The validation of the numerical data reduction methods also enabled their validation of to obtain GIIc.
Description
Keywords
Adesivos estruturais Ligações adesivas End-Notched Flexure (ENF) Four-Point End Notched Flexure (4ENF) Método de elementos finitos (MEF) Structural adhesives Adhesively-bonded joints Finite element Method (FEM)