The adhesion modeling process in Abaqus/Standard is a key tool for accurately simulating the mechanical and thermal properties of adhesive materials during curing. In the aerospace, automotive and electronics industries, structural adhesives play an important role due to their unique properties, such as corrosion resistance, high strength-to-weight ratio and minimization of thermal and mechanical damage compared to methods such as welding.
The curing process of adhesives, which usually involves the formation of chemical bonds through an irreversible reaction, leads to the material changing from a liquid to a solid. Simulation of this process can help design appropriate curing conditions to reduce residual stresses and strains.
Curing process modeling capabilities in Abaqus/Standard
Abaqus/Standard offers tools to accurately model the curing process of adhesives, allowing analysis of complex thermal and chemical phenomena. Key capabilities that can be used in simulations include:
- Curing process modeling - simulate the curing of adhesives by considering the kinetics of chemical reactions and thermal processes.
- TRS (thermorheologically simple) model - takes into account the temperature dependence of the viscoelastic material and the degree of cure.
- Definition of the coefficient of thermal expansion - using the coefficient of thermal expansion as a function of temperature and degree of hardening.
- Dependence of mechanical properties on thermal and chemical processes - full temperature-displacement coupled analysis taking into account these dependencies.
Example of the Watts test
The Watts test (Watts and Cash, 1991) serves as a validation of the modeling process of adhesive curing. It involves measuring the polymerization shrinkage of adhesive materials. In this test, a disk-shaped sample is sandwiched between two glass plates, and the material's shrinkage results are affected by both chemical reaction and thermal changes.
Modeling approach in Abaqus
The modeling of the Watts test in Abaqus involves three key stages:
- Curing stage - simulation of chemical reactions and thermal expansion.
- Cooling stage - analysis of material shrinkage during cooling to room temperature.
- Stress relaxation stage - quasi-static stress analysis after the cooling process.
Types of analysis
Simulations in Abaqus are based on fully coupled temperature and displacement analysis. This type of analysis takes into account not only the deformation of the material resulting from the curing process, but also the dependence of the mechanical properties on changes in temperature and chemical processes. During the quasi-static analysis, a stress relaxation analysis of the adhesive is performed after the curing process.
Mesh design and elements
Axisymmetric elements are used to model the Watts test. The adhesive sample is modeled with CAX4RHT elements, which are ideal for materials with nearly incompressible properties in the liquid state. Glass plates, a brass ring and an air gap are modeled using CAX4RT elements.
Materials
Kamal's equation describes the kinetics of the chemical reaction of the adhesive and allows controlling the rate of cure as a function of temperature and degree of cure. In addition, a viscoelastic material model based on DMA (dynamic mechanical analysis) describes the mechanical properties of the adhesive as a function of temperature and degree of cure.
Thermal-mechanical properties of adhesion (TRS displacement, tangential thermal expansion coefficient, Young's modulus, Poisson's ratio).
Initial conditions, boundary conditions and loads
The initial temperature of the entire assembly is 22°C, and the initial cure rate of the adhesive is 0.0018. Symmetric boundary conditions are applied at the axis of symmetry and at the lower nodes of the glass plates. Thermal loads are applied to the outer surfaces of the set to simulate adhesive curing conditions, where the sink temperature is 65°C for 30 minutes and then drops to 22°C during cooling.
Results and discussion
The simulation results show that the rapid rise in temperature and fast cure rate lead to significant shrinkage of the adhesive sample, which can be a problem in real-world designs where temperature-sensitive materials are used. However, Abaqus models enable accurate prediction of such phenomena, which allows optimization of manufacturing processes.
Change in the DOC (degree of cure) of an adhesive in a sample section during the curing process
Change in the DOC (degree of cure) and TEMP (temperature) at the center point of the sample
Deformation of the sample during the curing process
Modeling the curing process in Abaqus/Standard provides the ability to precisely understand and control the mechanical and thermal properties of structural adhesives, which is crucial for their proper application in various industries.
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