Abstract

Structural adhesives are widely used in many industries, including the automotive, aerospace, and electronics industries. To avoid damage to sensitive components, warpage of the final assembly, surface defects, and/or reduced bond life, the chemical shrinkage and residual stresses associated with the curing process of the adhesive must often be understood. In this presentation a finite element-based method for simulating curing processes is outlined and validated. The method developed uses a transient coupled thermal-structural analysis procedure. Using user-defined subroutines, the conversion rate is calculated as a function of temperature using the measured reaction kinetics of material, and an explicit update procedure is used to calculate the total conversion. The elastic (and, optionally, viscoelastic) properties of the material are then updated as functions of conversion and temperature, and the new stresses and material Jacobian are defined. In addition, at each time step the conversion and temperature-dependent chemical shrinkage and thermal strains are updated. Finally, the heat generation associated with the exothermic reaction is updated, and the equilibrium iteration or time stepping process is continued. The experimental methods required to characterize the conversion and temperature-dependent properties of the material (including density, thermal conductivity, specific heat, heat-of-reaction, reaction kinetics, chemical shrinkage, thermal expansion and elastic or viscoelastic properties) are also discussed. The experimental methods used include Differential Scanning Calorimetry (DSC), Fourier Transform Infrared spectroscopy (FTIR), and Dynamic Mechanical Analysis (DMA). Of particular interest is the use of a DMA testing procedure featuring a multi-wave drive signal that enables the measurement of a frequency-dependent response in a time span that is sufficiently short to assume that the conversion level is constant during the test cycle. This method enables the generation of master curves at multiple iso-conversion levels. In addition, by defining both temperature and conversion-dependent shift factors, a "master curve of master curves" can be constructed (the Time-Temperature-Cure Superposition (TTCS) principle). The experimental methods developed to validate the curing model will also be discussed. In addition, a native Abaqus implementation of the cure modeling procedure will be introduced.