Lifetime Prediction with Combined Hygro-Thermo-Mechanical Influences on Thick Layer Adhesives

Semi-structural bonded joints of steel components are exposed to long-term stresses in addition to water diffusion, temperature changes and varying external forces during operation. These hygro-thermomechanical loadings lead to successive degradation of stiffness and strength due to damage resulting from chemical aging and fatigue processes, causing failure of the bonded joint when its service life is reached. In the following contribution, a methodology is presented to predict the service life of hygro-thermo-mechanically loaded semistructural adhesive joints with transient FE simulation. The lifetime prediction is based on a constitutive model with a viscoelastic and a damage part. The first part for viscoelasticity consists of the generalized Maxwell model, in which the effects of temperature changes and varying humidity on the viscoelastic properties of the adhesive bond are captured by the time-temperature and time-water concentration shifts. The second part for the material damage is based on an ordinary differential equation for the damage evolution. It consists of a creep and a moisture damage part for the void developments caused by the local water concentration due to mechanical stress and chemical aging. Both damage parts are multiplied by an Arrhenius-type function to account for the effect of temperature change on the defect growth. The interaction of the creep and the moisture damage part is studied by applying the analytical and numerical solution of the damage differential equation. All parameters of the damage model are determined by using fracture times from creep tests under different climatic conditions and loading levels. The local water concentration in the adhesive is calculated by Fick's model and a concentration boundary condition. The diffusion parameters are determined by gravimetric measurements. A numerical example is setup to demonstrate the lifetime prediction methodology and to show the influence of viscoelasticity on the predicted lifetimes for different hygro-thermo-mechanical loading cases.