MODELING OF TIME-DEPENDENT BEHAVIORS OF ADHESIVES UNDER CREEP AND CYCLIC LOADINGS

Yi Chen

doi:10.7273/000003157

Due to high strength to weight ratio, excellent damping and vibration properties, and good bonding appearance and quality between dissimilar materials, adhesive bonding is replacing conventional joining methods in the aerospace industry to pursue long aircraft life and less maintenance cost. Because of the nature of polymers, most adhesives exhibit complex time-dependent behaviors under different loading types and paths, which are not fully understood and accurately predicted. This study aimed to describe the time-dependent responses to creep and cyclic loadings for a toughened and a standard structural aerospace adhesive.A one-dimensional nonlinear viscoelastic-viscoplastic model combining the Schapery nonlinear single integral model and Perzyna’s viscoplastic model was developed and solved numerically. The von Mises criterion and nonlinear kinematic hardening were considered in the viscoplastic component. This model successfully described the creep and cyclic responses of the adhesives in bulks. For the adhesives in scarf joints, the one-dimensional numerical model was extended to three-dimension and further simplified to a two-dimensional form considering plane strain condition. The permanent strain increased nonlinearly with increasing load duration of creep tests and was used to calibrate the viscoplastic parameters. The model was able to describe the creep-recovery responses of the adhesives systems. Given the reduced time-dependence observed from the cyclic response of the toughened adhesive in scarf joints, the model was recalibrated and compared with a nonlinear viscoelastic-plastic model. While both models showed good agreement with experiment for long exposure to cyclic loading, only the viscoelastic-viscoplastic model agreed with experiment for both short and long loading histories. Load-unload tests were conducted on the toughened adhesive in bulks and bonded joints to characterize the initial yield surface. Given the observed influence of hydrostatic stress, the Drucker-Prager yield criterion replaced the von Mises criterion in the viscoplastic component. It provided an accurate prediction of the ratcheting trend and a more satisfactory result of the recovery stage for the scarf joints subjected to reversed cyclic loading. The lack of considering hydrostatic stress in viscoelasticity and softening due to damage formation might contribute to the underestimation of ratcheting and recovery strain.

MODELING OF TIME-DEPENDENT BEHAVIORS OF ADHESIVES UNDER CREEP AND CYCLIC LOADINGS

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