Recent experimental efforts using glass fiber/epoxy microdroplet test specimens have shown that increased fiber surface roughness can simultaneously increase strength and energy absorption, i.e. material toughness, through mechanical interlocking. This research employs finite element (FE) modeling techniques to investigate the effect of fiber topography on the structural response of the microdroplet test. The characteristic length of the fiber topography is on the order of nanometers and the characteristic length of the microdroplet specimen is on the order of micrometers. Consequently, a multiscale numerical method is constructed to translate the effective nanoscale structural behavior to the microscale in order to simulate its effect on the microdroplet test. These numerical techniques are validated by comparing the simulated responses to published experimental data.

The multiscale translation process effectively maps the nanoscale modeling results into a set of cohesive contact parameters that control the progressive failure of the fiber-droplet interface during the microdroplet simulation. The nanoscale simulations show that the both the interphase thickness and elastic modulus have a notable effect on the effective microdroplet structural behavior. In the presence of increased surface roughness fibers, the shear strength and toughness is increased only when the characteristic protrusion length of the fiber topography is greater than the interphase thickness. Comparison of the microdroplet simulation results with experimental data indicates that that the multiscale translation process shows good performance up to peak load, which is often the primary metric used to rate structural performance. However, the multiscale process is less accurate in predicting the post-failure response because the nanoscale model does not accurately simulate failure of the interphase.