Material damping is important in the design of structures as it limits vibration amplitudes, increases fatigue life, and affects impact resistance. This is particularly true for composite materials, which are currently used extensively in applications that experience frequent dynamic loading. Furthermore, the damping capacity of composites can be significantly greater than that of standard engineering materials. Like other performance parameters of composites (e.g., stiffness, strength, density), the effective damping capacity of composite materials is dependent not only on the damping properties of the constituent materials, but also microstructural details such as fiber volume fraction, fiber orientation, ply stack up, fiber packing array, and weave pattern in woven composites. Therefore, like other performance parameters, composite damping capacity can be engineered.
The objective is to maximize the damping of structural composites while avoiding negatively impacting their mechanical properties. An extreme improvement (10×, and potentially more) in the damping properties of structural fiber-reinforced composite materials can be realized through engineering of the fiber/matrix interface.
The damping properties of unidirectional, laminated, and woven composites have been predicted using a multiscale implementation of the High-Fidelity Generalized Method of Cells (HFGMC) micromechanics theory. This model considers periodic repeating unit cell geometries on both the global and local scales, and utilizes the constituent material specific damping coefficients, mechanical properties, and local fields, along with the strain energy approach, to determine effective directional specific damping coefficients of the composite. In addition to comparisons of the HFGMC predictions with results from the literature, the effect of a degraded fiber/matrix interface was examined parametrically.
The multiscale HFGMC simulations presented in this work illustrate that the decrease in composite mechanical properties caused by such an engineered interface can be minimized when implemented within a technologically relevant laminate, while still maintaining an extreme improvement in the laminate damping properties. Strong maxima in the damping coefficients are present for a quasi-isotropic laminate, rather than simply for the transverse direction in a unidirectional ply. This is the first time that these maxima in the composite damping properties have been discovered, quantified, and analytically demonstrated. This could make possible significant improvements to the damping of real structural composite materials with only a small impact on the mechanical properties. This is important because this type of laminate is used extensively in structural applications, and often, modifications to the composite material can be made that improve the behavior of a ply oriented at 90 degrees, but the effect is washed out by the presence of the strong and stiff fibers oriented in other directions in the laminate. Key to the present innovation is that the damping improvements persist for a practical laminate, and only result in a small (8.5%) decrease in the mechanical stiffness of a quasi-isotropic laminate.
A triply-periodic HFGMC is first enhanced to enable coupled multiscale analysis, wherein both the local (fiber/matrix/interface constituent) and global (laminate/woven) scales are synergistically linked. The HFGMC method determines the strain concentration tensors, which are used to establish the macroscopic constitutive equations of the composite, and also to provide the local stress and strain fields throughout the composite. This enables the prediction of not only effective composite properties, but also the strain energy distributions (and thus specific damping coefficients) in response to given external loading.