Development of a dynamic model for subsurface damage in sandwich composites

Ethan R Brush, Purdue University

Abstract

Despite the advantages of lightweight composite materials over metals and metal alloys, many challenges remain in their inspection and failure prediction. Researchers have found that subsurface damage accumulation due to fiber breakage and delaminations can be detected by observing localized nonlinear vibrations in composite structures. A mathematical model to describe these nonlinearities does not exist in the literature. This research presents an experimental approach to identify a single degree of freedom nonlinear dynamic system model that represents subsurface damages in lightweight sandwich composites as localized stiffness or damping nonlinearities. First, damaging impacts with increasing energy levels are imposed on a set of carbon fiber laminated honeycomb core sandwich panels. Pulsed infrared thermography is used to verify the extent of subsurface damage in each specimen. A test fixture is designed to clamp the panels to constrain the response of the structure to that of an equivalent single degree of freedom system corresponding to the transverse oscillatory motion of the material at the impacted area. A linear vibration study is performed to quantify the changes in mass, damping, and stiffness properties of the material in the test fixture. Significant reductions in the nominally linear stiffness and damping coefficients are estimated after impact damage occurs, whereas the estimated change in mass is small. The forced vibration of the material is then investigated using restoring force analysis. The forced response is measured at single sinusoidal excitation frequencies to construct the relationship between the input force and displacement response. Restoring force analysis is used to develop a Bouc-Wen nonlinear hysteretic model for describing the transverse dynamic response. Although initial testing produces force versus displacement restoring forces that exhibit increased nonlinear behavior with increased impact energy levels, further testing shows that the nonlinear hysteresis observed in the carbon fiber panels is a feature of the experimental setup and not of the damaged material. The forced vibration of a fiberglass laminated honeycomb sandwich composite panel is also tested. The fiberglass panels are shown to be much more compliant than the carbon fiber panels resulting in less influence of the experimental setup on restoring force measurements. Damage is introduced by cutting away the bond between the face sheet and core material. Restoring force analysis of this material shows a clear bi-linear stiffness behavior for the damaged case and a parametric model of the nonlinear stiffness is fit to the measured data. The research concludes that nonlinearities due to composite material damage must be measured using experimental equipment that is matched to the material being tested.

Degree

M.S.M.E.

Advisors

Adams, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

COinS