The mechanical properties of thermosetting resins strongly depend on the curing process itself and, hence, it is important to understand the effects of temperature distributions and degree of cure on the mechanical behavior of polymeric matrix composites. Moreover, the understanding of rheological changes during the curing process is essential for the manufacturing process optimization . In this paper, the chemo-rheological behavior of AS/3501-6 graphite/epoxy during the cure process is presented. Dynamic mechanical analyses are employed to determine complex dynamic moduli of composites during the curing process. These experimental results reveal the strong material property dependence on temperature, frequency (time) and degree of cure.
Dynamic transient responses of plates with viscoelastic free damping layers are studied in order to evaluate free layer damping treatment performances. The effects of forcing frequencies and temperatures on free-layer viscoelastic damping treatment of plates are investigated analytically. Young’s modulus ratio of structures to viscoelastic damping materials and the damping layer thickness effects on the damping ability are also explored.
Abstract Dynamic transient responses of plates with free damping layers are studied in order to evaluate free layer damping treatment performances. The effects of forcing frequencies and temperatures on free-layer viscoelastic damping treatment of plates are investigated analytically. Young’s modulus ratio of structures to viscoelastic damping materials and the damping layer thickness effects on the damping ability are also explored.
Dynamic responses of linear viscoelastic flat panels (plates) un- der the influence of aerodynamic noise are formulated and examined. Three modes of potential failures are considered, namely panel flutter, creep buckling and material failure such as composite fiber delamination. Dierential, inte- gral or proportional servo-controlers are added in order to stabilize the system and extend its lifetime. Parametric studies of servo-controllers led to no gen- eral conclusion because of the highly complicated phase relations. However, the simulations indicate that an increase in servo control parameters does not necessarily lead to an increase in flutter velocity.
Complete three-dimensional, anisotropic, nonhomogeneous degree of cure, temperature-and time-dependent nonlinear thermal properties, and linear viscoelastic constitutive relations are formulated to describe composites during curing processes in autoclaves. Specific temperature, degree of cure, and stress profiles are determined for two distinct fiber orientations and two thicknesses by time-dependent finite-element analyses. Expressions for the probability of times to delaminate during cure under combined stresses are derived. Delami-nation onset survival times of laminated composites in the autoclave during manufacture are investigated in detail, and their dependence on temperature and thickness is shown. Specific areas are identified where critically needed deterministic and stochastic experimental data for thermal, viscoelastic, and failure properties during manufacturing cycles are unavailable.
Viscoelastic materials are known for their ability to dissipate energy. This property has been successfully used by the author and his colleagues to produce eective passive structural control for column and plate creep buckling, various vibratory modes, and aero-viscoelastic phenomena, such as torsional divergence, lifting surface and panel flutter, and attenuation of aerodynamic noise in panels. In self-excited systems the application of increased dissipation may stabilize or destabilize such systems depending on the influence of damping and all other forces on phase relations. Conventional design and analysis formulations call for use of the best available “o designer material protocols based on calculus of variation principles developed in [1] are formulated to determine the global best elastic or viscoelastic properties for specified service conditions. It has been previously established in [2] that in isotropic and anisotropic viscoelastic materials the shape of the relaxation curve is a major contributor to the material;s response performance. In particular, it has been shown that Region C and the ratio E0 / E1 of the relaxation modulus, as seen in Fig.1, are the most influential in dictating material dissipation rates. Consequently, such relaxation modulus functions are tailored through prescriptions of appropriate functionally graded viscoelastic materials to produce the desired designer material performance. Relaxation moduli are, of course, highly temperature sensitive and performances must be evaluated relative to operational demands. In this paper, an analytical study presents optimal sandwich combinations of high shear modulus auxetic [3] webs with composite faceplates of proper number of stacking sequences and fibers as well as their orientations, and their viscoelastic material properties. The constraints that can be imposed consist of one or more selected from weight, dimensions, cost, deformations, failure probabilities, survival / life-times, etc. Some preliminary results are presented. The delamination failure analyses are based on uniaxial viscoelastic experimental data found in [4] and the theoretical stochastic failure criteria developed in [5]. For the same structural weight, the optimized designer viscoelastic sandwich composite plate clearly shows substantial longer survival times and orders of magnitude smaller probabilities of delamination. Extensions of these analyses to multi-element structures, i. e. entire structures, are also presented.
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