Journal of Aerospace Engineering

Impact tests using a soft gelatin projectile were performed to identify failure modes that occur at high strain energy density during impact loading. Failure modes were identified for aluminum plates and for composites plates and half-rings made from triaxial carbon fiber braid having a $0∕\pm 60°$ architecture. For aluminum plates, a large hole formed as a result of crack propagation from the initiation site at the center of the plate. For composite plates, fiber tensile failure occurred in the back ply at the center of the plate. Cracks then propagated from this site along the $\pm 60°$ fiber directions until triangular flaps opened to form a hole. For composite half-rings fabricated with 0° fibers aligned circumferentially, fiber tensile failure also occurred in the back ply. Cracks first propagated from this site perpendicular the 0° fibers. The cracks then turned to follow the $\pm 60°$ fibers and 0° fibers until rectangular flaps opened to form a hole. Damage in the composites was localized near the impact site, while cracks in the aluminum extended to the boundaries.

This paper attempts to characterize the various methods that may be used in defining the vibration features of a plate dimensionally proportioned to represent a turbine compressor blade. The comparison is specifically devoted towards certain damping properties of a titanium plate coated with magnesium aluminate spinel (mag spinel). Two different coating thicknesses were tested: $0.13\mathrm{mm}\left(.005\mathrm{in.}\right)$ per side and $0.25\mathrm{mm}\left(.010\mathrm{in.}\right)$ per side [total thicknesses of $0.25\mathrm{mm}\left(.010\mathrm{in.}\right)$ and $0.51\mathrm{mm}\left(.020\mathrm{in.}\right)$ ]. In order for the reader to appreciate the experimental variations possible in this undertaking, the writers define various tests performed. Dynamic ping tests were conducted on all specimens to determine their resonance frequencies. Laser vibrometry was used to determine the mode at each resonance frequency. Damping ratios were determined through the use of sine sweeps. It is important that damping characteristics be measured very precisely. Thus, in the approach using a sine sweep, one must make sure that the rate of sweep is sufficiently slow to evaluate the proper frequency response function. A discussion of the proper rate is provided. This project demonstrated that, in order to represent the basic damping properties of a coating, one must consider the rate of the dynamic load application. The usual method associated with the damping coefficient evaluation is referred to as the half-power method. The true value of the damping coefficient is directly associated with the rate of experimental power versus frequency.

A previously developed analytical formulation has been modified in order to more accurately account for the effects of hydrostatic stresses on the nonlinear, strain rate dependent deformation of polymer matrix composites. State variable constitutive equations originally developed for metals have been modified in order to model the nonlinear, strain rate dependent deformation of polymeric materials. To account for the effects of hydrostatic stresses, which are significant in polymers, the classical $J_2$ plasticity theory definitions of effective stress and effective inelastic strain, along with the equations used to compute the components of the inelastic strain rate tensor, are appropriately modified. To verify the revised formulation, the shear and tensile deformation of a representative polymer are computed across a wide range of strain rates. Results computed using the developed constitutive equations correlate well with experimental data. The polymer constitutive equations are implemented within a strength of materials based micromechanics method to predict the nonlinear, strain rate dependent deformation of polymer matrix composites. The composite mechanics are verified by analyzing the deformation of a representative polymer matrix composite for several fiber orientation angles across a variety of strain rates. The computed values compare well to experimentally obtained results.

The reusable launch vehicle thrust cell liner, or thrust chamber, is a critical component of the space shuttle main engine. It is designed to operate in some of the most severe conditions seen in engineering practice. These conditions give rise to characteristic deformations of the cooling channel wall exposed to high thermal gradients and a coolant-induced pressure differential, characterized by the wall’s bulging and thinning, which ultimately lead to experimentally observed “dog-house” failure modes. In this paper, these deformations are modeled using the cylindrical version of the higher-order theory for functionally graded materials in conjunction with two inelastic constitutive models for the liner’s constituents, namely Robinson’s unified viscoplasticity theory and the power-law creep model. Comparison of the results based on these two constitutive models under cyclic thermomechanical loading demonstrates that, for the employed constitutive model parameters, the power-law creep model predicts more precisely the experimentally observed deformation leading to the “dog-house” failure mode for multiple short cycles, while also providing much improved computational efficiency. The differences in the two models’ predictions are rooted in the differences in the short-term creep and relaxation responses.

Lightweight and heavy-duty fiber-reinforced polymer (FRP) composite honeycomb sandwich structures have been increasingly used in civil infrastructure. Unique cellular core configurations, such as sinusoidal core, have been applied in sandwich construction. Due to specific core geometry, the solutions for core effective stiffness properties are not readily available. This paper presents a mechanics of materials approach to evaluate the effective stiffness properties of sinusoidal cores. In particular, the internal forces of a curved wall in a unit cell are expressed in terms of resultant forces, and based on the energy method and principle of equivalence analysis, the in-plane stiffness properties of sinusoidal cores are derived. Both finite-element modeling and experimental testing are carried out to verify the accuracy of the proposed analytical formulation. To illustrate the present analytical approach as an efficient tool in optimal analysis and size selection of sinusoidal cores, several design plots are provided and discussed. The simplified analysis and formulation presented for sinusoidal cores can be used in design application of FRP honeycomb sandwich and optimization of efficient cellular core structures.

The paper illustrates a new approach to the evaluation of the effect of z-pins on deformations and the strain energy release rate in composite double cantilever beams (DCB) subject to a standard fracture toughness test. The effect of z-pins is modeled by an elastic foundation, based on previously published work. The approach to the solution is based on a separate analysis of the intact and delaminated parts of DCB. The rotational stiffness of the intact part is obtained from the Rayleigh-Ritz solution for this part subjected to a force couple, rather than modeling the rotational restraint by introducing an elastic foundation, as has been done in the previous studies. Subsequently, the deformation of the delaminated part of DCB is analyzed exactly by solving the equation of equilibrium with the appropriate boundary conditions. Based on this solution, the compliance, the rate of change of compliance, and the strain energy of the specimen can be evaluated. The results illustrate the beneficial effect of z-pins on the resistance of DCB to delamination cracking.

This paper presents further development in a new time-domain computational technique developed for the determination of far-field acoustic radiation. This method, referred to as the transform potential-theoretic (TPT) method, solves linear time-dependent wave propagation problems in an unbounded medium, combining techniques involving numerical transforms/inverse transforms and potential theory. The end result is a robust procedure that is accurate and computationally efficient. The TPT technique is meshless, yet it can handle arbitrary geometries for the acoustic source. The validity of the procedure is demonstrated for the case of an acoustic wave propagation from a two-dimensional bounded surface embedded in a uniform flow. Both direct simulations as well as the current method(s) are compared. The single-layer potential theory based TPT method required fewer points on the boundary than the double-layer potential theory based TPT and performs better than direct simulation in high subsonic Mach number flows $(M=0.8)$ . The times required to compute these solutions were more than two orders of magnitude lower than direct simulation.