Quasi-static indentation and sound-absorbing properties of 3D printed sandwich core panels

The use of 3D printing to produce acoustic panels with good mechanical and acoustic properties was investigated in this paper. Various fiber layups of the fiberglass face sheet and core designs were fabricated and tested for their indentation resistance and acoustic absorption performance. It was found that the bidirectional face sheet layup exhibited the best indentation energy absorption recording 4.2 J, which is 37% more than the 45-degree layout and 66% more than the quasi-isotropic layup. The specific energy absorption of the hybrid honeycomb core is the best among the three core designs recording 404 J/kg, which is 56% higher than the corrugated triangle with horizontal beam core (359 J/kg) and 20% higher than double ellipse core (335 J/kg). Computed-Tomography (CT) scan was used to study the fracture behavior of the sandwich structures. It was found that the bidirectional layup exhibited a different failure mode as compared to the 45-degree and quasi-isotropic layup. In terms of the acoustic properties, the face sheets with various layup patterns have a low acoustic absorption coefficient with minimal differences from each other at low frequencies (500 Hz–3000 Hz) and have higher absorption coefficients with greater differences from each other at frequencies between 3000 Hz–6500 Hz. The absorption curve was significantly affected by the design of the core. The orientation of the core also comes into play if the core is asymmetrical. The hybrid honeycomb sandwich structure was the optimal structure among the three designs for balanced indentation resistance and acoustic insulation.

Evaluation of test methods and face-sheet thickness effects in damage tolerance assessment of composite sandwich plates

Composite sandwich materials provide high bending performance-to-weight ratios. However, these materials are vulnerable to impact damages which can drastically reduce their load-bearing capability. Presently there is a lack of standardised test methods for impact assessment. This study compares three different test methods for impact assessment; single skin compression after impact (CAI-SS), sandwich compression after impact (CAI-SW) and four-point bending-after-impact (BAI). The CAI-SS test method shows high compressive strength and strain at failure and the tesr is relatively easy to evaluate. For finite size plates with significant impact damage, the CAI-SS test method is recommended for post impact strength assessment. For large sandwich panels with relatively small impact damages the CAI-SW test method could be more relevant since it includes effects of panel asymmetry generated from the impact damage. The BAI test method may be recommended as an alternative to CAI but quite long specimens are required in order to assure compressive failure in the tested face-sheet, making the test both demanding and expensive. On the other hand, lower load levels are required to break the specimens and there is less need for precise machining during specimen manufacturing. A finite element model including progressive damage evolution was used to estimate the post impact strength. The simulations showed generally good agreement with the experiments.

Equivalent in-plane elastic modulus of honeycomb sandwich in the direction of cell vertical wall

The equivalent in-plane elastic modulus [Formula: see text] of a honeycomb sandwich in the direction of cell vertical wall can be assessed by the law of mixture from the modulus of face sheet [Formula: see text] and the equivalent modulus of honeycomb core [Formula: see text]. However, significant errors as large as 40% can be made depending on material and geometry parameters, when the used [Formula: see text] is obtained from a cellular model of core alone without considering the skin effect of face sheet. The main reason of the error is that the rigidity to deformation of y-direction is different greatly between the vertical cell wall of core and inclined cell wall of core. In the present paper, an analytical model is proposed to assess [Formula: see text] of honeycomb sandwich with considering the interference effect of the core with the face sheet. In the proposed model, the influence of the face sheet rigidity on [Formula: see text] is taken into account. The results demonstrate that the contribution of the core to [Formula: see text] is also dependent on the face sheet rigidity significantly. The validity of presented model is verified by comparing the results with numerical results of FEA.

Impedance models for single and two degree of freedom linings and correlation with grazing flow duct testing

Presented here is the development of a predictive model for impedance of single-degree-of-freedom (SDOF) and two-degree-of-freedom (2DOF) acoustic linings that is suitable for the design stage of suppression of inlet noise for turbo-fan engines. It is required that over a probable range of lining physical parameters and operating conditions the impedance spectrum is predicted with accuracy sufficient to support a lining design process and assessment of achievable attenuation. The starting point is a published impedance model for SDOF linings that primarily focuses on the transfer impedance of conventional and micro-perforate face sheets with grazing flow. This is expanded here to include 2DOF linings, introducing new issues related to transfer impedance of the inserted septum. Problems addressed are related to the septum insertion process that can change thickness, hole diameter and open area ratio of the uninstalled septum, and introduce blockage. Required empiricism is discussed and models for face sheet and septum-in-core transfer impedance are derived, applicable to a specific range of sheet thickness, hole diameter, and open area ratio. Manufacturing processes considered are mechanical drilling in the case of the carbon fiber laminate face sheet that is conventional perforate, and laser drilling in the case of the epoxy film micro-perforate septum material. Benchmarking is carried out by comparison of acoustic field predictions, using the proposed lining model in an FEM propagation code, with measured data from a grazing flow duct facility. Test samples include SDOF and 2DOF linings, including cases with three segments, each with distinct physical properties. Example results of comparisons are shown to highlight the fidelity of the impedance model over a frequency range compatible with the grazing flow duct geometry.

Slanted septum and multiple folded cavity liners for broadband sound absorption

The design of acoustic liners with complex cavities for a wide frequency range of attenuation using numerical method is investigated in this paper. Three novel liner concepts are presented, demonstrating predicted improvements in broadband sound absorption when compared with that for conventional designs. The liners include a slanted septum core, a slanted septum core with varying percentage open area, and a MultiFOCAL concept. A finite element model of a normal incidence impedance tube is developed using COMSOL Multiphysics modeling software to predict the acoustic properties (resistance and reactance) of liners at medium and high sound pressure levels, and to study the impact of variations in the liner design parameters. The impedance tube finite element model incorporates non-linear semi-empirical impedance equations, validated by comparing numerical results with measurements performed on a single-degree-of-freedom liner, with a perforated face sheet, at high sound pressure level. The design variables of the novel liner concepts are optimized using a hybrid automated optimisation procedure. The low-frequency optimum slanted septum core concept with an open area of 4.5% for the face sheet and 18% for the short slanted septum is predicted to have an absorption level of at least 14 dB in the frequency range of 400–1000 Hz for normally incident pure tone excitations at 150 dB. The slanted septum core concept with varying percentage open area, with broadband optimum design variables, is predicted to have good broadband sound absorption levels of at least 10 dB in the frequency range of 570–3800 Hz. Finally, the MultiFOCAL liner concept with optimised percentage open areas is predicted to have an excellent broadband sound absorption levels of at least 14 dB, for pure tone excitations at 150 dB, in the frequency range of 900–5300 Hz. This work will be followed by optimisation of the face sheet geometries of these novel liner designs in order to maximise lined duct attenuation for aircraft engine applications.

Failure analysis of 3D woven spacer sandwich composites with woven cross-links and face sheet thickening under compressive and flexural loading

The effect of face sheet thickening on the mechanical properties of sandwich structures reinforced with 3 D woven spacer fabrics having woven cross-links has been presented in this work. Spacer fabrics forming cells of rectangular (RECT), trapezoidal (TPZ) and triangular (TR) shapes were used as reinforcement of sandwich structures with non-thickened face sheets. While for thickened face sheet structures, additional layers of 2 D fabric were integrally woven with the top and bottom face sheets of spacer fabrics, having almost similar cell geometrical parameters as that of the former structures, in a single step of weaving. Composites manufactured from these structures were characterized for their out-of-plane compressive and flexural properties. The specific compressive load was observed to be lower for thickened face sheet structures. The flexural load was higher for thickened face sheet structures; but the specific bending load decreased for the same. Flexural stress was also found to decrease with thickening of the face sheets. The results obtained could be helpful in designing sandwich composites connected woven fabric cross-links with enhanced mechanical properties.