Abhendra K. Singh

Compression-After-Impact of Sandwich Composite Structures: Experiments and Simulation

A combined experimental and numerical study of compression-after-impact strength of honeycomb core sandwich composite panels is described. Barely-visible impact damage was induced using quasi-static indentation. Specimens consisted of 16-ply carbon-epoxy facesheets with an aluminum honeycomb core. The facesheet stacking sequence, core geometry and thickness were varied as was the indentor diameter to study the effect of these parameters on damage resistance and post-impact damage tolerance. Computational models of the quasi-static indentation and compression after impact are underway. Results to date compare the experimental and simulate indentations for 25.4 mm diameter indentors. Future work will include modeling the larger, 76.2 mm indentor as well as compression-after-impact.

Damage characterization of quasi-statically indented composite sandwich structures

The nature of quasi-static indentation damage is studied in aluminum honeycomb core sandwich panels with eight ply, quasi-isotropic, graphite/epoxy face sheets. Parameters that are varied include the core thickness, core density, face sheet layup, and indentor diameter. The majority of induced damage is in the vicinity of the barely visible threshold. The permanent dent in the panel is found to be always larger than the contact area of the indentor, and specimens with denser cores exhibit smaller dent diameters for a given dent depth. Regardless of specimen layup, delaminations occur essentially only at the 3rd, 5th, 6th, and 7th interfaces. Stiffer cores, either in terms of a higher density or, for those cores considered, a smaller thickness, result in lower dent depths, smaller dent diameters, and more face sheet delamination for a given indentation event. The manner in which core orthotropy influences the size and pattern of the delaminations is shown to depend on face sheet layup. Regardless of the core, larger delaminations occur in face sheets that contain 90° angle changes between the adjacent plies in comparison to those that contain only 45° angle changes. Results are compared to those previously reported in the literature, and mechanisms that are related to plate boundary conditions are described that reconcile what would appear to be conflicting results obtained by different studies. Findings are then discussed in the context of choosing the most damage-resistant structural configuration and how this translates to damage tolerance.

Effects of temperature, seawater and impact on the strength, stiffness, and life of sandwich composites

A study was conducted to determine the effects of subzero temperatures, seawater saturation, and low velocity impact on the flexural strength, stiffness, and life of sandwich composites. Specimens were tested statically and in fatigue using a four-point bending arrangement that included a combined metal and rubber load pad that spanned the inner loading heads and which prevented any local crushing damage. Both undamaged and impact damaged specimens were tested in room temperature (RT) dry, RT seawater saturated, -20°C dry, and -20°C seawater saturated environments. The impact energy level was chosen to produce damage that was observable, yet did not produce excessive permanent deformation. The primary results were that reducing the temperature tended to increase stiffness, strength, and fatigue life. Seawater saturation had minimal effects on strength, stiffness, or life, but influenced the static failure mode. Impact damage had little effect on the static results, but caused a significant loss in fatigue life. These results demonstrate that static test results cannot be used to infer fatigue behaviors, and indicate the need for the accurate determination of material and structural responses across the full range of expected usage environments.

Barely visible impact damage evaluation of composite sandwich structures

The nature of quasi-static indentation damage is st udied in aluminum honeycomb core sandwich panels with eight ply, quasi-isotropic, graphite/epoxy fac e sheets. Parameters that are varied include the co re thickness, core density, face sheet layup and inden tor diameter. The majority of induced damage is in the vicinity of the barely visible threshold. The perma nent dent in the panel is found to always be larger than the contact area of the indentor, and specimens with de nser cores exhibit smaller dent diameters for a giv en dent depth. Regardless of specimen layup, delaminations occur essentially only at the 3 rd , 5 th , 6 th and 7 th interfaces. Stiffer cores, either in terms of a higher density or, for those cores considered, a smaller thickness , result in more face sheet delamination for a given indentatio n event. Regardless of the core, larger delaminatio ns occur in face sheets that contain only 90° angle ch anges between adjacent plies in comparison to those that contain only 45° angle changes. Thus, when delamination area is considered as a damage resistance metr ic, a low density core with a [45/90/-45/0] s face sheet will provide the best results of those geometries considered. However, if dent area or dent depth is chosen as th e damage metric, this geometry will provide the lea st damage resistance, and a high density core with a [ ±45/0/90] s face sheet is best. These and similar results are discussed in the context of choosing the most damag e resistant structural configuration and how this translates to damage tolerance.

Damage resistance of aluminum core honeycomb sandwich panels with carbon/epoxy face sheets:

The damage resistance of composite sandwich structures with eight and 16 ply quasi-isotropic, carbon/epoxy face sheets and aluminum honeycomb core is evaluated. The external damage is induced quasi-statically, using spherical steel indentors under displacement control, to be in the vicinity of the barely visible threshold. In addition to the face sheet thickness, other parameters that are varied include the core thickness, core density, face sheet layup, and indentor diameter. The effect of these parameters on the extent of damage is evaluated using the damage metrics of dent depth, dent diameter, and planar area of delamination. When dent depth or dent diameter is considered as the damage metric, specimens containing a higher density core are always found to be the most damage resistant. When planar area of delamination is considered as the damage metric, the eight ply configuration comprised of a lower density core and face sheets containing only small ply angle changes are found to be the most damage resistant. However, this configuration is found to be the least damage resistant when this damage metric is applied to the 16 ply specimens. Rather, the best delamination resistance is provided by a 16 ply configuration with a high density core and face sheets that have ±45° ply groups at the beginning and ending of their stacking sequence.

Compressive strength of honeycomb-stiffened graphite/epoxy sandwich panels with barely-visible indentation damage

This study examines the parametric effects of core density, core thickness, face-sheet stacking sequence, and indentor diameter on the compressive strength of aluminum honeycomb-core sandwich panels stiffened with eight-ply, quasi-isotropic, graphite/epoxy face sheets. The sandwich panels contained damage at the threshold of visual detectability created through quasi-static indentation with 25.4 mm or 76.2 mm-diameter spherical indentors. During compression-after-indentation testing, failure occurred due to: dent deepening followed by localized, compressive micro-buckling of fibers in the 0° plies; localized buckling of the near-free-surface sub-laminates; or unstable dent growth in the direction lateral to the applied compressive load. Regardless of failure mode or face-sheet type, the compression-after-indentation strength increased with increasing core thickness and with decreasing core density. Additionally, panels containing face sheets with the 0° plies near the mid-plane and 45° angle change between subsequent plies exhibited greater undamaged compressive strength and higher compression-after-indentation strength relative to panels containing 90° angle changes between subsequent plies and 0° plies near the free surface. The compression-after-indentation strength was found to be relatively unaffected by the indentor diameter size and the resulting variations in the face sheet and core damage. These results imply that precise representation of the damage state in models to predict the post-indentation response of sandwich panels may not be necessary in order to make accurate average residual strength predictions.

Fatigue behavior of double-edge notched oxide/oxide ceramic matrix composite in a combustion environment

Tension–tension fatigue tests in a combustion environment were performed on double-edge notched oxide/oxide ceramic matrix composite specimens. The composite, designated as N720/A, constituted woven 0°/90° Nextel™720 fibers in alumina matrix. Monotonic tensile and cyclic loads at a frequency of 1 Hz and a stress ratio of 0.05 were applied on the specimens in a combustion environment. The maximum specimen temperature due to combustion flame impingement in the notch region was 1250 ± 50℃. A stiffness reduction of less than 10% evaluated for the run-out specimens showed the harsh combustion environment had a minimal effect on specimen degradation. The residual strength was evaluated to be ∼75%–85% the strength of non-fatigued (virgin) double-edge notch specimens in room temperature. The monotonic tensile strength and the fatigue limit for 90,000 cycles (run-out) were found to be ∼40 MPa less in the combustion environment when compared to published results for 1200℃ laboratory air environment. The damage mechanisms were also the same in the two environments. Finite element analyses showed that the reduction in strength and fatigue limit in the combustion environment (as compared to the laboratory air environment) was due to the presence of thermal gradient stresses because of non-uniform specimen temperature distribution.

Compressive strength of aluminum honeycomb core sandwich panels with thick carbon–epoxy facesheets subjected to barely visible indentation damage:

An experimental study of damage tolerance under quasi-static indentation (QSI) was performed for sandwich composite panels consisting of 16-ply carbon–epoxy facesheets bonded to an aluminum honeycomb core. To determine how indentation damage and compression strength after indentation depend on the facesheet layup, three facesheet stacking sequences were used, varying the maximum ply angle change and placement of the outermost 0° ply. Similarly, to determine the effect of core parameters on damage and strength following indentation, three cores with varying density and thickness were studied. Specimens were indented in QSI to the barely visible indentation damage threshold by spherical indenters of 25.4 or 76.2 mm diameters. Damaged specimens were tested to failure in compression to determine the post-indentation compressive strength and resulting failure mode. Compression-after-indentation (CAI) strength is compared to the undamaged strength obtained from edgewise-compression tests of specimens with the same geometry type. Three distinct failure modes were observed in the CAI experiments: compressive fiber failure, delamination buckling and global instability. Post-indentation compressive strength was independent of indenter size and there was no clear propensity for a particular failure mode dependent on a given specimen geometry. Specimens with a high core density and facesheets with a primary ply angle change of 90° were found to be the most damage resistant. Specimens with facesheets having the outer 0° plies closest to the center of the laminate were found to be the most damage tolerant.

Combined Experimental/Numerical Assessment of Compression After Impact of Sandwich Composite Structures

An integrated experimental and computational study of residual compressive strength of composite laminate sandwich structures after low velocity impact (CAI strength) is performed using samples consisting of 8 ply graphite/epoxy face sheets bonded to aluminum honeycomb core. The study encompasses characterizing the indentation damage (dent depth and laminate fractures), measuring the CAI strength for a range of layups and core densities and computational modelling of indentation and CAI strength.

Effect of UVB Light Exposure on Tensile Behavior of Carbon Nanotube Sheet

The effects of ultraviolet B (UVB) exposure on the tensile behavior of CNT sheet was investigated in this study. Two types of CNT sheet, one acid treated and one un-treated, were directly exposed to UVB light for 500 hours. The exposure was done using a UVB lamp inside a dark chamber under room temperature ambient condition. The microstructure of the CNT sheets were studied both prior to and after UVB exposure using a scanning electron microscope. Upon completion of the exposure duration, the CNT sheet test coupons were tested mechanically in tension using a microtester to evaluate the tensile strength and behavior. The results were compared to those of the CNT sheet test coupons that were not exposed to UVB light. It was observed that the strength of the acid treated CNT sheet decreased after UVB exposure while the strength of the un-treated CNT sheet increased. Apart from slight changes in stiffness, the overall mechanical behavior with increasing applied load did not show much change after the exposure to UVB light. The microscopic analysis showed evidences of morphological changes in the CNT microstructure upon UVB exposure. These changes supported the change in mechanical strength as a result of the UVB exposure.