Abstract Salt water environments are very harsh on materials that are used within them. Many issues are caused by either corrosion and/or internal degradation to the materials themselves. Composites are better suited for this environment due to their high strength to weight ratios and their corrosion resistance, but very little is known about the fracture mechanics of composites. The goal of this study is to gain a better understanding for the behavior of a composite boat hull under a shear loading, similar to the force water applies on the hull as the boat moves through the water; then attempt to strengthen the composite sandwich panel against the shear loading. A parametric study was conducted to investigate monotonic in-plane shear loading for composite sandwich panels used in commercial naval vessels. In order to model a conventional composite boat hull, test specimens were composite sandwich panels made of a Divinycell H100 foam core with four layers of fiberglass on both sides of the core. Specimens were tested under a monotonic loading with a rate of 0.2 in/min, and tested until complete failure using the standard test. Seawater specimens were manufactured in the same manner as the original test specimens, but then were submersed in either filtered seawater or the ocean. The differences between the filtered pieces and the ocean allowed us to determine if any changes found in the composite sandwich panels were related to environment conditions or if the changes were related to the saltwater interaction itself. To create these different environments the seawater specimens were taken to the Avila pier where 36 specimens were placed in a tub that was fed filtered saltwater, while 30 specimens were placed in a plastic mesh with weights and lowered to a depth of approximately 30 ft. in the ocean. Three specimens were then removed at monthly intervals from both filtered and ocean environments. Shear Keys were created as a method to strengthen the composite sandwich panels against the shear force that the previous specimens had been tested to. Eight Shear Keys were then placed into groves cut into the foam core (four on each side) and the four fiberglass layers were laid on top. Testing showed that the seawater did have an initial effect on the composite sandwich panels. The filtered pieces showed a decrease in yield strength and stiffness the longer they were subjected to the seawater. The raw unfiltered pieces placed in the ocean saw an even higher decrease in their yield strength and decrease in stiffness. However, for both the unfiltered and raw specimens there was an increase in the ultimate strength and fracture point of the specimens. The effects of the sea water seemed to taper off after the 3rd month however. The Shear Key specimens were tested with a 4mm and an 8mm Shear Key. The 8mm Shear Keys showed a decrease in shear strength, which was primarily due to removing too much material from the core and weakening the specimen. It was concluded that the decrease in area created a force concentration at the deepest part of the Shear Key causing the premature failure. The 4mm Shear Key showed an increase in the yield strength, ultimate strength, and fracture point. A finite model was built to simulate the original test specimen along with the 4mm and 8mm Shear Key cases, and the results were compared to the experimental results. The numerical results showed that it was possible to relate the experimental results to the linear or elastic portion of the plots. There was a difference between the maximum displacement of the model and the actual specimens, but this was attributed to potential inaccurate comparison of the loading on the model compared to the actual specimens. The correlation between the model itself and the experimental data was close enough to conclude that it could be used for predicting baseline trends. Further investigation of the specimens should include looking into the effects of a cyclic shear loading on the specimens. This combined with the seawater element used in this thesis would provide further insight to the initial degradation seen in the seawater specimens, and could potentially provide a closer relation to current hull failures. In addition to including a cyclic loading another numerical model should be created. A model that could be constrained both locally and globally would provide more accurate results. The FEM should also include the ability to run a crushable foam core model within the solver which would also increase the accuracy of the numerical solution.

Over the last three decades, the aerospace industry has gradually shifted from metals to composites in many different applications due to the lightweight properties of composite materials. Some benefits of composites include higher strength-to-weight ratio and corrosion resistance. At this point in time, the composite industry researchers are focusing on renewable and sustainable materials (bio-composites). By understanding the structural capabilities of bio-composites that have been used for centuries, new developments of sustainable materials will spark more interest throughout the industry. Bio-composites include fibers such as hemp, bamboo, flax, etc. The high demand for bio-composites in composite structures can also reduce raw material costs. This study investigated, through experimental and numerical analysis, the mechanical behavior of sandwich panels under edgewise compressive loading. The first task of the study was to use four different facesheet materials and the same Nomex honeycomb core. The number of facesheet layers consecutively increased from one layer to four layers on each side of the core for each material. The facesheet materials used were Hexply AGP280-5H Carbon Fiber Pre-Preg, B601 Plain Weave Hemp, D118DKBR Split Herringbone Weave Hemp, and NB308T 7725 Texalium Fiberglass Pre-Preg. The sandwich panels were cured using a composite heat press and followed the recommended cure cycle for the material’s resin matrix. The variation of the facesheet materials while keeping the core consistent showed how the edgewise strength and displacement of the composite sandwiches were affected under compressive loading. The second task of the study was to try and create a multifunctional hybrid composite sandwich with two different facesheet materials; using one hemp material and one pre-preg material. The goal of this task was to try and minimize the damage occured upon failure. Being that the pre-preg materials are more brittle than the hemp material, the hybrid composite sandwiches can potentially create a superior composite structure. The sequence of stacking of the facesheet materials was manipulated to study how changing the outer and inner layers affected the results. All the specimen were loaded at a rate of 0.05 in/min in a steel jig specifically made per ASTM C364 standard using an Instron 8801 to determine the mechanical behavior. These experimental results combined with results from theoretical and finite element analysis using Matlab and Abaqus, respectively, were used to compare composite sandwich designs under compressive loadings. Failure mode comparison between the individual material composites and the hybrid composites were also discussed.

The goal of this effort was to build an analysis tool for aircraft wings and incorporate it into a program that optimizes the wing structure for the lowest weight for the conceptual design phase. The analysis tool calculates the internal stresses in primary load carrying members using established analysis methods for semimonocoque beams. Via a Graphical User Interface (GUI) built in MATLAB, the user can define the structural layout and material properties of the load carrying members. The program requires a degenerate geometry model built with Vehicle Sketch Pad (OpenVSP) to define the outer mold line (OML) of the wing, and a section of the program in MATLAB calculates the geometric parameters of the wing structure based on the model and user inputs in the GUI. Another section generates a lift curve using a Schrenk distribution, the vehicle weight, and load factors as defined by the user. The GUI also allows the user to define other external loads in addition to the aerodynamic loads. With the loads and structural model defined, the program uses the analysis tool to find a minimum structural weight while maintaining positive structural margins for all structural members. The analysis tool was compared against examples in structural analysis books from Bruhn and Peery to validate the method. The average relative difference between the normal and shear stresses calculated by the tool and the sources was 1.6%. To test the program, a Cessna 210G wing was modeled in the program and using Finite Element software. The comparison showed the tip deflection of the MATLAB model was 1.4 times that of the Finite Element Model. When the areas of the structural members were multiplied by 1.4, the normal stress in the stiffeners had an average difference of 5.8 ksi and the shear stresses in the webs had an average difference of 0.33 ksi. The program estimated the weight to be 198 lbs, underestimating the weight when compared to other existing methods.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 102-111).

In order to design a successful space system, the unique challenges of the space environment it will operate in must be considered during the design process. Atomic oxygen (AO) is a detrimental environmental effect found in Low Earth Orbit (LEO) that affects spacecraft surfaces by oxidizing and eroding material over time, particularly polymers. Carbon fiber/epoxy composites are a commonly used spacecraft material affected by AO exposure. Carbon composites are used as a structural material, such as on solar panels; their large surface area therefore is a potential contamination source to sensitive components. The Space Environments and Testing Lab at California Polytechnic State University, San Luis Obispo (Cal Poly SLO) includes an apparatus that can simulate AO in the LEO environment. This apparatus was used to expose carbon composite samples to AO before being tested for short beam strength to measure the effect on material properties. Results showed no significant difference in short beam strength for a 24-hour AO exposure compared to unexposed samples, but a 4% decrease for samples with a 48-hour exposure. Previous work at Cal Poly SLO found that AO-exposed composite generated particulate contaminants. Tape lift tests and mass measurements of samples were conducted before and after AO exposure to characterize the particulate contamination generated and percent mass loss. It was found that AO exposure increased the percent mass loss by 1.5% for 24-hour exposure and 3% for 48-hour exposure. The tape lift percent area coverage increased by 2.5% near sample ends and 0.35% in the middle after AO exposure.

Polymer thin membranes are used in a variety of deployable structures that require large areas and compact stowage. Packaging membrane structures often involves creasing the membrane along predefined fold lines to enforce the desired kinematics under folding action. Inducing permanent deformation by folding to a high curvature is a common method to create creases, particularly in the design of solar sails. The distinct mechanical characteristics at the crease regions have a profound effect on the subsequent deployment and tensioning of the membrane structures. The mechanical and geometric properties at the crease are related to the crease formation process, but the relationship is not well understood due to the presence of viscoelasticity and plasticity. This thesis seeks to investigate the relation between permanent material deformation and creasing behavior. In particular, creasing experiments are performed on polyimide thin films to identify the conditions for creasing onset. Uniaxial tension yield tests are conducted to relate material yielding with creasing onset.

Lattice materials are used as the core of sandwich panels to construct light and strong structures. This thesis focuses on metallic sandwich structures and has two main objectives: (i) explore how a surface treatment can improve the strength of a lattice material and (ii) investigate the collapse response of two competing prismatic sandwich cores employed in ship hulls. First, the finite element method is used to examine the effect of carburisation and strain hardening upon the compressive response of a pyramidal lattice made from hollow tubes or solid struts. The carburisation surface treatment increases the yield strength of the material, but its effects on pyramidal lattices are not known. Here, it is demonstrated that carburisation increases the plastic buckling strength of the lattice and reduces the slenderness ratio at which the transition from plastic to elastic buckling occurs. The predictions also showed that strain hardening increases the compressive strength of stocky lattices with a slenderness ratio inferior to ten, but without affecting the collapse mode of the lattice. Second, the quasi-static three-point bending responses of simply supported and clamped sandwich beams with a corrugated core or a Y-frame core are compared via experiments and finite element simulations. The role of the face-sheets is assessed by considering beams with (i) front-and-back faces present and (ii) front face present, but back face absent. These two beam designs are used to represent single hull and double hull ship structures, and they are compared on an equal mass basis by doubling the thickness of the front face when the back face is absent. Beams with a corrugated core are found to be slightly stronger than those with a Y-frame core, and two collapse mechanisms are identified depending upon beam span. Short beams collapse by indentation and for this collapse mechanism, beams without a back face outperform those with front-and back faces present. In contrast, longbeams fail by Brazier plastic buckling and for this collapse mechanism, the presence of a back face strengthens the beam. Third, drop weight tests with an impact velocity of 5 m/s are performed on simply supported and clamped sandwich beams with a corrugated core or a Y-frame core. These tests are conducted to mimic the response of a sandwich hull in a ship collision. The responses measured at 5 m/s are found to be slightly stronger than those measured quasi-statically. The measurements are in reasonable agreement with finite element predictions. In addition, the finite element method is used to investigate whether the collapse mechanism at 5 m/s is different from the one obtained quasi-statically. The predictions indicate that sandwich beams that collapse quasi-statically by indentation also fail by indentation at 5 m/s. In contrast, the simulations for beams that fail quasi-statically by Brazier plastic buckling show that they collapse by indentation at 5 m/s. Finally, the dynamic indentation response of sandwich panels with a corrugated core or a Y-frame core is simulated using the finite element method. The panels are indented at a constant velocity ranging from quasi-static loading to 100 m/s, and two indenters are considered: a flat-bottomed indenter and a cylindrical roller. For indentation velocities representative of a ship collision, i.e. below 10 m/s, the predictions indicate that the force applied to the front face of the panel is approximately equal to the force transmitted to the back face. Even at such low indentation velocities, inertia stabilisation effects increase the dynamic initial peak load above its quasi-static value. This strengthening effect is more important for the corrugated core than for the Y-frame core. For velocities greater than 10 m/s, the force applied to the front face exceeds the force transmitted to the back face due to wave propagation effects. The results are also found to be very sensitive to the size of the flat-bottomed indenter; increasing its width enhances both inertia stabilisation and wave propagation effects. In contrast, increasing the roller diameter has a smaller effect on the dynamic indentation response. Lastly, it is demonstrated that material strain-rate sensitivity has a small effect on the dynamic indentation response of both corrugated and Y-frame sandwich panels.

This thesis presents a parametric study on the effects of how damage arrestment devices application interacts with a fastener in a composite sandwich panel. The primary objective of the damage arrestment device was to prevent the failure of the composite face sheet, such as crack propagation, around the hole/fastener joint. The damage arrestment devices are made of composite strips that are inserted under the face sheet to increase the overall structural strength of the panel and to prevent the propagation of failure along the hole. This was supposed to be a quicker and stronger alternative to potted inserts for composite sandwich panels for designer. The manufacturing curing cycle of the composite sandwich specimens has been carried out by using a Tetrahedron Composite Air Press. The press has been used to fabricate composite sandwich panels by applying constant pressure and variable heat to create panels with dimensions of 5” x 2” x .552”. The panels were stacked using a polyurethane foam, Last-A-Foam FR-6710 with two layers of a carbon-fiber/epoxy weave, LTM45, on both sides of the foam. The specimens were loaded under a compressive strain of 0.5 mm/min. The damage arrestment devices’ thickness was varied and tested under both monotonic and fatigue loading. The experimental results indicate that as the thickness of the device increased the overall strength of the part increased at a parabolic curve with it topping at a thickness of 0.065” and a strength increase of 109%. Under fatigue loading, a control group test case and damage arrestment device configuration case was tested. The experimental results indicate that both cases have similar fatigue trends but shows that the damage arrestment specimens are stronger due to the increase of structural strength. The experimental results were compared with numerical results or Finite Element Model. The results showed that numerical results can capture the linear or elastic portion of the experimental results having identical Elastic Modulus values. The models do differ in the maximum displacement of the specimen and the failure mode around the hole of the composite sandwich panel. The discrepancy in displacement and the failure mode was attributed to inaccurate loading on the hole of the composite sandwich panel and non-linear modeling of the solution. The correlation between the FEM and the experimental data was good enough in predicting the trends of the composite sandwich panels.

Thesis (Ph. D.)--School of Aerospace, Mechanical and Mechatronic Engineering, Graduate School of Engineering, University of Sydney, 2005. / Bibliography: leaves R1-R20.

Thesis (Ph. D.)--University of Sydney, 2000. / Title from title screen (viewed 27 February 2007). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Aeronautical Engineering. Includes bibliographical references. Also issued in print.