Sarah E. Mouring

Buckling and postbuckling of composite ship panels stiffened with preform frames

Abstract The U.S. shipbuilding industry recently has started incorporating composite materials in the construction of both military and commercial ships due to the advantages of composite construction. These advantages include the reduction in total life costs, corrosion resistance, high strength- and stiffness-to-weight ratios, and improved stealth for military applications. One disadvantage is the higher costs of composites compared to steel and other conventional materials. Therefore, new higher quality materials with lower costs and new fabrication methods need to be developed before composite materials will be fully accepted for the construction of large ships. A new composite preform framing technology shows promise in the reduction of fabrication costs for large ship construction. There already has been significant cost savings using this framing technology in the construction of small recreational boats and large yachts. This framing technology involves casting a dry glass fiber-reinforced plastic (GRP) fabric into shape in a closed mold with a foam core. One unresolved issue using this framing technology is the orientation of the fiber for the frames. This paper summarizes experimental results of the testing of composite panels stiffened with preform frames under in-plane uniaxial compressive loads. Biaxial (0,90), quadaxial (0,90,+45,−45), and triaxial (+45,−45,0) laminates were used in the frames.

Multibody Modelling of Wave Energy Converters Using Pseudo-Spectral Methods With Application to a Three-Body Hinge-Barge Device

Multibody wave energy converters are composed of several bodies interconnected by joints. Two different formulations are adopted to describe the dynamics of multibody systems: the differential and algebraic equations (DAEs) formulation, and the ordinary differential equations (ODEs) formulation. While the number of variables required for the description of the dynamics of a multibody system is greater in the DAE formulation than in the ODE formulation, the ODE formulation involves an extra computational effort in order to describe the dynamics of the system with a smaller number of variables. In this paper, pseudo-spectral (PS) methods are applied in order to solve the dynamics of multibody wave energy converters using both DAE and ODE formulations. Apart from providing a solution to the dynamics of multibody systems, pseudo-spectral methods provide an accurate and efficient formulation for the control of multibody wave energy converters. As an application example, this paper focuses on the dynamic modeling of a three-body hinge-barge device, where wave-tank tests are carried out in order to validate the DAE and ODE models against experimental data. Comparison of the ODE and DAE PS methods against a reference model based on the straightforward (Runge-Kutta) integration of the equations of motion shows that pseudo-spectral methods are computationally more stable and require less computational effort for short time steps.

Reinforced concrete beams externally retrofitted with advanced composites

This study investigates the effect of externally bonded carbon fiber reinforced polymer (CFRP) laminates on the ductility of reinforced concrete beams used in the repair of damaged bridge structures. Reinforced concrete structures deteriorate over time due to environmental aging, fatigue, excessive loading, chemical attack, and other factors. Strengthening and rehabilitating these concrete structures by externally bonding carbon laminates is one of many economical engineering solutions. Eight rectangular beams with varying internal steel reinforcement were retrofitted with CFRP strips on the tension faces and tested under four-point bending. The beams were instrumented to monitor strains, deflection, and curvature over the entire spectrum of loading, and determine the structural response of the beams. An existing analytical model using the discrete yield and ultimate values of the load-deflection and moment-curvature curves was modified to an energy-based model, and used to predict the ductility of the beams. Numerical results indicated an increase in strength, a decrease in ductility, and validated the analytical model. Ultimately, this study will aid in the development of design guidelines governing the use of CFRP.

Structural response of impact-damaged composite panels

Internal damage due to impulse events, such as impact or shock, may not be detected by visual inspection in composite structures. This internal damage can cause significant reductions in strength and may lead to catastrophic failure. Therefore there is a need first to understand completely the effects of an impact event on a composite material. As part of this research project, a simple energy model has been used to better quantify the behavior of glass reinforced polymers (GRP) panels subjected to impact. Sixteen 0.229 m times 0.178 m times 0.006 m woven roving GRP panels were impacted with varying energy levels and tup sizes to compare the experimental results to that of the energy model

Hybrid steel-to-composite joint behavior under tensile loading

Hybrid steel-to-composite joints are being used more commonly for load bearing applications. However, these hybrid joints usually entail geometry and material discontinuities which can induce stiffness mismatch and cause local stress concentrations. The shock impedance mismatch caused by the different wave propagation characteristics can also be crucial to the structural response of the hybrid joints under impulsive loads. Recent research at Imperial College London (ICL) and the U.S. Naval Academy (USNA) has focused on characterizing the behavior and ultimate load capacity of hybrid steel-to-composite joints with different configurations under various loading conditions. This paper presents results from tensile strength testing of steel-to-composite double lap joints, comparing pseudostatic strength with dynamic strength and comparing joints that exploit perforated steel plates with those manufactured with non-perforated steel plates. An intentional manufacturing flaw also was incorporated into half of the joints, both perforated and non-perforated joints, in order to assess the effect of this flaw type on joint strength. Finite Element Analysis (FEA) results are compared to experimental results for both perforated and non-perforated joints.

Measured and predicted response of a submerged towed sonar array to maneuvering input

There exists a necessity to increase a helicopter pilots safety during mine countermeasure operations. Models have been developed for scenarios involving a cable that is fully submerged in either air or water, but there has been very little testing when the cable is partially submerged. This change in fluid type along the length of the cable has a significant impact on the stability and drag forces that has not yet been extensively studied. A series of experimental investigations examining speed, acceleration, length of cable and path of motion were paired with predictions from a simulation for the purpose of developing a predictive tool. These tests characterized the stability of the body, indicating that acceleration is a governing factor in the operability of the towed sonar. The simulation developed to predict the behavior of the helicopter towed submerged body predicts the fundamental behavior of the body, but does not capture hydrodynamic instabilities that result in the body leaving the water.

Behavior of perforated metal-to-composite joints subjected to tensile loading

Advanced composite materials have many advantages compared to traditional metallic materials including high stiffness- and strength-to-weight ratios, corrosion resistance, damage tolerance, and improved stealth characteristics. Thus, they are being used more often as primary structural members in both civil and military applications. However these materials are not typically applied in isolation due to insufficient stiffness and ductility of composites compared to metallic materials, leading to a rapid expansion of interests in metal and composite combined structures. The design of hybrid metal-to-composite joints is one of the major structural challenges in this area. Hybrid metal-to-composite joints usually entail both material and geometry discontinuities which lead to stiffness mismatch and local stress concentrations. Current research at the U.S. Naval Academy and Imperial College London shows that among the various types of novel metal-to-composite joints, perforated hybrid steel-to-composite joints demonstrate significant potential in naval structural applications. Perforations are cut into the steel increasing the cohesion between steel and composite part of the hybrid joint thus improving the transfer of load between the two parts. In addition to the benefit of mechanical interlocking, it is believed that the perforated steel plate decreases the elastic mismatch between the stiff steel part and the relatively compliant composite part. This paper reviews an Office of Naval Research - sponsored research project focusing on a perforated metal-to-composite joint design subjected to tensile loading.