Joachim L. Grenestedt

Development of a new peel-stopper for sandwich structures

A device referred to as a peel-stopper with the purpose of eliminating the risk of catastrophic skin debonding in sandwich structures in general, and high-speed ships in particular, has been developed. Two major advantages of the new peel-stopper are its simplicity of manufacture, and its lack of connections through the sandwich core. The latter is essential for good UNDEX performance. Two different versions were developed. Quasi-static mechanical tests were performed to verify the function of the peel-stopper, and the strength of the structure incorporating the peel-stopper. Regarding functionality, the peel-stoppers performed as expected. Regarding quasi-static strength, it was shown that the peel-stoppers could be made such that no structural strength reduction occurs.

Dynamic Performance of a Peel Stopper for Composite Sandwich Ship Structures

Devices known as peel stoppers were manufactured in composite sandwich panels and tested under dynamic conditions. Peel stoppers aim to arrest peeling or debonding of the composite skin from the core in composite sandwich ship hulls. The present work addresses the effectiveness of the peel stopper when faced with dynamic peeling of the skin. Several ship panels were produced and tested. Some tests to investigate whether the debonding could behave in a way that could defeat the peel stopper were also performed. Various issues surrounding the manufacture of the device were addressed. Original contributions include the manufacturing procedure of sandwich panels with multiple peel stoppers, the testing of panels (as opposed to beams) containing peel stoppers, and dynamic testing of peel stoppers.

Closing the Loop in Dynamic Soaring

This paper examines closed-loop dynamic soaring by small autonomous aircraft. Wind eld estimation, trajectory planning, and path-following control are integrated into a system to enable dynamic soaring. The control architecture is described, performance of components of the architecture is assessed in Monte Carlo simulation, and the trajectory constraints imposed by existing hardware are described. Hardware in the loop simulation using a Piccolo SL autopilot module are used to examine the feasibility of dynamic soaring in the shear layer behind a ridge, and the limitations of the system are described. Results show that even with imperfect path following dynamic soaring is possible with currently existing hardware. The eect of turbulence is assessed through the addition of Dryden turbulence in the simulation environment.

Wind field estimation for autonomous dynamic soaring

A method for distributed parameter estimation of a previously unknown wind field is described. The application is dynamic soaring for small unmanned air vehicles, which severely constrains available computing while simultaneously requiring updates that are fast compared with a typical dynamic soaring cycle. A polynomial parameterization of the wind field is used, allowing implementation of a linear Kalman filter for parameter estimation. Results of Monte Carlo simulations show the effectiveness of the approach. In addition, in-flight measurements of wind speeds are compared with data obtained from video tracking of balloon launches to assess the accuracy of wind field estimates obtained using commercial autopilot modules.

Improving joints between composites and steel using perforations

The elastic mismatch between fiber reinforced composites and steel makes joining them difficult. One way to reduce this mismatch is to decrease the average stiffness of the steel by introducing perforations. Further, this has the possible advantage of mechanical interlocking. In the present study, stainless steel strips were perforated with circular or triangular holes. The hole sizes were smaller near the edge of the composite, and increased with increasing depth in the composite, thus creating a graded steel component. Various surface preparations, perforation fillers, and degrees of perforations were investigated. The steel strips were joined to the composite by inserting them between glass fiber fabrics before vacuum assisted resin transfer molding the composite. Tensile experiments were performed, and the quasi-static strength was used as the measure by which the different joints were compared.

Experimental evaluation of a balsa sandwich core with improved shear properties

End grain balsa is widely used as a core material in sandwich structures, especially in marine applications where the shear properties are of major importance. In a recent study, a new balsa core structure that better utilizes the anisotropy of balsa was designed and numerically analyzed. An improved shear stiffness was predicted with this new core. In the present study, manufacturing methods for this core were developed and more extensive experiments were performed to verify the improvement in shear stiffness as well as to evaluate the shear strength.

Influence of electric artwork on thermomechanical properties and warpage of printed circuit boards

The electric copper lines in printed circuit boards (PCBs) introduce anisotropy in the various layers in multilayered PCBs. This anisotropy in combination with a general lack of symmetry with respect to the PCB midplane is a cause for thermally induced warpage. An analytical model which can predict the thermomechanical properties and warpage of PCBs with various electric artworks was developed. The model is based on Kirchhoff plate theory. An effective medium assumption was employed for the layers consisting of copper lines and composite side by side. The properties of the effective medium were obtained by a combination of Voigt (constant strain) and Reuss (constant stress) predictions, in conjunction with three-dimensional finite element unit cell calculations to determine a few constants. The prediction of the model conform well to some recent experiments. The model was finally used to “tune” the electric artwork of a PCB to minimize thermally induce warpage by changing trace widths and spacings in a si...

Recent progress in CFD for naval architecture and ocean engineering

An overview is provided of CFDShip-Iowa modeling, numerical methods and high performance computing (HPC), including both current V4.5 and V5.5 and next generation V6. Examples for naval architecture highlight capability and needs. High fidelity V6 simulations for ocean engineering and fundamental physics describe increased resolution for analysis of physics of fluids. Uncertainty quantification research is overviewed as the first step towards development stochastic optimization.

Adhesive tongue-and-groove joints between thin carbon fiber laminates and steel

A tongue-and-groove joint geometry for thick composite laminates has been proposed in recent literature. The present study experimentally investigated nine geometrically different tongue-and-groove designs between a thin carbon fiber composite and stainless steel. The specimen width and tongue length were kept constant, but the number of tongues and their tapers were varied. One version with a lollipop like geometry was also investigated. The carbon fiber specimens were made by oven cured prepregs, the tongue and grooves were cut using a waterjet cutter, and the specimens were joined with an epoxy paste adhesive. Uniaxial tension tests were performed. The tongue-and-groove specimens were observed to fail by the adhesive regardless of joint type. Conventional single lap joints using the same materials, the same widths, and overlap lengths equal to the tongue lengths of the tongue-and-groove specimens, were also manufactured and tested. The tongue-and-groove specimens with large aspect ratio tongues were stronger than the conventional single lap joints.

Test of a redesigned glass-fiber reinforced vinyl ester to steel joint for use between a naval GRP superstructure and a steel hull

Abstract The joining of metal structures and composites has a number of applications in engineering fields, for example in naval ship structures. In the present research, the joint design type A from Clifford et al.’s recent paper [Compos. Struct. 57 (2002) 59] was replicated with different glass fiber, vinyl ester and core material. The joint was then redesigned by moving the steel edge away from an area of high stress concentrations. Specimens of both the original and new designs were made and tested. The new joint was lighter, and the limited number of tests suggests that it was stronger as well.