Steven Huybrechts

Manufacturing Theory for Advanced Grid Stiffened Structures

Abstract Lattices of rigidly connected ribs, known as advanced grid stiffened (AGS) structures, have many advantages over traditional construction methods, which use panels, sandwich cores and/or expensive frameworks. The technology behind these structures has progressed significantly during the past five years to the point where these structures are being integrated into operational systems. Two tooling methods for fabricating these structures using composite materials have proven to be highly effective at achieving high quality, low cost AGS structures: the hybrid tooling method and the expansion block method. Both methods rely on a precise understanding of tooling behavior during cure to achieve proper consolidation, often determined through trial and error. This paper proposes a theory governing the behavior of both tooling types during the cure cycle in order to minimize the trial and error required to understand tooling expansion during cure. The theory is validated by experimental data.

Validation of the Quadratic Composite Failure Criteria with Out-of-Plane Shear Terms:

The Tsai–Wu Quadratic Failure Criterion is a modified tensor polynomial criterion, that is widely used and the most readily accepted failure criterion for orthotropic materials. While it is an excellent criterion for the majority of composite structures, there is limited experimental validation of this criterion when predicting the failure envelope for “combined shear” failures or, better stated, failures that involve both 2-D (in-plane) and 3-D (out-of-plane) shear stresses in an orthotropic material or laminate. Since several common types of material failure involve complex stress states, the ability to incorporate the effect of 3-D shear stress into failure prediction is attractive. The 3-D Tsai–Wu quadratic failure criterion that incorporates multiple shear stresses is presented in this text and tests were conducted on several laminate specimens in an effort to validate this criterion. Failure load predictions were made to validate the modified 3-D criterion and compare the results with the 2-D Tsai–Wu criterion for specialized cases where multiple shear stresses were present at failure. This testing clearly shows strong agreement between the 3-D criterion’s predictions and experiments for these cases, implying that the standard 2-D Tsai–Wu quadratic failure criterion can be safely extended, through the addition of terms, to 3-D cases.

Processing Induced Warpage of Filament Wound Composite Cylindrical Shells

In this paper a systematic procedure is developed to eliminate the processing induced warpage in filament wound and fiber placed composite parts. This is accomplished by first developing a through-thickness strain model based on fiber/resin cure consolidation (also referred to as a compaction) and tooling thermal expansion. The lay-up or stacking sequence can be arbitrary (i.e., symmetric or asymmetric). The strain profile model is then integrated into classical laminate theory and solutions for predicting and eliminating warpage are obtained. The accuracy of both solutions is evaluated by comparison with experimental data. To facilitate this, cylindrical test specimens were manufactured and the cure consolidation and warpage measured. It was found that the predictions were accurate and the warpage could be reduced and eliminated in most cases. The majority of cure consolidation in composites results from resin bleed-out and evacuation of entrapped air (voids). The magnitude is dependent on manufacturing parameters including cure pressure, winding tension, and material characteristics (i.e., pre-preg fiber volume fraction, resin viscosity, etc.). The strain profile that develops is set once the resin cures and is therefore not a hygrothermal phenomenon and is independent of cure temperature, or finished part operational environment.

Optimal design of composite ChamberCore structures

Abstract An optimization procedure has been developed to uniquely and efficiently determine the “best” local geometry design of a new composite ChamberCore structure. This procedure is based on minimization of the total mass of a single composite ChamberCore subject to a set of design and stress constraints. The stress constraints are obtained in closed form based on the composite box-beam model for various composite lamination designs and loading conditions. The optimization problem statement is constructed and then solved using the VMCON optimization program, which is an iterative sequential quadratic programming (SQP) technique based on Powells algorithm. The sensitivity of the solution of the optimal geometry to the values of parameters that characterize the structural durability and the failure mechanism is discussed.

The Effect of Varying Thickness on the Buckling of Orthotropic Plates

Although stability failure of constant thickness plates is a fairly well understood problem, buckling of plates with varying thickness has seen little research. This is not a common problem, but buckling of varying thickness plates does occur, as in the case of Advanced Grid Stiffened structures. These structures are characterized by lattices of rigidly connected rib stiffeners. While real-world Advanced Grid Stiffened structure ribs are modeled as orthotropic plates, they often have complex cross-sections (i.e., varying thickness). Unfortunately, failure analysis techniques for these ribs have been limited to rectangular cross-sections. To address this shortcoming, a buckling theory is presented for orthotropic plates of varying thickness. Orthotropic plates with both linear and hourglass thickness variations are considered. These are common grid structure cross-section geometries resulting from existing manufacturing processes. For both geometries, results are given that allow designers to predict how these plates will behave, relative to a constant thickness plate, for a variety of material properties and plate aspect ratios.

Air force research laboratory's technology programs addressing deployable space optical systems

The US Air Force Research Lab (AFRL) has integrated several technology development efforts together to form a cohesive approach for enabling deployable optical systems in the future. Aperture size dominates the cost/architecture trades for space based laser systems for missile defense and tactical imaging system pursuing broad area coverage with local access. Larger apertures allow both systems to consider higher orbits, offering greater fields of regard. However, large monolithic apertures quickly run into launch vehicle faring volumetric and throw mass constraints. Several technologies may enable space deployable of optical segments to form a large primary mirror at a reduced mass, circumventing the launch vehicle constraints. However, to produce an optically phased wavefront, a combination of technologies, deployment mechanisms, lightweight structures and mirrors, mirror mount isolators and actuators, adaptive optics, and processing techniques, must be applied in concert. While this paper concentrates on the hardware development activities under the UltraLITE program, namely the Precision Deployable Optical Structure ground demonstration and the brassboard Deployable Space Telescope, it will also briefly cover and provide references to related technology programs on-going at the AFRL.

Structural design for deployable optical telescopes

Many future space missions will require the deployment of space-based telescopes with large (greater than 4 m) effective apertures and with surface accuracy of better than 0.1 micron RMS. Current systems are limited not only by the physical size of the launch vehicle shroud, but also by the quality and alignment of the system after incurring stresses induced by launch. The Air Force Research Laboratorys UltraLITE program is a multifaceted program designed to develop viable deployable telescopes for space missions. As part of this program, the first ever deployable telescope is being fabricated as part of a ground demonstration. This paper discusses the development and fabrication of the structure for this telescope, an effort that resulted in a unique structural configuration, the first of its kind ever developed.

Ultralightweight structures for deployed optics

The optical support boom and related facilities for the Ultra- Lightweight Imaging Technology Experiment (UltraLITE) program at the U.S. Air Force Research Laboratory are described in this paper as well as the implementation of a local feedback loop to control the booms first bending mode. The primary goal of the efforts described in this paper are to provide a relatively quiet vibration environment for optical active control experiments to be performed on the deployable optical support boom. The optical active control experiments to date are described in a companion paper.