Jeffry S. Welsh

Responsive satellites and the need for structural health monitoring

The United States is striving to develop an Operationally Responsive Space capability. The goal is to be able to deliver tailored spacecraft capabilities to the warfighter as needs arise. This places a premium on the timespan between generating that requirement and having a functioning satellite performing its mission on orbit. Although there is lively debate regarding how to achieve this responsive space capability, one thing remains undeniable; the satellite flight qualification and launch vehicle integration process needs to be dramatically truncated. This paper describes the Air Force Research Laboratorys attempts to validate the use of Structural Health Monitoring (SHM) in lieu of traditional structural flight qualification testing schemes (static and shock loads, random vibration, coupled loads analysis, thermal vacuum testing, etc.) for potential Responsive Space (RS) satellites.

An Integrated Systematic Approach to Linerless Composite Tank Development

The paper describes a program currently underway at Composite Technology Development, Inc. to dramatically improve the design and capabilities of lightweight linerless composite tanks. The program integrates material development and characterization, micromechanics-based analyses of composite materials and structural design and fabrication of prototype tanks. This integrated systematic approach, addresses the multi-scale and multi-disciplinary issues that are critical to linerless composite tank design by looking concurrently at material requirements, capabilities and tailoring, refinement of fabrication process, and structural design optimization. Unlike traditional composite over- wrapped pressure vessels, the linerless composite tanks depend on the composite shell itself to serve as a permeation barrier in addition to carrying all pressure and environmental loads. Designing these tanks requires accurate knowledge of the structural response of the tank on the macro-scale as well as the material behavior on the micro-scale. Limiting and managing the development of microcracks and microcrack-induced permeability in the composite shell dictates that new materials be tailored specially for this purpose. The paper describes how micromechanics-based analysis is used to: 1) define critical material- performance parameters that drive the development of new toughened matrices, and 2) predict microcrack formation and permeability in composite laminates under biaxial load. Key concepts are presented that help optimize the structural design of linerless composite tanks. Finally, the paper presents the progress to date in designing and fabricating linerless composite tanks using a newly developed, microcrack-resistant resin system.

Comparison of MCT Failure Prediction Techniques and Experimental Verification for Biaxially Loaded Glass Fabric-reinforced Composite Laminates

The inability to accurately predict the onset of failure in modern fabric-reinforced composite materials has hindered the implementation of these materials into the mainstream applications. Excessive qualification and structural testing must be performed on composite members as a result of uncertainty in the performance of the material, resulting in significantly higher expenses associated with these materials. This situation can only be improved through the rigorous development of both improved failure and material response prediction capabilities and improved experimental verification. The current study addresses these issues, as well as explores recent developments in numerical predictions and experimental techniques. A combined numerical investigation of damage initiation mechanics and experimental verification of predicted results for a woven glass–vinyl ester composite material is performed. More specifically, 18-oz biased (5 warp/4 fill rovings) plain weave E-glass–vinyl ester laminate with warp rovings oriented in [0/90]s and [0/90/±45]s configurations are investigated. Experimental data for the E-glass–vinyl ester [0/90]s laminate is summarized in a two-dimensional biaxial failure envelope. The feasibility of using thickness-tapered cruciform specimens for generating the experimental biaxial test data is also addressed. Finally, numerical predictions of failure of the fabric-reinforced laminate are developed and compared against the experimental failure envelope for the cross-ply laminate.

Fiber print-through mitigation technique for composite mirror replication

The quickest method for generating a lightweight composite optic is to replicate an optical-quality glass tool onto a carbon-fiber- reinforced polymer CFRP. However, fiber print-through creates an un- acceptable sinusoidal surface roughness on replicated CFRP mirrors; chemical and thermal shrinkage during cure are commonly hypothesized to be the dominant causes. In order to mitigate fiber print-through, two methods of generating a polishable resin layer were investigated. The first method employs the application of a resin film to the CFRP surface. The second method, which is a more unconventional approach, generates a cocured resin layer using magnetic fibers. The latter ap- proach is being developed to eliminate the application of additional resin layers to the CFRP surface, since additional layers present structural disadvantages. It was found that the magnetic fiber technique is compa- rable to the conventional approach in mitigating fiber print-through. Due to the presence of a 0.25-mm-thick buffer above the reinforcing phase, a final polishing step was used to attain optical quality features on all of the replicated specimens. CFRP and magnetic fiber samples were polished to within 50-A rms roughness 1-m to 1-mm bandwidth.

Development of a satellite structural architecture for operationally responsive space

The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RV) is developing a satellite structural architecture in support of the Department of Defenses Operationally Responsive Space (ORS) initiative. Such a structural architecture must enable rapid Assembly, Integration, and Test (AI&T) of the satellite, accommodate multiple configurations (to include structural configurations, components, and payloads), and incorporate structurally integrated thermal management and electronics, while providing sufficient strength, stiffness, and alignment accuracy. The chosen approach will allow a wide range of satellite structures to be assembled from a relatively small set of structural components. This paper details the efforts of AFRL, and its contractors, to develop the technology necessary to realize these goals.

Structural health monitoring: an enabler for responsive satellites

The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RV) is developing Structural Health Monitoring (SHM) technologies in support of the Department of Defenses Operationally Responsive Space (ORS) initiative. Such technologies will significantly reduce the amount of time and effort required to assess a satellites structural surety. Although SHM development efforts abound, ORS drives unique requirements on the development of these SHM systems. This paper describes several technology development efforts, aimed at solving those technical issues unique to an ORS-focused SHM system, as well as how the SHM system could be implemented within the structural verification process of a Responsive satellite.

THE EFFECT OF ADHESIVE BOND THICKNESS AND MATERIAL TYPE ON STRUCTURE STIFFNESS

In an effort to more accurately predict the behavior of adhesively bonded structural systems, a comprehensive study has been initiated in which many aspects of conventional numerical modeling techniques and assumptions have been re-evaluated. This paper details the current status of this ongoing study in which the desire to more accurately predict the dynamic response of a structural system at higher natural frequencies is discussed. The results of the numeric parametric study of a simple lap-shear specimen that led to this conclusion are presented and discussed, illustrating the need for more accurate modeling techniques. Based on this overriding objective, the full three-dimensional nonlinear material response of unidirectional IM7/977-2 carbon/epoxy and Hysol EA 9309.3NA material systems was experimentally determined and is presented. A discussion of the accuracy of the experimental data generated for the present study as well as the recommended use of these data is included. The current status and long-term research plan that has been proposed and initiated to develop an improved technique to numerically predict the behavior of adhesively bonded structural systems is also presented.

Prediction of Pressure Cycle Induced Microcrack Damage in Linerless Composite Tanks

Abstract : Linerless composite tanks made from continuous carbon fiber reinforced polymers will enable significant mass and cost savings over lined, composite overwrapped tanks. The key technical challenge in developing these linerless tanks will be to choose and/or design the material to resist microcracks that may lead to leakage. Microcracks are known to form in the matrix of a composite due to mechanical stresses transverse to the reinforcing fiber direction. This paper presents an approach for characterizing the accumulation of microcracks in linerless composite tank materials under cyclic mechanical loading associated with multiple fill-and-drain pressure cycles. The model assumes that the rate of microcrack-damage accumulation is related to the microcracking fracture toughness of the material through a modified Paris-law formulation. A key artifact of this model is that microcrack-damage accumulation under cyclic load can be predicted from only two material constants. This damage accumulation model is validated through a series of coupon tests, and an illustrative example is presented to demonstrate how the model can be used to predict the microcracking performance of a linerless composite tank subjected to fatigue cycles.

Recent Advances in Failure Predictions of Composite Laminates Utilizing Multicontinuum Technology

This study represents an effort to make substantial improvements in failure predictions for composite laminates and structures utilizing material constituent (fiber and matrix) information and the finite element method. Detailed numerical results are compared to experimental data obtained from the World Wide Failure Exercise [1]; biaxial testing of quasi-isotropic laminates performed at the Air Force Research Lab, Kirtland, NM; and combined thermal-mechanical testing of an unlined composite pressure vessel performed by Firehole Technologies, Inc.

FINITE ELEMENT METHODS FOR THE FREQUENCY RESPONSE PREDICTION OF BONDED COMPOSITE STRUCTURES

The present study examines the effect bonded lap joints have on the frequency response of a given structure. The test article developed consist of carbon fiber composite sections joined with epoxy single-lap joints. The motivation for this study stems from the initial findings of previous works conducted at the Air Force Research Laboratory. This work shows the key factors affecting the influence of bonded lap joints on the system frequency to be: section stiffness, bond configuration and type, boundary conditions, and mass distribution. In addition, sensitivity analyses conducted give direction to the level of detail necessary to model bonded lap joints with adequate efficiency and accuracy. To aid in determining an effective and accurate methodology, the following sensitivity studies are conducted: element formulation, composite ply angle, and mesh discretization. Validation of these studies with experimental data for various configurations allow for an uncertainty assessment of the numerical data presented.