Multi-Objective Topology Optimization of Wing Skeletons for Aeroelastic Membrane Structures

This work considers the multi-objective aeroelastic optimization of a membrane micro air vehicle wing through topology optimization. The low aspect ratio wing is discretized into panels: a two material formulation on the wetted surface is used, where each panel can be membrane (wing skin) or carbon fiber (laminate reinforcement). An analytical sensitivity analysis of the aeroelastic system is used for the gradient-based optimization of aerodynamic objective functions. An explicit penalty is added, as needed, to force the structure to a 0–1 distribution. Pareto trade-off curves are constructed by considering convex combinations of two disparate lift, drag, or pitching moment-based objective functions. The general relationship between spatial stiffness distribution (wing topology) and aerodynamic performance is discussed, followed by the Pareto optimality of the computed designs over a series of baseline wing structures. The work concludes with an experimental validation of the superiority of select optimal designs

Interlaminar shear test method development for long term durability testing of composites

The high speed civil transport is a commercial aircraft that is expected to carry 300 passengers at Mach 2.4 over a range of more than 6000 nautical miles. With the existing commercial structural material technology (i.e., aluminum) the performance characteristics of the high speed civil transport would not be realized. Therefore there has been a concerted effort in the development of light weight materials capable of withstanding elevated temperatures for long duration. Thermoplastic composite materials are such candidate materials and the understanding of how these materials perform over the long term under harsh environments is essential to safe and effective design. The matrix dominated properties of thermoplastic composites are most affected by both time and temperature. There is currently an effort to perform short term testing to predict long term behavior of in-plane mechanical properties E22 (transverse modulus of elasticity) and G12 (shear modulus). Out-of-plane properties such as E33, G13, and G23 are inherently more difficult to characterize. This is especially true for the out-of-plane shear modulus G23 and hence there is no existing acceptable standard test method. Since G23 is the most matrix dominated property, it is essential that a test method be developed. A shear test methodology is developed to do just that. The test method, called the double notched specimen, along with the previously developed shear gage was tested at room temperature. Mechanical testing confirmed the attributes of the methodology. A finite element parametric study was conducted for specimen optimization. Moire interferometry, a high sensitivity laser optical method, was used for full-field analysis of the specimen. From this work, material parameters will be determined and thus enable the prediction of long term material behavior of laminates subjected to general loading states

A Study of Loblolly Pine Growth Increments—Part V. Effects of Chemical and Morphological Factors on Tensile Behavior of Paper

Loblolly pine growth increments were divided into five fractions: two earlywood, a transition, and two latewood growth zones. Each fraction was kraft-pulped to four different time schedules, Valley beaten, made into handsheets, and investigated for tensile strength properties. Differences in tensile strength properties were related to inherent characteristics of individual tracheids. It was shown that the number of tracheids per unit volume of paper was the most important attribute to strength. Of secondary importance was the strength of the individual tracheid-to-tracheid bonds, which was influenced by residual lignin in the pulp. Using tensile energy values, the number of hydrogen bonds active in resisting tensile forces was estimated. This number was also related to the number of tracheids per unit volume as well as to residual lignin. The above variables were explained on the basis of the intraincremental chemical and anatomical properties of wood

Through-the-thickness tensile strength of textile composites

A series of tests was run to characterize the through-the-thickness tensile strength for a variety of composites that included 2D and 3D braids, 2D and 3D weaves, and prepreg tapes. A new test method based on a curved beam was evaluated. The through-the-thickness deformations were characterized using moire interferometry. Failures were significantly different between the 2D and 3D materials. The 2D materials delaminated between layers due to out-of-plane tensile stresses. The strength of the 2D textile composites did not increase relative to the tapes. The 3D materials failed due to the formation of radial cracks caused by high circumferential stresses along the inner radius. A circumferential crack similar to the 2D materials produced the final failure. Final failure in the 3D materials occurred at a lower bending moment than in other materials. The early failures were caused by radial crack formation rather than low through-the-thickness strength

A Bendable Load Stiffened Wing for Small UAVs

A bendable load stiffened wing, developed at the University of Florida, has the ability to load stiffen in the positive flight load direction while remaining compliant in the opposite direction, enabling UAV storage inside smaller packing volumes. The wing employs an under-cambered airfoil with a swept planform providing dissimilar stiffness in the flight load and the folding direction. A comparative experimental study is performed using two wing geometries; straight camber and swept camber. The load stiffening ability is tested by performing three point bend tests while monitoring the wing root airfoil shape change using a visual image correlation technique. For the wing utilizing a swept camber design, increase in the root airfoil camber with increased loading resulted in a load stiffening structure. Swept camber wing showed a higher load carrying capacity (7 g's load factor) over a straight camber wing design (2 g's load factor), still maintaining the compliant nature in the folding direction. Long term storage induced creep deformations are small in both of the wing geometries. By increasing the wing stiffness, sweepback helps in reducing spanwise residual creep strain. Wind tunnel tests at Re = 7×104 of both the straight camber and the swept camber wing show similar L/D ratios. The sweepback helps in improving the static stability of the wing. Thus the bendable load stiffened wing has a clear advantage of offering stiffness improvement and reducing storage induced creep residual strains while maintaining the aerodynamic efficiency and improving the static stability of the wing

Paper Session II-A - Mixed-Mode Interfacial Fracture Toughness of Sandwich Composites at Cryogenic Temperatures

Honeycomb sandwich composites are found in a wide range of structural applications due to their high strength and stiffness-to-weight ratio compared to other systems. Current use of sandwich composites ranges from secondary structures in commercial aircrafts to primary structures in military aircraft, helicopters, and reusable launch vehicles, e.g. Space Shuttle. One of the applications of sandwich construction is in the liquid hydrogen tank of future RL V\u27s. Because of their low density and high stiffness sandwich construction is attractive for LH2 tank. However, past tests shave shown that leakage of hydrogen through the composite face sheet and subsequent de bonding of the face-sheet is a major concern in using sandwich construction. This problem can be eliminated by thorough understanding of the fracture mechanics of face sheets under cryogenic conditions. This study aimed to understand the failure phenomena of sandwich composites constructed from carbon fiber/epoxy composite face sheets and Nomex honeycomb cores. Both experiments including testing ·under cryogenic conditions and finite element analyses are performed to understand the conditions under which debonding occurs and propagates. One of the major objectives of the study is to measure the critical energy release rate or fracture toughness of the face-sheet/core interface, which will be a strong function of mode-mixity and temperature. Furthermore, mode-mixity itself will depend up on the geometric factors such as crack length, face sheet and core thickness, and material stiffness parameters. Fracture tests similar to double cantilever beams will be performed on sandwich panels containing initial delaminations. The fracture toughness will be measured for various crack lengths. The loads at which crack propagation occurs will be applied in the finite element model of the panel to obtain the detailed stress field in the vicinity of the crack tip. From the results of the fracture tests and finite element analysis the interfacial fracture toughness of the sandwich panel under cryogenic conditions can be measured. Application of the results to the design of a LH2 tank will be demonstrated

Bayesian Identification of Elastic Constants in Multi-Directional Laminate from Moir\'e Interferometry Displacement Fields

The ply elastic constants needed for classical lamination theory analysis of multi-directional laminates may differ from those obtained from unidirectional laminates because of three dimensional effects. In addition, the unidirectional laminates may not be available for testing. In such cases, full-field displacement measurements offer the potential of identifying several material properties simultaneously. For that, it is desirable to create complex displacement fields that are strongly influenced by all the elastic constants. In this work, we explore the potential of using a laminated plate with an open-hole under traction loading to achieve that and identify all four ply elastic constants (E 1, E 2, 12, G 12) at once. However, the accuracy of the identified properties may not be as good as properties measured from individual tests due to the complexity of the experiment, the relative insensitivity of the measured quantities to some of the properties and the various possible sources of uncertainty. It is thus important to quantify the uncertainty (or confidence) with which these properties are identified. Here, Bayesian identification is used for this purpose, because it can readily model all the uncertainties in the analysis and measurements, and because it provides the full coupled probability distribution of the identified material properties. In addition, it offers the potential to combine properties identified based on substantially different experiments. The full-field measurement is obtained by moir\'e interferometry. For computational efficiency the Bayesian approach was applied to a proper orthogonal decomposition (POD) of the displacement fields. The analysis showed that the four orthotropic elastic constants are determined with quite different confidence levels as well as with significant correlation. Comparison with manufacturing specifications showed substantial difference in one constant, and this conclusion agreed with earlier measurement of that constant by a traditional four-point bending test. It is possible that the POD approach did not take full advantage of the copious data provided by the full field measurements, and for that reason that data is provided for others to use (as on line material attached to the article)

Static Aeroelastic Model Validation of Membrane Micro Air Vehicle Wings

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76488/1/AIAA-2007-1067-747.pd

Development of test methods for textile composites

NASA's Advanced Composite Technology (ACT) Program was initiated in 1990 with the purpose of developing less costly composite aircraft structures. A number of innovative materials and processes were evaluated as a part of this effort. Chief among them are composite materials reinforced with textile preforms. These new forms of composite materials bring with them potential testing problems. Methods currently in practice were developed over the years for composite materials made from prepreg tape or simple 2-D woven fabrics. A wide variety of 2-D and 3-D braided, woven, stitched, and knit preforms were suggested for application in the ACT program. The applicability of existing test methods to the wide range of emerging materials bears investigation. The overriding concern is that the values measured are accurate representations of the true material response. The ultimate objective of this work is to establish a set of test methods to evaluate the textile composites developed for the ACT Program

The effects of specimen width on tensile properties of triaxially braided textile composites

The objective of this study was to examine the effect of the unit cell architecture on the mechanical response of textile reinforced composite materials. Specifically, the study investigated the effect of unit cell size on the tensile properties of 2D triaxially braided graphite epoxy laminates. The figures contained in this paper reflect the presentation given at the conference. They may be divided into four sections: (1) a short definition of the material system tested; (2) a statement of the problem and a review of the experimental results; (3) experimental results consist of a Moire interferometry study of the strain distribution in the material plus modulus and strength measurements; and (4) a short summary and a description of future work will close the paper