Marc R. Schultz

Snap-Through of Unsymmetric Cross-Ply Laminates Using Piezoceramic Actuators

The paper discusses the concept of using a piezoceramic actuator bonded to one side of a two-layer unsymmetric cross ply [0/90] T laminate to provide the moments necessary to snap the laminate from one stable equilibrium shape to another. The results presented are considered an alternative to existing morphing concepts wherein actuators are used to elastically warp structures into a shape other than their natural and unique equilibrium shape. These existing concepts require the continuous application of power to maintain the warped shape. With the concept discussed here, the actuators are used only to change from one equilibrium shape to another, so continuous power is not needed. The paper discusses several phases of modeling, including bonding the actuator to the laminate and applying voltage to the actuator to effect the shape change, and experimental work. Two models are developed, a simple model and a more refined one. Both are based on the Rayleigh-Ritz technique and the use of energy and variational methods. The experimental phase of the study is discussed, particularly the measurement of the voltage level needed to snap the laminate. The voltage measurements are compared with predictions and the agreement between measurements and the predictions of the refined model are reasonable, both qualitatively and quantitatively. Suggestions for future activities are presented.

A Concept for Airfoil-like Active Bistable Twisting Structures

A novel type of morphing structure capable of a large change in shape with a small energy input is discussed. The considered structures consist of two curved shells that are joined in a specific manner to form an airfoil-like structure with two stable configurations. These configurations have a difference in axial twist, and the structure can be transformed between the stable shapes by a simple snap-through action. The benefit of a bistable structure of this type is that, if the stable shapes are operational ones, power is needed only to transform the structure from one shape to another. Several composites and one steel device are considered as proof-of-concept models and active control using piezocomposite actuators is demonstrated. Finite-element analysis is used to compare the predicted shapes with the experimental shapes, and to study how changes to the geometric input values affect the shape and operational characteristics of the structures. An interesting method for combining both continuous shape change with the bistable behavior is discussed.

A Morphing Concept Based on Unsymmetric Composite Laminates and Piezoceramic MFC Actuators

Bi-stable unsymmetric composite laminates controlled with the piezoelectric MFC actuators are considered as a possible morphing structure. The use of piezoelectric actuators to effect snap-through behavior in bi-stable unsymmetric laminates is studied two ways. First, an experimental actuator-laminate structure is discussed along with efforts to model the shapes and behavior of the structure using the Rayleigh-Ritz technique. The modeling shows promise: the shapes were closely predicted, but the snap-through voltage was under predicted. In the second part of the paper, the capability to model actuator-laminate structures was expanded to include finite-element modeling using the finite-element package ABAQUS. Two finite-element models of simple problems were developed and compared with Rayleigh-Ritz solutions. Agreement between the finite-element and the Rayleigh-Ritz solutions was generally good: the predicted shapes were very similar, but the finite-element solutions were able to capture behavior not seen in the Rayleigh-Ritz solutions.

Investigation of Self-Resetting Active Multistable Laminates

Elastically multistable structures, that is, structures possessing more than one elastically stable equilibrium configuration, are particularly attractive for advanced shape changing (morphing) aircraft applications because no control effort is required to maintain the structural shape in any specific stable equilibrium. For example, thin, unsymmetric, fiber-reinforced composite laminates (e.g., [0/90] T ) can have multiple equilibrium shapes, and such laminates can be changed from one stable shape to another by a simple snap-through action. Furthermore, previous work by the first author with others demonstrated the use of a planar piezocomposite actuator to snap a bistable laminate from one equilibrium shape to another, but not back again. Such a self-resetting capability is desirable in many practical applications. The present paper describes analytical and experimental efforts to model and demonstrate self-resetting, piezoelectrically controlled, multistable laminates. The work is based on a two-ply, [0/90] T graphite-epoxy laminate that is sandwiched between two piezocomposite actuators. A simplified analytical model of the structure was developed to fine-tune the design of an experimental test article and correlate with results from testing. The simplified model captures the global response of the experimental device and predicts self-resetting actuation. Differences between the analytical and experimental results are identified, and possible reasons for these differences are explored.

Static and Dynamic Energy-Absorption Capacity of Graphite-Epoxy Tubular Specimens

This article examines the energy-absorption capacity of 50k and 12k carbon fiber composite tubular specimens crushed axially, in both quasi-static and dynamic fashions. Round and square tubular specimens with - 45° and - 45°/0° fiber orientation schemes were studied. The fundamental issue was to compare the energy-absorption capacity of the lower-cost 50k material with the 12k material, and to examine the influence of specimen geometry, fiber orientation schemes, and dynamic effects on the energyabsorption characteristics. Specimens made from the 50k material absorbed less energy than similar specimens made with the 12k material, and the load ratios were generally higher for the 50k specimens. The square specimens tended to have lower values of specific energy absorption (SEA) than the circular specimens. In addition, the crush modes were somewhat different and the load ratios were generally higher for the square specimens than for the circular specimens. Changing the fiber orientation schemes did not hav...This article examines the energy-absorption capacity of 50k and 12k carbon fiber composite tubular specimens crushed axially, in both quasi-static and dynamic fashions. Round and square tubular specimens with - 45° and - 45°/0° fiber orientation schemes were studied. The fundamental issue was to compare the energy-absorption capacity of the lower-cost 50k material with the 12k material, and to examine the influence of specimen geometry, fiber orientation schemes, and dynamic effects on the energyabsorption characteristics. Specimens made from the 50k material absorbed less energy than similar specimens made with the 12k material, and the load ratios were generally higher for the 50k specimens. The square specimens tended to have lower values of specific energy absorption (SEA) than the circular specimens. In addition, the crush modes were somewhat different and the load ratios were generally higher for the square specimens than for the circular specimens. Changing the fiber orientation schemes did not have much of an effect on the SEA, nor on the crush modes, but the presence of 0° fibers led to higher load ratios for the 50k specimens. The specimens tested dynamically had lower SEAs and higher load ratios than similar specimens that were tested statically.

Compression Response of Fluted-Core Composite Panels

In recent years, fiber-reinforced composites have becomemore accepted for aerospace applications. For example, duringNASA’s recent efforts to develop new launch vehicles, compositematerials were considered and baselined for a number of structures, including dry barrel sections, which are primarily loaded in longitudinal compression. Because of mass and stiffness requirements, sandwich composites are often selected for these applications. However, there are a number of manufacturing and in-service concerns associated with traditional honeycomb-core sandwich composites that in certain instances may be alleviated through the use of other core materials or construction methods. A fluted core, which consists of integral angled webmembers with structural radius fillers spaced between laminate facesheets, is one such construction alternative. In this paper, two different fluted-core composite designs were considered: a subscale design and a full-scale design sized for a heavy-lift-launch-vehicle interstage. In particular, the longitudinal compression behavior of fluted-core composites was evaluated with experiments and finite-element analyses. Detailed branched-shell finite-element models were developed, and geometrically nonlinear analyses were conducted to predict both buckling andmaterial failures. Good agreement was obtained between test data and analysis predictions for both failure types. Though the local buckling events are not catastrophic, the resulting deformations contribute to material failures. Consequently, neither the local buckling behavior nor the material failure loads and modes can be predicted by either linear analyses or nonlinear smeared-shell analyses. Compression-after-impact performance of fluted-core composites was also investigated experimentally. Nondestructive inspection of the damage zones indicated that the detectable damage was limited to no more than one flute on either side of any given impact. More study is needed, but this may indicate that an inherent damagearrest capability of fluted core could provide benefits over traditional sandwich designs in certain weight-critical applications.

Test and Analysis of a Buckling-Critical Large-Scale Sandwich Composite Cylinder

Structural stability is an important design consideration for launch-vehicle shell structures and it is well known that the buckling response of such shell structures can be very sensitive to small geometric imperfections. As part of an effort to develop new buckling design guidelines for sandwich composite cylindrical shells, an 8-ft-diameter honeycomb-core sandwich composite cylinder was tested under pure axial compression to failure. The results from this test are compared with finite-element-analysis predictions and overall agreement was very good. In particular, the predicted buckling load was within 1% of the test and the character of the response matched well. However, it was found that the agreement could be improved by including composite material nonlinearity in the analysis, and that the predicted buckling initiation site was sensitive to the addition of small bending loads to the primary axial load in analyses.

An Efficient Analysis Methodology for Fluted-Core Composite Structures

The primary loading condition in launch-vehicle barrel sections is axial compression, and it is therefore important to understand the compression behavior of any structures, structural concepts, and materials considered in launch-vehicle designs. This understanding will necessarily come from a combination of test and analysis. However, certain potentially beneficial structures and structural concepts do not lend themselves to commonly used simplified analysis methods, and therefore innovative analysis methodologies must be developed if these structures and structural concepts are to be considered. This paper discusses such an analysis technique for the fluted-core sandwich composite structural concept. The presented technique is based on commercially available finite-element codes, and uses shell elements to capture behavior that would normally require solid elements to capture the detailed mechanical response of the structure. The shell thicknesses and offsets using this analysis technique are parameterized, and the parameters are adjusted through a heuristic procedure until this model matches the mechanical behavior of a more detailed shell-and-solid model. Additionally, the detailed shell-and-solid model can be strategically placed in a larger, global shell-only model to capture important local behavior. Comparisons between shell-only models, experiments, and more detailed shell-and-solid models show excellent agreement. The discussed analysis methodology, though only discussed in the context of fluted-core composites, is widely applicable to other concepts.

Evaluation of Analysis Techniques for Fluted-Core Sandwich Cylinders

Buckling-critical launch-vehicle structures require structural concepts that have high bending stiffness and low mass. Fluted-core, also known as truss-core, sandwich construction is one such concept. In an effort to identify an analysis method appropriate for the preliminary design of fluted-core cylinders, the current paper presents and compares results from several analysis techniques applied to a specific composite fluted-core test article. The analysis techniques are evaluated in terms of their ease of use and for their appropriateness at certain stages throughout a design analysis cycle (DAC). Current analysis techniques that provide accurate determination of the global buckling load are not readily applicable early in the DAC, such as during preliminary design, because they are too costly to run. An analytical approach that neglects transverse-shear deformation is easily applied during preliminary design, but the lack of transverse-shear deformation results in global buckling load predictions that are significantly higher than those from more detailed analysis methods. The current state of the art is either too complex to be applied for preliminary design, or is incapable of the accuracy required to determine global buckling loads for fluted-core cylinders. Therefore, it is necessary to develop an analytical method for calculating global buckling loads of fluted-core cylinders that includes transverse-shear deformations, and that can be easily incorporated in preliminary design.

Compression Behavior of Fluted-Core Composite Panels

In recent years, fiber-reinforced composites have become more accepted for aerospace applications. Specifically, during NASA s recent efforts to develop new launch vehicles, composite materials were considered and baselined for a number of structures. Because of mass and stiffness requirements, sandwich composites are often selected for many applications. However, there are a number of manufacturing and in-service concerns associated with traditional honeycomb-core sandwich composites that in certain instances may be alleviated through the use of other core materials or construction methods. Fluted-core, which consists of integral angled web members with structural radius fillers spaced between laminate face sheets, is one such construction alternative and is considered herein. Two different fluted-core designs were considered: a subscale design and a full-scale design sized for a heavy-lift-launch-vehicle interstage. In particular, axial compression of fluted-core composites was evaluated with experiments and finite-element analyses (FEA); axial compression is the primary loading condition in dry launch-vehicle barrel sections. Detailed finite-element models were developed to represent all components of the fluted-core construction, and geometrically nonlinear analyses were conducted to predict both buckling and material failures. Good agreement was obtained between test data and analyses, for both local buckling and ultimate material failure. Though the local buckling events are not catastrophic, the resulting deformations contribute to material failures. Consequently, an important observation is that the material failure loads and modes would not be captured by either linear analyses or nonlinear smeared-shell analyses. Compression-after-impact (CAI) performance of fluted core composites was also investigated by experimentally testing samples impacted with 6 ft.-lb. impact energies. It was found that such impacts reduced the ultimate load carrying capability by approximately 40% on the subscale test articles and by less than 20% on the full-scale test articles. Nondestructive inspection of the damage zones indicated that the detectable damage was limited to no more than one flute on either side of any given impact. More study is needed, but this may indicate that an inherent damage-arrest capability of fluted core could provide benefits over traditional sandwich designs in certain weight-critical applications.