Wall effect and scale effect in v/stol model testing
Wind-tunnel wall effect on tilt-wing and fan-in- wing aircraft models
FAN-IN-WING AIRCRAFT↗
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Scaling effects of defects in fiber-reinforced composites
Material defects may be introduced willingly or unwillingly during material manufacturing and structural component fabrication stages. Their presence in the material plays a dominant role in determining the material's strength and the associate failure mechanisms. In the sense that the size and the number of defects may increase with the volume of the material, the effect of dimensional scaling may manifest itself in the dependence of material strength on volume. Or, alternatively, there may exist a scaling effect of material defects. In fiber-reinforced composites, manufacturing or fabrication defects may come in several forms: matrix voids, matrix microcracks, fiber misalignment, broken fibers, or interface disbonds, just to mention a few. These are interacting and competing defects in the sense that one type of defect may become dominant under one stress condition and another type of defect may become dominant under a different stress condition. This happens because the fiber reinforcement network, together with the distribution of defects, constitutes the prime microstructure of the composite, and there exist continued interactions between the evolving microstructure and the distribution of defects. In the process, the scaling effects of defects are complicated by this interaction. In this presentation, the scaling effects of defects in fiber-reinforced composites will be briefly discussed with the introduction of the concept of effective defects. It is then shown with the aid of some actual experimental and analysis results that the scaling effects are very much present, but they are regulated by the characteristic dimension of the composite microstructure due to the aforementioned microstructure-defect interaction effect.
Wang, A. S. D.↗
Cavitation erosion size scale effects
Size scaling in cavitation erosion is a major problem confronting the design engineers of modern high speed machinery. An overview and erosion data analysis presented in this paper indicate that the size scale exponent n in the erosion rate relationship as a function of the size or diameter can vary from 1.7 to 4.9 depending on the type of device used. There is, however, a general agreement as to the values of n if the correlations are made with constant cavitation number.
Rao, P. V.↗
Scaling effects in sublaminate-scaled composite laminates
A series of tensile tests have been carried out to determine the effect of specimen size on the mechanical response of composite laminates which were scaled at the sublaminate level. Two material systems were studied AS4/3502 and AS4/PEEK. The effect of specimen size was examined with respect to first ply failure stress, delamination stress, ultimate stress, and strain at failure. In general, the strength of scaled specimens increased with increasing specimen size. Several models from the published literature were applied with varying degrees of success to predict the first ply failure and delamination stress in scaled composites. It was found that the effect of ply constraint is not addressed properly in the existing theories.
Kellas, Sotiris↗
Scaling effects in the static and dynamic response of graphite-epoxy beam-columns
Scale model technology represents one method of investigating the behavior of advanced, weight-efficient composite structures under a variety of loading conditions. It is necessary, however, to understand the limitations involved in testing scale model structures before the technique can be fully utilized. These limitations, or scaling effects, are characterized. in the large deflection response and failure of composite beams. Scale model beams were loaded with an eccentric axial compressive load designed to produce large bending deflections and global failure. A dimensional analysis was performed on the composite beam-column loading configuration to determine a model law governing the system response. An experimental program was developed to validate the model law under both static and dynamic loading conditions. Laminate stacking sequences including unidirectional, angle ply, cross ply, and quasi-isotropic were tested to examine a diversity of composite response and failure modes. The model beams were loaded under scaled test conditions until catastrophic failure. A large deflection beam solution was developed to compare with the static experimental results and to analyze beam failure. Also, the finite element code DYCAST (DYnamic Crash Analysis of STructure) was used to model both the static and impulsive beam response. Static test results indicate that the unidirectional and cross ply beam responses scale as predicted by the model law, even under severe deformations. In general, failure modes were consistent between scale models within a laminate family; however, a significant scale effect was observed in strength. The scale effect in strength which was evident in the static tests was also observed in the dynamic tests. Scaling of load and strain time histories between the scale model beams and the prototypes was excellent for the unidirectional beams, but inconsistent results were obtained for the angle ply, cross ply, and quasi-isotropic beams. Results show that valuable information can be obtained from testing on scale model composite structures, especially in the linear elastic response region. However, due to scaling effects in the strength behavior of composite laminates, caution must be used in extrapolating data taken from a scale model test when that test involves failure of the structure.
Jackson, Karen E.↗
Scale effects on supercritical airfoils
A survey of research relative to scale effects on supercritical airfoils has been conducted. The results of this survey indicated that Reynolds number scale effects have a significant impact on airfoil design and performance. Further, this impact is greater for supercritical airfoils than for conventional airfoils. It was found that low Reynolds number drag data could be extrapolated to high Reynolds number conditions provided the flow was attached and the pressure distribution shape did not change appreciably. Airfoil lift and pitching-moment data obtained at low Reynolds numbers cannot be extrapolated to full-scale values. Viscous theoretical transonic analysis methods currently under development will significantly improve the ability of the designer to account for scale effects. Boundary-layer manipulation in low Reynolds number facilities using natural transition or aft located transition strips to simulate high Reynolds number conditions was shown to be an uncertain test procedure and reliance should be made on high Reynolds number facilities if available.
Blackwell, J. A., Jr.↗
Experimental observations of scale effects on bonded and bolted joints in composite structures
The objective is to observe size (scale) effects in (1) fiber dominated laminates and bolted joints, (2) adhesive (matrix) dominated bonded joints with fiber dominated laminate adherends, and (3) matrix dominated laminates. Selected literature on scale effects is reviewed with comments and test data from one source that is analyzed for predicted and actual scale effects utilizing uniaxial loaded static strength, spectrum fatigue residual strength, and spectrum fatigue lifetime test results. Causes of scale effects are discussed, the results are summarized, and conclusions are made.
Grimes, Glenn C.↗
Evaluation of some scale effects in the response and failure of composite beams
The feasibility of using scale model testing for predicting full-scale behavior of composite beams loaded in tension and flexure was investigated. Classical laws of similitude were applied to fabricate and test replica model beams to identify scaling effects in the load response, strength, and mode of failure. Experiments were conducted using graphite-epoxy composite beams having different laminate stacking sequences and a range of scaled sizes. Results indicated that the elastic response of scaled composite beams was independent of specimen size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled beams of certain laminate stacking sequences. Weibull statistical and fracture mechanics based models were applied to predict the strength scale effect since standard failure criteria cannot account for the influence of absolute specimen size in failure.
Jackson, Karen E.↗
Scale effects in the response and failure of fiber reinforced composite laminates loaded in tension and in flexure
The feasibility of using scale model testing for predicting the full-scale behavior of flat composite coupons loaded in tension and beam-columns loaded in flexure is examined. Classical laws of similitude are applied to fabricate and test replica model specimens to identify scaling effects in the load response, strength, and mode of failure. Experiments were performed on graphite-epoxy composite specimens having different laminate stacking sequences and a range of scaled sizes. From the experiments it was deduced that the elastic response of scaled composite specimens was independent of size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled specimens of certain laminate stacking sequences. A Weibull statistical model and a fracture mechanics based model were applied to predict the strength scale effect since standard failure criteria cannot account for the influence of absolute specimen size on strength.
Jackson, Karen E.↗
Scaling effects in the static large deflection response of graphite-epoxy composite beams
Scaling effects in the large deflection response of graphite-epoxy composite beams was investigated. Eight different scale model beams ranging from 1/6 to full-scale were subjected to an eccentric axial compressive load to promote large bending deformations and failures. Beams having laminate stacking sequences including unidirectional, angle ply, cross ply, and quasi-isotropic were tested to examine a wide variety of composite response and failure modes. The model beams were loaded under scaled test conditions until catastrophic failure. Data acquired included load, end displacement, and strain measurements, and qualitative failure measurements. The experimental data is compared to a large rotation beam analysis and a finite element model analysis. Results from the tests indicate that the beam response becomes nonlinear. Failure modes are consistent between scale models within a laminate family, however, a significant scale effect is observed in strength of the scaled beams.
Jackson, Karen E.↗
Size scale effect in cavitation erosion
An overview and data analyses pertaining to cavitation erosion size scale effects are presented. The exponents n in the power law relationship are found to vary from 1.7 to 4.9 for venturi and rotating disk devices supporting the values reported in the literature. Suggestions for future studies were made to arrive at further true scale effects.
Rao, P. V.↗
Workshop on Scaling Effects in Composite Materials and Structures
This document contains presentations and abstracts from the Workshop on Scaling Effects in Composite Materials and Structures jointly sponsored by NASA Langley Research Center, Virginia Tech, and the Institute for Mechanics and Materials at the University of California, San Diego, and held at NASA Langley on November 15-16, 1993. Workshop attendees represented NASA, other government research labs, the aircraft/rotorcraft industry, and academia. The workshop objectives were to assess the state-of-technology in scaling effects in composite materials and to provide guidelines for future research.
Karen E Jackson↗
Experimental studies of scale effects on oscillating airfoils at transonic speeds
Scale effects are discussed with reference to a conventional airfoil (NACA 64A010) and a supercritical airfoil (NLR 7301) at mean flow conditions that support both weak and strong shock waves. During the experiment the Reynolds number was varied from 3 x 10 to the sixth power at time history data are presented over the range of reduced frequencies that are important in aeroelastic applications. The experimental data show that viscous effects are important in the case of the supercritical airfoil at all flow conditions and in the case of the conventional airfoil under strong shock wave conditions. Some frequency dependent viscous effects were also observed.
Davis, S. S.↗
Scaling Effects in Carbon/Epoxy Laminates Under Transverse Quasi-Static Loading
Scaling effects were considered for 8, 16, 32, and 64 ply IM-7/8551-7 carbon/epoxy composites plates transversely loaded to the first significant load drop by means of both a quasi-static and an equivalent impact force. The resulting damage was examined by x-ray and photomicroscopy analysis. Load-deflection curves were generated for the quasi-static tests and the resulting indentation depth was measured. Results showed that the load-deflection data scaled well for most of the various thicknesses of plates. However, damage did not scale as well. No correlation could be found between dent depth and any of the other parameters measured in this study. The impact test results showed that significantly less damage was formed compared to the quasi- static results for a given maximum transverse load. The criticality of ply-level scaling (grouping plies) was also examined.
Nettles, Alan T.↗
Scale effects in the use of cloud statistics for earth oriented space experiments.
Scale effects in use of ground observation cloud statistics in analysis of cloud cover distribution on basis of space experiments
Barnes, J. C.↗
On nature's scaling effects
This presentation afforded the opportunity to look back in the literature to discover scaling effects in nature that might be relevant to composites. Numerous examples were found in nature's approaches to wood, teeth, horns, leaves, eggs, feathers, etc. Nature transmits tensile forces rigidly with cohesive bonds, while dealing with compression forces usually through noncompressible hydraulics. The optimum design scaling approaches for aircraft were also reviewed for comparison with similitude laws. Finally, some historical evidence for the use of Weibull scaling in composites was reviewed.
Wilkins, Dick J.↗
Empirically determined wind and scale effects on hot gas recirculation characteristics of jet V/STOL aircraft
Empirically determined wind and scale effects on hot gas recirculation characteristics of jet V/STOL aircraft
Cosgrove, W. J.↗