Search NASA⌕ Search

Engineering topics

Prabhakaran, R.

Publications and source records attributed to Prabhakaran, R..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

An investigation of the Iosipescu and Asymmetrical Four-Point Bend tests for composite materials

The results of an extensive analytical and experimental investigation of the Iosipescu and Asymmetrical Four-Point Bend (AFPB) test methods are summarized. Finite element analyses were used to assess the influence of notch parameters and load locations on the stress state in the specimens. The shear moduli and shear strengths were experimentally measured for quasi-isotropic graphite-epoxy laminates using both the Iosipescu and the AFPB test methods with various combinations of notch parameters and load locations. The test results indicate that changes in the notch geometry and load locations aimed at improving the stress distribution in the test section result in unexpected changes in the failure mode.

Spigel, B. S.↗
NASA NTRS1984

Structural efficiency studies of corrugated compression panels with curved caps and beaded webs

Curved cross-sectional elements are employed in structural concepts for minimum-mass compression panels. Corrugated panel concepts with curved caps and beaded webs are optimized by using a nonlinear mathematical programming procedure and a rigorous buckling analysis. These panel geometries are shown to have superior structural efficiencies compared with known concepts published in the literature. Fabrication of these efficient corrugation concepts became possible by advances made in the art of superplastically forming of metals. Results of the mass optimization studies of the concepts are presented as structural efficiency charts for axial compression.

Davis, R. C.↗
NASA NTRS1983

Simultaneous elastic and photoelastic calibration of birefringent orthotropic model materials

Transmission photoelastic analysis of composite models has been shown in recent years to be a viable stress analysis method for composite structures. However, difficulties in the fabrication of the model materials require the development of efficient mechanical and optical calibration procedures. In this paper, the methods for measuring elastic and photoelastic constants of composites are briefly reviewed. Then a new method, utilizing a relatively small calibration specimen, is described. In this method, electrical resistance strain gauges are mounted at selected points of a half-plane model and photoelastic measurements and strain readings are simultaneously taken. The data are processed by a least-squares procedure to yield the elastic and photoelastic constants. Comparison of results obtained by the proposed method with the results from conventional tests shows good agreement.

Prabhakaran, R.↗
NASA NTRS1982

Photo-orthotropic elasticity - A new technique for stress analysis of composites

This paper deals with the application of photoelastic methods to glass fiber reinforced plastics. The basic principles governing the photoelastic response in birefringent orthotropic composites are briefly reviewed. Then, three important aspects of photo-orthotropic elasticity, namely, photoelastic calibration, separation of principal stresses, and application to problems of engineering importance, are considered in detail. A new calibration specimen is proposed for determining the three independent photoelastic constants: an orthotropic half-plane subjected to a concentrated edge load. From the isochromatic response, the three stress-fringe values can be obtained by a least-squares procedure based on the theoretical stress distribution. Next, four methods of separating the principal stresses in birefringent composite models are proposed: the method of drilling small holes at the points of interest, the method of oblique incidence, the method of combining the transmitted photoelastic response with the reflected photoelastic response, and the method of birefringence dispersion.

Prabhakaran, R.↗
NASA NTRS1982

Extension of the hole-drilling method to birefringent composites

A complete stress analysis and reliable failure criteria are essential for important structural applications of composites in order to fully utilize their unique properties. The inhomogeneity, anisotropy and inelasticity of many composites make the use of experimental methods indispensable. Among the experimental techniques, transmission photoelasticity has been extended to birefringent composites in recent years. The extension is not straight-forward, in view of the complex nature of the photoelastic response of such model materials. This paper very briefly reviews the important developments in the subject and then describes the theoretical basis for a new method of determining the individual values of principal stresses in composite models. The method consists in drilling very small holes at points where the state of stress has to be determined. Experiments are then described which verify the theoretical predictions. The limitations of the method are pointed out and it is concluded that valuable information concerning the state of stress in a composite model can be obtained through the suggested method.

Prabhakaran, R.↗
NASA NTRS1981

Determination of principal stress in birefringent composites by hole-drilling method

The application of transmission photoelasticity to stress analysis of composite materials is discussed.The method consists in drilling very small holes at points where the state of stress has to be determined. Experiments are described which verify the theoretical predicitons. The limitations of the method are discussed and it is concluded that valuable information concerning the state of stress in a composite model can be obtained through the suggested method.

Prabhakaran, R.↗
NASA NTRS1981

Extension of oblique incidence method to photo-orthotropic-elasticity

With the interpretation of isochromatic and isoclinic fringes in transparent, birefringent, orthotropic composites established, efforts were in progress to determine the individual values of the principal stresses or strains. Several methods utilize the photoelastic results only partially and rely on numerical procedures. Others attempt to obtain the required information based entirely on experimental data. The classical oblique incidence technique as applied to transversely isotropic birefringent composites, is presented. The proposed extension is verified by applying it to the problem of an orthotropic half-plane subjected to an edge-load.

Prabhakaran, R.↗
NASA NTRS1981

Photoelastic techniques for the complete determination of stresses in composite structures

Three methods of photoelastic analysis are examined. The first method is an extension of the oblique incidence technique in which the model (or the light beam) is rotated about one of the material symmetry axes. In the second method, transmission and reflection photoelastic responses are combined. The third method requires the drilling of small holes and the determination of the fringe orders at selected points on the hole boundary. The three methods are applied to an orthotropic circular disk under diametral compression. Results are compared with strain gage data.

Prabhakaran, R.↗
NASA NTRS1981

Photoelastic techniques for the complete determination of stresses in composite structures

Three new methods are proposed for determining the individual values of the principal stresses or strains in an orthotropic birefringent model using transmission photoelastic analysis. One of the methods is an extension of the oblique incidence technique in which the model (or the light beam) is rotated about one of the material symmetry axes. In another method, transmission and reflection photoelastic responses are combined, while in the third method small circular holes are drilled at points of interest and the isochromatic fringe order is measured at selected points on the hole boundary. The three methods are applied to an orthotropic disc under diametral compression and results are compared with strain gage data.

Prabhakaran, R.↗
NASA NTRS1980

Photoelastic tests on models of thermal protection system for space shuttle orbiter

The thermal protection system (TPS) of the space shuttle orbiter vehicle, consisting of ceramic tile/adhesive/strain isolator pad/adhesive/aluminum substructure, was modeled photoelasticity. A highly sensitive photoelastic material was used in the models to show the nature of the stress-transfer between the strain isolation pad (SIP) and the ceramic tile through the RTV-adhesive layer. Isochromatic fringe patterns were obtained for models subjected to tension and combined tension and bending. Tests indicated that the load-transfer between the SIP and the photoelastic material occurred at discrete locations causing stress concentrations in the photoelastic material. Stress concentration factors of the order of 1.9 were measured, but as the observed photoelastic response was an integrated effect through the model thickness, the local stress concentration factors at the SIP/tile interface could be even higher.

Prabhakaran, R.↗
NASA NTRS1979

Photoelastic studies of advanced lap joint concept

Photoelasticity was introduced as a quantitative tool in the experimental stress analysis of anisotropic birefringent composite model materials. The investigation was divided into three parts: (1) separation of principal stresses or strains, (2) photoelastic calibration, and (3) development of model-prototype relations.

Prabhakaran, R.↗