Showing papers in "Experimental Mechanics in 1986"

Prediction of the long-term creep compliance of general composite laminates

Abstract: An accelerated viscoelastic characterization procedure for use with polymer-based composite materials is presented which employs short term test data obtained using unidirectional specimens to predict the long term viscoelastic behavior of general composite laminates. This procedure is here illustrated using the Schapery (1966, 1969) nonlinear theory as the required viscoelastic constitutive model, as well as classical lamination theory for the lamination scheme. The technique is applied to T300/5208 graphite/epoxy.

A review: Acoustic emission, a tool for composite-materials studies

Abstract: The technique of acoustic emission has two broad applications areas. The first is nondestructive evaluation. The second is as a tool in studies or research which are not fundamentally directed towards acoustic emission. It is this second application with which we are concerned here. Acoustic emission is a very useful tool in this role because of its high sensitivity, real-time capability, volume-monitoring approach, and sensitivity to any process or mechanism which generates sound waves. This paper presents a comprehensive review of areas where acoustic emission (AE) has been used for materials studies on composite materials. The following fields, among others, will be covered: (1) time-dependent composite properties, (2) impact studies, (3) correlation of AE with stress level, (4) application to matrix cure studies, (5) relationship of AE-detected damage to other measures of damage, (6) studies of the effects of matrix material, (7) application to differences in second phase, (8) interface studies, (9) AE and dimensional stability, (10) AE applied to orientation studies, and (11) environmental effects. This review will emphasize the roles that AE can play as a tool for the materials scientist: (1) discovery of damage mechanisms, (2) characterization of damage progression with increasing time or stress, (3) optimization of fabrication variables, and (4) reduction in the numbers of test specimens required in various studies.

Shear characterization of unidirectional composites with the off-axis tension test

Abstract: The influence of end constraints on accurate determination of the intralaminar shear modulus G(12) from an off-axis tension test is examined both analytically and experimentally. The Pagano-Halpin (1968) model is employed to illustrate that, when the effect of end constraints is properly considered, the exact expression for G(12) is obtained. When the effect of end constraints is neglected, expressions for the apparent shear modulus G(12)-asterisk and apparent Young's modulus Exx-asterisk are obtained. Numerical comparison for various off-axis configurations and aspect ratios is carried out using typical material properties for graphite/polyimide unidirectional composites. It is demonstrated that the end-constraint effect influences accurate determination of G(12) more adversely than it affects the laminate Young's modulus E(xx) in the low off-axis range. Experimental results obtained from off-axis tests on unidirectional graphite/polyimide specimens confirm the above. Based on the presented analytical and experimental evidence, the 45-deg off-axis coupon is proposed for the determination of the intralaminar shear modulus G(12).

The influence of fiber length and fiber orientation on damping and stiffness of polymer composite materials

Abstract: This paper describes the theoretical analysis, the experimental results and the curve-fitting of the analytical model to the experimental results on the influence of fiber length and fiber orientation on damping and stiffness of polymer-composite materials. The experimental results show that, as predicted, very low fiber aspect ratios are required to produce significant improvements in damping. Measurements and predictions also indicate that the control of lamina orientation in a continuous fiber-reinforced laminate may be a better approach to the improvement of damping than the control of the fiber aspect ratio.

Residual-stress determination through combined use of holographic interferometry and blind-hole drilling

Abstract: Holographic interferometry is used to determine in-plane radial displacements due to release of residual stresses by hole drilling. A method is derived for relating radial displacements measured in three directions of illumination to the state of residual stress, analogous to relations used in the conventional strain-rosette technique. Residual stress is produced by an interference fit of two circular tubes. Agreement between stress determined holographically with a computed value and with that determined by the conventional technique is good. Advantages of the holographic technique in overcoming various shortcomings of the conventional technique are discussed. A modification of the holographic technique involving data collection in only two directions of illumination is described.

A new technique for heating specimens in split-Hopkinson-bar experiments using induction-coil heaters

Abstract: A new technique to heat metallic specimens, in split-Hopkinson-bar experiments, is presented. The heating is achieved with induction coils surrounding the specimen. The main advantages of this technique are (1) a relatively fast heating rate, (2) localization of the heated volume, and (3) heating achieved without touching the specimen. Experimental results for several metals, in terms of high strain-rate stress-strain curves at elevated temperatures, are presented and discussed.

A study of the axial-torsional coupling effect on a sagged transmission line

Abstract: Two mechanisms have been proposed to explain the low-frequency phenomenon of power-transmission-line galloping: the Den Hartog theory and the torsional theory. The current study proposes that the axial-torsional coupling parameter provides one tentative explanation for the torsional theory. Simpson's dynamic model was used as the basic model with modifications made to incorporate the mechanical coupling effects between axial tension and torsion of the conductor. Both the cubic-spline and the Runge-Kutta numerical techniques were used in a computer simulation of conductor dynamics. The validity of the computer-simulation results was checked against the taut string model, Shea's critical-sag criterion, and experiments with a thin-wire catenary. The results from this study show that vertical motion can be mechanically induced by the torsional motion of the cable in the absence of any air flow.

Residual-stress analysis of an epoxy plate subjected to rapid cooling on both surfaces

Abstract: This paper presents theoretical and experimental methods of finding the residual stress in an epoxy plate subjected to rapid cooling on both surfaces. The theoretical residual-stress distributions in a plate are calculated by using the fundamental equations based on the linear viscoelastic theory. The specimens in the experiment are subjected to rapid cooling. The residual stresses are measured by the layer-removal method. The theoretical and experimental results are compared and discussed.

Fractional moiré strain analysis using digital imaging techniques

Abstract: A new method for analyzing low-order moirefringe patterns of displacement fields is presented. This method adapts the techniques of half-fringe photoelasticity to moire and extracts continuous displacement information in the regions between integral fringes. The effectiveness of the technique is illustrated with three examples: a uniform uniaxial field, a tapered specimen in tension, and a disk in diametral compression.

Experimental and analytical characterization of multidimensionally braided graphite/epoxy composites

Abstract: This paper presents results of an investigation of a novel, through-the-thickness fiber-reinforced composite material. The generic name for this composite technology is multidimensional (X-D) braiding. X-D braided composites consist of a net-shaped, densely braided fiber skeleton which is rigidized with a structural epoxy-resin system. This material is an alternative to the conventional laminated composite and has the potential for being more resistant to delamination and matrix cracking. This paper describes results of the mechanical characterization of one graphite fiber system which was braided into panels in which two braid parameters could be investigated. The variables investigated included the effect of edge condition and braid pattern on the tensile, compressive and flexural properties of the braided panels. These properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated, 1×1, 3×1, and 1×1×1/2 F, the 3×1 braid pattern showed superior tensile performance, while the 1×1×1/2 F braid pattern exhibited superior flexural properties. The development of an analytical method for modeling the tensile performance of the multidimensionally (X-D) braided composite is also presented. The fiber geometry in X-D braids was modeled based on the braid parameters used in the construction of these composites. By the nature of the symmetry of the resulting braided structure, an analytical model based on classical lamination theory was used to determine the extensional stiffness in the three principal geometric directions of a braided composite. These analytical results are shown to compare favorably with those obtained experimentally. Finally, to further validate the ability of this material to contain damage, multidimensionally braided and conventionally laminated panels were impacted and the resulting damage was nondestructively determined. The multidimensionally braided material was shown to reduce the area of damage caused by impact by a factor of three for the energy levels tested.

‘SPATE’ stress studies of plates and rings under in-plane loading

Abstract: In this paper, a new experimental, computercontrolled, stress-analysis technique (SPATE) based on the measurement of infrared emissions from the surface of a cyclically loaded body, has been used to study the stress distributions in rectangular steel plates and circular rings under cyclic in-plane loading A typical test procedure for quantitative stress measurement is described The effects of surface obliquity and the thickness of the paint coating applied to the specimen surface on the received signal are described and discussed Results are given for three series of test specimens and compared quantitatively with relevant theoretical solutions Attention is drawn to the changes in the received signal as the applied load range increases beyond the elastic limit of the material A new application of the technique to the determination of the stress-intensity factor in acracked body is also illustrated

Toward more accurate resudual-stress measurement by the hole-drilling method: Analysis of relieved-strain coefficients

Abstract: In residual-stress measurement by the holedrilling technique, accuracy can be enhanced if the proper values for the relleved-strain coefficients are employed. These values are obtained by double integration of the relieved strain over the area of the grid. In this paper, several methods customarily used in the literature for the calculation of these coefficients are discussed. A comparison of the coefficients from these methods and those derived by double integration demonstrate the increased accuracy of the latter.

Coherent-light-shadow spot of a crack under mode I loading: theory and experiment

Abstract: The coherent-light-shadow field formed by crack-tip deformation under Mode 1 loading is studied theoretically and experimentally. First-order approximation of geometrical optics and higher order expansions are examined in the theoretical development. The wave-optical analysis shows that the interference fringe spacing around the caustic is approximately proportional toKsu−2/15 for the near-tip singular field, and, that the peak amplitude of the light-intensity distribution around the caustic is proportional toK 4/15. The effects of diffraction on the measurement of the caustic diameter are examinediin detail. The analysis and accompanying experiments show that the diffraction effect alone could lead toK estimation errors of ±20 percent or more, if the edge of the shadow spot or it the peak-intensity point is used to determine the caustic diameter. An alternate measure of caustic diameter is therefore suggested for practical use. Effects of the size of the nominal initial curve and crack-tip bluntness onK measurements are included. Finally, possible applications of the coherent-light-shadow spot in fracture mechanics are outlined.

A laser interferometer dilatometer for thermal expansion measurements of composites

Abstract: A high precision Fizeau type, laser interferometer dilatometer system has been developed for low expansion composite materials. The strain resolution is about 1 microstrain. The system is automated to operate over a large temperature range and record data during the test in real time. A technique has been developed to reduce the fringe data in real time to length changes. The dilatometer system is described and thermal expansion measurements for several fiber-reinforced and particle filled composites are presented.

Wrinkling of elastoplastic circular plates during stamping

Abstract: Circular plates that are stamped into a shallow, biaxially curved die by a matching punch develop radial wrinkles near the periphery when the edge is not clamped. Thin ductile metal plates develop these wrinkles after some plastic deformation occurs at the center of the plate. In comparison with elastic wrinkling, the center deflection to thickness ratio for wrinkling is increased as a consequence of the plastic deformation. In elastoplastic plates, this critical deflection ratio is a decreasing function of the plate thickness parameter ξo

Energy methods for fracture-toughness determination in concrete

Abstract: The concepts for evaluating the energetic fracture parameter,G ic, experimentally for concrete have been disputed because of the inconsistency and diversity of the results obtained. In order to re-examine the validity of this fracture parameter, experimental investigations were conducted to compare various methods for determiningG ic with considerations of effects of microcracking and slow crack growth prior to the onset of instability leading to failure. These studies were made using small beams in three-point bending and utilizing compliance calibration techniques to estimate crack lengths. The following approaches were used to determine the critical energy-release rate: (1) stress-intensity-factor method which uses linear-elastic fracture mechanics to relateG ic toK ic; (2) theJ-integral concept in which energy change per unit crack extension was measured experimentally; and (3) Petersson's Method (modified) in which the total energy absorbed to instability is divided by the uncracked cross-sectional area to obtain the energy per unit area during the fracture process. All of these approaches resulted in excellent agreement and also good consistency. Invariancy witha/W was obtained when extended crack lengths were considered.

Determination of tensile flow stress beyond necking at very high strain rate

Abstract: The objective of this effort was to extend the Bridgman analysis of tensile necking to obtain stress-strain data beyond the point of onset of necking from a split Hopkinson bar. For this purpose, combined analytical and experimental techniques were considered. The analytical efforts were focused on validating the use of Bridgman solutions for high rate of deformation through a finite-element analysis of a tapered tensile specimen. The experimental technique involved the development of a photographic system using a light-emitting diode and a 35-mm rotating drum camera for the observation of necking during dynamic tensile tests conducted with a split Hopkinson tension bar. The developed new technique was successfully used to measure neck profiles of 6061-T6 aluminum, HY100 and 1020 steel tensile specimens. The measured profiles were used with the Bridgman analysis and stress-strain data were obtained to over 70-percent strain.

Examination of the computational model for the layer-removal method for residual-stress measurement

Abstract: An analytical solution is presented to examine the accuracy of the ‘layer-removal’ method for measuring localized residual stresses. In this approach, strips, which may have been cut from a pipe or a plate, have strain-gage rosettes placed on one face and layers removed from the other face. The strain measurements are used to deduce the residual stress in the layers removed. The stress measured is that along the axis of the strip. It may vary rapidly with axial distance, as for example when the strip is taken from a welded part. The present analysis shows that the actual stress distribution may be quite different from that predicted by the computational model normally used in the layer-removal method. It shows that the difference increases as the ratio of the heighth of the strip from which a layer is removed to the half dimensiona of the localized residual-stress zone increases. It is recommended that the layer-removal method can be used for measuring residual stresses for cases in which the ratio ofh/a is less than or equal to unity.

Image-processing techniques in laser-speckle photography with application to hybrid stress analysis

Abstract: Six techniques for analyzing speckle halo fringes for fringe spacing and orientation using a digital image analyzer are presented. Each of the techniques were tested for range, accuracy, and computer run-time through analysis of a known case of rigid-body motion. Application of these techniques to two-dimensional hybrid stress analysis is described where speckle-displacement data around the high-stress region of a notched bar in tension is used as input data to a plane-stress finite-element program.

Analysis of Elastic-Plastic Ball Indentation to Measure Strength of High Strength Steels

Abstract: Ball-indentation experiment ware performed with A723 steel of 100- to 1200-MPa ultimate strength. Results are compared with conventional tension tests and with an elastic-plastic finite-element model of the ball indentation. Finite-element analysis shows the ball-indentation process to be insensitive to friction effects. Comparison of indentation and conventional tests shows that slip-line field analysis closely predicts the ball contact stress. The indentation tests gave an accurate measure of ultimate tensile strength because of the following test procedures: using a large ball size and a fixed ratio of indentation depth to ball size; accounting for directional material properties; accounting for extraneous system deflections.

Experimental eigenvalues and mode shapes for flat clamped plates

Abstract: Experimental eigenvalues of both square and rectangular clamped flat plates were measured using digital spectrum analysis. Individual mode shapes were recorded experimentally using holographic interferometry. Plate spectra showing the first 35 modes of vibration for each of the square and rectangular plates were recorded, allowing the experimentally determined eigenvalues to be compared with published theoretical predictions. Over 25 modes for a square plate and 16 modes for a rectangular plate with aspect ratio of 2/3 were recorded holographically. Selected recorded mode shapes are compared with beam mode shapes as well as with modified Bolotin mode shapes, both of which are popular assumed mode shapes in current numerical techniques. It was found that both of these assumed mode shapes agree favorably with the experimental results. The beam mode shapes agree better in some modes; the modified Bolotin mode shapes agree more favorably in others.

Crack growth in unidirectional graphite-epoxy under biaxial loading

Abstract: Center-notched coupons of 16-ply unidirectional AS4/3501-6 graphite-epoxy were loaded in tension to failure in order to determine the direction of initial crack growth, the load required to cause initial crack growth, and the load required to cause failure. Fifteen-degree off-axis tensile coupons provided a far-field biaxial state of stress away from the notch. Specimens of three different length-to-width ratios were tested in order to vary the biaxial stress states. Two notch configurations were used, one perpendicular to the loading direction, the other perpendicular to the material fiber direction. In all of the cases studied experimentally, crack growth was parallel to the fiber direction. An anisotropicelasticity crack-tip stress analysis was subsequently implemented in order to compare experimental crack-growth directions to those predicted by three crack-extension-direction criteria. Of the three criteria studied, (Strain-energy density, the tensor polynomial, and the normal stress ratio), only one, the normal-stress-ratio criterion, provided crack-growth-direction predictions that agreed with the experimental results.

Determination of dynamic yield strengths with embedded manganin gages in plate-impact and long-rod experiments

Abstract: The dynamic yield strengths of three steels were determined at strain rates of about 103 s−1 and 106 s−1. The measurements at 103 s−1 were obtained by a new technique based on measurements of large amplitude elastic waves in long bars struck by rigid flyer plates. Embedded manganin gages were used to measure stress, and the gage records were long enough to observe subsequent reverberations between the bar free end and the plastically deformed impact end. The measurements at 106 s−1 were made with a slightly modified version of a conventional flyer-plate impact configuration. The data are combined with static results to show the behavior of these steels at strain rates of 10−3 s−1 to 106 s−1.

A dynamic method for measuring the critical loads of elastic flat plates

Abstract: A dynamic method is described for determining the linear buckling loads of elastic, perfectly flat, rectangular plates. The proposed method does not require the application of in-plane loads; it requires only vibrational excitation of the plate. The buckling load is determined from the measured normal modes of vibration. The method is applicable to isotropic as well as anisotropic plates with any type of edge support. The accuracy of the dynamic method was evaluated by tests in which buckling loads of aluminum and graphite fiber-reinforced-epoxy composite plates were determined both by the dynamic method and by imposing static in-plane loads on the plates. The results of the dynamic and static tests agree closely.

The effect of a centrally located midplane delamination on the instability of composite panels

Abstract: The buckling loads of eight-ply graphite-epoxy cylindrical panels with midplane delamination were determined experimentally. The study included two different ply orientations, two different aspect ratios, two different delamination sizes, and one set of boundary conditions; clamped along the top and bottom edges and simply supported along the vertical sides. The experimental test results are compared to the linear bifurcation and nonlinear collapse loads of panels with square cutouts obtained from the STAGSC-1 finite-element computer code.

The Plastic Compressibility of 7075.T651 Aluminum.Alloy Plate

Abstract: The change in volume, and therefore the change in mass density, of an aluminum alloy was measured in uniaxial tension using clip-on extensometers. The experimental data do not agree with the assumption of plastic incompressibility found in the classical theories of plasticity. In fact, the elastic and plastic volume changes are of the same order of magnitude. Plastic anisotropy is thought to be the prime cause of this plastic compressibility.

Acoustic-Backscattering Studies of Transverse Cracks in Composite Thick Laminates

Abstract: The distribution of first-ply transverse cracks in graphite/epoxy 0.5-in. thick laminates was scanned by the acoustic-backscattering technique. The backscattering scans reveal a discrete number of characteristic dark bands in the case of specimens subjected to static loads or a low number of fatigue-load cycles. A dark band extending along the length of the specimen was obtained in specimens subjected to a high number of fatigue-load cycles. These dark bands reveal the three-dimensional distribution of transverse cracks situated in several adjacent planes. X-ray radiography shows the projection of the three-dimensional distribution of transverse cracks in the laminate plane and hence it may provide a low estimate of the distribution of transverse cracks in composite thick specimens.

An engineering failure envelope for adhesive joints

Abstract: An engineering failure envelope (EFE) for adhesive joints is presented and discussed. It is based on local nominal deformation parameters near the free edge of the bond where delamination may start. Tested on a double CLS specimen, this EFE exhibits an easy-to-apply failure envelope. Linear-fracture-mechanics tools show limited capability of predicting the failure load, since the singular region, especially for the interlaminar peeling stresses, is very small. Tests include simultaneous bending and stretching at different ratios in order to simulate realistic loading conditions.

Experimental observation of stress waves propagating in laminated composites

Abstract: The theoretical research on stress waves propagating in laminated composites has been reported by many authors. However, there has been little work on experimental studies of stress waves in those materials. This paper presents an experimental investigation on stress waves propagating parallel to the layers of a laminated composite. A sandwich laminated composite consisting of two aluminum facings and an epoxy core is used as a specimen. The stress wave in the specimen is observed by use of high-speed holographic interferometry with a pulsed laser. In order to obtain the relative fringe orders, the interference fringe pattern in the reconstructed image is treated as an image-processing system with a personal computer. For the calculation of the in-plane displacement, an approximate relative-fringe-order method is used. The in-plane displacements obtained at some sampling points on the surface are smoothed by using a spline function. Distributions of the in-plane displacement and the shear stress are then obtained quantitatively over the whole analyzed field.

Cumulative creep damage for polycarbonate and polysulfone

Abstract: Creep to failure tests performed on polycarbonate and polysulfone under single and two step loadings are discussed. A cumulative damage law or modified time fraction rule is developed using a power law for transient creep response as the starting point. Experimental results are approximated well by the new rule. Damage and failure mechanisms associated with the two materials are suggested.