Showing papers in "Experimental Mechanics in 1975"

Ballistic-penetration resistance and its measurement

Abstract: Heretofore, the resistance of a barrier to ballistic penetration has been characterized chiefly by a specific ballistic-limity V50, the velocity at which a specified projectile has an even (50–50) chance of perforating a plate in a specified manner. Conventional procedures for measuringV50 involve statistical sampling. For each situation, a number of rounds must be fired; each round must have the same geometrical attitude at impact.

Using the scanning infrared camera in experimental fatigue studies

Abstract: Materials under cyclic loading dissipate energy in the form of heat due to hysteresis effects in the material. At locations of high stress levels, more heat is released than elsewhere, resulting in a local temperature rise in those areas. The scanning infrared camera has been used in this study to visualize the surface-temperature field on steel and fiberglass-epoxy composite samples during fatigue tests. The information achieved in this manner allows one to predict the probable location of the greatest fatigue damage well before such damage becomes visible in the form of a crack. The use of the scanning infrared camera for monitoring traveling cracks and mapping the temperature fields resulting from stress concentrations in cyclically loaded materials is also demonstrated. The results indicate that this instrument is of value in both nondestructive testing and crack-propagation studies.

Analysis of Rigid Polyurethane Foam as a Shock Mitigator

Abstract: In problems of impact, the compressive-stress wave generated at the surface of contact can be quite severe In certain instances, it may be necessary to mitigate the peak stress Many materials, eg, rigid polyurethane foam, exhibit elastic, nearly perfectly plastic stress-strain relations Stresses greater than the yield stress cannot be transmitted until compaction occurs

Critique of Southwell plots with proposals for alternative methods

Abstract: The theory of the Southwell plot is outlined and its history traced. Examples are given in which the technique fails to give a straight line; these are classified into low-load and higher-load nonlinearities. It is shown that the former may be due to inherent zero errors in the deflection measurement and that this can cause serious errors in the apparent critical loadP c. A new approach involving “pivot points” is introduced and used to formulate plots which may help to linearize the data; an application to one of Karman's classical column tests is given. For plates, use is made of a general post-critical equation of which Donnell's result for a simply supported square plate is a special case. A new graphical technique is proposed for findingP c for plates with small imperfections, and applied to example data from published test results. A more general numerical technique is also proposed and applied to results from a test during which the experimental boundary conditions were changing.

Experiments on the postbuckling behavior of circular cylindrical shells under compression: Precise experimental results are presented clarifying the whole aspect of the postbuckling behavior of circular cylindrical shells under compression for a wide range of shell geometries

Abstract: Detailed experimental studies are performed on the postbuckling behavior of circular cylindrical shells under compression, by using polyester test cylinders with the geometric parameterZ ranging from 20 to 1000. In each case, variations of the equilibrium load, circumferential wave number and maximum inward and outward deflections, with applied edge shortenings, are clarified. Contour lines for typical postbuckling configurations are also shown. It is found that, as the cylinder is compressed beyond the primary buckling, secondary bucklings take place successively with diminishing wave numbers, and that postbuckling equilibrium loads become significantly lower than those at buckling asZ increases. Further, for short shells withZ≦100, the buckled waveforms are always symmetric with one-tier diamond buckles, while for longer shells, asymmetric postbuckling patterns with two tiers of buckles dominate.

On photoelastic response of composites

Abstract: Using the theory of the unitary system of retarders, it is shown that two seemingly different theories of photoelasticity (Sampson’s phenomenological theory and Dally and Prabhakaran’s stress-proportioning concept) are identical if the heterogeneous nature of the composites is respected. It is also shown that the optical isoclinic parameter can be accurately predicted even if the initial birefringence is present in the unloaded specimen.

On wave and fracture propagation in Rock Media - Transient deformations and wave and fracture propagation were studied in marble and granite plates loaded explosively, by means of photoelastic coating methods, and moire and high-speed photographic techniques

Abstract: Experimental-stress-analysis techniques were used to study wave and fracture propagation in rock media. Marble and granite plates were loaded explosively. Wave propagation was observed through isochromatic-fringe patterns on bonded photoelastic coatings and moire-fringe patterns. These patterns were recorded with a Beckman and Whitley camera operating at rates from 250,000 to 1,000,000 frames per second. Dilatational, shear and Rayleigh wave velocities were determined. The leading part of the pulse is compressive and shows appreciable attenuation. The trailing part goes into tension, causing widespread tensile fracture. The velocity of propagation of this fracture zone in marble was nearly equal to the theoretical terminal velocity. In the case of induced cracks in marble and granite, the velocities of crack propagation were appreciably lower than the theoretical terminal values. Experimental results obtained were discussed and interpreted for their relevance to the rapid-excavation process in rock.

Three-dimensional strain rosettes: Pattern selection and performance evaluation

Abstract: The configuration of three-dimensional strain rosettes is shown to be directly related to precision and reliability. Equations are given for computing these parameters, and several rosette configurations are compared accordingly.

The strain-rate behavior of ductile polymers

Abstract: The stress-strain-strain-rate behavior of polycarbonate is presented. It is demonstrated that the material does not exhibit a double-yield-point phenomenon, as others have reported, if true stress is plotted against actual strain. Also, the behavior of polycarbonate is presented under constant strain rate and relaxation conditions. The observed behavior of the material is discussed in relation to elements of recent theories of viscoelasto-plasticity due to Nagdi and Murch and Crochet. Simple mechanical models of the Bingham type are presented and are discussed with respect to the constitutive equation characterization of polycarbonte. The advantages and disadvantages of more general models are mentioned. Finally, the strain-rate behavior of PMMA (Polymethylmethacrylate) and a poylester given by others is presented and discussed, relative to polycarbonate and the characterization procedures used.

Determination of residual stresses in aluminum-alloy sheet material

Abstract: A method has been developed for the determination of residual stresses in thin sheet material. Thin layers are removed by chemical etching, which induces an increasing curvature of the sheet. The initial distribution of the residual stress is derived from curvature measurements. Details of the etching procedures and the equations required for the calculations are presented. Results are given for as-rolled sheet material and for spot-peened material.

The size of plastic zones in cracked plates made of polycarbonate

Abstract: The influence of the thickness of a cracked plate, made of Polycarbonate of Bisphenol A, on the plastic zones developed at the crack tip was studied. The three-dimensional character of yielding at the vicinity of the crack tip is revealed. The study of the thickness variation in the plastic zones is made by applying the shadow-moire method. It has been derived that the thickness variation in the plastic zones is discontinuous with successive dimples of different thickness. The influence of the overall thickness of the specimen on the size and length of the plastic zone was studied for different crack lengths. Irwin's theory for small yielding, as well as the simple and modified Dugdale-Barenblatt models, were applied for the study of plastic zones. By comparing the plastic-zone lengths in relation to their shape, with those given by the above theories, it may be concluded that, for small loads where the applied stress σ is not exceeding 0.40 of the yield stress $$\sigma _o $$ in tension and small values of the ratio of crack lengtha to plate thicknesst, (a/t<4) a state of plane strain dominates at the vicinity of the crack tip, while, by increasing the load and ratioa/t, the plastic zones approach in shape and size those given by the modified Dugdale-Barenblatt model.

Determination of slope and strain contours by double-exposure shearing interferometry

Abstract: An interferometric technique is used to determine directly patterns of derivatives of displacements. In the case of plate models, the fringes are loci of equal slope; for specimens provided with surface gratings, one forms patterns of partial derivatives of in-plane displacements. A theory for the interpretation of the interference fringes in terms of displacements is outlined; some experimental applications illustrate the paper.

An analytical and experimental study of vibrating equilateral triangular plates

Abstract: A general description and a historical discussion of trilinear coordinates is given. These coordinates are then used to develop the solution of a simple supported freely vibrating equilaterial triangular plate. The solution gives fundamental frequencies, mode shapes, nodal lines and a general expression for displacements. An experimental program is described to validate the analytical solution. Nodal lines, mode shapes and displacements are determined by holographic interferometry and compared to theoretical results. Excellent agreement is obtained. A second experimental technique, a fiber-optic device, is used to determine displacements with the results being compared to the displacements obtained by both theoretical and holographic analysis.

Strain gages for long-term high-temperature strain measurement

Abstract: The capabilities of resistance and capacitance strain gages for use in the temperature range of 1100–1400°F for durations up to 10,000 h are reviewed. Characteristics are given in summary and tabular form along with a list of references. Resistance strain gages offer some capability under these conditions, but the newer capacitance gages show promise for both laboratory and field use at high temperatures.

Measurement of fatigue damage in composite materials: Fatigue-damage initiation sites in a composite material can be related to regions of high strain-energy density and can be monitored using nondestructive-testing techniques

Abstract: Numerical results for the stress state around a circular hole in a [0/±45/0]s boron-epoxy plate under tensile loading are presented This serves as a model for the initial stress state around the hole during fatigue loading Comparison is drawn with experimental results for a fatigued specimen obtained from thermography and radiography Using these results, an interpretation of the effects of the initial stress state on the thermal behavior and on failure initiation is given This interpretation shows that the circumferential normal stresses are responsible for the initial heat generation and failure initiation in the fatigued specimen

A shadow-moiré method with two discrete sensitivities

Abstract: A shadow-moire method using a composite grating with two discrete pitches is proposed whereby the range of measurable deflection is enlarged as compared to the ordinary shadow-moire method.

On the stress-optic law for orthotropic-model materials in biaxial-stress fields

Abstract: The stress-optic law for othotropic-model materials, proposed by Sampson on the basis of a simple analogy to the isotropic-model materials, is examined for biaxial-stress fields. The stress-optic law is reduced to a simple form for special cases. It is also shown that the zero-order isochromatic fringe corresponds to an isotropic state of stress only in the case of balanced laminates. A glass-fiber-reinforced plastic disk with the glass fibers in only one direction is examined under diametral compression photoelastically and by means of strain-gage rosettes, with the loading direction perpendicular and at 45 deg to the reinforcement direction. The fringe order along the horizontal diameter is computed from the simplified stress-optic law making use of stress values from strain-gage readings and compared with the observed fringe order. Based on a fairly good agreement of the fringe orders, it is shown that a circular-disk specimen can be used to calibrate an orthotropic-model material. The three independent material-fringe values,f L ,f T ,f LT , can be found from measurements of the fringe order and the strains at the center of the disk for the three cases of loading perpendicular, parallel and at 45 deg to the reinforcement direction.

Investigation of stress-concentration factors in tensile strips

Abstract: A photoelastic and a numerical investigation has been carried out to determine the stress-concentration factors at the edge of a central circular hole in a tensile strip for different ratios of hole diameter to width of the strip. The photoelastic data and the numerical results indicate that the stress-concentration factor at the minimum cross-sectional area tends to a value of two if the ratio of the hole diameter to the strip-width approaches a value of one.

Ambulatory-force measurement using an instrumented-shoe system: Lower-leg loading envelopes occurring during common activities in nonlaboratory situations can be determined by using a shoe-mounted force and position-measuring system

Abstract: A knowledge of the external forces acting on the human lower limbs is necessary for the effective formulation of safety precautions A new approach of determining lower-leg forces that is more portable than existing measurement systems has been developed Shoe-mounted load cells measure the orthogonal components of foot-to-ground forces and moments while goniometers continuously monitor the position of the lower leg The output signals are transposed to a system of loads acting at the lower leg The axial torque calculated to occur during common turning maneuvers under various conditions is presented and discussed

Experimental cross verification of damping in three metals

Abstract: In order to ensure a valid damping measurement based on a proposed mathematical model, various laboratory techniques may be employed which require relatively simple instrumentation from which cross correlation of the data may be obtained. Measurements were made (utilizing an accelerometer, strain gages, and a sound-level meter) under forced (3-dB frequency band or amplification factor Q) and free-vibration conditions at the fundamental and higher axial modes. Analysis (based on the viscous and complex-moduli theories) of the material damping of an aluminum alloy 2024-T4 in longitudinal vibrations shows good cross correlation between theories and experimental methods. These methods were extended to damping measurements of yellow brass and commercial steel. A multiple regression analysis was employed in the damping data reduction and was found to be useful in eliminating the data scatter. A sound-level meter was used as a noncontacting test method which proved useful even though its use does have drawbacks.

High-temperature fatigue-life estimation: Extension of a unified theory

Abstract: A procedure is presented for predicting the fatigue behavior at elevated temperature by extending the unified theory of fatigue damage previously proposed for room temperature. The method predicts the experimental results of high-temperature push-pull tests under isothermal conditions, using the total strain range. The analysis is based on parameters obtained from short-term tensile tests in which the temperature and the strain rate are the same as for the fatigue test. The procedure is applied for fatigue of a stainless steel at 650°C under cyclic axial strain. It has also been applied to published data for three austenitic stainless steels. In general, the present procedure gives estimates closer to experimental results than those obtained from other known methods.

Off-table holography: A technique that permits the practice of holography in a vibration environment

Abstract: A technique for practicing holography in a vibration environment is described and illustrated with examples. Advantages would consist of the ability to examine small areas of large structures at their installed site, ability to examine deformation in a local area of a structure despite large motions of the overall object, ability to practice holography in a vibrating environment, such as an industrial shop, without vibration-isolation equipment, ability to reconstruct holograms with white light rather than laser light and a much lower equipment cost as compared with the usual laboratory holography system.

Acoustic emission from composite-reinforced metals

Abstract: Acoustic-emission (AE) count rates are presented for tensile loading of unidirectional boron-epoxy and for aluminum sheets reinforced with unidirectional boron-epoxy. It is shown that different prepreg materials have different characteristic AE patterns. Results from composite-reinforced metal specimens show that early failures are accompanied by a sharp increase in AE count rate at the knee of the bilinear stress-strain diagram. It is further shown that the count rates are a function of specimen fabrication and that higher total counts do not necessarily correspond to early failures.

Photoelastic analysis of interlaminar matrix stresses in fibrous composite models: Through the use of scattered-light photoelastic methods, high-matrix stress magnitudes are observed in multilaminar fibrous composite models

Abstract: Scattered-light methods are presented for analysis of interlaminar matrix stresses between the fiber layers of composite models. These include data-smoothing techniques and a least-squares solution to utilize the excess information available. Applications are given to determine all stress components in the matrix of a two-layer model and to determine the interlaminar shear stress in the matrix of a four-layer model. The results indicate that matrix stress magnitudes are a function of proximity to the fibers and that they are significantly higher than composite stresses obtained using mathematical models.

Development of a low-frequency-vibration calibration system: The development and calibration of an electrodynamic vibration exciter was developed specifically to calibrate accelerometers at frequencies from 55 to 2 Hz

Abstract: The development of the low-frequency-vibration apparatus described in this report was done in response to a need to establish and verify vibration-pickup performance at infrasonic frequencies. The exciter design is an extension of the Dimoff type exciters1 and, in fact, many of its components are identical.

Buckling experiments on stiffened cast-epoxy conical shells

Abstract: This paper describes a casting technique for fabricating high-quality plastic structural models and presents results on the use of such specimens to parametrically study the effect of base-ring stiffness on the critical buckling pressure of a ring-stiffened conical shell. The fabrication technique involves machining a metal mold to the desired configuration and vacuum drawing the plastic material into the mold. A room-temperature-curing translucent thermoset epoxy was the casting material selected. The casting technique allows many high-quality specimens to be produced and each specimen is capable of being repeatedly tested without failure. The conical shell was modified for successive tests by machining the epoxy base-ring configuration to reduce its stiffness. A shell-of-revolution computer program which uses a nonlinear axisymmetric prebuckling strain field to obtain a bifurcation-buckling solution was used to guide the selection of configurations tested. The shell experimentally exhibited asymmetric collapse behavior and the ultimate load was considerably higher than the analyticalbifurcation prediction. The asymmetric buckling-mode shape, however, initially appeared at a pressure near the analysis-bifurcation solution. Comparison of experimental and analytical prebuckling strains at pressure magnitudes below the initiation of asymmetric collapse showed good agreement.

Vibration and stability in thermally stressed roatating disks

Abstract: Paper presents the experimental confirmation of a vibration and stability model of a thermally stressed rotating disk. Attention is directed to the rotating-disk convective-heat-transfer problem. Measured temperature distributions are used in the computation of the theoretical characteristic frequency spectrum which is then verified by the experimental frequency spectrum. The theoretical characteristic spectrum is shown to be effective in predicting the likelihood of a critical speed instability of the thermally stressed rotating disk. Optimal control of disk operation using this model is proposed.

Maximum stress in a tensile strip with a large hole

Abstract: Measurements were made to decide between two solutions of the title problem. In question was the limiting value of the stress-concentration factor at the edge of a large hole in a tensile strip as the hole diameter approached the strip width. The results indicate that the stress-concentration factor is near two and support one of the solutions with a minor qualification.

Use of a curved grating in shadow moiré

Abstract: The shadow-moire method has been extended to study the large displacements of a uniformly loaded hyperbolic-paraboloid shell. The new technique employs a curved grating which is everywhere equidistant from the initial position of the surface, as in the application of a plane grating in the study of plates. The theory of the curved grating is developed. Experimental verification is reported and a significant increase in accuracy over the equivalent plane grating is observed. The proposed improvement can be applied to the study of any ruled surface.