Showing papers in "Experimental Mechanics in 1995"

Towards RGB photoelasticity: Full-field automated photoelasticity in white light

Abstract: In this paper a new full-field method for the automatic analysis of isochromatic fringes in white light is presented. The method, named RGB photoelasticity, eliminates the typical drawbacks of the classical approach to photoelasticity in white light which requires a subjective analysis of colors and an experienced analyst to acquire and interpret the results.

A pressure-shear plate impact experiment for investigating transient friction

Abstract: A pressure-shear plate impact experiment is introduced to study time-resolved friction at interfaces subjected to high sliding speeds under relatively high normal pressures. The conditions of slip at the interface are varied by changing the surface roughness of the impacting plates and by varying the applied normal to shear stress ratio. The configuration offers the simplicity of allowing the interpretation of the experimental data by using the frame-work of one-dimensional plane wave analysis. The interfacial material pairs investigated in the present study are comprised of a wear-resistant grade of tungsten carbide and either an AISI 4340 steel or a Ti-6AI-4V alloy. The experimental results indicate that the coefficient of friction increases with the increase in surface roughness of the tungsten-carbide plates and with cumulative slip at the interface.

An assessment of residual-stress measurements around cold-worked holes

Abstract: An X-ray diffraction technique was employed to determine the residual stresses introduced by cold working a fastener hole in a 6-mm thick 2024-T351 aluminum plate. The radial and tangenital residual stresses were measured at both faces of the plate and the measurements compared with the results from a two-dimensional axisymmetric finite-element model. The comparisons were favorable, although modifying the finite-element model to simulate the X-ray process provided better agreement. Experimental determinations of residual stresses showed differences between the two faces of the plate. This feature was attributed to the directional nature of the cold-working process.

Two methods for determining impact-force history on elastic plates

Abstract: Two methods are presented to detect the impact-force history on elastic plates. The first method involves construction of a Green’s function to relate the strain responses to an impact force acting axisymmetrically on a circular plate. The classical plate theory and a series of Bessel functions were used to obtain the Green’s functions in the time domain. The gradient projection method was employed to search for the optimal force history. Examples using circular plates with free and fixed boundary conditions are demonstrated. On the other hand, the second method is purely experiment-based and involves no Green’s function. Thus, this method can be applied to structures of various geometries, boundary conditions and material properties. The very satisfactory agreement between the measured and the detected force histories using both methods shows that they are reliable for impact-force determination purposes.

Thermoelastic stress analysis of orthotropic composites

Abstract: The temperature variation in a cyclically loaded orthotropic composite is proportional to a linear combination of the changes in the normal stresses in the directions of material symmetry. An effective method is presented here to determine the individual stresses from measured thermoelastic data in a region adjacent to an arbitrarily shaped fraction-free boundary of loaded orthotropic composites. The method, which is based on equilibrium and compatibility, uses complex-variable formulations involving conformal mappings, analytic continuation and numerical techniques.

The spatial and pressure resolution of fuji pressure-sensitive film

Abstract: Due to its ease of application, Fuji prescale pressure-sensitive film is currently one of the more popular methods, within the biomechanics community, for assessing contact areas and pressures within articulating joints—in addition to its use in industry. This material produces a stain on the application of pressure due to the rupture of microscopic bubbles releasing a liquid which, in turn, causes patches of color to be formed; a greater pressure produces a darker stain. These stains are often converted into digital images and manipulated to produce false-color pressuremaps, an approach which is beyond the simple methods of analysis suggested by the manufacturer. Due to the granular nature of Fuji film stains, the two user-defined variables which will determine the accuracy of any pressure-map are: (a) the size of the sample-area used to capture data from the original stain during the digitization process and (b) the number of pressure-intervals identified on each map; the chosen values should match the spatial and pressure resolutions of the film. Despite the importance of these factors, the literature presents a bewildering array of values, particularly for the number of pressure-intervals, with no validation of those chosen; consequently, little guidance is provided for other potential users of Fuji film. This paper discusses the relationship between sample-area and pressure-interval and introduces a method for examining their effect on the resulting pressure-maps. The results obtained using ‘Super Low’ grade Fuji film suggest that the authors of some previously reported methods may have been overambitious in their choice of sample-area and pressure-intervals. Finally, a series of suggested values of sample-area size and pressure-intervals are provided.

Phase-stepped ESPI and moiré interferometry for measuring stress-intensity factor and J integral

Abstract: Electronic-speckle-pattern interferometry and moire interferometry have been used to calculateK 1 andJ for compact tension specimens. Automated-fringe-pattern analysis enables the full-field of data to be used with the minimum of operator intervention. Measurements are shown to be accurate to within 10 percent. TheJ-measurement procedure employed could form the basis of an automatic-fault detection system.

Determining the size and location of transverse cracks in beams

Abstract: The spectral element method, which is very suitable for solving inverse dynamic problems, is combined with a stochastic genetic algorithm to give a scheme that can locate and size cracks in structural components. The mechanical model is based on an approach that separates the global structural dynamics from the local crack-tip zone dominated by singular stresses. The global model, consisting of connected waveguides, describes the structural dynamics using spectral elements. The local model, describing the crack-tip behavior, is based on the relation between the stress-intensity factor and the stored strain energy representing the crack region. The results are demonstrated with experimental data from an aluminum beam with a transverse crack.

Determination of stress-intensity factors by the method of caustics in anisotropic materials

Abstract: This paper studied the applicability of the method of caustics to anisotropic materials under Mode I and mixed-mode static-loading conditions and introduced the procedure to obtain stress-intensity factors (SIF) in anisotropic materials by the method of caustics.

Experimental strain analysis of the losipescu shear test specimen

Abstract: An experimental strain analysis of the losipescu shear test specimen was performed, utilizing a 20-ply AS4/3501-6 carbon/epoxy unidirectional composite. Using three-element strain-gage rosettes, it was shown that the presence of loading-point-induced transverse normal strains in the gage section do not affect the measured shear strain. Thus, the shear modulus determined using the standard notch specimen is not affected. Likewise, modulus determination is not influenced by cracking at the notch tips, since this occurs at strains beyond the range over which modulus is determined. To further evaluate the effect of notch-tip cracking, material was removed adjacent to the standard V-notches where these cracks initiate. The measured shear strength was unaffected by removing this material, although the shear modulus was reduced slightly (by as much as eight percent for the more grossly exaggerated geometries).

A dropped-weight apparatus for low-speed impact testing of composite structures

Abstract: A dropped-weight test apparatus has been developed that can be used to perform low-speed impact tests on composite aircraft structures. This vertical drop-weight test apparatus is simple, compact, inexpensive and has precision impact and self-arresting design features similar to the more sophisticated, expensive test machines. The test apparatus has been used to perform low-speed impact response studies on laminated composite plates to understand the influence of impactor and target parameters on structural response and to develop a validated analysis method. Some of the experimental results generated by using this test apparatus for composite laminated plates are presented in the present paper and compared with the corresponding analytical results.

Additive-subtractive phase-modulated shearography with synchronized acoustic stressing

Abstract: A robust electronic shearography technique is described that is immune to ambinet noise. In this technique, additive speckle-interference images containing information about the same two states of deformation of a test object undergoing acoustic stressing are acquired in each video frame. When a spatially nonuniform phase modulation is introduced in every other frame during the video sequence (using a tilting mirror) and the additive interference images are subtracted sequentially using a real-time image processor, the self-interference component of sheared-speckle interference pattern is removed. The susceptibility of this method to environmental noise caused by induced thermal noise and low frequency vibration is demonstrated to be lower than that of conventional substractive shearography methods; fringe visibility and contrast are significantly improved as well. The ability of this technique to work in a turbulent environment is demonstrated, and application to detection of structural defects in adhesively bonded structures, a problem of interest in nondestructive evaluation of structures, is shown.

Grating Interrogations: from Small to Large Strain Measurement

Abstract: Our purpose is the direct strain measurement from the interrogation of a crossed grating marked on the surface of a specimen. The observation of the object through a master grid (as in moire method) is replaced by a direct characterization of the pattern using a Fourier transform. This gives direct access to the modification of the pitches which leads to quantification of the strain without the intermediary of a fringe pattern. The comparison between the undeformed and deformed states allows the determination of the magnitude and orientation of principal strains and of the local rigid-body rotation. We describe three analysis techniques, one using the diffraction phenomenon, another utilizing a numerical spectral evaluation and third combining diffraction and phase-shifting procedure. These grid interrogations have different domains of application and a suitable choice of these analysis techniques allows a very large measurement range (10−5 to high strain according to the grating resistance).

Determining reliable edge isopachic data from interior thermoelastic measurements

Abstract: The maximum stresses in a plane-stressed component typically occur on the boundary. However, it is generally difficult to obtain reliable experimental data at an edge and thermoelastic stress analysis is no exception. The inability to measure reliable edge isopachic stresses has caused many previous thermoelastic stress analyses to be more qualitative than quantitative. This paper develops and implements an effective iterative least-squares method for calculating reliable edge isopachic stresses from measured interior values. The method is based upon the plane-stress isotropic compatibility equation. A regularization scheme is employed to minimize the sensitivity to measurement error and to improve the stability of the algorithm by controlling the rate of convergence. An illustrative example with actual measured thermoelastic data is included. The processes thermoelastically determined results compare well with those obtained using strain gages.

Design of a focal-plane array thermographic system for stress analysis

Abstract: A state-of-the-art thermographic system has been developed based on a 512×512 focal-plane array thermal imager. The system, dubbed FAST (for focal-plane array for synchronous thermography) is able to deliver high-resolution full-field thermoelastic stress scans in minutes rather than hours. The paper itemizes the hardware components together with their functions.

Buckling of slender string in cylindrical tube under axial load: experiments and theoretical analysis

Abstract: Buckling of drill and tubing strings in drill or casing holes will affect the life of the string and cause difficulties in drilling and in oil production. A number of theoretical models have been developed to study the buckling load and the post-buckling configuration of the strings under some idealized conditions. However, verification of these theories in practice or in laboratory has not been extensively reported.

Performance of the FAST system for stress analysis

Abstract: A state-of-the-art thermographic system has been developed based on a 512×512 focal-plane array thermal imager. The system was primarily designed to perform stress analysis, but has applications in other areas where small cyclic temperature variations are encountered. A detailed analysis of its performance relative to an existing equipment (SPATE) revealed that, at least for low frequency applications, the proposed system out performs SPATE by some two to three orders of magnitude such that high-resolution area scans can now be achieved in minutes instead of hours. It is suggested that higher frequency applications (i.e., order of 102 Hz) can be achieved with more up-to-date imagers.

The losipescu in-plane shear test: Validation on an isotropic material

Abstract: The purpose of this paper is to compare the value of the shear modulus of an isotropic material (PMMA) obtained from both tensile and losipescu shear tests. A finite-element model is introduced to account for a nonuniform stress distribution in the losipescu coupon. A correction accounting for nonuniform strain values over the strain-gage section is also introduced. Rigorous statistical data processing leads to an estimation of the error between the moduli obtained by the two tests. Some points relevant to the losipescu test are reviewed, particularly concerning the sample preparation. Provided that a careful procedure is followed, excellent agreement is achieved between the shear moduli measured by the two methods.

Characterization of piezo-film sensors for direct vibration and impact measurements

Abstract: Piezo-film sensors were employed in determining the dynamic response of [(0 deg/90 deg)4]s s-glass/epoxy laminates and 2024 aluminum specimens. Simple beam- and plate-type sensor equations were derived based on classical plate theory incorporating the linear piezoelastic constitutive relationship. A series of vibration and impact tests were conducted for the determination of structural dynamic response. Piezo-film sensors, with a thickness and area of 110 μm and 1×1 cm2, respectively, were connected directly to a voltage measurement device in these tests. The first three bending frequencies of the glass-fiber-reinforced plastic (GFRP) cantilever specimen were examined. Experimental results and those simulated by the MARC finite-element code were found to be in good agreement, with the difference between the two being less than five percent. At frequencies above 3 kHz, piezo-film transducers are capable of closely detecting structural dynamic response in the absence of charge amplification. At frequencies lower than approximately 3 kHz, however, the voltage measured directly from a piezo-film sensor underestimates structural response. A modified piezo-film sensor equation is thus proposed for lower frequency measurements. Effect of frequency and piezo-film's size on lower frequency attenuation is explicitly formulated based on a simple RC circuit analogy. Drop tests were also performed on clamped [(0 deg/90 deg)4]s GFRP laminates and aluminum targets, with nine piezo-film sensors being glued to the specimen's distal surface in order to determine the low-speed impact response. Specimen transient central deflection subjected to impact loading was identified based on test findings and the plate-type piezo-film sensor equation. The results were found to be in good agreement with the numerical solution obtained from the MARC finite-element code.

Enhanced Sensitivity Residual-stress Measurements Using Taper-hole Drilling

Abstract: A novel method for enhancing the strain sensitivity of the hole-drilling method for measuring uniform residual stresses is examined. Such enhanced strain sensitivity is important because it improves the accuracy of the residual-stress evaluation. The new method involves enlarging the effective hole size by drilling a reverse taper hole. A simple practical technique for drilling reverse taper holes is described. The strain sensitivity for this new method is compared with that of the conventional hole-drilling method. Experimental results show excellent correspondence with theoretical results. The reasons for the sensitivity improvement are explained.

Crack-tip Caustics in Mechanically Anisotropic Materials

Abstract: This contribution focuses on the theoretical development of the method of caustics and its applicability to anisotropic materials. The method displays its full power when employed in conjunction with interactive numerical data reduction and evaluation procedures. For the analysis the selection of data points along the experimentally recorded caustic curve is done automatically, the selected points are marked on the screen and if necessary interactive correction of the positions is possible. Final proof of the correctness of the result of the automatic data point selection is achieved by comparing for acceptable coincidence the numerically generated caustic determined on the basis of the results of the data-reduction technique with the experimentally recorded caustic.

Response Spectrum Methods in Tank-vehicle Design

Abstract: Existing design rules for tank vehicles have proved insufficient, because vibrations often cause fatigue cracks. Measurements have been performed on the tank to provide a picture of the influence of different road types and filling ratios. Shock response spectrum analysis (SRS) was used to obtain a measure of single-dynamic events. To get a basis for dimensioning against fatigue a calculation of fatigue-damage response spectra (FDRS) has been performed. This relates the vibrations in the tank to the risk of fatigue damage. In lieu of cycle counting by rain-flow count techniques, which has certain disadvantages, a new model, the so called HdM model, for fatigue-life assessment based on level crossing has been used. Further development of the results of the analysis can be used to improve design criteria in transport tank regulations, as well as to determine optimum inspection intervals for, in particular, tanks for hazardous materials.

Characterization of Shock Accelerometers Using Davies Bar and Laser Interferometer

Abstract: In this paper, we propose a novel method for evaluating the frequency response of shock accelerometers using Davies bar and interferometry. The method adopts elastic wave pulses propagating in a thin circular bar for the generation of high accelerations. The accelerometer to be examined is attached to one end of the bar and experiences high accelerations of the order of 103∼105 m/s2. A laser interferometer system is newly designed for the absolute measurement of the bar end motion. It can measure the motion of a diffuse surface specimen at a speed of 10−3 ∼100 m/s. Uncertainty of the velocity measurement is estimated to be±6×10−4 m/s, proving a high potential for use in the primary calibration of shock accelerometers. Frequency characteristics of the accelerometer are determined by comparing the accelerometer's output with velocity data of the interferometry in the frequency domain. Two piezoelectric-type accelerometers are tested in the experiment, and their frequency characteristics are obtained over a wide frequency range up to several ten kilohertz. It is also shown that the results obtained using strain gages are consistent with those by this new method.

Three-jack solution to obtain a truly stable and symmetric tensile concrete test

Abstract: Testing equipment, made up of three orthogonally arranged jacks, which is able to apply a purely tensile load to a concrete specimen is proposed. The secondary flexural stresses are kept under control and constitute a degree of error comparable to the values allowed for normal testing apparatus. When the crack formation tends to produce asymmetry in the system, the counter-reaction intervenes, annulling the load eccentricity. In this way the true tensile properties (strength and fracture energy) of the material may be measured.

Estimation of Flexible Cylinder Displacements in Wave-basin Experiments

Abstract: This paper describes a method developed to estimate the three-dimensional displacement field of long, flexible cylinders subjected to wave loading. The method makes use of the relationship between curvature and displacement of a uniform beam. The technique was implemented in a large-scale experimental study to investigate the response of a pair of flexible cylinders in close proximity, with an emphasis on the collision behavior between them. The model cylinders were designed to be representative of risers or tendons of a tension-leg platform in approximately 1000 m of water. The cylinder models, approximately 17-m long and 0.03-m diameter, were instrumented to measure the cylinder curvature at discrete locations along their length. At each time step the displacement field was computed from the curvature measurements using a ‘shoot-to-fit’ numerical-integration scheme. This procedure yielded a complete description of displacement along the cylinder as a function of time. The number and location of the curvature measurements were optimized by simulating the wave-flexible cylinder interaction with a finite-element model that was also used to demonstrate the accuracy of the method. The effectiveness of the technique is demonstrated by results from the experimental program, and it appear to be feasible to adapt this approach for field experiments.

Approaches for the generation of stress waves in concrete

Abstract: This paper reviews several existing, as well as potential, stress-wave-generating techniques for concrete. The generating techniques under study include standard ultrasonic pulse generators, high-energy ultrasonic toneburst systems, and simple impact. In addition, the use of laser sources and ultrasonic lithotripters are proposed. The novelty of this review is that the methods are analyzed considering applications to existing concrete NDE methods. The strengths and weaknesses of each generation technique are discussed. Some of the methods are demonstrated through laboratory tests on a variety of specimen types. A description of each technique and conclusions concerning the appropriateness of each technique are also made.

Instability of confined rings : an experimental approach

Abstract: The stability of a ring confined by a rigid boundary, subjected to circumferential end loads is investigated by an experimental approach. Dimensional analysis was used to establish the basic relationship of the critical buckling stress with material properties, geometric dimensions and initial geometric imperfections. The coefficients in the relationship were obtained experimentally. Compared to other theoretical methods, this approach is very simple, straightforward and accurately fits the experimental data. Therefore, it is suitable for practical applications.

An experimental investigation of the structural response of reactor fuel plates

Abstract: In some reactors, thin fuel plates are cooled by water flowing through thin channels on either side of the plates. There is a need to know the amount of deformation in the fuel plates due to the coolant flow so that failures can be avoided. A verifiable solution to this problem in the past has not been available. An experimental-analytical solution to this problem for design purposes has been developed and is herein presented.

Fatigue crack-tip deformation, crack growth and life prediction in polycarbonate

Abstract: Low-cycle fatigue fracture of polycarbonate is investigated Local strain in the vicinity of the notch root and crack tip has been measured in real time by using a fine-grid method The relationships among the local strain, crack initiation from the notch root and crack propagation of the crack tip are studied, and a method for more precise life estimation is suggested

Measurement of residual stress in the weld overlay piping components

Abstract: In general industry, especially in the nuclear industry, welding overlay repair is an important repair method mainly used to rebuild piping systems suffering from intergranular stress-corrosion cracking (IGSCC).