Showing papers in "Experimental Mechanics in 1983"

The losipescu shear test as applied to composite materials

Abstract: A history of the losipescu shear test as applied to composite materials is presented along with a description of the test fixture and specimen design. Iosipescu's shear test is compared to similar test techniques, including the asymmetrical four-point bending (AFPB) test. Finally, in-plane and through-the-thickness shear properties measured using the losipescu shear tests are presented for a variety of materials, including a unidirectional graphite/epoxy, random and continuous-fiber sheet molding compounds, and two polymer materials.

Fragmentation of metal rings by electromagnetic loading

Abstract: A method is described for performing fragmentation studies on rapidly expanding metal rings A fast-discharge capacitor system generates magnetic forces which accelerate the rings to maximum radial velocities of approximately 200 m/s corresponding to circumferential-strain rates of approximately 104/s at fragmentation Streak-camera techniques are used to record the time-resolved motion of the rings Fracture-strain and fragmentation experiments have been performed on samples of OFHC copper and 1100-0 aluminum

Half-fringe photoelasticity: A new approach to whole-field stress analysis

Abstract: This paper presents a new method for whole-field stress analysis based on a symbiosis of two techniques—classical photoelasticity and modern digital image analysis. The resulting method is called ‘half-fringe photoelasticity (HFP)’. Classical photoelasticity demands materials with high birefringence, which leads to extensive use of plastics as model materials. Since the behavior of these materials is often different from that of the prototype materials, their use distorts the similitude relationships. In many contemporary problems this distortion is untenable. HFP offers a way out of this dilemma. It permits materials and loads to be chosen so that no more than one half of a fringe order appears in the area of interest. Thus, for example, glass, which behaves linearly up to high stress levels and over a wide range of temperatures, could be used as model material. Alternatively, models from polymeric materials could be used under very low load in order to stay within the linear part of the stress-strain diagram and to prevent large deformations. The half-fringe-photoelasticity system, which is described here, utilizes the resulting low levels of birefringence for effective stress analysis. This paper describes the system. It outlines a calibration routine and illustrates its application to two simple problems using glass models.

Observation of damage growth in compressively loaded laminates

Abstract: An experimental program to determine tie phenomenological aspects of composite-panel failure under simultaneous compressive n-plane loading and low-velocity transverse impact [C-75 m/s (0-250 ft/s)] is described High-speed photography coupled with the shadow-moire technique is used to record the phenomenon of failure propagation The information gained from these records, supplemented by plate sectioning and observation for interior damage, has provided information regarding the failure-propagation mechanism The results show that the failure process can be divided roughly into two phases In the first phase the plane is impacted, and the resulting response causes interlaminar separation In the second phase the local damage spreads to the undamaged portion of the plate through a combination of laminae buckling and further delamination

Moiré interferometry at VPI & SU

Abstract: A relatively easy technique for producing high-frequency gratings on specimens extends moire techniques into the high-sensitivity domain. Whole-field patterns of inplane displacement components are obtained with grating frequencies of 1200, 2400 and 4000 l/mm (30,480, 60,960 and 101,600 l/in.). Moire interferometry is a case of two-beam interference, characterized by extensive range, excellent fringe contrast and fringe localization on the specimen surface. It is a reflection technique, compatible with opaque specimens and live observation of deformation.

Hybrid experimental-numerical stress analysis

Abstract: The hybrid experimental-numerical stress-analysis technique, which saw limited applications during the 1950's, has been resurrected with the vastly improved numerical techniques of the 1970's. By inputing the experimental results as initial and boundary conditions, modern computer codes are executed in its generation and application modes to yield results which are unobtainable when only one of the two techniques is used. The hybrid technique thus exemplifies the complementary role of the experimental and numerical techniques.

Dynamic Crack Curving - A Photoelastic Evaluation.

Abstract: A dynamic-crack-curving criterion, which is valid under pure Mode I or combined Modes I and II loadings and which is based on either the maximum circumferential stress or minimum strain-energy-density factor at a reference distance ofr 0 from the crack tip, is verified with dynamic-photoelastic experiments. Directional stability of a Mode I crack propagation is attained when $$r_o= \frac{1}{{128\pi }}(\frac{{K_r }}{{\sigma _{ox} }})^2 V_o^2 (C,C_1 ,C_2 ) > r_c $$ wherer c =1.3 mm for Homalite-100 used in the dynamic-photoelastic experiments.

Buckling of cylinders with cutouts under axial compression

Abstract: An experimental study was carried out to clarify the effects of circular holes on the buckling of circular cylinders under axial compression. The effect of reinforcements was also examined by placing thin annular plates around the cutouts. Tests were performed on polyester shells with radius-to-thickness ratio of 400 and 100 and with two diametrically opposed circular holes.

Simultaneous measurements of stress intensity and toughness for fast-running cracks in steel

Abstract: Simultaneous measurements of the dynamicstress-intensity factorK I dyn and the dynamic-fracture toughnessK ID were made in a high-strength steel to investigate the relation between energy delivered to and energy absorbed by rapidly propagating cracks. Values ofK I dyn were obtained intermittently during the propagation history by the shadow optical method of caustics from high-speed photographs of the moving crack tips. Values ofK ID were calculated from temperature maxima recorded by thermocouples near the crack path. The results indicate that for fast-running cracks, the change in energy available at the crack tip can be significantly less than the energy absorbed in crack extension, suggesting that currently used dynamic-energy-balance methods for determining dynamic-fracture toughnesses may provide erroneous values.

In-plane shear test of thin panels

Abstract: Efficient application of thin-gage composite materials to helicopter fuselage structures necessitates that the materials be designed to operate at loads several times higher than initial buckling load Methods are required to accurately measure and predict the response of thin-gage composites when subjected to these loads This paper presents the results of an analytical and experimental study of the behavior of thin-gage composite panels subjected to in-plane shear loads Finite-element stress analyses were used to aid in the design of an improved shear fixture that minimizes adverse corner stresses and tearing and crimping failure-modes characteristic of commonly used shear fixtures Tests of thick buckle-resistant aluminum panels and thin aluminum and composite panels were conducted to verify the fixture design Results of finite-element stress and buckling analyses and diagonal-tension-theory predictions are presented Correlation of experimental data with analysis indicated that diagonal-tension theory can be used to predict the load-strain response of thin composite panels

An edge-cracked Mode II fracture specimen

Abstract: A proposed edge-cracked Mode II fracture specimen is analyzed using an extended photoelastic-numerical procedure. It is shown that the Mode II stress-intensity factor is significantly higher for the longest edge-crack considered and the Mode I deformation is negligible, demonstrating the merits of this specimen for Mode II fracture-toughness testing.

A high-strain biaxial-testing rig for thin-walled tubes under axial load and pressure

Abstract: It is well know that high-strain biaxial testing with axial force and pressure on thin-walled tubes can be interesting from several points of view: capability of testing over the whole range of strain or stress ratios, great versatility of testing. and because of the physical system, the measured strains and stresses are principal. Nevertheless, there are few experimental-biaxial-fatique data due to the complexity of the testing equipment involved. In this paper, an experimental testing rig capable of subjecting thin-walled tubes to combined dynamic loadings in tension-compression with torsion and tension-compression with internal-external pressure is presented. The following components are described: the mechanical and hydraulic system, servohydraulic control, specimen configuration, strain-measuring equipment and data-acquisition system. The computation of stresses and strains, as well as examples of data acquisition, are also shown.

Further studies on dynamic crack branching

Abstract: The newly derived dynamic-crack-branching criterion with its modifications is verified by the dynamicphotoelastic results of dynamic crack branchings in thinpolycarbonate, single-edged crack-tension specimens. Successful crack branching was observed in four specimens and unsuccessful branching in another. Crack branching consistently occurred when the necessary conditions ofK I =K I b =3.3 MPa\(\sqrt m\) and the sufficiency condition ofr o =r c =0.75 mm were satisfied simultaneously. In the unsuccessful branching test, the necessary condition was not satisfied sinceK I was always less thanK I b .

Experimental technique for measurement of stress-acoustic coefficients of Rayleigh waves: An experimental technique is described for determining the two stress-acoustic coefficients relating Rayleight-wave velocity to biaxial surface stress in elastic solids. The method is applied to two aluminum alloys

Abstract: Two stress-acoustic coefficients,K1 andK2, are required to determine the state of biaxial surface stress from ultrasonic Rayleigh-wave velocity or time of flight measurements in elastic, initially isotropic solids. An experimental technique is described for the precise measurement of these two coefficients in uniaxially stressed test specimens. The technique is applied to aluminum 2024-T351 and 6061-T651 alloys. The influence on measurement results of various parameters such as material anisotropy and temperature is considered.

Nondestructive residual-stress measurement in a wide-flanged rolled beam by acoustoelasticity

Abstract: X-ray stress analysis is a standard nondestructive stress-measurement technique, but its use is limited in the sense that only a surface layer is surveyed. Recently, acoustoelasticity has emerged as a technique for nondestructive stress analysis. Acoustoelasticity makes use of stress-induced acoustic-birefringent effects. It gives stress distributions averaged through the thickness of a specimen. This technique is attractive because it does not require a transparent plastic model as photoelasticity does. However, much should be done before this method is established as a standard nondestructive technique of stress analysis. The most important among them is to separate stress-induced birefringence from that introduced by texture structure. For special cases, such as axisymmetric stress distributions and when a stress-free region is knowna priori, residual-stress distributions can be evaluated nondestructively. In this paper, we measured residual-stress distribution in a wide-flanged rolled beam by using a recently developed T-type transducer. The results were compared to those obtained from conventional destructive methods.

The monomode fiber—A new tool for holographic interferometry

Abstract: Monomode fibers are used to minimize modal interactions which ordinarily occur during holographic deformation studies based on multimode fiber-optic access. Test results show that holograms have better diffraction efficiency than those generated using multimode fibers and that stringent requirements for vibration isolation associated with holographic testing can be somewhat relaxed.

Developments in the design and use of liquid-metal strain gages

Abstract: The design, construction and use of liquidmercury strain gages are described. Gage-fabrication techniques and associated circuitry are discussed. A theoretical description of gage operation is also presented. Application of the liquid-mercury strain gage to the study of cruciate-kneeligament deformation is described.In vitro measurements on canine stifle joints and human-cadaver knees are presented.In vivo measurements on canines are also given.

Stress analysis by combination of holographic interferometry and boundary-integral method

Abstract: The paper describes a hybrid experimental and numerical method analysis of bodies. It consists of the experimental method of double-aperture speckle interferometry and the boundary-integral method. The interference patterns allowing evaluation of the displacement vector are obtained by the speckle interferometry. The boundary displacements obtained experimentally are conveniently used for the calculation of stresses in the body by the boundary-integral method. Some examples bear witness of the effectiveness and accuracy of the hybrid technique.

Displacement measurements around cracks in three-dimensional problems by a hybrid experimental technique

Abstract: Diffraction gratings were applied to slices taken from three-dimensional stress-frozen models of bodies with cracks. The slices were then annealed to relax the reversible strains and moire interferometry was used to determine the deformation of the slice. A virtual reference grating of 1200 l/mm (30,000 l/in.) was used to provide the required high sensitivity. The moire pattern in the LEFM (linear-elastic fracture-mechanics) zone was photographed with 20 × magnification. Results were compared with those obtained photoelastically.

Determination of dynamic properties of elastomers over broad frequency range

Abstract: Support excitation of a small spring-mass system, consisting of a massm and two thin wafers in shear, is used as a simple and reliable method to determine the dynamic properties (storage-modulus and complex-modulus loss factor) of elastomeric materials over a fairly broad frequency range. No special equipment or instrumentation is required since ordinary vibration equipment is used. Microminiature accelerometers are used to monitor the support and mass accelerations which eliminates the measurement of small forces and displacements. The frequency range of interest over which experimental data can be obtained fromone specimen can be varied by the thicknessh of the elastomeric waters and/or the size of the massm. The method discussed has been used to determine the dynamic properties of a wide variety of elastomers with frequencies as high as 6000 Hz. Typical results demonstrating repeatability of the method are shown for a carboxyl-terminated butadiene acrylonitrile (CTBN) specimen over the frequency range of 500–3000 Hz. The small test setup is easily adaptable for use in small temprature-controlled chambers for temperature/frequency studies of elastomers. The mathematical model developed for reduction of experimental data is based on the complex shear modulus for the damping model of the elastomer where the spring-mass system is subjected to support excitation.

A semi-automated in-plane loader for materials testing

Abstract: A universal testing machine is described for applying general in-plane loading as combinations of tension or compression, shear and in-plane rotation. It is composed of three computer-controlled hydraulic actuators connected to a movable head free to execute translation and rotation motions. Test specimens 1 in.×1.5 in. (25 mm×38 mm) are loaded through hydraulic grips for which the gripping pressure can be programmed. Other components include a mechanical specimen loader, a video digitizer for defining the specimen geometry, and a computer system that controls the test and gathers and stores the test data. A large number of tests can be conducted inexpensively over a broad range of loading conditions in this system, as opposed to the more generally available complex loading systems for which specimen costs are higher and testing time considerably longer. With rather limited changes, the system could be fully automated. The system has been used to characterize the fracture behavior of a series of carbon/epoxy composites over a broad range of in-plane loads.

Experimental stress-intensity distributions in three-dimensional cracked-body problems

Abstract: For over a decade, the first author and his associates have worked towards the development of an optical-experimental-modeling technique for predicting both the flaw shape and the stress-intensity-factor distribution in three-dimensional cracked-body problems where neither are knowna priori. The application is associated with subcritical flaw growth, the precursor to most service fractures.

Crack-tip stresses as computed from strains determined by stereoimaging

Abstract: Measurements of the three in-plane elements of the strain tensor near a crack tip have been combined with the material constitutive behavior to compute the three inplane elements of the stress tensor and the mean stress. The method is illustrated with fatigue-crack plasticity results for 7075-T651 aluminum alloy.

A consistent-splitting model for experimental residual-stress analysis

Abstract: Destructive laboratory procedures for measuring through-thickness residual stresses in metals frequently involve some combination of three types of cutting steps. Deformation data, recorded after each step, are used in back-computation procedures to produce an estimate of the original residual-stress distribution. Questions arise concerning the applicability of simplifying assumptions embodies in existing back-computation schemes for each of the three types of steps. This paper focuses on one of these steps generally called the splitting step. Two existing back-computation schemes were evaluated. One existing scheme, applied to a typical spliting problem, estimated residual stresses that were in error by as much as 35,000 psi. In another case, the second existing back-computation scheme led to errors of about 30 percent. An improved back-computation scheme is developed for the splitting step based on a ‘consistent-splitting model’. Verifications and applications of the model are presented. In the verification studies, results obtained from the consistent-splitting model for two cases showed excellent agreement with finite-element reference solutions.

Photoelastic determination of mixed-mode stress-intensity factorsKI, KII andKIII

Abstract: On the basis of existing photoelastic methods for the determination ofKI andKII, this paper presents an experimental method for determiningKIII with photoelastic data, and a photoelastic method for comprehensively determiningKI,KII andKIII under the complex stress condition. A frozen three-dimensional photoelastic model is first used to determineKI andKII from the slice perpendicular to the flaw edge. Then, from that slice, a sub-slice is taken to determine the factorKIII. This method is examined by comparison with two test models.

Photoelastic determination of stresses in multiple-pin connectors - Stresses in the inner lap of double-lapped connectors are determined using transmission photoelastic models

Abstract: The design, fabrication, and testing of photoelastic models of double-lap, multiple-pin connectors are discussed. Interest is in the stresses in the inner laps. These stresses are determined by constructing models with photoelastic inner laps and transparent-acrylic outer laps. The connectors have two pins, in tandem, parallel to the load direction. A photoelastic-isotropic point is shown to permit the evaluation of load sharing between the two pins. A numerical scheme, utilizing the isochromatic- and isoclinic-photoelastic data and a finite-difference representation of the planestress equilibrium equations, is used to compute the stresses around the two pins. Representative stress distributions and stress-concentration factors are shown.

Application of fiber optics to speckle metrology—a feasibility study

Abstract: The development of fiber optics provides a potential means of simplifying coherent-light-metrology techniques and, simultaneously, increasing their realms of applicability by suppressing the negative effects of hostile mechanical and thermal environments. This report describes a series of experiments which explore some of the possibilities for applying fiber optics to speckle metrology, and, at the same time, demonstrates the use of both photoelectronic-numerical and conventional optical systems for recording and correlating the resulting speckle fields.

Reduction of elastic stress concentrations in end-milled keyed connections

Abstract: Frozen-stress, three-dimensional models were used to study the elastic stress distribution in both metric and inch rectangular, parallel, standard keys, shafts and hubs with end-milled keyways, loaded in torsion The coefficient of friction between the shaft and the hub was measured and the surface conditions were arranged to simulate typical prototype friction

Plane-stress fracture testing of finite sheets under biaxial loads: A combined analytical and numerical procedure is employed in the analysis of finite-element specimens containing two radially collinear cracks emanating from a circular hole under biaxial loads

Abstract: A procedure which combines the Williams series-type stress- and displacement-field expressions at the crack-tip neighborhood with a suitable numerical scheme away from the crack-tip was employed in the determination of the plane-stress fracture properties of four finite 7076-T6 aluminum sheets containing cracks emanating from a circular hole under four biaxial loads. The compatibility of the analytical and numerical displacements at the nodal points along the boundary of the crack-tip neighborhood was utilized in formulating displacement-continuity expressions containing some undetermined constants which solution depends on the nature of the boundary loading conditions. By linear superposition of the displacement due to remote uniaxial load and the displacements due to remotely applied transverse load in the neighborhood of the crack-tip, biaxial-displacement-continuity expressions containing these important fracture properties—namely, the opening Mode I stress-intensity factorK, the nonsingular stress term associated with the stresses in the direction parallel to the plane of cracksA and the integration termB associated with the displacement in this direction—were evaluated.