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Results for “BIAXIAL TESTING”

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NASA NTRS2012

The Design of Cruciform Test Specimens for Planar Biaxial Testing of Fabrics for Inflatable Aerodynamic Decelerators

A preliminary analytical study was conducted to investigate the effects of cruciform test specimen geometries on strain distribution uniformity in the central gage section under biaxial loads. Three distinct specimen geometries were considered while varying the applied displacements in the two orthogonal directions. Two sets of woven fabric material properties found in literature were used to quantify the influence of specimen geometries on the resulting strain distributions. The uniformity of the strain distribution is quantified by taking the ratio between the two orthogonal strain components and characterizing its gradient across the central area of the gage section. The analysis results show that increasing the specimen s length relative to its width promotes a more uniform strain distribution in the central section of the cruciform test specimen under equibiaxial enforced tensile displacements. However, for the two sets of material properties used in this study, this trend did not necessary hold, when the enforced tensile displacements in the two orthogonal directions were not equal. Therefore, based on the current study, a tail length that is 1.5 times that of the tail width is recommended to be the baseline/initial specimen design.

Corbin, Cole K.↗

NASA NTRS1974

Miniature load cell instrumentation for finite deformation biaxial testing of elastomers

Accuracy of biaxial test equipment to examine the nonlinear mechanical behavior of thin sheet specimens of elastomeric materials has been hampered by lack of precise determination of the force distribution along the sides of the specimen. It has been necessary to use an effective width established by approximate means to obtain the stress from the overall force applied along the edges. To avoid this experimental difficulty, individual miniature proof-ring load cells utilizing semiconductor strain gages have been developed and applied to the support hooks for the thin sheet specimens. Typical results are shown for time-dependent stress distributions for all degrees of biaxiality. An accurate evaluation of the effective specimen width is now possible.

Arenz, R. J.↗

NASA NTRS2002

Biaxial Testing of 2195 Aluminum Lithium Alloy Using Cruciform Specimens

A cruciform biaxial test specimen was used to test the effect of biaxial load on the yield of aluminum-lithium alloy 2195. Fifteen cruciform specimens were tested from 2 thicknesses of 2195-T8 plate, 0.45 in. and 1.75 in. These results were compared to the results from uniaxial tensile tests of the same alloy, and cruciform biaxial tests of aluminum alloy 2219-T87.

Johnston, W. M.↗

NASA NTRS1997

Biaxial Testing of 2219-T87 Aluminum Alloy Using Cruciform Specimens

A cruciform biaxial test specimen was designed and seven biaxial tensile tests were conducted on 2219-T87 aluminum alloy. An elastic-plastic finite element analysis was used to simulate each tests and predict the yield stresses. The elastic-plastic finite analysis accurately simulated the measured load-strain behavior for each test. The yield stresses predicted by the finite element analyses indicated that the yield behavior of the 2219-T87 aluminum alloy agrees with the von Mises yield criterion.

Dawicke, D. S.↗

NASA NTRS1981

Biaxial tests of flat graphite/epoxy laminates

The influence of biaxially applied loads on the strength of composite materials containing holes was analyzed. The analysis was performed through the development of a three dimensional, finite element computer program that is capable of evaluating fiber breakage, delamination, and matrix failure. Realistic failure criteria were established for each of the failure modes, and the influence of biaxial loading on damage accumulation under monotonically increasing loading was examined in detail. Both static and fatigue testing of specially designed biaxial specimens containing central holes were performed. Static tests were performed to obtain an understanding of the influence of biaxial loads on the fracture strength of composite materials and to provide correlation with the analytical predictions. The predicted distributions and types of damage are in reasonable agreement with the experimental results. A number of fatigue tests were performed to determine the influence of cyclic biaxial loads on the fatigue life and residual strength of several composite laminates.

Liebowitz, H.↗

NASA NTRS2019

Improved Methods for Hyperelastic Material Characterizations Based on Uniaxial, Radial-Biaxial, and Strip-Biaxial Test Data

This report concerns the different analytical methods used for material characterizations of hyperelastic silicone rubber, a potential candidate elastomer for aerospace applications. To establish the constitutive equations for the silicon rubber, strain-energy gradients (material constants) must be determined from experiments. Earlier experiments by Blatz-Ko showed that the material constants of natural rubber determined from different stress fields were different. Therefore, in the current material characterization, uniaxial, equalbiaxial, and strip-biaxial tensile tests were conducted. Based on the resulting stress-stretch data, the material constants were determined by using a trendline on the stress-difference plots and energy plots, and also the MSC-Marc ® curve-fitting method. The results confirm that the material constants determined from uniaxial and biaxial stress fields are indeed different. The material constants determined from the equal-biaxial energy plots and from the MSC-Marc ® combined-curve-fitting method were found to be extremely close, and can be considered as universal material constants for establishing the constitutive equations for the silicone rubber under any stress field. The results of this report show that a uniaxially stretched specimen became slightly anisotropic due to crystallization of polymer molecular chains, and that equal-biaxial and strip-biaxial testing must be conducted to fully understand the material properties of hyperelastic materials.

William L Ko↗

NASA NTRS1968

Development of biaxial test fixture includes cryogenic application

Test fixture has the capability of producing biaxial stress fields in test specimens to the point of failure. It determines biaxial stress by dividing the applied load by the net cross section. With modification it can evaluate materials, design concepts, and production hardware at cryogenic temperatures.

Helf, J. C.↗

NASA NTRS1988

An Experimental and Analytical Evaluation of a Biaxial Test for Determining Shear Properties of Composite Materials

The results of an experimental and analytical investigation of a biaxial tension/compression test for determining shear properties of composite materials are reported. Using finite element models of isotropic and orthotropic laminates, a specimen geometry was optimized. A kinematic fixture was designed to introduce an equal and opposite pair of forces into a specimen with a one inch square test section. Aluminum and several composite laminates with the optimized geometry and a configuration with large stress gradients were tested in the fixture. The specimens were instrumented with strain gages in the center of the test section for shear stiffness measurements. Pure shear strain was measured. The results from the experiments correlated well with finite element results. Failure of the specimens occurred through the center of the test section and appeared to have initiated at the high stress points. The results lead to the conclusion that the optimized specimen is suitable for determining shear modulus for composite materials. Further revisions to the specimen geometry are necessary if the method is to give shear strength data.

Kennedy, John M.↗

NASA NTRS1994

Results of uniaxial and biaxial tests on riveted fuselage lap joint specimens

As part of an FAA-NLR collaborative program on structural integrity of aging aircraft, NLR carried out uniaxial and biaxial fatigue tests on riveted lap joint specimens being representative for application in a fuselage. All tests were constant amplitude tests with maximum stresses being representative for fuselage pressurization cycles and R-values of 0.1. The parameters selected in the testing program were the stress level (sigma(sub max) = 14 and 16 ksi) and the rivet spacing (0.75 and 1.0 inch). All specimens contained 3 rows of countersunk rivets, the rivet row spacing was 1 inch and the rivet orientation continuous.

Vlieger, H.↗

NASA NTRS2001

Biaxial Testing of High-Strength Fabric Improves Design of Inflatable Radar Domes

Large radar installations around the globe continuously watch the skies, unobtrusively providing security to the United States; these systems have been in active use for the past 50 years. Often situated in extreme environments, the radar dishes require shielding from the harsh elements. Air-inflated domes (over 100 ft in diameter) are one structure of choice for providing this essential protection. The radomes are constructed from highstrength fabric that is strong enough to withstand the inflation pressure, high winds, and other environmental loads, yet transparent to the microwave signal to allow precise radar mapping. This fabric is woven from glass fibers for high strength and embedded in a polytetrafluoroethylene resin matrix, akin to the nonstick coatings used on cookware.

Krause, David L.↗

NASA NTRS2001

Specimens and Reusable Fixturing for Testing Advanced Aeropropulsion Materials Under In-Plane Biaxial Loading: Results of Conceptual Design Study - Part 1

A design study was undertaken to investigate the feasibility of using simple specimen designs and reusable fixturing for in-plane biaxial tests planned for advanced aeropropulsion materials. Materials of interest in this work include: advanced metallics, polymeric matrix composites, metal and intermetallic matrix composites, and ceramic matrix composites. Early experience with advanced metallics showed that the cruciform specimen design typically used in this type of testing was impractical for these materials, primarily because of concerns regarding complexity and cost. The objective of this research was to develop specimen designs, fixturing, and procedures which would allow in-plane biaxial tests to be conducted on a wide range of aeropropulsion materials while at the same time keeping costs within acceptable limits. With this goal in mind. a conceptual design was developed centered on a specimen incorporating a relatively simple arrangement of slots and fingers for attachment and loading purposes. The ANSYS finite element code was used to demonstrate the feasibility of the approach and also to develop a number of optimized specimen designs. The same computer code was used to develop the reusable fixturing needed to position and grip the specimens in the load frame. The design adopted uses an assembly of slotted fingers which can be reconfigured as necessary to obtain optimum biaxial stress states in the specimen gage area. Most recently, prototype fixturing was manufactured and is being evaluated over a range of uniaxial and biaxial loading conditions.

Ellis, J. R.↗

NASA NTRS1975

Biaxial compression test technique

Fixture and technique have been developed for predicting behavior of stiffened skin panels under biaxial compressive loading. Tester can load test panel independently in longitudinal and transverse directions. Data can also be obtained in combined mode.

Hansard, E. T.↗

NASA NTRS2016

A Biaxial-Bending Test to Observe the Growth of Interacting Delaminations in a Composite Laminate Plate

It is not easy to isolate the damage mechanisms associated with low-velocity impact in composites using traditional experiments. In this work, a new experiment is presented with the goal of generating data representative of progressive damage processes caused by low-velocity impact in composite materials. Carbon fiber reinforced polymer test specimens were indented quasi-statically such that a biaxial-bending state of deformation was achieved. As a result, a three-dimensional damage process, involving delamination and delamination-migration, was observed and documented using ultrasonic and x-ray computed tomography. Results from two different layups are presented in this paper. Delaminations occurred at up to three different interfaces and interacted with one another via transverse matrix cracks. Although this damage pattern is much less complex than that of low-velocity impact on a plate, it is more complex than that of a standard delamination coupon test and provides a way to generate delamination, matrix cracking, and delamination-migration in a controlled manner. By limiting the damage process in the experiment to three delaminations, the same damage mechanisms seen during impact could be observed but in a simplified manner. This type of data is useful in stages of model development and validation when the model is capable of simulating simple tests, but not yet capable of simulating more complex and realistic damage scenarios.

McElroy, Mark↗

NASA NTRS1977

On the form of the strain energy function for a family of SBR materials

Styrene-butadiene materials with varying crosslink densities are analyzed through use of a strain energy function of the type introduced by Valanis and Landel (1967). A form of the strain energy function derived from strip biaxial tests proves to be accurate when checked against uniaxial and other biaxial test results.

Arenz, R. J.↗

NASA NTRS1971

Static and dynamic load measurements in aerospace decelerator canopy fabrics with metal foil strain gages.

A test program was conducted to determine the feasibility of using conventional metal foil strain gages to measure load-time relationships on thin fabric membranes while these membranes were loaded under simulated aerodynamic decelerator conditions. Uniaxial and biaxial tests were made at fabric strain levels up to about 10%. Loadings were made both statically and dynamically, with the fastest load time being 0.015 second for zero to full load on uniaxial test specimens. For the biaxial tests, plane strain conditions were assumed, and by using experimentally determined strain-load relationships, principal loads were determined from the perpendicularly oriented strain-gage pairs. Although the complex stress-strain behavior of decelerator fabrics prevents the attainment of normally expected strain-gage accuracy, utilization of the techniques described can lead to meaningful measurements for the decelerator stress analyst.

Hoffman, I. S.↗

NASA NTRS1980

Uniaxial and biaxial tensioning effects on thin membrane materials

Thin laminated membranes are being considered for various surface applications on future large space structural systems. Some of the thin membranes would be stretched across or between structural members with the requirement that the membrane be maintained within specified limits of smoothness which would be dictated by the particular applications such as antenna reflector requirements. The multiaxial tensile force required to maintain the smoothness in the membrane needs to be determined for use in the structure design. Therefore, several types of thicknesses of thin membrane materials have been subjected to varied levels of uniaxial and biaxial tensile loads. During the biaxial tests, deviations of the material surface smoothness were measured by a noncontacting capacitance probe. Basic materials consisted of composites of vacuum deposited aluminum on Mylar and Kapton ranging in thickness from 0.00025 in (0.000635 cm) to 0.002 in (0.00508 cm). Some of the material was reinforced with Kevlar and Nomex scrim. The uniaxial tests determined the material elongation and tensile forces up to ultimate conditions. Biaxial tests indicated that a relatively smooth material surface could be achieved with tensile force of approximately 1 to 15 Newtons per centimeter, depending upon the material thickness and/or reinforcement.

Hinson, W. F.↗

NASA NTRS2001

Viscoplastic Model Development to Account for Strength Differential: Application to Aged Inconel 718 at Elevated Temperature

The magnitude of yield and flow stresses in aged Inconel 718 are observed to be different in tension and compression. This phenomenon, called the Strength differential (SD), contradicts the metal plasticity axiom that the second deviatoric stress invariant alone is sufficient for representing yield and flow. Apparently, at least one of the other two stress invariants is also significant. A unified viscoplastic model was developed that is able to account for the SD effect in aged Inconel 718. Building this model involved both theory and experiments. First, a general threshold function was proposed that depends on all three stress invariants and then the flow and evolution laws were developed using a potential-based thermodynamic framework. Judiciously chosen shear and axial tests were conducted to characterize the material. Shear tests involved monotonic loading, relaxation, and creep tests with different loading rates and load levels. The axial tests were tension and compression tests that resulted in sufficiently large inelastic strains. All tests were performed at 650 C. The viscoplastic material parameters were determined by optimizing the fit to the shear tests, during which the first and the third stress invariants remained zero. The threshold surface parameters were then fit to the tension and compression test data. An experimental procedure was established to quantify the effect of each stress invariant on inelastic deformation. This requires conducting tests with nonproportional three-dimensional load paths. Validation of the model was done using biaxial tests on tubular specimens of aged Inconel 718 using proportional and nonproportional axial-torsion loading. These biaxial tests also helped to determine the most appropriate form of the threshold function; that is, how to combine the stress invariants. Of the set of trial threshold functions, the ones that incorporated the third stress invariant give the best predictions. However, inclusion of the first stress invariant does not significantly improve the model predictions. The model shows excellent predictive capability for nonproportional load paths. Additionally, it reduces to the well-known models of Mises Drucker and Drucker-Prager. The requisite experiments involve reasonably simple load paths in the axial-shear stress plane and hence can be performed on a variety of different materials: be they metallic, geological. polymeric, ceramic or granular. The general form of the threshold function allows representation of inelastic deformation in a range of materials.

Iyer, Saiganesh↗

NASA NTRS1994

Long duration biaxial cylinder testing of balloon grade polyethylene films with emphasis on automated data acquisition and control

Characterization of a 2-dimensional material constitutive model in the practical biaxial state for engineering utility of balloon envelope structural analysis is achievable with laboratory cylinder tests. Hoop (circumferential, and longitudinal strains are time-wise measured on axially loaded pressurized cylinders. Pre-programmed ramped loading changes as well as precise long term pressure control are achieved through cascaded proportional, integral derivative (PID) flow control loops that are part of an integrated data acquisition and process control system. Real time data reduction and analysis allow monitoring of unattended tests that have durations of up to a month. Measured and controlled parameters and variables are discussed; date analysis techniques are reviewed.

Martone, Michael↗