Constitutive models are necessary to predict the mechanical behavior of biological tissues. However, biological materials present challenges in constitutive modeling due to their complex mechanical behavior. Their oriented fibrous structures often exhibit pronounced mechanical anisotropy.
Due to anisotropy, stress-strain data generated from uniaxial tests cannot be used to extrapolate to generalized three-dimensional constitutive equations. Since biological tissues are generally considered incompressible, planar biaxial testing allows for a two-dimensional stress state that can be used to characterize their mechanical properties and validate the constitutive models.
ADMET planar biaxial testing systems are engineered solutions based on specimen sizes, specimen elongations, test speeds, force capacities, environmental conditions, and gripping requirements. Each system comes equipped with ADMET’s biaxial version of the MTESTQuattro Materials Testing System capable of force, displacement, strain control (using DIC system) in addition to sine, square, triangular cyclic waveforms, and independent or synchronized control of X & Y actuators.
Planar biaxial testing applies force in both the x and y directions, allowing researchers to analyze the anisotropic behaviors of materials. Biaxial test results can help manufacturers find the right material at the right quantity without quality loss in their products.
Anisotropic materials, such as fiber-reinforced composites and wood, have different mechanical properties in different directions. This affects their strength and stiffness depending on the direction of force.
An anisotropic material’s Young’s Modulus changes with direction along the specimen.
Forces in uniaxial and biaxial testing are shown in the two figures below. When tested biaxially, four perpendicular forces are applied to the specimen; due to the material anisotropy, the weaker region will fail quicker than other regions the same force is applied to.
Planar Biaxial Testing Specimen Geometry
Biaxial testing is often performed on flat, cruciform shaped specimen, where the four arms of the specimen are pulled by the four perpendicular forces shown above. This allows a homogenous strain distribution in the thickness direction yielding in the center of the specimen which is then further investigated. The cruciform shaped specimen can be prepared in multiple variations; curved arms, straight arms, or with slits to avoid flexural forces in the plane of interest, shown below.
Note that forces can be applied at the same magnitude as well as at a variety of magnitudes in different directions. The stress distribution and the influence of such misalignment can be studied for further analysis.
Planar Biaxial Specimen Material
Types of materials tested in biaxial loading include soft biological tissues, silicone elastomers, composites, metal sheets, films, coated fabrics, and textiles, all subjected to orthogonal stress-strain fields. Test specimens are often prepared as explained in the section above. To adequately characterize the mechanical properties and the stress-strain behavior of the material, specimens are biaxially tested under force or deformation control with a configured planar biaxial testing machine and software.
Planar Biaxial Testing & Equipment
The primary reason for running biaxial tests, as opposed to running standard tensile tests, is to determine a specimen’s mechanical properties in its different points and analyze the stress and strain distribution. To do so, a planar biaxial testing system with multiple actuators that can also be configured with different strokes and speeds is used. Each actuator represents the different axes of the planar biaxial test and moves in equal or opposite directions so that the sample center point remains stationary. First, the biaxial specimen is mounted on the tester and the arms are clamped. We recommend securing two opposite sides first to ensure proper alignment. ADMET offers special biaxial grip and fixture setups and a variety of techniques, including suture racks, clamps and pin arrays, to match specimen dimensions and geometry as well as planar biaxial test setups with fluid baths and optional temperature control.
Biaxial testing systems can be equipped with various load cell capacities and strain measurement devices including contact and video extensometers to provide accurate force-strain data. Contact extensometers mount on the load frame and the slide mounting brings the extensometer into contact with the cruciform specimen. ADMET biaxial extensometers are capable of measuring combined in-plane strains of biaxial specimens while eliminating crosstalk between axes. Contact extensometers can also be configured for use in high temperatures.
Video extensometry is also often used in biaxial testing applications. Non-contact video extensometers are able to measure variables such as 3D displacements, strain, strain rates, velocities, as well as user-defined variables of cruciform specimens and specimens with complex geometries under complex loading conditions.
Samples can be loaded in static or cyclic fatigue tension, compression, flexion, or shear using the MTESTQuattro testing software to run planar biaxial test procedures where each axis is programmed for independent or coordinated motion under force or deformation control. The MTESTQuattro controller provides a camera trigger output for syncing test data with third party microscope/camera images. At the end of the tests, test reports are created automatically based on selected analyses.
Planar Biaxial Testing Case Studies
Planar Biaxial Fatigue Testing with Seawater Bath
ADMET recently designed a customized planar biaxial fatigue testing system for a customer testing materials in a seawater bath.Comparative Biomechanics of Hagfish Skins & Planar Biaxial Testing
ADMET eXpert 8600 Planar Biaxial testing system was used to conduct planar biaxial tests on hagfish skin samples.Case Studies: Material Testing in the Electronics Industry
Read more about the technology trends in the electronics industry and how material testing is used for electronics applications via three case studies.
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