New Biaxial Testing Machine for Biomechanics

Biaxial tests are used to characterize the anisotropic properties of elastic tissues. The new biaxial testing machine was developed specially for mechanical testing of natural and artificial elastic tissues. Planar mechanical tests can also be performed and uniaxial, equibiaxial, non-uniform loadings determined with extremely high accuracy.

During uniaxial tests there is a possibility of fibres straightening along the measurement axis, causing the mechanical characteristics of the specimen to change. A clamping system is therefore required which holds the tissue securely during the test without damaging it. Unrestricted lateral movements are necessary in order to ensure homogeneous specimen deformation under biaxial loading, while the strain measurement device must on no account cause damage to the specimen and must be able to record strain in all loading directions.

Zwick’s new materials testing machine provides an ingenious solution to these requirements, using four high-resolution linear drives (travel 50mm) which can be position, load or strain-controlled completely independently of each other. Load measurement takes place via four load cells (two load cells each in X and Y directions) with a maximum test load of 100N. Control is provided by test control measurement and control electronics.

Strain control and/or centre point control is via the laserXtens extensometer. This requires no gauge marks, but uses the structure of the specimen surface, which is as unique as a human fingerprint, to generate virtual gauge marks. Laser beams aimed at these are reflected in various directions corresponding to the surface texture, forming a specific speckle pattern which is captured by a video camera. Continuous measurement and evaluation allows selected measuring points to be followed, tracking the change in the surface structure – and that of the speckle pattern – during specimen deformation.

The advantages of a non-contact extensometer are obvious when dealing with specimens which react critically to knife edges, cause damage to contact measuring extensometers at specimen break, and for non-markable specimens. With a resolution of at least 0.15µm, laserXtens is the right choice if very high accuracy at low strains is required.

A fixture supplied with the machine allows the specimen to be mounted away from the test device and inserted into the measuring system ready assembled, simplifying specimen preparation and reducing downtime.

A height-adjustable saline bath also accompanies the system, with optional heating to allow tests on human and animal specimens to be carried out under simulated body conditions.

Reliable and flexible testing to international standards or internal company standards is easily achieved through testXpert® II software, a proven success in both research and industry. The ‘Expanded Traceability’ software option provides complete digital logging of individual actions and changes relating to the test (meets the requirements of FDA 21 CFR Part 11). Bi-directional exchange of data with SAP and other laboratory databases is also possible.

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