Over the last few years, the demand for testing solutions that accommodate smaller and smaller specimens has grown. Sometimes specimens are small because they are taken from components with small geometries and sizes, or sometimes it is useful to examine the scaling effects of the material characteristics. Even in the fields of standardization, standard mini-specimens have already been defined for various materials.
In this context, Zwick has begun a joint project with the Lucerne University of Applied Sciences and Arts aimed at optimizing how such mini-specimen are handled.
For this project, a new specimen grip for micro tensile specimens has been designed as part of a bachelor's thesis. The paper examines both the handling and alignment of small specimens.
In order to better accommodate specimens, the entire specimen grip is removed from the machine and placed ergonomically on the table. The specimen is positioned on a carriage and centered in the grips. The carriage is then removed and the upper jaw is moved toward the specimen and fixed in place.
Now the entire specimen grip can be hung by means of a special mount in the testing machine. This procedure, with the help of a linear guide, ensures that angled pulling and other parasitic forces are minimized.
By using laserXtens compact HP, even the smallest specimens with gage lengths of less than 3 mm can be characterized mechanically in their entirety.
Direct strain measurement is particularly important. A high resolution and a high degree of accuracy is vital since the absolute deformation often equals only a few µms due to the dimensions of the specimen. That is why laserXtens compact HP is the ideal extensometer. Only one non-contact measuring system can deliver reliable test results for specimens with these dimensions.
The image below shows a titanium alloy specimen from the IISc Bangalore with a complete stress-strain curve. The black curve was measured by laserXtens compact and the red curve was measured with the moving crosshead.
Zwick decided to design a fixture for a biaxial tensile test based on a publication submitted by Nihon University for the 2011 Zwick Science Award. In contrast to a standard biaxial testing machine, the multi-axial stress state is achieved in a universal testing machine by means of a deflector.
The deflector is mounted in a standard testing machine and a compressive force is applied across four arms to a cruciform specimen in the X and Y directions. The force values are recorded by four load cells, and the deformation of the specimen is recorded from below by a videoXtens or laserXtens system.
Fx, Fy, εx and εy, as well as all dimensions derived from these, can be determined. Furthermore, this fixture fulfills all requirements of ISO 16842 "Metallic materials - Sheet and strip - Biaxial tensile testing method using a cruciform test piece" of 2014.
At the Institute of Metal Forming and Casting at the TU Munich a prototype for this application is being tested for further examination of the ductility of sheet metal. The goal of this is to find out to what degree this fixture can be used to determine the forming limit of sheet metal. and improve both the specimen geometry specified in the ISO standard and the test setup. www.utg.de
Biaxial test fixture
Biaxial test fixture for mounting in a standard materials testing machine