Automatic Strain Measurement - Forming Limit Curve (FLC) via AutoGrid vario

AutoGrid vario

The AutoGrid product family enables for strain analysis on samples and components in sheet metal industry. The new AutoGrid vario has been designed to meet our customers needs for highest flexibility in the optical setup, measurement volume and local resolution. The direct, fast, and accurate measurement is based upon the automatic evaluation of grid patterns that consist of electrochemically-marked orthogonal lines spaced 1-5 mm. The system is able to determine a field of strain values in the range of 0.5 % to more than 100 % without any unambiguity. The AutoGrid systems proved well suited for a variety of applications in deep drawing and hydroforming as well as for different materials including aluminium alloys, stainless steel and fibre reinforced composites.

The AutoGrid vario is fitted with an in-process facility, i.e. an extension that allows the recording of deformations and strains during a forming process. This type of operation facilitates the measurement of forming limit curves (FLC) if the pre-calibrated measuring head is mounted on a sheet metal testing machine (Erichsen or others). All four cameras observing the specimen from different perspective views are exactly synchronized and the image sequences are recorded with a rate of up to 15 x 4 frames per second. Electronic shuttering is combined with the progressive scanning mode of the cameras with variable exposure time to avoid any blur or disturbance due to the object motion. Employing latest digital camera technology AutoGrid vario provides 5.7 million pixel per recording. In case of the standard testing velocity of 1-2 mm/s, a punch depth resolution of less than 0.1 mm can be achieved. In many practical cases some lower frame rates can be chosen. The length of the sequence recorded fulfils all practical requirement by appropriate memory configuration of the notebook PC used.

In addition, the in-process facility supports the data acquisition of signals from the testing machine. Measuring data, e. g. of path and force pictures in the sequence and the recording can be controlled automatically by corresponding thresholds.

After the test, the whole sequence of image ensembles is stored in memory and may be inspected immediately or saved to disk for further analyses if desired. The sequence can be displayed step by step, zoomed and also connected to force/path data in order to identify the moment and the location where the critical necking occured first.

The results of a specimen for the forming limit diagram are available about 5 min after the test run. A variety of stress conditions is required for a complete forming limit curve. They are generated by differently shaped specimens and, using AutoGrid vario, all the results can be conveniently combined in one diagram to define the points of a FLC interactively. Polygon or spline functions can be used for interpolation. Data export is given by ASCII format.

Qualitest provides a fast and accurate tool for the determination of forming limit curves (FLCs) of sheet metal materials. Directly connected to the testing machine, the system gives precise access to the maximum strain value just before necking and subsequent cracking occurs. Comfortable software allows to create FLCs from a set of specimens very effectively. Furthermore, the customer have the option to create free editable FLD reports for documentation. Of course, AutoGrid vario can also be used e.g. for bulge or tensile tests. The unsurpassed performance allows for materials testing and component analysis with one system, which makes AutoGrid vario the ideal solution for quality assurance purposes in sheet metal industry.