The paper presents a method of measuring deformations of cylindrical samples on the testing machine for free tube hydroforming experiments. During experiments a sample made of a thin-walled metal tube is expanded by the internal pressure of the working liquid and additionally subjected to axial compression. This results in a considerable circumferential deformation of the tube and its shortening. Analysis of the load cases and their impact on the deformations can be helpful in determining e.g. tube material properties or general limiting conditions in the tube hydroforming process. In connection with the above, the value of deformations and knowledge of their course during experiment has become one of the most important problems related to the issue described above.

This article presents a linear strain measurement method insensitive to temperature variations and using fibre Bragg gratings. Two Bragg gratings were applied with periods selected to obtain partial coverage of their spectrum characteristics. One of the gratings was subjected to a tension strength. Placing both gratings in one thermal chamber allowed - through ensuring the same thermal conditions - to obtain insensitivity of the entire scheme to temperature variations. The gratings were recorded on the same optical fibre and reacted to temperature variations in the same degree. Value of strain was indicated based on the transmission spectrum characteristic of two grating schemes. The use of transmission, not reflectance, characteristics of the gratings allowed for a direct measurement of the spectrum, without having to use a coupler or optical circulators, and at the same time, this allowed to simplify the strain detection scheme. We proposed applying the continuous wavelet transform with characteristics of the spectrum scheme of two gratings for improvement of static properties. Especially, the thermal linearity and stability of the sensor was improved. For a strain range up to 750 μe, the non-linearity error of processing characteristic obtained was 0.45%. Thermal stability of the scheme proposed was defined as relative sensitivity of the power to temperature variations. The mean value of such relative sensitivity of the scheme proposed in the temperature scope of 21ºC-95ºC, amounted to 0.195.

Incremental Sheet metal Forming (ISF) Process is a suitable process which helps to produce various parts used in automotive sector by rapid prototyping. This method of producing a prototype helps industry in reducing the production cost. In ISF process, a final product is evolved through local deformation of the sheet metal made by the tool. Usually better formability is obtained when the tool makes a better contact with the sheet metal throughout the process. Improved formability elevates dimensional accuracy of the product, thus increases the market value of the product. A new tool with multiple ball ends capable of making multiple mating points over sheet metal was used in this research to enhance the efficiency of formability and surface finish. Ability of the new Multi-Point Incremental Forming Tool (MPIF) was investigated and compared to the existing Single Point Forming Tool (SPIF) based on the formability and surface finish. Forming Limit Diagram (FLD), Strain Distribution (SD) and Scanning Electron Microscope (SEM) were used to examine the formability of the sheet metal. The SEM & 3D-Surface roughness profilometer were used to observe the sheet metals surface finish. In addition to these experimental techniques a simulation results were also used to predict the stress and strain rate during forming process. The experimentation and simulation outcome shows that the MPIF provides superior formability and surface finish.