The Laboratory of Experimental Mechanics has a long experience in mechanical measurements, including: force and moment, position, displacement, velocity, acceleration, temperature and strain. These measures have been used to test materials and structural components at room temperature or in various environments (very high or low temperatures or chemically aggressive environments).
Measurements are usually performed by PC-based automated testing systems.
Standard test equipment
Several conditioning devices (HBM and Sint) are available for displacement sensors (LVDT) and load cells.
Different types of load cells are available for forces in the range of a few Newtons to 100 kN. Load cells for specific measurements can be designed and manufactured. Temperature can be measured with thermocouples either in the cryogenic region (type K from -200°C) or at high temperature (type T up to 1200°C). Recording devices include: computerized data acquisition systems, XY paper recording devices, analog and digital oscilloscopes.
Long-term experience in strain measurement with electric strain gauges has been accumulated. Load cells made of both metal and non-metallic (mainly composite and ceramic) materials have been used to measure strain under various conditions, including fatigue loading or aggressive environments.
The laboratory is also equipped with digital image processing systems designed for automated measurements of strain distribution using optical methods of experimental mechanics.
Residual stress
In recent years, several experimental and theoretical measures have been developed in the field of residual stress (RS) measurement and modeling.
A technique based on the initial strain distribution (ISD) has been proposed to model the full RS field in a component. RS measurements to estimate the ISD were performed using a progressive slice and multiple strain gauge measurements. The method was applied to estimate RS in laser-welded wafers and clad components.
The hole drilling (HD) technique for local RS measurement has been applied in several activities. A portable, computer-controlled hole drilling device (from Sint) is also available for field measurements.
The HD technique has been the subject of extensive research activities aimed at extending the applicability of existing standards (mainly ASTM E837). In particular, the effect of plasticity on measurements was considered and a procedure was proposed to increase the current limit of applicability from 0.5 to 0.9 of the yield strength of the material. This procedure is currently being considered for inclusion as an Annex to the new version of ASTM E837. In this area, a new four-gauge socket has been proposed for high RS measurements.
A new analytical approach to solve the problem of variable RS thickness has also been proposed.
Ultrasonic applications
An ultrasonic (ultrasonic) device (from Panametrics) is available to detect cracks and damage in material and components. Different types of ultrasonic probes can be used for different materials (including metal and composites).
C-scan.
US devices coupled to a digital oscilloscope (from LeCroy) were the hardware basis for the C-scan device, designed and manufactured to create an automatic complete US response map for the airframe.
The C-scan device also includes a pair of stepper motors and a PC that controls the movement of the probe and receives the signal. This system can also be used to perform tomographic analysis, producing a three-dimensional map of the body.
The US device has been used to assess various types of damage in laminate composites under fatigue loading.
Optical grid method
Grid methods offer a means of measuring displacements and deformations directly on the surface of structural components. The technique consists of transferring a grid to the surface of the part and determining the position of the grid node points before and after deformation. The availability of digital image measurement systems has given a powerful impetus to the optimization of this technique. The laboratory is currently developing innovative automated meshing methods based on image processing that provide high accuracy and speed.
The automated meshing method has been applied to material testing, design optimization, and computer model comparison.