titexperimental.gif (19739 byte)

 

LABORATORIES AND EXPERIMENTAL ACTIVITIES

The Experimental Mechanics Lab has a long time experience in mechanical measurements including : force and moment, position, displacements, speed, acceleration, temperature and strain. Those measures have been applied for material and structural components testing either at room temperature or in different environments (very high or low temperatures or chemically aggressive environments) .

Measurements are commonly performed by PC based Computer Aided Testing systems.

Standard test facilities

Several conditioning devices (HBM and Sint) are available for displacement (LVDT) and strain gauges transducers.

Different kinds of loading cells are available for forces ranging from few Newtons up to 100kN. Loading cells for particular measurements can be designed and manufactured. Temperature can be measured by thermocouples either in the cryogenic region (K type from -200°C) or at high temperature (T type up to 1200°C). Recording devices include : PC acquisition systems, X-Y paper recorders, analog and digital oscilloscopes.

A long experience has been gained in the strain measurement by electric strain gauges. Strain gauges were applied in both metallic and non metallic (mainly composite and ceramic) materials for measuring the strain in different conditions including fatigue loading or aggressive environment.

The laboratory is also equipped with digital image processing systems dedicated to automated measurements of strain distributions using optical methods of experimental mechanics.

Residual stress

In the last years several experimental and theoretical activities were developed in the field of residual stress (RS) measurement and modelling.

A technique based on the Initial Strain Distribution (ISD) was proposed for modelling the complete RS field in a component. RS measurement for the evaluation of the ISD was performed by the progressive cut and several strain gages measurements. The method was applied to the evaluation of RS in laser welded plates and in cladded components.

The Hole Drilling (HD) technique for local RS measurement has been applied in several activities. A PC controlled transportable hole drilling device (by Sint) is available also for in field measurements.

The HD technique has been the object of an extensive research activity aimed at extending the limits of applicability of the present standards (mainly the ASTM E837). In particular the effect of plasticity on the measurement was considered and a procedure for increasing the present limit of applicability from 0.5 to 0.9 the material yield stress was proposed. At present this procedure is under review for being included as an Appendix in the new release of the ASTM E837 standard. In this field a new four gages rosette for high RS measurement was proposed.

A new analytical approach was also proposed for facing the problem of variable through thickness RS.

 

Ultra sound applications

An ultra sound (US) device (by Panametrics) is available for fracture and damage detection in material and components. Different kind of US probes can be used for different material (including metallic and composites).

C-scan

The US devices coupled and a digital oscilloscope (by LeCroy) were the hardware basis for a C-scan device designed and manufactured in order to produce an automatic complete map of the US response for a plane body.

 

The C-scan device includes also a couple of stepping-motors and a PC which controls the probe motion and makes the signal acquisition. By this system a tomography analysis can also be performed thus obtaining a three-dimensional map of the body.

The US device was applied to the evaluation of different kind of damage in laminate composites under fatigue loading.

 

The optical grid method

Grid methods offer a means of measuring displacements and strains directly on the surface of structural components. The technique consists of transferring a grid on the surface of a component and determining the position of nodal points of the grid before and after the deformation. The availability of digital image measurement systems has given a vigorous impulse to the optimisation of this technique. The laboratory is currently involved in developing innovative image-processing-based automated grid methods which allow high accuracy and speed.

The automated grid method has been applied for material testing, construction optimisation and comparison of computer models.

 

Automated photoelasticity

Photoelasticity is a full field optical method which enables experimental stress analysis of two- and three-dimensional components to be performed by analysing the photoelastic fringes, i.e. isochromatics and isoclinics. The classical procedure of analysis is based on manual location of the centres of isochromatic and isoclinic fringes and on assignment of the relative parameters, i.e. fringe order and isoclinic value, by a well-trained operator. The use of digital image processing technique in photoleasticity has been the object of an extensive research activity aimed at developing automated systems which allow the experimentalists to speed up the rate of analysis and to perform more complex investigations. The laboratory is endowed both with a transmission and with a reflection polariscope coupled to a digital image processing equipment.

 

Main Current Research Activities

Residual stress evaluation of large diameter welded pipes for gas transportation (sponsored by ILVA)

Experimental validation of the plastic correction procedure for the hole drilling technique

Study of optimal techniques for variable through-thickness residual stress evaluation

Full field strain evaluation of steel thin plates for cold working up to failure

Computer graphic applications to displacement and strain evaluations.

 

atwht.gif (7507 byte)CONTACT PEOPLE : Prof. Leonardo BERTINI, Prof. Marco BEGHINI, Ing. Sandro BARONE

 

REFERENCES

M.Beghini, L.Bertini "Residual stress modelling by experimental measurements and finite element analysis" Journal of Strain Analysis, Vol. 25, n.2, 1990, pp. 103-108.
M.Beghini, L.Bertini, P.Raffaelli "Numerical Analysis of the Plasticity Effect in Hole Drilling Residual Stress Measurement" ASTM Journal for Testing and Evaluation, Vol. 22, n. 6, 1994, pp. 522-529.
M.Beghini, L.Bertini, C.Carmignani, F.Frendo "Damage evaluation in composite laminates by ultra-sonic tomography", Proc. of 2nd Scientific meeting on 'New material technologies and their applications in the naval field' Livorno, 4-7 Aprile 1995 pp. 207-220, In Italian.
M.Beghini, L.Bertini, P.Raffaelli "An account of plasticity in the hole drilling method of residual stress measurement" Journal of Strain Analysis Vol. 30 n.3, 1995, pp.227-233.
M.Beghini,, L. Bertini, F. Frendo "Analysis of some methods for determining in plane elastic modulus of ceramic coatings", Proc. of 25th AIAS International Conf. on Material Engineering Gallipoli ,1996, pp.407-414.
S. Barone, E.A., Paterson "Full field separation of principal stresses by combined thermo- and photo-elasticity" Experimental Mechanics, vol.36, n.4, 1996, pp.318-324.
M.Beghini, L. Bertini "Recent advances in the hole drilling method for residual stress measurement" Journal of Materials Engineering and Performance, Vol. 7, No 2, 1998, pp. 163-172.
S. Barone, M.Beghini, L. Bertini "An Image Processing Based Automated Grid Method Using Square Pattern Grids" Proceedings of the 1998 SEM Spring Conference, Houston, 1-3 June 1998.