According to the Romanian Dictionary, the experimental stress analysis with strain gauges represents the total testing methods used for determining the stress in the finite parts subject to efforts by measuring the strains in several points on the surface of the component.
Experimental stress analysis with strain gauges is the method used for measuring deformations and the extension of a body subject to stress by using transducers which transform mechanical deformations into variations of an electrical variable, a principle discovered by Kelvin in 1850. The Wheatstone Bridge used in experimental stress analysis was already been discovered by Samuel Hunter Christie since 1833, but it was improved and made known by Charles Wheatstone in 1843. As method, the experimental stress analysis is part of the methods generated by the general electrical measuring methods of non-electrical variables. With the help of the principles used in the Theory of elasticity, specific measured deformations are transformed into stresses (1). The foundations of experimental stress analysis with strain gauges were posed after 1930 in parallel with the development of electronic amplifiers; with their emergence, the first measurement devices have been manufactured by E. Simmons and A. Ruge which are considered the modern inventors of experimental stress analysis with strain gauges.
The advantages presented by the electrical experimental stress analysis with strain gauges as testing method are:
• the method is non-destructive (consequently, the form and dimensions of the component or the structure of material are not affected);
• it permits carrying out measurements in conditions of real operation (both statically and dynamically);
• by using the measurement device, it ensures a higher sensitivity and precision than mechanical, acoustic, optical methods etc.;
• since the device used while measuring is electronic, it does not present inertia which permits the measurement and recording of phenomena whose variation is rapid.
The transducers used in experimental stress analysis can be classified according to the electrical variable in which the mechanical variable is rendered (resistive, capacitive, inductive transducers) or according to the mechanical variable measured (motion transducers, strains, speed, acceleration transducers etc.).
The experimental analysis of specific stress/strain of railway structures through the method of electrical resistive experimental stress analysis uses electrical resistive transducers (TER). Using this type of transducers is also imposed by reference documents used in the tests carried out on railway vehicles, structures or different components. Resistive strain gauges installed on railway structures are usually mounted quarter-bridge; full-bridge transducers are used at load cells (for example, for measuring the loads of the vehicles or vehicle axle loads). For monoaxial stress body state single measuring directions transducers are used – strain gauges and for plane stress are used rosettes for measuring strains.
In Romania, the experimental stress analysis with strain gauges as testing method is used for the railway vehicles manufactured in the 1960s (2). The first experimental stress analysis laboratory was established in the current Romanian Railway Authority – AFER. The tests conducted in AFER’s laboratories (certified by RENAR – Romanian Accreditation Association – National Accreditation Body) can be static – conducted on AFER’s bench test and dynamic – performed at Făurei Railway Testing Centre (collision tests and traffic tests).
There are several systems used for fastening rail tracks on sleepers.
Figure 1 shows the main rail fastening system.
The quality of a railway line is determined according to the geometric, weight and speed characteristics of the rolling stock running on that line. When vehicles pass, the railway track is statically and dynamically stressed and the track imperfections affect the vehicle’s traffic quality, stability and even safety (3).
Until 1990, the Romanian railway network used an indirect means for fixing rail tracks to sleepers – the K fastening system. Over the years, this means proved insufficient which led to the use of other elastic fastening systems based on the experience of other railway managers. The most important advantage of this fastening system is the fact that it requires much lower costs than any other type.
On the pan-European Corridor IV, the Romanian section now under rehabilitation uses compressed concrete sleepers with a PANDROL FASTCLIP or VOSSLOH W14 fastening systems, homologated and certified in the past years and designed for speeds of 200 km/h and a axle load of 25 t (4).
This paper is aimed at presenting the method used for determining the damping of impact forces at the rubber pads underneath the rail tracks through experimental stress analysis with strain gauges.
2. How the tests were made
Dynamic tests have been conducted on elastic fasteners. Measurements requires Hottinger strain gauge to be applied on the sleeper (fig. 2) and wire-connected to the Hottinger MGCplus dynamic acquisition system. Acquisition systems have been connected to a laptop; the interface was created in the Catman 4.5 software (produced by Hottinger). Tests have been carried out according with the SR EN 13146 .
The sleeper has to be made of non-cracked and intact concrete for tests with support surfaces correctly dimensioned for the fastening system to be subject to tests
The sleeper has been equipped with two resistive electrical strain gauges with a nominal measuring base between points of 100-200mm fixed on the lateral sides of the sleeper, symmetrically to the line which crosses the centre of the support surface, perpendicularly on the sleeper base.
The transducers have to be parallel to the sleeper base, one should be placed as closer to the sleeper support surface as possible, but avoiding the edge or the joint, while the other should be placed at a distance of more than 10 mm but less than 25 mm away of the sleeper base, as shown in figure 2.
3. Standard calibration of the system
By gluing the two strain gauges, the sleeper becomes a transducer (load cell), which makes necessary the standard calibration of the entire system.
For carrying out the standard calibration, the Hottinger MGCplus dynamic acquisition system was used, the standard calibration being carried out with known ascending loads and maintaining them for a specific period of time.
Figure 3 presents the standard calibration curve for the two tensometric transducers.
Dynamic tests have been carried out by applying a shock resulted from the fall of a mass on the surface of a rail track fixed on a concrete sleeper. The resulted shock effect is measured through the stress felt in the concrete sleeper. Damping the shock characteristic to a fastening system is evaluated by comparing the strains produced when using the reference pad with reduced damping and the fastening system plate respectively. In case a reference pad is installed in the fastening system, the stress caused by the shock do not have to exceed, at the level of strain gauges, 80% of the cracking stress calculated from the sleeper resistance point at the level of the track support surface (Mdr according to EN 13230-1). The falling mass, the falling height and the resilience of the bolt are thus adjusted so as the allowable stress will not be exceeded. The procedure is thus repeated for the pad subject to loads, without the modification of the deforming mass, fall height and bolt resilience.
The fastening system and the rail are installed using the test pad. A shock is applied to a rail track through the free fall of a mass and the stress is recorded. Recording is initiated at least 3ms before the impact and continued at least 5ms after the impact. Five shocks are applied on the test pad. Then the deformation during three consecutive shocks is recorded.
The integrity of the tested sleeper has to be controlled after every shock test by comparing the report of the stress measured by tensometric sensors located at the top and base section of the sleeper with the report corresponding to a similar sleeper subject only to a static load.
The static load has to be in conformity with the test load of the support surface under EN 13230-2 and EN 13230-3.
If the difference between the report obtained during the shock test and the report obtained during the static test is 10% of the latter, the measurements have to be rejected and the test repeated on another sleeper.
Table no. 1 presents the figures obtained during the test presented in figure 5.
As referred to the table below, it should be mentioned that the “minimum value” was referred to as the minimum value of real numbers and the “maximum value” was referred to as the maximum value of real numbers.
1. Mănescu, T. Ş., Jiga G. G.., Zaharia N. L., Bîtea C. V. Basic notions of strength of materials and theory of elasticity, Publishing house Eftimie Murgu, ISBN 978-973-1906-67-6, Reşiţa, 2010
2. Zaharia N. L., Tensometric tests at the underframe of locomotive 150239, a page in the history of the tensometry staff, Buletinul AFER nn. 6/2010, ISSN 1583-3143, Bucharest, 2010
3. Sebeşan I., Dynamics of railway vehicles, Editura Tehnică, ISBN 973-31-0919-3, Bucharest 1996
4. Mirea S., Mărunţiş H., New types of sleepers, Jurnal Feroviar no. 11/2002, Bucharest, 2002
5. SR EN 13146-3 Railway applications Testing methods for rail fastening systems Part 3: Determining the damping of impact forces
dr. ing. Nicuşor Laurenţiu ZAHARIA
Autoritatea Feroviară Română – AFER