Because of the continuous evolution of the market in terms of quality and performance, the car production industry is being subjected to more and more pressing technological challenges. In this framework the use of an advanced measurement technique such as thermoelasticity allows the engineers to have a fast and reliable tool for experimental investigation, optimization and validation of the finite element method (FEM) of those critical parts, such as parts of car-frame tables (Marsili and Garinei, 2013; Ju et al., 1997). In this work it is shown how the thermoelastic measurement technique can be used to optimize a Ferrari car frame, as a method of experimental investigation and as a technique of validation of numerical models.
The measurement technique developed for this purpose is described together with the calibration method used in the test benches normally used for fatigue testing and qualification of this car's components. The results obtained show a very good agreement with FEM models and also the possibility of experimentally identifying the concentration levels of stress in critical parts with a very high spatial resolution and testing the effective geometry and material structure.
Car frame under analysis and FEM analysis results.
In this work, in order to characterize a car frame, we propose a new measurement technique, and we have designed and realized a test bench to reproduce the real conditions of the use of the car.
The hydraulic shakers furnish the frames with the necessary solicitations to reproduce, in a few hours, the forces that the car will receive during its life.
The complex frame structure and the presence of notches and of weldings cause sudden fatigue fractures for the strain concentrations not always foreseen by the FEM model (Tomlinson and John, 2015; Brouckaert et al., 2012; Becchetti et al., 2010).
Moreover, for the safety coefficient growth, it is not possible to increase the material sections indiscriminately. In this way, in fact, the car would be heavier and its performance on the road would decrease.
Commonly, the mechanical characterization of the car frame is realized in two steps: the first one concerns the development of numerical models. In order to test the numerical solution and to increase the calculation speed, different types of solvers have been used, in linear and non-linear fields. The second step regards their experimental validation by using strain gauge techniques and accelerometers. Unfortunately this technique furnishes local information only on discrete points. Moreover, the measurement volume depends on the strain gauge dimensions, and often it is not lower than 1 mm (D'Emilia et al., 2015; Speranzini et al., 2016).
The thermoelastic measurement technique has been used to validate the FEM models in terms of stress distribution. The advantage of this technique is to determine, on the experimental bench, in very low time, the qualitative and quantitative stress distribution on all the car frames.
Typical thermoelastic result.
The phenomenon of material changing temperature when it is stretched was
first noted by Ghough in 1805, who performed some simple experiments using a
strand of rubber, but the first observation in metals of what is now known as
the thermoelastic effect was made by Weber in 1830: he noted that a sudden
change in tension applied to a vibrating wire did not cause the fundamental
frequency of the wire to change as suddenly as he expected, but that the
change took place in a more gradual fashion. He reasoned that this transitory
effect was due to a temporary change in temperature of the wire as the higher
stress was applied. In 1974, the Admiralty Research Establishment approached
Sira Ltd to determine the relationship between stress and the temperature
changes that may be produced by an applied load. Sira confirmed the
feasibility and, over the next 4 years, with funding from the English
Ministry of Defence, developed a laboratory prototype called Spate (Stress
Pattern Analysis by measurement of Thermal Emissions) for applied research.
The scientific development of the thermoelastic effect, which is well known
on gases, where a temperature variation gives a pressure variation, has been
known in solid materials for a short time because of the small variation of
temperature induced (in the steel where the stress level is near the yield
point, the temperature increases by 0.2
In order to obtain the stress distribution in terms of quantitative values, a calibration process is required.
This latter can be realized by using a common strain gauge, placed in a zone where the stress gradient is the smallest possible.
In this case the calibration factor
Generally, in order to perform the calibration, a double-axis strain gauge is used, with the aim of acquiring the sum of the two principal strains.
In this work we have analysed the mechanical behaviour of some components of a Ferrari car frame, which has proven critical in the experimental road tests. Figure 1 shows the photo of the frame under study and the FEM model developed. The presence of notches, of weldings and of brazing causes strain concentrations, as highlighted in the numerical analysis (Marsili et al., 2005; Marsili and Garinei, 2014a, b).
A car bracing analysis.
Typical FEM result.
On these more stressed points, the use of the classical measurement techniques based on strain gauge or magnetic test are very difficult because of the non-planarity of the surface, the insufficient superficial finish and the small dimensions (Cardelli et al., 2015). The use of thermoelasticity, a measurement technique without contact for the surface distribution of solicitations, provides very important information (Marsili et al., 2009).
Thermoelastic stress analysis.
The test bench, equipped with the hydraulic shaker, generates a cyclical load on the frame which has been painted with a dull black paint, in order to make uniform the thermal emissivity of the body (Grigg et al., 2000; Lesniak et al., 2013; Offermann et al., 1997; Speranzini et al., 2014).
An electrical strain gauge has been pasted on the frame to convert the thermographic frame into a stress frame, as well as to generate the reference signal necessary to the TSA system, to synchronize the frame grabber with the dynamic cycle load.
In fact the Delta Therm 1550 uses the lock-in amplifier technique to acquire only the temperature change synchronous with the applied load.
At the same time it allows us to improve the signal–noise ratio. Figure 2
presents a typical result that can be obtained using this type of measurement
technique. In the same picture it is also possible to determine the stress
concentrations near the constraint section and the zones where the
shut
is present. The thermoelastic stress map has been scaled by means of a
calibration factor
The previous TSA image is very useful for validating the FEM distributions in terms of the sum of principal stresses. In this case the correlation between FEM analysis and experimental results appears clearly. Drawing an interrogation line as shown in Fig. 2, it is possible to evaluate the stress trend along the same line as reported to the right of the same Fig. 2. From the analysis of two typical interrogation lines, the gap between FEM and thermoelastic analysis is evaluated. The maximum difference found is 3 MPa. The qualitative and quantitative coincidence of the experimental and numerical results now allows the use of the model to change the geometry and the sections of the frame, or to insert a bracing, in order to reduce at the maximum the concentration of strains, without repeating the experimental tests, with economic and time advantages.
Figure 3 shows an example of a bracing welded on the frame. By the numerical analysis it is possible to see the strain concentration in correspondence to the bracing that could cause fatigue breaks of the component.
The same strain concentration is also seen in the experimental analyses by the thermoelastic system (Fig. 5). The thermoelastic experimental analysis highlights an elevated concentration of strains also around the screw not predicted, instead, by the FEM analysis.
Normally in the thermoelastic measurement the calibration is based on the
measurement of the deformation by a strain gauge rosette in a point of the
structure. To put on the strain gauge rosette we have considered certain
points with a high and regularly distributed solicitation. Repeated
measurements by stain gauge of the principal strains and the relative measure
of the infrared intensity radiation allow us to estimate the calibration
factor
The composed uncertainty in the value of the thermoelastic constant
In this work a Ferrari car frame has been characterized from the mechanical point of view to single out the areas with higher concentrations of stress. Firstly, a model of numerical simulation has been validated using classic measurement techniques based on the use of a strain gauge and through thermoelastic techniques.
This last analysis has confirmed the results obtained from the numerical point of view and in certain cases we have identified areas with tension concentrations not foreseen with the FEM analysis.
The use of a strain gauge, as an instrument of reference, has given us the possibility of calculating the calibration constant and estimating the measurement uncertainty.
No data sets were used in this article.
The authors declare that they have no conflict of interest. Edited by: Rosario Morello Reviewed by: two anonymous referees