Strain Gauges

Chapter 5

its characteristic dimensions. The greater dimensionchas, of course, to be smaller than the perimeter of the bars in order to be attached to it. However, if this dimension is too small, it will be difficult to manipulate this components, attach them to the bars and wire them. The solution is to find an available strain gauge whose dimension is a good compromise between these two conditions. Given so, the chosen extensometer hasc= 13mmandd= 6mm.

Another important aspect lies on the type of connection to the wires. For this type of strain gauges, there are, mainly, three types of possible wirings that are to be considered for this specific application:

pre-wired strain gauges, strain gauges with integrated solder pads beneath the measurement grid and strain gauges with leads beneath the measurement grid. All these alternatives present pros and cons and the final cost is highly dependent on this characteristic. Ideally, the best option would be the pre- wired strain gauges since it would simplify the assembling process and the linkage to the data acquisition system, however, this solution is very expensive. The cheaper solution is the strain gauge with integrated solder pads, however, this welding process is hard and requires extremely high precision. Given this, the best solution was found to be the strain gauges with leads, since they are cheaper than the pre-wired ones and easier to operate when compared to the strain gauges with solder pads.

Considering all the specifications referred previously, the following extensometer was selected: 1- LY13-6/350 by HBM. The first digits1-LY1refer to the type of extensometer, in this caseLstands for linear, Y stand for universal foil strain gauges for stress analysis and1 stand for the type of wiring in this case with leads beneath the measurement grid. The next digit3 refers to the material in which the extensometer is supposed to be attached, in this case aluminium. The last two numbers 6 and 350 represent, respectively, the measuring grid length, in milimeters, and the nominal resistance, inΩ. This value was found to be the best option, after analysing the specifications of the data acquisition system related with the full bridge inputs and excitation voltages [33].

Since the extensometers are supposed to be attached to the sensing bars, it is necessary to guaran- tee that they are capable of measuring the range of deformations that the bar is expected to have, both in axial and tangential directions. These two deformations are particularly important given the way that the extensometers are supposed to be attached to the sensing bars. As referred previously in section 2.3.1, in order to maximize the measuring efficiency of the strain gauges, they will be connected as Wheatstonebridges where one pair of strain gauges will be measuring the axial deformation of the bar and the other pair will be measuring the tangential displacement of the bar. It is important, also, to note that when one of the pairs is in tension the other is in compression.

Figure 5.2: Schematic representation of the strain gauges attachment to the sensing bars Figure 5.2 shows how the strain gauges are supposed to be attached to the sensing bar in order to

form theWheatstonebridges. In the figure, only the front view of the bar is represented but two more strain gauges are attached in the back in a similar manner so that, in the end, there will be two strain gauges aligned with the axial direction and another two perpendicular to it.

Considering the selected strain gauges, their maximum elongation (max) is50000µm/m, that is 5×10⁻², and so it must be guaranteed that, for all the critical loading conditions presented previously, the sensing bars do not deform (both axially and tangentially) past this value of strain. Table 5.1 presents the maximum expected values of strain for the critical loading conditions at the middle of the bars, where the strain gauges will be attached, as depicted in figure 5.3. It is possible to see that the range of values is below the maximum admissible value of strain for this type of strain gauge (50000µm/m) and so, they can be used to measure the values of strain in the bars for all the operating conditions.

Table 5.1: Maximum absolute values of microstrain in the bars for the maximum expected loading con- ditions

Maximum micro-strain

values (µm/m) Axial Tangential Maximum Admissible

Plane model 105 34

50000

Half-wing model 395 130

Figure 5.3: Strain measurement at the middle of the sensing bar

A first observation of the table 5.1 leads to conclude that the maximum expected values of microstrain are considerably below the maximum admissible value the strain gauge is capable of measuring. Based on these values, the issue that arises is that it is necessary to guarantee that the strain gauges have enough resolution to measure such small quantities as the maximum expected values of displacement are less than1%of the maximum admissible elongation of the strain gauges. In fact, these sensors are highly sensitive and can be used to measure extremely low values of strain. A brief research showed

that it is common for a strain gauge to have an extension of 0.1%, which corresponds to 1000µm/m and so one can conclude that the expected values are within this range [34]. In addition, the use of Wheatstonebridges compensates this effect as it would be extremely hard to quantify the change in the resistance of one isolated strain gauge when loaded.