Determination of r max for Mode I Cases
6.3 Experimental verification of optimal strain gage locations
6.3.3 Details of experiments
In the present investigation, the experimental verification of optimal strain gage locations (determined by the present approach) has been demonstrated using experimental values of SIFs under mode I and mixed mode loading conditions. In order to determine mode I and mixed mode SIFs experiments have been conducted with mode I and mixed mode (I/II) test specimens made of PMMA with different configurations as shown in Fig. 6.1 and Table 6.3. The specimens have been made from the same PMMA sheet which was used for determination of material properties of PMMA.
All the specimens of mode I and mixed mode have been loaded with a closed loop servo hydraulic INSTRON 8801 machine with 100 kN capacity under the displacement control with an actuator speed 0.1 mm/min. Clevis grips have been used for transferring the tensile load from the machine to the specimen.
Strain measurements on all the loaded specimens have been carried out using
Details of the specification of strain gages used are given in Table 6.6. Fig. 6.16 shows the photograph of a typical strain gage. Utmost care has been taken while pasting the strain gages on the test specimens, to ensure perfect and defect free bonding following the standard procedures for strain gage pasting.
Table 6.6 Details of TML strain gage
Parameter Specifications
Type : FLA-1-11-3LT
Gage length : 1 mm
Gage factor : 2.12 1 % Gage resistance : 119.5 0.5 Transverse sensitivity : 0.9%
Lead wires : 10 / 0.12 3W pre-wired 3 m long Test condition : 23 0C 50%RH
Figure 6.16 A typical 1 mm gage length, pre-wired TML strain gage
Number of strain gages (depending upon mode I and mixed mode) have been pasted very carefully at specific selected locations along the gage lines (at angle and ).
The radial position and orientation of the strain gages have been maintained while pasting and have been cross-checked using a profile projector. Fig. 6.17 shows the picture of a profile projector showing clearly the orientation of a typical strain gage pasted on to the specimen.
Figure 6.17 Profile projector for verification of orientation and location of a strain gage
The measured strains have been acquired, digitized and processed using NI Data Acquisition System comprising of cDAQ9178 chassis. A universal analog input module (NI 9219 having 4 channels 24 Bit) has been used for the measurement of force from INSTRON machine in terms of voltage signals. This has been achieved by connecting BNC cable between the load cell of the INSTRON and the NI 9219 module. In this way the load has been measured simultaneously along with the strains.
Strain measurements have been carried out using four number of NI 9237 (4 channels 24 Bit, half-full bridge analog input module).
In the present investigation, in all the experiments, quarter bridge Wheatstone bridge circuit has been employed for the measurement of strains. For this purpose, NI 9944( Quarter bridge completion accessories) has been added to the NI 9237 strain gage module. The sampling rate for data in all of these modules has been set to 100 Hz. LabVIEW 9 has been used to interface the DAQ System with the digital computer and this software is also used for processing and storing of experimental data. In all the experiments, offset-nulling and shunt calibration of strain gages has been done before the actual data acquisition during loading. An excitation voltage of 2.5V is set in all the experiments.
Fig. 6.18 shows the snap shot of a typical virtual instrumentation using DAQ assistant used in one of the experiments. Once the instrumentation part required for data acquisition is ready, the specimen with the strain gages pasted on it is put into the clevis grips of INSTRON machine using pins. In order to avoid bending of the specimen during tensile loading and to ensure that the specimen is subjected to only axial tensile loads, spacer blocks are employed in both the top and bottom clevis as shown in Fig. 6.19.
Figure 6.18 DAQ assistant of LabVIEW programming for a typical experiment
Spacer blocks
Figure 6.19 Specimen with the spacer blocks in clevis
The dimensions of the spacer blocks are machined so as to provide perfect alignment of the specimen. The alignment of the specimen is also checked using a plumb before loading as shown in Fig. 6.20. During loading both force and strain data have been simultaneously stored in the computer for further processing. Fig. 6.21 shows the photograph of the complete experimental setup highlighting all the individual elements as described in this section.
Figure 6.20 Checking of alignment of a specimen using plumb INSTRON 8801
Processor
INSTRON Monitor 8801 controller
Strain gage lead wire connection to DAQ
LabVIEW programming
cDAQ 9178 Spesimen
As stated earlier, appropriate radial locations of strain gages are not only important for accurate determination of SIFs but also desirable to eliminate erroneous strain measurements due to the three dimensional effects, strain gradient effects and plasticity effects. In general, all these effects can be greatly minimized by placing strain gages as far as possible from the crack-tip. The three dimensional effects can be eliminated by placing the gages beyond t/ 2 [21]. In the present experimental study all the gage locations have been selected to satisfy the above condition in both mode I and mixed mode loading conditions.
The plasticity effects is avoided in the present investigation by choosing PMMA as the specimen material which is a linear elastic and brittle material at room temperature. The measurement errors due to the strain gradients can be drastically reduced by selecting strain gages with very small gage length and width [62, 63, 64].
In the present investigation very small gages of 1 mm gage length have been employed for strain measurements. In addition, the analysis of errors due to the strain gradients on the determination of mode I SIF by Dally and Sanford [22] clearly showed that these errors could be drastically reduced if the ratio of radial distance of center of a strain gage to the length of the gage is more than four. All the gage locations for both mode I and mixed mode experiments in the present experimental study are selected to satisfy the above requirement for avoiding the strain gradient effects.