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There is insufficient engineering data about the Young’s modulus of R11 resin under small scale conditions, and the elastic value of the material varies to a certain extent from different micro-characterisation methods. In order to provide reference value for the tensile test result, another characterisation was carried out using a commercial Deben microtest module with a similar methodology (Figure 5.31). The Deben microtest module is based on a tensile/compression/bending step motor-driven stage primarily designed for use in confined space such as SEM chamber (Deben 2005).

Force Cell

Clamping Device

Figure 5.31: The 2kN Deben Tensile compression and horizontal bending stage (left) and specimen mounting (right)

The Deben microtest module employs a stepper motor to applying a tensile load up to 2 kN to deform the specimens. The motor speed is controlled by optical encoders from 0.55µm/s to 6.67µm/s with sample time from 100 ms to 5 s. The force reading comes through a custom miniature load cell of 660 N embedded into one end of the moving stage (Figure 5.31). The strategy for mounting specimen is to use hard mechanical jaws clamping on each ends of the specimen. At first, the specimen is horizontally mounted onto the stage and hard clamped by a pair of mechanical jaws that are supported on stainless steel slide bearings. A dual threaded leadscrew drives the jaws symmetrically in opposite directions, keeping the specimen centrally aligned. A summary about the performance of Deben tensile stage is given in Table 5.6.

Table 5.6: Typical performance of the Deben microtest module

Module type 2KN tensile tester

Loadcell calibration value 660 N

Gear box 1526:1

Minimum step motor speed 0.55 µm/s

Minimum sample time 100 ms

Extensometer range 2058 – 62328 µm

Applying the same specimen design strategy for the MSL tensile test, the specimens for Deben microtest module (Figure 5.32) was similar to the ones used in the previous tensile test-rig with bulk end regions for clamping and built-in protection bars at each side. The thickness was designed to be 2 mm and the curing exposure was set to be 9.5 s in order to obtain a high flatness. The test-beam in the central had the same dimension of 10.50.1 mm3 _{with bulk end dimension of 2762 mm}3

to fit the dimensions of the mechanical jaws.

Figure 5.32: The dimensions of a specimen for Deben microtest module

The Deben microtest module allows real time observations of the mechanical behaviour of different samples (Figure 5.33). The rigid hard mechanical jaws clamping in the Deben microtest modules introduce substantial stress to the specimen after the screw is firmly tightened. Therefore, the specimen protected structure should be gently cut-off to release

the specimen and the loadcell offset should also be re-set after mounting. During the test, the step motor runs to exert a steadily increasing force upon the sample.

Figure 5.33: The interface of Deben microtest module software

The stress-strain result of the specimen is shown in Figure 5.34. The Young’s modulus of the test beam was measured as about 0.8 GPa within the strain of 0.1, namely 100 µm elongation. This test result from Deben microtest module agreed broadly with the value of preliminary result from the new tensile test-rigs. It was observed that with faster rate of increasing load the specimen tended to be broken early.

This additional test can be used only as general confirmation that the new test-rig behaved consistently with these materials, because it had inherent disadvantages as well as good features. The Deben microtest module can take a wide range of loadcells and is capable of producing a full-view of tensile stress-strain behaviour of MSL specimens, where as the new tensile test-rig had limited force range of a few newtons because of the

relatively high stiffness flexure spring. Similar to the previous specimen clamping strategy, the specimen under hard clamping was likely to be subject to large external stress, which might not be so well accommodated by smaller areas. The large face-to-face mechanical jaw clamping required an extremely high flatness and can result in uncertainty in the early off-setting process. Small wobbles had been observed sometimes in the initial stress region due to the uncertainty in the hard clamping. Moreover, the accuracy of this method was limited by the step-motor precision which was only a few tens of micrometre level.

Figure 5.34: The test of typical beam Deben result (S01-02 0.55µm/s; S01-01 1.66 µm/s; S01-03 3.33µm/s)

0 20 40 60 80 100 120 140 160 180 200 0 0.2 0.4 0.6 0.8 1 1.2 St re ss (M ρ a ) Strain

Deben Test result of typical size specimens

5.11 Conclusion

The new test-rig offered a practical solution to the unusual challenges of the current measurements. Dividing applied load between the delicate sample and a guide flexure provided robustness to protect the delicate specimen. The flexure allowed an operationally useful Abbe offset in the extension measurement: attaching small capacitors directly to the specimen, as used here for calibration, was impracticably slow and tedious for routine use. The specimen design worked well with friction clamping followed by cutting of the support structures. There was no evidence of slippage at the clamps sufficient to significantly degrade extension measurements. Nevertheless, over-constraint of the specimen was not ideal and further refinements will be explored in parallel with using the instrument to study MSL materials. The results from the Deben microtest module confirmed that this test-rig produced reliable results within small-scale elongations. The details of tensile tests on specimens under various fabricated conditions will be described in Chapter 6.

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Chapter 6: The tensile results of MSL specimens