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Uniaxial compression tests were used to evaluate the Young's modulus and Poisson ratio of a number of materials. Cylindrical specimens were placed upon a flat perspex plate which rested on the base of the machine. Loads were applied by a similar plate rigidly coupled, via a threaded studding, to the load cell which was mounted on the crosshead of the machine. Applied load versus deflection responses gave values of Young's modulus whereas the Poisson ratio was calculated using measurements of the specimen diameter from marks on the compressing plate at a various levels of applied strain.

Some open and closed cell foams were found to have sufficiently low stiffness but these exhibited little lateral straining and were in general nonlinear, probably due to effects of the voids that they contain. All rubbers and elastomers tested showed good linearity but were mostly rather stiff. A room temperature vulcanised (RTV) silicone elastomer (Dow Coming Q3-3320) was found to have suitable mechanical properties and was used for all physical models of soft tissues discussed in this thesis. The main advantage of this type of material is that the base and catalyst mixture used to produce each batch may be poured to mould specimens of all shapes and sizes. Preparation involved the mixing of the two components (base to catalyst 20:1 by weight) followed by degassing in a vacuum chamber for a full half hour. Throughout the degassing process the container holding the highly viscous mixture was agitated vigorously to help expel air. Early specimens which did not undergo this process were of inconsistent modulus and exhibited nonlinearity of true stress versus strain. A disadvantage was that not all of the specimens could be prepared in a single batch since the volume was limited of the base/catalyst mixture that could be degassed at one time and slight variations existed between batches.

For indentation tests on Q3-3320 specimens, cylindrical indenters with flat or round tips were manufactured from extruded acetal. Indenters of both type were made with diameters of 15, 20 and 25 mm. The flat ended indenters were turned on a lathe and the edges were left sharp on machining, the rounded indenters had hemispherical tips and were manufactured using a ball turning tool. These all screwed directly into the coupling of the test machine's load cell.

Elastomer specimens for the indentation tests were cylindrical and a number were prepared to evaluate various aspect ratios of the material beneath the indenter, d/h (indenter diameter divided by layer thickness). These were formed in moulds with

Chapter 4 - Experimental indenter tests

diameters of 102 mm. After curing the thickness of specimens was checked with digital calipers and in each case was within ±0.2 mm of the expected value.

Once the test apparatus had been checked for 'play' some of the phenomena were investigated which were considered likely to introduce measurement errors. These were friction at the interfaces, viscoelasticity and age-hardening of the elastomer and the effects of temperature variations of the specimens.

For a qualitative assessment of frictional effects, a series of tests observed the magnitude of uniaxial compressive load versus deflection response of a specimen of Q3-3320 under a number of interface conditions (Figure 4.1). The aspect ratio (diameter divided by height) of the specimen was approximately 4, its thickness was 20 mm. In test (a) the specimen was compressed between dry perspex plates whereas in tests (b) and (c) the interface between the specimen and the compressing plates was lubricated with KY jelly, a medical lubricating jelly, and a light oil respectively. In test (d) the interface was roughened by placing sheets of 720 grit abrasive paper between the specimen and the compressing plates.

2000 Load, N 1500 1000 500 Deflection, mm 1 4 1 2 2 4 6 8 1 0 0

Figure 4.1 - Effect of interface conditions upon measured compressive loads (a) dry perspex plates.

(b) KY jelly. (c) lubricating oil. (d) abrasive paper.

Chapter 4 - Experimental indenter tests 89

These results show that care must be taken if frictionless conditions are assumed in experimentation since a roughly 50% increase in applied load was required to cause a nominal strain of -0.5 when lubrication was omitted. Other tests showed that compression between plates made of perspex required slightly lower loads than with plates of steel or polypropylene. Lubricating oil was used in all tests discussed in this chapter.

The rate of applied straining of Q3-3320 was found to have a small effect on experimental measurements. Figure 4.2 is a plot of the load versus deflection response of an indentation test using a round-tipped indenter. Data from tests at strain rates of 0.2, 0.02 and 0.002 s"l are shown.

800 0 .2 /s 0.02 /s Load, N 600 _{0.002 /s} 400 200 Deflection, mm 0 1 0 1 5 20 0 5

Figure 4.2 - Effect of strain rate upon indenter loads for Q3-3320 elastomer.

A difference in the loads required to cause a given strain of the order of only 6% exists across this wide range. The speed of the crosshead movement of the Zwick machine may be controlled and a nominal strain rate of 0.05 s'* was used in all tests presented within this chapter.

Low viscoelasticity in the elastomer was also demonstrated by stress relaxation tests. Figure 4.3 is a plot of the applied indenter load versus log(time) for a test involving constant strain rate (0.05 s 'l) loading followed by stress relaxation at a fixed strain due to a hemispherical tipped indenter.

Chapter 4 - Experimental maenter tests