Compression testing

In compression testing a compressive stress is applied to either a crystal or compacted specimen and the corresponding strain measured, the ratio of the stress to strain being a measure of the compressive Young's modulus of elasticity.

The loading system depends on the size of specimen, ranging from a micro tensile testing instrument providing a load up to 5 N and a minimum displacement of 5 nm, as used for single crystals by Ridgway etal. (1969a), to an Instron physical testing instrument providing a load up to 50 kN, as used for flat-faced cylindrical compacts (8 mm diameter) by Kerridge and Newton (1986). The Young's modulus in compression can be determined using the following equation:

where L is the length of the specimen, X is the slope of strain versus stress, C is the machine constant (strain versus stress for loading of the machine without a specimen) and A is the cross-sectional area of the specimen.

While for crystals single measurements are all that is necessary, for compacts measurements on specimens prepared at different porosities are required. In the latter test the Young's modulus at zero porosity can be determined using equation 10.

Values for the compressive modulus of a variety of materials in Table 3 are lower than those measured by other techniques although the trend of decreasing modulus with increasing particle size is the same as that reported from flexure testing (Bassam et al., 1990). Furthermore, for the tests involving crystals Ridgway etal. (1969a) indicated that cracks were important and they attributed this factor to variation in the results. This factor may also account for the low values of modulus when compared with those from flexure testing. In view of this and the fact that the compressive modulus should always be greater than the tensile modulus, the results from these tests must be viewed with some scepticism.

Table 3 Young's modulus measured by compaction testing

Young's

Particle

modulus

Material

Method

size 0*m)

(GPa)

Reference

Aspirin

Crystal

-

0.1

Ridgway etal. (1969a)

Aspirin

Compact

+180-250

2.5

Kerridge and Newton (1986)

Aspirin

Compact

+ 250-355

2.3

Kerridge and Newton (1986)

Sodium chloride

Crystal

-

1.9

Ridgway etal. (1969a)

Potassium chloride

Compact

-

9.2

Kerridge and Newton (1986)

Avicel PH102

Compact

+ 250-350

4.7

Kerridge and Newton (1986)

Sucrose

Crystal

-

2.2

Ridgway etal. (1969a)

Salicylamide

Crystal

-

1.3

Ridgway etal. (1969a)

Hexamine

Crystal

-

0.9

Ridgway etal. (1969a)

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