For measuring small, fast deformations on a micrometre scale optical methods are common. Various forms of interferometry are available, and holography is another possibility. However, if there are complications such as limited space and hazardous environments special demands arise. In our case a granite block was subject to a detonation from a small explosive charge, sending shock waves through the block. In some cases there was risk of expulsion with flying rock shards, with possible damage to expensive optics. Therefore a robust solution with no expensive, fragile parts near the test surface was needed.
A fast four-channel fibre-optic sensor for measuring dynamic surface displacements was developed. Light from an LED is sent through a plastic optic fibre and reflected from the observed surface back into the fibre. A photodetector is used to measure the intensity of the reflected light, which gives the distance to the surface. Based on simple principles, micrometre and microsecond resolution was achieved. The only parts near the test surface were the fibres, easy to replace.
The sensor was used for measuring the propagation velocity of the shock wave, the surface displacement and its particle velocity. One of the most important factors for the wave transmission in granite is the water content of the rock material. The pores in the granite allow for about 0.1% of the granite’s weight to come from liquid water. This is enough to affect the aforementioned measured quantities, from a few percent in some cases to over 100% in other cases. Therefore an investigation was undertaken aiming to control the water content of the granite blocks. Several series of small blocks were both soaked in water and dried, the water content measured with a high performance balance, to ensure well-controlled blasting experiments.
On a larger scale, the aim with the project of which this thesis is a part was to gain systematic knowledge on the effects of penetrating weapons on granite rock shelters. This thesis shows how the raw data was obtained, presents the data itself and exemplifies its application.
Stockholm: KTH , 2005. , 53 p.