Preparation of Test Specimens

Scope:-

This is done for valid results from tests on brittle materials. This required careful and precise sample preparation processes which include collection of samples, storage of samples, and avoidance of contaminations, selection, coring, sawing, end preparation and specimen check.

There are some standards of ASTM (American Society for Testing and Materials) or ISRM (International Society of Rock Mechanics) societies for specifying the details of any sample. So to check these requirements we have to prepare the samples for tests according to these standards.

Main steps of sample preparation are described as follows;

Collection and Storage of Samples:-

Test material is normally collected from the field in the form of drilled cores. Field sampling procedure should be rational and systematic, and material should be marked to indicate its original position and orientation relative to identifiable boundaries of the parent rock. Ideally sample should be moisture proofed immediately after collection either by waxing, spraying, or packing in polyethylene bags and sheets.

For making sample moisture proof we can wrap the sample in a clear thin polyethylene such as GLAD WRAR or SARAN WRAP. We can also wrap the sample in cheese cloths. We can coat a layer of LUKEWARM wax mixture to an approximate thickness of 0.25 in.

Avoidance of contamination:-

The deformation of and fracture properties of rock may be influenced by air, water, and other fluids in contact with their internal surface that may be cracks and pores. If these internal surface contaminated with oils and other sub-stances, their properties may be altered and give wrong results. Of course a cutting fluid is required with many types of specimen preparation equipments. Clean water is the preferred fluid.

Hard, dense rock and low porosity will not normally be affected by moisture. Drying at temperature above 49 C is not recommended as excessive heat may cause an irreversible change in rock properties.

Some shale and rocks containing clay will disintegrate if allowed to dry. Usually the disintegration of diamond drill cores can be prevented by wrapping the cores as they are drilled in a moisture proof material such as aluminum foil or chlorinated rubber, or sealing them in moisture proof containers.

Mud shale and rock containing bentonites may soften if the moisture content is too high. Most of softer rocks can be cored or cut using compressed air to clear cutting and to cool the bit or saw.

It is imperative to determine very early in the test the moisture sensitivity of the material and take steps to accommodate the requirements throughout the test life of the selected specimens.

Selection:-

All tests are done in laboratory and for this purpose we need small size of samples as compared to field. This small size sample should represent the properties of whole or a large section of the field. In non-homogeneous geological formations, under the complex system of induced stresses, the selection of specimen which represents the best features of a foundation which influence the analysis or design of a project is very important step. Selection of sample can be done from different points in the field or can also be done through many points in different sections in the fiels according to our nature of work.

Coring:-

Coring of the sample can be done directly in the field or it can also be done in the laboratory. In laboratory coring can be done on the lumps of ores with thin-wall diamond rotary bits, which may be detachable or integral to the core barrel. The usual size range for laboratory core drills is from 6 inch dia. down to 1 inch outside diameter. For some uniaxial tests, sample diameter is 2.125 inch.

In coring there is a general trend that the speed of drill increases as drill diameter decreases. Also higher drill speeds are sometimes used on softer rocks. Usually the range of drill speed lies between 200 to 2,000 rpm.

Sawing:-

For heavy sawing, a slabbing saw is adequate for most purposes. For exact sawing, a precision cutoff machine, with a diamond abrasive wheel about 10 inch in diameter and a table with two-way screw traversing and provision for rotation are recommended. The speed of the wheel is usually fixed, but the feed rate of the wheel through the work can be controlled. Clean water, either direct from house supply or circulated through a settling tank, is the standard cutting and cooling fluid. For cutting, core should be clamped in a vee-block slotted to permit passage of the wheel. By supporting the core on both sides of the cut, the problem of spalling and lip formation at the end of the cut is largely avoided. Saw cuts should be relatively smooth and perpendicular to the core axis in order to minimize the grinding or lapping needed to produce end conditions required for the various tests.

End Preparation:-

Due to the rather large degree of flatness required on bearing surfaces for many tests, end grinding or lapping is required. Conventional surface grinders provided the most practical means of preparing flat surfaces, especially on core samples with diameters greater than approximately 2 inch. Procedures are essentially comparable to metal working.

The lathe can also be used for end-grinding cylindrical samples. A sample is held directly in the chuck, rotated at 200 to 300 rpm, and the grinding wheel, its axis inclined some 15 degrees to the sample axis, is passed across end of the sample with rotating at 6000 to 8000 rpm. The “bite” ranges from about 0.003 inches maximum to less than 0.001 inch for finishing and the grinding wheel is passed across the sample at about 0.5 in. per minute. For core diameters of 2-1/8 in. or less, a lap can be used for grinding flat end surfaces on specimens, although producing a sufficiently flat surface by this method is an art.

To end-grind on the lap, a cylindrical specimen is placed in a steel carrying tube which is machined to accept core with a clearance of about 0.002 in. (0.0508 mm). At the lower end of this tube is a steel collar which rests on the lapping wheel. The method requires use of grinding compounds and hence is not recommended where other method are available.

Specimen Check:-

In general tests, test specimens should be straight, their diameter should be constant and the ends should be flat, parallel and normal to the long axis. Sample dimensions should be checked during machining with a micrometer or vernier caliper; final dimensions are normally measured with a micrometer and reported to the nearest 0.01 in. Tolerances are best checked on a comparator fitted with a dial micrometer reading to 0.0001 in. There is a technique for revealing the roughness and planes qualitatively. Impressions are made by sandwiching a sheet of carbon paper and a sheet of white paper between the sample end and a smooth surface. The upper end of the sample is given a light blow with a rubber or plastic hammer, and an imprint is formed on the white paper. Areas where no impressions are made indicated dished or uneven surfaces. The importance of proper specimen preparation cannot be over emphasized. Specimens should not be tested which do not meet the dimensional tolerances specified in the respective test methods.