Duration of a blade.
The duration of a blade depends very much on the nature of the soil. Dense, abrasive sand with angular grains wear considerably the blade. Clays wear the blade very slowly. Some users have blades 20 years old that are in good shape. Intensive use in dense abrasive sand with angular grains may wear out the blades in a couple of months. A solution can be to increase the price of testing in very abrasive soils.
When it is time to discard a blade.
Wear makes the blade thinner. Wear also decreases the length of the lower tapered portion of the blade. Comparative tests have shown that such changes have nearly negligible effects on DMT results. Yet it is helpful to have some criterion to guide deciding when it is time to discard a blade.
A new blade has the “pedestal” of the membrane 0.25 mm lower than the surrounding steel plane. Due to wearing, the blade surface outside the membrane will get lower and lower, while the pedestal, protected by the membrane, will not undergo erosion. As wearing progresses, the pedestal will get higher (rather than lower) than the outside plane. This change in relative elevation has light influence on the results. However the membrane, being higher than the surrounding plane, will be subject to more intense wear (and will have to be replaced more frequently). Hence a worn blade will have to be scrapped not because of invalid results, but because the membranes will fail more and more frequently.
A reasonable limit for wearing can be : discard a blade when the pedestal sticks 0.10 mm out of the surrounding plane (at that time the total surface erosion is 0.25+ 0.10 = 0.35 mm, i.e. the blade is 0.7 mm thinner than originally).
The following figure shows a tool that can be used for measuring the depression of the pedestal of the blade.