Robert F. MillerEngineers are comfortable when they deal with numbers. To put a number on a certain materials attribute, such as tensile strength, yield strength, impact energy, or hardness not only lends credibility to the attribute, many times it can be used in calculations and formulations. And so it is no different when it comes to wear properties. While the literature abounds with all types of wear tests, few if any, give us real hard numbers to use in calculations and formulae. Most of the time the established tests give us relative numbers which impart some ranking to the materials in question. Since wear is a weight loss phenomenon, these tests generally are reported as weight losses or volume losses.
One of the most popular wear tests is the ASTM G-65 Dry Sand Rubber Wheel Abrasion Test. This test measures the weight or volume loss in a very controlled environment and simulates what is commonly referred to as Low Stress Abrasion or Scratching Abrasion. Scratching Abrasion is principally characterized by the lack of any fracturing of the abrading material. In other words, the sand particles in the G-65 test retain their shape and size throughout the test procedure. This characteristic is unlike its cousin abrasion wear mechanism, High Stress Abrasion, or Grinding Abrasion where the sand particles are actually fractured into smaller pieces. The newly created particles are very sharp and angular, and coupled with the high degree of stress, imparts an entirely different type of wear. But back to Scratching Abrasion. Scratching Abrasion can be easily illustrated by sand sliding on a dump truck bed liner.
The ASTM G-65 test is quite helpful in ranking materials according to their relative abrasion resistance. Below is a representation of the basic test.
Typical results of the test might yield the following:
Weight Loss (gm)
|400 BHN Wear Plate||
|Hardened Tool Steel||
|Chrome Carbide Plate||
From the above, it would be easy to conclude that the substitution of Chrome Carbide Wear Plate for Mild Steel would yield a life expectancy of 13 times in a field application. This not the case however. It must be kept in mind that the G-65 wear test is a highly controlled test and does not simulate actual field conditions. Most applications involve a number of wear mechanisms such as corrosion, impact, frictional wear, and erosion, just to mention a few. Humidity, particle speed, shape, and dynamic forces, all impart their contribution to the deterioration of the component. It is not uncommon for the actual component life to be increased by only 3 to 4 times, while the G-65 test indicates a much greater factor.
Despite this discrepancy between lab and field, the test is an invaluable tool in the ranking of materials and predicting component wear life.