Things to Consider
Laboratory tests have the potential to provide considerable insight into various factors that contribute to a material's performance. One chief advantage is that a laboratory test provides highly controlled conditions, which allows you to isolate individual characteristics and ignore distracting variables.
Before performing any physical property test:
Understand why you are performing the test and what you intend to do with the data.
Confirm the end point is understood and defined. For subjective end points, it is recommended to have an agreed upon rating chart between the interested parties to minimize interpretation differences of what is acceptable.
Train laboratory personnel to reduce operator error.
Clarify any ambiguity in the test method.
It is critical to recognize that physical property testing may be influenced by conditions to which the whole system is exposed. For example, resistance to abrasion is affected by conditions of the tests such as the nature of abradant; variable action of the abradant over the area of specimen being abraded; tension of the specimen; the pressure between the specimen and abradant; and the dimensional changes in the specimen. For a laboratory test, you should accept the fact that you may not be able to accurately identify all of the influences that create conditions that you are trying to replicate. Most standardized test methods will specify the parameters that must be adhered to when conducting tests. If you are not following an established abrasion test, give careful consideration to the following influences:
- contact geometry
- length of exposure
- interacting material surfaces
- normal force
- sliding speed
- environmental conditions
- material composition and hardness
Do not become distracted when attempting to isolate and replicate the influence of each parameter.
To demonstrate the complexity of recreating wear, consider the following example presented by Adam Varley of Vartest Laboratories. When a person sits on an upholstered chair, the weight of their body presses down on the fabric. The cushioning materials pushes up against the upholstery fabric, compressing it, and the body of the user provides a force that drives the fabric they are wearing across the surface of the upholstery fabric causing abrasion to occur. At the same time shear or tension forces are placed on the fabric as it pulls against its support.
Imagine the enormous variation with body sizes and shapes that should be considered. Add to that, different clothing materials that should be considered. Most textile technologists agree that wear on textiles encompasses all interacting factors that may cause a fabric to fail; rubbing, flexing, snagging, stretching, twisting, and exposure to chemicals, sunlight, heating, etc. The resistance to abrasion is also influenced by factors specific to the fibers (fiber content, fiber mechanical properties, fiber shape and dimension, yarn structure, yarn twist, yarn size, yarn ply, and the type, kind and amount of finishing materials added to the fibers or yarns); fabric construction; thickness; thread count; and weave type.
1) Whether or not a particular type of abrasion test correlates with end-use performance depends not only on a similarity of abrading mechanisms, but also on the extent to which that mechanism is maintained during the course of the abrasion test.
2) Abrasion resistance is a combination of basic properties of the specimen such as elasticity, hardness, strength (both cohesive, tensile, and shear strength), toughness and especially in the case of wear resistance, thickness.
3) The mechanism of wear depends upon the topography of the counterface abradant. It is preferable in most cases to use an abrasive only once, unless it can be refreshed.
4) Different types of wear can occur in combination or on different areas of the same component.
5) Particles responsible for abrasion or erosion are typically between 1 µm and 500 µm in size.
6) Materials may not exhibit the same relative order of resistance to abrasion when tested by different methods.
7) Test methods that lack critical procedural information could introduce problems with reproducibility.
8) When evaluating multilayered systems, the substrate plays an important role. Use caution when evaluating results of tests on different substrate materials.
9) Humidity and loading systems have an effect on test system dynamics.
10) Do not assume that the wear rate is always a linear function of time or number of contact cycles - it depends very much on the materials, type of wear, and the contact conditions.
11) Ensure steady-state conditions are achieved. Certain materials may require a run-in period.
12) Extrapolate results ONLY with great caution.
13) Do not use loads that exceed the ultimate strength of a material or speeds that can cause a thermal condition on the test specimen.