Month: October 2018

Archard Wear

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Wear is typically defined as the gradual removal of material by contacting surfaces in motion. Though there isn’t a definite way to qualitatively measure the life of a component from wear alone, it is  important to energy loss and linked to frictional processes.

Wear can be classified into several types. The more mechanistic ones are:

  • Abrasive Wear
  • Adhesive Wear
  • Corrosive Wear

You can also have wear due to fatigue, melting transitions, chemical reactions among many others. The models most often used in DEM (discrete element methods not dust extinction moisture, new term I learned recently) use an abrasive wear or adhesive wear method. The Finnie Wear Model  can be considered an abrasive wear type. A common adhesive wear type is the Archard Wear model.

Used by some of the commercial DEM codes, or variations of it, the Archard wear model states that the volume of material removed is proportional to the work done by friction forces. It is a function of the sliding distance, normal load, hardness number of the softest contacting material, and a dimensionless wear factor or coefficient.

In its basic form, Archard wear is expressed in terms of wear depth. However, wear volume is the more popular approach. Since there are a number of versions to Archard wear and similar models done by Holms and Khrushchov it’s best to look through each paper. Here are some that may move you in the right direction:

  • Archard, J. F. Contact and rubbing of flat surfaces. J. Applied Phys 24:981-988, 1953.
  • Kauzlarich, J. J., Williams, J. A., Archard wear and component geometery. Proc Instn Mech Engrs. 215: 387-403, 2016.
  • Rabinowicz, E. Friction and wear of materials. John Wiley, New York. 1965.

for a quick intro:

https://en.wikipedia.org/wiki/Archard_equation

 

Granular Dynamics or Contact Mechanics

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I have recently delved deeper into DEM and Powder Mechanics and have spent hours reading conference proceedings and studies. After a while I had to take a step back and answer the basics. Am I studying granular dynamics or contact mechanics?

In truth, both. Granular dynamic is studied as particle kinematics where we obtain the incremental displacements at contact from the contact reactions. During these interactions we look at particle-particle slip, rotation, normal and tangential forces, energy damping forces, chemical and body forces, among other factors which combined reorient the particle and updates it position and displacements.

Contact mechanics is labeled as the theoretical methods used to describe that force-displacement behavior. There are so many theories and I have mentioned a few in previous post. Most restrict the particle considered to spheres and study the force-displacement behavior as dependent on the material properties, size, surface conditions and in some cases the medium in which these material are interacting. Think soil movement vs a fluidized bed.

Needless to say, which model you use will depend on the problem application being studied.  I keep on hand about nine different models that are switched out depending on the study being performed. These models also have a number of variation in behavior if I am also observing cohesive/adhesive forces.

Particle size or scale is one of my driving factors when selecting a theory. If the material we are testing and modeling is being handled at the a small sieve size (a few millimeters in diameter) then a model such one by Deresiewicz, Tsuji, or Hertz is used. If we are working at a larger scale, say pellets or bigger, we will look at models by Luding or Walton. It all depends on the material and how it is being handled. Pneumatic conveying, fluidized beds, overland conveying all have their own set of requirements and challenges.

The study of DEM is all about knowing what methods are at your disposal, knowing when and how to use them, and filling in the gaps with further research. I have no one recommended reading, however, Colin Thornton has studied the various methods extensively and recorded his findings. A good place to start is with his Particle Technology Series: Granular Dynamics, Contact Mechanics, and Particle System Simulations–A DEM study.

I am currently reading The Springer Particle Technology Series Volume 24. It may be a little dense if you are just starting your DEM studies but it is a good resource towards other papers that may be better tailored to your application.

Good luck in your studies! Let me know how it is going and reach out on LinkedIn or here.