# research

## Equivalence of Virial stress to Continuum Cauchy Stress

Calculating stresses in MD simulations is a controversial topic. There are two different schools of thought about the equivalence of the virial stress to the continuum Cauchy stress; for and against. Some argue based on momentum balance, that only the potential contribution to the virial stress should be considered as the continuum Cauchy stress. However, others assert that the total virial stress that contains both the kinetic and potential parts is indeed the quantity that corresponds to the Cauchy stress in continuum mechanics. We used a simple thermo-elastic analysis to verify the validity of using the total virial stress as the continuum Cauchy stress and found that the total virial stress is indeed the continuum Cauchy stress.

## Guide vanes flutter/vibration

Choose a channel featured in the header of iMechanica:

Greetings co-researchers,

I am currently designing a full-scale Impulse turbine (2 to 3 m diameter, 0.5 hub to tip ratio) for extracting of energy from waves.The turbine will be connected to a shore based device oscillating water column (OWC), so the airflow through the turbine is bidirectional (i.e. reverses as the wave enters and recedes in the OWC). This means we have to use symmetrical entry and exit guide vanes. These vanes are fixed, not movable.·        The guide vanes are slender, approximately; height 700mm, chord length 600mm and thickness 2 to 5mm. Their role is to redirect the air flow from the axial direction to an angle of 60o ·        The rotor rotation speed is low (100 RPM to 1000 RPM). ·        The airflow is incompressible (Mach number < 0.3) and unsteady as it is related to the wave energy, which means the mass flow inlet to the turbine changes randomly (from zero to a maximum value say 10kg/s). One of the good approximations to this airflow is a sinusoid, but even this is extremely difficult to simulate in Fluent 6.2 CFD.I have done some preliminary forced vibration response analysis of the guide vanes. As far as I can see, the main cause of any vibration of the guide vanes would be the changes on pressure caused by the chopping of the flow by the rotor (i.e. the passing frequency of the rotor/guide vanes assembly). I plan to measure these pressure variations using pressure tapings on an experimental turbine test rig.

Please could you comment whether in your experience the main source of guide vane vibration would be the chopping of the fluid flow by the rotor. Also I would appreciate it if you have done any experimental or analytical data on this problem.

## Crack Propagation

Choose a channel featured in the header of iMechanica:

Hi

I have an investigation on Crack Propagation.

How can i predict the path of a crack.

## friction and plasticity: new avenues of research?

Choose a channel featured in the header of iMechanica:

Based on some recent results by Anders Klabring, myself and Jim Barber, showing rigorously that Melan’s theorem only works for a very restricted class of frictional problems, we suggest possible ave

## IS THERE NO PULL-OFF FOR ADHESIVE FRACTAL SURFACES?

Choose a channel featured in the header of iMechanica:
Free Tags:

In this short note we remark that, at least for the theory of Fuller & Tabor for the adhesive contact of rough random surfaces, fractal surfaces have a limiting zero pull-off force, for all fractal dimensions or amplitudes of roughness. This paradoxical result raises some questions. I ask the iMechanica community for opinions, comparisons of experiments, etc.

## review on KLJ's most loved areas in contact mechanics

Choose a channel featured in the header of iMechanica:
Free Tags:

If we read Ken Johnson’s Timoshenko medal 2006 speech also posted in iMechanica, the subjects Ken mentions in his brief and humorous speech are:-

1. corrugation of railway rails,
2. the damping at clamped joints,
3. Hertz contact under the action of tangential friction forces,
4. ‘tribology' (word invented by David Tabor along with F.P.Bowden in Cambridge),
5. Atomic Force Microscope, Surface Force Apparatus & friction on the atomic scale,
6. Relation between adhesion and friction.

These are probably the subjects Ken is most attached to. Some are older (but perhaps not solved, lke corrugation, for which the “short-pitch” fixed wavelength mechanism is still unclear despite Ken’s 40 years of efforts (!), and some are certainly fashionable today (like adhesion and friction at atomic scale). In starting this forum, why not start from here? Should we prepare a 1 page summary on each of these topics? Since I start this, I will do the effort on corrugation I promise in the next week or so!

Regards, Mike

## shakedown in friction --- where should we send it to?

Anders Klarbring, Jim Barber and I are preparing a paper on the subject of shakedown in elastic contact problems with Coulomb friction. In particular, we establish the (rather limited) conditions under which a frictional equivalent of Melan's theorem can be applied, and we counterprove the theorem in all other cases.There is no plasticity here - the contacting bodies are linear elastic - but the analogies between the Coulomb friction law and elastic plastic deformation make us think the plasticity community might be interested in the results.

## modern explosion science and engineering

This blog focuses on the behaviors of energetic materials (such as solid rocket propellants, high explosives), shock waves,  and explosions. And also on the protections, from the design of protecting materials and structures.

Lecture notes: