Hi everyone,
When we apply a traction force to a boundary, which field should i look to track it ?. I checked both pressure and stress, but it will change with time. So, Should the applied traction ( for example from a ASCII file) be constant over the model run time ? 
Best,
Chameera
Hi Chameera,
A traction is a force applied across a specific surface. A single traction does not uniquely define the pressure or the stress.
To answer your Question 1: You can calculate the traction \boldsymbol{t} across any given surface from the (Cauchy) stress tensor \boldsymbol{\sigma} via the equation t_i = \sigma_{ij} n_j, where \boldsymbol{n} is the unit normal to the surface. You can get paraview (for example) to do this calculation.
To answer your Question 2: You can apply a constant or time-dependent traction condition to your model. See Boundary traction model — ASPECT 2.6.0-pre
Does this answer your questions?
Best wishes,
Bob
Hi Bob,
Thank you for the reply . Just to clarify, because I apply traction from an ASCII file ( Now i think this represent constant traction ) which has values defined for Y coordinate. When i calculate traction from Cauchy stress tensor, i should be seeing constant value for each depth level ( @ y = 200e3 should have a constant value ) for every time step forward from applied traction time step.
calculated traction will be represented by 2 by 1 matrix [ stress[0,0] stress[1,0] ] where normal vector is [1 0] for the right boundary.
Is this the right way of understanding your answer to the question 1 ? Or can you explain a little bit more about the traction values that i should be getting over the model time considering the model depth, if you think i did not understand your answer 
?
Best,
Chameera
Yes, that is correct - the boundary traction model constrains one or more elements of the traction vector at every point on the prescribed boundary/boundaries (Boundary conditions — ASPECT 2.6.0-pre)
Best wishes,
Bob
Thank you Bob.
Best,
Chameera