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Dear Aydın, "Traction" is a vector representing force per unit area acting on a surface. If we talk in three dimensions, this vector has three components. "Stress" is the collection of tractions acting in all the possible different planes anchored in a single point. Therefore, is not a single vector, and it is expressed as a tensor. If we are talking in three dimensions, this tensor has 9 components. This 9 components are sufficient to define the stress state at this point. As it happens, many times we use stress where we should be using traction, which probably adds confusion to the topic... You can find a superb explanation in two textbooks: "Structural geology", of Twiss and Moores (2007), and "Structural geology or rocks and regions", of Davis and Reynolds (1996, although a new edition is coming in November!) A much better explanation than mine can be found online in: http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/stress.cfm (Look carefuly where it says " Surface tractions, or stresses acting on an internal datum plane, are typically decomposed into three mutually orthogonal components. One component is normal to the surface and represents direct stress. The other two components are tangential to the surface and represent shear stresses.") I hope that helps and I haven't complicated it even more!!! Jorge Ginés Geologist, MSc Fugro NPA Limited Telephone: +44 (0) 1732 865023 / Fax: +44 (0) 1732 866521 E-mail: [log in to unmask] <mailto:[log in to unmask]> / Website: www.fugro-npa.com <http://www.fugro-npa.com/> Crockham Park, Edenbridge, Kent TN8 6SR, United Kingdom From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of aydin CICEK Sent: 27 September 2011 15:18 To: [log in to unmask] Subject: About stress and traction Dear GeoTectonics Community, What is the difference between stress and traction in terms of structural geology ? I found a bunch of information about them. But, I am not satisfied. Any help will be appreciated! Kind regards, Aydın ÇİÇEK Mineral Research & Exploration of Turkey Geothermal Energy & Hot Springs Surveys Unit