{"id":28751,"date":"2013-05-21T18:43:37","date_gmt":"2013-05-21T13:13:37","guid":{"rendered":"http:\/\/www.kopykitab.com\/blog\/?p=28751"},"modified":"2013-05-21T18:43:37","modified_gmt":"2013-05-21T13:13:37","slug":"solid-mechanics-stress-notes","status":"publish","type":"post","link":"https:\/\/www.kopykitab.com\/blog\/solid-mechanics-stress-notes\/","title":{"rendered":"Solid Mechanics : Stress Notes"},"content":{"rendered":"

Solid Mechanics : Stress Notes<\/h1>\n\n\n\n\n\n\n\n\n
The concept of\u00a0stress<\/b>\u00a0originated from the study of strength and failure of solids. The stress field is the distribution of internal “tractions” that balance a given set of external tractions and body forces.\"\"<\/span><\/p>\n

First, we look at the external traction\u00a0T<\/b>\u00a0that represents the force per unit area acting at a given location on the body’s surface. Traction\u00a0T<\/b>\u00a0is a\u00a0bound vector<\/em>, which meansT<\/b>\u00a0cannot slide along its line of action or translate to another location and keep the same meaning.<\/span><\/p>\n

In other words, a traction vector cannot be fully described unless both the force and the surface where the force acts on has been specified. Given both\u00a0D<\/span>F<\/i>\u00a0and\u00a0D<\/span>s<\/i>, the traction\u00a0T<\/b>\u00a0can be defined as<\/span><\/p>\n

 <\/p>\n

\"\"<\/span><\/div>\n

\"\"<\/span><\/p>\n

The internal traction within a solid, or stress, can be defined in a similar manner. Suppose an arbitrary slice is made across the solid shown in the above figure, leading to the free body diagram shown at right. Surface tractions would appear on the exposed surface, similar in form to the external tractions applied to the body’s exterior surface. The stress at point\u00a0P<\/i>\u00a0can be defined using the same equation as was used for\u00a0T<\/b>.<\/span><\/p>\n

Stress therefore can be interpreted as internal tractions that act on a defined internal datum plane. One cannot measure the stress without first specifying the datum plane.<\/span><\/p>\n

 <\/td>\n<\/tr>\n

\"\"<\/span><\/td>\n<\/tr>\n
The Stress Tensor (or Stress Matrix)<\/b><\/td>\n<\/tr>\n
\"\"<\/span><\/td>\n<\/tr>\n
<\/td>\n<\/tr>\n
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\u00a0direct stress<\/i>. The other two components are tangential to the surface and represent\u00a0shear stresses<\/i>.<\/span>What is the distinction between normal and tangential tractions, or equivalently, direct and shear stresses?\u00a0Direct stresses<\/b>\u00a0tend to change the volume of the material (e.g. hydrostatic pressure) and are resisted by the body’s bulk modulus (which depends on the Young’s modulus and Poisson ratio).\u00a0Shear stresses<\/b>\u00a0tend to deform the material without changing its volume, and are resisted by the body’s shear modulus.<\/span><\/p>\n

\"\"<\/span><\/p>\n

Defining a set of internal datum planes aligned with a Cartesian coordinate system allows the stress state at an internal point\u00a0P<\/i>\u00a0to be described relative to\u00a0x<\/i>,\u00a0y<\/i>, andz<\/i>\u00a0coordinate directions.<\/span><\/p>\n

For example, the stress state at point\u00a0P<\/i>\u00a0can be represented by an\u00a0infinitesimal<\/em>\u00a0cube with three stress components on each of its six sides (one direct and two shear components).<\/span><\/p>\n

Since each point in the body is under static equilibrium (no net force in the absense of any body forces), only nine stress components from three planes are needed to describe the stress state at a point\u00a0P<\/i>.<\/span><\/p>\n

These nine components can be organized into the matrix:<\/span><\/p>\n

 <\/p>\n

\"\"<\/span><\/div>\n

where shear stresses across the diagonal are identical (i.e.\u00a0s<\/span>xy<\/i><\/sub>\u00a0=\u00a0s<\/span>yx<\/i><\/sub>,\u00a0s<\/span>yz<\/i><\/sub>\u00a0=\u00a0s<\/span>zy<\/i><\/sub>, and\u00a0s<\/span>zx<\/i><\/sub>\u00a0=\u00a0s<\/span>xz<\/i><\/sub>) as a result of static equilibrium (no net moment). This grouping of the nine stress components is known as thestress tensor<\/b>\u00a0(or stress matrix).<\/span><\/p>\n

The subscript notation used for the nine stress components have the following meaning:<\/span><\/p>\n

 <\/p>\n

\"\"<\/span><\/div>\n\n\n\n\n\n
\u00a0\u00a0\u00a0\u00a0Note:<\/span><\/td>\nThe stress state is a second order tensor since it is a quantity associated with two directions. As a result, stress components have 2 subscripts.
\nA surface traction is a first order tensor (i.e. vector) since it a quantity associated with only one direction. Vector components therefore require only 1 subscript.
\nMass would be an example of a zero-order tensor (i.e. scalars), which have no relationships with directions (and no subscripts).<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/td>\n<\/tr>\n
<\/td>\n<\/tr>\n
<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\n\n\n\n\n\n
\"\"<\/span><\/td>\n<\/tr>\n
Equations of Equilibrium<\/b><\/td>\n<\/tr>\n
\"\"<\/span><\/td>\n<\/tr>\n
<\/td>\n<\/tr>\n
Consider the static equilibrium of a solid subjected to the body force vector field\u00a0b<\/b>. Applying Newton’s first law of motion results in the following set of differential equations which govern the stress distribution within the solid,<\/span><\/p>\n

 <\/p>\n

\"\"<\/span><\/div>\n

In the case of two dimensional stress, the above equations reduce to,<\/span><\/p>\n

 <\/p>\n

\"\"<\/span><\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n","protected":false},"excerpt":{"rendered":"

Solid Mechanics : Stress Notes The concept of\u00a0stress\u00a0originated from the study of strength and failure of solids. The stress field is the distribution of internal “tractions” that balance a given set of external tractions and body forces. First, we look at the external traction\u00a0T\u00a0that represents the force per unit area acting at a given location … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"fifu_image_url":"","fifu_image_alt":""},"categories":[4773],"tags":[],"amp_enabled":true,"_links":{"self":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts\/28751"}],"collection":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/comments?post=28751"}],"version-history":[{"count":0,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts\/28751\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/media?parent=28751"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/categories?post=28751"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/tags?post=28751"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}