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Displacement field (mechanics)
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<h2 id="formulation">Formulation</h2> <p>Before considering displacement, the state before deformation must be defined. It is a state in which the coordinates of all points are known and described by the function: R → 0 : Ω → P {\displaystyle {\vec {R}}_{0}:\Omega \to P} where </p> <ul><li> R → 0 {\displaystyle {\vec {R}}_{0}} is a placement vector</li> <li> Ω {\displaystyle \Omega } are all the points of the body</li> <li> P {\displaystyle P} are all the points in the space in which the body is present</li></ul> <p>Most often it is a state of the body in which no forces are applied. </p><p>Then given any other state of this body in which coordinates of all its points are described as R → 1 {\displaystyle {\vec {R}}_{1}} the displacement field is the difference between two body states: u → = R → 1 − R → 0 {\displaystyle {\vec {u}}={\vec {R}}_{1}-{\vec {R}}_{0}} where u → {\displaystyle {\vec {u}}} is a displacement field, which for each point of the body specifies a <a href="/facts/Displacement_vector/jkmEfS3C">displacement vector</a>. </p> <h2 id="decomposition">Decomposition</h2> <p>The displacement of a body has two components: a <a href="/facts/Rigid_body/THezBlq2">rigid-body</a> displacement and a deformation. </p> <ul><li>A rigid-body displacement consists of a simultaneous <a href="/facts/Translation_(physics)/KlpWmnfX">translation</a> and rotation of the body without changing its shape or size.</li> <li>Deformation implies the change in shape and/or size of the body from an initial or undeformed configuration κ 0 ( B ) {\displaystyle \kappa _{0}({\mathcal {B}})} to a current or deformed configuration κ t ( B ) {\displaystyle \kappa _{t}({\mathcal {B}})} (Figure 1).</li></ul> <p>A change in the configuration of a continuum body can be described by a displacement field. A <i>displacement field</i> is a <a href="/facts/Vector_field/ccFNuPPp">vector field</a> of all displacement vectors for all particles in the body, which relates the deformed configuration with the undeformed configuration. The distance between any two particles changes if and only if deformation has occurred. If displacement occurs without deformation, then it is a rigid-body displacement. </p> <h2 id="displacement-gradient-tensor">Displacement gradient tensor</h2> <p class="note">See also: <a href="/facts/Lagrangian_and_Eulerian_specification_of_the_flow_field/sYLvgboC">Lagrangian and Eulerian specification of the flow field</a></p> <p>Two types of displacement gradient tensor may be defined, following the Lagrangian and Eulerian specifications. The displacement of particles indexed by variable i may be expressed as follows. The vector joining the positions of a particle in the undeformed configuration P i {\displaystyle P_{i}} and deformed configuration p i {\displaystyle p_{i}} is called the <a href="/facts/Displacement_(vector)/jkmEfS3C">displacement vector</a>, p i − P i {\displaystyle p_{i}-P_{i}} , denoted u i {\displaystyle u_{i}} or U i {\displaystyle U_{i}} below. </p> <h3>Material coordinates (Lagrangian description)</h3> <p>Using X {\displaystyle \mathbf {X} } in place of P i {\displaystyle P_{i}} and x {\displaystyle \mathbf {x} } in place of p i {\displaystyle p_{i}\,\!} , both of which are vectors from the origin of the coordinate system to each respective point, we have the <a href="/facts/Continuum_mechanics/vI7LpFDk">Lagrangian description</a> of the displacement vector: u ( X , t ) = u i e i {\displaystyle \mathbf {u} (\mathbf {X} ,t)=u_{i}\mathbf {e} _{i}} where e i {\displaystyle \mathbf {e} _{i}} are the orthonormal <a href="/facts/Unit_vectors/GyaXneDn">unit vectors</a> that define the <a href="/facts/Basis_(linear_algebra)/89IPoN6c">basis</a> of the spatial (<a href="/facts/Lab_frame/WdBs8wgh">lab frame</a>) coordinate system. </p><p>Expressed in terms of the material coordinates, i.e. u {\displaystyle \mathbf {u} } as a function of X {\displaystyle \mathbf {X} } , the displacement field is: u ( X , t ) = b ( t ) + x ( X , t ) − X or u i = α i J b J + x i − α i J X J {\displaystyle \mathbf {u} (\mathbf {X} ,t)=\mathbf {b} (t)+\mathbf {x} (\mathbf {X} ,t)-\mathbf {X} \qquad {\text{or}}\qquad u_{i}=\alpha _{iJ}b_{J}+x_{i}-\alpha _{iJ}X_{J}} where b ( t ) {\displaystyle \mathbf {b} (t)} is the displacement vector representing rigid-body translation. </p><p>The <a href="/facts/Partial_derivative/JWHsBkAu">partial derivative</a> of the displacement vector with respect to the material coordinates yields the material displacement gradient tensor ∇ X u {\displaystyle \nabla _{\mathbf {X} }\mathbf {u} \,\!} . Thus we have, ∇ X u = ∇ X x − R = F − R or ∂ u i ∂ X K = ∂ x i ∂ X K − α i K = F i K − α i K {\displaystyle \nabla _{\mathbf {X} }\mathbf {u} =\nabla _{\mathbf {X} }\mathbf {x} -\mathbf {R} =\mathbf {F} -\mathbf {R} \qquad {\text{or}}\qquad {\frac {\partial u_{i}}{\partial X_{K}}}={\frac {\partial x_{i}}{\partial X_{K}}}-\alpha _{iK}=F_{iK}-\alpha _{iK}} where F {\displaystyle \mathbf {F} } is the <i><a href="/facts/Material_deformation_gradient_tensor/rEjpC9oD">material deformation gradient tensor</a></i> and R {\displaystyle \mathbf {R} } is a rotation. </p> <h3>Spatial coordinates (Eulerian description)</h3> <p>In the <a href="/facts/Continuum_mechanics/vI7LpFDk">Eulerian description</a>, the vector extending from a particle P {\displaystyle P} in the undeformed configuration to its location in the deformed configuration is called the <a href="/facts/Displacement_(vector)/jkmEfS3C">displacement vector</a>: U ( x , t ) = U J E J {\displaystyle \mathbf {U} (\mathbf {x} ,t)=U_{J}\mathbf {E} _{J}} where E i {\displaystyle \mathbf {E} _{i}} are the unit vectors that define the basis of the material (body-frame) coordinate system. </p><p>Expressed in terms of spatial coordinates, i.e. U {\displaystyle \mathbf {U} } as a function of x {\displaystyle \mathbf {x} } , the displacement field is: U ( x , t ) = b ( t ) + x − X ( x , t ) or U J = b J + α J i x i − X J {\displaystyle \mathbf {U} (\mathbf {x} ,t)=\mathbf {b} (t)+\mathbf {x} -\mathbf {X} (\mathbf {x} ,t)\qquad {\text{or}}\qquad U_{J}=b_{J}+\alpha _{Ji}x_{i}-X_{J}} </p><p>The <a href="/facts/Spatial_derivative/2cmoCezd">spatial derivative</a>, i.e., the partial derivative of the displacement vector with respect to the spatial coordinates, yields the spatial displacement gradient tensor ∇ x U {\displaystyle \nabla _{\mathbf {x} }\mathbf {U} \,\!} . Thus we have, ∇ x U = R T − ∇ x X = R T − F − 1 or ∂ U J ∂ x k = α J k − ∂ X J ∂ x k = α J k − F J k − 1 , {\displaystyle \nabla _{\mathbf {x} }\mathbf {U} =\mathbf {R} ^{T}-\nabla _{\mathbf {x} }\mathbf {X} =\mathbf {R} ^{T}-\mathbf {F} ^{-1}\qquad {\text{or}}\qquad {\frac {\partial U_{J}}{\partial x_{k}}}=\alpha _{Jk}-{\frac {\partial X_{J}}{\partial x_{k}}}=\alpha _{Jk}-F_{Jk}^{-1}\,,} where F − 1 = H {\displaystyle \mathbf {F} ^{-1}=\mathbf {H} } is the <i><a href="/facts/Spatial_deformation_gradient_tensor/rEjpC9oD">spatial deformation gradient tensor</a></i>. </p> <h3>Relationship between the material and spatial coordinate systems</h3> <p> α J i {\displaystyle \alpha _{Ji}} are the <a href="/facts/Direction_cosine/6YbPD687">direction cosines</a> between the material and spatial coordinate systems with unit vectors E J {\displaystyle \mathbf {E} _{J}} and e i {\displaystyle \mathbf {e} _{i}\,\!} , respectively. Thus E J ⋅ e i = α J i = α i J {\displaystyle \mathbf {E} _{J}\cdot \mathbf {e} _{i}=\alpha _{Ji}=\alpha _{iJ}} </p><p>The relationship between u i {\displaystyle u_{i}} and U J {\displaystyle U_{J}} is then given by u i = α i J U J or U J = α J i u i {\displaystyle u_{i}=\alpha _{iJ}U_{J}\qquad {\text{or}}\qquad U_{J}=\alpha _{Ji}u_{i}} </p><p>Knowing that e i = α i J E J {\displaystyle \mathbf {e} _{i}=\alpha _{iJ}\mathbf {E} _{J}} then u ( X , t ) = u i e i = u i ( α i J E J ) = U J E J = U ( x , t ) {\displaystyle \mathbf {u} (\mathbf {X} ,t)=u_{i}\mathbf {e} _{i}=u_{i}(\alpha _{iJ}\mathbf {E} _{J})=U_{J}\mathbf {E} _{J}=\mathbf {U} (\mathbf {x} ,t)} </p> <h3>Combining the coordinate systems of deformed and undeformed configurations</h3> <p>It is common to superimpose the coordinate systems for the deformed and undeformed configurations, which results in b = 0 {\displaystyle \mathbf {b} =0\,\!} , and the direction cosines become <a href="/facts/Kronecker_delta/gcGAy0bm">Kronecker deltas</a>, i.e., E J ⋅ e i = δ J i = δ i J {\displaystyle \mathbf {E} _{J}\cdot \mathbf {e} _{i}=\delta _{Ji}=\delta _{iJ}} </p><p>Thus in material (undeformed) coordinates, the displacement may be expressed as: u ( X , t ) = x ( X , t ) − X or u i = x i − δ i J X J {\displaystyle \mathbf {u} (\mathbf {X} ,t)=\mathbf {x} (\mathbf {X} ,t)-\mathbf {X} \qquad {\text{or}}\qquad u_{i}=x_{i}-\delta _{iJ}X_{J}} </p><p>And in spatial (deformed) coordinates, the displacement may be expressed as: U ( x , t ) = x − X ( x , t ) or U J = δ J i x i − X J {\displaystyle \mathbf {U} (\mathbf {x} ,t)=\mathbf {x} -\mathbf {X} (\mathbf {x} ,t)\qquad {\text{or}}\qquad U_{J}=\delta _{Ji}x_{i}-X_{J}} </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Stress_(mechanics)/ErAXDbbd">Stress</a></li> <li><a href="/facts/Strain_(mechanics)/xK7R59oI">Strain</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Continuum Mechanics - Kinematics". School of Engineering. Brown University. Retrieved 2018-07-25. <a href="https://www.brown.edu/Departments/Engineering/Courses/En221/Notes/Kinematics/Kinematics.htm" target="_blank">https://www.brown.edu/Departments/Engineering/Courses/En221/Notes/Kinematics/Kinematics.htm</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"2.080 Lecture 3: The Concept of Stress, Generalized Stresses and Equilibrium" (PDF). MIT OpenCourseWare. Retrieved 2018-07-25. <a href="https://ocw.mit.edu/courses/mechanical-engineering/2-080j-structural-mechanics-fall-2013/course-notes/MIT2_080JF13_Lecture2.pdf" target="_blank">https://ocw.mit.edu/courses/mechanical-engineering/2-080j-structural-mechanics-fall-2013/course-notes/MIT2_080JF13_Lecture2.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> </ol>