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Pure shear

Pure shear is a fundamental type of deformation in in which a body experiences equal and opposite extensions and contractions along principal directions, without any accompanying , often preserving volume. This irrotational contrasts with other modes by isolating the symmetric component of the deformation, transforming shapes like spheres into ellipsoids with aligned axes. Mathematically, pure shear is represented by a symmetric stretch tensor V with principal stretches \lambda, $1/\lambda, and 1 (for plane ), where the deformation gradient F = V R incorporates only R externally if needed. In , the strain tensor \varepsilon takes the form \begin{pmatrix} \varepsilon & 0 & 0 \\ 0 & -\varepsilon & 0 \\ 0 & 0 & 0 \end{pmatrix} in principal coordinates, with zero indicating no volumetric change. For the associated stress state, pure shear corresponds to a Cauchy tensor with zero, such as principal values \sigma, -\sigma, and 0, representing purely deviatoric loading without hydrostatic . Pure shear differs from simple shear, which involves a nonsymmetric deformation like F = \begin{pmatrix} 1 & \gamma & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix}, introducing both shear and rigid . While simple shear produces an infinitesimal tensor \varepsilon = \begin{pmatrix} 0 & \gamma/2 & 0 \\ \gamma/2 & 0 & 0 \\ 0 & 0 & 0 \end{pmatrix}, pure shear eliminates the rotational component for a cleaner measure of material distortion. In nonlinear settings, simple shear does not align with a pure shear state, highlighting their incompatibility in advanced elasticity models. In applications, pure shear models tectonic deformations in , where it describes non-rotational flattening of rock layers during . In engineering, it approximates plane strain conditions in long elastomer specimens under tension, enabling fracture energy analysis independent of crack length. Materials like amorphous solids exhibit unique yielding and under pure shear, informing studies of deformation mechanisms at the atomic scale.

Fundamentals

Definition

Strain refers to the deformation of a , quantifying changes in its or due to applied forces. deformation, a subset of , involves the sliding or angular distortion of adjacent layers within the . Pure shear is a three-dimensional homogeneous flattening of a body without . It represents an irrotational , characterized by in one principal direction accompanied by of equal in the orthogonal principal direction, while the intermediate principal direction experiences zero . This process maintains constant volume and aligns with principal axes that do not rotate during deformation. Pure shear can be represented by equal and opposite normal forces applied to a layer, causing displacement parallel to the layer plane without inducing rotation. In contrast to simple shear, which incorporates rotational components, pure shear remains purely deformational.

Physical Characteristics

Pure shear represents a constant-volume deformation process, characteristic of incompressible materials where there is no net dilation or change in volume. This volume conservation arises because the extension in one principal direction is precisely balanced by an equal and opposite contraction in the perpendicular direction, ensuring the determinant of the deformation gradient equals unity. In terms of along the axes, pure shear involves principal stretches where one experiences (λ₁ > 1), the remains unchanged (λ₂ = 1), and the third undergoes shortening (λ₃ < 1), with the relation λ₁ λ₃ = 1 maintaining incompressibility in plane strain conditions. This symmetric stretching and compressing occurs without any rotation of material points, distinguishing pure shear as an irrotational deformation where material elements deform symmetrically around fixed principal axes. Under the infinitesimal strain approximation, pure shear manifests as pure extension in one direction coupled with pure compression in the orthogonal direction, with zero shear strain components in the principal coordinate system. For visualization, consider a square element aligned at 45 degrees to the principal axes: under pure shear, it deforms into a diamond (rhombus) shape while preserving its area and without any tilting or rotation of its sides relative to one another. Similarly, a sphere subjected to pure shear flattens into an oblate aligned with the principal axes, illustrating the balanced elongation and shortening without volumetric change.

Mathematical Formulation

Deformation Gradient

The deformation gradient tensor \mathbf{F} provides the fundamental kinematic description of pure shear as the two-point tensor that maps infinitesimal line elements from the reference configuration to the deformed configuration, defined as the Jacobian matrix of the deformation mapping \mathbf{x} = \boldsymbol{\chi}(\mathbf{X}), where \mathbf{X} is the position in the reference state and \mathbf{x} is the position in the deformed state. In two dimensions, for pure shear aligned with the principal axes, \mathbf{F} takes the diagonal form \mathbf{F} = \begin{pmatrix} 1 + \epsilon & 0 \\ 0 & 1 - \epsilon \end{pmatrix}, where \epsilon > 0 is the principal strain parameter representing extension in one direction and equal contraction perpendicular to it, assuming approximate incompressibility for small \epsilon. When expressed in coordinates rotated by 45 degrees relative to the principal axes (aligned with the shear direction), the deformation gradient adopts the symmetric form \mathbf{F} = \begin{pmatrix} 1 & \gamma \\ \gamma & 1 \end{pmatrix}, where \gamma is the shear parameter, related to \epsilon by \gamma \approx \epsilon for infinitesimal deformations. For infinitesimal strains, the displacement gradient tensor \mathbf{H} = \nabla \mathbf{u} = \mathbf{F} - \mathbf{I} (with \mathbf{I} the ) reduces to its symmetric part only, comprising the infinitesimal strain tensor, while the antisymmetric part—the tensor—is zero, reflecting the absence of rotation in pure shear. This irrotational nature is confirmed by the of \mathbf{F} = \mathbf{R} \mathbf{U}, where \mathbf{R} is the orthogonal tensor (here, \mathbf{R} = \mathbf{I}) and \mathbf{U} is the right stretch tensor, which coincides with \mathbf{F} due to its symmetry and , possessing principal values $1 + \epsilon and $1 - \epsilon along the principal directions. In the context of for steady pure shear deformations, the velocity gradient tensor \mathbf{L} = \dot{\mathbf{F}} \mathbf{F}^{-1} remains symmetric, ensuring no and consistent irrotational flow throughout the deformation process. This symmetry of \mathbf{L} underscores the purely deformative character of pure shear, distinguishing it kinematically from rotational deformations.

Strain Tensors

In pure shear, the infinitesimal tensor \boldsymbol{\varepsilon} is the symmetric part of the displacement gradient tensor, capturing small deformations where higher-order terms are neglected. In the principal aligned with the directions of maximum extension and compression, \boldsymbol{\varepsilon} is diagonal with components \varepsilon_{xx} = \varepsilon, \varepsilon_{yy} = -\varepsilon, \varepsilon_{zz} = 0, and all off-diagonal components zero, where \varepsilon > 0 represents the magnitude of the principal . In the sheared rotated by 45° relative to the principal axes, the tensor takes the form with zero diagonal components and off-diagonal shear \varepsilon_{xy} = \varepsilon_{yx} = \gamma/2, where \gamma = 2\varepsilon is the engineering shear , reflecting the angular distortion without rotation. For finite deformations, the Green-Lagrange tensor \mathbf{E} = \frac{1}{2}(\mathbf{F}^T \mathbf{F} - \mathbf{I}) provides a measure derived from the deformation \mathbf{F}, accounting for large stretches while remaining objective. In pure shear, where \mathbf{F} is symmetric and volume-preserving (\det \mathbf{F} = 1), the principal components of \mathbf{E} are \frac{1}{2}(\lambda^2 - 1), \frac{1}{2}((1/\lambda)^2 - 1), and 0, with \lambda > 1 the principal stretch. For small strains (\varepsilon \ll 1, \lambda \approx 1 + \varepsilon), the principal components approximate to \varepsilon and -\varepsilon (with corrections such as \varepsilon + \frac{1}{2}\varepsilon^2 and -\varepsilon + \frac{3}{2}\varepsilon^2) in the principal directions in , capturing the leading linear terms and nonlinear corrections beyond the \boldsymbol{\varepsilon}. In the sheared coordinates, for \mathbf{F} = \begin{bmatrix} 1 & \gamma \\ \gamma & 1 \end{bmatrix}, the exact form is \mathbf{E} = \begin{bmatrix} \frac{\gamma^2}{2} & \gamma \\ \gamma & \frac{\gamma^2}{2} \end{bmatrix}, which for small \gamma approximates the tensor. The Euler-Almansi strain tensor \mathbf{e} = \frac{1}{2}(\mathbf{I} - \mathbf{F}^{-T} \mathbf{F}^{-1}) offers an Eulerian counterpart, referenced to the deformed . For pure shear, its principal values mirror those of \mathbf{E} in magnitude but differ in sign for the nonlinear terms, yielding similar approximations for small deformations where \mathbf{e} \approx \boldsymbol{\varepsilon}. In the sheared frame, the full expression involves the inverse stretches, but principal values remain \varepsilon_1 = \varepsilon, \varepsilon_2 = 0, \varepsilon_3 = -\varepsilon in the linear limit. Key strain invariants for pure shear underscore its incompressible nature: the first invariant I_1 = \operatorname{tr} \boldsymbol{\varepsilon} = 0 for the infinitesimal tensor, reflecting no volumetric change; in 2D, the second invariant I_2 = 0 characterizes the pure deviatoric state; and the third invariant I_3 = \det \mathbf{F} = 1 preserves across finite formulations. The principal strains are \varepsilon_1 = \varepsilon, \varepsilon_2 = 0, \varepsilon_3 = -\varepsilon, with extension along one axis balanced by equal compression perpendicularly in the plane.

Comparison with Simple Shear

Pure shear and shear represent distinct kinematic states in , differing primarily in their rotational components despite sharing similar measures in the infinitesimal limit. shear is characterized by a deformation gradient tensor F_{\text{simple}} = \begin{pmatrix} 1 & \gamma \\ 0 & 1 \end{pmatrix}, where \gamma is the amount of , which incorporates both deformation and rotation. In the small approximation, the corresponding infinitesimal tensor exhibits off-diagonal components \epsilon_{xy} = \gamma/2, with no strains along the shear axes (\epsilon_{xx} = \epsilon_{yy} = 0); the principal strains are then \epsilon_1 = \gamma/2 and \epsilon_2 = -\gamma/2, indicating balanced extension and but without alignment to the coordinate axes. In contrast, pure shear is an irrotational deformation, obtained by superposing a rotation of magnitude \omega = -\gamma/2 onto the shear to eliminate the rotational component. Specifically, rotating the by an \theta = 45^\circ (or, in general, \theta = \frac{1}{2} \atan\left( \frac{2 \epsilon_{xy}}{\epsilon_{xx} - \epsilon_{yy}} \right), which simplifies to $45^\circ here) aligns the axes with the principal directions, transforming the shear state into one with only normal strains and no components. This reveals the principal strains, while the superposed rotation compensates for the antisymmetric part of the displacement gradient in shear. The key kinematic distinction lies in the tensor: pure shear has a zero rotation tensor (the antisymmetric part of the displacement gradient is zero), ensuring no net rotation of material elements, whereas shear features a nonzero rotation \omega = \gamma/2. This difference is vividly illustrated using for . For simple , the is centered at the origin with radius \gamma/2, reflecting the shear dominance and principal strains at \pm 45^\circ to the shear axes. Rotating to the principal axes yields the pure shear representation, where the circle shows pure normal strains with no shear stress on the principal planes, emphasizing the absence of in those orientations. These invariants of the tensor remain consistent between the two, as noted in the mathematical formulation of .

Relation to General Strain States

Pure shear constitutes a specialized case within the general theory of strain states in , defined as a coaxial deformation wherein the principal axes of the strain tensor remain aligned with the material coordinates throughout the deformation history. This coaxiality ensures that the orientation of maximum and minimum extension directions does not rotate relative to the deforming body, distinguishing pure shear from non- strain paths. As a deviatoric strain, pure shear exhibits a trace of zero for the infinitesimal strain tensor, \operatorname{tr}(\boldsymbol{\varepsilon}) = \varepsilon_{11} + \varepsilon_{22} + \varepsilon_{33} = 0, which corresponds to no volumetric change and thus incompressible behavior in the distortion sense. In the broader classification of general strain states, pure shear represents a of irrotational, incompressible deformations, in contrast to rotational strains that incorporate or dilational strains that alter volume. This irrotational property arises from the symmetric nature of the strain rate tensor in pure shear, with zero antisymmetric () component. Within yield criteria such as the von Mises distortion energy theory, pure shear achieves a maximum octahedral shear strain of \gamma / \sqrt{3}, where \gamma is the engineering shear strain magnitude; this value quantifies the critical distortion level for material yielding under pure shear loading. The octahedral shear strain is given by \gamma_{\mathrm{oct}} = \frac{\sqrt{2}}{3} \sqrt{ (\varepsilon_1 - \varepsilon_2)^2 + (\varepsilon_2 - \varepsilon_3)^2 + (\varepsilon_3 - \varepsilon_1)^2 }, and for the principal strains in pure shear (\varepsilon_1 = \varepsilon, \varepsilon_2 = -\varepsilon, \varepsilon_3 = 0), it simplifies to \gamma_{\mathrm{oct}} = \gamma / \sqrt{3}, emphasizing pure shear's efficiency in inducing shape change without volume alteration. For finite deformations, pure shear preserves the fixed principal directions, enabling commutative superposition of incremental pure shears along the same axes, in contrast to general hypoelastic formulations where path-dependent rotations can misalign principal directions. This property facilitates accurate modeling of large-strain scenarios while maintaining the irrotational and deviatoric characteristics observed in approximations.

Applications and Examples

In Engineering and Materials Science

In and , pure shear is employed in experimental testing of rubber and hyperelastic materials to characterize nonlinear behavior under large deformations. A common setup involves clamping the ends of a thin, rectangular strip specimen—typically short and wide to minimize —and applying uniaxial perpendicular to the length, inducing a homogeneous shear state. The G (or initial modulus \mu) is determined from force-displacement curves, where the applied load F relates to the nominal \tau = F / (L_0 W_0), with L_0 and W_0 as initial length and width, and displacement data fitted to hyperelastic models like Ogden or Mooney-Rivlin. This method, pioneered by Rivlin and in their seminal work on rubber rupture, provides accurate shear response data for calibrating constitutive models in applications such as and . Pure shear also plays a key role in plasticity models for ductile metals, particularly through the von Mises yield criterion, which predicts yielding based on distortional energy. In pure shear, the yield stress \tau relates to the uniaxial yield stress \sigma_y by \tau = \sigma_y / \sqrt{3}, reflecting the criterion's equivalence of multiaxial states to uniaxial tension in terms of effective stress \sigma_e = \sqrt{3} \tau. This relation, derived from the second deviatoric stress invariant, enables accurate prediction of plastic flow in components like shafts and pressure vessels under combined loading, where pure shear approximates torsional or biaxial conditions without volumetric changes. Experimental validation in metals such as steel confirms von Mises outperforms simpler criteria like Tresca for shear-dominated yielding. In of composites, pure shear loading simulates mode II (in-plane shear) conditions to assess and , often compared to mode I (opening) for mixed-mode . For / laminates, pure shear induces interlaminar sliding, with mode II critical release rate G_{IIc} typically exceeding mode I G_{Ic} (e.g., 1334 J/m² vs. 208 J/m²), highlighting shear's higher resistance but faster under equivalent . This equivalence in loading allows unified criteria like G_c = G_{Ic} + (G_{IIc} - G_{Ic})(G_{II}/G_T)^\eta (where \eta \approx 4.5) to predict failure paths in structures, using end-notched flexure tests to isolate pure shear effects. Strain tensors from such analyses serve as inputs for broader modeling. Finite element modeling of pure shear in software like ABAQUS facilitates stress analysis in materials under controlled deformation, enforcing principal strains through tailored boundary conditions. For instance, in slender plate simulations, axially free edges with panel extensions (e.g., half-depth beyond supports) and initial geometric imperfections (1/1000 of depth) achieve homogeneous shear without spurious normal stresses, solved via nonlinear arc-length methods like Modified Riks. This setup accurately captures post-yield behavior in metals or hyperelastic responses in polymers, validated against experiments for buckling and plasticity in shear-loaded panels.

In Geology

In geology, pure shear is a key mechanism in crustal deformation, particularly during tectonic processes such as and , where it models homogeneous flattening and thickening of rock layers under compressive forces. This type of deformation involves symmetric shortening in one direction accompanied by extension in perpendicular directions, often leading to the development of planes oriented normal to the principal shortening axis. For instance, in zones, pure shear contributes to the distributed thickening of the overriding plate, as observed in non-collisional orogens where uniform vertical crustal thickening produces steep s without significant lateral shear. Similarly, during orogenic events like those in the mountains, vertically coherent pure-shear shortening dominates intracontinental mountain building, resulting in widespread crustal flattening inferred from seismic and structural data. Pure shear is graphically represented in Flinn's fabric diagram, where it corresponds to the line k=1, signifying plane with zero and equal magnitudes of principal extension and axes. This plane condition implies no volume change and deformation paths, distinguishing pure shear from constrictional or flattenable states commonly analyzed in . In natural settings, such as high-strain zones in basement terranes, mylonitic fabrics record pure shear dominance when vorticity numbers (Wm) are low (≤0.4), with ellipsoids plotting near k=1, indicating symmetric deformation without substantial . Examples of pure shear are evident in mylonite zones, where intense ductile deformation produces fine-grained, foliated rocks reflecting symmetric , as seen in the Lawhorne Mill zone with approximately 70% contraction across the shear plane. During lithospheric extension, pure shear models symmetric thinning of the crust and mantle, generating melt through adiabatic decompression, as applied to basins where uniform stretching contrasts with asymmetric simple shear. markers, such as deformed fossils or ooids, provide quantitative evidence of pure shear when their elliptical distortions align with principal strain axes, allowing reconstruction of original shapes to estimate finite strain ratios in the XZ plane. These markers are particularly useful in low-strain rocks adjacent to s, revealing progressive pure shear without rotational components. In natural progressive deformation, pure often couples with simple , forming general shear paths that transition between coaxial and non-coaxial components, as described by deformation matrices that integrate both for transpressional or transtensional . This coupling explains heterogeneous in shear zones, where initial pure shear flattening evolves into rotational simple shear under changing boundary conditions, such as during oblique convergence in orogens. Such transitions are common in the , where pure shear accommodates early symmetric thickening before simple shear localizes along faults.

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