---
title: "Edge Dislocation Glide: When Does Shear Tear a Dipole Apart? Interactive Lab | Sandbox Physics"
description: "How much shear does it take to pull apart two edge dislocations that attract each other?"
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language: "en"
page_type: "lab"
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  zh-CN: "https://sandboxphysics.com/zh/labs/edge-dislocation-glide/"
last_modified: "2026-10-31"
---

FR39 · Crystal defects

# Edge Dislocation Glide: When Does Shear Tear a Dipole Apart?

**How much shear does it take to pull apart two edge dislocations that attract each other?** A shear above the passing stress, the peak pull one dislocation exerts on the other. For a pair on glide planes four atomic spacings apart in a metal with Poisson ratio 0.3, that is about 1.4 percent of the shear modulus. Halving the spacing doubles it, so narrow dipoles are the hardest to break.

Shear a crystal holding edge dislocations. Watch their stress fields, see a dipole break above its passing stress, and drag a dislocation onto another plane.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

Model assumptions

Each dislocation is a straight edge dislocation in an isotropic crystal, described by the Volterra displacement and stress fields of linear elasticity. The atoms are drawn at their displaced positions, so the inserted half-plane above or below each core is visible. A dislocation glides along its slip plane at a speed proportional to the Peach–Koehler force of the applied shear plus the stress of the other dislocations; climb, the core structure and lattice friction are left out. The passing stress is the largest stress one dislocation of the pair can exert on the other, derived from the same field. Everything shown is computed from these formulas; no measured data are shown.

Applied shear and pair offset over time

Glide force between the two neighbours (elastic theory)

More numbers and how they are computed

- **Plastic slip carried by the dislocations**: $0$
- **Model time**: $0$
- **Loading cycles completed**: $0$
- **Dislocations in the crystal**: $0$
- **Shear amplitude over passing stress**: $0$
- **Plane spacing and Poisson ratio**: $0$

The interaction curve is the closed-form glide stress of one edge dislocation on a parallel plane, drawn for the pair nearest each other; it is a reference, not a measurement. The offset chart records the simulated positions. The exported file lists the settings and the full position history.

Physics tutorial

## A crystal deforms one row of atoms at a time

**Background** A perfect crystal would need an enormous stress to slide one half over the other all at once. Real metals yield at a far smaller stress, because they hold line defects, dislocations, that let the slip happen one atomic row at a time.

**Why it matters** Dislocations carry long-range stress fields, so they push and pull on each other. Those interactions decide whether a metal work-hardens, how dislocation walls form during recovery, and how much stress a pair of trapped dislocations can resist.

Start with the essentials

- **Focus question**: How much shear does it take to pull apart two edge dislocations that attract each other?
- **One-sentence intuition**: Two edge dislocations of opposite sign on parallel glide planes $h$ apart settle at 45 degrees, where their mutual glide force vanishes. As the offset grows, the attraction first rises and then falls; its peak, the passing stress $\tau_{\mathrm p}=\mu b/[8\pi(1-\nu)h]$, is the largest applied shear the pair can resist.

### Core mathematical model

#### Shear stress around an edge dislocation

$$
\sigma_{xy}=\frac{\mu b}{2\pi(1-\nu)}\,\frac{x\,(x^2-y^2)}{(x^2+y^2)^2}
$$

The field falls off as one over distance and changes sign on the lines $x=0$ and $x=\pm y$, which gives the four-lobed pattern in the scene.

#### Peach–Koehler glide force

$$
f_{\mathrm g}=\pm b\,\bigl(\tau+\sigma_{xy}^{\mathrm{others}}\bigr),\qquad v=M f_{\mathrm g}
$$

Only the shear stress resolved on the glide plane pushes an edge dislocation along it. The sign follows the Burgers vector, so the same shear drives the two members of a dipole in opposite directions.

### Common difficulties

#### A larger dipole is weaker, not stronger

**Typical misconception** Dislocations farther apart must be harder to separate, since more distance has to be covered.

**Better mental model** The stress field weakens as one over distance, so the peak pull between the planes scales as one over their spacing. Doubling the spacing halves the passing stress.

### Run the experiment

1. 01

   #### Tear a dipole apart

   Start with A dipole torn apart and watch the offset chart against the dashed passing-stress lines.

   **What to observe:** Each time the applied shear climbs past the passing stress, the pair snaps through the vertical and the two dislocations glide far apart. They return when the shear reverses.

2. 02

   #### Halve the spacing

   Choose A narrow dipole holds, which keeps the same shear amplitude with planes two Burgers vectors apart.

   **What to observe:** The passing stress doubles past the applied amplitude, and the pair only rocks around its 45 degree position.

3. 03

   #### Make the pair annihilate

   Drag one dislocation onto the glide plane of its partner.

   **What to observe:** The two glide together and vanish, and the lattice around them becomes perfect again.

Physical conclusion

Dislocations move under the resolved shear on their glide plane, and their own fields make them attract or repel. A dipole resists any applied shear below its passing stress, which falls as one over the plane spacing.

## Related experiments and articles

Crystal defects and microstructure · Reading path

### From the Burgers vector to alloy dendrites

Articles on this path that use this experiment, followed by the other experiments they suggest.

1. [Article **The Burgers vector and the stress field of an edge dislocation**](https://sandboxphysics.com/stories/crystal-defects-microstructure/burgers-vector-and-edge-dislocation-field/)
2. [Article **The Peach–Koehler force makes like dislocations stack into walls**](https://sandboxphysics.com/stories/crystal-defects-microstructure/peach-koehler-force-and-glide/)
3. [Article **Dislocation dipoles and the stress that tears them apart**](https://sandboxphysics.com/stories/crystal-defects-microstructure/dislocation-dipoles-and-passing-stress/)

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)