Why is the movement of dislocation compared to Caterpillar?

Why is the movement of dislocation compared to Caterpillar?

Dislocation motion is analogous to movement of a caterpillar. The caterpillar would have to exert a large force to move its entire body at once. Instead it moves the rear portion of its body forward a small amount and creates a hump.

Which Lattice will have the higher critically resolved shear force and why?

In general BCC metals have higher critical resolved shear stress values compared to FCC.

What is the difference between resolved shear stress RSS and critical resolved shear stress CRSS )?

Sir, please see this, resolved shear stress is component of the applied stress that causes shear on a given plane in a given direction. Whereas the critical resolved shear stress is the minimum shear needed to cause dislocation motion for a certain material in a certain state condition.

What is the critical resolved shear stress for a pure metal single crystal?

The stress required to initiate slip in a pure and perfect single crystal, the critical resolved shear stress (CRSS) is a constant for a material at a given temperature. This rule, known as Schmid’s Law, has been experimentally proven for a large number of single crystals.

What is dislocation movement?

A dislocation is a linear crystallographic defect or irregularity within a crystal structure which contains an abrupt change in the arrangement of atoms. The movement of mobile dislocations allow atoms to slide over each other at low stress levels and is known as glide or slip.

Where does dislocation motion occur?

Dislocation motion during plastic deformation is concentrated in slip bands within the grains. If such a slip band impinges on a grain boundary, it causes a stress concentration which might nucleate a cavity as indicated in Fig. 13.3. Many researchers observed this phenomenon.

What are the factors that affect the critical shear stress?

Factors Influencing critical shear stress: The rate of deformation and the extent of initial deformation will also raise critical shear stress. Purity in metals reduces the critical stress. Surface effects like surface films greatly increase the critical shear stress.

On which factors does the critical resolved shear stress depend?

It was also observed that the magnitude of H depends on the type of stress, i.e., tensile or compressive. Thus the critical resolved shear stress (CRSS) for slip depends on orientation, temperature, and the type of stress whether tensile or compressive.

Which of the following stresses is required for the slip to occur?

7. Which of the following stresses is required for the slip to occur? Explanation: A critical resolved shear stress is that component of shear stress which is in the slip direction and hence needed for a slip to occur. Tensile or compressive stress aren’t needed for a slip.

How does slip in single crystals occur and what is the role shear stress plays?

Slip occurs by dislocation motion. To move dislocations, a certain stress must be applied to overcome the resistance to dislocation motion. This critical shear stress is related to the stress required to move dislocations across the slip plane.

How are dislocations created?

Dislocations are generated by deforming a crystalline material such as metals, which can cause them to initiate from surfaces, particularly at stress concentrations or within the material at defects and grain boundaries.

Which metals generally require a high stress to move dislocations?

1 Slip Bands. Dislocation motion during plastic deformation is concentrated in slip bands within the grains. If such a slip band impinges on a grain boundary, it causes a stress concentration which might nucleate a cavity as indicated in Fig. 13.3.

What happens when shear stress is applied on a dislocation?

With that dislocation in there, when we apply the shear stress, it pushes on the adjacent plane of atoms. If the stress is high enough, the original bonds on the third plane are broken, and they snap back and rejoin with the second plane. The dislocation is still there; it’s just shifted over.

Why is there a shear component to the tensile stress?

If you think about it, there is still a shear component to this tensile stress because of the crystal structure and randomly oriented grains. When a material fails by fracture or tearing, this is when the atomic bonds rip apart. You may recall that two main types of linear crystallographic defects were talked about: edge and screw dislocations.

Do shear lattice strains exist around edge dislocation lines?

™ Consider the edge dislocation shown in the figure. Some atomic lattice distortion exists around the dislocation line. Consequently, there are regions in which compressive, tensile, and shear lattice strains are imposed in the neighboring atoms.

Why do dislocations allow deformation at a lower stress than perfect crystal?

Dislocations allow deformation at much lower stress than in a perfect crystal because slip does not require all bonds across the slip line to break simultaneously, but only small fraction of the bonds are broken at any given time. Some theory: Schmidt’s Law