Deformation Mechanisms Questions and Answers

Q: The strain velocity becomes zero in ........................... and is constant under ......................... .

The key's answer is Elastic flow, Plastic flow: the strain rate is zero in elastic deformation and constant in plastic flow.An elastic body under a constant stress deforms at once to a fixed strain set by Hooke's law and then stops deforming, so its rate of strain (strain velocity) falls to zero; the strain is recovered when the load is removed. Once the stress exceeds the yield strength, a plastic material keeps flowing, and under a constant stress it deforms at a steady rate, with the strain permanent.Reversing the pair contradicts these two behaviours. Rheology is the general study of flow and deformation, not a type of flow, and creep is slow time-dependent deformation rather than the behaviour that defines zero strain rate.

Q: Material that flows as perfectly viscous, but only above a certain yield stress is called :

This is a Bingham material.A Bingham material (Bingham plastic) behaves as a rigid solid until the stress exceeds a yield stress, after which it flows like a viscous fluid, with strain rate proportional to the excess stress. Debris flows, some lavas and wet concrete approximate this behaviour.A Newtonian material is viscous at any stress, with no yield stress. A Kelvin (Kelvin-Voigt) material is visco-elastic, an elastic spring in parallel with a viscous damper, and 'Prahl material' is not a standard rheological model.

Q: Mylonite in which grain growth has occurred either during or after the deformation, such mylonite are termed as ............................ .

Such a mylonite is called blastomylonite.The prefix blasto- marks recrystallisation and grain growth. A blastomylonite is a mylonite in which the fine-grained, deformed matrix has recrystallised and the grains have grown during or after shearing, so that the mylonitic fabric is partly overprinted.Blastophitic describes a metamorphic rock that keeps relics of an original ophitic igneous texture. Diatexite (printed Diaxtexite) is a strongly melted migmatite, and granoblastic describes a metamorphic texture of equant interlocking grains, not a kind of mylonite.

Q: In carbonate and siliclastic sedimentary rocks uniaxial strain can be accommodated by chemical compaction, by the process of :

The process is pressure solution.Under burial load, grains dissolve preferentially where they press against one another, because solubility rises with stress at contacts. The dissolved material moves away and is redeposited as cement in pore spaces. The rock shortens in the direction of the load without fracturing, a chemical compaction that produces sutured grain contacts and stylolites in limestones and quartz-rich sandstones.Screw dislocations are crystal lattice defects that allow plastic deformation within grains, not chemical compaction. Kink banding is a form of folding in layered or foliated rocks. 'Wedge deformation' is not a recognised compaction process.

The strain velocity becomes zero in ........................... and is constant under ......................... .

A.Elastic flow, Plastic flow
B.Plastic flow, Elastic flow
C.Elastic flow, Rheology
D.Plastic flow, Creep

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

The key's answer is Elastic flow, Plastic flow: the strain rate is zero in elastic deformation and constant in plastic flow.

An elastic body under a constant stress deforms at once to a fixed strain set by Hooke's law and then stops deforming, so its rate of strain (strain velocity) falls to zero; the strain is recovered when the load is removed. Once the stress exceeds the yield strength, a plastic material keeps flowing, and under a constant stress it deforms at a steady rate, with the strain permanent.

Reversing the pair contradicts these two behaviours. Rheology is the general study of flow and deformation, not a type of flow, and creep is slow time-dependent deformation rather than the behaviour that defines zero strain rate.

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Material that flows as perfectly viscous, but only above a certain yield stress is called :

A.Newtonian material
B.Kelvinian material
C.Bingham material
D.Prahl material

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

This is a Bingham material.

A Bingham material (Bingham plastic) behaves as a rigid solid until the stress exceeds a yield stress, after which it flows like a viscous fluid, with strain rate proportional to the excess stress. Debris flows, some lavas and wet concrete approximate this behaviour.

A Newtonian material is viscous at any stress, with no yield stress. A Kelvin (Kelvin-Voigt) material is visco-elastic, an elastic spring in parallel with a viscous damper, and 'Prahl material' is not a standard rheological model.

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Mylonite in which grain growth has occurred either during or after the deformation, such mylonite are termed as ............................ .

A.Blastomylonite
B.Blastophitic
C.Diaxtexite
D.Granoblastic

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

Such a mylonite is called blastomylonite.

The prefix blasto- marks recrystallisation and grain growth. A blastomylonite is a mylonite in which the fine-grained, deformed matrix has recrystallised and the grains have grown during or after shearing, so that the mylonitic fabric is partly overprinted.

Blastophitic describes a metamorphic rock that keeps relics of an original ophitic igneous texture. Diatexite (printed Diaxtexite) is a strongly melted migmatite, and granoblastic describes a metamorphic texture of equant interlocking grains, not a kind of mylonite.

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In carbonate and siliclastic sedimentary rocks uniaxial strain can be accommodated by chemical compaction, by the process of :

A.Wedge deformation
B.Screw dislocation
C.Pressure solution
D.Kink banding

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

The process is pressure solution.

Under burial load, grains dissolve preferentially where they press against one another, because solubility rises with stress at contacts. The dissolved material moves away and is redeposited as cement in pore spaces. The rock shortens in the direction of the load without fracturing, a chemical compaction that produces sutured grain contacts and stylolites in limestones and quartz-rich sandstones.

Screw dislocations are crystal lattice defects that allow plastic deformation within grains, not chemical compaction. Kink banding is a form of folding in layered or foliated rocks. 'Wedge deformation' is not a recognised compaction process.

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