EFFECT OF STRAIN RATE

PRESENTED BY SANDEEP NAIR(ASE,COIMBATORE) CB.EN.P2MFG15018

EFFECT OF STRAIN RATE

STRAIN RATE:Strain rate () is the time interval it takes to accumulate a certain amount of strain; and it is defined as the elongation per time. Strain rate is expressed in s-1. Deformation speed of material in a process of forming is expressed in m/s. = e/t = l/l0t t in secondshear strain rate preferred to be used rather than strain rate.The relation between shear strain rate () and strain rate is: = 2

True strain rate is =V/LTrue strain rateis dependent on velocity and instantaneous length. In order to maintain constant strain rate during tensile test, velocity of the cross head of the tensile machine has to be increased. Increasing the strain rate increases the tensile strength. Increasing the temperature reduces the strengthAt higher temperatures, the sensitivity of strength to strain rate increases strength becomes more sensitive to changes in strain rate.

The flow stress is strongly dependent on strain rate at elevated temperatures.

Strain rates and velocities in some forming operations:ProcessTrue strainDeformation speed, m/sStrain rate, s-1Cold forging, rolling0.1-0.50.1-1001-10Cold wire drawiNG0.05-0.50.1-1001-10Hot forging, rolling0.-0.50.1-301-10

Hot extrusion2 - 5 0.1-11-10

Sheet metal forming1-100.05-21 - 10

General behavior: the creep curveThree creep regimes are obtained:I. Primary or transient creep (decreasing strain rate)II. Secondary or steady-state creep (constant strain rate)III. Tertiary or accelerated creep (increase strain rate

Strain rate dependence of flow stress or tensile strength, can be represented by the expression:=C()^m m is strain rate sensitivity parameter and is strain rate In general, the value of m decreases as strength increases. With higher values of m, a material can undergo more plastic deformation before necking or failure. The material near the necking becomes stronger due to work hardening, at the instance of necking. Strain rates near necking are also high. As a result, Necking gets delayed. There is large uniform deformation before failure.Elongation after necking also increases due to large values of m.

Superplastic behavior of some materials is possible only if the strain rate sensitivity for a material is high 0.3 to 0. 85Superplastic behavior is the ability of materials to undergo very large amounts of elongations upto 1000% elongation, before failure. There is also no necking in such behavior as m values are high. Examples are thermoplastics, hot glass, fine grained titanium alloy, zinc-aluminium alloy. Strain rate also affects the strain hardening exponent of materials. Strain hardening exponent decreases with increase in strain rate.

1. Decreasing the strain rate results in decreased rock strength and increased ductility (viscosity)2. T. changes produce similar effects as strain rate variations in rocks experiments (h. T s. )

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