HYDROSTATIC PRESSURE , RESIDUAL STRESS AND WORKABILITY.

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INTRODUCTION OF HYDROSTATIC PRESSURE

Hydrostatic pressure is what is exerted by a liquid when it is at rest.


The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure. 

HYDROSTATIC PRESSURE

The hydrostatic properties of a liquid are not constant and the main factors influencing it are the density of the liquid and the local gravity.

Both of these quantities need to be known in order to determine the hydrostatic pressure of a particular liquid. 

HYDROSTATIC PRESSURE

The formula for calculating the hydrostatic pressure of a column of liquid in SI units is: 
Hydrostatic Pressure (Pa, N/m2) = Height (m) x Density(kg/m3) x Gravity(m/s2)

The density of a liquid will vary with changes in temperature


Residual Stress.

Residual stress is that which remains in a body which is stationary and at equilibrium with its surroundings.

Residual stresses affect a part similarly to externally applied stresses. If they are strong enough to overcome the structural integrity of the part, the part will warp upon ejection, or later crack, when external service load is applied.

Residual stresses are more difficult to predict than the in-service stresses on which they superimpose. For this reason it is important to have reliable methods for the measurement of these stresses and to understand the level of information they can provide

Thermal-induced residual stress

Material shrinks as the temperature drops from the process settings to the ambient conditions reached when the process is complete.
The material elements experience different thermal-mechanical histories (e.g., different cooling rates and packing pressures) as the material solidifies from the mold wall to the center.
Changing pressure, temperature, and molecular and fiber orientation result in variable density and mechanical properties.
Certain mold constraints prevent the molded part from shrinking in the planar directions.