Flexural modulus is most easily thought of as ‘resistance to bending’, and as such applies whether or not the part has previously been bent. What is important is the strain applied relative to the initial shape.
Provided that the strain is relatively small, that is if the assembly shows only a small degree of bow or twist, it can be distorted in the opposite direction, in order to create a flat board, by imparting an appropriate amount of stress in the opposite direction. As with the test beam, the amount of stress that needs to be applied for a given deformation will depend on the stiffness of the assembly and its dimensions, in particular its thickness.
In order that the assembly should be maintained in this flat position, which is actually different from its rest (zero-strain) condition, force needs to continue to be applied, and this can only be done by distorting the enclosure sufficiently to create a permanent stress on the assembly inside. Whether or not that stress transferred to the enclosure will be accompanied by a strain sufficient to be visible will depend on the stiffness of the enclosure relative to the assembly inside.
If the distortion is too great, then the action of clamping the assembly flat may stress the board sufficiently to cause permanent damage due to non-elastic distortion of the structure.
Because flexural strength reduces with increase in temperature,
less force needs to be applied for a given degree of flattening.
However, unless the material recovers in some way, due to the equivalent
of ‘creep’, the resultant stress in the structure once
it has returned to room temperature will be identical.
The only cure to warped boards is prevention: this is all to do
with the balance of materials within the structure, as will be
discussed later in this module and also in Design for eXcellence