Another way to view how much deformation will be required to accurately position the plate on the hull is to display a map of strain in the plate. You can display this by turning on items in the Strain menu. A colour map of strain in the plate will be displayed; areas of positive strain (stretching) are shown in red, areas of negative strain (shrinking) in blue and areas of no strain in green.
The strain is calculated by dividing the plate up into small quadrilateral elements. In each area the principal direct strains and the shear strains are computed. The strains are computed by comparing the elemental lengths of the expanded and unexpanded plate. Negative strains indicate that the expanded plate must be shrunk for it to fit onto the vessel; positive strains indicate that it must be stretched. There are a number of relationships between the strains:
· The strain in the forming direction is always greater than or equal to the strain perpendicular to the forming direction.
· The strain in the forming direction is equivalent to the maximum magnitude principal strain.
· The strain perpendicular to the forming direction is equivalent to the minimum magnitude principal strain.
· If the strain ratio is 100%, the strain is isotropic and the maximum and minimum principal strains are equal. In this case the shear strain is zero.
· If the strain ratio is 0%, all the strain is in the forming direction; the minimum principal stain is zero and the shear strain is a maximum.
· If the plate is developable, all strains are zero.
The palette of colours at the top right corner of the Part window indicate the percentage strain corresponding to the colours in the map.
The amount of strain allowable must be decided by the designer and builder and will depend on the stiffness and thickness of the plate material. The maximum strain at any point in the plate is displayed at the bottom of the list of girth information at the top right corner of the Part window.
Three different of strains may be displayed, these are essentially the principal maximum and minimum direct strains and the shear strain. Each is chosen by selecting the appropriate item from the strain sub-menu under the Display menu.
(Maximum principal direct strain) This displays the strain in the direction which the plate will be formed. This is the direction in which the maximum direct strain will occur. Note that if the General method is used, the forming direction is displayed by an arrow; if either Old Developable or Fabric methods is used, the forming direction is the direction from the first to second plate edge selected when defining the plate (i.e. from butt to butt).
Perpendicular to Forming Direction
(Minimum principal direct strain). At 90° to the maximum principal direct strain will be the minimum principal direct strain. If the strain ratio is 100% then both will be equal, otherwise the minimum principal strain will be reduced, if the strain ratio is 0%, the minimum principal strain will be zero.
If the principal strains are not the same, there will be some shear strain present. This is a measure of how much the angles are distorting in the plate.