Fully Automatic Local Shrinkage Compensation

The main steps in the fully automatic evaluation of local shrinkage compensation have been described above. However, there are certain preparatory steps taken before the actual evaluation, for example,

• "Fictitious" marking traces are created for all welds along the edges of the plate part. They will be considered in the calculation of the shrinkage directions and will contribute to the longitudinal shrinkage compensation.

• Curved marking traces will be replaced by straight ones if they have a restricted curvature.

• Many (short) marking traces in line with each other will be combined to one for the shrinkage evaluation.

Since somewhat different rules apply when a plate is rectangular compared to when it is triangular it is necessary to check the plate in this respect. This is done in the following way, see the figure below. The TOL_ variables used in the expressions below are default parameters by which a customer can control this evaluation. They are described in detail in a separate paragraph below.

Figure 1:2. Surrounding rectangle in a simple case.

1. Calculate the extension of the plate contour in the shrinkage direction. Let the result be called LEN. Calculate the extension of the plate in the direction perpendicular to the shrinkage direction and let the result be called BRE. The area of the surrounding rectangle is then LEN*BRE (=RA).

2. Calculate the area (=PA) of the plate (skip holes). In case PA>RA*TOL_RECT_AREA the plate is supposed to be rectangular, otherwise in case PA<RA*TOL_TRIA_AREA the part is supposed to be triangular.

3. If the shape of the plate is not classified according to previous conditions, check the distances from the 4 corners of the circumscribed rectangle to the plate contour. Calculate also the length (=D) of the diagonal of the same rectangle. If any of the 4 calculated distances is larger than TOL_RECT_DIAG*D then the part is considered as triangular.

4. If still not classified, check D against length of the shortest edge of the rectangle (any of BRE or LEN, BRE is used in the formula). If D>TOL_RECT_EDGE*BRE then the part is considered as triangular.

5. If still not classified, assume rectangular shape.

   A number of default parameters, all starting in TOL_ and used in expressions below, control whether a certain marking trace should be considered to contribute to the shrinkage or not. They have all certain predefined values which can be changed by the customer. The figure below shows a rectangular plate with a number of marking traces of different lengths.

   

   Figure 1:3. Part with marking traces of different lengths.

   Let LEN be the length of the two edges of the circumscribed rectangle parallel to the shrinkage direction.

6. For rectangular plates check like this for all combined marking contours:

   1. Project a trace on the shrinkage direction. Call the projected length PL<i>. Fictitious marking traces will be skipped for perpendicular shrinkage. If PL<TOL_K1SFT*LEN then the corresponding trace should not take part in the perpendicular shrinkage calculation, otherwise it should. If PL<TOL_K1SFL*LEN then the marking line should not take part in longitudinal shrinkage calculation, otherwise it should.

   2. Calculate an average PL (=PLA) for the traces contributing to perpendicular shrinkage. If PLA<TOL_K2SFT*LEN then skip the perpendicular shrinkage compensation altogether. If PLA<TOL_K2SFL*LEN then skip longitudinal shrinkage compensation altogether.

   For triangular plates the check is slightly modified.

7. Project traces as described above. Intersect the outer contour of the plate with a line parallel to the shrinkage direction and through the midpoint of the current marking line. Calculate the distance (=TOT<i>) between the utmost of these intersection points (normally there should be two). If PL<TOL_K1SFT*TOT then the corresponding marking line should not take part in the perpendicular shrinkage calculation, otherwise it should. If PL<TOL_K1SFL*TOT then the marking line should not take part in longitudinal shrinkage calculation, otherwise it should.

8. Calculate an average PL (=PLA) for those marking contours that will take part in the perpendicular shrinkage and an average of all TOT<i>:s (=TOTA). If PLA<TOL_K2SFT*TOTA then skip perpendicular shrinkage compensation altogether. If PLA<TOL_K2SFL*TOTA then skip longitudinal shrinkage compensation altogether.

   Fictitious marking lines for welding along plate edges are considered in the evaluation of the longitudinal shrinkage. Accept them only if their direction deviates less than 45 degrees from the shrinkage direction and if PL>TOL_K2SFL*LEN (LEN is replaced by TOT for triangular plates).

   The longitudinal shrinkage will be applied proportionally all over the plate as a scaling in the shrinkage direction. Some calculations are made to decide if there will be any longitudinal shrinkage compensation or not.

   This evaluation is done in the following way:

   • Project the midpoints of all accepted traces on a line perpendicular to the shrinkage direction.

   • Sort the points along this line.

   • DIST<i> is the distance between two adjacent points.

   • The distance between the extreme points is denoted TOT_EXT.

9. If any DIST<i> exceeds TOL_MAXPART_IND or if the average of the DIST<i>:s exceeds TOL_MAXPART_AVE then skip the longitudinal shrinkage compensation altogether. Do the same if (BRE -TOT_EXT) / 2 > TOL_MAXPART_IND, that means, if there are big regions of the plate without any traces (for BRE, see figure above).

   As already pointed out the length of stiffeners are modified so that they get the same change of length as the marking traces they have caused on plate parts (even if a certain stiffener should be the origin of marking traces in several plates parts).