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Angular distortion can occur in butt welds as the individual weld beads are added to the joint. e contraction of each bead tends to reduce the included angle between the plates on either side of the weld. Angular distortion of fillet-welded stiffeners is due to differing amounts of thermal expansion and contraction through the plate thickness. Angular distortion is less evident in thin panels due to a smaller temperature dierence through the plate thickness during welding. As plate thickness increases, the temperature dier- ential through the thickness is increased, and angular distortion is increased. Angular distortion produces maximum displacements for moderate thicknesses, approximately 3/8 in. to 3/4 in. (9.5 mm to 19 mm.) For thicker plates, the bottom side of the panel remains cool and provides sucient stiness to resist the angular dis- tortion produced by the llet weld. Buckling distortion often dominates the distortion of thin panels that are less than 3/8 in. (9.5 mm) thick. Buckling is a particu- larly dicult problem to deal with due to the large out-of-plane deformations that can be several inches high. Buckling distortion is inherently unstable and may exhibit multi- ple modes. Buckling most often occurs at free edges of the plate as well as between stien- ers. e two primary factors that inuence buckling distortion are the shrinkage forces on the panel and the buckling resistance of the panel. Buckling occurs when the com- pressive forces along the length of the weld (longitudinal to the welding direction) exceed the buckling resistance of the panel. The shrinkage forces that can cause buckling can be expressed as a compres- sive applied weld load (AWL) applied along the length of the weld. is shrinkage force is primarily inuenced by weld size, weld- ing parameters (heat input), and cooling rate (plate thickness). Figure 2 shows AWL as a function of welding heat input for dou- ble llet-welded stieners and a range of plate thicknesses. The figure shows that AWL increases with weld heat input and weld size for a given plate thickness. The weld sizes in the gure correspond to typical heat inputs for double-llet welded stien- ers in a shipyard. e structural stiness of the panel that resists the applied weld load (shrinkage force) depends on the thickness of the plate, size, and spacing of the stieners; size of the panel; and restraint. e ability of the panel to resist buckling can be calculated and the largest load the panel can support without buckling expressed as the critical buck- ling load (CBL) of the panel. Compressive loads that exceed the CBL cause the panel to buckle; the CBL of a panel varies as a func- tion of plate thickness. Buckling will occur when the AWL exceeds the CBL of the panel. Control of distortion Reducing distortion begins with attention to design factors such as material type and Welding Distortion continued Rotational distortion Longitudinal bending Angular distortion Bucklingdistortion Transverse shrinkage Longitudinal shrinkage FIGURE . TYPES OF DISTORTION FIGURE . APPLIED WELD LOAD AS A FUNCTION OF HEAT INPUT FOR FOUR STEEL PLATE THICKNESSES 00255075103/8? 1/4? 3/16? 1/8? 1/8 in. 3/16 in. 3/16 in. 1/4 in. 2030 HEAT INPUT (kJ/in) LASER WELDING PLATE THICKNESS APPLIED WELD LOAD (kips) April 2011www.sname.org/sname/mt (mt notes)