Plates and sections are bent by mechanical force using hydraulic machines either manually or through some automated process. The bending is carried out at ambient temperature, i.e. without application of any heat.
12.1.1
Roller Bending and Hydraulic Press
Plate Bending
Plate bending through mechanical means is carried out using 3 or 4 roller bending machines and hydraulic press. The bilge plates generally have cylindrical, i.e. single curvature. These are developable surfaces. This bending is done by roller bending machines. However for other complicated compound curvature plates in addition to roller bending they are further processed through the hydraulic press to achieve the required shape. Progressive bending of the plate is carried out. Prior to bending of the plates, the frame positions are marked on the plate. As the bending progresses, tame plates at the frame positions are used to check the correctness of the shape being obtained. This process of bending is fully manual and highly worker’s skill dependent.
Stiffener Bending
The stiffener bending is carried out by 3-point frame bending machines. The stiffener is progressively fed to the machine from one end and through the strokes of the side rams of the frame bender the required curved shape is obtained. A schematic of a typical frame bending machine is shown in Fig.12.1. Here also the above referred tame plates are used for the respective frames to check the curvature being attained progressively. This process of frame bending is also fully
Fig. 12.1 Schematic of a typical frame bending machine
manual and worker’s skill dependent. A typical frame bending (concave, web in compression) operation is shown in Fig.12.2.
These stiffeners are of rolled steel sections. Where a stiffener/girder of higher section modulus is required which is not available as a standard section, this is fabricated by welding face plate to a web. This face plate and web are cut in suitable dimensions from standard steel plates. These fabricated stiffeners/girders are required either as straight or curvilinear sections. There can be two ways of fab- ricating these curvilinear sections:
• by cutting the web to the required shape and welding the face place to it; • by cold forming of the straight section in a frame bending machine.
For carrying out cold forming of the sections, the following needs to be checked beforehand to achieve successful bending of the frames [1,2]:
• Fillet weld strength needs to be assessed to ensure that web-flange weld failure does not take place.
• Buckling of web due to compressive stresses in concave bending. • Roofing of face plate due to secondary stresses in concave bending. • Thinning of web edge caused by tensile stresses due to convex bending.
For assessing thefillet weld strength, an index called the Fillet Weld Loading Factor (FWLF) can be used. This index depends on the section dimensions,fillet throat thickness, radius of curvature and bending machine parameters [1]. Depending
Fig. 12.2 A typical frame (concave) bending operation
on the yield stress of the parent metal and the tensile strength of the weld deposit, limiting values of the FWLF are determined. By comparing the FWLF of the T sections with that of the limiting ones it can be predicted whether thefillet joining the flange to the web will be able to sustain the load during cold bending.
12.1.2
Universal Press for Plate Bending
The concept of matched die has been used in devising universal press. In ship- building, there is no scope of large scale mass production. Hence it is not feasible to keep large number of expensive dies for plate forming. However if the die can be madeflexible, such that it can easily take the required curvature. Then plates having different curvatures can easily be formed using this pair offlexible die. The uni- versal press essentially consists of a pair offlexible die, which are computer con- trolled. Using universal press, plate bending operation can be automated.
• In this method two arrays of hydraulic jacks are used, one at the ground and the other hanging from top. In this system of jacks, each jack can be operated independently. In this method the array of these jacks form the male and female dies as shown in the Fig.12.3.
Fig. 12.3 Schematic presentation of universal press
• The curved shape of the plate is first generated on the bottom array of jacks. • The plate to be bent is kept over this surface and pressed by the upper array of
jacks.
• Under this pressure the stress level exceeds the yield stress of the plates leading to bending of the plates.
• Thus almost any curved shape as relevant in shipbuilding can be achieved through this method.
Difficulties in This Process of Automatic Plate Bending
When the jacks are retracted after bending, the bent shape deviates from the shape under pressure. It is known as spring back action. The extent of spring back that may take place depends on mechanical properties of the given plate, i.e. yield stress and modulus of elasticity. These values are not unique for a given plate. They generally lie within a range. Hence exact amount of spring back cannot be calcu- lated. Hence in this process getting the exact bent shape will require monitoring of the plate shape after each stroke and evaluating the difference with that of the target shape and again apply the bending stroke. Thus it may require several strokes to finally get the desired bent shape.