Plate bending can be split into two main categories, plate rolling and press braking(plate bending).
Bending Plate and Rolling Plate
Bending steel plates is defined as deforming steel around a straight axis. We use force to manipulate steel until it reaches a desired angle or shape. The difference between bending versus rolling depends on the radius. For larger radii, we call it plate rolling, and for smaller radii, we use to call it plate bending.
Here are several key factors to take into consideration for the bending of steel plates:
The material tends to want to bend back to its original shape – this is commonly known as “spring back”. Every piece of material has to be evaluated and taken into consideration when bending. In some cases, the dies used to roll the material will bend the material further than the desired radius, but the over bend will be compensated by the material´s spring back.
What is Spring Back Plate Rolling
When rolled into a ferrule or arched shape, the sheet or plate will spring back a certain amount, and that amount depends on myriad factors. In fact, as highlighted by studies and tests carried out in various parts of the world, the precise amount of workpiece spring back can only be determined experimentally.
How to eliminate the spring back in the rolling process
The tensile strength and thickness of material can cause materials to have a lot of spring-back. Springback is a term in bending and curving of steel which refers to the way the material will attempt to open back up towards its original position after being formed. We think of steel as something stiff and rigid, but there are still elastic properties at work that cause the material to relax a little after bending.
When roll bending a plate metal we must overbend the material to get the desired radius. The unique properties of some plate metals ( such as AR400 or AR360) cause the material to have a lot of spring-back. For these abrasion-resistant materials, they must be bent to a tighter initial radius than standard mild steel to end up with the same finished radius.
The curvature radius has an important impact on thickness variations. One bend with no inside radius could imply material fractures and we should always avoid this kind of bend. Below are two examples of how stretching from press compression relates to thickness reduction:
If the radius equals the thickness of the steel plate, the stretching of the material due to the tightness of the bend causes the plate thickness to decrease up to 20%.
If the radius equals up to 5 times the thickness, the material does not stretch nearly as much, and material thickness decreases by less than 5%.
When a steel plate is bent, the inside surface of the bend is compressed and the outside surface of the bend is stretched. Somewhere in between lies the neutral axis which is a line in the steel that is neither compressed nor stretched. The tighter the radius the bigger the opposing forces become – bringing up the concept of minimal radius.
Usually, the neutral axis is slightly closer to the inside of the bend. For practical purposes, it is located at half of the thickness for thickness values less than 0.08 inch with a minimal error.
To calculate the blank length of a bent piece, we first have to determine the position of the neutral axis. The developed length of the piece will be the developed length of its neutral axis.
The bend must always be performed perpendicular – or at most 45 degrees – to the grain lamination direction. This will increase bent resistance and reduce the risk of fractures.
The crystals or ‘grains’ of which iron and steel are composed are built up of exceedingly small cubes built up of atoms. The length of each side of a cube is less than a tenth of a millionth of an inch, too small to be seen under even the most powerful microscope, but what can be seen are irregularly shaped crystal or crystal-grain, and each is built up of a huge number of cubical units.
The regular cubic arrangement of the atoms result in the formation of planes, either parallel with the three axes of the cube or along diametrical planes along which atoms can slide more easily over one another known as cleavage planes which are a real cause of weakness.
At the junction or boundaries of different crystals there are ‘spare’ atoms, not occupying regular or symmetrical positions. These ‘spare’ atoms occupy random or indeterminate positions, which actually form the crystal boundaries, therefore there are no cleavage planes and so the boundaries do not split easily i.e. they are stronger than the crystals themselves. This explains why fracture most readily takes place along cleavage planes i.e. through the crystals and not along their boundaries. Further, the smaller the crystal size, the larger the number of boundaries that must be broken across before fracture can take place. A small or fine grained structure is therefore always stronger than a larger or coarse-grained structure of the same metal or alloy.
Metal bending and grain direction
Applying this to metal bending, be it plate bending, press braking or any other type of metal forming, consideration must be given to the grain direction before any process is carried out. As a general rule, the grain line must run perpendicular to the bend to avoid the potential for cracking or fracture. In cases where there are multiple bends in different directions, causing the grain line to run parallel with the bend, it is always worthwhile considering your material choice and thickness and also the size of the radius to limit cracking.
The plate bending of much tighter radii, therefore the risk of fracture is much greater and the need to bend perpendicular to the grain increases. Limitations on the minimum internal radius exist and set rules have been laid out in Europe in the advent of CE Marking which is specifically designed, through initial type testing, to ensure that the risk of fracture is reduced where the bent plate is being used in structures deemed to have some element of risk to human safety, for example tread plate on steel stairs.
- “Three Factors for Bending Steel Plate”, CMRP, Visited 19 Aug, https://www.cmrp.com/blog/plate-rolling/three-factors-for-bending-steel-plate.html
- “PLATE ROLLING: BASICS AND TIPS FOR THE PRODUCT ENGINEER”, Angle Roller, Visited 19 Aug, https://www.angleroller.com/bending-of-plates/plate-rolling-basics-and-tips-for-product-engineer.html
- “UNDERSTANDING GRAIN STRUCTURE AND DIRECTION WHEN PLATE BENDING”, BARNSHAWS, Visited 19 Aug, https://www.barnshaws.com/information/articles/understanding-grain-structure-and-direction-when-plate-bending