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INTRODUCTION
Sheet metal is a critical material for vehicle design due to design veracity
and Manufacturability. Low carbon steel sheet has long been the workhorse
material in auto and consumer industries it can be stamped into inexpensive,
complex components at very high production rates.
1.1. PROJECT BACKGROUND:
In the broadest, most general sense, bending process is an important process
in the sheet metal forming in many industries. The main problem of the bending
process is spring-back phenomenon after removing the punch. Any parts that
Involve bending are subjected to spring back. Spring back will affect the part.
Accuracies. In study analysis to experimental design a suitable punch or dies to
over come the spring back of the part that required in U and V bending. Variables
and their effect on spring back are harder sheet metal have greater degrees of
spring back due to a higher elastic limit and the resulting larger elastic band at the
bend. Then, a sharper or smaller bend radius reduces spring back by creating a
larger plastic zone and could cause tearing, due to higher stresses in the out side
surface. After that, as metal is bent through greater degrees of bend, the plastic
zone is enlarged and spring back is reduces for each degrees of bend. However, the
thicker sheet metals have less spring back because more plastic deformation occurs
considering no change in the die radius.
On the other hand several methods are used to overcome spring back. These
are over bending, bottoming or setting and stretch bending. The sheet metal is
often over bent amount sufficient to produces the desired degree of bend or bend
angle after spring back. For the over bending may be accomplished by using cams,
by decreasing the die clearance.
Setting the punch or die steel at smaller angle than required in the case on V
die. When the clearance is reduces below the sheet metal thickness, the burnishing
action wipes the metal against an undersized punch or die steel. Although the
bottoming or setting consist of striking the metal severely at the radius area. This
places the metal under high compressive stresses that set the metal past the yield
strength. Bottoming is accomplished by placing a bead on the punch at the bend
area. The pad must bottom against the shoe or backing plate so that the punch may
set the metal at the bend. It would be useless to bottom against the flat area of the
sheet metal because they are not stressed and do not cause spring back. Bottoming
must be carefully controlled when adjusting press ram or the forced involve will
rise at rapid rate.
Stretch bending consists of stretching the blank so that all the metal is
stressed past the yield strength. The blank is then forced over the punch to obtain
the desired contour. This pre stressing before bending results in very little spring
back. Only relatively large radius is bent because sharp radius would take the
pre stressed metal beyond the ultimate tensile strength. The sheet metal must be
uniform in strength. Stretching bending is most frequently done with a special
hydraulic machine rather than with a die in a press.
1.2. SHEET METAL FORMING OPERATIONS
In sheet metal forming a shape in produced from a flat blank by stretching
and shrinking the dimensions of all its volume elements in three mutually
perpendicular principal directions. A large variety of shapes such as singly curved
parts, contoured flanged parts, curved sections, deep recessed drawing parts, etc.
1.3. JAPAN AIRLINES (FLIGHT 123) CRASH
In 1985, Japan Airlines suffered an unexpected disaster due to the crash of
its Boeing 747 flight 123 at Gumma, Japan [2.20]. A dimensional flaw due to
spring back in the manufactured and assembled rem pressure bulkhead was
established as the primary factor of the accident. The disaster resulted in severe
repercussions for the airline.
The jetliner rear pressure bulkhead disintegrated and ruptured at an elevation
of 32000 ft., allowing air to flow through the aircraft deck and pressure to be lost.
Just before the crash, the aircraft conducted a 360-degree right turn and brushed
against a tree-covered area bursting into flames. Only four passengers among the
524 people aboard including the flight crew survived, leading to one of the highest
deaths ever in a flight crash.
The Boeing jetliner maintenance history reports revealed that the rear
bulkhead had previously been replaced. However, the newly manufactured
bulkhead did not conform to the required dimensional specifications mainly due to
excessive spring back after the stretch forming process. The margins around the
bulkhead rivet holes at the splice of the upper and lower webs were less than the
design specifications.
1.4. BENDING OF SHEET METAL
Bending is one of the most important sheet metal forming operations by
which a straight length is transformed into a curved one with the help of suitably
designed die and punch. It is very common process of changing sheet and plate
into channel, drums, tanks, car bodies, aircraft fuselages etc. In addition is a part of
deformation in many other forming process.
MOTIVATION AND PROBLEM DEFINITION
Spring-back is a common phenomenon in sheet metal forming, caused by
the elastic redistribution of the internal stresses during unloading. It has
beenrecognized that spring-back is essential for the design of tools used in sheet
metal forming operations. Recently, in automotive industry many companies,
abroad, are trying to form the body panels in single stamping operation while
keeping the material costs and the crap down by using IFHS blanks.
The push for usage of IFHS in the auto industry results from the need of fuel
conservation, safety mandate, customer demands, and environmental concerns to
design lighter automobiles that are more fuel efficient, produce lower emissions.
With improved handling and overall improvement of structure the vehicle.
Spring-back causes following problems in sheet-metal forming:
1) The assembly of the sheet metal components becomes problematic
There by increasing the assembly time and reducing the productivity.
2) Rolling direction affects the spring-back as the strength of the sheet-metal
is different in different directions i.e. IFHS sheet metal is anisotropic.
3) In automobile industry different punch corner radius are used for different
bending operations which in turn affects the spring-back in components.
4) A wide range of thickness is used in sheet-metal components which again
affects the spring-back.
5) High strength sheets are preferred for automotive body as to reduce the
thickness which results in reduction of the overall weight of the vehicle. Lighter
vehicles are in demand for higher fuel efficiency.
However, spring-back characteristic of IFHS has not been investigated
widely and very little information is available about its behavior during bending
operations.
1.8. OBJECTIVES
In the view of above mentioned facts the spring-back has been analyzed in V
bending process with the following objectives:
1. To determine the effect of punch corner radius on spring-back of IFHS steel
sheet with reference to the rolling direction (60◦)and to include the effects of
anisotropy of sheet metal in V-bending operation.
2. To determine the effect of thickness on spring-back of sheet metal (i.e. 0.5mm.
0.7mm and 1.0mm) in V Bending.
3. Simulation of V-bending process using Experimental to predict the spring-back
in the above cases.
4. Validation of experimental results with FEA results.
5. To suggest a model to compensate the spring-back in v- bending process.
2. LITERATURE REVIEW
Bending of sheet metal is important manufacturing process. Large number of
studies has been carried out to gain a deep understanding of this process. A brief
summary of the available literature on various aspects of sheet bending is given
below:
2.1 INTERNATIONAL STATUS
2.1.1 BENDING THEORIES
Hill (1950)(6) presented a complete solution for pure bending in which
deformation of sheet metal is achieved by a couple applied along its length. In the
analysis, hill predicted the movement of neutral surface but no change in the
thickness for rigid perfectly materials under plane strain bending.
Gardiner,F, (1957)(7) analyzed, in a manner similar to hills, the bending of
rigid perfectly plastic materials in cases of both plane stress and plane strain , and
they predicted no change in material thickness by assuming that the surfaces,
including the neutral surface.
Gau,Jenn-Terng(2001)(8) considered the effect by assuming the constant
yield surface on reverse straining by fibers overtaken by the neutral surface. And
he predicted obvious thickness thinning of rigid-strain-hardening metal sheets.
Johnsons,W,(1981)(9) conducted an analysis on plane strain plastic bending
of rigid –work hardening materials and they proposed two models. In model 1, a
numerical solution based on a linear “sigma”-epsilon” approximation or fibers in
reversed loading has been presented. Model 2 is an extension of proses analysis to
the case of materials described by ludwik’s equation. It has been demonstrated that
the result of these models are generated in close agreement.
The difference between all these theories of pure bending was in choosing
the strain hardening models or considering Bauschinger effect.