FORMABILITY

Metallic materials have the ability of undergoing plastic deformation without damage. Thus these materials can be shaped into desired geometries of semifinished (e. g. by rolling, extrusion) or finished products (e. g. by forging, rollforming, stamping, hotstamping, packaging, hydroforming).

Plastic deformation occurs in the way that the shape of bodies can be changed without any volume change. This mechanism can be explained by a rearrangement of atomic structure with the shear stress being the driving force for such dislocation movements. The plastic deformation capacity of metallic materials however has its limits. There are several modes of failure that can occur and this is where the formability as material property comes in. Thus:

The metal formability is a measure of its ability to deform plastically during a forming process or it is the relative ease with which a metal can be shaped through plastic deformation.

Fracture strain

A very general parameter that indicates the formability and the ductility of a material is the fracture strain from a

uniaxial tensile test .The strain taken from this test is defined by the elongation with respect to a reference length .It has to be noted that up to the uniform elongation the deformation is homogeneous, subsequently localized straining takes place until fracture occurs. The fracture strain is not a physical strain since within the reference strain there is an inhomogeneous distribution of the deformation within the reference length. Nevertheless the fracture strain is a rough indicator of the formability of a material. Typical values of the fracture strain are 7% for ultra high strength material and well over 50% for mild strength steel.

Forming limits for sheet forming

One main failure mode is caused by tearing the material. This is typical for sheet forming applications. At a certain forming stage a neck may be appear. This is an indication of localized plastic deformation. Whereas in the early stable deformation stage a more or less homogeneous deformation takes place in and around the subsequent neck location, almost all deformation concentrates in the neck zone in this quasistable and instable deformation phase which leads to failure of the material by tearing. Forming limit curves display extreme but still safe deformation of sheet material may undergo at any location in the stamping process. These limits depend on the deformation mode, the ratio of the surface strains. The major surface strain has a minimum value when plane strain deformation occurs, this means that the corresponding surface strain is zero. Forming limits are a specific material property.

Deepdrawability

A classic form of sheetforming is deep drawing. This is done by drawing a sheet using a punch tool (acting in the inner region of the sheet) whereas the material from the side which is held by a blankholder can draw inside. It has been observed that materials with outstanding deepdrawability behave anisotropic (anisotropy). The plastic deformation in the surface is much more pronounced than in the thickness. The lankford coefficient ( r ) is a specific material property which displays the ratio of the width deformation versus the thickness deformation for the uniaxial tensile test. Materials with very good deepdrawability have a r value of 2 and above. One can understand this positive aspect of formability with respect to the forming limit curve (forming limit diagram) in the way that the deformation paths of the material are concentrated on the very left side of the diagram where the forming limits become very large.

Ductility

Another failure mode that may occur without undergoing the tearing mode is ductile fracture after plastic deformation (ductility). This may happen due to shear deformation (inplane or shear through the thickness) or due to bending. The failure mechanism may be understood by void nucleation and expansion on a microscopic level. Microcracks and subsequent macrocracks may appear when deformation of the material between the voids has exceeded its limit. Research has been very active in recent years in order to understand and model ductile fracture. The approach taken is to identify ductile forming limits using various small scale tests which exhibit different strain ratios or stress triaxialities

Use of formability parameters

Knowledge of the material formability is very important for the layout and design of any industrial forming process. Here simulation with the finite element method along with the use of formability criteria like the forming limit curve (forming limit diagram) enhances and in some cases is indispensable for certain tool design .

FORMABILITY LIMIT DIAGRAM

A forming limit diagram, also known as a forming limit curve, is used in sheet metal forming for predicting forming behaviour of sheet metal. The diagram attempts to provide a graphical description of material failure tests, such as a punched dome test.

In order to determine whether a given region has failed, a mechanical test is performed. The mechanical test is performed by placing a circular mark on the workpiece prior to deformation, and then measuring the post-deformation ellipse that is generated from the action on this circle. By repeating the mechanical test to generate a range of stress states, the formability limit diagram can be generated as a line at which failure is onset.

The semi-axes of the ellipse formed in this circle allow for the measurement of relative strain in two primary directions, known as the major and minor directions, which correspond to the major and minor semi-axes of the ellipse. Under the assumption of path independent strain, the relative strains will reach a critical value at which deformations occurs. Through repeated experimental measurements, the shape of the curve can be obtained experimentally. Alternately, a formability limit diagram can be generated by mapping the shape of a failure criterion into the formability limit domain. However the diagram is obtained, the resultant diagram provides a tool for the determination as to whether a given forming process will result in failure or not. Such information is critical in the

design of forming processes, and is therefore fundamental to the design of sheet metal forming processes. Through the establishment of forming limit diagrams for range of alloys, the forming process and alloy behaviour can be matched at the metalworking design time by the process engineer.

CONCEPT

Strain analysis by grid marking is a useful method, which has been used effectively to solve the problems inmetal forming. When sheet metal is formed, its surface is subjected to different stresses. This results into non

uniform strains to be developed in the formed part. Thus there will be regions of high strains as well as lowstrains, which may lead to wrinkling or fracturing of the material. By the grid marking method the areas ofhigh strain can be easily identified. The sheet is marked with the grid before forming process is carried out.After the sheet metal is deformed into desired shape, strain distribution can be visualized and critical areas ofstrain will be found by FLD (forming limit diagram) and control can be planned by varying the formingparameters. GRID

Many types of circle grid patterns have been used, such as square arrays of contacting or closely spaced noncontacting circles and arrays of overlapping circles. With small closely spaced circles, it is possible todetermine strain gradients accurately. After deformation the circle is transferred into ellipse. The direction ofthe strains is indicated by the major and minor axis of the ellipse. Circles of 2.5mm diameters have been foundto be a good size.

GRID MARKING METHODS There are various techniques available for applying the grids. Circular grids are normally made in twodifferent ways. They can be made electro-chemically or photo-chemically, both processes having particularadvantages and disadvantages.

STRAIN MEASUREMENT After sheet metal is formed the marked circles will deform into ellipses of different sizes. Strain is calculatedfrom the following formula.

1) Dividers and steel rule - This is the most simple and quick method. This method is suitable formeasurement on more or less flat surface. On curved surface the measured dimension will be less i.e. itwill measure the chord length rather than arc length. The accuracy is also limited.

2) Mylar Tape – this is a transparent scale to measure the strain directly. This tape has diverging lines scaledto read directly in percent strain. This scale is produced by photographic printing from a negative on tofilm. The scale is placed over an ellipse over a sharp radius and then shifted until the diverging lines lineup with the major axis of the ellipse. The percent strain is measured directly from the scale. The scale isnext turned 90 degrees to read the minor strain.

Mylar Tape

3) Travelling microscope - This is the most widely used method for measuring the changes in the dimensionof grid circles. There are two right angle slides on which work is mounted. The work is positioned underthe microscope. Cross wire is aligned at one end and the measurement is taken. The cross wire is thenaligned on the other end by moving the work table and the measurement is taken. The difference betweenthe two readings gives the absolute measurement. This is an accurate method. Two persons can getdifferent readings because of error in aligning the two axes.

4) Grid Circle Analyser (GCA) - They use a solid state digital array camera with a built in light source, acomputer, keyboard, and CRT display. The image of given deformed circle is displayed on the CRT and aleast squares curve fitting program selects the most suitable ellipse, which is displayed simultaneously.The major and minor strains computed from the equation for ellipse and the diameter of the original circle

are displayed on the screen.

FORMING LIMIT DIAGRAM PROCEDURE

To reduce experimentation through trial and error method which is both expensive and time consuming,Keeler and Goodwin proposed grid strain analysis. This involves etching a pattern of fine circles on the sheetmetal before pressing. After pressing the circles will be deformed into ellipses which can be measured toindicate major and minor strains produced in the component. An estimate of how close the metal is to failureis obtained by reference to the FLD, which is a plot of the major and minor strains at fracture over a widerange of conditions. Forming limit diagrams indicate the limiting strains that sheet metals can sustain over a wide range of major – to – minor strain ratios.

Two types of tests are used to determine these limiting strains. The first category of test involves stretching

test specimens over a punch – for example the hemispherical punch method producing some out of planedeformation. The hemispherical punch method for determining FLD uses circle gridded strips of the testmaterial ranging in width from 25.4 to 203 mm that are clamped in a die ring and stretched to fracture by asteel punch (102 mm dia). The strains are measured in and around regions of visible necking and fracture. Theforming limit curve is drawn above the strains measured outside the necked regions and below those measuredin the necked and fractured regions.The second test produces only in – plane – deformation.and does not involve any contact with thesample within the gage length. This can be achieved by using uniaxial tensile test or marciniak biaxialstretching test with elliptical or circular punches.

Example: A grid of 2.5 mm circles is electroetched on a blank of sheet steel. After forming into a complex shape the circle in the region of critical strain is distorted into and ellipse with major diameter 4.5 mm and minor diameter 2.0 mm. How close is the partto failing in this critical region?

Major strain

Minor strain

USE OF GRID ANALYSIS FOR PRESS SHOPS

FLD can be used in conjunction with grid analysis to estimate how closely areas of high strain in a shell are tofracture. Thus the criticality of a press forming operation can be determined. Areas of high strain in the pressformed component can be located and by studying the development of the strains during the press forming thecause of the strains can be usually found and remedied. Grid analysis can be used -1) For die development2) For selection of material and lubricant.3) For fault finding.4) For production control.5) For monitoring die work.

ACKNOWLEDGEMENT

I would like to express my sincere thanks to MR. SUNIL.D.T, Lecturer, Govt. Engg. College for his guidance and encouragement for preparing this report.

I also express my gratitude and respect to all those who have guided and inspired in completing this report.

CONTENTS

1.) FORMABILITY

2.) FORMABILITY LIMIT DIAGRAM

3.) CONCEPT AND STRAIN MEASUREMENT

4.) FORMABILITY LIMIT DIAGRAM PROCEDURE

5.) BIBLIOGRAPHY

BIBLIOGRAPHY

1.) STRAIN STUDIES IN SHEET METAL STAMPINGS, By, Amit Mukund Joshi

2.) THEORY OF PLASTICITY, By Sadhu Singh

3.) PRODUCTION ENGINEERING , By Made Easy Publications 4.) WWW. GOOGLE . COM

5.) WIKIPEDIA .ORG

FORMABILITY AND FORMABILITY LIMIT DIAGRAM

A SEMINAR REPORT

Submitted by

N. ANAND BALUROLL: ETAIEPEO14

S6

DEPARTMENT OF PRODUCTION ENGINEERING

GOVERNMENT ENGINEERING COLLEGE THRISSUR