finite element analysis of shear punch testing ppt

7/29/2019 Finite Element Analysis of Shear Punch Testing ppt

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Finite element analysis of shear

punch testing and experimental

validation

Nishant gaurav

Division :B

Roll no : 60

Guided by :Mr. V. D. Padalkar


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Index

Introduction to shear punch test

Introduction of finite element analysis

Ideology

literature review

Recent developments


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Why shear punch test ????

It requires very small volume of material as comparedto conventional tensile tests. So its an efficient testtechnique for evaluating mechanical properties whenthe material availability is limited.

The advantage of using the SPT technique for materialswith limited availability has attracted the nuclearindustry for assessing the properties of irradiatedmaterials.

This technique is used where conventional machinetests are practically not used like in weld joints .


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What is Finite Element Analysis???

FEM: Method for numerical solution of fieldproblems.

Description:

-FEM cuts a structure into several elements(pieces of the structure).

-Then reconnects elements at nodes as if nodeswere pins or drops of glue that hold elements

together. -This process results in a set of simultaneous

algebraic equations.


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Ideology

Many engineering phenomena can be expressed bygoverning equations and boundary conditions

Governing Equation (Differential equation)

L() + f=0

Boundary ConditionsB() + g=0

These equation lead us to a set of simultaneous

algebraic equations .

[K]{U}={F}In mechanics problem , K is the stiffness , U is the

displacement and F is the force .


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Literature review

In previous research involving the use of the shearpunch test, it was assumed that the displacementof the punch tip was only slightly different than thecrosshead displacement.

The shear punch test done by Toloczko(1) suggestedthat punch tip displacement is much less thanpreviously assumed, and that for the test frames whichhave been used, crosshead displacement is over anorder of magnitude greater than punch tip

displacement and It altered the slope of the loadingcurve masking the true yield point . These suggestionswere made based on the analysis done using FEA.


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Figure 1. a) Shear stress versus crosshead

displacement trace for a real shear punch test

of a cold-worked steel, and b) shear stress

versus punch tip displacement trace for an

FEA simulated shear punch test on the same

steel.


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The FEA of ShP testing by Guduru et al.[2] showed

that the elastic loading lines of FEA generated LDC

was found to be much steeper than theexperimental curve.

Its important to note that both Toloczko et

al.[1]and Guduru et al.[2]had measured the punch

displacement by a displacement sensor coupled tothe moving punch.

Fig 2:

Displacementsensor

coupled to

the moving

punch.


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In one of our recent work [3], a modified

shear punch experimental setup in which

displacement was measured at specimenbottom directly using a Linear Variable

Differential Transformer (LVDT) has been

demonstrated.

Fig : LVDT is

coupled to thespecimen

bottom.


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The intent of current work is to

simulate the shear punch test using FEA

and compare the initial loading curveswith the experimentally generated

curves.

We will see that the elastic portion ofthe FEA generated loaddisplacement

curve overlaps with the corresponding

experimental curve only when the

fixture compliances are eliminated in

experiments.


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Shear punch test :experimental

procedure The shear punch test is a small specimen test

technique for extracting yield strength,

ultimate strength.

A punch of 3 mm diameter and die of 3.04 mm diameter

was used for the present study. Small disc specimens were cut by Electric Discharge

Machining (EDM) from the various materials and theirsurfaces were gently ground .

Ideally, the load on the punch is measured as a function of

punch tip displacement, but due to the difficulty in actuallymeasuring the punch tip displacement, it has beenpreviously assumed that crosshead displacement isapproximately equal to punch tip displacement, and thus,the load on the punch has been measured as a

function of crosshead displacement.


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Fig:

Sketch of the shear punch

fixture.


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Shear punch load versus crosshead displacement

traces are similar in appearance to uniaxial tensile test

traces.

Yield is measured from these traces at deviationfrom linear elastic loading, and ultimate is measured at

the peak load.

The loaddisplacement data is converted to stress-

normalized displacement data using the following

expressions.

where P is the applied load, t is the specimen

thickness, r is the average of punch and lower die radius


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Experimental: The Finite Element

Analysis Simulation

The MARC finite element analysis software was used tosimulate the shear punch test. To reduce computingtime and costs, an axisymmetric mesh was utilized andis shown in Fig.

There are three main components to the mesh: thespecimen, the punch, and the receiving die.

The specimen was modeled as elastic-plastic. In aneffort to make the simulation as realistic as possible,the punch and the receiving die were modeled as

elastic. This allows for a small amount of elasticdeformation in these components which alters thestress distribution in the specimen mesh.


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Fig :

Sketch of the mesh used in the

present study.


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As with any finite element analysis, material

properties must be inserted into the model. For

this simulation, the punch and the receivingdie were assigned the elastic properties of BCC

steel (E = 200 GPa, = 0.28). The specimen was

assigned the uniaxial deformation behavior of

several different materials of interest .

The FEA boundary conditions were as

follows:

1) Translations in the radial direction

were prevented because the

model is axisymmetric.


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2) The bottom of the receiving die was held

stationary in the axial direction.

3) An immovable boundary was placed in contactwith a portion of the top of the specimen to

simulate the presence of the upper-half of the shear

punch fixture. No clamping force was applied to

the specimen with this boundary condition.

4) Friction between the components was set

equal to zero. The previous FEA based study has

shown the effect of friction between components tobe minimal.


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Under the assumption that real shear punch tests

are performed in the strain rate independent realm,

the FEA simulations were run in static mode.

Due to the difficulty in simulating the cutting and

failure behavior which occur in a real shear punch

test, the FEA simulations were run to only a small

amount beyond yield.

Simulated shear punch tests were performed on

several different materials. Different materials

were tested by assigning true stress versus trueplastic strain data and elastic deformation properties

from different materials to the specimen elements.


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During a simulation, the MARC program keeps

track of the load on the punch and the

displacement of the top of the punch.

To obtain the punch tip displacement, it was

necessary to run simulations using a rigid punch and

receiving die.

By comparing the elastic loading obtained from arigid component test to an elastic component test, it

was possible to measure the compliance of the

punch and receiving die.


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This compliance value was then

used to estimate the punch tip displacement as a

function of the displacement at the top of

the punch using the following formula:

where x is the punch tip displacement, x is thedisplacement at the top of the punch, P is the load

on the punch, and C is the measured compliance of

the punch and receiving die.

Using the calculated punch tip displacement data,

load versus punch tip displacement traces

were created.


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Figure 1. a) Shear stress versus crosshead

displacement trace for a real shear punch test

of a cold-worked steel, and b) shear stress

versus punch tip displacement trace for an

FEA simulated shear punch test on the same

steel.

C li


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Compliance :

The key aspect of this comparison is that the elastic

loading slope for the FEA simulation is 2 orders of

magnitude steeper than the elastic loading slope of the

real shear punch test (look at the x-axis scales in figures).

Since the main difference between an FEA simulationand a real shear punch test is the location at which

displacement is measured, it is reasonable to assume

that this is leading to the dissimilar traces. This could be

determined by measuring the compliance of the testmachine, but performing such a compliance

measurement is not a simple task because of the

geometry of the shear punch test.


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Following the idea that the differences in the traces

for the FEA simulations and the real shear punch

tests is due to test machine compliance, a

compliance was added to the FEA punch tip

displacement data. By adding a compliance, the

punch tip displacement was converted to a

hypothetical crosshead displacement. The

equation which describes this hypothetical

crosshead displacement is

where is the hypothetical crosshead displacement, x

is the displacement of the punch tip, P is the load on the

specimen, and C is the estimated test machine compliance


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C was found by comparing the elastic loading slope

in shear punch test traces obtained from real tests

and from FEA simulations.

The resulting shear stress versus hypothetical

crosshead displacement trace is compared to the

real test trace in Fig. The FEA generated trace has

been transformed into a trace that looks nearlyidentical to the real trace.

This further confirms the idea that crosshead

displacement measured in real tests is much larger

than the actual punch tip displacement.

It also shows that a large amount of compliance can

strongly alter the appearance of a load versus

displacement trace.


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Fig: Comparison of a real trace and an FEAsimulated trace where the FEA punch tip

displacement data has been converted to a

hypothetical crosshead displacement.


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Yield stress management

Initiallyyielding takes place at the punch and die

corners and interior is

still elastically deformed. The transition from elastic to plastic deformation

in the specimen occurswithin a few microns of

punch displacement. As the punch penetrates the specimen, the

plastic deformation extends throughoutthe

specimen. Yielding in the specimen was assumedwhen theplastic deformation takes place

through the thickness.


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It is seenfromFig. that at a stress corresponding

to 0.15% offset of normalized displacement, fully

developed plastic deformation spreadsthrough the

thickness of the sample. We conclude that the stressevaluated at 0.15% offset truly represents the shear

yield stressbased on through section plasticity.


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Using the experimental data, shear yield strength is

computed at offsets of 0.15% and 1%. It has been

found that the

experimentally obtained shear yieldstrength at 0.15% offset compares well with that

obtained from FEA curves for all the materials

studied.

h l h ld l b d f


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The tensileshear yield correlation obtained from

the experimental data using 0.15% offset definition

is seen to have high correlation coefficient for a

linear fit Fig. as compared to that obtained using1% offset values .


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It also obeys the von Mises yield relation.

A material is said to start yielding when its von Mises

stress reaches a critical value known as the yield

strength . The von Mises stress is used to predict yielding of

materials under any loading condition from results of

simple uniaxial tensile tests.


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Conclusion: FEA of the shear punch testing indicated a large influence

of the punch compliance on the elastic portion of load

displacement plot. The elastic loading lines of experimental curve obtained

with displacement measured at specimen bottom matcheswell with the FEA generated curve.

The yield strength based on through thickness plasticity

corresponded to an offset of 0.15% of normalizeddisplacement. The experimental shear yield strengthevaluated at 0.15% offset compares well with the FEAgenerated value.

The tensile-shear yield correlation obtained using 0.15%

offset definition was found to obey the von Mises yieldrelation.

The results of FEA are thus verified and validated withexperimental data.


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References

[1] Hamilton ML, Toloczko MB, Lucas GE.Recent progress in shear punch testing.In:

Hans Ullmaier, Peter Jung, editors.miniaturized specimens for testing ofirradiated materials. IEA international

symposium; 1995. p. 4651

Guduru RK, Nagasekhar AV, Scattergood RO,

Koch CC, Murty KL. Finite element analysis of ashear punch test. Metall Trans A

2006;37:147783


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Thank you

finite element analysis of shear punch testing ppt

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