lecture on bend testing

8/3/2019 Lecture on Bend Testing

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Flexure Testing Principles

± Flexure is the bending of a material specimen under load.Bending action in beams is often termed flexure, referringto transverse loading of the beam.

± Bending/flexural forces occur when load is applied to abeam or rod that involves compressi on for c es on one sideof a beam and tensile for c es on the other side.

± Bending can be accompanied by direct stresses, transverseshear, or torsional shear, depending on loading.

± The deflection of the specimen is the displacement of apoint on the neutral axis of the beam from its original

position under the action of the applied loads.



Flexure Testing Principles

± Strength that material exhibits is a function of the flexuralmodulus of the material and the cross-sectional geometry.

Example, rectangular beam of 1 x 4 (W) will exhibithigher flexural strength than a 2 by 2 square beam of

the same material modulus

± Properties are the same as in tensile testing.

Strength, deflection, modulus, ultimate strength, etc

± Specimen is loaded in a 3-point / 4 point bending test

Load is applied vertically in the middle at 1 or 2 pts

bottom goes in tension and the top goes in compression



Three-Point and Four-Point Bend Tests

± The modulus of elasticity is determined by load-deflection

measurements at stresses below the proportional limit

± The bending proof strength is determined by a load-unload sequence

until a specified permanent strain is measured on unloading

± The bending proof stress is the nominal stress in the outer fibers of a

beam that results in permanent strain in the outer fibers upon

unloading the specimen

The apparatus for three- and four-point tests

consists of two adjustable supports and a means for measuring deflection and

applying load.

Three- or four-point bend fixtures containing supports and load applicators are

normally used

The fixtures can be used with a material testing system, such as a universal testing

machine (UTM), to apply the load to the test specimen.

A deflection-measuring device, such as a deflectometer can be used to measure

deflection at midspan. The load at a given deflection is determined from the loadcell

Flexure Testing - Procedure



Test Specimens.

± The minimum specimen strip thickness for three- and four-point

bend specimens is 0. 25 mm

±

The span length should be 150 times the nominal thickness inthe range 0.25 to 0.51 mm and 100 times the nominal thickness

in the range exceeding 0.51 mm

± The specimen width should be 3.81 mm in the thickness range

0.25 to 0.51 mm and 12.7 mm in the thickness range exceeding

0.51 mm ± Total specimen length should be 250 times the nominal

thickness in the range 0.25 to 0.51 mm and 165 times the

nominal thickness in the range exceeding 0.51 mm





Performed on a Universal Testing Machine In flexure or bend testing, the rectangular specimen

is typically loaded flat on two solid support rods

A third rod is used for loading.

This setup helps insure three-point loading which

allows the tensile forces to act from the center loading point outward

Once the specimen has been accurately measured

using proper instruments and the machine properly

set up, loading continues in a slow, steady manner.

A load-versus-deflection curve or stress-straincurve can be plotted based on the data.

The flexure strength and modulus of rupture may be

calculated based on these data.

The stress at fracture in bending is known as the modulus of rupture, flexural strength, or transverse rupture strength.

3 pt bending

4 pt bending




The load, Pp, corresponding to deflection, p, is determinedfrom a linear interpolation of the data points above and

below the exact value of p desired.

The bending proof strength, p, for 0.01% strain in the outer

fiber is determined by:

h

b

Configuration of a three-point loading




The permanent deflection p, w

hich

produces th

e permanentstrain in the outer fiber of 0.01%, is obtained by:



Test Procedure. ± The average modulus of elasticity in bending is determined for a minimum

of six specimens, half of which are tested with the marked side facing

upward and half with the marked side facing downward.

± The modulus of elasticity in bending is calculated from the load increment

and the corresponding deflection increment between the two points onthe straight line as far apart as possible, using the formulas:


where

a is d ist anc e f r om t he su pport t o t he

l oad a ppli c at or (for four- poi nt l oad i ng);

P is l oad i nc rement as measured f r om prel oad, N ( lb f); and

is d e f lec ti on i nc rement at mi d s pan

as measured f r om prel oad, mm ( i n.).



Bend Ductility Test

Bend Ductility Tests are conducted to determine the ductility

or strength of a material

Bend tests for ductility differ fundamentally from other

mechanical tests in that most mechanical tests are designed

to give a quantitative result

Bend ductility tests give a pass/fail result with a subjectiveendpoint;

± the test operator judges whether a surface has undergone cracking.

± developed as a shop-floor material inspection test because of its

pass/fail qualities

± the simplicity and low cost of the required tooling.



Bend Ductility Test Bending ductility tests determine the smallest radius around which a specimen

can be bent without cracks being observed in the outer fiber (tension) surface. This forming limit commonly is called the minimum bend radius and is expressed

in multiples of specimen thickness, t .

A materi al w it h a mi ni mum bend radius of 3t c an be bent w it hout c r ack i ng

through a radius equal to three times the specimen thickness.

It thus follows that a material with a minimum bend radius of 1t has greaterductility than a material with minimum bend radius of 5t .

Alter natively , t he bend radius can be fixed, and the angle of bend at which

fracture occurs noted



Bend Ductility Test Bending test apparatuses include wrap, wipe, V-block, and soft tooling devices that

may have interchangeable die radii and are able to bend test specimens to several

preset angles. The pins, mandrels, rollers, radiused flats, and clamping devices must be longer than

the specimen width, and they must be strong and rigid enough to resist deformation

and wear.

Wrap bending devices

grip the test specimen at one end; a mandrel, reaction pin, or block with the

desired bend radius is positioned at mid-

length

A roller that sweeps concentrically around

the bend radius applies the bending force.

The distance from the mandrel to the loading

roller generally is equal to the thickness or

diameter of the test piece, plus clearance.

The clearance is adjusted to allow the test

specimen to bend to the desired radius or

angle without scuffing, smearing, or galling of

strip and die surfaces.



Bend Ductility Test

Wipe bending devices (Fig. 3) are similar to wrap bending devices, exceptthat the bending force is applied by a mandrel or roller that moves

perpendicular to the clamped specimen.



Bend Ductility Test V-block bending devices consist of a mandrel and a bottom block (Fig. 4a) or

specimen supports (Fig. 4b).

The sample rests on supports or on the bottom block and is not clampedduring the test.

The distance between supports is selected to force the specimen to conformto the mandrel radius without excessive interference.

This clearance is often the mandrel diameter, d, plus three times the specimen

thickness, t.

Bending force is applied at the center of the specimen.

The bottom block normally is a V or U shape. Bends made with conformingbottom block radii are bottoming or closed-die bends (Fig. 4a);

those without conforming bottom block radii are air or free bends (Fig. 4b).



Applicability of Bend Ductility Test

Free-Bend Test

The FREE-BEND TEST is designed to measure the ductility of the welddeposit and the heat-affected area adjacent to the weld

Also it is used to determine the percentage of elongation of the weldmetal.

The first step in preparing a welded specimen for the free-bendtest is to machine the welded reinforcement crown and rootflush with the surface of the test plate




Free-Bend Test

The next step in the free-bend testis to scribe two lines on the face of thefiller deposit. Locate these lines 1/16 inch from each edge of the weldmetal, Measure the distance

Then bend the ends of the test specimen until each leg forms an angle of 30 degrees to the original centerline

If a crack more than 1/16 inch develops during the test, stop the bendingbecause the weld has failed; otherwise, bend the specimen flat.




Guided-Bend Test

You use the GUIDED-BEND TEST to determine thequality of weld metal at the face and root of awelded joint.

This test is made in a specially designed jig.

The test specimen is placed across the supports of the die.

A plunger, operated from above by hydraulicpressure, forces the specimen into the die.

To fulfill the requirements of this test, specimenmust bend the specimen 180 degrees-the capacityof the jig.

No cracks should appear on the surface greater

than 1/8 inch. The face-bend tests are made in this jig with the

face of the weld in tension (outside)

The root-bend tests are made with the root of theweld in tension (outside)

lecture on bend testing

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