Universidad Tecnolgica de Altamira.
Name of Investigation: Bending Test
Prepare by:Ramrez Trejo Moiss de Jess. Garca Crdenas Ivn
Alfredo.
Teacher: Ing. Francisco Javier Estrada Castillo.
Name of the Subject:Destructive Testing.
Grade and Group:IMI-8-A
Industrial Area Maintenance.
Altamira, Tamps. June, 2015
Introduction.
Method for measuring ductility of certain materials. There are
no standardized terms for presenting the results of bending tests
in broad classes of materials; on the other hand, terms associated
with trials of bending to specific material types or forms. For
example, materials specifications sometimes require that a specimen
is flexed to a specified inside diameter (ASTM A - 360, steel
products). In ASTM E-190 provides a bending test to check the
ductility of welded joints. A description of the fracture or
photographs are used to report the results of the tests of wood.
(ASTM D-1037).
Bending Terminology.
Here are some diagrams and terms that you may find useful.
Arc - The degree of bend for the curved portion of an pipe, tube
or beam.
Bevel - A type of end finishing for a pipe, tube or beam.
Centerline Radius (CR) - Distance from the center of curve to
the centerline axis of the pipe, tube or beam. Abbreviated as
CLR.
Cold Forming - Using cold shaping methods to bend a pipe or
other object.
Degree - An increment of angle to form a bend above the plane of
0 degrees, to which the bend is formed (i.e. 10, degrees, 45
degrees, 90 degrees, 180 degrees, etc.)
Extrados - The exterior curve of an arc.
Easy Way (EZ) - Bending of a rectangular pipe, tube or beam
along its shortest dimension of length.
Hard Way (HW) - Bending of a rectangular pipe, tube or beam
along its thickest dimension of length.
I.D. - The Inside diameter of the pipe or tube.
Intrados - The interior curve of an arc.
Neutral Axis - The unbent portion of an object (pipe, tube or
beam) that is neither compressed or under tension due to bend.
O.D. - The Outside diameter of a pipe, tube or beam in
inches.Out of Plane - The deviation from a horizontal plane of
rectangular object by a single bend, between its tangent points and
the centerline of the bend.
Ovality - The distortion of pipe or tube from it's a circular
shape caused by bending.
Material Grade - Manufacturers' specification of material for
pipe, tube or beam material, (i.e. A53B, T304W SS).
Plain End - Square cuts made to a pipe, tube or beam prior to
bending. Roll Past - Degree or fraction of degree that a pipe, tube
or beam is bent beyond a specified point.
Rough Cut - Pipe, tube or beam end cuts that are not required to
be straight.
Square Cut - End cuts to a pipe, tube or beam that are square to
the centerline of a bend after the bending process.
Tangent - The straight portion of material on either side of arc
of a bend.
Tangent Point - A point where the bend of a pipe, tube or beam
begins or ends.
Taper Bore - A milling or grinding out of the inside diameter of
the end of a bend to create a tapered pipe or tube diameter.
Wall - The thickness in inches of the outside wall of a pipe or
tube.
Wrinkles - Ribbed or wavy indentations in the inner bend angle
of a bend of a pipe, tube or beam.
Behavior of Materials in Bending.
Many machine and structural parts are often subjected to bending
and shear stresses. In most cases, it is highly desirable to test
such parts under the conditions that simulate the actual service
loading. The static bending and shear properties of materials are
not of the same universal interest as the static tension and
compression properties. This is because such properties might be
determined directly, or at least some insight may be gained to
these properties from tensile test data (for instance.The
preparation and adequate testing of tension test specimens might be
difficult, hence simpler shear and bending tests are often
employed.
When a member of a structure or machine is subjected to bending
load, it acts as a beam which has a primary function of resisting
this loading.
In bending, both tensile and compressive stresses are induced
over a cross section of the beam. Thereby, bending tests are less
severe than tensile tests, but more severe than compressive
tests.
In fact, a bending test does not provide extra information on
mechanical behaviour of a material unless bending causes special
failure. Its value is defined as a direct means of evaluating the
behaviour of beams under loading or as a simple means to determine
the strength and ductility.
Since the loads required to cause failure relatively small and
easily applied, bending tests can be made with simple and cheap
apparatus. Moreover, test specimens are simple and easy to prepare,
also gripping problems are eliminated and deflection data are more
easily obtained.
There are two common types of bending tests:
1. 3-point bending (Fig. 1a): applying a concentrated load at
the span centre.2. 4-point bending (Fig. 1b): applying half-loads
equally distant from supports.
3-point bending method is often used due to its
simplicity.However, the 4-point bending (pure-bending) method
characterises the material properties better due to constant
bending moment which occurs between inner load points.
3-Point Bending. 4-Point Bending.
Bending tests areintended forbrittle materialswhen the scope of
test isto determine the strength of material.Thebending strengthof
a material is defined by various titles such as:flexural
strength,cross-breaking strength,transverse strength,modulus of
rupture, andcoefficient of bending strength.To determine the
bending strength, the beam must be so proportioned that it will not
fail in shear or by lateral deflection before reaching its ultimate
flexural strength. Usually, long specimens with higher length to
depth ratio (L/h > 10) are recommended. Likewise, a shorter beam
(L/h < 6) is intended for shear failure testing in
bending.Hence, for a rectangular part in a 3-point bending test,
bending strength is the highest stress at the moment of
rupture:
Bending tests are notably employed for cast iron (based on ASTM
A48), concrete, wood and certain plastics (according to ASTM
D790-66, BS 2782 and DIN 53452) using circular or rectangular
specimens.The flexural strength of brittle materials obtained from
bending tests would be higher than that determined from tensile
tests. As failure is approached, the neutral axis shifts toward the
compression face, and thus tends to strengthen the beam.Bending
tests cannot be employed to determine the flexural strenght ductile
materials since specimens cannot be fully bent without rupture.
Therefore, as in compression tests, it becomes impossible to
determine the ultimate strength and plasticity.
The resistance to deformation in bending within elastic range is
called stiffness in bending. A measure of this property is the
modulus of elasticity in bending which is defined by
load-deflection relation (Fig. 2).Deflection is dependent not only
the material but also the configuration of cross-section and
unsupported length. Hence, stiffness in bending for identical
specimens tested under identical conditions can be compared.The
load-deflection measurements are carried out using 3-point bending
test according to the standards mentioned in previous slide, and
elastic modulus in bend is defined from initial (straight) portion
of the curve:
For ductile materials, cold-bend and folding tests are used to
determine whether the material can be bent sharply without cracking
and serves as a simple acceptance test with respect to this form of
ductility.
The scope of such tests is to check ductility for particular
type of service or to detect loss of ductility under certain types
of treatment.
The test consists in sharply bending a bar through a large angle
and noting if cracking occurs on the outer surface. The aim is to
determine the angle () at which cracking starts (Fig. 3a). If no
cracks produced while specimen is bent around the pin, testing is
continued by compressing the specimen on itself between the
compression platens referred as folding (Fig. 3b).
Such tests are required in specifications for many steels such
as structural steel (ASTM A36-74), boiler rivet steel and rivets
(ASTM A141), pressure vessel plates (ASTM A285-72).
Minimum ductility that must be possessed by a material is
defined according to Tetmajers bending limit (Bg) :
Pin diameters and corresponding Bg values are given in the
table:
3-point bend test of flat specimens (30-50 mm wide) is covered
in DIN 1605. Bending load is applied slowly and steadily, and the
ductility is defined as angle () until which specimen can be bent
without cracking on the tension side (Fig. 4).
The inner distance between supports should be D+3h, and the
outer supports must have a radius of 25 mm (for h12 mm).
1.- Notched-Bar Test: It is used to specify the resistance of a
materialagainst shock, and its ability to withstand stress
concentration.Similar to tension test, the work done in bending the
test piece through about a right angle (using Monsanto Tensometer
in Fig. 5) can also be employed as toughness index number, which is
expressed as product of force applied by the nose and the distance
through which it moves.
2.- Weld Test: Similar in concept to 3-point bending test, weld
test is carried out by subjecting a butt-welded specimen to
transverse loading in a fixture. The load is applied slowly and
steadily until either cracks are produced on the tension side or
the specimen is bent to extreme limit in the fixture (in such case,
specimen is removed and testing is continued as in folding test).
Fig. 6a and 6b show the free-bend test fixtures for testing the
ductility of welds according to ASTM E16-64 and DIN 50121,
respectively.
3.- Fiber-Strain Measurements: made in connection with weld
tests.Tension side of specimen is marked over a distance and
percent elongation of outer fiber is specified through the use of
flexible tape. Hence, original (L0) and final length (Lf) are used
to calculate ductility (R):
4.- Hot-Bend Test: made with specimens heated to red-hot
temperature to determine suitability of material to
hot-working.This test is also employed for welded joints to test
blue brittleness.Plain carbon steels experience discontinuous
yielding within 230-370 C (known as blue brittle region) as steel
heated in this range shows a lower tensile ductility and higher
notch sensitivity.
5.- Quenched-Bend Test: used in connection with plates used for
boilers.
The specimen is heated to 650 C and held at this temp. about
half an hour. It is then quenched in warm water around 28 C and
subjected to bend test. The aim is to detect traces of nitrogen
present in the metal, indicated by fracture.Aplications of Bending
Test.The bending test is used to determine the resistance of a
material by the application of force to the point of maximum
tension and see how he reacts under pressure. Normally, the bend
test measured ductility, the ability of a material to change shape
under pressure and maintain that shape. In some cases, the bending
test can determine the tensile strength. When using bending test
for this purpose, testers examine that part of the material breaks
first to see what kind of force has the material. It also allows
them to know what kind of pressure is capable of enduring.
Ductility describes to what extent a material, usually metal,
can stretch and maintain your new shape. Steel, for example, is
very ductile. If pressure is applied it extends into a new form,
which will be maintained even when the pressure is removed. This
feature is known as ductility and is a desirable feature for metals
and other building materials.
To determine how ductile is a material used in laboratories the
bending tests. Force in ascending amounts applies to a piece of
material at a specific angle and for a specific amount of time. The
material is then twisted a certain diameter due to the use of
force. Once bending test is finished, the material is examined to
see until it is able to maintain its shape once removed the
pressure, and whether or not the material is cracked when pressure
is applied.
This test can also determine the tensile strength. The test may
be used with materials more fragile to test its resistance. These
fragile materials, those who can not stand well in a normal test
for tensile strength, are used in a bending test. This essay
applies in the same way as usual, bending the material while the
force is applied, and then the results are examined. If the
material has cracks in the bent part, it shows that material
supports better compression than tension.
Welding is an area where the bend test is used very often. A
double guide test is used to determine that strong is a weld after
it's been built. A special test machine is used to perform guided
bending test. The material must be able to bend to a specific
angle, such as 180 degrees, for example, without any kind of cracks
that appreciate. If this happens, welding has passed the test and
the material can be used without problems in your destination,
either construction, naval or steel industry.
The bending stress is a form of stress that occurs when loads
are applied perpendicular to an object, making it necessary to
divert the load. The degree of curvature that an object is able to
tolerate until there is a permanent deformation varies, depending
on the materials of construction, size, and other variables. Test
products to determine their tolerances to the bending stress is an
important part of the safety tests, especially for things such as
elements that are used in construction, where deformation under
tension can lead to structural collapses and fatal
consequences.
Many types of objects are tested with this type of procedure, a
simple example being the cabinets. Cabinets typically have one or
more grab bars at each end. When these bars are loaded with
clothes, shoes and other belongings, castings can deviate under the
weight of the load. A solid hanging bar will recover its position
when the load is removed.
Finally, the burden can become so high that the rod is
permanently bent or appear cracks in pressure as a result of the
bending stress. Bending tests help determine how much load and what
types of materials are suitable for use in the manufacture of
products such as cabinets and see daily in our daily lives.
ScopeThese test methods cover bend testing for ductility of
materials. Included in the procedures are four conditions of
constraint on the bent portion of the specimen; a guided-bend test
using a mandrel or plunger of defined dimensions to force the
mid-length of the specimen between two supports separated by a
defined space; a semi-guided bend test in which the specimen is
bent, while in contact with a mandrel, through a specified angle or
to a specified inside radius (r) of curvature, measured while under
the bending force; a free-bend test in which the ends of the
specimen are brought toward each other, but in which no transverse
force is applied to the bend itself and there is no contact of the
concave inside surface of the bend with other material; a bend and
flatten test, in which a transverse force is applied to the bend
such that the legs make contact with each other over the length of
the specimen.
After bending, the convex surface of the bend is examined for
evidence of a crack or surface irregularities. If the specimen
fractures, the material has failed the test. When complete fracture
does not occur, the criterion for failure is the number and size of
cracks or surface irregularities visible to the unaided eye
occurring on the convex surface of the specimen after bending, as
specified by the product standard. Any cracks within one thickness
of the edge of the specimen are not considered a bend test failure.
Cracks occurring in the corners of the bent portion shall not be
considered significant unless they exceed the size specified for
corner cracks in the product standard.The values stated in SI units
are to be regarded as standard. Inch-pound values given in
parentheses were used in establishing test parameters and are for
information only.This standard does not purport to address all of
the safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish
appropriate safety and health practices and determine the
applicability of regulatory limitations prior to use.
Regulations of the Bending Test.The standards ASTM E290, ISO
7438 are required and JIS Z2248 describes the requirements for Flex
testing to see the ductility of metallic materials. The bending
test helps to provide a visual indication of the ductility of the
material. The guided test method requires load is applied at the
center point of the sample while it is supported at the ends. The
specimen is folded up to a default angle or until it is fractured.
The convex side of the sample is inspected visually for cracks or
defects in, and the failure is determined by the size of the cracks
and imperfections that are permitted by the specifications of the
material.
For this test, a team in the series SATEC KN, DX, or HDX model
with a Flex W-6810 accessory is recommended. This accessory allows
you to adjust the distance between supports and is supplied with
various sizes of chucks of load to accommodate different
thicknesses specimens. Load brackets have some reinforcements to
provide greater rigidity during charging. DX or HDX are models that
have a double test area, so Flex is done at the bottom where the
compressions are performed and is always the top free to make
traction.
Trial software packages include Flex applications that allow a
simple configuration of the assay. The software generates the load
curves in real time and can determine the end of the test
automatically when there is a break or when it comes to the proper
angle. Our advanced control electronics offers the best response,
precision data and resolution. In addition, the system supports the
interconnection of external signs of control or measurement.
Bending test of wood (ASTM-D143, ISO 3133, 310).The wood is
usually used as an engineering material in construction and in the
furniture industry. With its wide range of physical and mechanical
properties, you can choose wood of different species of trees to
adapt to the specific requirements of an application.
The resistance of the wood is influenced by factors such as the
types of loading, direction and duration of load, temperature and
humidity. Standards such as ASTM D143, define the test methods for
determining the mechanical properties, including resistance to
bending, tensile strength and resistance to shear of wood. This
allows engineers to choose which best fits the needs.Different
standards may have slightly different requirements and a challenge
is to try to comply with different regulations. For example,
ASTM-D143, claims that the lower support brackets of the bending
attachment to three points should have bearings and that the load
should be applied in the Centre of the sample by a rigid upper
block. However, ISO 3133 says that the support and attachment of
the load application must be a specified diameter rollers.
To ensure arising the first break of the sample of traction or
compression and not of the shear stress, many wood bending test
standards require that the attachment has a relationship
anchura-profundidad minimum of 14. A typical configuration of the
test would be to use either our 3300 or 5500 series
electromechanical machine configured with beam series 2820 base and
a fixture three points for this essay. In addition, a falling
weight deflectometer is available to measure deviations from
bending to the axial axis, as specified by the standard
ASTM-D143.
The 2820 bending of wood accessories series is designed
following common international rules for bending tests on a range
of wood products. The modular design of this series of accessories
provides maximum flexibility, offering a range of optional plates
and four-point conversion kits, in order to meet different
standards. This allows that accessories can be configured to meet a
wide range of standards at the same time that share as many common
parts as possible.
References.
1. Willems Nicholas, Easley John T, Stanley T. Rolfe.
Resistencia de materiales. 1984, McGraw Hill.
2. Hibbeler R. C. Mecnica de materiales. Quinta Reimpresin,
2004, CECSA.
3. Craig Roy R, Jr. Mecnica de materiales. Primera Edicin en
Espaol, 2002, CECSA.
4. Rossmanith H.P. Fracture mechanics and materials testing:
forgotten pioneers of the early 20th century. 1999, Blackwell
Science Ltd. Fatigue Frac. Engng Mater Struct 22, 781-797.
5. Comentarios Histricos sobre la teora de flexion. 1983,
Journal of structural engineering, Vol. 109.
6. http://www.dyna.mess.de/es/produkte/maschinen.html. Sistemas
de ensayos.
7. http://www.fceia.unr.edu.ar/ensayosnormalizados/. Maquinas de
laboratorio de ensaye.
8.
http://www.imnc.org.mx/laboratoriosdeensayoydecalibracion_c_111.html.
Instituto Mexicano de Normalizacin y Certificacin A.C.9.
http://dimei.fi-b.unam.mx/udiatem/instalaciones.htm. Departamento
de Materiales y Manufactura, Unidad de Investigacin y Asistencia
tcnica en Materiales (UDIATEM), laboratorio de pruebas
mecnicas.
10. Laboratorio de Ensaye de Materiales, Departamento de
Ingeniera Civil y Topografa, (CUCEI) UDG.
11. http://www.gunt.de/static/s7_3.php?p1=&p2=&pN.
Mecnica Aplicada y Ensayo de Materiales.
12.
http://www.tq.com/teachequip_products.asp?choice=1&menu=1.
Materials Testing and Properties.
13. Seely Fred B, Smith James O. Resistencia de materiales.
1974, UTEHA.
14. Monroe Gere James, Timoshenko Stephen. Resistencia de
materiales. 2002, Thomson Learning Ibero.
15. Hibbeler Russel C. Mecnica vectorial para ingenieros.
Esttica. Dcima Edicin, 2004, Pearson Educacin.