Evaluation of the mechanical properties through 3 points bending and tensile test

Department of Industrial Engineering
Master Degree in Materials and Production Engineering
Engineering Properties of Materials
Exercitation 2: Evaluation of the mechanical properties
through 3 points bending and tensile test
Daniele Rigotti Ph.D.
Academic Year 2021/2022
UNIVERSITY OF TRENTO

Summary
• From load – displacement curves to stress – strain curves
• Overview of Origin software
• Graphs
• Importing graphs in MS Word
• Linear fit
• Integration of experimental data
• All these thinghs in R
2

Symbols/units settings
3
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Change the regional settings
To change regional settings, follow the steps described for your
computer's operating system.
In Windows 10
1.Click the Start button, and then click Control Panel.
2.Click Change date, time, or number formats.
3.The Region dialog box appears.
4.On the Formats tab, select the format you want to change
from the drop-down list. Now click the Additional Settings
button.
5.The Customize Format dialog box appears.
6.Click the tab that contains the settings that you want to
modify, and make your changes. For example, to change part or
all of a number format, click the Numbers tab and change the
setting that you want.

Evaluation of mechanical properties: which method?
Depends on:
• Application
• Material
• possibility of obtain specimen of the
desired shape
6
Methods:
• Tensile
• Flexure
• Compression
• Torsion
• Impact

Evaluation of mechanical properties: which are the
most important?
•Stiffness
•Fracture
•Strain at break
•Stress at break
•Energy at break
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Evaluation of mechanical properties: How we choose?
• Ceramic brittle material -> Flexure test
• Polymeric brittle material (PMMA, PS) -> Flexure test
• Polymeric «elasto-plastic» material (PP, PE) -> Tensile test
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The shoulders are large so they can be readily gripped, whereas the
gauge section has a smaller cross-section so that the deformation and
failure can occur in this area
Gripping force has to be high enough to prevent
slippage of the specimen during the test

Material testing
• You don’t have to invent anything, just follow the right
standard.
• Standard are the recipe to perform a test.
• Different standard for the same test (ASTM, ISO, DIN,…)
• Always report the standard you follow and the modification you
had done.
• In the .zip file, I provide you, there are all the standard needed
for this lab activity.
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Flexural test
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In engineering mechanics, flexure or bending characterizes the behavior of a
slender structural element subjected to an external load applied perpendicularly to
a longitudinal axis of the element.
Typical Materials used for Flexural Test
• Polymers
• Wood
• Composites
• Brittle Materials

Flexural test
• In this test a specimen with rectangular or flat cross-section is placed on two
parallel supporting pins. The loading force is applied in the middle by means loading
pin.
• The supporting and loading pins are mounted in a way, allowing their free rotation
about:-axis parallel to the pin axis;-axis parallel to the specimen axis.
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Types of flexure test
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• 3-point or four-point bend fixture.The main advantage of a three-point flexural test is the ease of the specimen
preparation and testing. However, this method has also some disadvantages: the results of the testing method are
sensitive to specimen and loading geometry and strain rate.
• 4Point bending test is very similar to the three-point bending flexural test. The major difference being that with the
addition of a fourth bearing the portion of the beam between the two loading points is put under maximum stress, as
opposed to only the material right under the central bearing in the case of three point bending.

Flexure test
A flexure test produces tensile stress in the convex side of the specimen and
compression stress in the concave side. This creates an area of shear stress along
the midline. To ensure the primary failure comes from tensile or compression stress
the shear stress must be minimized.
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ASTM D790 standard provides a guideline minimize it
by controlling the span to depth ratio; the length of
the outer span divided by the height (depth) of the
specimen.
For most materials L/d=16 is acceptable. Some
materials require L/d=32 to 64 to keep the shear
stress low enough.

ASTM D790 – testing parameters
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From extensive to intensive values
• According to IUPAC, an intensive quantity is one whose magnitude is
independent of the size of the systemwhereas an extensive quantity is one
whose magnitude is additive for subsystems.
• An intensive property does not depend on the system size or the amount of material
in the system. It is not necessarily homogeneously distributed in space; it can vary
from place to place in a body of matter and radiation.
• By contrast, extensive properties such as the mass, volume and entropy of systems
are additive for subsystems.
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Extensive Intensive
Displacement Strain
Force Stress
Stiffness Elastic modulus

From extensive to intensive values
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0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
0
100
200
300
400
500
Load
(N)
Extension (mm)
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007
0
10
20
30
40
50
60
Stress
(MPa)
Strain (mm/mm)
Curve that I get from the
machine
Curve relative to intensive
properties of the material
f(d, W, L)

Toe compensation
• Toe compensation shall be made in accordance with Annex A1 unless it can be shown that the toe region of
the curve is not due to the take-up of slack, seating of the specimen, or other artifact, but rather is an
authentic material response.
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Sample: Group 1 Specimen:A
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Specimen name
Position of the cross
head at specified time
Force measured by
the load cell at the
given cross head
position

Tensile test
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▪ Tensile Testing is a form of tension testing and is a destructive engineering and materials science test whereby
controlled tension is applied to a sample until it fully fails.
▪ This is one of the most common mechanical testing techniques. It is used to find out how strong a material is and also
how much it can be stretched before it breaks. This test method is used to determine yield strength, ultimate tensile
strength, ductility, strain hardening characteristics, Young's modulus and Poisson's ratio.

Tensile testing
• Tensile testing provides details of the tensile mechanical properties of a
material. These properties can be plotted on a graph as a stress/ strain curve
to show details such as the point at which the material failed as well as
providing details of properties such as the modulus of elasticity, strain and
yield strength.
• Tensile testing has a variety of uses, including:
• Selecting materials for an application
• Predicting how a material will perform under different forces
• Determining whether the requirements of a specification, contract or standard are met
• Demonstrating proof of concept for a new product
• Proving characteristics for a proposed patent
• Providing standard quality assurance data for scientific and engineering functions
• Comparing technical data for different material options
• Material testing to provide evidence for use in legal proceedings
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Tensile test specimen
• Tensile test specimens are prepared in a variety of ways depending on the test
specifications.
• Most specimens use either a round or square standard cross section with two
shoulders and a reduced section gauge length in between.
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• The shoulders allow the specimen to be gripped
while the gauge length shows the deformation and
failure in the elastic region as it is stretched under
load. The reduced cross section gauge length of
specific dimensions assists with accurate
calculation of engineering stress via load over area
calculation.

Dumbbell specimen
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Width and
thickness have to
be measured
Gauge lenght is needed
for non – instrumented
test
An indication to
perform the test

Calculation of elastic modulus
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Calculation of elastic modulus
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Evaluation of the mechanical properties through 3 points bending and tensile test

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