Compression, Flexural and Tensile tests

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Compression, Flexural and Tensile tests for
concrete, steel and timber
Dr. Mohanad Talal Alfach
City, University of London
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Learning Objectives
After this lesson, students should be able to

• Define engineering stress, engineering strain,
  Poissons ratio and modulus of elasticity.
• Explain a typical engineering stress-strain
  diagram of an elastic material and its important
  features.
• Determine elastic modulus, yield strength, and
  tensile strength from an engineering
  stress-strain diagram.
• Understanding the reasons of material failure.

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Engineering Connection
When designing structures, engineers carefully
choose the materials by anticipating the forces
the materials (the structural components) are
expected to experience during their lifetimes.
The mechanical properties of these materials are
the most important properties because all service
conditions and most end-use applications in
involve some degree of mechanical loading. The
material selection for a variety of applications
is quite often based on mechanical properties
such as tensile strength, modulus, elongation and
impact strength. Usually, ductile materials such
as steel and other metals are used for components
that experience tensile loads. Brittle materials
such as concrete are used for components that
experience compressive loads.
Definitions
Stress is a ratio of applied load to the
original cross-sectional area.
A
F
F
4
Elongation The increase in the length of a
specimen produced by a tensile load.
Strain The ratio of elongation to the original
length of the test specimen.
Lo
e
L
Elastic Modulus (Youngs modulus) the slope of
the tangent to the stress-strain curve.
Yield point The first point of stress-strain
curve at which an increase the strain occurs
without the increase in stress.
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Compression Test
The most common test performed on hardened
concrete. This test is performed on cylindrical
specimens standardized by ASTM C39. The standard
specimen size is 6 in. in diameter and 12 in.
high. (the compressive strength of normal-weight
concrete is between 21 MPa to 34 MPa)
Compression stress, s
?? ?? ?? 4 ?? ?? ?? 2
FC
Area, A
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• Procedure of Concrete Compression Test
• Step1 - Preparation Check all the things you
  need are ready. Check concrete compression
  machine is in working order.Step2 - Safety
  Wear hand gloves and safety goggles.Step3 -
  Taking measurement Take the measurement of
  concrete specimens (which are sent to laboratory
  for testing). Calculate the cross-sectional area
  (unit should be on mm2) and put down on paper. Do
  the same for each specimen. Step4 - Start
  machine Turn on the machine. Place one concrete
  specimen in the center of loading area.Step5 -
  Lowering piston Lower the piston against the top
  of concrete specimen by pushing the lever. Don't
  apply load just now. Just place the piston on top
  of concrete specimen so that it's touching
  that.

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• Step6 - Applying load Now the piston is on top
  of specimen. It is the time to apply load. Pull
  the lever into holding position. Start the
  compression test by Pressing the zero button on
  the display board.
• Step7 - Increasing pressure By turning pressure
  increasing valve counter-clockwise, adjust the
  pressure on piston so that it matches concrete
  compression strength value. Apply the load
  gradually without shock.Step8 - Test is
  complete Observe the concrete specimen. When it
  begins to break stop applying load.Step9 -
  Recording Record the ultimate load on paper
  displaying on machine's display screen.Step10 -
  Clean the machine When the piston is back into
  its position, clean the creaked concrete from the
  machine.

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• Step11 - Turning off machine Match your record
  once again with the result on display screen. The
  result should still be on display screen. And
  then turn off the machine.Step12 - Calculate
  concrete compressive strength The result we got
  from testing machine is the ultimate load to
  break the concrete specimen. The load unit is
  generally in lb. We have to convert it in newton
  (N). Our purpose is, to know the concrete
  compressive strength.
• Compressive strength Ultimate load (N) cross
  sectional area (mm2).

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Shear stress, t
Area, A
Fs
Fs
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Tensile Test
A tensile test is a scientific test process
involving the application of tension to a
specimen until it fractures. It is an important
type of test for determining a materials tensile
strength, yield strength and ductility.
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Procedure of Tensile Test

• Before starting the test for tensile strength,
  use a Tensile Preparation ASTM E8 and mold the
  sample material. Once the mold is whole, the
  sample will take on the shape of a slim dog bone
  or dumbbell.
• Position the lower and upper clamps in their
  proper position to accommodate the length of the
  test sample. Next, place the material between the
  tensile clamps. Vertically align the sample from
  the upper clamp (the fixed grip) to the lower
  clamp (the grip in charge of applying tension.
• After securing the sample, attach the
  extensometer to its length. While it undergoes
  testing, the extensometer will be monitoring and
  measuring any changes in the material.
• To begin the tensile stress test, slowly separate
  the tensile clamps at a constant speed.
• During the test, the specimen will slowly
  elongate with the standardized speed. The data
  gathering software will present the materials
  test parameters, as well as the changes in the
  gage length.
• While the substance undergoes tension, the
  elongation is occurring in the process. The
  change in length brought about by the pulling
  forces is a measurement called strain.
• Eventually, the specimen will begin to deform in
  the middle of its length. Changes in the
  stress-strain curve will begin to appear during
  this phase. Once the specimen breaks, the tensile
  testing has officially ended.
• After the fracture, unlatch the specimen piece
  from the tensile clamps. The tensile testers
  will calculate the tensile strength, yield
  strength and ductility of the material.
• The tensile strength will determine the
  materials maximum tensile stress and Yield
  strength.

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Tensile Strength Comparison
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Flexural Strength Test
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• The procedures for conducting the
  flexural-strength test are as follows
• Assemble the loading device. Turn the test beam
  so that the finished surface is to the side and
  centered in the loading assembly. Operate the
  testing apparatus until the loading blocks are
  brought into contact with the upper surface of
  the beam.
• Apply the test load at a rate such that the
  increase in extreme fiber stress in the beam is
  between 125 and 175 pounds per square inch per min
  ute. Obtain readings on the proving-ring dial and
  convert them to corresponding total loads in
  pounds by applying the proving-ring
  constant. Aside from the reading used to control
  the rate of application of the load, the only
  reading necessary is the one that corresponds to
  the maximum load applied to the beam.
• After the specimen has broken, obtain dimensions
  of the cross section at which failure occurred to
  the nearest 0.1 inch. These dimensions represent
  the average width and average  depth  of  the
   section  in failure.

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• The flexural strength, expressed in terms of
  modulus of rupture,  is given in psi, and can be
  calculated as follows
• If the specimen broke within the middle third of
  the span length, use the following equation
• ?? ???? ( ???? ?? )
• Where
• R modulus  of  rupture , MPa (psi)
• P maximum  applied  load, N  (pounds)
• L span  length, mm  (in  inches)
• b average width of specimen, mm (inches)
• d    average depth of specimen, mm (inches)

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Stress-Strain Diagram

ultimate tensile strength
3
necking
Strain Hardening
SlopeE
Fracture
yield strength
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2
Elastic region slopeYoungs(elastic) modulus
yield strength Plastic region ultimate tensile
strength strain hardening fracture
Plastic Region
Stress (F/A)
Elastic Region
4
1
Strain ( ) (e/Lo)
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Youngs Moduli Comparison
Graphite Ceramics Semicond
Metals Alloys
Composites /fibers
Polymers
E(GPa)

109 Pa
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Questions?