Mechanical Property of Bio-material

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Mechanical Property of Bio-material

• Physical Properties of Bio-Materials (III-B)

Poching Wu, Ph.D. Department of Bio-Mechatronic
Engineering National Ilan University
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Compression Test

• The sample is deformed uniaxially in one
  direction only and the result used as a measure
  of the texture of the food.
• The probe is usually cylindrical or rectangular
  and must be of greater area than the test
  product. If the sample has a larger surface area
  than the probe then the test must be considered
  to be puncture or penetration.
• High uniaxial compression usually causes the
  product to rupture, spread, fracture, or break
  into pieces. This type of compression is the
  basis of the Texture Profile Analysis (TPA) test.

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Compression Test of Bio-Materials with Convex
Shape

• Bio-yield Point A point where an increase in
  deformation result in a decrease or no change in
  force.
• Point of Inflection A typical force-deformation
  curve is first concave up and then concave down.
  The point at which the rate of change of slope of
  the curve becomes zero is called the point of
  inflection.
• Rupture Point The point on the force-deformation
  curve at which the loaded specimen shows a
  visible or invisible failure in the form of
  breaks or cracks.

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Hertz Problem of Contact Stresses

• Heinrich Hertz (1896)
• The maximum contact stress, being at the center
  of the surface of contact, is denoted by Smax and
  is given by

• Where a and b are the major and minor semi-axes
  of the elliptic contact area.
• The maximum contact stress is 1½times the average
  pressure on the surface of contact.

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Modulus of Elasticity Calculated from force and
deformation Data

• E modulus of elasticity, Pa
• F force, N
• D elastic deformation at both loading and
  supporting point of contact, m
• m Poissons ratio
• R1, R1, R2, R2 radii of curvature of the
  convex body at the points of contact, m
• D diameter of the spherical indenter, m

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Tension Test

• Tensile tests are used to measure the adhesion of
  a food to a surface. In this type of test the
  sample of food has a probe pressed onto it after
  which the extraction force is measured.
  Important textural characteristics such as
  elasticity of spaghetti and extensibility of
  dough are further examples of tensile tests.
• Tensile tests have mainly been performed for meat
  analysis where breaking strength is the best
  parameter for predicting tenderness in cooked
  meat.

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Extrusion Test

• Backward Extrusion The sample is contained in a
  cell with a solid base and an open top. A loose
  fitting plunger is then forced down into the
  container until the food flows up through the
  annulus between the plunger and the container
  walls.
• Forward Extrusion the sample is placed in a
  container with an open top. However the base of
  the container accommodates a disc containing a
  central hole. The tightly fitting plunger acts as
  a piston to compress the sample causing forward
  flow.
• This technique has been applied to butter,
  margarine and other fats in an attempt to measure
  firmness and Spreadability. Other materials
  commonly tested are fruits, vegetables, gels, and
  some viscous liquid products.

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Adhesion Test

• Adhesion is the force that resists the separation
  of two bodies in contact.
• Tensile tests are used to measure the adhesion of
  a food to a surface. In this type of test the
  sample of food has a disk pressed onto it after
  which the force required to pull it off is
  measured.

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Fracture Test

• Fracturability is a parameter that was initially
  called Brittleness. It is the force with which
  a sample crumbles, cracks or shatters.
• Foods that exhibit fracturability are products
  that possess a high degree of hardness and low
  degree of adhesiveness.
• The degree of fracturability of a food is
  measured as the horizontal force with which a
  food moves away from the point where the vertical
  force is applied.
• Another factor that helps determine
  fracturability is the suddenness with which the
  food breaks.

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Cutting Shearing Test

• There are many single blade or multi- bladed
  fixtures that cut or shear through the sample of
  food. The maximum force required and the work
  done is taken as an index of firmness, toughness
  or fibrousness of the sample.
• Although the term Shear is used to describe the
  action of such fixtures, both compression and
  tension forces are developed as well.
• Cutting and shearing is a usually used on food
  with a fibrous structure including meat, meat
  products and vegetables such as asparagus.

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Bending Snapping Test

• Bending is a combination of compression, tension
  and shear.
• Snap, meaning to break suddenly upon the
  application of a force, is a desirable textural
  property in most crisp foods, such as fresh green
  beans and other vegetables, potato chips and
  other snack items. The ability to snap is a
  measure of the temper of chocolate, the moisture
  content of crisp cookies, the turgor of fresh
  vegetables and the amounts of shortening in baked
  goods.
• The sharp cracking sound that usually accompanies
  snapping is the result of high-energy sound waves
  generated when the stressed material fractures
  rapidly and the broken parts return to their
  former configuration.

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Shear and Three-Point Bending Test of Animal
BoneASAE S459

• This test is designed for use in determining the
  mechanical properties of animal bones such as the
  ultimate shear strength, ultimate bending
  strength, apparent modulus of elasticity, and
  fracture energy.
• Shear and bending tests of intact animal bones
  provide an objective method for evaluating the
  effects of age, sex, nutrition, contamination,
  and environment on the physical condition of the
  animal.

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• The type of test selected, sear or three-point
  bending, will be dependent on the size and shape
  of the bone. The three-point bending tests
  should be used only when the bone is straight,
  has a symmetrical cross section, and has a
  support length to diameter ratio greater than 10.
• The shear test is good for any size or shape of
  bone.
• Any of the these mechanical properties can be
  used for the purpose of evaluation, and it is
  recommended that more than one property be used.

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Test Specimen and Testing Condition

• Specimens will be tested in their original size
  and shape.
• They can be tested under 3 different conditions
  (1) fresh, (2) frozen and thawed, or (3) cooked
  and dried.
• Tests on fresh bone specimens must be conducted
  before the time of exposure to air exceeds 10 min
  in order to avoid changes caused by drying of the
  specimen.
• Frozen specimens must be thawed, brought to room
  temperature (22 2?), and tested before drying
  occurs.

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• Cooked specimens should be air dried for a
  minimum of 24 hours at room temperature before
  testing.
• Because of the large variance inherent in bone
  specimens, each experiment must be statistically
  designed to have enough test specimens for an
  acceptable level of confidence in the results. A
  minimum of 25 specimens should be used.
• For the shear test, a crosshead speed of 5 mm/min
  should be used.
• For the bending test, a crosshead speed of 10
  mm/min should be used.

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Three Point Bending
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Stable Micro SystemsTexture AnalyserModel
TA-HD50 kg Loadcell250 kg Loadcell
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3-Point Bending TestA/3PB - 3 Point Bending Rig
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Chicken Femur
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The Ultimate Bending Strength
Where ? ultimate bending stress, MPa F
applied force, N L distance between
supports, m C distance from neutral axis to
outer fiber, m I moment of inertia, m4
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Apparent Modulus of Elasticity (E, Pa)
F
L

• Where ? deformation, m

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Moment of Inertia
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Most bone cross sections can be modeled as either
a hollow ellipse or a quadrant of an ellipse. The
moment of Inertia for a hollow ellipse is
Where B outside major diameter , m b
inside major diameter , m D outside minor
diameter , m d inside minor diameter , m
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For a quadrant of a ellipse
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The Ultimate Shear Strength
Where t shear stress, Pa F applied
fracture force, N A initial cross-sectional
area, m2
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Mechanics of Impact

• The concept of impact is differentiated from the
  case of static rapid loading by the fact that the
  forces created by the collision are exerted and
  removed in a very short period of time (duration
  of impact) and that the collision produces stress
  waves which travel away from the region of
  contact.

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Four Phases of Impact

• Initial elastic deformation during which the
  region of contact will be deformed elastically
  and will recover fully without residual
  deformation.
• Onset of plastic deformation during which the
  mean pressure exceeds the dynamic yield pressure
  of the material and the resulting deformation
  will not be fully recovered.
• Full plastic deformation during which the
  deformation continues from elastic-plastic to
  fully plastic until the pressure falls below the
  dynamic yield pressure.
• Elastic rebound during which a release of elastic
  stresses stored in both bodies takes place.

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Plastic Impact

• If the impact is not purely elastic , the kinetic
  energy is converted into permanent deformation of
  the material and eventual dissipation of this
  energy in the form of heat.
• The Meyers Law

Where D the central indentation
k, n constants
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Impact Energy Consumed, Eab

• Where e the coefficient of restitution
• W sample weight
• H height of drop

The mechanical parameters considered were
potential energy, energy consumed, and rebound of
the impacting product.
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The measured Parameters of Drop Test

• Drop Height, h
• Duration of Impact, t
• Maximum Force, F
• Impulse, P (the integral of the force along time)
• Initial Momentum, mv
• Bruise Volume, B (the volume of damaged tissue)

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Puncture Penetration Test

• In a penetration or puncture test the probe
  penetrates into the test sample by a combination
  of compression and shear forces that cause
  irreversible changes in the sample.

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Puncture Penetration Test

• The force necessary to achieve a certain
  penetration depth is measured and used as a
  measure of hardness, firmness or toughness.
• Puncture test measures the force required to
  reach a specified depth.
• Penetration test measures the depth of
  penetration is measured under a constant load.
• Puncture and penetration tests are commonly used
  in the testing of fresh fruits and vegetables,
  cheese, confectionery and the spreadability of
  butter and margarine. Penetration tests have
  also been used extensively for testing the
  rigidity of gels, such as the Bloom test.

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Elasto-plastic Hysteresis

• The major part of the residual deformation is due
  to initial setting which may be caused by the
  presence of pores or air spaces, weak ruptured
  cells on the surface, microscopic cracks in
  brittle materials, and other discontinuities
  which may exist in the structure of the material.
• In the case of corn, the higher the moisture
  content, the greater was the hysteresis loss.
  This would be expected because the addition of
  water increases the plasticity of the grain which
  in turn will increase the hysteresis loss.

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Elastic Hysteresis

• If in the process of loading and unloading, there
  is a complete cycle resulting in a closed loop,
  like in the case of rubber, the behavior is
  called elastic hysteresis.
• Some energy is lost in the process of loading and
  unloading. The energy loss, referred as
  hysteresis loss, is obtained by taking the
  difference between the work of loading and the
  work of unloading. The relative amount of
  hysteresis loss is a measure of elasticity.

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Convex
Concave
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Force Deformation Behavior

• The initial part of the force-deformation curves
  are usually concaved towards the force axis.
  This is exactly opposite the force-deformation
  curves for polymeric materials which is usually
  convex towards the force axis.
• The presence of moisture in bio-materials offers
  little resistance to shear stresses causing
  relatively large deformations in response to
  small initial stresses.
• Plants with greater number of air chambers show
  greater elasticity and thus are less stiff and
  have smaller modulus of elasticity.

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Force Deformation Behavior

• The presence of a sigmoidal shape
  force-deformation curve in bio-materials means
  that a modulus of elasticity calculated on the
  basis of the slope of the force-deformation curve
  would always be, up to a point, greater for
  heavier loads or larger strains than for lighter
  loads and smaller strains.
• The tangential modulus of soft biological tissues
  is almost zero at small strain but increases
  exponentially as the strain increases.
• A statement of modulus of elasticity of a
  bio-material must always be accompanied by the
  load or strain level at which the value of the
  modulus was calculated.

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Texture Profile Analysis (TPA)
A closer look at this popular way of
characterising the structure of foods
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Hardness

• The force necessary to attain deformation given
  as the final peak of the TPA curve, which is the
  force value corresponding to the first major
  peak. The maximum force during the first cycle
  of compression. Is also known as the firmness.

Fracturability
The force which the material fractures (height of
first significant break in the peak of TPA
curve) a sample with a high degree of hardness
and low cohesiveness will fracture This can also
de called Brittleness. Fracturability is the
force value corresponding to the fracturability
peak (if there is one).
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Springness

• The height that the food recovers during the time
  that elapses between the end of the first cycle
  and the start of the second cycle.
• The rate at which a deformed sample goes back to
  its un-deformed condition after deforming force
  This can also de called Elasticity.

Stringness
Defined as the distance that the product is
extended during de-compression before separating
from the probe.
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Cohesiveness

• The quantity to simulate the strength internal
  bonds making up the body of the sample if
  Adhesiveness lt Cohesiveness then probe would
  remain clean as the product has the ability to
  hold together.

Adhesiveness
The quantity to simulate the work necessary to
overcome the attractive forces between the
surfaces of the sample and surface of the probe
with which the sample comes into contact if
Adhesiveness gt Cohesiveness,then part of the
sample will adhere to the probe.
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Chewiness

• The quantity to simulate the energy required to
  masticate a semi-solid sample to steady state of
  swallowing (Hardness Cohesiveness
  Adhesiveness).

Gumminess
The quantity to simulate the energy required to
disintegrate a semi-solid sample to a steady
state of swallowing (Hardness Cohesiveness).
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Selecting the Correct Testing Procedure

• Nature of the Product - The kind of material
  (crisp, aerated, homogeneous, plastic, brittle,
  heterogeneous) affects the type of test principle
  that should be used.
• Purpose of the Test - Is the test to be used for
  quality control, for product development, for
  setting legal official standards or basic
  research? The answer to these questions is an
  integral part of the selection process.

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• Accuracy Required - A larger sample size or a
  greater number of replicate tests give a higher
  degree of accuracy but this required more
  product, a higher force range and more time to
  perform the test. In most applications a
  compromise is made between the cost and time of
  the test and the degree of accuracy required.
• Destructive or Non-destructive - Destructive
  tests ruin the structure of the sample rendering
  it unsuitable for repeating the test or for using
  the product for other purposes. Non-destructive
  tests leave the food in a condition close to its
  original state so the test can be repeated on the
  same item. Both types of tests are used in the
  food industry.