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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. Compression Test.

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Mechanical Property of Bio-material

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mechanical property of bio material

Mechanical Property of Bio-material

Physical Properties of Bio-Materials (III-B)

Poching Wu, Ph.D.

Department of Bio-Mechatronic Engineering

National Ilan University

compression test
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.
compression test of bio materials with convex shape
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.
hertz problem of contact stresses
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.
modulus of elasticity calculated from force and deformation data
Modulus of ElasticityCalculated 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 = Poisson’s 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
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.
extrusion test
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.
adhesion test
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.
fracture test
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.
cutting shearing test
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 orfibrousness 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.
bending snapping test
Bending & SnappingTest
  • 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.
shear and three point bending test of animal bone asae s459
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.
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.
test specimen and testing condition
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.
Cooked specimens should be air dried for a minimum of 24 hours at room temperaturebefore 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.

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
apparent modulus of elasticity e pa

Apparent Modulus of Elasticity (E, Pa)


Where  = deformation, m



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:

WhereB = outside major diameter , m

b = inside major diameter , m

D = outside minor diameter , m

d = inside minor diameter , m


The Ultimate Shear Strength

  • Wheret = shear stress, Pa
  • F = applied fracture force, N
    • A = initial cross-sectional area, m2
mechanics of impact
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.
four phases of impact
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.
plastic impact
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 Meyer’s Law:

Where D = the central indentation

k’, n’ = constants

impact energy consumed e ab

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.

the measured parameters of drop test
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)
puncture penetration test
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.
puncture penetration test70
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.
elasto plastic hysteresis
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.
elastic hysteresis
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 isa measure of elasticity.



force deformation behavior
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.
force deformation behavior75
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.
texture profile analysis tpa







Texture Profile Analysis (TPA)

A closer look at this popular way of characterising the structure of foods




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”.

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).



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.


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




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

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 > Cohesiveness,then part of the sample will adhere to the probe.




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

The quantity to simulate the energy required to disintegrate a semi-solid sample to a steady state of swallowing; (Hardness × Cohesiveness).

selecting the correct testing procedure
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.
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.