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 • 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 • 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 • 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 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
TensionTest • 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 • 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 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 • 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 • 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 & 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 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 • 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.
Stable Micro SystemsTexture AnalyserModel TA-HD50 kg Loadcell250 kg Loadcell
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) F Where = deformation, m L
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 • 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.