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03 A - MECHANICAL PROPERTIES OF MATERIALS


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Mechanical properties
PROPERTIES THAT DETERMINE MATERIAL'S BEHAVIOUR UNDER MECHANICAL STRESS, such as elastic modulus, ductility, hardness, etc.
Design-manufacturing trade-off
To achieve design function and quality, the material must be strong; for ease of manufacturing, the material should not be strong, in general.
Stress-strain relationships
Relationship between the stress and the strain in the material, Hooke's curve. Materials can be subjected to three types of static stresses: tensile, compressive, and shear.
Tensile properties
Determined by a tensile test which plots relative elongation or strain versus the stress in the part.
Modulus of elasticity
Measure of inherent stiffness of the material (E = σ/ε; σ = E*ε ).
Engineering stress-strain curve
DEFINED RELATIVE TO THE ORIGINAL AREA AND LENGTH OF THE TEST SPECIMEN. These values are of interest in design because the designer expects that the strains experienced by any component of the product will not significantly change its shape. The components are designed to withstand the anticipated stresses encountered in service.
Tensile strength
The tensile strength is the maximum load experienced during the tensile test divided by the original area.
Yield strength
THE STRESS AT WHICH THE MATERIAL BEGINS TO PLASTICALLY DEFORM. It is usually measured as the 0.2% offset value, which is the point where the stress-strain curve for the material intersects a line that is parallel to the straight-line portion of the curve but offset from it by 0.2%. YIELD POINT MARKS TRANSITION TO PLASTIC REGION
Fracture stress
Stress calculated immediately before failure.
Necking
After the point of tensile strength the deformations localize to a certain area of the part, the neck, instead of elongating uniformly. NECK NARROWS DOWN UNTIL FRACTURING.
Ductility
AMOUNT OF STRAIN MATERIAL CAN ENDURE BEFORE FAILING, the ability of the material to plastically strain without fracture, can be taken as elongation or area reduction. Alluminum: 8% - 20%, steel: 10% - 30%, Ceramics: 0% >>> MATERIAL IS DUCTILE UNTILL THE FRACTURE POINT.
True stress-strain curve
REAL RELATIONSHIP BETWEEN STRESS AND STRAIN. Division of the load by the instantaneous area which decreases as the specimen stretches. Provides true stress and true strain which considers the plastic deformation of the part. VERY SIMILAR TO ENGINEERING STRESS-STRAIN CURVE IN ELASTIC REGION.
Strain hardening
INCREASE OF MATERIAL STRENGTH DUE TO PLASTIC DEFORMATION. Strain hardening also reduces the ductility of the material, just like cold rolled still and bent paper clip would become harder at the deformed regions >>> MATERIAL STRAIN HARDENS UNTIL THE UTS POINT.
Flow curve
POWER FUNCTION THAT CHARACTERIZES THE PLASTIC REGION, a straight line in a log-log true stress-strain curve, until the start of the necking, that is until point of tensile strength. Flow curve provides good approximation of how metals behave in the plastic region - the higher the slope (strain hardening exponent), the better the material at strain hardening.
Types of stress-strain relationships
[1] ELASTIC - Material is elastic until it fractures: ceramics, cast irons, thermosetting polymers. [2] ELASTIC AND PERFECTLY PLASTIC - after yield poit materil deforms plastically at same stress level: heated metals above recrystallization temperature, lead. [3] ELASTIC AND STRAIN HARDENING - obeys hookes law in elastic region, starts to flow after the yield point: most ductile metals
Compression test
COMPRESSION OF CYLINDRICAL SPECIMEN BETWEEN TWO PLATENS, reducing the height and increasing the cross sectional area of the sample. Same flow curve parameters apply in compression as in tension. Due to friction with platens at the end of the material and the barreling phenomena, additional energy is applied and that requires additional force by the machine.
Flexural test
A bending test to determine the transverse rupture strength of brittle materials, since it's tricky making a tension test sample out of them. At the outermost fibers tension and compression occurs, the stress value, for homogeneous material will be the same as in a tensile test. >>> BEND IT TILL IT RUPTURES.
Shear properties
By using a torsion test, shear stress, strain, shear modulus of elasticity (G ≈ 0.4E), shear stress at fracture (S ≈ 0.7*σ_uts ) are determined.