Tests For Materials
Mechanical testing of materials Monocyclic strength tests Cyclic strength tests Monocyclic fracture toughness tests Cyclic fracture toughness tests
Test for small, brittle specimens microhardness transverse strength biaxial flexure strength diametral tensile strength
Hardness: Tests which measure a material’s resistance to penetration by an indentor.
Knoop microhardness test: The Knoop test is frequently used in dental biomaterials research. A tiny diamond indentor is impressed in the material at a specified load and loading rate. The length of the indentation is measured. NOTE: the shorter the indentation, the harder the material.
Knoop microhardness test: eyepiece a system of weights, levers, and a dashpot produce a repeatable loading rate objective indentor Moveable stage (the spec-imen is moved under the indentor and then moved under the microscope’s objective.
Knoop microhardness test: The specimen is under the indentor, which is pressed in its surface. The specimen is moved under the objective and the length of the Knoop indentation is measured.
Knoop microhardness test:
Microhardness shows change in properties with position in a specimen. Unfortunately, hardness is not easily correlated to strength or elastic modulus. Such correlations are the subject of a great deal of current research.
Test for small, brittle specimens microhardness transverse strength biaxial flexure strength diametral tensile strength
300 200 100 0.2 0.4 0.6 strain (%) X Ultimate strength yield strength point 200 100 0.2 0.4 0.6 strain (%)
Tests of fracture resistance: ultimate strength tests slow strain rate (if viscoelasticity is important) high strain rate (if not interested in viscoelasticity
Transverse Tensile Strength: b d Transverse strength = 3 F L / ( 2 b d2 ) F compressive stresses tensile stresses Fracture occurs on tensile side; tensile stresses open cracks making many materials weaker in tension.
Test for small, brittle specimens microhardness transverse strength biaxial flexure strength diametral tensile strength
Biaxial Strength Test: Advantages: A major advantage of this test is that the load is applied well away from the edges of the specimen. This, minimizes the effects of stress concentrations at the edges. For materials that are condensable into molds biaxial test specimens are simple to make.
Test for small, brittle specimens microhardness transverse strength biaxial flexure strength diametral tensile strength
Diametral tensile test: A tensile test for brittle materials in which a compressive load is placed parallel to the diameter of a cylindrical specimen. Note: To be valid, the material to be tested must be very brittle. Diametral fracture will not occur if there is plastic flow.
Mechanical testing of materials Monocyclic strength tests Cyclic strength tests Monocyclic fracture toughness tests Cyclic fracture toughness tests
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Fatigue Tests A test in which a material is repeatedly subjected to stresses below it yield strength. The fatigue life is the number of load applications required to produce failure at a given stress.
stress 103 104 105 cycles the materials fails at lower and lower stresses as the number of cycles increases. 103 104 105 cycles
Tests of crack resistance: impact tests (toughness tests) un-notched notched fracture toughness tests single-edge-notch bending (SENB) compact tensile (CT)
Impact test
Tests of crack resistance: impact tests (toughness tests) un-notched notched fracture toughness tests single-edge-notch bending (SENB) compact tensile (CT)
Fracture toughness – compact tensile specimen:
The End