Mechanical Tests A Flashcards
two types of test
destructive and non destructive
cite mechanical properties (3)
strength, hardness and ductility
what is the size effect
The larger the test specimen, the more likely it is
have flaws of greater severity.
Hence, the more likely it is to exhibit a lower fracture strength.
describe the tensile test
In this test, a sample is elongated at a constant rate and the load necessary to produce a certain elongation is measured as a dependent variable.
formula of engineering stress
sigma=P/A, sigma=stress, P=ratio of load, and A=cross sectional area
formula of engineering strain
espilon=change of length / original length
describe Hookes law
The relation between stress and strain in the elastic region is linear for metals and ceramics
sigma=E*epsilon, with E being Young’s Modulus
What is Young’s Modulus
Young’s modulus, also known as elastic modulus, can be determined from a stress-strain curve as
E = deltaSigma / deltaEpsilon
Materials behave in one of two primary modes:
1) Fracture of Brittle Materials
2) Yielding of Ductile Materials
describe strenghts of brittle materials
Their tensile strengths are thus low and usually vary from sample to sample.
These same materials can nevertheless exhibit high compressive strengths
yielding of ductile materials
Whether a material behaves in a brittle or ductile manner depends on: (2)
- Atomic or molecular structure
- Service conditions
Primary service conditions affecting behavior: (3)
- Temperature
- Strain Rate
- Degree of Triaxiality
What happens during plastic deformation (describe necking)
There is no change in the volume of the specimen when measured in the unloaded state.
A*L=constant
The length may increase, but the cross-sectional area becomes smaller, keeping the above equation constant.
When do ductile materials tend to fail in tension
Ductile materials tend to fail in tension by shearing on planes at approximately 45o to the direction of the applied tensile stress.
This generally leads to a local contraction or ‘necking’ of the materials
how do brittle materials tend to fail
Brittle materials tend to fail due to crack propagation
perpendicular to the axis of loading. Because of their lack of ductility, they exhibit little or no necking.
Describe the compression test
In the compression test, the cross- section of the specimen increases, therefore, necking does not occur.
Materials that are extremely ductile are seldom tested in compression because the sample is constrained by friction at the points of contact with the platens of the apparatus.
Describe potential problems in bending tests (4)
- High localized stresses at loading points
- Frictional forces at loading points
- Torsional loading
- Shear effect
Define hardness
“The ability of a material to resist permanent deformation of its surface, in the form of scratching, indentation, abrasion or cutting.”
All Hardness tests are entirely empirical.
Not a Fundamental Property
Describe the hardness test
In materials testing, the hardness test measures the resistance of a material to an indenter or cutting tool.
The indenter is usually made from a material such as hardened steel, sintered tungsten carbide, or diamond, and takes the shape of a ball, pyramid, or cone.
In most standard hardness tests, a load is applied by slowly pressing the indenter at a right angle to the surface of the sample for a given period of time.
For more accurate results, these tests are never taken near or at the edge of a test specimen.
Describe the impact test
An impact test measures a material’s resistance of damage during an impact event.
Since impact events are random by nature, standard impact tests are empirical in design.
Not a Fundamental Property
Describe the Charpy Test
The Charpy Test uses a notched rectangular bar, which is subjected to an impulse load, giving a relative indication of the notch toughness of a material under shock loading.
Define fracture energy
The amount of energy that is irreversibly absorbed in the process of fracturing
What are the three factors that can induce transition from ductile to brittle behavior
- Temperature
- Strain Rate
- Triaxiality
