07 - Mechanical Tests Flashcards
Tests to determine the properties of different materials can be
- destructive
- nondestructive
what is size effect
- the larger the test specimen, the more likely it is to have flaws of greater severity
- hence, the more likely it is to exhibit a lower fracture strength
(bigger will fail under a higher load, but not a higher stress)
describe the tensile test
- sample is elongated at a constant rate and the load necessary to produce a certain elongation is measured as a dependent variable
- data plotted on a load-elongation or stress-strain curve
engineering stress and strain
is the ratio of (load, change in length of speciment) to the (original cross-section, the the original length)
The relation between stress and strain is linear for metals and ceramics in which region
elastic region - described by Hooke’s Law stress = Ee
for brittle materials, the tensile strength is low or high and why
- brittle materials contain microscopic cracks which propagate relatively easily when they are under tensile stress
- so tensile strengths are low
whether a material behaves in a brittle or ductile manner depends on
- atomic or molecular structure
- service condition
what are the primary service conditions affecting behaviour
- temperature
- strain rate
- degree of triaxiality
If you increase the temperature, decrease stain rate, decrease triaxiality
material is more ductile
what is the most common speciment used for tension testing and why
- cylindrical specimen with threaded ends
- bc better stress distribution
- if test result is outside of gauge length then inconclusive
during plastic deformation there is no change in what
- the volume of the specimen when measured in the unloaded state
- necking: length increase but A becomes smaller
how do ductile materials fail in tension
- by shearing on planes at 45º to the direction of the applied tensile stress
- leads to local contraction of necking of the materials
how do brittle materials fail in tension
- due to crack propagation perpendicular to the axis of loading
- because of their lack of ductility there is little to no necking
due to the nature of standard tension testing technique, the resulting stresses calculated from these test results (eng stress) are _____ than stresses actually carried by specimen
lower
is there necking in compressive testing
no because the A of the specimen increases
extremely ductile materials are how often tested in compression?
- rarely
- bc the sample is constrained by friction at the points of contact with the platens of the apparatus
what are the potential problems in bending tests
- high localized stresses at loading points (prevent with round support)
- frictional forces at loading points (prevent with round support and round force application piece)
- torsional bending (avoid twisting use square cross-section)
what is hardness
- not a fundamental property
- ability to resist permanent deformation of its surface in the form of scratching, indentation, abrasion or cutting
what do the hardness tests measure
- resistance of a material to an indenter or cutting tool
what happens during hardness test
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 from the hardness test what should you do
never take the test near or at the edge of a test specimen
what is an impact test
- measures a material’s resistance of damage during an impact event
- empirical in design bc impact events are random by nature
- not a fundamental property
what do impact test measure
energy required to fracture a standard specimen under specified conditions
what is the charpy test
- uses a notched rectangular bar, which is subject to an impulse load, giving a relative indication of notch toughness of a material under shock loading
