Mechanical Tests Flashcards
What are the two types of tests? What do they mean?
Destructive and nondestructive. Pretty self explanatory. Destructive tests need a lot more specimen. Example of nondestructive: using sound waves.
What is size effect?
Applies in real life. The larger the specimen, the more likely it is to have flaws of greater severity and have a lower fracture strength. Important to take into consideration when sizing up from a lab experiment. Larger specimen fails at higher load but not at a higher stress. Reason why small perfect wood is stronger than small concrete.
What is the tensile test?
About the most useful. A sample is elongated at constant rate and the load is the dependent variable.
What is the difference between linear and nonlinear elastic stress strain curves?
Linear elastic has loading and unloading along a straight line. Nonlinear has loading and unloading in a curved line and there is a gap between them. That is energy dissipation.
KNOW ALL SOLID MECH REFERENCES
Hookes law
Young modulus (modulus of elasticity)
Strain vs stress
Why is there a zigzag line at yield point? For steel
We have upper yield points and lower yield points. The stress is “moving around” between weakest points and this causes a zigzag at yield point until we reach ultimate yield
At what percentage offset do we assume yielding?
At 0.2% of strain offset, we consider the material to have yielded (at same young modulus slope)
How is the tensile strength of brittle materials?
Brittle materials fail in tension because their cracks propagate easily under tensile stress. They are strong in compression (closing of cracks and fingers)
Look at L7 slide 17 for failure modes
What determines whether a material will break in a brittle or ductile manner?
Atomic or molecular structure
Service conditions
-> temp, strain rate, degree of triaxiality
What happens at increase of temp? At increase of strain rate? At increase of triaxiality?
Respectively:
More ductile
More brittle
More brittle
Is there a change in volume during tension testing during plastic deformation?
No! Length increases as area shrinks (necking) therefore volume remains constant
How do ductile materials fail in tension?
They fail at 45° to the direction of applied tensile tress in tension by shear.
How do brittle materials fail in tension?
Tend to fail due to crack propagation perpendicular to axis of loading. No necking. Flat break
What is the difference between engineering and true curve?
Engineering curve doesn’t account for the cross section area getting smaller therefore it’s inaccurate. The true curve does account for that and that’s why it always goes upward, engineering curves goes down after ultimate strength but that’s not accurate
We use engineering and that’s okay because we never want to go past yield anyways and until yield eng and true are same
What are the different types of tension testing and how do they work?
Direct tension test : directly applying tensile loads. They have to be perfectly aligned and have no friction in order to avoid uni axial loads
Splitting tension test: inaccurate because we test only the middle of the specimen but the weakest point might be on the sides
Pressure tension test: gas pressure applied at curved surface (ends are free) it fractures across a single transverse plane
Why aren’t ductile materials tested in compression?
The sample is constrained by friction at points of contact which gives rise to complicated stress distribution. There is no be king occurring.
Can concrete fail in compression?
No! It always fails in tension like all brittle materials. Hourglass shape!
What are three types of elastic modulo?
Tangent, chord, secant
When are the different types of modulo used?
Elastic: ductile materials. Gives too high of an estimation for max strain so we use,
Chord: max allowed exposure is 40% of ultimate strength
Tangent: when there are already strains and stresses and we want to see how much more we can put on the specimen
Secant: linear elastic until a specific stress we are looking at, ie yield point
What is the anti clastic curve and what is it due to?
During flexing. It is due to poisson effect. The region that was compressed now has residual tension and other way around.
What is the difference between three-point bending and third-point bending?
Third-point bending aka four-point bending. Three point as max bending in the centre therefore it’ll fail in the middle and not at weakest point. Four point has a higher area under max moment so more likely to fail at weakest point.
Flexural strength is the modulus of rupture.
What are some potential problems in the bending test?
Doesn’t account for size effect (what’ll happen when we scale up?) With a rounded support, we have less friction and less stress concentration but it will slide.
We have shear stress which can cause increased deflection.
We have high localized stresses at loading points and can have torsional loading
