unit 4 Flashcards
how is displacement measured during material testing?
measured with either
- Linear Variable Differential Transformers (LVDT’s)
- Strain Gauges
what is a tensile test in metal testing?
-force used to pull sample apart, causing elongation and reduction in diameter
-most common test for studying stress strain relationship
-used to find out strength, ductility, toughness, elastic modulus, and strain-hardening capability
-test specimen is solid and round, with an original length Lo and original cross sectional area Ao
how does a tensile test work?
- tensile force is applied to a specimen
- strain gauge or extensometer used to measure elongation
- elongation and original length used to calculate STRAIN
- loading data and strain data used to plot Stress v Strain graph, stress-strain curve, finds mechanical properties
how does engineering stress and strain relate to the qualities found in tensile testing?
because:
engineering stress is defined as instantaneous force divided by original area
engineering strain defined as change in length divided by original length
what are the four stages of tensile testing?
- LINEAR ELASTIC: uniform elastic elongation, finds Young’s Modulus if the graph is linear enough
- PLASTIC ELONGATION: straight graph first begins to bend with a big decrease in the slope of the curve, represents plastic deformation and max load is reached
- NECKING: graph begins to slope down after max load reached, load begins to decrease
- FRACTURE: sample breaks and fracture stress is found
What are some important parameters in a tensile test?
Proportional Limit (Sigma p): happens after linear elastic region, represents the stress at which graph isn’t elastic anymore
Elastic Limit: soon after proportional limit, when actual plastic (permanent) deformation starts happening
Yield Point (Sigma o): stress when the material yields, immediately after elastic limit (sometimes they’re hard to tell apart)
Ultimate Tensile Strength (UTS) (Sigma u): the max stress achieved on the curve
Fracture Point: stress at which the sample breaks
in practice how do engineers easily determine the Yield Point?
0.2% strain offset method is used. draw a straight line parallel to the linear elastic section until it hits the real curve, and that is the yield strength at 0.2% offset
when does strain hardening happen on the curve?
in the plastic region (upside down parabola) right after the initial linear elastic region where the max load is reached then stress slowly decreases
Strain Hardening ratio is found by = Ultimate Stress/Yield Stress
What is toughness when discussing stress strain curves?
toughness is the total area under the stress strain curve, this measures energy absorbed by the specimen during the testing process
what role does temperature play in tensile testing?
as temperature increases, ductility and toughness increase, but the yield stress and modulus decrease
increase in temperature decreases the strain hardening ratio (Ultimate Stress/Yield Stress)
What are some methods to find the True Stress and Strain from the Engineering Stress and Strain?
the True values take into account the change in cross sectional area as testing occurs.
one method is using Bridgman’s formula (nahh)
what is a compression test?
some materials behave very differently in compression versus tension, so it’s useful to test it that way too (rock, concrete)
brittle materials fracture immediately after reaching ultimate strength
ductile materials wont fracture but deform into large thin pancakes until the machine can’t deform it anymore
Compression machines can be used to test tension indirectly, (Brazilian tensile test)
what is hardness testing?
BRINELL: press a steel ball against sample surface at various loads, then measure the dimension of the indentation.
finds how well sample resists scratches and wear, harder surfaces indent less than softer ones
VICKERS: uses pyramid shaped diamond instead of ball but still measures indentation
ROCKWELL: also uses diamond indenter, but depth is measured rather than diameter of indentation
what is flexural testing?
used for brittle materials like ceramics and carbides which are hard to test in other ways
a rectangular specimen is supported at both ends and loaded vertically in the middle at one point or at two points
stress at fracture is called the modulus of rupture, flexural strength, or transverse rupture strength
Why would you choose a 4 point bending test over a 3 point
bending test?
4 point is chosen over 3 point when
-uniform stress distribution over larger area is needed
-materials behavior under pure bending without shear is important to know
-material is brittle or sensitive to stress concentrations
-large specimen needs to be analyzed
