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VIDEO 3 (MATERIAL - STRESS-STRAIN )

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may go mechatronics welcome to the
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mechatronics bootcamp series for mechanical engineering
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today room talking about materials stress and strain
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throughout the series we're going to cover topics and stress and strain
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ductility corrosion hardening he treatments
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filler now says surface finish and thermal dynamics
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recover in stress and strain today
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so stockwell with stresses
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the units of stress express
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as a pressure like the pressure your tire
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atmospheric pressure so that if force per unit
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area and the units are
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here in the States pounds per square inch kasi which is
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PSI divided by 1000 or the metric system we have news per square meter
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known as Pascal's make a pass girls and take a pass girls
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soul let's say we have this
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material here this column and we apply
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10 pounds to it was figure out the stressful stresses
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the Lotus 10 pounds we know that but about the area graph to calculate the
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area
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so here some other formulas the simi square square rectangle same thing likes
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time with see the triangle circle where so here let's
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say it's a half inch by half ish 0.5
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times .5 is can you give us point 25
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so berliner formula 10 pounds about about
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point 25 messy area
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and that's going to give us forty pS I've stressed with in that column
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that's why something like a thumbtack compress into a wall so easily because
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that point so tiny that will affect the formula here
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and increase the PSI very high number
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and are moving on to strain
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strain doesn't have any unit unit list because
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it is air ratio of 2 likes that's good equal
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change in like dealt to change
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divided by the initial link the supplies
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anything you want to calculate the percent change ur you take that
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changed by the bay the initial and
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but also legal stress about a by the modules elasticity
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I'll that material so all materials are classified
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with some sort of modules are Young's modulus a
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thing and you can see I we flip that
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formula around because Young's modulus is stressed about buy straight
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ballistic are sample column here
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once a week poll on this think we stretch it out a little bit
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so we started with four inches
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we stretched it .0 50 fifty thousand 7-inch
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not very much so can we're gonna
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take that change for not taking a the total length over
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on so 50 found about about 4 inches
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that's going to give us a strain of .0 125
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or 1.25 percent
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so we change that link by 1.25 percent that strain
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so
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how our material stressed so here
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are the three basic ways material will be stress
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compression re trying to smash acecomm together
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like your billing snowball pressing
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tansel pulling on it really hard like when you
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poll on a rope it's under tension
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and then sharing sick in Seattle's arrows are slightly offset
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and that's that the shares of the parents were in line that would be
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compression
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ansel let's say we have a beam
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and on
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with banners beam so this is experiencing more than
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once truss more one title stress could be under tension
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top in under compression
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at the bottom see an example here the piece a paper
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seats gonna tear at the top because it under tension that's
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took now so fails under a bending moment like this
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NEC on the bomb site falls up because it's on a compression
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so
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how to dougie measures stress and strain the most popular way is a
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tansel test so we have this test specimen
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a standard cell standardized size
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and we put in between to joz furlough gauge on their to measure
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the change in distance and you pull apart
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so here's an example real-life one
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with some self locking jobs there see that gauge
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so the specimens typically the flat ones
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ir for polymer materials synthetics
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and then for the medals we have little bit larger piece there
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showing this video I one on hand
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its peace love %uh aluminum are stocking machine down
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and to a standard diameter so
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that's a half-inch and we can use that to compare against
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other half inch samples and
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how you machine that just depends on the machine using some use
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threaded ends so it's quite important so don't slip in the job other
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so we'll see how what we're measuring relates to you
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the infamous stress and strain curve
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which is really the point to this video
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so we're going to take this stress because we're going to calculate the
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area
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other specimens Elizabeth min the rectangular you'd
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find the area that where the round one we use the
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areas circle so we know the force because we're measuring
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and then we input the unit area Minister Alain over there
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and for the distance measurement again this week the computer him on these
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calculations
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change in length divided by the initial links
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between those measuring little
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were measuring jobs there yeah we're gonna
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platter on the strain curves at the bottom
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so here's a stress-strain curve again looking at the bottom
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you could see house train lines up there so
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gone across the top that's about four hundred make a Pasco
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stress and we're getting a small amount of strain
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her moving across to about 700 make a pass causes stress and you can see you
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for we have significantly more strain happen more
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more change in a link and then week
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some materials will experience is where you require
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that's a maybe 650 MPA
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and we're way over on the strain
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and then until fractures matter breaking
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breaking strength right there
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so there's some important attributes to the stress-strain curve
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that we need to understand first where the Alaska region
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and this is essentially the spring
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bringing a southern materially concern the slope
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riz related to the stiffness so steeper the slope
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stickers you can see has a piece of
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still wear your impending and
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it doesn t form Sona put down by the ruler
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is still straight so we we stayed in the evil elastic
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region and that's the last look like a rubber band
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because you can stretch a rubber in and returns shape
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and then appears the yield point so
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we're beginning our little have hockey stick here
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and that's when the material yield says hey Aki handle this
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and it starts to deform permanently in the now
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elastic region to NCAA have this
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still around again and I will
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apply stress above the your point
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and then obviously permanently deformed
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and then at the end to this curve is feel your point
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so we have the elastic region which is very important
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me where returns to its original shape and then we have the plastic deformation
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pass the your point this is where permanent deformation occurs
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looking at this graph to compare some different materials
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CA is very strong
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be is very brutally see there's
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that doesn't even yield a jazz brakes
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and then see is none not quite as stiff but it is
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quite doctor lee CO far we had to strain before
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that broke so stretching out like a workpiece gum
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and then the is not very stiff
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low strength and kinda ductile
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so ultimate shrank this is something that you need to know
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and when comparing materials looking up material properties
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will be listed along with the yield strength so
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the ultimate strength is gonna be
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the maximum stress that experience sternest test
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so you can see the different
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maximums here so it's not just the breaking point
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with be yield and ultimate strength
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the same we can see how
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see continued after the ultimate shrinks in
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and this is what so good about on
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stressed rankers weaken visually pair materials
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very easily here because were using stress
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which is standardized Loper area
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and and strain is a ratio
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so we can take data from some other tests
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and actually over later even if the tested in a little bit different way
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and various allies issues steals her up top stainless
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never gone down to aluminum miles
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steelman magnesium right at the bottom
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you on cation this is important
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now show you how that's measured so before the
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the specimens polled
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ritika a measurement of some known mark so
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maybe I'll take sharpy some time to you could put a dimple in there something
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and then after that ass you can see a how it is longer because
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neck down and pull the park and then
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failed so we can actually put those pieces back together
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after broke and the measure that change shoulders
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on actually strain so if you look at the stress-strain curve
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and at the breaking point and then you go down to the strain
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your CDL elongation percentage there so
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percent to be longish in another proxy to
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compare materials
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and there's they're still other tests and I'm not covering
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every single material to steer is covering
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on the most popular ones here's torsional testing
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so we take the torque no man cut that the stress
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and then the angle change that will mom the party the strain calculation
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and share testing appear concha
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test share directly so where initial condition
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and then you see after that to us her during the test was being strained
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sore cuts across their use in cities and shares a cut something
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so here's a working example love
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a bolt underage shear stress that something you test for fast news you'd
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wanna
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the shear strength when you're looking at the fastener that will be under shear
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load
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in neck
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includes are stress and strain tutorial
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thank you for watching

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