05 The Tensile Test
Summary
TLDR本视频展示了如何通过实验确定材料的应力-应变曲线,特别是在弹性区域。使用黄铜样品进行拉伸试验,样品呈特定的“狗骨”形状,中间部分较窄,两端较宽,以确保在拉伸过程中中间部分承受最大应力。实验中,通过在样品上安装应变片来测量应变,同时使用便携式拉伸试验机施加应力。视频详细记录了从弹性阶段到塑性变形的整个过程,并展示了材料在达到弹性极限后如何继续增强,直至最终断裂。
Takeaways
- 📏 拉伸样品通常是狗骨头形状,中间较窄,末端较宽,以便于夹持。
- 🔧 中间较窄的区域被称为“减小截面”,这是材料变形和失效的主要区域。
- 🎯 应变计安装在减小截面区域,用于精确测量应变。
- 📊 通过拉伸试验可以生成应力-应变曲线,其中弹性变形部分表现为直线。
- ⚙️ 拉伸试验中,初始加载曲线可能有“曲线尾巴”,是由设备的机械特性引起的。
- 🧠 在应力-应变曲线的弹性区域末端,材料开始发生塑性变形,通常在400兆帕附近。
- 🔄 卸载时,材料会发生弹性恢复,曲线的斜率仍然等于杨氏模量。
- 🏋️ 通过塑性变形,材料的强度增加,但杨氏模量保持不变。
- 🔄 拉伸试验还展示了材料的滞后现象,这是一种实验伪影。
- 💥 最终,在持续施加应力下,材料断裂,显示出延展性和弹性恢复。
Q & A
什么是应力-应变曲线?
-应力-应变曲线描述了材料在受到拉伸或压缩时的应力和应变之间的关系,特别是在弹性区和塑性区的表现。
为什么样品的中间部分被设计成较窄的截面?
-样品的中间部分被设计为较窄是为了集中应力,并确保材料的塑性变形和断裂发生在这一区域,从而便于研究和获取数据。
什么是‘狗骨样品’?
-‘狗骨样品’是一种用于拉伸测试的样品,它因形状类似于狗骨头而得名,通常在中间部分变窄,两端较宽,方便夹紧。
应力和应变是如何被测量的?
-应力是通过测量样品所承受的载荷来计算的,应变则通过附着在样品上的应变片精确测量。
什么是弹性恢复?
-弹性恢复指的是当材料在应力解除后恢复到原来形状的能力,通常发生在材料的弹性区域。
拉伸测试中的‘塑性变形’是什么意思?
-塑性变形是指材料在超出其弹性极限后发生的永久变形,即材料不会完全恢复到原来的形状。
‘屈服点’是什么?
-屈服点是指材料从弹性变形转变为塑性变形的应力水平,通常在应力-应变曲线上表现为曲线开始偏离线性部分的地方。
拉伸测试中的‘断裂’意味着什么?
-断裂是指材料在拉伸测试中承受的应力达到极限后发生的断裂或破坏,测试结束时材料通常会变得比原来更长。
应力-应变曲线中出现的‘滞后现象’是什么?
-滞后现象是指在加载和卸载过程中,材料应力-应变曲线之间形成的闭环,通常由于材料的内部摩擦或实验过程中的误差引起。
如何通过塑性变形增强材料的强度?
-通过对材料施加塑性变形,材料的强度会增加,这是由于材料内部结构发生了变化,如黑smith锻打金属时通过反复敲打增强强度的原理。
Outlines
🛠️ 如何制作拉伸应力-应变曲线
本段详细介绍了如何通过拉伸实验来制作应力-应变曲线,重点在于解释样本形状的设计以及测试的过程。样本是铜制拉伸试件,形状类似哑铃,被称为“狗骨试件”,用于确保应力集中在中间较窄的截面。中间部分是“减少截面”,在拉伸过程中发生的塑性变形和断裂将集中于此。文中还简要提及应变规的使用,该仪器用于精确测量中间截面的应变。
📊 通过实验获取应力-应变数据
这一段介绍了如何通过软件记录拉伸实验的应力和应变数据。应力和应变的计算基于样本的横截面积,实验结果显示了线性弹性阶段的应力-应变曲线。虽然在实验初期可能会有一些微小的偏差,但整体曲线表现良好。作者还解释了曲线的“脚趾”现象,这种现象与实验过程中设备的微小调整有关。
🔄 弹性恢复与材料强化
本段讨论了材料在加载和卸载过程中的弹性恢复现象。作者指出,当加载超过某一应力值时,材料会发生塑性变形,但当卸载时,材料会弹性恢复,曲线的斜率仍然等于杨氏模量。通过重复加载,材料逐渐变得更强,这是由于塑性变形引发的强化效果。作者将这种现象与锻造过程中的强化作用作了类比,强调了杨氏模量不会随塑性变形而改变。
💥 拉伸测试的最终断裂
这一段记录了实验的最后阶段,材料在经过多次加载后发生断裂。作者展示了试件在断裂后的伸长情况,并解释了“弹性回弹”现象,即试件在断裂前经历了弹性恢复,导致了可见的间隙。作者对此实验表现出极大的兴奋,并暗示将在后续视频中进一步讨论这个现象。
Mindmap
Keywords
💡应力-应变曲线
💡拉伸试样
💡弹性区域
💡杨氏模量
💡塑性变形
💡减少截面
💡应变片
💡标距
💡塑性强化
💡弹性恢复
Highlights
介绍了拉伸实验中的应力-应变曲线,并展示了如何生成这些曲线。
演示了如何使用一个小型拉伸样品进行实验,这是一个典型的狗骨试样。
讲解了为什么样品的中间部分更窄,这被称为减少区域,是材料塑性变形和断裂发生的地方。
强调了使用应变仪测量减少区域的应变,该应变仪是精密的电子设备,用于准确测量应变。
通过实验展示了金属材料的应力-应变曲线,包括弹性变形和塑性变形的阶段。
提到了在曲线开始部分的非线性区域,称为“曲线趾”,这是实验装置固有的误差。
通过手动加载展示了应力-应变曲线的形状,并指出这类金属材料的典型应力-应变行为。
讨论了杨氏模量的重要性,解释了尽管材料变得更强,但杨氏模量不变。
展示了材料在塑性变形后的弹性恢复现象,并指出了弹性恢复与杨氏模量的关系。
引入了应力松弛现象,并通过卸载和重新加载实验展示了滞后效应。
通过继续加载展示了材料的进一步强化,并强调通过塑性变形使材料变得更强。
实验显示了材料在不断加载下断裂的过程,并解释了断裂发生的机制。
通过对比实验前后的试样长度,展示了塑性变形对材料形状的影响。
讨论了断裂后材料的弹性恢复现象,并强调了材料回弹的可视化效果。
总结了实验的重要性,并预告了后续课程将进一步探讨材料强度和杨氏模量的关系。
Transcripts
okay so in this video I'd like to show
you how we actually determine create one
of these stress-strain curves that we've
been talking a little bit about at least
in the elastic region so far so what
we're gonna do without further ado is
we're gonna test one of these samples
this is a little bit of a bit of brass
it's a tensile specimen you might call
it it's got a certain shape I'll sketch
it out for you it looks something like
this
okay sort of narrows down a bit like
that it's gonna figure reason at the
ends yeah that's not too bad there you
go so that's a what we call a tensile
specimen coupon or sample sometimes if
you want to be hip you call it a dog
bone specimen because it kinda looks
like a female or something right um no
kidding anyway it is so what why do we
have this shape why is it narrower here
and bigger at the ends what's up with
these big regions well you'll see in a
moment that these larger regions at the
end of where we're gonna grip it I'm
gonna tighten it down in this machine
quite quite hard and we have to make
sure it doesn't slip there we also have
to make sure that the section here in
the middle which has a smaller
cross-sectional area right this
cross-sectional area here is smaller
that's also where we define our a knot
for our stationary stress so this is
this region with the smaller
cross-sectional area is called often
that the reduced section okay reduced
because the cross-sectional area is
reduced so that's where all the good
stuffs gonna happen you know when we
start to stretch this thing out that's
where the plastic deformation is going
to occur and ultimately that's where
it's gonna fail so we as engineers can
study it and by making that reduce
section in there smaller like that in
cross-section we know all the good
stuffs gonna happen there and that's
where we can study it and get the data
so that's the reduced section and the
final thing is that somewhere in this
region doesn't have to be actually in
the middle but somewhere in here usually
what we do is we clip on a little
instrument called a strain gauge and
it's a delicate little piece of
electronics that accurately measures
this
and so that's called the gauge length
that gauge length okay that's where we
define L naught usually you clip on this
little strain gauge it's got a couple of
razor blades that just touch into the
surface them don't slip and it could be
anywhere in that reduced section because
of course a knots the same through this
reduced section so the stress is the
same all right so without further ado
I'd like to actually show you a tensile
test and I'd like you to think about
something we haven't talked about yet
that's this if this is a stress-strain
curve and we've talked about this linear
elastic region what is the rest of the
curve gonna look like is it gonna look
like that you know is it gonna look like
this sort of straight up and then maybe
braking or is it gonna look like say
this and then break somewhere let's see
which one of those do you think it's
gonna be take a moment think about that
so here we have tensile specimen I'm
just gonna show you if this camera round
like this so you can see this tensile
tester okay and this is a portable
tensile tester normally for material
testing you'd use a bigger machine than
this but this one's pretty good for a
little demonstrations and hands-on
activities so there I've already mounted
the little brass specimen in there okay
I've got another one this is the one I
was showing you and I've mounted that
one and it's gripped on these ends okay
so that's where it tightened these bolts
down so it grips on the grip region
reduced section right through the middle
enough for this simple test we're not
going to put a strain gauge on we're
just going to use the length of that
reducer section for our strain and over
here you'll see there's a little rock
around this is a little unconventional
but what happens is it's just the way
that this particular little machine
records the load so this is a load cell
and it's going to record the load that
the results from the application of
strain and the strain
apply by turning this little wheel over
here on the right-hand side of the
screen so I'm gonna crank that it will
impose strain on this and we will record
the stress men let me show you here the
software will plot I've already coded
into the software the sample dimension
is a cross sectional area so over here
you'll see it's gonna calculate the
stress for us and it's gonna calculate
the strain now this strain I don't know
why it says F there just ignore that and
we're actually not gonna see any
negative values it just happens to it
auto scales the axes but it starts off
and you're gonna collect if we're gonna
collect the first data point right here
in the middle but of course it this is
the software likes to put those negative
ones I'm just so it's right in the
middle but we're gonna get just positive
values okay so I'm going to zero the
load cell I'm gonna start the test and
here we go and start cranking the handle
here and look at this we start to
accumulate some data and so this is now
the brass sample starting to go through
it stress drinkers I'm gonna stop right
there and now you might look at that and
say wait a second that doesn't look
linear right but experimentally this is
beautiful linear the linear curve so
right through here that's where you
would calculate the Youngs modulus sure
there's a bit of scatter but some of
that's probably from inconsistencies in
the way I was turning the crank the
other thing you'll notice is down here
there's a little toe of the curve we
often call it and that's mostly from
bolts in the in the machine actually
settling in a bit to there threads a bit
of strain there so that's kind of an
artifact of the experimental test but
right through here this is good data and
we can see if I continue to deform it
like this it continues to go to higher
higher values of stress but you can see
now what's happened it's done I guess
what we had called C right it's curving
off like this always go back over here C
is this generalized shape for the
stress-strain curve for a metal okay and
that's what we're doing there so if you
had to guess where would you say the
material first
exceeded the elastic region first
started to deform permanently and I
think if you look at this you would
probably say well somewhere in here
maybe around 400 mega pascals or
something I want to show you something
interesting what's gonna happen now I
continue to load it continue to load it
what's gonna happen if I unload it I
start to reduce the stress which what do
you think it's gonna go is it gonna go
off you know maybe this way straight
down back this way well in fact you
remember from the previous video that we
talked about the young's modulus being
this really important structure
independent property so although we've
changed the strength let me show you
that we've changed the strength it's
exciting it's really exciting
where was this material first
permanently deforming and we'll explore
this later in more detail in a separate
video
but where did it first stop being
elastic well you'd probably guess
somewhere around here near the end of
the straight line so say around we'll
call it 400 mega Pascal's if I unload
here well look at this I'm I can't
ignore the fact that that's got a slope
to it that slope is the same as the
slope over here why because it's elastic
recovery I unload it I reduce the stress
on sample and it recovers elastically it
springs back it pulls back even though
it has also plastically or s right not
permanently deformed okay so this slope
is the same as this slope because it's
no it's the Youngs modulus and even
though we've strengthened it we haven't
changed the Youngs modulus why have we
strengthened it well let me show you
that if I unload it all the way here
unload it right down to zero and now if
I bring it back up I load it up again
you can see there's a little bit of this
loop we call it hysteresis that's really
more of an experimental artifact here in
this case and I'm gonna see that it's
suddenly right there where we left it
off last time up here close to 500 mega
Pascal's is when it starts to
plastically deform the second time so
the first time around 400 mega Pascal
strength and then 500 we've made it
stronger we've made it stronger and this
is in fact an actual way that the
through planet
deformation like this you know you see
one of these blacksmiths hammering away
on a sword or something like that or a
horseshoe or or whatever that that
horseshoe is actually getting stronger
through that deformation operation so
we're strengthening we'll learn about it
more in more detail later in the course
but of course the whole time the Youngs
modulus does not change it's not
exciting I think it is anyway I hope you
do is well now look at this I can
continue to deform it will continue to
get stronger the little saucer just from
the way I'm turning it I'm trying to
turn this smoothly as I can but I pause
for a second and the Machine just
relaxes a bit and if we are lucky here
we'll actually get this to break before
I run out of strain on a machine so it's
getting quite strong getting quite
strong I'm going up to 600 mega Pascal's
here right I could unload at any time if
I wanted and it'll still have the same
Young's modulus loaded back up again
same Young's modulus continued to load
it continued load it
it's a it's deforming permanently
absolutely it's deforming permanently
now I can actually show you here on this
camera you can see that's the size it
was to start with now it's it's
substantially longer what's gonna happen
is the stress on it what's gonna happen
what's gonna happen am I gonna be able
to get this to happen in this video is
it kind of break is it gonna break this
is a very ductile there we go okay I
stopped right away look at that so it
broke it fractured and look at this look
how long it is but also pay close
attention to this see that little gap in
there what is that what's that gap well
that's the elastic spring back or
recovery we can explore that more in a
subsequent video okay but I hope that
was exciting here it was for me okay
thank you
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