07 Introduction to Permanent Deformation

Summary

该视频讨论了弹性变形和永久变形的区别。通过拉伸橡皮筋和金属回到原始形态来解释弹性行为，而当金属受到过度应力时，会发生不可逆的永久变形。讲者还提到应力-应变曲线，指出在直线末端附近，永久变形最有可能发生。此外，介绍了'塑性'的定义，解释其来源于希腊语'plastikos'，意为'成形'，并澄清了'塑性变形'与'塑料'材料无关。最后，他将该术语与'整形外科'联系起来，展示了它们的共同词源。

Takeaways

• 😊 弹性变形后，物体可以恢复到原来的尺寸，就像橡皮筋被拉伸后能恢复原状。
• 🔨 金属在弹性变形后也会恢复，但如果变形过大，就会产生永久变形。
• 📏 永久变形意味着当卸载后，物体不会恢复到原来的尺寸。
• 🔄 永久变形的另一个特征是卸载后应变不会回到零。
• ⚛️ 永久变形时，原子会移动到新的平衡位置。
• 📉 永久变形通常发生在应力应变曲线的线性区域结束时。
• 🧩 永久变形有时也被称为塑性变形。
• 🎨 “塑性”一词来源于希腊语“plastos”，意为“塑造或雕刻”。
• 💉 “塑性”不仅指材料类别，也用于描述如整形手术中的永久形变。
• 🏗️ 塑性变形在材料科学和工程学中广泛应用，用于描述金属、陶瓷和聚合物的形变过程。

Q & A

什么是弹性变形？

弹性变形指材料在外力作用下变形，但在移除外力后，材料可以恢复到其原始形状和尺寸。例如，橡皮筋拉伸后会回到原来的形状。

什么是塑性变形？

塑性变形是指材料在外力作用后无法恢复原状，发生永久变形。举例来说，金属弯曲后即使移除外力，也不会恢复原来的形状。

塑性变形的一个简单定义是什么？

塑性变形可以简单定义为在移除外力后，材料不能回到其原始尺寸和形状。

塑性变形与原子的位置变化有什么关系？

在塑性变形过程中，材料内部的原子会移动到新的平衡位置，这种变化是不可逆的。

应力-应变曲线中的线性区域代表了什么？

应力-应变曲线中的线性区域代表材料的弹性变形区域，在这一阶段，材料在移除外力后可以恢复原状。

塑性一词的来源是什么？

塑性一词来源于希腊语“plastos”，意思是“塑造”或“雕刻”，与塑性变形的概念一致。

塑性变形发生在哪个区域？

塑性变形通常发生在应力-应变曲线的线性区域结束的地方，这个点标志着弹性变形的结束和塑性变形的开始。

为什么塑性变形的定义比永久变形更加准确？

塑性变形不仅描述了材料无法恢复原状，还强调了原子层面的不可逆变化，因此比仅仅描述形变的永久变形更全面。

塑料与塑性变形有关系吗？

尽管“塑料”这个词经常用于指代聚合物，但在材料科学中，塑性变形指的是材料形变的性质，而不特指某一种材料类别。

应力-应变曲线的断裂点代表什么？

应力-应变曲线的断裂点代表材料在经历最大应力后无法承受更多负荷，最终断裂。

Outlines

00:00

📏 弹性变形与永久变形的比较

第一段介绍了弹性变形和永久变形的区别。以橡皮筋为例，描述了弹性物体在拉伸后可以恢复原状的特性，并比较了金属的变形行为。当施加的应力超过一定限度时，金属会发生永久变形，无法恢复到原始形状。通过对回形针的实验，解释了永久变形的概念，并引出了讨论应力-应变曲线和金属在该曲线下的表现。

05:01

🛠️ 塑性变形与其来源

第二段详细解释了塑性变形的概念及其与永久变形的关系。塑性变形是指物体在应力作用下发生不可逆变化的现象。虽然'塑性'一词常被误解为指代聚合物，但实际上它来源于希腊语'plastikos'，意为'可塑性'或'可雕刻性'。段落进一步解释了该词的词源，并提到了'整形手术'中使用的塑性一词的相同含义。

Keywords

💡弹性变形

弹性变形是指物体在外力作用下产生形变，但在卸载后能恢复到原来形状。在视频中，用橡皮筋和金属作为例子来说明弹性变形的特性。即物体在一定限度内被拉伸或压缩后能回到其原始尺寸。

💡永久变形

永久变形指物体在超出弹性限度的外力作用下，无法恢复到原始形状的现象。视频中的回形针在过度施力后，无法恢复原状，这就是永久变形的例子。永久变形是不可逆的，体现了材料在极端条件下的反应。

💡应力-应变曲线

应力-应变曲线展示了材料在受力下的表现，通常包括弹性区、屈服点和断裂点。在视频中，讲解者使用该曲线来展示金属材料的变形过程，包括线性弹性区和永久变形区域。

💡塑性变形

塑性变形是指材料在永久变形后的形变行为。视频解释了塑性一词来源于希腊语，意为“雕刻”或“塑造”，强调的是形变的不可逆性。塑性变形通常用于形容金属在超过弹性限度后发生的形变。

💡加载与卸载

加载与卸载分别指物体在外力作用下的变形和外力解除后的反应。在视频中，加载时回形针发生形变，而卸载后它无法恢复原状，这体现了永久变形的概念。

💡屈服点

屈服点是指材料从弹性变形转变为塑性变形的临界点。在应力-应变曲线上，屈服点标志着直线弹性区域的结束。视频中提到，屈服点附近的区域是永久变形开始的地方。

💡原子位置

原子位置指材料内部原子在受力变形时的移动。在塑性变形过程中，原子从原本的平衡位置移到新的平衡位置，导致材料无法恢复原始形状。视频中提到，永久变形的发生与原子移动到新的位置有关。

💡回弹

回弹是指材料在去除外力后恢复原来形状的能力。弹性变形中材料回弹至原始形状，而在永久变形中，材料失去了回弹能力。视频中通过回形针的实验展示了回弹和永久变形的对比。

💡弹性限度

弹性限度是指材料能够承受外力而不发生永久变形的最大应力。在视频中，讲解者展示了如何在弹性限度内对材料施加外力，一旦超过这个限度，材料将永久变形。

💡塑性（词源）

塑性这个词源自希腊语 'plastikos'，意为“可以塑造”。视频中解释了塑性一词不仅指塑料这种材料，而是指材料在永久变形中的形变能力，这一词义也体现在“整形手术”这一术语中。

Highlights

Elastic deformation is described using an elastic band example where the material returns to its original dimensions after being stretched.

Metals also exhibit elastic behavior, but at some point, permanent deformation occurs if overstressed.

A metal paper clip was used to demonstrate permanent deformation after exceeding its elastic limit.

The concept of permanent deformation is introduced, defined as when a material does not return to its original dimensions upon unloading.

Permanent deformation is explained as a process where the strain does not return to zero after the load is removed.

At the atomic level, permanent deformation occurs when atoms move to new equilibrium positions.

The region near the end of the linear section of the stress-strain curve is typically where permanent deformation begins.

Plastic deformation is introduced as the more commonly used term for permanent deformation.

The term 'plastic' in plastic deformation comes from the Greek word 'plastos,' meaning to shape or sculpt.

Plastic deformation refers to a permanent change in the material's shape, not specifically related to the material class (e.g., polymers).

Plastic surgery uses the same term 'plastic,' derived from the idea of shaping or sculpting anatomical structures.

Plastic surgery can involve both reconstructive and cosmetic procedures, but the term 'plastic' refers to shaping, not the materials used.

In reconstructive surgery, biodegradable materials like polylactic acid are used, but the term 'plastic' does not relate to these materials.

When discussing plastic deformation in materials science, it is important to differentiate between the lay public’s understanding of 'plastic' and its technical meaning.

The lecture concludes by reiterating that plastic deformation refers to permanent structural changes in materials, unrelated to their classification as polymers.

Transcripts

okay so let's look a little bit more

about at what happens after elastic

deformation so here's an elastic band we

stretch it out and we use this to

describe elastic behavior stretch it or

turn it back to its original dimensions

same thing it's less obvious immediately

it happens with a metal

now you can deform it a little bit and

take it off it's the same geometry is

whatever you started but at some point

you take it to too many sheets of paper

and inserts to form a little bit you

know that wasn't a pretty strong paper

clip it I thought it was going to deform

more obviously than that so let's say I

hit it with a big hammer kind of thing

and there you go look at that and that

gorgeous it that was too much

deformation it permanently deformed so

let's explore this in a little bit more

detail okay so here's what I want to do

I want to think about this pencil test

okay so we've got this stress-strain

curve and we're gonna look at at this

point metals okay metals and you'll

recall that we had a curve that looks

something like this linear elastic and

limit deform over like this and then it

fractured okay so what I want to do is

well how are we going to define this

permanent deformation and let's in fact

get up there here title permanent

deformation I think that's a fairly

descriptive title permanent in fact we

could elaborate on one of the possible

definition if we're going to come up

with definition it's kind of like we did

for elastic deformation what would

permanent deformation be well permanent

I mean tells us that and I think it

should be fairly obvious upon unloading

just think with this paper clip

I release the load and it did not go

back to

it's original paper put I destroyed that

paperclip so it's a upon upon no

unloading sample it does not return to

original dimensions and intuitively

that's a fairly good description in

practice we're gonna see that it's it's

a little insufficient but intuitively

it's not bad okay um what else could we

say then well another way of explaining

it really a restatement of the first

point there is that we'd say strain if

we just looked at the strain strain does

not return to zero upon unloading right

and finally if we want to be consistent

with what we we discussed for for

elastic deformation we could discuss

what happens to the atoms like the atoms

must move to new equilibrium positions

move to new positions right

they must I mean when I stretched this

poor little paperclip physically Shane

shaped macroscopically so I had to have

moved to atoms I certainly did I moved

to new equilibrium positions or new

positions in fact you know what I'm

gonna add another one because when we

were looking at this curve we I think

it's fairly intuitive to many

to you perhaps that somewhere in that

region you know near the end of the

straight line near the end of the

straight line is probably where it's

going to deform and that's actually

accurate near

and of linear region the straight line

is probably where this permanent

deformation is going to occur and that

is as I said that's accurate okay and

linear not all that practical it turns

out because it's very difficult to

determine where that straight line is

but these are all reasonable intuitive

descriptions for plastic deformation but

I keep throwing in the sort of plastic

so let's let's go over that right now so

permanent deformation is certainly this

this word that is intuitive to

understand and it's used sometimes but

more frequently what is used is this

term that I've thrown in a little bit

here and that's plastic let me give

myself a little bit of room okay

plastic plastic deformation and you may

you may look at that and think to

yourself why plastic does that refer to

a material class you know metal ceramics

and polymers and in fact I it's

certainly misleading in a way because

the lay public uses the term plastic to

mean polymer but actually what plastic

means is something a little different it

refers to the deformation and you can

impress your friends at your next party

by the etymology of the word plastic in

in this context and it is in fact from

the Greek plus Tico's all right there

you go you'll be extremely popular at

the parties and you say that plastic

owes right it comes from tests eCos

which meaning means to shape or sculpt

right permanently and if if this is not

exciting enough for you that is if if

you know a stress-strain curve and

permanent deformation there it's the

it's the same plastic as in plastic

surgery

so plastic surgery is the branch of

stuffs surgery medical specialty

involved are so concerned with changing

structures anatomical structures whether

for reconstructive purposes or elective

cosmetic purposes they certainly do use

some polymeric materials important are

polymers some biodegradable poly lactic

and stuff for example in reconstructive

surgery for sutures and things like that

but that's not where the term plastic

comes from plastic comes from a Greek

custom coast there you go so impress

your friends and certainly when you hear

me say plastic in this course you'll

know that I'm not referring to a

material class but instead I'm referring

to this permanent deformation