《材料力学 一》第二章 拉伸、压缩与剪切
Engineering Mechanics
第二章
Engineering Mechanics
§2-1轴向拉压的概念及实例
(Concepts and examples of axial tension &第二章拉伸、压缩与剪切Chapter2Axial Tension and Compression §
(Calculation of internal force)
§
(Stress and strength condition)
Engineering Mechanics
§2-4 材料在拉伸和压缩时的力学性能(Material properties in axial tension and compression)
§2-5拉压杆的变形计算
(Calculation of axial deformation)
§
axially loaded members)
§
calculation for shear and bearing)Engineering Mechanics
§2-1轴向拉压的概念及实例(Concepts and example problems of axial tension & compression)
一、工程实例
(Engineering examples)
Engineering Mechanics Engineering Mechanics
内
燃
机
的
Engineering Mechanics 三、变形特点(Character of deformation)
二、受力特点(Character of external force)
外力的合力作用线与杆的轴线重合。
F
轴向压缩(axial compression)
轴向拉伸(axial tension)
Engineering Mechanics
一、求内力(Calculating internal force)
§2–2内力计算
(Calculation of internal force)
Engineering Mechanics 在求内力的截面m -m 1.截面法(Method of sections)(1)截开
m
F
F
Engineering Mechanics
对研究对象列平衡方程F N = F
(3)平衡
m
m
F
F
Engineering Mechanics 若取右侧为研究对象,则在截开面上的轴m
m
F
F
m
Engineering Mechanics
2.轴力符号的规定
(Sign convention for axial force)m
F
F
m
Engineering Mechanics 二、轴力图(Axial force diagram)
用平行于杆轴线的坐标表示横截面的位置,用垂直于杆轴线的坐标表示横截面上的轴力数值,从而绘出表示轴力与横截面位Engineering Mechanics
例题1 一等直杆其受力情况如图所示,作杆的轴力图.
A
Engineering Mechanics A
解:求支座反力
kN
100
202555400
R R ==+-+--=?A A x F F F Engineering Mechanics
求AB 段内的轴力
40kN 55kN 25kN 20kN F R A
Engineering Mechanics 求BC 段内的轴力
20kN C
A
B
D
E
40kN 55kN 25kN F R A
Engineering Mechanics
求CD 段内的轴力
40kN 55kN 25kN 20kN F R A
Engineering Mechanics 求DE 段内的轴力
40kN
55kN 25kN
20kN
F R A
Engineering Mechanics
F =10kN (拉力)C
A
B
D 600
300500
400
E
40kN
55kN 25kN 20kN Engineering Mechanics C
A
B
D E
40kN
55kN 25kN 20kN
F N1=10kN (拉力)F N2=50kN (拉力)F = -5kN (压力)轴力的计算规律:
Engineering Mechanics
§2-3应力及强度条件(Stress and strength condition)
一、横截面上的正应力(Normal stress on cross section)
Engineering Mechanics 2.平面假设(Plane assumption)
变形前原为平面的横截面,在变形后仍保持为平面,且仍垂直于轴线.
Engineering Mechanics
4.正应力公式(Formula for normal stress)
A
F N =
s
Engineering Mechanics A
F N
=
s 的适用条件:1、只适用于轴向拉伸与压缩杆件,杆件受力区域稍远处的横截面。即外力的合力作用线与杆件的轴线重合。
a /2
Engineering Mechanics
讨论:如图所示两根杆件的正应力分布情况。
F
F
()()
F x A x s =
Engineering Mechanics 圣维南原理:
力作用于杆端的方式不同,但只圣维南像
只适用于离杆件受力区域稍远处的横截面。Engineering Mechanics
力作用方式不同产生的影响
Engineering Mechanics F
k
F
二、斜截面上的应力(Stress on an inclined plane)
1. 斜截面上的应力(Stress on an inclined plane )
以p α表示斜截面k -k 上的Engineering Mechanics
沿截面法线方向的正应力s a
沿截面切线方向的切应力t a
a
将应力p α分解为两个分量:2cos cos p a a s a s a
=×=sin sin22
p a a s
a a
=×=p α
F
k
k
a
F
F
k
k
x
n
a
p α
s a
t a
Engineering Mechanics (1)α角2.符号的规定(Sign convention)F
k
k
a
F
逆时针时a 为正号自x 转向n
a
t a
Engineering Mechanics
讨论
2cos cos p a a s a s a
=×=s
Engineering Mechanics 例题1 杆OD 左端固定,受力如图,OC 段的横截面面积是CD 段横截面面积A 的2倍。求杆内最大轴力,最大正应力,最大切应力及其所在位置。
3F
4F
B C Engineering Mechanics
1、作轴力图
3F
2F
F
F
F N 3=\max (在OB 段)
O
3F
4F
2F
B C D +
-
+
Engineering Mechanics 2、分段求max
s ,OB N OB A
F
A F 232==
s F
F 23F
2F
F
F N
+
-
+
Engineering Mechanics
三、强度条件(Strength condition)
杆内的最大工作应力不超过材料的许用应力1.数学表达式(Mathematical formula)
max N
Engineering Mechanics 例题2 一横截面为正方形的砖柱分上、下两段,其受力情况、各段长度及横截面面积如图所示.已知F = 50kN ,F
A
C
F
F
3000
240
1
Engineering Mechanics
F A
B
C
F
F
3000240
1
50kN
(2)求应力
..N 24
024050000
111=′-==
A F s Engineering Mechanics 例题3 简易起重设备中,AC 杆由两根80′80′7等边角钢组成,A
B 杆由两根10号工字钢组成. 材料为Q235钢,许用应力[s ]=170MPa. 求许可荷载[F ].
B
Engineering Mechanics
B
C
m
y
F N1
Engineering Mechanics 结点A 的平衡方程为
A
x
y
F N1
03001=-=?F F F y o sin N 00
=-=?o
cos30F F F Engineering Mechanics
(2)许可轴力为
A
F ][max N s £F F 21N =kN .][][N 2436911==A F s s
Engineering Mechanics 例题4 刚性杆ACB 有圆杆CD 悬挂在C 点,B 端作用集中力F =25kN,已知CD 杆的直径d =20mm,许用应力[s ]=160MPa ,试校核CD 杆的强度,并求:
A
D
Engineering Mechanics
解:
(1)求CD 杆的内力
F
A
D
C
F F M CD
A 2
30
N ==?Engineering Mechanics [F ]=33.5kN
F
A
D
C
B
2
3F A F CD =
£][N s 得
(3)若F =50kN ,设计CD 杆的直径
Engineering Mechanics
§2-4材料在拉伸和压缩时的力学性能
材料的力学性能——材料在外力作用下表现出来的变形、破坏
等方面的特性。
Engineering Mechanics 实验试件:(a)圆截面标准试件:或d l 10=d l 5=矩形截面标准试件(截面积为A ):或A l 3.11=l .5=金属材料非金属材料
Engineering Mechanics
2.试验设备(Test instruments)(1)微机控制电子万能试验机(2)引伸计
Engineering Mechanics 3.实验原理
Engineering Mechanics
二、拉伸试验(Tensile tests)
1. 低碳钢拉伸时的力学性质——含炭量在0.25%以下的碳素钢。(Mechanical properties for a low -carbon steel in tension)d
l
标距
Engineering Mechanics (2)拉伸图( F -D l 曲线)
拉伸图与试样的尺寸有关.为了消除试样尺寸的影响,把拉力F 除以试样的原始面积A ,得正应力;同时把D l 除以标距的原始长度l ,得到应变.
表示F 和D l 关系的曲线,称为拉伸图(tension diagram )
F
O
e
f
h a
b c d d ′g
f ′Δl 0
Engineering Mechanics
(3)应力应变图
表示应力和应变关系的曲线,称为应力-应变图(stress -strain diagram)s
f
Engineering Mechanics b 点是弹性阶段的最高点.
弹性极限
(elastic limit)e
s s
f
a
b c 45
Engineering Mechanics
(c )强化阶段
过屈服阶段后,材料又恢复了抵抗变形的能力,要使它s
f
c e
Engineering Mechanics (d )局部变形阶段
过e 点后,试样在某一段内的横截面面积显箸地收缩,出现颈缩(necking)现象,一s s
f
c e
Engineering Mechanics
试样拉断后,弹性变形消失,塑性变形保留,试样的长度由l 变为l 1,横截面面积原为A ,断口处的最小横截面面积为A 1 .
(4)伸长率和断面收缩率
Engineering Mechanics b
f
d
e
s
Engineering Mechanics
(5)卸载定律
卸载定律(unloading law)
若加载到强化阶段的某一点d s
c
e
f
d Engineering Mechanics 实验表象
参考值
四个阶段1、同时存在塑性和弹性变形;
屈服极限:σs
1、只有弹性变形;
2、有符合胡克定律σ=E ε的线性阶段;
3、试样无明显表象。
比例极限:σp 弹性极限:σe 弹性阶段
(段)
oa 、变形多集中在横截面积迅速收缩的某一小强度极限:σb
滑移线颈缩
Engineering Mechanics
四个阶段试件的变化:
形状为杯锥状。
Engineering Mechanics 其它形状断面
Engineering Mechanics
三、材料压缩时的力学性能(Mechanical properties of materials in axial compression)
1.实验试样(Test specimen)
d
h
F
F
Engineering Mechanics s
压缩的实验结果表明
低碳钢压缩时的弹性模量E 屈服极限s s 都与拉Engineering Mechanics
3.铸铁压缩时的s -e 曲线(Stress -strain curve for cast iron in compression)
s 铸铁压缩时破坏断面与横截面大Engineering Mechanics 1. 极限应力(Ultimate stress)
四、安全因数和许用应力
(Factor of safety & allowable stress)
材料的两个强度指标s s 和s b 称作极限应力或危险应力,b
n Engineering Mechanics
五、应力集中(Stress concentrations)
应力集中(stress concentrations).
F s F
max
s
Engineering Mechanics Engineering Mechanics
?应力集中因数(stress -concentration factor )
max
s s max
=K Engineering Mechanics 几点说明:
(3)可以利用应力集中达到构件较易断裂的目的。(4)不同材料与受力情况对于应力集中的敏感程度不同。
(1)截面尺寸改变越急剧,角越尖,孔越小,应力
集中的程度越严重。
(2)在构件上开孔、开槽时采用圆形、椭圆或带圆
角的,避免或禁开方形及带尖角的孔槽,在截面改变处尽量采用光滑连接等。
Engineering Mechanics
(a )静载荷作用下:
塑性材料所制成的构件对应力集中的敏感程度较小;
s s s s s
s Engineering Mechanics 脆性材料所制成的构件必须要考虑应力集中的影响。即当达到时,该处首先产生破坏。
max s b s b
s Engineering Mechanics
§2-5拉压杆的变形计算
(Calculation of axial deformation)
F
b h
h 1
b 1
l
l 1
l
Engineering Mechanics
F F
b
h h1
b
1 l
l
1Engineering Mechanics
四、胡克定律(Hooke’s law)
实验表明工程上大多数材料都有一个弹性阶段,在此弹性范围内,正应力与线应变成正比.
Engineering Mechanics
§2-6拉压超静定问题
(Statically indeterminate problem of axially loaded members) Engineering Mechanics
1.超静定的次数(Degrees of statically indeterminate problem )未知力数超过独立平衡方程数的数目,称作超静定的次数.二、超静定问题求解方法(Solution methods for statically indeterminate problem)
(4)联立补充方程与静力平衡方程求解
Engineering Mechanics
例题8 设1,2,3 三杆用绞链连结如图所示,l
1= l
2
= l,A
1
= A
2
= A,E
1= E
2
= E,3杆的长度l
3
,横截面面积A
3
,弹性模量
C
B D
三、一般超静定问题举例
(Examples for general statically indeterminate problem)Engineering Mechanics
C
B D
a a
12
3
x
y
F
N2
F
N3
F
N1
a a
C
B D
a a
12
3
Engineering Mechanics a
a A
1
2
3
a C B D a a
1
2
3C B D a a
1
2
31
l Δ3
3Engineering Mechanics
(4)联立平衡方程与补充方程求解
C
B D a a
1
2
3A'
a
cos A E 2
1N N F F =0
321=-++F F F F N N N cos cos a a Engineering Mechanics 例题9 图示平行杆系1、2、3悬吊着刚性横梁AB ,在横梁上作用着荷载F 。各杆的截面积、长度、弹性模量均相同,分别为A ,l ,E .
试求三杆的轴力F , F , F .
F
Engineering Mechanics
A
B
C
3a
a
l
21解:(1)平衡方程
?=0x F 0
=x F ?=0F 0
Engineering Mechanics A
B
C
3a
a
l
21A ¢B ¢
C ¢
l D 3
l D 2
l D 1
A B
C 321Engineering Mechanics
A
B
C
3a
a
l
21A ¢
B ¢
C ¢
l D 3
l D 2
l D 1
A
B
C 321
Engineering Mechanics 四、装配应力(Initial stresses)(Statically indeterminate structure with a misfit)
B
C
D
2
1
3
Engineering Mechanics
B
C
D
a
a 2
1
3
l
3l Δ代表杆3的伸长
1Δl 代表杆1或杆2的缩短
D 代表装配后A 点的位移
3
31
1A E A E Engineering Mechanics (3)补充方程
B
C
D
a
a 2
1
3
l
d a
=+21133cos N1N3A E l
F A E l F (4)平衡方程
Engineering Mechanics
例题10 两铸件用两根钢杆1. 2 连接,其间距为l =200mm. 现要
将制造得过长了D e =0.11mm 的铜杆3 装入铸件之间,并保持三根杆的轴线平行且等间距a ,试计算各杆内的装配应力. 已知:钢杆
直径d =10mm,铜杆横截面积为20′30mm 的矩形,钢的弹性模量A
B C
B A
C 3
C 1
C'D e
Engineering Mechanics )变形几何方程为
C 1
C''
D l 3
B C 1
B 1
C 1A
D l 1D l 2
=Engineering Mechanics
(3)补充方程
EA
l F l 11N1Δ=
3
33A E l F l N3Δ=
(2)物理方程
C'A'
B'F N1即可得装配内力,进而求出
Engineering Mechanics 五、温度应力(Thermal stresses or temperature stresses)
温度变化将引起物体的膨胀或收缩.静定结构可以自由变形,不会引起构件的内力,但在超静定结构中变形将受到部分或全部约束,温度变化时往往就要引起内力,与之相对应的应力A
B
l
Engineering Mechanics
解:这是一次超静定问题
变形相容条件是杆的总长度不变.
A
D l T
A
B
l
B'0
=l ΔA B
F R B
Engineering Mechanics (1)变形几何方程
A
B
l
A
D l T
=-=F T l l l ΔΔΔl
F l B R Δ=l
T l ×=ΔΔa (2)物理方程
T
E A
l T ××==
B'
A B
Engineering Mechanics
一、基本概念和实例(Basic concepts and examples)
§2-7剪切和挤压的实用计算
(Practical calculation for shear and bearing)
F
Engineering Mechanics m
齿轮(gear)
(3)键块联接(Keyed connection)(4)销轴联接(Pinned connection)
F
B
Engineering Mechanics
(合力)
2.受力特点(Character of external force)以铆钉为例
构件受两组大小相等、方向
Engineering Mechanics 4.连接处破坏的几种形式:
(Several types of failure in connections)(1)剪切破坏沿铆钉的剪切面剪断,如沿n -n 面剪断.
n
n (合力)
F F
F
Engineering Mechanics
4.连接处破坏的几种形式:
(Several types of failure in connections)(2)挤压破坏铆钉与钢板在相互接触面上因挤压而使n
n (合力)
F
F
Engineering Mechanics 4.连接处破坏的几种形式:
(Several types of failure in connections)(3)拉伸破坏钢板在受铆钉孔削弱的截面处,应力增大,易在连接处拉断.
n
n (合力)
F F
Engineering Mechanics
4.连接处破坏的几种形式:
(Several types of failure in connections)(4)剪豁破坏当钢板或耳片的边距不够时,有时会n
n (合力)
F
F
Engineering Mechanics F S
二、剪切的应力分析(Analysis of shearing stress)
1.内力计算(Calculation of internal force)
F
m
m
S =-=?F F F x Engineering Mechanics
3.强度条件(Strength condition)
F
F
m
m
[]
t t £=
A
F S
Engineering Mechanics 螺栓与钢板相互接触的侧面上,发生的彼此间的局部承三、挤压的应力分析
(Analysis of bearing stress)
F
F
Engineering Mechanics
(1)螺栓压扁
(2)钢板在孔缘压成椭圆2.挤压破坏的两种形式(Two types of bearing failure)
F
F
[s bs ]-许用挤压应力(allowable bearing stress)
Engineering Mechanics (1)当接触面为圆柱面时, 挤压面积
A bs 为实际接触面在直径平面上的投影面积
h
实际接触面
挤压面的面积计算
Engineering Mechanics
四、强度条件的应用(Application of strength conditions)
[]
t t £[]
bs bs s s £(Check the intensity)
1.校核强度
u
Engineering Mechanics 例题12 齿轮与轴由平键连接,已知轴的直径d =70mm,键的尺寸为b ×h ×L =20 ×12 ×100mm,传递的扭转力偶矩M e =2kN ·m,键的许用切应力为[t ]= 60MPa ,许用挤压应力为[s bs ]= 100MPa.试校核键的强度.
b
M e
d
M e
h
2
1070′d Engineering Mechanics
(2)校核剪切强度
F
F =S M e
d
F
b
h l
A
Engineering Mechanics 例题13 一销钉连接如图所示,已知外力F =18kN,被连接的构件A 和B 的厚度分别为d =8mm 和
d =5mm ,销钉直径d =15mm ,F
Engineering Mechanics
F
解: (1)销钉受力如图b 所示
F
剪切面
Engineering Mechanics d
F
(2)校核剪切强度
2
S F F =
由截面法得两个面上的剪力
2
d p bs d d
A 剪切面
Engineering Mechanics
D
h
(1)销钉的剪切面面积A
(2)销钉的挤压面面积A bs
思考题
Engineering Mechanics 挤压面
D
h
挤压面
d
d
A =Engineering Mechanics
例14 冲床的最大冲压力F =400kN,冲头材料的许用压应力[s ]=440MPa,钢板的剪切强度极限t u =360MPa,试求冲头能冲剪的最小孔径d 和最大的钢板厚度d .
F
Engineering Mechanics
F
冲头
d
钢板
F
F
4
d
A p Engineering Mechanics
F
冲头
d
钢板
F
F
d
p d
A
Engineering Mechanics
例题15 一铆钉接头用四个铆钉连接两块钢板. 钢板与铆钉材料相同. 铆钉直径d=16mm,钢板的尺寸为b=100mm,
d=10mm,F= 90kN,铆钉的许用应力是[t] =120MPa,
[s] =120MPa,钢板的许用拉应力[s]=160MPa. 试校核铆钉Engineering Mechanics
F
F d
d
Engineering Mechanics
F F
b
F/4
F/4
4
d
A p Engineering Mechanics
(2)校核铆钉的挤压强度
每个铆钉受挤压力为F/4
F/4
F/4
挤压面
[]
===£
bs bs
bs
4
141MPa
s s
d
F F
A d