bài giảng môn sức bền vật liệu




 !"#$#"%&'(
Chương 1:CÁC KHÁI NIỆM
)*+,-
I. Nhiệm vụ và đối tượng nghiên cứu
1 Nhiệm vụ
./.012134560.70892::5;:7
./<=20>;?
./<=.@AB0C;?
'<D;E4@0FG9;*;:7:5;/H;=
IJ;:7:5;E;/;K3LMN?9O8.7<MK7P
;9392202Q50;1R;1R;1H;=?9012<34
560.708
STIUEL
IUEL?97<M03V.@A7?8.09
&9DJK/QCW592>M5C2X
Y@Q76V9?9995?99C762Z<KC3P
9
6V9?993P3P98.U;6357<4?99
STS0/07
&971J@EA539@57142ZYK.@43[98D
@8J35/<9M9553,F7?9K<N3L71;5A;\5∆
35/<9R[42Z<<KC;\5∆S

TS&:3,
TSSI=+9
&2E2]:00@PL5R0@P5/:5M0S
TSTS)F5A
&:3,^7_23R_2FUR757W232FU

T
II Ngoại lực Nội lực và ứng suất
1 Ngoại lực
1.1Định nghĩa
5A_D_0;1`7<3\L598`/0L;9ab
1.2 Phân loại
5A_^7
&:3,;c.3C
):_20MA0L/. 
2 Nội lực
Định nghĩa
dC0@P?95A_J=.@ARD902*e?9a]L7
2*_0@Pf;JUA0@P?95A_S)*_;K,1_
2 Phương pháp mặt cắt
X
J9=205A41_FGC5A_S"4.;J/:5M01_9
@g220276VM9
"=_N3A0FG
IJ471_A9NE@g762Z
π
B9V39792*hR.
ib2*hFG@C0@P?905A_
1 2
,P P
→ →
_0@Pf`0
2*01_
1_2FULP3L@QY?976V
C
B

A
P3
Pn
P1
P2
A
P1
P2
F
3. Ứng suất
\;1?91_A71;J75;K3L76V;E,M]
j
&35Q509\2FM]52*
P
→
39792*
P
∆Ρ
C
σ
τ
P
1
P
2
σ
τ
P
P3
Pn

A B
&2*<KC76V,M]202R
σ
-M]202
&2*G73576V,M].2R
τ
-M].2
2 2
P
σ τ
= +

4. Các thành phàn nội lực trên mặt cắt ngang
'_@,kl
'_VmaRm8l
<7WUaR8l
<7Wa5VkS
n
Nz
Qy
My
X
Y
Qx
Mx
Mz
 
nS.@A
nSSb5b
1_N76V9;C71_@,

k
nSTSV3ER@2
1_N76V9;C71_9m
8
nSXSi5V
1_N76V9;C71o_K7<7WkG73576V
p
nSjS(U
Uốn thuần tuý-1_N76V9;C71o
_K7<7Waq568rS(ốn ngang-m8RaqmaR8r
pS0:.:
pSS&Q;^?9J
"3V;E,đàn hồi q983sRđàn hồi tuyệt đốirnếu có khả
năng phục hồi hoàn toàn hình dạng và kích thước vốn có sau khi ngoại lực thôi tác dụng,
biến dạng được khôi phục hoàn toàn sau khi hết ngoại lực được gọi là biến dạng đàn hồi.
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Bài 2
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2 Ứng suất trên mặt cắt ngang
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"83L76V9?99uK2*M]202…
k
[2*
M].2G/<S
3 Biểu thức liên hệ giữa ứng suất pháp và lực dọc
n
<QM]202
F
N
z
nk
±=
R

σ


,k n
s
-/†7
T

k
-03=_@,A76V;9ab-
Y-@Q76V9-7
T
R7
T
σ
@]qzr9=/b5
@]qtr9=b
4"| dQ
b

S
T
S b h
=

h
B
S
T
B b

S h
+
=
R R
T
S
x
d
S
π
=
p
b
SS b h
=
d
2
4
d
S
π
=
4 Biến dạng tính độ giãn dài của thanh :
4.1 Biến dạng
1
n
zi i
i
i i
N l

l
E F
=
´
D =
´
å
qxtXr
l
∆
-';1@c@
-@9;*?99
-I5A
‡-<;;^?9//b5ˆb
-&3=MU?9_@,
&QMU‡SY,;1?99//b598b
l
l
z
∆
=
ε
qxtjr
z
ε
-I1.@A@,;U
w
4.2 Định luật Húc
&352A7.@A;^?9RM]/b5bqbruC.
@A;U

ε
εσ
SE
=
5 Điều kiện bền
5.1 Ứng suất cho phép
O
[ ]
σ
IJ;:7:59535_.\9\Me@P7103=M]>
M]8J7,M]52b2R
0
n
σ
σ
 
=
 
qxtnr
€
σ
M]8J7
‰-MU95
z@ƒ5-
[ ] [ ] [ ]
n
ch
nk
σ
σσσ

===
"@[-$M]52b2/=b-
[ ]
n
b
n
n
σ
σ
=
n
b
n
σ
-CA/b

x

n
b
k
σ
$M]52b2/=/b5-
[ ]
n
b
k
k
σ
σ

=
n
b
k
σ
-CA//b5
5.2 Điều kiện bền của thanh chịu kéo nén đúng tâm
IUCF@ƒ5

max
z
N
F
σ σ
 
= ≤
 
qxtpr
IUC@[
[ ]
k
z
F
N
σσ
≤=
79a
qxtwr
79a
σ

-$M]/b5C]qQ3L2*@?9J;^r
[ ]
n
z
F
N
σσ
≤=
7
7
σ
-$M]b>]qQ3L2*F7?9J;^r
5.3 Bài toán cơ bản
1 Kiểm tra bền
max
z
N
F
 
= ± ≤
 
σ σ
2 Chọn diện tích mặt cắt
}
[ ]
σ
z
N
F
≥

&`@QYQ39/QC76V?99
3 Xác định tải trọng cho phép
.
,F
σ
 
 
R
[ ]
σ
S
79a
FN
z
≤

Bài tập ứng dụng
Bài 1
&9b2=0_)

•)
T
•T€l)
X
•j€K76V9/<;HY•€7
T

P
3
1m

1m
P
2
P
1
III
II I
2m
"|J;^_@,
TJ;^M]
X&Q.@A@,8;U?995‡•TS€
j
/†7
T
:
"|J;^_@,
€Š•

ŠT7
2
1 1
20 /
z
N P KN cm
= − = −

TŠ•
T
ŠX7
T€

2 1 2
2
20 20 0 /
z z
N N P
KN cm
= + =
− + =
XŠ•
X
Šj7
3 2 3
2
0 40 40 /
z z
N N P
KN cm
= +
= + =
P
3
1m
1m
P
2
P
1
III
II I
2m

20KN
40KN
T&QJ;^M]
2
1
1
20
2 /
10
z
N
KN cm
F
= = − = −
σ

2
2
2
0 /
z
N
KN cm
F
= =
σ
2
3
3
40

4 /
10
z
N
KN cm
F
= = =
σ
X&Q.@A@,
1
n
zi i
i
i i
N l
l
E F
=
´
D =
´
å
T
1 1
1 1
1
1 1
4
4
2 2

2
4
4
3 3
3
4
1 2 3
4
.l
2.2
2.10
2.10
.l
0.1
0
2.10
.l
4.1
2.10
2.10
( 2 0 2).10 0
z
N l
l
E F E
cm
l cm
E
l cm
E

l l l l
cm
s
s
s
-
-
-
´
D = = =
´
- =-
D = = =
D = = =
D =D +D +D =
- + + =
5.4 Bài toán áp dụng
59b2Z=_K/QC4.)

•T€/l)
T
•w€/lY
h
•
T7
T
lY

•j7
T


"|J;^_@,
k

TJ7399.K
[ ]
T
p€
MN
m
σ
=

X&Q.@A@,8;U‹R
.
n
T
TS€=
MN
E
m
TT
1m
1m
P
1
B
C
A
P

2
1
1
2
2
:
P
1
B
C
A
P
2
1
1
2
2
+
-
20KN
50KN
"|J;^_@,
IJ|J;^_@,
k
9@g220276V
I5Ah@g76VtRab2*3LK

k
•t)


•tT€SI5Ah=bS
I5A@g76VTtTRK

kT
•
k
z)
T
•ˆT€zw€•n€S
TX
I5A=/b5S
J;^
k
4
TJ739W5<
IH
Y

•T7
T
•TS€
tj
7
T
lY
T
•j7
T
•jS€
tj

7
T

max
z
N
F
 
= ± ≤
 
σ σ
X

j T
T€S€
€€
TS€
−
−
= − = −
σ
MN
m

X
T
j T
n€S€
Tn
jS€

−
−
= =
σ
MN
m
[ ]
79a
T T
Tn p€
MN MN
m m
σ σ
= < =
"89;:7:5;/
T&Q.@A@,

n
zi i
i
i i
N l
l
F E
=
×
∆ =
×
∑
z

N
F
σ
=
j
 

n
S
€€S
nS€
TS€
−
∆ = = − = −
σ
l
l m
E
Tj

j
T T
T
n
S S
TnS
pRTnS€
YS‡ TS€
−
∆ = = = =

σ
z
N l l
l m
E

 T
j j
qpRTn nrS€ RTnS€
− −
∆ = ∆ +∆ =
− =
l l l
m
T
59b2=_4|K0_)

•X€/l)
T
•n€/l)
X
•x€/l
Y

•TS€
tj
7
T
l
Y

X
•Y
T
•jS€
tj
7
T
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