close

Вход

Забыли?

вход по аккаунту

код для вставкиСкачать
Lecture #5
Thin-walled structures.
Normal stresses
FOREWORD
Modern aircraft structures are usually thin-walled
beams (shell beams).
“Thin-walled” means that one of dimensions
(thickness) is much smaller than others. Thus, the
middle surface could be specified.
Shells could be stiffened and not stiffened.
Stiffeners are used to:
a) support the skin from buckling;
b) optimize the stress state.
2
TYPES OF SHELL
General shell
The stress state is formed
by membrane and bending
stresses
Less effective
General case
Membrane shell
Stresses are uniformly
distributed along the
thickness (only membrane
stresses)
Most effective
Hard to realize in practice,
but typical for thin shells
3
TYPES OF SHELL
Vertical displacement plot
Not stiffened shell
Stiffened shell
4
TYPES OF SHELL
Bending stress plot
Not stiffened shell
Stiffened shell
5
TRUSS STIFFENING OF THE SKIN
Junkers F.13,
Germany, 1920
6
GEODESIC STIFFENING OF THE SKIN AT FUSELAGE
Vickers Wellington,
Great Britain, 1938
7
THIN-WALLED STRUCTURES (modern design)
8
THIN-WALLED STRUCTURES (modern design)
9
STRUCTURAL MEMBERS
Longitudinal
Lateral
Wing
Spar caps, stringers
and skin (panel)
Ribs
Fuselage
Stringers and skin
(panel), beams
Frames
10
HISTORICAL PROGRESS OF AIRCRAFT STRUCTURES
1903-1920. Truss structures, unstressed skin
The skin is not much stressed
11
HISTORICAL PROGRESS OF AIRCRAFT STRUCTURES
1920-1930. Monoplanes and corrugated skin introduced
The skin carries only shear stresses
Tupolev TB-3, Soviet Union, 1932
Take-off mass 19 500 kg, wingspan 39.5 m
12
HISTORICAL PROGRESS OF AIRCRAFT STRUCTURES
1930-1940. Aluminium extensively used, stressed skin.
The skin carries both normal and shear stresses
Messerschmitt Bf.109, Germany, 1935
Take-off mass 3 375 kg,
max. speed 720 km/h
13
NORMAL STRESSES IN THIN-WALLED BEAMS
The distribution of normal stresses obeys the
hypothesis of planar cross sections:
w  x, y  a  b  x  c  y
For the case of uniform linear material, it comes to be:
 z  x, y 
Nz
A

M
Iy
y
x
M
Ix
x
y
14
CROSS SECTION DISCRETIZATION
The discretization of real cross section is usually used
to possess the calculations of moments of inertia and
other geometrical properties:
- small but complex elements like stringers are
substituted by point areas;
- skins are substituted by center lines;
- complex center line is substituted by polygonal
curve.
15
CROSS SECTION DISCRETIZATION
The problem is to find the moment of inertia.
Dimensions:
a = 60 mm; h = 22 mm; d1 = 4 mm; d2 = 6 mm;
H = 120 mm.
16
CROSS SECTION DISCRETIZATION
One option is to
substitute the
real cross
section by center
lines with
appropriate
thicknesses.
Result:
1000 cm4
(exact value:
975 cm4;
2.5% error).
17
CROSS SECTION DISCRETIZATION
Another option is
to use
concentrated
areas instead of
stiffeners and
webs.
Result:
997 cm4
(exact value:
975 cm4;
2.5% error).
18
WHERE TO FIND MORE INFORMATION?
Megson. An Introduction to Aircraft Structural Analysis. 2010
Chapter 15
… Internet is boundless …
19
TOPIC OF THE NEXT LECTURE
Normal stresses.
Method of reduction coefficients
All materials of our course are available
at department website k102.khai.edu
1. Go to the page “Библиотека”
2. Press “Structural Mechanics (lecturer Vakulenko S.V.)”
20
1/--страниц
Пожаловаться на содержимое документа