Bruhn - Analysis And Design Of Flight Vehicles Structures Episode 3 part 12 pot

These so-called softer rivets have less shear strength, but since a great deal of aircraft construction involves thin sheet, bearing is often critical and thus rivet shear is not cri

and M, = +200 x 1.75 - 1000 x 25 = -600 in.1lb

and Py act through the 3olt will be reacted equally by each load on each bolt due to Py = 1b.; H load on each dolt due to

The load produced on saca bolt due to the

moment load My = -6C0 in.1b will vary directly

as the distance of the bolt from the canter of

resistance which coincides with the bolt group

cantroid,

the Let rg equal the distance from bolt

group centroid to bolt (a) Then the resisting

acaeant developed dy bolt (a) equals rarg, where

Fe equals the load on bolt (a) Since the bolt

ads are proportional to their distance from

Hence, I = Ir? = 4x 1.257 = 6.25 in

Therefore the moment load Fy on gach bolt W111 equal

Fig C4.31 shows the resulting H, V and moment loads applied to each bolt The resultant load can be found graphically by drawing the force polygons as showm in Fig Cl.23 The resultant bolt loads can likewise be determined analytically For example consider bolt (c)

TH = 250 + 120 x 1.25 ke + 0 = 3546 1b

iV = ~50 - 120 x + 0 5 ~122 1b,

1.25

Case where Bolts or Rivets are of Different Diameters

367 1b

when the joint bolts or rivets are not all the same size, the moment load on each bolt is proportional to the bolt area times its distance

to the bolt group centroid Thus the bolt areas must enter equation (15)

n = number of bolts of each size

Since theory of loads on 2 multiple bolt

group is only approximate, reascnable margins

of safety should be maintained

The Protruding Head type of rivet

(2) The Flush tyve rivet

Fig D1.24 illustrates the protruding head type of rivet ig 01.25

illustrates a number of modifications of the pro-

truding head type of rivet chat nave been used in ths past

=

Fig Di 24

For many years the round head rivet was

used for all interior work and before the era

of nign speeds it was used as 2 surface rivet

as well when wind tunnel experiments showed

that such rivets gave appreciable drag,

designers turned to rivets with less head

protrusion, thus the development of the 3razier

and modified Brazier type of rivet head Then

as the age of relatively high airplene speeds

rrived a flush surface was needed, particularl

on certain sensitive portions of the airplane

surface, thus various modifications of the

countersunk head involving press and machine

countersinking of the sheets wers developed

Pig D1.26 tliustrates the flush type of

rivet As fllustrated in Fig Dl.26, this

flush type rivet can be used in several differ-

ent ways, thus the method shown in Fig Bl.26a

is referred to as the machine countersunk type;

that in Fig Dl.26b as the oress countersunk

double dimpled type; and that in Fig Dl.26c as

the combined press and machine countersunk type

or the dimpled machine countersunk type

Approx Shset Limitations For countersunk Rivets (AN-426)

Approx Limitations For Press Countersunk

or Double Dimpled Rivets (AN-d26)

Fig DI 2T

459

DI, 16

DI.19 Rivet Materlal

Since aluminum alloy ts by far the most

widely used matertal in the aircraft industry,

it follows that aluminum alloy is the material

most Widely used for rivets Table D1.5

{column 1) lists the 5 aluminum alloys used

for rivets and the ultimate shearing stress

Fsy for each material Rivets made from

2017-13 (Foy “ 34000) and 2024-7381 (Fgy =

41000) are rivets that must be driven soon

after heat treatment or before age hardening

takes place The aging or hardening is slowed

oy Keeping rivets in refrigeration after heat

treatment The other rivet material 1s less

hard or less brittle in the aged state and thus

can be stored in air and driven anytime These

so-called softer rivets have less shear

strength, but since a great deal of aircraft

construction involves thin sheet, bearing is

often critical and thus rivet shear is not

critical Most surface or skin riveting

involves the softer rivet, usually 2117-T3

(Fey = 30,000)

D1 20 Strength of Rivets Protruding Head Type

Rivets are widely used in airplane

structures to fasten or tle together two or

more structural units Standard methods of

stress analyses of riveted joints consider two

primary types of failure, namely, the shear of

the shank of the rivet and the bearing or

compressive failure of the metal at the point

where the rivet bears against the connecting

Sheet or plate

Fig 01.28 illustrates the main forces on

a rivet in transferring a load from one plate

to another The load is transferred to the

rivet from the plate by bearing of the plate

on the rivet The load 18 then transferred

along the rivet and resisted by bearing action

on the other plate Since the plate bearing

forces on the rivet are not in the same line,

the forces tend to shear and bend the rivet

Bending of the rivet is usually neglected if

there are no intermediate filler plates In

Fig (a) o? D1.28, the rivet is in single shear,

whereas in Fig (b) the rivet is in double

The ultimate shear strength of a rivet is

Ziven by the following equation:-

BOLTED AND RIVETED

ultimate shear strength of rivets

nm = number of shear areas per rivet

Reference (17) shows that the shear strength of protruding head aluminum alloy rivets is affected

by increasing D/t (Diameter of rivet over sheet thickness) ratios The conclusions in Reference (17) are as follows:-

Rivets in Single Shear:- For values of D/t up ta 3:- Single shear strength = basic allowable single

of the D/t influence on the rivet shear strength Table D1.8 gives the allowable bearing strengths between the protruding head rivet and the various aluminum alloy sheet and Plate material The bearing values are given for two e/D ratios, namely 1.5 and 2.0, where

9 1S the edge distance measured from che center

of the hols to the edge of the plate Any reduction in edge distance may cause bulging

or the edge of the sheet due to driving energy

Edge distance should not be less than e/D = 1.5

D1,21 Strength of Rivets, Flush Type -

Since flush rivets

on the flush end of the flush riveting involves

or press countersinking or both, the strength of

the flush type rivet is different than the

common protruding head type

have no protruding head rivet and also since machine countersinking

ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES Fig D1.29 illustrates a machine counter-

sunk rivet Due to the 2ull P on the two

sheets ahich are held together by the rivet and

induced force Py 13 produced on the sloping

Side of the nead of the rivet This induced

force tends to shear and bend the portion 1-1

of the rivet head The sharp edge of the

countersunk Sheet at point (a) tends to cut

into the rivet These combined influences tend

to cause excessive deflections and finally

fatlure as roughly illustrated in Fig D1.30

II AE

Ttg DI.30

In the press countersunk or dimpled type

of flush rivet connection, see Fig D1.26b and

%, because of the interlocking of the sheets

due to the dimple, the joint could transmit a

load without a rivet if the sheets were held

together Since there is no clearly defined

bearing or shear surface in this type of joint,

the manner in which the loads are transferred

is quite complex As a result resort must be

made to tests to establish design allowables

Tables D1.?, D1.11 and D1.12 give the ultimate

and yield strength of flush type rivets

(Ref 2)

D1 22 Blind Rivets,

The name "Blind" rivet is given to that type of rivet which can be completely installed

from one side of the joint, and is therefore

almost exclusively used where it 1s impossibla

or impractical to drive the normal rivet, which

requires access to both sides of the joint,

There are two general types of blind rivets,

namely where the inside or blind head is formed

mechanically or where it is formed by an

D117

Ww

Inserted Tnstalled Fig D1 32 Jo Bolt

INSTALLED

ARAZIER HEAD (6951)

INSERTED WSTALLED Fig D1.34 Deutch Type

WSSSy

INSTALLED

Fig D1.34a Huck Lock Bolt

D1, 23 Riveted Sheet Splice Information

In splicing or connecting two sheets to- gether by means of rivets or bolts, the joint

or connection may fail in the various ways as explained in detail for single and muitiple bolt fitting units Thus one must check the shear strength of the rivets; bearing of rivets on the sheets; tear out of the sheet edges and tension

on sections through the rivet holes

Types of Sheet Splices or Connections

Fig D1.35 tllustrates the various types

of sheet splices In the offset lap splice between two sheets of different gauges, the of2set should be 1n the heavier matertal

a single shear butt splice, the butt splice plate should be equal to the thinnest of the two sheets being spliced and Likewise in the

For

_~ oo oy

rivets The com

5/32, 3/16 and i/ meters

sizes Snould not be used Sheet sclices

unless there Dacking uD structure as

buckls under the driving of the

a structural desisn s sint is one in whict

gest oractical size

Spacing Shest Sdge Distances

The allowadls

given in (Rez 1)

of two diameters Therefore in general

cistance in a joint should be less thân 2

diameters for protruding aead rivets, and

The larger

1 Strength ind Dearing sheets are practically

relative to ven splice, because derations usually

sine rivet should not be nat given

in Tables 4 and 8,

Table A

In gener ral the minima rivet row spacing should be such as co made the distance detveen any two rivets in the two rows not less than the minimun rivet spac the rivet size being used

Splice Sheet Tension =Erfi wnen a sheet is spliced by mean:

the sheet is weakened since the rivet holes cut away a part of the sheet malerial Tne ratio of the tension strength of the spliced sheet to the unspliced sheet ts called the sheet tension efficlency of the joint [If the minimum rivet spacing is used ani only one row of rivets the sheet efficiency will be around 70 to 75 sercent The designer should strive for a higher efficiancy

D1, 24 Tlustrative Problems Involving Use of Rivets

ANALYSIS AND DESIGN OF FLIGHT VEHICLE

mital cable pull of 400 1b, can

an equivalant force system at

@ 0i aterline o? the tubs, consisting of a

rsional moment of 400 x 5 = 2000 in.1lb., and

Load per rivet due to

388 1b, @ the rivets are in double shear

the shear strength of one rivet would 5e 2 x

s88 = 778 15 Referring to the table at the

bottom of Table DL.5, we find a rivet factor

a2 935 to apply for a 1/é rivet on 963 sheet

th TheraZore rivet strength ts 935 x

The rivets gre in singla shear

shear strength 2f 2 5/32 rivet from Table 91, 5

= 596 x ,99S = 594 12 (The value of the correction from middls

M.S = (594/334) -2 = 179 Bearing strength on 96) Table D1.S for 050 thickne:

ts 795 1D correcting to 043

730 For £2024-T3 tubs mater

we obtain a material factor of 1 Ther3 fore Dear strangth of one rivet on Subs wall is 1.24 X 780 = 967 1b,

M.S = (967/334) -l1 = 1.9

PhosLz

Fig D1.37 shows a plate fittt

to a double channel section by 6 - rivets The design fitting loads are show in the figure The rivetsd connection will be checked for strength under tha given design fitting loads

Solutton:-

The given force system will be replaced by

an equivalent force system acting at the center

of gravity of the rivet group This force

system will consist of:

Table D1.5 Shear Strengths of Protruding and Flush-

BOLTED AND RIVETED

Head Aluminum-Alloy Rivets

192 0.714

883 818 985 883 874 8đ85 1.000 1.000

NơtE: Values of shear strength should be multiplied by

the factors given herein whenever the D/¢ ratio is large enough

to require such s correction

Shear values are based on sress corresponding to the

nominal hole diameters specified in table 8.1.1.11(đ), note «

* The -T31 designation refers to rivets that hava been heat-treated and then maintained in the heat-treated condition until driving,

Shear stresses in table 8.1.1.11(đ} corresponding ta 90 percent probability data are used wherever available, Sheet thiekness is that of the thinnest sheet in single-shear joints and the middle sheet in doubleshear joints

Nominal hole diameter, (in.) ) 0.067 | 0.096 | 0.1285 | 0.159 | 0.191 | 0.257 | 0.323 | 0.386

Table D1.7 Ultimate and Yield Strengths of Sotid 100° Machine-Countersunk Rivets

17583 | 1,925

Nore: The values in thia table are based on “good” manu-

facturing practice, and any deviation from this will produce

significantly reduced values

ở Sheet gage is that of the countersunk sheet In cases

where the lower sheet is thinner than the upper, the sbear-

bearing allowable for the lower sheet-rivet combination shouid

be computed

2 Increased attention should be psid to detail design in cases

where D/t>>4.0 becnuse of possibly greater incidence of dif-

fieulty in ‘service

¢ Xleld values of the sheet-rivet combinations are ‘ese than 2/3 of the indicated uitimste values

D1 22

FITTINGS ann CON Table DI.8 Alưmin (K = ratio o

NECTIONS, lum-Alloy Sheet ang

f actual bearing stre: Plate Bearing Factors* ngth to 100 ksi)

BOLTED anp RIVETED

(heat treated by || 250- 499] 1.18] của |g

user), -ð00-1.000Ì 114| -010-.062| 114] 90Ì 58

loo! ÿ8 Chủ 2024 T3 { 063-249} 250-499] 1201 95] 74

120] 195} .74 Clad 2024 T4 { -300-1.000] 1.16] 192] 'ấy

<.03| 122| [98] | 90 Clad 202¢-Ts1 l| 5.063} 127! xool lee

tad 2024-T86 |; 5.068; 1.35| 1.06] 1.09

016.089] 144) 114} 106 o40- 249) 1.49] 216} 1.0

TO7S-TB .068-.488| 1.39! 1.08! 1.00

5001.000 | 1.42} 1.10! 1.04 015-.089 1.33] 1.05| 98 040-.062| 1.37) 1.08] Lor oe3-.187/ 1.391 1.10] 1.03

Clad7072-T6 || 250-.430 | 1435| 5001.000] 18g 240] tae] 1.39] cael 1.05| ios 98

1.08] 90]

os 044) 15s] tas} nit] oo 045-240) 160/ 126] 1181 L

T178-T6 aso 499 ã00-1.000| .015-.044] 1s! lãi | Lis} 1441 nist 114] nar 112

ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES

Table D1.9 Unit Bearing Strength of Sheet on Rivets, For 2 100 ksi

a Bearing values are besed on areas computed using the

Table D1.10 Unit Bearing Strengths for Pin Size Indicated; 1b *

nominal hole diametera apecified in table 8.11.11(4},

D1, 24

Since rivets are same size, all rivets are

assumed to share equally in resisting H and V

Loads

= 3000/6 = 1335 direction and to the right Load

From equesion (15), the load on a rivet

due to Họ on rivet group 2quals F = Mr/I

Ls ir? = 1.6257 x4+9.625"x2 = 11.4

Consider rivet marked c;

r= 1.625 = arm to c.g of bolt group

Po = Mr/I (3000 x 1,625)11.4 = 1280 1b,

Since rivets b, 4d and e are the same

distance as rivet c from the c.g., the moment

load on these bolts will also equal 1280 Fig

1.38 shows the H, V, and M loads on the rivets

Bb, c, dands Since the arm r to the rivets

f and g is only 0.425, the load due to moment

Will be constderably smaller and thus these

rivets will not be critical Observation of

Fig 51.58 shows rivet c is the rivet with the

largest resultant load

From Table D1.5, Single shear value =

1760 lb or double shear strengta = 3520 lb

Bearing strength of 1/4 rivet on the.071

4-73 clad channel section from Tables D1.9

1.4 ts 1325 x 1.20 = 2190 Since rivet

on two channels, dearing strength of

lyet = 2x 2190 = 4380 lb Rivet shear

a problem for the reader, change rivets

diameter and cetermine whether

ng still shows a positive margin

Fig 01.39 shows a lap joint involving

‘wo rows of rivets as shown Sheet material is

2024-73 clad, and rivets are 3/22 diameter and

FITTINGS AND CONNECTIONS

BOLTED AND REIVETED

2117-13 material and of the protruding head type

The ultimate design tension load in the sheet including a 1.15 fitting factor of safety 1S 1000 1b./inch, The limit fitting load is 2/3 x 1000 = 667 1b./in

The margin of safety of the sheet splice

AS an analysis unit, a width of sheet equal

to the rivet pitch of 1 tnen will be used Thus load on 1 inch unit = 1000 lb

Check Tension in Sheet at Section Through Holes

Rivets are in single shear and two rivets act in the 1l inch unit which was assumed From Table D1.5, single shear strength for 5/32,

21018 et is 596 1b The strength factor middle tadle of Tatle D1.5 for 04 sheet thickness 13 hinh Thus for two rivets the shear strength is 2x 96é¢4 x 596 = 1150 1b

a ultimate bearine 100,000 psi for 5/32

farring to

ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES Table O1.8 for 2024-TS clad material and an

D ratio of 2.0, we find correction factor K

= 1.14 Ther re rivet bearing strength is

1.14 x 636 x 2 = 1450 1b

M.S = (1450/1000) -1 2 45

Check Rivet Shear Out

distance is 5/16 in or e/D = out strength is satisfactory

Since dg2

2.0, shear

2OBLEM 4

Assume rivets are changed to the solid

100° dimpled type What would be the M.S for

the rivets Referring to Pig DlL.27, we find

the sheet thicknesses are such as to prevent

double dimpling From Table D1.11 and 51.12,

we obtain the ultimate and yleld strength of a

5/32 rivet on 04 sheet as 635 and 506 lbs

NOTE: In checking tensile strength of sheet

tnrough nole section, the drill size for dimpled rivets is slightly larger than for protruding head type

PROBLEM 5

This 1s a typical problem involving the

rivet loads in 4 sheet-stringer type of

gonstruction as {llustrated in Fig D1l.40

Before the rivet size and spacing at the points

(1) to (10) can be determined, the rivet loads

at these points must be known The shear flow

in direction and magnitude on the webs and skin

are shown on the figure and are in lbs per

inch, These values represent the results in

one of the flight conditions The structural

designer must look at all the shear flows in

the vartous flight and landing conditions in

order to obtain the critical rivet leads It

is assumed the shear flows as shown include

any diagonal tension effect in the various

sheet panels

The rivet loads in lbs./in

i to 10 witli be as follows:-

ae line Since 0S vertical web ands

+ point dene shear flow of 1075 lbs./in

Cl Lys

in the vertical web must obviously be reacted

by the rivets in rivet line (1), thus load on

rivet line (1) 1s 1075 158./1n

Rivet line (2) By the same reasoning since

S4ia ands at soint (2), the load on rivet line

(2) equals shear flow in panel 2-3 or 575

Since the summation of the forces parallel to

the stringer must equal zero, it is observed

that the load transferred to the stringer 18

150 1bs./⁄1n

ivet lines (4) and (5) Since the sheets end over the stringer, th load in rivet lines (4) and (5) are 425 and 275 lbs./in respectively

is 150 from equilibrium Thus the worst shear load on the rivet is 150 lps./in which is greater then the shear on another cross-section

of the rivet which equals 125 lbs./in as the shear flow in panel 6-7

Rivet Load at (3) Rivet Load at (9)

Rivet Load at (10

175 - 25 = 150 lbs./in

175 lps./in

) = $75 los./in „u DI.35 Rivets in Tension

Great judgnent should be used in using rivets in tension There is a general saying,

"Never use 2 rivet in tension." If this re- quirenent was strictly followed, it would be difficult to design 2 conventional airplane

For example, the skin on the upper surface of

the wing, due to the upward suction af places the rivets shat hold the skin to stringers and ribs in tension, however these tension loads in most cases are relitively small

The following general criteria apply relative to rivets in tension