Fasteners for structural bolted joints to be assembled in remote handling

Fusion Engineering and Design

Fusion Engineering and Design 29 ( 1995) 347-357

Fasteners for structural bolted joints to be assembled in remote handling

A. Colaiuda a, F. Amelotti a, E. Di Pietro b, C. Alessandrini b, L. Bettinali b >

G. Malavasi ‘, M. Sironi ‘, D. Maisonnier ‘, F. Olezza d, G. Ghia d, R. Cusolito e, G. Merckling e, L. Crippa e

a Associazione EURATOM-ENEA sulla CRE “E. Clementel”, via Martiri di Monte Sole 4, 40129 Bologna, Italy b Associazione EURATOM-ENEA sulla Fusione, CRE Frascati CP 65, 00044 Frascati, Roma, Italy

’ The NET Team, c/o Max-Plank-Institut fir Plasmaphysik, Boltzmannstraje 2, 85748 Garching, Germany d FIAT CIEI via Cuneo 20, PO Box 500, 10152 Torino, Italy e Istituto Ricerche Breda, v.le Sarca 336. 20126 Milano, Italy

Abstract

Fasteners are generally considered a matter of a mature technology with a variety of solutions ready to meet a wide range of applications, but their use in remote handling needs accurate considerations.

The fastening system has a substantial effect on the total time to perform maintenance operations, and therefore care should be taken from the beginning of the project to its standardization.

The set of requirements to be satisfied by a qualification activity on fasteners to be remotely handled within the environmental conditions relevant to the NET-ITER machine (high temperature, radiation, vacuum, cryogenic temperature) deals with shape and dimensions, materials properties, surface finish of mating parts, lubricant compatibility, precautions against galling and seizure, provisions of remedial actions from an emergency situation,

fabrication process specification, methods of preload application and control of its persistence under machine operation.

The qualification of high strength bolts at cryogenic temperature (4 K) and relevant fracture mechanics tests, and the qualification of suitable coatings and lubricants has been extensively investigated on both specimens and full size

components. In this paper we report final results of a three-year development programme on fastener technology.

1. Introduction

Within the framework of the NET-ITER technolog- ical programme ENEA has been assigned the “Qualifi- cation of Remote Handling Standard Components: Mechanical Fasteners” (Task RST l/l). The aim of this task was to obtain a database on fasteners’ dimensional

standardization, mechanical properties, lubricants- coatings-surface finish, capturing methods and locking devices, torque appliance methods, and effect of irradia- tion to be introduced in the remote handling manual.

The fastening system will have a substantial effect on the total time to perform maintenance operations; therefore care should be taken in the design of the

0920-3796/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved

SSDI 0920-3796(94)00308-4

348 A. Colaiuda et al. 1 Fusion Engineering and Design 29 (1995) 347-357

system. The environmental conditions encountered in the NET-ITER machine (high temperature, radiation, vacuum, cryogenic temperatures) require a detailed in- vestigation of the materials and mechanical properties of fasteners. It is essential that materials, surface finishes, tolerances and overall design of mating components are chosen to minimize the likelihood of galling and seizure. In the case that seizure occurs, remedial actions must be foreseen.

2. Why qualify fasteners?

Specified operating and environmental conditions make certain standards for components; in particular fasteners are widely used in mechanical and electrical engineering and their production is governed by design, geometrical and manufacturing codes. On account of their extensive use, they should be characterized by the greatest reliability in terms of structural capability. Although assessed design and manufacturing rules in fastener production form the basis of their reliability, the specific requirements of a tokamak or an equivalent nuclear fusion machine to be operated remotely introduce a new need for standardization and qualification. High mechanical strength and cryogenic temperatures

are the most demanding structural features and are in conflict with conceived practice. Basically remote handling may introduce specific standardization criteria relevant to component assembiy and dismantling: -to avoid a large number of tools the geometries have

to be suitably restricted and standardized; -to guarantee the correct fitting and to limit the

handling device precision requirements particular care has to be taken in the verification of the geomet-

rical interfaces between screws, bolts, studs and the components to be fastened (clearances, tolerance class of the threadings, lead-in);

-accessibility to the fasteners should be whenever possible by a straight-on motion of the manipulator and their operability by a straight line motion of the manipulator or by simple rotation about a fixed axis compatibly with the narrow space available;

-repeatability of torque vs. preload of the fasteners must be guaranteed to avoid to incur into unex- pected manoeuvres during maintenance and repairs;

-predictability of the friction coefficient between components must be ensured even after the required machine operating time;

-the fastener material has to be selected in accordance with the mechanical characteristics of the machine components to be fastened and preferably one mate-

rial should sustain alone the stress conditions over the full range of operating temperatures;

-special features in the fastener shape have to be foreseen in order to make possible remedial actions in case of seizure and/or deformation after operation.

Industrial experience in the field must be collected to a wide extent in order to avoid oversophisticated products manufactured using laboratory techniques imply- ing overrunning of costs, but equally excessively coarse products that do not represent a correct base for the qualification activity must be avoided.

3. Qualification programme

3.1. Overview

The main items of the programme were as follows: (A) fastener geometrical and dimensional characteriza-

tion; (B) fastener material mechanical properties characteri-

zation; (C) assessment of the surface finish, platings, coatings

and lubricants; (D) design of the capturing method and locking

devices; (E) assessment on the torque application methods and

preloads. The programme has been divided in two stages. After (i) the preliminary screening by literature on the above- said fields, (ii) the selection of applicable codes, and (iii) the recognition of suitable industrial practice the second

stage has been addressed to the engineering, the material procurement and the completion of the test campaign on the basis of the first-stage results. The past experience gained in the field of nuclear fission as well the background of space technology have been a conve- nient reference for orienting the programme.

Table 1 shows the share of functions in order to maintain a close cooperation between ENEA, industry and laboratories.

Table 1 Work organization

Project management Engineering and technical

supervision Fastener production

Mechanical tests up to 4 K

Coating characterization and

torque vs. load tests

ENEA-FUS, Bologna ENEA-FUS, Bologna FIAT-CIEI, Torino OME Erba

IRB, Milan0

ENEA-FUS, Frascati

A. Colaiuda et al. /Fusion Engineering and Design 29 (1995) 347-357 349

3.2. Project goals

3.2.1. Fastener geometrical and dimensional characterization

The goals were as follows: -to prepare tables of data and drawings of the pro-

posed fasteners, showing size, pitches, tolerance class, bolt thread, lead-ins, head with reference to IS0 standard;

-to define fasteners outside the standard (if any) with indication of the used criteria;

-to investigate the appropriate tools to screw-un- screw the bolts-nuts in order to ensure that the heat is correctly sized and can match standard remote handling tools available on the market;

-to investigate the thread rolling process and verify its beneficial effects.

3.2.2. Fastener mechanical properties characterization The goals were as follows:

-to make a literature survey in order to collect all the

data necessary for the materials’ mechanical characterization;

-to confirm through mechanical tests the selected materials and the dimensional characterization;

-to assess the effects of cryogenic and high temperature on the fasteners’ mechanical properties;

-to collect a set of data of the fasteners’ mechanical properties.

3.2.3. Surface finish, platings, coatings and lubricant The goals were as follows:

-to produce an assessment on the surface finish, plat- ing, coatings and lubricant to be used in the different environmental conditions of the NET machine (the choices must be done to minimize the likelihood of galling and seizure, to allow easy dismantling of threaded connections and to minimize outgassing in high vacuum) ;

-to perform a test programme on fasteners in order to simulate the main environmental conditions and to identify the best combination of surface finish-coating-lubricant;

-to produce a set of data concerning platings-coatings-lubricants.

3.2.4. Capturing methods, locking devices The goals were as follows:

-to prepare detail drawings, material part list and description of the used methods, in order to make fasteners captive and to reduce the time of assembly and disassembly;

-to prepare tests to verify the performance of some

proposed methods.

3.2.5. Torque application methods and preload The goals were as follows:

-to produce an assessment on the torque-applying methods;

-to prepare a set of drawings-data of the applicable

tools; -to produce an assessment on the torque-measuring

methods; -to test specific joint configurations; -to find a correlation of torque vs. preload for some

selected standard fasteners.

3.3. Main results

The present paper, in order to respect the size con- straints assigned by the editors, deals to some extent with fastener geometrical and dimensional characterization, mechanical properties characterization and capturing methods. The preliminary results of the assessment on surface finish, platings, coatings and lubricants have been described in Ref. [ 11; for the assessment on the torque application and fastener preloading the test activity is under progress, and final results are expected for our next publications.

3.3. I. Fastener geometrical and dimensional characterization

The fasteners’ geometry has been identified and standardized by considering the following main aspects: -methods and equipment of tightening and preloading; -methods and equipment of preload measurement; -space limitations; -operation in remote handling conditions. Both type (screws, studs, nuts, washers, washers with electrical insulation, spherical washers) and features (thread form, pitch, recommended length, thread tolerance class, leads-in, shear grooves, thread insert) were selected to give the fasteners the best operability in assembly, repair and maintenance conditions.

The following terminology makes clear the character- istic issues to which the fasteners for remote handling are recommended to conform (Fig. 1): -lead-in will align the screw with the threaded hole to

minimize the possibility of cross-threading during assembly;

-feathered edge will be removed to prevent thread damage during initial engagement;

-rounded leading face will help to prevent damage of the tapped hole during entry;


ALlGNrENTAIO ,EXTENO FLAT

/

/THREAO \ lOENTlFlCATlON MARK

\UNDERCUT SHANK

Fig. 1. Key words for remote handling fastener terminology.

-parallel lead-in section will aid alignment of the screw with the threaded hole;

-undercut shank provides a good elasticity, no loosen- ing at low and high temperature and better fatigue resistance;

-shear groove provides where necessary a means of breaking the fastener in an emergency, where the

threads become seized during operation; -socked lead aids the assembly of the tool; -extended flat is foreseen for a more positive engage-

ment of the tool and reduces the tendency to round the corners. The sizes M12, M16, M20, M24, M30 are standard-

ized for screws and studs, and the sizes M36, M42,

M48, M56, M64, M72, M80, M90, MlOO, M110, M125, M140, Ml60 are generally recommended only for studs and restricted for screws (examples are shown

in Fig. 2). The screw head types that have been standardized are

-hexagonal, -hexagonal with hole for electric heater (for stretch-

ing),

Fig. 2. (part I)

A. Colaiuda et al. 1 Fusion Engineering and Design 29 (199.5) 347-357 351

If=

\

f

Fig. 2. Some examples using fasteners for bolted joint design.

-bihexagonal, and - bihexagonal with hole for electric heater (for stretch-

ing). The bihexagonal head is used to increase the load-bear- ing area of the head-wrench contact without increasing the height. This feature reduces the possibility of round-

ing the head during assembly-disassembly; it is pre- ferred in the case of space shortage because its radial dimensions are smaller than those of the hexagonal head.

The double-end studs have been standardized according to the methods of the preload application both by an electric heater (a bore for the electric heater is

provided concentric with the stud axis) and by a hy- draulic tensioner (an overlength for the tensioner engagement is provided concentric with the stud axis).

3.3.2. Fastener mechanical properties characterization

The fasteners have to guarantee the tightening functions: the clamping force, while providing a locking

effect to avoid unscrewing, is essential to ensure tight- ness in front of gas or fluid pressure.

Nevertheless the clamping force value is a factor strictly related to external events (loads). The machinability of the materials is a parameter to be included by considering the production of pieces in series.

Four materials have been selected and supplied: In- cone1 718; ASTM A453 grade 660; ASTM B196; Hiduron Marine1 (alloy with 73% copper 18% nickel from Langley Alloys Ltd., UK). A set of mechanical and physical tests have been performed on standard specimens and full-size bolts of these materials in order to allow a correct evaluation. Taking into account the high quantity of the fasteners necessary for the NET- ITER machine, the material choice should be based on the most favourable cost:ratio benefit.

The selection of the most suitable material for the fasteners has been carried out according to the previ- ous experiences and, in large measure, to the test results

PI.


Table 2

Mechanical characteristics of ASTM A453 grade 660 evalu-

ated on the statistical basis of qualified procurements

Strength ASTM

reference

value

(MPa)

Statistical

assessment

(MPa)

Increment

W)

Yield 585 710 21.3

Tensile 895 1038 15.9

Although Inconel 718 is normally considered to be one of the best cryogenic alloys, it is interesting to observe that the effect of low temperature on the mechanical characteristics is much more beneficial for the ASTM A453 grade 660 than for the Inconel 718. For that reason, considering also the costs vs. performance of the two materials mentioned above, the ASTM A 453 grade 660 seems the best balanced among the materials considered within the qualification activity.

Table 3

Main results of the test campaign on samples

Temperature Tensile Yield Elongation

(K) strength strength

(MPa) (MPa)

(W

293 1061.5 836.6 25.3

923

723

77

77

77-293

293-723

4

4

1382.6-1401.7 819.3-1036.9 35.8-38.4

1490.4- 1562.4

150881527 955-965 30.0-32.9

1328- 1429 904-988 22.4-28.3

-

Considering that ASTM A453 grade 660 has been the object of remarkable development by European manu facturers, the reference properties of ASTM seem to be too conservative in comparison with the real capability of the material.

A statistical evaluation on the basis of ASTM A 453 grade 660 material procurement for the construction of fission reactors that has interested several steel makers has revealed an opportunity to take new values as the mechanical characteristics reference for the design of fasteners. Table 2 shows the comparison of the data of the yield and tensile strengths between ASTM reference values and those found from the statistical evaluation.

The above characteristics have been confirmed by qualification tests during material procurement.

The coefficients of thermal expansion of the A453 grade 660 and AISI 316 LN (machine structural material) are very close, and therefore the preload variation with the temperature is low. The preload variation, going from room temperature to cryogenic temperature (4 K), is only -5%. The fasteners fabricated with ASTM A453 grade 660 have sufficient ductility and

Hardness Reduction Stress LTEC a Specimen

(HB) of area rupture ( x 10e6 K-‘) dimensions

(W (MPa) (mm)

300-310 31.6 Diameter, 12.7;

length, 50.8

385 Notched

specimen

641 Notched

specimen

41.7-44.3 Smooth

specimen

Notched

specimen

13b, 13.5’

17.5-17.7

40.4-41.0 11.0=

Smooth

specimen

(diameter, 12.5)

Material: ASTM A 453 grade 660. Chemical composition: Cr, 14.61; Ni, 24.70; Ti, 2.12; MO, 1.16; Mn, 0.44; Si, 0.37; V, 0.22;

C, 0.035; P, 0.17; S, <0.005; Al, 0.29; B, 0.0038. Condition: samples.

a Linear thermal expansion coefficient.

b IRB test value. ’ ENEA test value.

Tab

le

4

Mai

n re

sults

of

th

e te

st

cam

paig

n on

sc

rew

s

Tem

pera

ture

T

ensi

le

Yie

ld

Elo

ngat

ion

Mag

netic

H

ardn

ess

Red

uctio

n of

Im

pact

Pr

oof

Spec

imen

(K)

stre

ngth

st

reng

th

W)

perm

eabi

lity

(HB

) ar

ea

test

lo

ad

dim

ensi

ons

( MPa

) (M

Pa)

0%)

WJ)

(J

) (N

) (m

m)

b 9 29

3

123

293

293

293

293 71

293

1053

.4-

1077

.0

929.

4-

942.

1

786.

1-

23.6

-

817.

6 25

.0

146-

162

14-1

5

270-

285

42.8

-

46.2

43.7

Dia

met

er,

9.00

; le

ngth

, 36

.0

Dia

met

er,

10.0

; le

ngth

, 10

0

53-6

1

1164

-

188

335-

380

1.00

5

Alig

nmen

t te

st,

1239

.8-

1283

.8;

wed

ge

test

,

1183

.0-

1237

.4

Alig

nmen

t te

st,

1030

.7-

1056

.6;

wed

ge

test

,

935%

986.

7

54.3

WV

sp

ecim

en)

-

10 x

10

Dia

met

er,

12.7

; le

ngth

25

.4

448

000

Nut

s,

M30

Dia

met

er,

12;

leng

th,

200

Scre

w.

M30

448

000

Scre

w,

M30

Mat

eria

l A

STM

A

45

3 gr

ade

660.

C

hem

ical

co

mpo

sitio

n:

Cr,

14.6

1;

Ni,

24.7

0;

Ti,

2.12

; M

O,

1.16

; M

n,

0.44

; Si

, 0.

37;

V,

0.22

; C

, 0.

035;

P,

0.

17;

S,

<0.0

05;

Al,

0.29

; B

, 0.

0038

. C

ondi

tion:

sc

rew

s.

354 A. Colaiuda et al. / Fusion Engineering and Design 29 (1995) 347-357

therefore low notch sensitivity also at cryogenic temperature. ASTM A453 grade 660 has a good machinability and an elongation (about 25%) high enough to allow the thread rolling process. The difference in hardness in the mating surfaces has been considered a driving char- acteristic to avoid galling-seizing. The fastener in A453 grade 660 with a Brine11 hardness of 300 (minimum) is coupled with the structure in AISI 316 LN with a Brine11 hardness of about 170.

Table 5

Main results of the test campaign on samples

Temperature

(K)

Tensile strength

@@a)

Yield strength

( MW

Elongation

(%)

293 686.6-801.1 623.5-773.4 29.5-39.7

77

4

293

293 975-1016

293 655-786 515-579 25-40.1

293 1203- 1220

77 1452-1460 684-733 41.8-42.2

77 l655- 1679

77 1475- 1486 870-880 46.4-48

77

4

4

655-697 482-515 25-46.4

1812-1849

1278- 1509 719-978 37.4-43.8

1351-1566 951-1093 32-41.7

When for mechanical joints severe preloadings are not required or when fasteners operate at temperature not much higher than room temperature, it is conve- nient to consider the use of bolts and screws made of ASTM Al93 B8M2.

ASTM A453 grade 660 mechanical and physical characteristics, as obtained in the test campaign, for samples and full-size bolts are summarized in Tables 3 and 4.

Hardness Reduction LTEC a

(HR) of area ( x lO-6 K-‘)

(%) -

Specimen dimensions

(mm)

250-280 69.6-70.5

13.9

11.4

Diameter, 12.7;

length, 50.8

M24

smooth specimen

(diameter, 12.5)

M24

notched specimen

(diameter, 9. IS)

Ml2

smooth specimen

(diameter, 6.35)

Ml2

notched specimen

(diameter, 6.36)

M24

smooth specimen

(diameter, 12.5)

M24

notched specimen

(diameter, 9.15)

Ml2

smooth specimen

(diameter, 6.35)

Notched specimen

(diameter, 6.36)

Smooth specimen

(diameter, 9.15)

Smooth specimen

(diameter, 6.35)

Material: ASTM A 193 B8M2. Chemical composition: C, 0.041; Cr, 16.78; Ni, 10.7; MO, 2.09; Mn, 1.75; Si. 0.49; P, 0.027; S,

0.024. Condition: samples. a Linear thermal expansion coefficient.

A. Colaiuda et al. / Fusion Engineering and Design 29 (1995) 347-357 355

For small size fasteners (from Ml2 to M24) the use of the above material seems to be very advisable. In fact its yield strength and tensile strength are sufficiently high and the thermal expansion coefficient is similar to that for AISI 316 LN (machine structural material); therefore very low preload variation with the temperature is obtained.

The method chosen to capture a fastener depends on the specific machine configuration. The general requirements are that -assembly and disassembly must be suitable to be

performed by remote handling, -design must be as simple as possible with the lowest

number of items, and ASTM Al93 B8M2 mechanical and physical char-

acteristics, as obtained in the test campaign, for samples and full-size bolts are summarized in Tables 5 and 6.

3.3.3. Capturing methods, locking devices

There are several types of locking devices; the com- mon function is to bind (or wedge) the nut thread to the bolt threads or the screw head to the upper flange.

-operability during the several operating conditions of the machine must be ensured.

The specific requirements are as follows: -disengagement from the component and engagement

by the capture device must be verifiable by the operator (this will normally be done by confirming that a bolt has been withdrawn to a predetermined position);

Table 6

Main results of the test campaign on screws

Temperature

(K)

Tensile

strength

(MPa)

Impact

test

(J)

293 Aligned test,

709-714;

wedge test,

7066712

190.7-215.7

(KV specimen)

293

293 218 000

293

II Aligned test, 1426-1434;

wedge test,

13441358

138.3- 142.0 (KV specimen)

56 600

254 400

77

11

71

4

4

Aligned test,

793-801; wedge test,

8033806

Aligned test, 1441-1530;

wedge test, 1349-1373

Proof

load

(N)

211 000

Specimen

dimensions

(mm)

Screw M24

(diameter, 12.5)

147.5 (KV specimen)

50 100 Screw Ml2

(diameter, 6.35)

128.7

(KV specimen)

75 000

316 000

88 000

143.7-151.7

935-986.7 132-135

Nut M24

(diameter, 12.5)

Nut Ml2

(diameter, 6.35)

Screw M24 (diameter, 12.5)

Screw Ml2 (diameter, 6.35)

Nut M24

(diameter, 12.5)

Nut Ml2 (diameter, 6.35)

Bolts M24

(diameter, 12.5)

Bolts Ml2 (diameter, 6.35)

Material: ASTM A 193 B8M2. Chemical composition: C, 0.041; Cr, 16.78; Ni, 10.7; MO, 2.09; Mn, 1.75; Si, 0.49; P, 0.027; S, 0.024. Condition: screws.

3.56 A. Colaiuda et al. 1 Fusion Engineering and Design 29 (1995) 347-357

SECTION A - A

TACK WELDED STRIP FOR NUT REMOVAL ONE WAY

-RETENTION PLATE

Fig. 3. “T” slot captured nut.

-washers, locking tabs etc. should either be integral with the bolt or remain attached to it;

-when a groove has been activated the individual parts of the fastener should remain attached. Attention has been paid in particular to the method

of the “T” slot captured nuts developed at the JET (Fig. 3). Table 7 shows the standardized dimension of the device.

Nuts are made captive by enclosing them in a T-slot and closing the ends of the T by means of a retention plate and removal tab made of AISI 316L. The size of the spot welds and the strip thickness have been checked and qualified with the aim of ensuring that the strip remains in place during machine operation and can be removed by means of a manipulator when the nut needs to be withdrawn. Adequate small clearance between strips and nut corners must be provided in order to keep the nut and the flange hole on the same axis.

4. Concluding remarks on the qualification activity

The reference points have been fixed to evaluate what kinds of deviations and for what amplitudes the fasten-

Table I Geometrical standardization of the nut-capturing device

Nut A B C D E F

size (mm) (mm) (mm) (mm) (mm) (mm) -

Ml2 25 22 3

Ml6 33 29 4

M20 39 35 4

M24 47 42 5

M30 59 53 6 M36 73 64 8

M42 87 76 10

M48 101 87 12

MS6 114 99 13

M64 126 110.5 14

M72 138 122 15

M80 151 134 16

M90 170 151 18

Ml00 189 168 20

Ml10 199 179 20

Ml25 228 207 22

Ml40 254 230 25

Ml60 274 253 25

3 13 14

3 17 19

3 20 22

4 23.5 26

4 29 32

6 34 38

7 37 44

8 41 51

9 49 59

9 55 67

IO 62 75

11 69 93

12 77 94

14 85 105

14 93 115

15 105 130

17 117 145

17 133 165 __~.__ _ _~ ~

ers should have exhibited from the standard rules. Once the fastener production technology has been postulated, both structural suitability and operational reliability have been qualified; the one analytically by some stan-

dard dimensioning code, the latter by live tests. After the geometrical standardization of the fasteners the prototypes and mock-ups have been produced on the basis of assessed industrial processes by a qualified industrial supplier governed by proven quality assur-

ance rules. A point of fundamental importance was to decide the

extension of the test-associated statistics viable for the phase of the machine design. If the design were defin- itely assessed the approach could be more focused on operational reliability by large-scale statistics tests on a well-restricted set of fasteners, but this was not our case; therefore it was decided that each test should have been repeated three times for a light statistics largely sufficient to support the verification activity. In conse- quence the results of the present qualification activity can be easily used as a guideline for the designer but they do not exclude the opportunity and necessity to specialize the qualification on the real components be- fore the construction decision. In fact the prerequisite of the qualification activity is that the components must be as close as possible to those which will actually be used in the machine. Several experiences demonstrate that, even when reference codes are complied with,

A. Colaiuda et al. 1 Fusion Engineering and Design 29 (1995) 347-357 351

products and their structural capabilities may be quite different.

References

[I] C. Alessandrini, L. Bettinali and E. Di Pietro, NET me-

chanical fasteners-measurement of the tensile properties

of materials at 4 K temperature and evaluation of anti-

seizure behaviour of coatings and lubricants, in C. Ferro,

M. Gasparotto and H. Knoepfel (eds.), Fusion Technol-

ogy 1992, Elsevier, Amsterdam, 1993.

[2] R. Cusolito and G. Merckling, Tests for the qualification

of bolting material in the mainframe of the NET-Project,

IRB Rep. 93363/PM/420, Istituto Ricerche Breda, Milan,

July 1993.

Documents

Fasteners for structural bolted joints to be assembled in remote handling