New experimental procedure for analysis

Tiré à Part

New experimental procedure for analysis
of rivets material mechanical properties
L. Patronelli, B. Langrand, E. Deletombe, E. Markièwicz*, P. Drazètic*
Crash questions (CEAS)
Naples (Italy), February 14-16, 2000
TP 2000-32

New experimental procedure for analysis of rivets material mechanical properties
N ouvelle procédure pour l'analyse des propriétés mécaniques
du matériau constituant les rivets
par

L. Patronelli, B. Langrand, E. Deletombe, E. Markiewicz*, P. Drazétic*
* UMR CN RS 8530 (Mechanical Engineering Research Group) Université de Valenciennes (France)

Crash questions (CEAS)
Naples (Italy), February 14-16, 2000

Résum é : L'analyse par élément finis de la tenue des assemblages rivetés nécessite la

connaissance des lois de comportement. N éanmoins, la petite taille des rivets (8 mm de long) ne
permet pas de procéder à des essais de traction classiques pour caractériser proprement la loi de
comportement du matériau rivet.
Afin de donner accès à cette loi de comportement, l'O nera a développé un essai original de microtraction. Dans un premier temps, des expérimentations sont réalisées sur un alliage d'aluminium
AU4G-T4 afin de valider le nouveau moyen d'essai. Lors de l'opération de rivetage, des contraintes
et déformations plastiques résiduelles (5 %) sont introduites (dues à un chargement en
compression). Aussi, dans un second temps, l'objectif est de mettre en évidence l'influence de ces
contraintes résiduelles de compression sur le comportement du matériau sous sollicitation de
traction. Pour cela, le fût est extrait du rivet pour être comprimé afin d'introduire les déformations
plastiques résiduelles (environ 5 %). Par la suite, des éprouvettes de micro-traction sont débitées
des fûts comprimés, pour être testées. Les résultats sont comparés avec ceux obtenus
précédemment. Il est montré que le matériau des rivets subit un écrouissage cinématique, les
caractéristiques mécaniques sont donc modifiées.

NB : Ce document comporte

12

pages

Ce Tiré à part fait référence au Document d’Accompagnement de Publication DMSE0002

NEW EXPERIMENTAL PROCEDURE FOR ANALYSIS
OF RIVETS MATERIAL MECHANICAL PROPERTIES
L. Patronelli, B. Langrand, E. Deletombe, E. Markièwicz¥, P. Drazètic¥
Department of solid and damage mechanics, Structural resistance and design section,
ONERA-Lille, 5 Bvd Paul Painlevé, 59045, Lille, France. Email : patronelli@imflille.fr. ¥ Industrial and human automatic control and mechanical engineering
laboratory, Mechanical engineering research group (UMR CNRS 8530), University of
Valenciennes, Valenciennes, France
Abstract: The study of airframe crash behaviour becomes a necessary step in aircraft design (for
possible future certification requirements). An airframe is composed of many parts (e.g. skin, frame,
stringers, clippers) mainly assembled using riveting techniques. These complex assemblies lead to
several basic non-linear rupture phenomena observed in case of crash events. Since 1995, the
ONERA-Lille Research Centre and LAMIH from the University of Valenciennes have been leading
numerical and experimental basic studies to analyse local material and global joints behaviours and
improve the riveted joints design. The final objective is to model full scale crash events using
accurate but macroscopic FE joints models.
Intrinsic material laws have to be characterised to perform numerical analyses of the joint strength.
Nevertheless for the rivet materials, it is impossible to undertake uniaxial tensile test using classical
specimens because of the small size of the rivet itself (8mm, long). To give access to the rivet

material law, the ONERA-Lille Centre has developed an original micro-tensile test rig. In the first
part of the paper, experiments are performed on a AU4G-T4 aluminium alloy. Results obtained with
the new procedure and with the classical uniaxial tensile test are compared to validate the microtensile test concept. In the second part, the rivet material (7050 aluminium alloy) is then
characterised with the new test rig.
The riveting process introduces residual stresses and plastic strains (> 5%) into the rivet (which
comes from the compression loading). So the objective of the third part of the paper is to highlight
the influence of compression residual plastic strains on the rivet material behaviour when tensile
load is applied. First, rivets are crushed to introduce compression plastic strain (about 5%). Second,
micro-tensile specimens are manufactured from the crushed specimens and micro-tensile
experiments are undertaken. Results are then compared with the compression and tensile material
laws of the 7050 aluminium alloy to highlight the influence of kinematic hardening on the
mechanical properties of this material.
This experimental work would prove to be useful to accurately improve the design of new riveted
joints techniques. An original experimental procedure has been developed to characterise material
parameters and to analyse kinematic hardening effects on material laws. The ultimate aim of these
studies is also to improve the joints F.E. models for airframe crashworthiness

Keywords : Tensile test - Riveted joints - Characterisation - Kinematic hardening
Notation :
Latin

A,B,n power model parameters
E
Young modulus
Fmax maximum load
SN
Gaussian standard deviation
Vimp loading velocity
f(zi) convergence criterion
fc
critical void volume fraction at coalescence
fF
critical void volume fraction at ductile fracture
fi
initial void volume fraction
fN
nucleated void volume fraction

qi
porous material parameter
zi

parameter vector
Greek.
εN
nucleated effective plastic strain
σe.
yield stress
σmax. maximum stress
δres
residual displacement
1.

INTRODUCTION

An airframe is composed of many parts (sheet metal plates, beam...), assembled by
riveting techniques. The aim of crash studies is to improve the behaviour of airframe in
term of energy absorption and cabin deformation. These studies are leaded with
numerical tools which decrease cost compared to experimental procedure [1].
Before performing this kind of complex crash study it is necessary to characterised the
rivets macroscopic behaviour (non-linear behaviour, failure criterion) [2]. Indeed,
airframes are built with numerous rivets which influence the ruin mechanisms in case

of crash [3], but it is impossible for obvious reasons of computing costs to refine the
mesh at the rivet geometrical level in airframe models [4]. For this reason, thin
simulations of rivets under mixed mode loading are previously undertaken and lead to
the determination of different parameters for simplified link models (equivalent
elements proposed by FE codes) [5]. To compute this kind of thin simulations, the
behaviour of the crushed rivets material must be known up to failure (eg. Gürson
damage model). Previous works have shown the high plastic strain level which
develops in rivets during the forming process (εp from 5% up to 17% [6]). This is the
reason why it is proposed to characterise the effect of this first loading (riveting
process) on rivet material, to be used in simulation (and more particularly the use on
kinematic or isotropic hardening).
For that purpose, it is proposed to develop a new experimental characterisation
technique which consists in doing micro-tension tests : indeed the size of rivets, which
are about 8 mm long, raises problems considering classical characterisation techniques.
First, it is impossible to use a normalised tension test specimen (l=5.65√S). Second,
the geometry of the test specimen must be specifically designed for this kind of
application. Third, it is necessary to use and adapt micro-gauge to measure the strains.
As a first step, numerical tools help to select the geometry of the test specimen. Then
the experimental investigation is divided in two steps. A first validation of the
experimental procedure is presented by comparing test results with classical available

ones on AU4G-T4 aluminium alloy. Then other tests are performed on “ on the shelf
rivet ” extracted test specimens. Both results permit to undertake an identification of
damage parameters of Gürson’s model with an inverse method (optimiser coupled to
FE code Pam-solidTM [7]).
In a second step, to take into account the forming effects on material behaviour
introduced during the riveting process (plastic strains from 5% up to 17%), microtension tests are performed on specimens taken from crushed rivets. Material
parameters are identified and results are compared to the previous ones.

2.

DESIGN OF TEST SPECIMENS GEOMETRY

2.1
GEOMETRICAL CONSTRAINTS
In the present case, the aim is to extract a test specimen from the rivet itself (small size
of the rivet). The raw material available to manufacture the test specimen is a 4 mm
diameter and 8.2 mm long cylinder. A test specimen is usually designed with two
different parts. A working part where the material behaviour is captured (elasticity,
plasticity, damage) during the loading phase. A second one which permits to attach the
test specimen to the tensile machine. Considering the very small size of the specimens,

we must consider a third one, which is the area between the two previous ones, in
which geometrical effects may amplify and interact with the working part behaviour
[8]. The objective is to take care about and minimise local embrittlement and stress
concentration phenomena in that area if necessary.
The final diameter of the working part of the specimen is chosen equal to 2 mm. Total
length of test specimen is still 8.2 mm and it is proposed to use a working length of
1.5 mm. The final test specimen is shaped by turning process (better machined
tolerance in this case). The grip system between the test facility and the specimen is a
self-closing system (Figure 1).
Two shapes for the junction area are investigated (chamfer or junction rounding-off ,
see Figure 1). To check and choose between the two solutions, a F.E. code is used.
2.2
INFLUENCE OF THE JUNCTION AREA SHAPE
Both presented simulations are realised with an explicit FE code Pam-SolidTM. The
nodal boundary conditions and the modelling simplifications are presented on Figure 2
(only 1/8th of the geometry is modelled). A rigid body is defined to model the massive
parts (grip system). The master node of this rigid body is pulled by a rigid wall, the
displacement of which is imposed by a velocity curve Vimp= f(t). The use of such a
rigid body gives access to the global load and displacement. In the first simulation, the
junction area is a chamfer (α=60°). In the second, it is a junction rounding-off (r=2.12

mm).
The constitutive material law which is used is described as a power-model (hardening
behaviour) coupled to the Gürson damage model (softening behaviour). Since the aim
of this part of work is just to compare both specimen geometry response, Gürson
damage parameters are taken from the literature [2].
Figure 3 shows the Von Mises stresses obtained in both simulations. Plastic strains are
localised in the working part and no stress concentration phenomenon in the area of
separation between working length and junction area is observed. Figure 4 gives the
global load versus local strain. Finally the smallest deviation obtained between both
curves shows that it is possible to take the simplest geometry for the specimens to be
tested (chamfer instead of junction radius).
3.

EXPERIMENTS

3.1
DESCRIPTION OF THE NEW PROCEDURE
Micro-tension tests are performed on tension/compression machine INSTRON 1195.
A special rig is adapted on the machine to attach the test specimen (Figure 8). The
shape and the geometry of the test specimen are given on

Figure 5. Micro-gauges (L=0.5mm, h=0.38mm) are used and bonded in the working
length area to measure directly the material deformation. This kind of micro-gauges
can catch up to 20% strain levels before failure. The global load F is measured with a
piezo-electric load cell (Kystler 9077). The displacement of the rigid rig is measured
on the Schenck G-Nr 911 test facility sensor.
Quasi-static and dynamic tests performed on single lap riveted joint test specimens
have highlighted no influence of strain rates on global behaviour and failure mode of
the assembly (failure of 7050 aluminium rivet) [9]. As this step of the study, we will
then consider for simplicity a quasi-static load velocity of Vimp =0.2mm/min.
To validate the experimental procedure and check if the specific geometry of the test
specimens enables us to get intrinsic mechanical properties of the material, a first
series of test specimens is realised in a well known aluminium AU4G-T4 [10].
3.2

TESTS RESULTS

3.2.1 AU4G-T4 aluminium alloy
Results are presented in Table. 1 in terms of Young modulus, maximum and yield
stress. Figure 6 shows the load versus displacement diagram for the AU4G-T4

aluminium alloy (the error linked to the setting of test specimens is corrected on the
diagram). In fact the linear elastic behaviour is extrapolated on the basis of the
measured behaviour between 0.07 and 0.1 mm of displacement). The obtained results
show that AU4G-T4 aluminium alloy is slightly ductile. The local variables are
presented on a stress-strain diagram in Figure 7 (real stress versus effective plastic
strain). Global and local results show that the experimental procedure is validated in
comparison with the literature for the AU4G-T4 aluminium alloy, except for the
maximum stress which is known to be specimen sensitive.
Table. 1 Mechanical characteristics of AU4G-T4 aluminium alloy.

Source
micro-tension
Literature [19]
deviation %

E (MPa) σmax (MPa) σe (MPa)
75000
74000
1.35

465
420
9

300
280
6

Α%

18
18
0

3.2.2 7050 aluminium alloy
Results are presented in Table. 2 in terms of ultimate load and total displacement at
failure (measured on test specimen after failure). The yield stress and the maximum
stress σy and σmax are also reported in Table. 2. Figure 6 shows the load versus
displacement diagram obtained for the 7050 aluminium alloy. As for the AU4G-T4
aluminium alloy, the linear elastic behaviour at the very start of the test is
extrapolated. This diagram shows that the 7050 aluminium alloy is more ductile than
AU4G-T4. The failure path of the test specimen is inclined at 45° (Figure 8). The load
versus displacement diagram enables to consider the three successive steps related to
plastic and damage mechanisms. Local variables are also exploited in stress-strain
diagrams (Figure 7). For the 7050 aluminium alloy a negative hardening is observed
after a strain limit of 9%. Beyond this strain level the working section can no more be
estimated with the theory of constant volume, because of the localisation phenomenon
in the working section of the test specimen.

Table. 2 Tests results (7050 aluminium alloy).

7050

Fu (kN)

δres (mm)

σe- (MPa)

σmax (MPa)

test_1
test_2
test_3
test_4
Average

1
1.1
0.97
0.92
0.9975

0.56
0.53
0.61
0.62
0.58

329
325
315
320
322.5

530
520
508
510
517

3.3
SYNTHESIS
The micro-tension test has been used to characterise two aluminium alloys (7050 and
AU4G-T4). The mechanical properties of the AU4G-T4 aluminium alloy obtained
from the micro-tension tests enable to validate the experimental procedures. It proves
its capacity to characterise constitutive material law for rivets in tension (or more
generally small pieces).
The tests performed on 7050 aluminium alloy show the more ductile property of this
aluminium. The obtained local results (stress-strain curve) permit to identify
parameters of a power-model for εp