The design of new grips for multiaxial fatigue tests

21 ème Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013
The design of new grips for multiaxial fatigue tests
V. I. PRISACARI a , P. GABORIT a , M. PONCELET a , B. RAKA a , J.M. VIRELY a
a. Laboratoire de Mécanique et Technologie (LMT-Cachan),
ENS de Cachan / UMR 8535 CNRS / Univ. Paris 6 / PRES UniverSud Paris,
61 Avenue du Président Wilson, F-94235 Cachan Cedex
Résumé:
L’amélioration des simulations numériques de tenue à la fatigue nécessite des modèles multiaxiaux et leurs
essais d’identification. Nombre d’entre eux se font sur des éprouvettes « en croix » depuis la démocratisation
des machines de traction biaxiale [1]. La majorité des mors utilisés actuellement pour ce type d’essai a
plusieurs avantages : robustesse vis-à-vis des charges radiales, montage simple et rapide des éprouvettes.
Généralement, ils ont comme défauts communs une précision d’alignement insuffisante (induisant une
flexion statique au montage) et potentiellement une flexion sous charge. Ces défauts sont évidemment de
première importance lorsque des essais de fatigue sont réalisés. D’autres mors suppriment une partie de ces
inconvénients, mais habituellement au prix d’un encombrement significatif. En outre, les techniques de
mesure mises en place au sein des machines biaxiales ont beaucoup évolué depuis leur apparition [2-7]:
Corrélation d’Images Numériques, Stéréo-corrélation, Thermographie, DRX. Il est donc nécessaire de
prendre en compte ces nouvelles contraintes en plus des défauts des mors actuels. La solution proposée offre
une plus grande raideur en flexion tout en conservant un encombrement très réduit, permettant de
nombreuses instrumentations. En outre, elle permet l’alignement du mors par rapport à son palier pour
diminuer au maximum les mouvements hors plan (ce qui est vérifié avec une éprouvette étalon instrumentée).
Plusieurs capteurs peuvent être embarqués sur le mors pour répondre à différentes fonctions, en particulier
des capteurs de déplacement LASER offrent une mesure directe de l’écart entre mors et de leur inclinaison
relative.
Abstract:
Most of the planar biaxial machine grips currently used for cross-shaped specimens have several advantages:
stiffness to radial loads, fast and easy sample mounting. Generally, they lack a sufficient alignment precision
(inducing a static bending during clamping) and might exhibit flexion during load. These shortcomings are
obviously of major importance when it comes to fatigue tests. Other types of grips avoid some of these
problems, but usually at the cost of a significantly larger size. Moreover, measurement techniques used with
biaxial testing machines have evolved considerably: Digital Image Correlation, Stereo Correlation,
Thermography, X-Ray Diffraction. It is thus necessary to take into account these new requirements along
with the defects of the existing grips. The considered solution offers a higher bending stiffness without
considerable increase in size, permitting significant additional instrumentation. Also, it allows the
measurement and the adjustment of the alignment of the grip with respect to its hydrostatic bearing, thus
reducing considerably the out of plane motions. Auxiliary devices can be attached to the grips, particularly
LASER displacement sensors that provide direct measurement of the distance between the grips and of their
relative tilt.
Keywords: multiaxial, fatigue, testing machine, grips
1 Introduction
Due to the increasing complexity of systems used in aeronautics, spatial, automobile industries, etc. the need
for more sophisticated fatigue models to describe their behavior has grown in the past years [1]. This implies
that experiments manage to get as close as possible to the multiaxial loading states encountered in service. In
this sense, studies performed on biaxial samples have grown in number and in complexity.
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21 ème Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013
The LMT Cachan laboratory owns a tri-tension/compression testing machine named ASTREE. It has three
orthogonal axes, each of them containing two hydraulic actuators working in pairs. This machine is
particularly interesting because of its mobile architecture, allowing the mounting of considerably large
samples and permitting experiments where the center of the specimen is kept motionless.
Most of the experiments performed in ASTREE are biaxial tension-compression tests on planar cross-shaped
samples [2-7]. Many sets of grips have been used for these experiments [3-7] but the majority fails in terms
either of stiffness, size, or ultimate load. The design of new grips for ASTREE aims to fulfill the following
needs: improving the stiffness of the grips, controlling unexpected solicitations, evaluating the quality of the
alignment of the actuator lines, mounting of smaller and thicker samples. Moreover, the instrumentations and
types of tests performed in the laboratory have evolved considerably making it necessary to adapt the grips to
these complex conditions.
This study will first provide a brief description of the existing grips and the reasons why they fail the
aforementioned criteria. The architecture of the new grips will be then presented along with their innovations.
Finally, the validation procedure and conclusions are stated.
2 Existing grips
The current configuration contains four grips (fixed and mobile part), each one connected to the hydraulic
actuator by means of a spacer (Fig 1a). These grips (Fig 2a), designed by J. Lemaitre and J.M. Virely (and
called in the following L&V grips), have been used for most biaxial tests on ASTREE because of their clear
advantages: sufficient effort transmission, small size and easy sample mounting. Nevertheless, some tests
have shown non-negligible bending values (Fig. 1b). Moreover, the alignment of the actuators is not properly
evaluated (inducing static bending during mounting) and the compatibility with other types of samples is
quite limited both in size and thickness.
On the grips developed by Instron and used at the Centre des Matériaux laboratory an extra tightening,
perpendicular to the load axis, has been added in order to ensure the contact between the fixed and the
mobile part. The MTS standard grips used in uniaxial tensile machines have a more complex design: two
wedges supported on the fixed part, with a mobile shaft which clamps these wedges. They are available in
“hydraulic” or "mechanical" versions. As shown in Fig. 1b, these types of grips endure high loads and induce
almost no bending. However, their large diameter prevents the use of a stereo-DIC setup or a mobile X-ray
diffraction (XRD) machine that was developed in LMT Cachan.
L&V
LMT
Instron
CdM
MTS
Hydr.
MTS
Mech.
Diameter (mm) 150 225 204 170
Axial efforts (kN)
250 100 100 100
- 250 - 100 - 100 - 100
Radial efforts (kN) 200 110 - -
Bending (mm) > 0,1 ≈ 0 ≈ 0 ≈ 0
(a)
(b)
Fig. 1 – (a) Current configuration used in ASTREE, (b) Basic specifications of existing grips
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21 ème Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013
3 Chosen design
Different architectures have been investigated. The chosen one is a combination between the L&V grips (Fig.
2a) and to the Instron ones. It is simple (only two main parts) and stiff (because of its nearly symmetric
geometry (Fig 2b), compactness and large ribs) despite its small size. Its rough shape and size derive directly
from the collision limits with the neighboring devices or their size.
(a)
Fig. 2 – (a) L&V grips (b) Diagram of the new grips
(b)
The upper mobile part will be attached during tests to the lower fixed one directly, using 8 M12 screws, and
through sample contact using 8 M8 screws or less depending on the specimens. The height of the grips
allows the XRD machine to be used. This machine will be mounted above the horizontal plane of the grips in
order to measure the stress state and phase changes while performing biaxial tension tests. The seemingly
trivial aspect of the “steep” front end makes the region of interest more accessible in terms of visibility
(lighting, XRD) and allows the use of shorter samples (no collision with neighboring grips).
Another aspect that had to be taken into account was the cooling of the grips (controlling the heat flux from
the grips to the samples or in the opposite way in case of high temperature tests). The four cooling pipes that
go right through the grips allow maintaining their temperature at 20°C (the temperature of the actuator being
about 60°C) without noticeably reducing their stiffness.
Another clear improvement is the wider range of possible sample thicknesses (from 3 to 10mm) that allows
coherence between the sample mid-plane and the actuators plane. One need only use the corresponding jaws.
These jaws, which complement the tightening M8 screws in terms of adherence, can be designed with
different contact surfaces (smooth, spiked, striated, etc.) depending on the material and on the loading type.
4.1 Alignment solution
The design of the grips aims to accomplish two purposes: on the one hand to allow the alignment of each
grip with respect to its bearing in order to reduce as much as possible the out of plane movements that come
from bearing rotation and on the other hand to allow the alignment of the grip assembly to avoid sample
bending. Because of the inability to rigorously position the actuators vertically, these two conditions cannot
be stricto sensu satisfied. An alternative is to verify the relative position of the actuators and then to modify
the position of the grips with respect to the spacers. For the relative position verifications, one of the
proposed solutions is to use a cross-shaped gauge with cylindrical ends placed on specific surfaces of the
grips (Fig 2). Using this method, the measure is independent with respect to the rotation of each bearing and
simplified (no iterations between each possible combination of grips).
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21 ème Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013
(a)
(b)
Fig. 2 – Alignment measuring system: (a) Support for the alignment cross which is coaxial with respect to
the axis of the grip (b) Alignment cross gauge
4.2 Out of plane motion measurement
In order to measure the relative position of each grip with respect to its opposite neighbor, LASER sensors
(Fig 3a) have been chosen. They offer high precision (e.g. 2 microns of measure incertitude at 10Hz, for a
working range of 300-500 mm) without interfering with the other measurements. Because of their
considerable size they could only be placed on top and on the bottom of the grips. Such a pair of sensors
along with their corresponding “screen” on the opposing grip would allow the monitoring of the distance
between the grips and the system’s alignment during loading (Fig 3).
(a) (b) (c)
Fig. 3 – Distance measuring using LASER: (a) Principle of LASER triangulation (b) Side view (c) Top view
4.3 Numerical validation of the design
In order to quantify the gain in stiffness with respect to the L&V LMT grips, numerical simulations of the
two systems were performed. The purpose was to find the critical areas and to compare the way each set of
grips distributes efforts. Thus, the two systems are elastically loaded up to the maximum service values. To
come closer to the real behavior, the efforts weren’t directly applied onto the grips but through a quarter of a
typical cross-shaped sample. For the L&V grips an assembly with the fixed part, the mobile part and a
sample in between was created (perfect contacts were used). For the new grips, in order to simulate the worst
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21 ème Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013
clamping scenario, a clearance was introduced between the upper grip and the lower one so that the efforts
are only transmitted through the 5 M12 screws.
The first simulation was simple tension, i.e. the most commonly encountered solicitation during grips usage.
Thus, a force of 100 kN (the maximum load that can be induced by one actuator) was applied on the tip of
the quarter sample (Fig. 4). The upper limit of the represented stress range was set to 150MPa, i.e. far lower
than the fatigue limit of the material (about 600 MPa). The new grips are below this value once optimization
was done, whereas the L&V ones are considerably above in the connecting area and the lower ribs.
Moreover, the deflection of the new grips is approximately 15 times smaller than of the L&V ones (Tab. 1).
(a)
Fig. 4 – Simple tension numerical simulations (100kN): (a) L&V grips (b) New grips
Secondly, accidental radial loads were simulated, where a force of 200kN was applied in the loading plane
but perpendicular to the axis of the grip (Tab. 1 - Radial force 1). Although the found efforts are unrealistic,
they validate qualitatively the gain in stiffness. Finally, system behavior was tested in the case of an
accidental load perpendicular to the loading plane (Tab. 1 - Radial force 2). The considered load was 500kN,
representing the maximum impact of an actuator from the vertical axis. As before, it can be seen that the
compact design and the contribution of the mobile part are responsible for the considerably better response
of the new system. In both cases, it is clear that the weakest link will be the M12 screws in such accidental
loadings.
(b)
Measured value
Axial force
(100 kN)
Radial force 1
(200 kN)
Radial force 2
(500 kN)
L&V grips
New grips
Deflection (mm) 0.3 - -
Maximum stress (MPa) 350 2000 10000
Deflection (mm) 0.02 - -
Maximum stress (MPa) 140 600 4000
Tab. 1 – Numerical simulations results for the L&V grips and the new grips
5 Conclusions and perspectives
The study has presented the design of new grips for biaxial tension tests. The main focus was to resolve the
issues found in most existing grips: improving the stiffness of the grips, controlling unwanted solicitations,
evaluating the quality of the alignment of the actuator lines, mounting of smaller and thicker samples.
Moreover, the complex techniques used while performing biaxial tests (Digital Image Correlation, Stereo
Correlation, Thermography, XRD) created further needs that were partially covered (providing cooling pipes,
easy LASER sensor attachment, mobile XRD machine accessibility, etc.).
At the writing time of this proceeding, a validation procedure for the new grips was foreseen in a few weeks.
The main purpose of the validation is to quantify the bending due to loading for the L&V and the new grips.
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21 ème Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013
This comparison will be performed using different measuring techniques (strain gauge rosettes, Digital
image correlation, LVDT) during cyclic proportional loadings with a stress ratio of -1 using two loading
frequencies (20 Hz and 1 Hz). In order to perform these tests, two reference test samples [8] were machined.
One of them will be instrumented with 10 strain gauge rosettes (5 on each side) thus obtaining the averaged
plane strain field in these areas of interest (Fig. 5a). The surface of the second one will be painted with a
black and white speckle in order to obtain the full displacement field using Digital Image Correlation (Fig.
5b).
(a)
Fig. 5 – Reference test samples used for grip validation using different measuring techniques: (a) strain
gauge rosettes (b) Digital Image Correlation. The positioning system using centering pins is represented:
primary contact (red) and secondary contact (blue).
(b)
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