Search

International audience

The main aim of the present work is to study the effect of axial preload on the durability of aerospace fastened joints. For that purpose, experimental and numerical approaches are adopted to demonstrate the beneficial effect of axial preload on the fatigue life of representative aerospace fastened joints. In the experimental approach, an innovative tightening technique, based on iterative tightening, is used to reduce the scatter habitually induced on preload when the conventional torque tightening technique is employed. The main benefit, besides the use of this alternative tightening technique, is to allow an accurate preload to be introduced without geometry or stiffness modification, thus enabling the preload effect to be determined precisely. The results show the effect of preload on the static resistance, fatigue life and failure mode of fastened joints. In the numerical approach, a 3D finite element model is developed and the analysis of its behavior confirms and explains experimental observations. Analysis of longitudinal stresses through critical paths shows that the effect of high preload is beneficial because it reduces hydrostatic pressure and shear stress amplitude, thus leading to longer fatigue lives. The correlation between experimental observations and the numerical results is studied using SWT and Crossland multiaxial fatigue criteria.

International audience; In this paper, the mechanisms of damage of an oxide/oxide ceramic matrix composite bearing are studied with a new experimental setup, the balanced quarter hole device (BQH). This test was designed to allow direct observation of the damage development on a material subjected to bearing failure. In a standard bearing experiment, real time monitoring is extremely difficult to set up, and post mortem observations of the bearing plane can be biased by the cutting operation. High speed cameras were used to take pictures of the bearing plane so that the damage development could be studied and a damage chronology established. The validity of the setup was verified by comparing the results obtained with those of a standard bearing test. Two different stacking sequences were studied, and the influence of the material microstructure and composite machining was investigated. It was shown that the first critical damage, matrix cracks, appeared before the load drop, and then led to delamination and kink bands, causing the final failure of the material.

International audience; A high‐pressure device, reaching an axial pressure of 1000 MPa, intended to the extraction of the pore solution of rigid and slightly porous materials, has been developed to improve the efficiency of extraction. This paper gives an application of fluid extraction from mortars made with Portland cement. It includes an experimental study of the performance of the apparatus, and an analysis of the results in terms of efficiency of extraction, repeatability of measurement, and effect of the squeezing pressure on the pore solution composition. Results shows that: (1) the squeezing efficiency using our apparatus is higher than those found in the literature; (2) the measurement uncertainty ranges between 1.5% and 14%; (3) no significant effect of pressure (up to 1000 MPa) is observed for concentrations of Ca, Na, K, and Si. This paper suggests conducting extraction at 1000 MPa, especially on old concrete or concrete made with low W/C ratios.

PurposeThis paper seeks to deal with a new modelization method which aims at fatigue sizing of preloaded bolted joints. Industrial design offices indeed need new models which, on the one hand, take bending of the bolts and geometrical non‐linearity into account and, on the other hand, run fast enough to be used for preliminary design stages. Usual sizing procedures derive from VDI recommendations, which makes them inaccurate. On the contrary, classical finite element methods are revealed to be very costly.Design/methodology/approachThe first task lies in reducing the physical problem down and model the structure using axisymmetrical elements. Then, the core of the method lies in modifying the stiffness matrix of a tube element, in order to modify the axial compression stiffness to the one used by preloaded assembly classical computations. Eventually, a 2D finite element model is programmed which takes advantage of the modified element. A mounting was built to reproduce the typical loading of a slewing bearing. Experimental tests were carried out in order to help analyse the problem and to check finite element simulation results.FindingsSample experimental results are presented which confirm the need for new models and validate the 2D model that was developed.Research limitations/implicationsThe new finite element, as well as the set of hypotheses that are used, appear to be usable for other bolted joints.Practical implicationsA software was produced for the industrial partners, which is usable by non FE‐specialists.Originality/valueThis work may serve as a basis for building fast and accurate finite element models of other types of bolted joints.

International audience; The fatigue behaviour of bolts under axial load has always been considered from the component point of view for which fatigue limit is usually taken equal to 50 MPa, and few results are available to designers for limited lifetimes. Here, we take up this problem from a material point of view using a local approach. For each case of fatigue testing, using finite‐element (FE) model of the bolt, we determine the stabilized local stress at the root of the first thread in contact with the nut. To characterize bolt behaviour with these numerical results, we use Dang Van multiaxial fatigue criterion for which we extend application to the medium fatigue life. These results can be correlated with the experimental numbers of cycles to failure to determine material parameters of the generalized criterion. Using statistical Gauss method, we can make lifetime predictions for any level of risk of failure. In addition, we propose an analytical model to rapidly determine the local stress condition from nominal loading data (mean stress and alternating stress). This model dispenses us from a new modelling if the bolt is stressed in the same manner as the bolts used for behaviour characterization. Using this model and the generalized criterion, it is extremely easy to make lifetime predictions whatever the risk considered.

International audience; Self-loosening of bolts is a problem encountered in many industrial fields, and especially in aviation. The purpose of this paper is to study the self-loosening phenomenon numerically through an assembly consisting of two plates assembled by a bolt and subjected to a transversal dynamic load. First, a 3D finite element model is developed, with contact elements and all geometrical details, including the real thread geometry with its helix angle. This leads to many degrees of freedom and long calculation time. Then a 1D simulation is made, based on a simplified model with only 4 degrees of freedom and few contact conditions. Calculation is nearly instantaneous. Both models describe the succession of sticking and slipping on contact surfaces and predict loosening of the assembly. They provide close results. The next step will be to compare their behaviour with an experimental approach, in order to validate and fit the models.