•We develop a behavior model for a semi crystalline polymer with full thermo-mechanical couplings.•The model is based on the molecular chain network description with parameters physical meaning.•We ...assume that inelastic phenomena result in an increase of entropy related to “de-entanglement”.•The model is identified on a large experimental basis based on tensile and shear tests.•We report very good results on prediction using a reduced number of parameters.
A visco-hyperelastic constitutive model, based on an original approach initially developed by (Billon, 2012) and applied to amorphous rubbery polymers for a one-dimensional formalism, was extended in this study to three-dimensional constitutive equations based on a thermodynamic framework. The model was applied to a semi-crystalline polyamide polymer, PA66. The experiments included tension and shear testing coupled with synchronized digital image correlation and infrared measurements device for capturing the time, temperature, and stress state dependence, as well as the complex thermomechanical coupling exhibited by the material under large deformation. A notion of equivalent strain rate (based on the time–temperature principle superposition) was also introduced to show its capability to build master curves and therefore decrease the number of testing needed to build a material database. The model is based on the Edward Vilgis theory (1986) and accounts for chains network reorganization under external loading through the introduction of an evolution equation for the internal state variable, η¯, representing the degree of mobility of entanglement points. The model accounting for the equivalent strain rate notion was calibrated using master curves. The thermomechanical model agreed well with the experimental mechanical and temperature measurements under tension and shear conditions. The approach developed in this study may open a different way to model the polymer behavior.
•A phenomenological cohesive zone model for fracture of direct bonded assemblies.•Tests performed to characterize fracture propagation and initiation on a free edge.•Macroscopic CZM is atomistically ...motivated and identified on experiments.•The finite elements model implemented has a good correlation with experiments.
Fused silica direct bonding is of particular interest for optical system manufacturing for spatial applications. However, in order to validate the European Space Agency standards, a better understanding of the assemblies mechanical behavior is required. Therefore, it is important to develop some predictive tools to determine numerically mechanical strength of complex assemblies. In this paper, a cohesive zone model is proposed to model the direct bonding interface behavior. In order to determine the mechanical strength of the interface, a propagation test, and an initiation test on a free edge the cleavage test have been performed on direct bonded fused silica samples. The FIT test (Flexible Initiation Test) is also used to identify the properties of the direct bonding joint. At the end, a comparative analyses is proposed between experimental results and finite elements models of the propagation and initiation tests.
Direct bonding is based on molecular adhesion. This bonding technique consists to join two surfaces into direct contact without the use of any adhesives or additional material. This process requires ...clean surfaces with a nanometric roughness, sufficiently close together to initiate bonding. Mechanical characterization of this type of assembly with classical mechanical test as for instance wedge test, cleavage test or double shear test present a highly scattering on mechanical results. This paper presents the Flexible Initiation Test (FIT test), an original test designed to characterize fracture initiation in mode I, and to decrease scattering in fracture initiation load measurements, in particular for adhesive with brittle behavior. This new test has to take into account the industrial framework: to be easy to manufacture, easy to execute repeatedly and efficient to provide experimental data for numerical models (stress criteria applications for instance). The paper proceeds first with an explanation of the main initial ideas to introduce the concept of this new test. Next, a numerical analysis is proposed to validate the concept and to determine the optimal geometry of the tests. Then the experimental device is set up and the concept is validated on three different adhesives with the same substrate (a brittle cyanoacrylate adhesive, a ductile and a brittle epoxy adhesives). To conclude, the FIT test is applied on direct bonded samples (an extreme case nanometric interface and very brittle behavior) to determine the fracture initiation load and to compare scattering of measurements.
Last years analytical or finite element models of milling become more efficient and focus on more physical aspects, nevertheless the milling process is still experimentally unknown on a wide range of ...use. This paper propose to analyse with accuracy milling operations by investigating the cutting forces values, shape of cutting forces curves obtained for different cutting speeds, and related phenomena as tool wear or tool run-out. These detailed experimental data in milling constitute a suitable experimental basis available to develop predictive machining modelling. All the tests have been conducted on the 304-L stainless steel in many cutting configurations and for different tool geometries. The machinability of the 304-L stainless steel with different tools geometries and configurations in shoulder milling is defined by three working zones: a conventional zone permitting stable cutting (low cutting speed; under 200–250 m.min
−1
), a dead zone (unfavourable for cutting forces level and cutting stability; between 250 and 450 m.min
−1
), and a high speed machining zone (high cutting speed; up to 450–500 m.min
−1
). All the used criteria (cutting forces, chips, wear) confirm the existence of these different zones and a correlation is proposed with cutting perturbations as tool run-out, cutting instability, ploughing, and abrasive wear.
The presence of defects in adhesive materials influences the mechanical behaviour of bonded composite structures. Indeed, the initial defects in the adhesive joint damage the mechanical properties of ...the interface and reduce the mechanical strength of the structure.
An in-depth study of the influence of initial porosity in the bonded joints on the mechanical properties of the rigid adhesive material has been experimentally measured. Different joint geometries are investigated by varying the thickness and the bonding area. A modified Arcan device is used to load the bonds in tension (initiation in Mode I). An imperfect interface model capable of considering the presence of initial diffuse cracks according to the Kachanov-Sevostianov material definition is then used to predict the mechanical properties of these adhesive joints. In this model, the presence of defects in the adhesive layer is described by an initial damage parameter: the porosity rate. Prior to testing, the porosity rates of the bonded joints are measured using μCT scans. Finally, the mechanical properties obtained experimentally are compared with those predicted by the imperfect interface model.
In this paper a methodology for studying the mechanical behaviour of complex structures made of GFRP structural members joined by means of adhesive connections and subject to variable intensity loads ...is presented. The fundamental equations are derived by an asymptotic approach applied on composite structures made of two elastic solids bonded together by a third thin one represented by a thin layer with a nonlinear behaviour. The adhesive layers are considered micro cracked according to the Kachanov’s assumptions.
Within this framework, to calibrate the parameters of the imperfect interface model, the mechanical properties and damage evolution of an epoxy adhesive have been experimentally evaluated under cyclic loadings. The experimental tests have been performed on aluminium cylinders considering different thicknesses of adhesive. The experimental results evidence how the adhesive thickness influences the strength, stiffness and consequently the initial damage parameter (initial cracks) of the bonded connections.
Finally, the robustness and accuracy of the imperfect interface model is demonstrated by the excellent comparison with experimental results of a GFRP hollow column to built-up beam adhesive connection, under static, cyclic and fatigue loads using a description of damage evolution.
The paper proposes to compare static and dynamic properties of three structural adhesives selected for the integration of optical space systems. These comparisons are based on an original methodology ...using static test and drop weight impact test to determine the best shock-strength candidate. In a first section, the design of the drop weight impact rig integrating a modified Arcan fixture is described. Then validity of the impact test rig is discussed on a simplified case thanks to an analytical model of “soft impact” available in literature. In a second section, static tests results are presented and the adhesive's critical stresses are identified with a point stress criterion. Finally, impact tests results are used to discuss the dynamic tensile and shear behaviour of each adhesive with different stress concentrations. Static critical stresses and impact strengths of the adhesives are reported. In conclusion a strategy of adhesive choice is discussed.
Direct bonding is of particular interest for optical system manufacturing for spatial application. This technology is already used in terrestrial optical system manufacturing because of the very high ...precision of the process and complex geometries are able to bond. However, even if a first prototype already passed with success space environment test, quantification and improvement of the mechanical strength of assemblies are essential to validate the assembly's life expectancy and to validate the European Space Agency standards. So, this work proposes to study the influence of process parameters, such as roughness, relative air humidity during room temperature bonding, annealing time and temperature, on mechanical strength of an elementary mechanical structure using a double shear test procedure and cleavage tests. At the same time, the performances of fused silica and Zerodur
®
glasses are compared. For the process considered in this study, a parallel is made between chemical phenomena, surface roughness and mechanical strength. In the end, cleavage tests confirm the choice of the optimal process parameters and highlight a damaging phenomenon of bonded interfaces with successive re-adhesion.
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