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Assemblies of many materials, especially composites, laminates, interlayers, and assemblies of composites with metals are made preferentially with a structural adhesive. The mechanical behavior of an adhesive thin film, 100 to 500 micrometers of thickness, embedded by adjacent stiff surfaces, is not the behavior of the same free massive material. The plasticity and the rupture of those adhesive films subjected to complex state of stresses can be studied to get the intrinsic properties of the adhesives. Plasticity and rupture criteria depend on the stress invariants I1 (hydrostatic pressure) and J2 (Von Mises equivalent stress). The Arcan-Mines test gives those plasticity and rupture criteria of thin adhesive films when submitted to complex loading. Those criteria are modified Drucker-Prager criteria. The rupture criterion obtained with Arcan-Mines test is compared with results obtained with linear fracture mechanic applied to the fracture of the adhesive film. [ABSTRACT FROM AUTHOR]

In the frame of the European project PMJoin, researches lead to the development of a new manufacturing laser process in order to assemble metal and glass fibber reinforced polyamide 6-6 composite, without extra filler. Knowledge on the mechanical behaviour and characterization data for both material components, and evaluation of the assembly performances, (Arcan-Mines tests), permit the strength calculation and the numerical simulation of the impact failure of a hybrid assembly. [ABSTRACT FROM AUTHOR]

For different welded plastics assemblies by laser TWIST® we experimentally establish a plastic and breaking intrinsic stress criterion. Lot of plastic materials has been studied. We present here, the characterization of ABS assemblies and PA66 assemblies. The yield surfaces which characterize the limit of elasticity and the rupture, have been described by a modified Drucker-Prager criterion identified in the case of complex mixed modes state of stresses coupling tensile and shear stresses. For design purpose of welded zones, the proposed Drucker-Prager criterion has been used for the calculation and the simulation of breaking welded ABS plates and welded PA66 stepped plates. The comparison between experimental and numerical simulation values shows a relatively good agreement. [ABSTRACT FROM AUTHOR]

Numerical calculations become a more and more efficient way to estimate the lifetime of structures under thermomechanical loading. But these life estimations cannot be reliable if the needed parameters have not been correctly identified and measured and if not all the damage causes have been considered. The disk testing under gas pressure is an experimental technique used for the mechanical characterization of thin sheets with various conditions: monotonous loading (biaxial rupture tests at strain rates from 10-6 to 10° s-1), constant loading under high stresses (sustained load) at elevated temperature (creep tests), cyclic loading (mechanical fatigue tests); the temperature may be chosen between 20 and 900 °C. The disk testing quickly reveals the damage when crossing cracks through the membrane create leakages detected by a mass spectrometer. Besides, we have developed a calculation method in order to measure the true mechanical properties of the pressurized disk. We evidence the quality of the results obtained with this method in the thermomechanical characterization of thin sheets and its sensitivity to secondary parameters. The analytical method of calculation will be optimized in order to obtain the best modelling of thermomechanical behaviour of materials loaded at temperature and strain rates within very large ranges and subject to different environments as gaseous hydrogen. [ABSTRACT FROM AUTHOR]

A mechanical modelling is proposed in order to describe viscoplastic behaviour without hardening of a nickel-base super alloy loaded at high temperature (900 °C) with strain rates varying within a wide range (from 10−1 to 10−4 s−1). A mathematical law is associated to the viscoplastic model; the parameters of the law are identified from monotonic biaxial tests of membranes loaded by pressure of inert gas (disk pressure testing under helium). The viscoplastic law provides calculated stresses with accuracy better than 1% at the highest strain rates and 4% at the lowest strain rates; the identified yield stress is a logarithmical function of strain rate as for other metallic alloys studied in the bibliography. The parameters identified from biaxial tensile tests of disks have been successfully used to calculate the stresses during stabilized tension–compression loops of cylindrical specimens. The proposed experimental method and behaviour model are interesting because the disk biaxial testing is much more easily performed at high temperature than the tension–compression testing of cylindrical specimens. [ABSTRACT FROM AUTHOR]

Since three decades, the French nuclear centre (CEA) has defined and systematically used the disk pressure testing for studying and measuring the metals sensitivity to gaseous hydrogen. In 1990, this experimental method became a French standard to select metallic alloys used to construct high-pressured vessels under gaseous hydrogen. The disk pressure testing compares the pressure resistance of membranes similarly tested under helium and hydrogen. The experimental method is simple and sensitive and its results have always been sure; the disk testing allows original and various studies and reveals parameters that are not significant with less sensitive methods. First, we present an overview of different experimental results: the influence of the mechanical resistance on the hydrogen embrittlement, the influence of the chemical composition and the mechanical or thermal treatments of the materials, the influence of the gas composition (hydrogen content and impurities content)... Then, we present different types of metals damage: the metallurgical embrittlement of different alloys used at medium and high temperatures, the influence of the thermal ageing on gaseous hydrogen embrittlement, the embrittlements by gaseous and internal hydrogen when hydrogen enters the material before or during mechanical loading, the influence of the initial hardening mechanism on thermal ageing and on the hydrogen embrittlement, the shortened fatigue life due to thermal ageing and gaseous hydrogen and the synergy induced by the mutual presence of both these phenomena. The embrittlements are discussed and extrapolated to the loading and the extreme environment found within a space launcher engine: high temperatures with low partial pressure of hydrogen, rapid cooling, hydrogen super-saturation and high thermal cyclic strains. [ABSTRACT FROM AUTHOR]

A mechanical modelling is proposed in order to describe the viscoplastic behaviour without hardening at high temperature (900 °C) of a nickelbase super alloy with strain rates varying within a large range (from 10-1 to 10-4 S-1). A mathematical law is associated to the viscoplastic model; the law parameters are identified from monotonic biaxial tests of membranes loaded by pressure of inert gas (disk pressure testing under helium). The viscoplastic law provides calculated stresses with accuracy better than 1% at the highest strain rates and 4% at the lowest strain rates for the whole strain range; the identified yield stress is a logarithmical function of strain rate as for other metallic alloys studied in bibliography. The parameters identified from biaxial tensile tests of thin sheets have been used to calculate accurate stresses during stabilized tension-compression loops of cylindrical specimens. The proposed experimental method is interesting because the disk biaxial testing is much easier to be performed at high temperature than the tension-compression testing of cylindrical specimens. [ABSTRACT FROM AUTHOR]

Numerical calculations are becoming more and more efficient in estimating the lifetime of structures under thermomechanical loading. However, these life estimations cannot be reliable if the necessary parameters have not been correctly identified and measured and if all the causes of damage have not been considered. Disk testing under gas pressure is similar to oil bulging testing. However, disk testing can easily be used for the mechanical characterization of materials subject to more varied solicitations: monotone loading (biaxial rupture tests at strain rates from 10−6 to 100 s−1), constant loading under high stresses (sustained load) at elevated temperature (creep tests), cyclic loading (mechanical slow fatigue tests); the temperature may be chosen between 20 and 900 °C and the environment may be studied by comparing the results obtained with either an inert gas or reactive gas. Moreover, disk testing reveals light damage since crossing cracks through the thin membrane create leakages detected by a mass spectrometer. In this paper, we present an original method of calculation developed to determine the true mechanical properties of the pressurized disk; the method of calculation is validated because its numerical results are identical to the measured tensile properties. In addition, the range of uniform deformation is correctly determined; this property is needed to establish sheet formability which is not clearly determined by oil bulging. Of course, the mechanical behaviour can be determined within the whole ranges of temperature and strain rates; such wide ranges cannot be tested by other techniques such as tensile testing or oil bulging. As disk edges are not stressed during testing, the results are very reproducible at any temperature and at any strain rate while the machining or cutting defects initiate very scattered ruptures of tensile specimens tested at high temperature or at high strain rate. The disk and its loading simulate real applications with thin walls embedded by thick parts such as thermal exchangers or spatial engines. The analytical method of calculation may be used for identifying the needed parameters of thermomechanical modelling; it will be optimized by finite elements methods and it would allow a rational quantification of hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

Abstract: The ability of structural adhesive to withstand combination of cyclic loading and service environment makes them suitable for a wide range of applications. It is nevertheless true that their mechanical and micromechanical behaviour is still little understood. The purpose of the work reported here is to study the fracture process and the role of talc fillers of an epoxy adhesive subjected to oligocyclic fatigue. Tests were conducted by using a set-up based on the Arcan fixture and experimental results have been reported for both tensile and shear cyclic loadings. The cohesive failure of the adhesive seems to set off at a specific amplitude level, whatever the loading value. By an incursion at the microstructural scale using scanning electron microscopy, we suggest an explanation of these phenomena. The foliated form of talc fillers are of major importance and seem to provide a deterministic evolution of damage. [Copyright &y& Elsevier]

We propose a non-local failure criterion to predict the failure of composite laminated structures submitted to local stress concentration. The twill glass/polyamide 6 composite specimens with an open-hole as well as a bolted joint have been investigated. Material behavior was modeled using Hill's yield criterion to describe its anisotropic non-linearity. To determine a structure failure, based on the classical characteristic distance approach, the proposed method predicts the structure failure using the characteristic volume identified by experimental testing and numerical simulations. The average values of failure criteria indices over this characteristic volume were used to determine the failure. The damage initiations were predicted and showed a good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

This study deals with the characterization of two adhesive joints under quasi static and fatigue loading. Different testing systems were involved. For quasi static loading, the complex behaviour model based on modified Drucker-Prager criterion taken into account hydrostatic stress and rate dependence was proposed to describe the elasto-visco-plastic experiments with the aid of finite element simulations (FEM). For fatigue loading, the lifetime of adhesives was studied. S-N curves was established and extended a failure criterion for any number of cycles. Endurance limit was determined for any combination of applied load for one adhesive, while another adhesive showed the endurance limit only in shear loading. [ABSTRACT FROM AUTHOR]