•Post-impact damage propagation in tension in woven carbon/epoxy laminates is studied.•The monitoring of damages is based on Digital Image Correlation and RX Tomography.•The propagation of cracks is ...governed by the resin damages and by delamination.•A FEM based on Semi-Continuous strategy is developed to represent these propagations.•This new modelling strategy well represent the damage initiation and propagation.
This article concerns the experimental and numerical study of post-impact damage propagation in thin carbon/epoxy woven composite laminates loaded in fatigue tension. Low velocity normal drop weight impact tests are first performed. Post-impact fatigue tensile tests are then carried out. They are controlled in displacement. The monitoring is based on Digital Image Correlation and RX tomography. The influence of the impact energy and the tensile fatigue loading on the post-impact damage propagation is studied. The damage propagation is governed by matrix damage, with the emergence of tows/resin splittings and intra-tows crackings as well as delamination when the plies have different orientations. When the impact energy or the displacement level increases, the post-impact damage initiates sooner and propagate faster. In some cases, that can lead to a quasi-instantaneous failure identical to that observed for quasi-static tensile loading. The FEM based semi-continuous approach, initialy developped for the modelling of impact damage, is extended to fatigue loading for carbon/epoxy woven laminates. Fatigue damage laws, based on experimental observations are implemented. The modelling well correlates the experimental results in terms of damage propagation scenario and speed depending on the number of cycles for laminates made up of plies with the same orientation.
The room-temperature fluorescence properties of DNA nucleoside and nucleotide aqueous solutions are studied by steady-state and time-resolved spectroscopy. The steady-state fluorescence spectra, ...although peaking in the near-UV region, are very broad, extending over the whole visible domain. Quantum yields are found to be mostly higher and the fluorescence decays faster than those reported in the literature. The fluorescence spectra of the 2‘-deoxynucleosides are identical to those of the 2‘-deoxynucleotides, with the exception of 2‘-deoxyadenosine, for which a difference in the spectral width is observed. The steady-state absorption and fluorescence spectra do not show any concentration dependence in the range 5 × 10-6 to 2 × 10-3 M. All fluorescence decays are complex and cannot be described by monoexponential functions. From the zero-time fluorescence anisotropies recorded at 330 nm, it is deduced that after excitation at 267 nm the largest modification in the electronic structure is exhibited by 2‘-deoxyguanosine. In the case of purines, the fluorescence decays and quantum yields are the same for 2‘-deoxynucleosides and 2‘-deoxynucleotides. In contrast, for pyrimidines, the fluorescence quantum yields of nucleotides are higher and the fluorescence decays slower as compared to those of the corresponding nucleosides showing that the phosphate moiety affects the excited-state relaxation.
This paper deals with static indentation of sandwich beams with a foam core. An analytical model is presented assuming that the specific response of foams in compression can be assimilated to an ...elastic–perfectly plastic behavior. The elastic part is represented using Vlasov’s model. The displacements and stress calculated with this two parameters model are compared with results given by Winkler’s theory and Finite Element Method. Vlasov’s idealization gives more accurate results that Winkler’s model. A complete study of the influence of the parameters of Vlasov’s model is performed. Then, plasticity is added to the model to represent the non-linear response of the core. The load–displacement response of sandwich beams under static indentation is calculated and compared to experimental and Finite Element results. A good correlation is found. The size of the area where the foam is crushed given by the developed model is in good agreement with Finite Element analysis.
In aerospace acamedic and industrial world, soft impacts are commonly used to replace bird strike tests for the validation of materials and structures as well as the calibration of numerical models. ...However in general, the analysis reported show only a few part of the experimental information available. In this paper, three laminate composites made of epoxy resin reinforced by glass or carbon fibres are tested under gelatin impact at several velocities up to complete failure. A detailed analysis based on 3D Digital Image Correlation (3D DIC) and visual inspection of the three laminates is provided for a total of 21 tests with impact velocities in the range 60-112 m/s. DIC extraction provides accurate quantitative displacement fields of the rear face in both time and space. Moreover, specific failure scenarios are identified for each laminate. The results obtained provide a suitable database for the development of numerical models. In addition, all experimental data from DIC extractions are opened to the readers on the Recherche Data Gouv website for comparisons with their own tests or numerical models.
•Pure mode I tests are carried out on several woven composites configurations.•Carbon/epoxy and glass/epoxy woven fabrics have been studied.•Each interface has been tested in both static and dynamic ...mode I (DCB test).•The influence of ply orientation, fiber nature and adhesive has been evaluated.
Woven composites are well-known for their good transverse properties and for their high fracture toughness. The damage mechanisms leading to delamination in woven composites are identified in mode I. The influence of several parameters, including the draping sequence and the fiber/matrix interface on the fracture toughness of woven composite laminates is studied. Pure mode I tests are carried out on several carbon/epoxy and glass/epoxy woven composites configurations and the differences observed are discussed from a fractographic point of view. A novel experimental method is designed to perform dynamic pure mode I tests. The study illustrates the high fracture toughness of the composites made of woven fabrics as well as the influence of the orientation of the plies, the nature of the fibers and the addition of an adhesive film on the fracture toughness in mode I. The dynamic tests prove that, on the configurations tested and for crack velocities up to 100m/s, the crack propagation velocity has a limited effect on the value of GIc.
The gold standard end point in randomized clinical trials in metastatic breast cancer (MBC) is overall survival (OS). Although therapeutics have been approved based on progression-free survival ...(PFS), its use as a primary end point is controversial. We aimed to assess to what extent PFS may be used as a surrogate for OS in randomized trials of anti-HER2 agents in HER2+ MBC.
Eligible trials accrued HER2+ MBC patients in 1992–2008. A correlation approach was used: at the individual level, to estimate the association between investigator-assessed PFS and OS using a bivariate model and at the trial level, to estimate the association between treatment effects on PFS and OS. Correlation values close to 1.0 would indicate strong surrogacy.
We identified 2545 eligible patients in 13 randomized trials testing trastuzumab or lapatinib. We collected individual patient data from 1963 patients and retained 1839 patients from 9 trials for analysis (7 first-line trials). During follow-up, 1072 deaths and 1462 progression or deaths occurred. The median survival time was 22 months 95% confidence interval (CI) 21–23 months and the median PFS was 5.7 months (95% CI 5.5–6.1 months). At the individual level, the Spearman correlation was equal to ρ = 0.67 (95% CI 0.66–0.67) corresponding to a squared correlation value of 0.45. At the trial level, the squared correlation between treatment effects (log hazard ratios) on PFS and OS was provided by R2 = 0.51 (95% CI 0.22–0.81).
In trials of HER2-targeted agents in HER2+ MBC, PFS moderately correlates with OS at the individual level and treatment effects on PFS correlate moderately with those on overall mortality, providing only modest support for considering PFS as a surrogate. PFS does not completely substitute for OS in this setting.
An explicit finite element modelling of the 5-harness satin woven composite material is proposed in this paper. It is based on the semi-continuous approach which consists in separating fibre and ...matrix mechanical behaviours. The bundles are modelled with rod elements and a specific damageable shell element is used to stabilize this truss structure. As the woven pattern geometry plays a key role in damage initiation and propagation, a modification has been made in the failure strain of the rods located at the crimp regions where warp and weft yarns cross each other.
The method has been implemented into the explicit finite element code RADIOSS and is computationally efficient to model low to medium velocity impact (1–200m/s) at the structure scale. The modelling strategy is validated by representing a drop weight test and an oblique impact test and provides good prediction of impact force history and damage size and location.
High speed dynamic loadings such as small engine fragments, bird strike, tyre impact or ice debris are a concern for many aeronautical structures, as they can create severe damages raising safety ...issues. A strategy to develop dedicated mechanisms for energy absorption of high speed dynamic impact debris at sub-component level is therefore proposed by means of several reinforced foam-woven composite structures. Among the tests for evaluating the mechanical performances, dynamic crushing tests were performed on a slice of such reinforced composite structures to evaluate their energy absorption. Using simultaneously load signal and fast camera imaging, the tests were analyzed to provide important informations such as damage mechanisms and displacement-load-energy absorption values. At the end, quantitative criterions are presented in order to distinguish the designs that have a good potential for absorbing shock energy and for getting a better understanding for designing reinforced composite structures.
Adhesive bonding is a commonly used method in multi-materials assemblies dedicated to the transport fields. In order to ensure structures integrity and users safety, the knowledge of the mechanical ...behaviour of structural adhesives used in these assemblies under impact conditions appears to be an essential prerequisite. To date, numerous tests combining usual specimens geometry e.g. single lap joint, butt joint, etc. and high velocity testing rigs exist and are used. Among these, most allow comparative studies and a few provide a partial identification of the material properties of the investigated adhesive.
In this study, an experimental method dedicated to the dynamic characterization of structural adhesives under drop weight condition is proposed. On the basis of existing works, a modified Arcan specimen and a dynamic tensile testing mean were developed and are presented. The Arcan geometry allows to test the adhesive under various loading directions and so to obtain its mechanical response envelope. Design strategies are also implemented in order to obtain time stable and quasi-homogeneous stress distributions in the adhesive during the tests.
At last, the dynamic characterization of a Dow® BetamateTM 1496 V adhesive is proposed. Results are repeatable and show a strain rate dependent behaviour validating the appropriateness of the experimental approach.
The characterization of the adhesive of bonded assemblies under combined and dynamic loading cases appears to be crucial for the development of the future structures dedicated to the transport ...industry. To date, most of the tests on adhesive joints are dedicated to comparative studies and only a few ones to characterization. Among these, the stress concentration-free bonded Arcan Tensile/Compression-Shear test specimen (Arcan TCS) developed by Créac’hcadec et al. allows to characterize the adhesive of bonded joints under combined quasi-static loading cases while minimizing the edge effects. This paper deals with an extension of the use of this specimen under dynamic loadings.
In a first part, an experimental study of the Arcan TCS device under drop weight conditions is made. The mechanical behaviour of the adhesive appears to be non-linear and clearly dependent of the strain rate. Also, stress-strain curves highlight a significant influence of tests conditions. In particular, the way the kinetic energy is transmitted by the falling mass to the testing device plays a significant role on the vibrational behaviour and the loading rate of the specimen.
In a second part, a dedicated finite element model is built under the plane stress and elastic assumptions. Results extracted from this numerical study are in agreement with several experimental observations. Moreover, they allow a better understanding of the loading seen by the adhesive.
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