A Study of the interlaminar fracture toughness of unidirectional flax/epoxy composites

Having environmental and economic advantages, flax fibers have been recognized as a potential replacement for glass fibers as reinforcement in epoxy composites for various applications. Its widening applications require employing failure criteria and analysis methods for engineering design, analysis, and optimization of this material. Among different failure modes, delamination is known as one of the earliest ones in laminated composites and needs to be studied in detail. However, the delamination characteristics of unidirectional (UD) flax/epoxy composites in pure Mode I has rarely been addressed, while Mode II and Mixed-mode I/II have never been addressed before. This work studies and evaluates the interlaminar fracture toughness and delamination behavior of UD flax/epoxy composite under Mode I, Mode II, and Mixed-mode I/II loading. The composites were tested following corresponding ASTM standards and fulfilled all the requirements. The interlaminar fracture toughness of the composite were determined and validated based on the specific characteristics of natural fibers. Considering the variation in the composite structure configuration and its effects, the results of interlaminar fracture toughness fit in the range of those reported for similar composites in the literature and provide a basis for the material properties of this composite

Effect of porosities on brazed martensitic steel tensile properties: 2D and 3D pre-mortem vs post-mortem characterizations

International audienceA quantitative investigation of the porosity and its effects on mechanical properties was conducted on brazed materials presenting macroscopically brittle behavior with various defect levels. Three different techniques for evaluating the porosity and the fracture mechanisms were compared: (i) pre-mortem X-ray computed tomography (XCT), (ii) post-mortem laser scanning confocal microscopy (LSCM), and (iii) post-mortem scanning electron microscopy (SEM). The apparent surface area of the pores was evaluated by segmenting the height maps produced by LSCM and the electron backscattered images produced by SEM. The results show good agreement for different porosity levels. The LSCM height maps for opposite fracture faces were virtually stitched together in order to reconstruct three-dimensional (3D) images of the whole brazed joints. These proved to be similar to the XCT 3D segmented images, yet with some discrepancies due to local plastic deformation. Quantitative volume fraction and spatial distribution of pores were assessed as well as the identification of fracture mechanisms. Interestingly, it was noted that two specimens with different ultimate tensile strengths presented the same porosity amount, but different distributions. Two distinctive features were identified in the broken specimens: plastically teared pillars and walls that were separating the pores before fracture, and plateaus of approximately constant height with micro-sized dimples showing interfacial fracture. Although the specimens exhibited a macroscopic brittle behavior, these features are characteristic of ductile fracture. The most probable scenario for fracture was conjectured as follows: (i) Final fracture occurs in the near-surface region of the samples and conversely early damage develops in the volume, (ii) Interfacial fracture within plateaus is responsible of the rupture/strength of the brazed specimen but detrimental effect is induced by the tearing if highly voided regions are present in the joint

Etude par microtomographie de la nocivité de la porosité sur la tenue en fatigue de pièces en Al-Si9Cu3

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Microtomographic study and finite element analysis of the porosity harmfulness in a cast aluminium alloy

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Compression plane à chaud d'un alliage de zirconium.

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Mikrostruktura stopu Zircalloy-4 przetwarzanego w podwyższonych temperaturach

The microstructure of commercial purity Zircalloy-4 (Zry-4) compressed at temperatures 650°C and 750°C up to strains of 0.8 was characterized over a wide range of scales, using optical metallography, scanning (SEM) and transmission (TEM) electron microscopy. The typical microstructure after warm deformation consisted of the α-phase matrix and the second phase particles (SPP) enriched in Fe and Cr. Two kinds of these particles were observed. The large isolated SPP were situated along boundaries of lamellae of α-phase, whereas very fine intermetallic particles were nearly homogeneously distributed inside the lamellae. The intensity of recrystallization was main factor deciding on microstructure development in this material. The efficiency of that process increased quite rapidly with temperature or decreasing strain rate. The appearance of twins was the second important feature of the "warm deformed" structure with the prevailing {1102} - type system of twins. They were occasionally observed in all samples after the deformation at 650°C and 750°C temperatures.W pracy dokonano wieloskalowej charakterystyki mikrostruktury stopu Zircalloy-4 przerabianego plastycznie w zakresie pośrednich temperatur, tj. 650°C-750°C, w zakresie odkształceń logarytmicznych do 0.8, z wykorzystaniem technik mikroskopii optycznej oraz skaningowej i transmisyjnej mikroskopii elektronowej. W obrazie mikrostruktury obserwowano płytki fazy α oraz wydzielenia cząstek drugiej fazy wzbogaconoej w Fe oraz Cr. Obserwowano dwa typy wydzieleń. Duże izolowane cząstki usytuowane były wzdłuż granic płytek fazy α, podczas gdy wydzielenia dyspersyjne rozmieszczone były równomiernie w strukturze stopu. Głównym parametrem który decydował o zróżnicowaniu w strukturze stopu była intensywność zachodzenia procesu rekrystalizacji, która silnie uzależniona była od temperatury i prędkości odkształcenia. Drugim istotnym zjawiskiem obserwowanym po odkształceniu w obydwu temperaturach było pojawienie się obszarów zbliźniaczonych, przy czym dominował system zbliźniaczenia na płaszczyźnie {1102}

IN-PLANE and out-of-plane deformation at the SUB-GRAIN scale in polycrystalline materials assessed by confocal microscopy

International audienceHigh-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near Σ3 twin boundaries with bulk locations in the elastic domain (macroscopic stress as low as 80% of the 0.2 % offset yield strength (Y.S.0.2%)). This intense irreversible strain localization occurs with either a high Schmid factor (μ > 0.43) or a significant elastic modulus difference between the pair of twins (ΔΕ > 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the real glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide