Robust improvement of the asymmetric post-buckling behavior of a composite panel by perturbing fiber paths

The buckling behavior of structures is highly sensitive to imperfections, i.e., deviations from the geometry and material properties of the ideal structure. In this paper, an approach is presented in which the effects of spatially varying fiber misalignments in composite structures are assessed through random field analysis and are subsequently used to improve the structure while simultaneously making it more robust to fiber misalignments. Effects of misalignments are quantified by applying random fields on the structure, which represent fiber misalignments. Using analyses of the effect of the random local stiffness changes due to fiber misalign- ments, a pattern of the relative influence these local changes have on the buckling load is created. By applying a small change to local fiber orientation corresponding to this pattern to the original structure, the performance of the design is improved. Additional stochastic analyses are performed using the improved design, reanalyzing the effects local fiber misalignments have on the structural performance and the subsequent changes in robustness. Stochastic results show an overall increase in the mean buckling load and a reduction in the coefficient of variation in the analysis of the perturbed structure. The approach is applied to a composite panel exhibiting asymmetric post‐buckling behavior, i.e., having an unstable post‐buckling branch and an (initially) stable branch. Results show that perturbations in the fiber path can nudge a structure into a more stable post‐ buckling path by promoting a post‐buckling path using local changes in structural stiffness. The robustness of improved designs can also increase, making structures less susceptible to local fiber misalignments

Influence of reversed fatigue loading on damage evolution of cross-ply carbon fibre composites

Microcrack formation and delamination growth are the main damage mechanisms in the fatigue of composites. They lead to significant stiffness loss, introduce stress concentrations and can be the origin of subsequent damage events like buckling or fibre breakage, especially in case of shear and compression stresses during load reversal. Fatigue experiments of carbon fibre reinforced laminates were conducted at several stress ratios and analysed in terms of crack and delamination growth. These investigations were accompanied by microscopic imaging, digital image correlation and finite element modelling to take into account the effects of residual stresses and crack closure. It was found that residual stresses significantly change the local stress ratio in off-axis layers and lead to residual crack opening of inter fibre cracks. These cracks remain open and close under high compression loadings only. Furthermore, crack formation under pulsating compression loading turned out to be driven by residual stresses leading to perpendicular cracks as observed under pure tension loading. The experimental findings further confirm the severe detrimental effect of tension-compression loading on crack formation and delamination growth compared to pulsating tension-tension or compression-compression loads

Effect of spatially varying material properties on the post-buckling behaviour of composite panels utilising geodesic stochastic fields

The post-buckling behaviour of panels can be very sensitive to imperfections or variations in materials or geometry. This paper presents an ecient numerical model to calculate the eects of material stiffness variations on the non-linear response of a structure. This is done by first defining a geodesic mesh on which a unit variance random field is generated. This field uses the true geodesic distance on the structure to calculate how points in the field should be correlated. The fields generated are projected onto a 3D structural mesh which is used for assembly and post-processing of the structural model. The structural model, based on the Unified Formulation is capable of accurate non-linear calculations of both straight and curved elements. Baseline results generated using the implementation are compared to those in literature, and verified using Abaqus. Random material variations are then applied to the structure in a Monte Carlo analysis. The analyses show that the local variation of stiffness can have a variety of effects on the non-linear response of structures. Aside from the change of mean stiffness causing a change in bifurcation or limit point load, the different stiffness distributions can affect and trigger competing buckling modes and post-buckling modes and affect their corresponding post-buckling load-deflection paths

Progressive Failure Analysis Using Global-Local Coupling Including Intralaminar Failure and Debonding

Composite laminate stiffened panels are often used in aircraft fuselage design because of their favorable properties. To assess the failure load of these thin-walled structures and to exploit their reserves, a reliable simulation capability for their postbuckling behavior is often necessary. To perform a realistic failure analysis and to accurately detect final collapse, material degradation should be considered. Global-local approaches are computationally efficient techniques to perform a progressive failure analysis and to examine localized damaged areas in detail. In this paper, a two-way coupling global-local approach is presented, including a combination of different damage modes, such as matrix cracking, fiber damage, and skin-stringer debonding. An accurate exchange of information concerning the damage state between global and refined local models is performed. From the global to the local model, the displacements are transferred through a submodeling procedure. Afterward, the degraded material properties obtained from the local model analysis are returned to the global model with a special mapping technique that accounts for the different mesh sizes at the two levels. The two-way coupling procedure is applied to the progressive failure analysis of a one-stringer composite panel loaded in compression. Finally, the numerical results of the procedure are compared with experimental results

Analysis of skin-stringer debonding in composite panels through a two-way global-local method

According to various experimental results, stiffened panels under compressive loading are prone to debonding between the skin and the flange of the stringer. In this paper, a novel two-way global-local coupling approach is presented that is able to model progressive separation of the skin and stringer in stiffened CFRP panels under compression. The main goal of this methodology is to examine skin-stringer debonding at two levels of accuracy, taking advantage of the fast calculations at the global level and assessing in detail the damage propagation at the local level. First, critical areas are defined in a global model with a standard mesh, and local models with a considerably finer mesh are created by means of a submodeling technique. Secondly, a local model analysis is conducted, in which cohesive elements are applied to simulate debonding. Particularly important is the appropriate information exchange in both directions between the different steps of the coupling analysis. Averaged degraded properties are defined at the local model level and transferred back to the global level. The applied compressive load is increased and induces a progression in skin-stringer separation. The global-local coupling loops are repeated until panel failure occurs. The approach is applied to a case of a representative one-stringer stiffened panel and to a stiffened panel for which test results are available. A good correspondence with reference results and test results demonstrates the effectiveness of the global-local approach presented

Multicomponent Synthesis of 3,6-Dihydro-2H-1,3-thiazine-2-thiones

Non-fused 3,6-dihydro-2H-1,3-thiazine-2-thiones constitute a so far rather unexplored class of compounds, with the latest report dating back more than two decades. Thiazine-2-thiones contain an endocyclic dithiocarbamate group, which is often found in pesticides, in substrates for radical chemistry and in synthetic intermediates towards thioureas and amidines. We now report the multicomponent reaction (MCR) of in situ-generated 1-azadienes with carbon disulfide. With this reaction, a one-step protocol towards the potentially interesting 3,6-dihydro-2H-1,3-thiazine-2-thiones was established and a small library was synthesized

Imminent brain death: point of departure for potential heart-beating organ donor recognition

Contains fulltext : 88186.pdf (publisher's version ) (Closed access)PURPOSE: There is, in European countries that conduct medical chart review of intensive care unit (ICU) deaths, no consensus on uniform criteria for defining a potential organ donor. Although the term is increasingly being used in recent literature, it is seldom defined in detail. We searched for criteria for determination of imminent brain death, which can be seen as a precursor for organ donation. METHODS: We organized meetings with representatives from the field of clinical neurology, neurotraumatology, intensive care medicine, transplantation medicine, clinical intensive care ethics, and organ procurement management. During these meetings, all possible criteria were discussed to identify a patient with a reasonable probability to become brain dead (imminent brain death). We focused on the practical usefulness of two validated coma scales (Glasgow Coma Scale and the FOUR Score), brain stem reflexes and respiration to define imminent brain death. Further we discussed criteria to determine irreversibility and futility in acute neurological conditions. RESULTS: A patient who fulfills the definition of imminent brain death is a mechanically ventilated deeply comatose patient, admitted to an ICU, with irreversible catastrophic brain damage of known origin. A condition of imminent brain death requires either a Glasgow Coma Score of 3 and the progressive absence of at least three out of six brain stem reflexes or a FOUR score of E(0)M(0)B(0)R(0). CONCLUSION: The definition of imminent brain death can be used as a point of departure for potential heart-beating organ donor recognition on the intensive care unit or retrospective medical chart analysis.1 september 201

Transanal endoscopic microsurgery versus endoscopic mucosal resection for large rectal adenomas (TREND-study)

Background: Recent non-randomized studies suggest that extended endoscopic mucosal resection (EMR) is equally effective in removing large rectal adenomas as transanal endoscopic microsurgery (TEM). If equally effective, EMR might be a more cost-effective approach as this strategy does not require expensive equipment, general anesthesia and hospital admission. Furthermore, EMR appears to be associated with fewer complications. The aim of this study is to compare the cost-effectiveness and cost-utility of TEM and EMR for the resection of large rectal adenomas. Methods/design. Multicenter randomized trial among 15 hospitals in the Netherlands. Patients with a rectal adenoma 3 cm, located between 115 cm ab ano, will be randomized to a TEM- or EMR-treatment strategy. For TEM, patients will be treated under general anesthesia, adenomas will be dissected en-bloc by a full-thickness excision, and patients will be admitted to the hospital. For EMR, no or conscious sedation is used, lesions will be resected through the submucosal plane i