Ives De Baere

Damage-Resistant Composites Using Electrospun Nanofibers: A Multiscale Analysis of the Toughening Mechanisms

Today, fiber-reinforced polymer composites are a standard material in applications where a high stiffness and strength are required at minimal weight, such as aerospace structures, ultralight vehicles, or even flywheels for highly efficient power storage systems. Although fiber-reinforced polymer composites show many advantages compared to other materials, delamination between reinforcing plies remains a major problem limiting further breakthrough. Traditional solutions that have been proposed to toughen the interlaminar region between reinforcing plies have already reached their limit or have important disadvantages such as a high cost or the need for adapted production processes. Recently, electrospun nanofibers have been suggested as a more viable interlaminar toughening method. Although the expected benefits are numerous, the research on composite laminates enhanced with electrospun nanofibrous veils is still very limited. The work that has been done so far is almost exclusively focused on interlaminar fracture toughness tests with different kinds of nanofibers, where typically a trial and error approach has been used. A thorough understanding of the micromechanical fracture mechanisms and the parameters to obtain toughened composites has not been reported as of yet, but it is crucial to advance the research and design highly damage-resistant composites. This article provides such insight by analyzing the nanofiber toughening effect on three different levels for several nanofiber types. Only by combining the results from different levels, a thorough understanding can be obtained. These levels correspond to the hierarchical nature of a composite: the laminate, the interlaminar region, and the matrix resin. It is found that each level corresponds to certain mechanisms that result in a toughening effect. The bridging of microcracks by electrospun nanofibers is the main toughening mechanism resulting in damage resistance. Nevertheless, the way in which the nanofiber bridging mechanism expresses itself is different for each scale and dependent on parameters linked to a certain scale. The multiscale analysis of the toughening mechanisms reported in this paper is therefore crucial for understanding the behavior of nanofiber toughened composites, and as such allows for designing novel, damage-resistant, nanofiber-toughened materials.

Sensor design for outdoor racing bicycle field testing for human vibration comfort evaluation

This paper is concerned with the vibrational comfort evaluation of the cyclist when cycling a rough surface. Outdoor comfort tests have so far only been done through instrumenting the bicycle with accelerometers. This work instruments a racing bicycle with custom-made contact force sensors and velocity sensors to acquire human comfort through the absorbed power method. Comfort evaluation is assessed at the hand–arm and seat interface of the cyclist with the bicycle. By means of careful finite-element analysis for designing the force gauges at the handlebar and the seat combined with precise calibration of both force and velocity sensors, all sensors have proven to work properly. Initial field tests are focused on the proper functioning of the designed sensors and their suitability for vibration comfort measurements. Tests on a cobblestone road reveal that the outcome of the absorbed power values is within the same range as those from laboratory tests found in the literature. This sensor design approach for outdoor testing with racing bicycles may give a new interpretation on evaluating the cyclists comfort since the vibrational load is not only quantified in terms of acceleration but also in terms of force and velocity at the bicycle–cyclist contact points.

Flexural mechanical properties of porcine aortic heart valve leaflets

Freshly excised porcine aortic heart valve cusps were subjected to a uni-axial flexural indentation test, from which the rupture characteristics and a functional stiffness parameter were determined. It was found that the flexural mechanical properties of aortic valve cusps (i) are unaffected by their coronary position and (ii) are sensitive to the effect of mechanical preconditioning. The resulting values of the cusps flexural mechanical properties are intended as a set of reference properties which scaffolds, meant for the tissue engineering of heart valves, must approximate in order to be considered as a functional replacement.

High Strain Monitoring during Fatigue Loading of Thermoplastic Composites Using Imbedded Draw Tower Fibre Bragg Grating Sensors

This paper presents the experimental study of fibre Bragg grating sensors for measuring strain inside composite laminates during fatigue loading. The optical fibres are imbedded inside thermoplastic CFRP test-coupons which have an ultimate strain of about 1.1%. Tension – tension fatigue cycling at a rate of 5Hz is been carried out at 314MPa with a maximum strain of 0.51%. At such extreme strain levels the use of high strength sensors becomes inevitable. Neither the sensor nor the composite test-coupons showed any significant degradation even after more than 500000 cycles. Fibre optic Bragg grating sensors are known to be very accurate strain sensors but one should be very careful interpreting their response once they are imbedded inside composite materials. In this study high strength fibre Bragg grating sensors with coating are imbedded in composite test coupons and a pretty good correlation was found between the strain measurements of an electrical extensometer and the imbedded sensor during the complete cycling. The high strength sensor show to be very feasible for extreme and long term strain measurements.

Pressure measurement on the surface of a rigid cylindrical body during slamming wave impact

Among all kinds of loads that floating and fixed marine constructions experience, water wave slamming can be considered as one of the most critical. To prevent naval constructions from failing due to slamming impact, slamming loads should be carefully investigated. Besides analytical and numerical calculations, experimental data is of crucial importance. Slamming loads can be measured by performing pressure measurements on the surface of the object during impact. Previous publications showed that precise and correct measurements are very difficult to perform, especially for slamming events with small deadrise angles. Large scatter mostly characterizes these measurements. This research focuses on improving the accuracy and reproducibility of the pressure recordings. Therefore, slamming drop tests are performed on a rigid cylindrical body. Most attention is paid to the bottom of the cylinder where the deadrise angle is 0 degrees.

Experimental study on the impact loads acting on a horizontal rigid cylinder during vertical water entry

This paper experimentally studies the local and global loads acting on a rigid cylinder subjected to water wave slamming. Local loads are hereby expressed in terms of pressure on the cylindrical surface while global loads are investigated in terms of force acting on the complete cylinder. Global impact loads may be better suited for use in design processes. An experimental setup to perform vertical drop experiments to approximate wave slamming is presented and the necessary measuring equipment is described. The experimental results are firstly discussed in the time domain to understand what exactly is happening during the water entry and in what stage the maximum loads occur. The measurements learn that the time scale of the pressure and the force histories is considerably different. Secondly, the attention is focused on the peak values of the time plots. These impact pressures and impact forces are represented as function of the impact velocity. The pressure is hereby given for different positions along the circumference of the cylindrical surface. The experiments show that the impact pressure and force increase very fast with growing impact velocity, indicating that large loads accompany waves with large velocities. Wave slamming is thus an important design criterion for all kind of cylindrical structures when exposed to harsh sea conditions.

Testing and modeling of diffusion bonded prototype optical windows under ITER conditions

Glass-metal joints are a part of ITER optical diagnostics windows. These joints must be leak tight for the safety (presence of tritium in ITER) and to preserve the vacuum. They must also withstand the ITER environment: temperatures up to 220 °C and fast neutron fluxes of ∼3·109 n/cm2·s. At the moment, little information is available about glass-metal joints suitable for ITER. Therefore, we performed mechanical and thermal tests on some prototypes of an aluminium diffusion bonded optical window. Finite element modeling with Abaqus code was used to understand the experimental results. The prototypes were helium leaking probably due to very tiny cracks in the interaction layer between the steel and the aluminium. However, they were all able to withstand a thermal cycling test up to 200 °C; no damage could be seen after the tests by visual inspection. The prototypes successfully passed push-out test with a 500 N load. During the destructive push-out tests the prototypes broke at a 6–12 kN load between the aluminium layer and the steel or the glass, depending on the surface quality of the glass. The microanalysis of the joints has also been performed. The finite element modeling of the push-out tests is in a reasonable agreement with the experiments. According to the model, the highest thermal stress is created in the aluminium layer. Thus, the aluminium joint seems to be the weakest part of the prototypes. If this layer is improved, it will probably make the prototype helium leak tight and as such, a good ITER window candidate.

Finite Element Modeling of Thermal Stress in ITER Prototype Optical Windows and its Influencing Parameters

Glass-metal joints are needed for the optical windows in ITER to perform diagnostics. These joints must be leak tight for the safety (presence of tritium in ITER) and to preserve the vacuum. They must also withstand the ITER environment: temperatures around 250 °C and neutron fluxes of 109 n/cm2.s. At the moment, little information is available about glass-metal joints suitable for ITER. Therefore, we set-up a 2D elastic model of prototype Al diffusion bonded optical windows using Abaqus code to model temperature effects on the windows. With this model we analyzed the influence of different parameters like the joint area and the braze thickness on the mechanical properties of the joint. Calculations of the thermal stress created by a temperature field of 150 °C (normal ITER temperature) showed that the Al-bond is the weakest part of the window. To find a way of reducing the thermal stress, the influence of some parameters has been studied. In particular, a specific thickness of the Al layer can result in a minimum of stress in the Al bond while the joint area and the thickness of the glass have only a small influence on the stress in the windows. The calculations allowed to propose an optimized design for the windows prototypes.