Danièle Waldmann

Shear stresses in honeycomb sandwich plates: Analytical solution, finite element method and experimental verification

Honeycomb composite structures are used in airplanes, railway cars and vehicles. The sandwich panels consist of two stiff face sheets of aluminium, which are bonded to a very lightweight honeycomb core of aluminium. Compared to normal plates, sandwich panels have a very high stiffness and simultaneously a low weight. The core of these structures is mainly subjected to shear stresses. The shear stresses depend strongly on the angle of the load application. The distribution and the level of the shear stresses are investigated using analytical calculations. The load direction which induces highest stresses in the honeycomb core is derived. This direction is not the W-direction, which is the most compliant one. When doing finite element simulations of honeycomb cores, often the core is homogenized in order to reduce the calculation time. In this article, some equations are derived in order to calculate the real shear stresses from the shear stresses of the homogeneous core. The equations are validated by finite element simulations and partially by tests. Three-point bending tests and additionally some Food Cart Roller Tests were conducted in order to test the panels in different angles.

Fatigue in the core of aluminum honeycomb panels: Lifetime prediction compared with fatigue tests:

In comparison to their weight, honeycomb composite structures have a high bending stiffness, which makes them very suited for every application such as airplanes, railway-cars and vehicles, where little weight is important. The sandwich panels consist of two thin and stiff aluminum face sheets, which are bonded to a thick and lightweight aluminum honeycomb core. These structures are subjected to dynamic loading. However, in the literature, there are hardly any fatigue properties of the honeycomb core described. The fatigue properties of the core are investigated using the finite element method and experiments. Depending on the load application, the honeycomb core fails either through core indentation or shear failure. For a fatigue prediction, both failure modes have to be investigated. Additionally, the physical behavior of the honeycomb core is depending on the orientation of the core. Hence, fatigue tests were conducted in three directions of the core: the stiffest direction, the most compliant direction and the direction with the highest stresses. A three-point bending test setup was built to study the fatigue properties of the honeycomb core. Several fatigue tests were carried out with a load ratio of R = 0.1 (maximum load 10 times bigger than minimum load) and the fatigue diagrams being illustrated. Additionally, food-cart roller tests (wheels of a cart rolling in a circle on a floor panel) were done to dynamically test the panels in every angle. The sandwich structures were modeled with the ANSYS finite element software. The simulations, which were used to determine the stress amplitudes in the specimens, are described later. In addition, buckling analyses were used to examine core indentation failure. Based on these simulations, failure predictions can be made. The fatigue life of the examined specimens is successfully approximated in this manuscript, with the lifetime analysis being based on the FKM guideline (error less than 14% in load amplitude).

Mechanical Material Properties of Polymethylmethacrylate (PMMA) for Medical Applications

Abstract Polymethylmethacrylate (PMMA) also known as bone cement is a frequently used material for medical applications. This study investigated the material properties of the bone cement Palacos R. Flexural, tensile, and compression tests on Palacos R, following common standard, revealed typical brittle material behaviour with high compression (83 MPa) by rather low tension stress (35 MPa). No significant difference was measured between dry stored and 37°C saline solution stored specimens regarding tensile, compression and flexural properties. As bone cement can be considered as a visco-elastic material, additional tests under varied strain rates revealed an increasing Youngs modulus and ultimate strength of Palacos R with increasing strain rate. To evaluate long-term load-cycling behaviour an S-N curve was determined. The material parameters of Palacos R may help to predict and evaluate failure of bone cement and improve its medical application.

Design, Repeatability, and Comparison to Literature Data of a New Noninvasive Device Called “Rotameter” to Measure Rotational Knee Laxity

The present paper deals with the design, the repeatability, and the comparison to literature data of a new measuring device called “Rotameter” to characterize the rotational knee laxity or the tibia-femoral rotation (TFR). The initial prototype P1 of the Rotameter is shortly introduced and then modified according to trials carried out on a prosthetic leg and on five healthy volunteers, leading therefore to an improved prototype P2. A comparison of results obtained from P1 and P2 with the same male subject shows the enhancements of P2. Intertester and intratester repeatability of this new device were shown and it was observed that rotational laxities of left and right knees are the same for a healthy subject. Moreover, a literature review showed that measurements with P2 presented lower TFR values than other noninvasive devices. The measured TFR versus torque characteristic was quite similar to other invasive devices, which are more difficult to use and harmful to the patient. Hence, our prototype P2 proved to be an easy-to-use and suitable device for quantifying rotational knee laxity. A forthcoming study will validate the Rotameter thanks to an approach based on computed tomography in order to evaluate its precision.

Primary energy used in centralized and decentralized ventilation systems measured in field tests in residential buildings

ABSTRACT Ventilation systems can save heat energy by using heat recovery, but use electrical energy to power the fans. In practice, the energy efficiency of those systems can be lower than expected. In this paper, results of field tests with 20 centralised and 60 decentralised ventilation systems for residential buildings and calculations a of the primary energy savings of those devices are presented. Parameters like volume flow unbalances, shortcuts, heat recovery efficiency and specific fan power have been addressed and used as input data to calculate the primary energy balance. Every system showed positive primary energy savings. The mean value for centralised systems was 2.92 Wh/m3 with a high standard deviation of 2.23 Wh/m3, while the decentralised systems showed higher savings of around 4.75 Wh/m3 with a standard deviation of 0.01 to 0.15 Wh/m3. In general, the calculated savings in field tests were significantly lower compared to those calculated from nominal values.

Numerical investigation of bridges with the aim of condition assessment in applying the Deformation Area Difference method (DAD-method) and selecting appropriate measurement techniques

Condition assessment of existing road bridges gains ever increasing importance today as bridges are getting older and the inflow of heavy traffic is constantly increasing. The further development of recognized techniques and the development of new methods for early and accurate detection of damage to the structure are made possible by means of innovative technological progress. In this contribution, the principles of Deformation Area Difference Method (DAD-Method) for condition assessment of bridges are presented. This method is based on the further processing of measured and computed deformation values. The application of the DADMethod requires a precise recording of the deflection of a load-deflection test. On the basis of theoretical calculations, this method has allowed to identify as well as to localise damage to a structure. The DAD-Method is independent of a reference measurement and insensitive to global influences such as temperature fluctuations. For precise detection of deformations, the most modern measuring instruments and methods like photogrammetry, total stations, displacement sensors, strain gauges and levelling are compared to each other. In collaboration with the appropriate measurement technology, the localisation of damage in bridges becomes possible. measuring techniques in combination with the DADMethod, different modern measuring instruments and methods are tested using laboratory tests. In this report, the results of the conducted investigations are presented. The test specimen is a reinforced concrete beam (Figure 1). In a first step, the beam was lifted on one side in order to compare the measuring techniques out their resolution to each other. Figure 1. Test specimen: reinforced concrete beam 2 APPLIED MEASUREMENT TECHNOLOGIES 2.1 Digital Levelling The levelling instrument employed in this study is a “Leica DNA03”, which is generally utilised for precise height measurements (Figure 2). This instrument compares on internal reference barcode with the barcode registered on the levelling rod, which is placed vertically on a point to be measured. Through a correlation process the height reading is determined. Several automatic internal readings are performed for each height measurement whereby a direct control of the verticality of the rod is given during the time of measurement. According to the instrument specification the standard deviation for the height measurement is 0.30 mm for a 1 k double levelling (Leica Geosystems AG, 2006). The significant advantages of a digital level are the high accuracy, the simple and efficient measurement, and the quick data evaluation. The measurement time for the 24 points is about 1020 min and the evaluation takes about 30-60 min, whereby the latter could be automated to a high degree. Figure 2. Digital Level Leica DNA03 (Leica Geosystems AG, 2006).

Masonry blocks from lightweight concrete on the basis of Miscanthus as aggregates

More and more concrete element producers have recently undertaken efforts to reconsider their global operations by expanding their range of market products to offer innovative, value-added solutions. To achieve this objective, companies aim to develop sustainable composite materials guaranteeing equal or higher performance than existing products; improvement of the environmental footprint; and recycling of waste from manufacturing and products at the end of life.

Use of a Computed Tomography Based Approach to Validate Noninvasive Devices to Measure Rotational Knee Laxity

The purpose of this study is to validate a noninvasive rotational knee laxity measuring device called “Rotameter P2” with an approach based on Computed Tomography (CT). This CT-approach using X-rays is hence invasive and can be regarded as a precise reference method that may also be applied to similar devices. An error due to imperfect femur fixation was observed but can be neglected for small torques. The most significant estimation error is due to the unavoidable soft tissues rotation and hence flexibility in the measurement chain. The error increases with the applied torque. The assessment showed that the rotational knee angle measured with the Rotameter is still overestimated because of thigh and femur displacement, soft tissues deformation, and measurement artefacts adding up to a maximum of 285% error at +15 Nm for the Internal Rotation of female volunteers. This may be questioned if such noninvasive devices for measuring the Tibia-Femoral Rotation (TFR) can help diagnosing knee pathologies and investigate ligament reconstructive surgery.

Acoustics During the Vibratory Pile Driving of Sheet Piles: Measurement Conditions and Key Parameters of the Noise Generation

This paper studies the sound generation during the vibratory pile driving of double sheet piles for different cross-sections with the aims to give guidelines how to make high-quality acoustic measurements and to find the key parameters of this noise generation. These objectives are reached by studying the assumed hypotheses when performing the acoustic measurements and by using a beamforming system; details about the vibratory pile driving are also given to obtain reproducible and representative measurements. The experiment results confirm some previously presented hypotheses and show two key parameters involved in the sound generation: the space between the welds of the common interlock of a double sheet pile and the wall height above the soil of a neighbouring wall.