Laurent Warnet

Thermally-induced shapes of unsymmetric laminates

Models to describe the shapes of unsymmetric laminates produced with a flat mould have been already developed by several researchers. These models are reviewed and on some points theory is extended. The experimental work done so far to confirm the developed theoretical models is rather limited. This especially applies for angle-ply laminates, on which no experimental work has been reported yet. This paper presents new experimental work that has been conducted both on cross-ply and angle-ply laminates over a wide range of laminate dimensions. The experiments show that the influence of the side length on the curvature is very well predicted both for cross-ply and angle-ply laminates, also the bifurcation point behaviour is clearly observed. Possible causes of differences between theory and experiment are outlined. The theoretical predictions are especially sensitive to the transverse thermal expansion coefficient of the individual layers.

A novel numerical mechanical model for the stress–strain distribution in superconducting cable-in-conduit conductors

Besides the temperature and magnetic field, the strain and stress state of the superconducting Nb3Sn wires in multi-stage twisted cable-in-conduit conductors (CICCs), as applied in ITER or high field magnets, strongly influence their transport properties. For an accurate quantitative prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the strain distribution along all individual wires is required. For this, the thermal contraction of the different components and the huge electromagnetic forces imposing bending and contact deformation must be taken into account, following the complex strand pattern and mutual interaction by contacts from surrounding strands. In this paper, we describe a numerical model for a superconducting cable, which can simulate the strain and stress states of all single wires including interstrand contact force and associated deformation. The strands in the cable can be all similar (Nb3Sn/Cu) or with the inclusion of different strand materials for protection (Cu, Glidcop). The simulation results are essential for the analysis and conductor design optimization from cabling to final magnet operation conditions. Comparisons are presented concerning the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties and thus on the eventual strain distribution in the Nb3Sn filaments when subjected to electromagnetic forces, axial force and twist moment. Recommendations are given for conductor design improvements.

Transient forces generated by projectiles on variable quality mouthguards monitored by instrumented impact testing

Objectives—(a) To determine the force-time trace that occurs when a spring mounted simulated upper jaw is impacted; (b) to examine if mouthguards of variable quality have significant influence on such force-time traces; (c) to attempt to relate physical events to the profile of the force-time traces recorded. Methods—A simulated jaw, consisting of ceramic teeth inserted into a hard rubber arch reinforced with a composite jawbone, was fitted with various mouthguards as part of a previous round robin study. A clinical assessment distinguished good, bad, and poor mouthguards, and these were each fitted to the jaw, which was then submitted to instrumental impact tests under conditions expected to produce tooth fractures. The force-time trace was recorded for such impact events. Results—The spring mounting method caused two distinct peaks in the force-time trace. The initial one was related to inertia effects and showed an increase in magnitude with impactor velocity as expected. The second peak showed features that were related to the differences in the mouthguards selected. Conclusions—The use of a force washer within a conical ended impactor enabled force-time traces to be recorded during the impact of a spring mounted simulated jaw fitted with mouthguards of variable quality. The spring mounting system causes an initial inertial peak followed by a second peak once the spring mount has fully compressed. Good fitting guards, which keep most teeth intact, result in high stiffness targets that in turn generate high reaction forces in the impactor. If the spring mounting is omitted, the two peaks are combined to give even higher reaction forces. The force-time trace offers some potential for assessing both overall mouthguard performance and individual events during the impact sequence. Mouthguards with good retention to the jaw remained attached during the impact event and helped to preserve the structural integrity of the target. This in turn developed high forces in the second part of the force-time trace. Guards that detached during impact and allowed tooth fractures showed lower forces in the second part of the test. The force profile measured offered some quantitative support to, and agreement with, the observed clinical quality of the mouthguards.

Influence of physical aging on impact embrittlement of uPVC pipes

Abstract Most failures of unplasticised poly(vinyl chloride) (uPVC) pipes used in the Dutch gas distribution network originate from third party damage. Brittle pipes should therefore be replaced to ensure safe operation of the network. In this study, the relation between physical aging and embrittlement of uPVC is investigated using instrumented falling weight impact tests. The ductile to brittle transition temperature was first measured for a water pipe grade uPVC at different stages of aging. As a hypothesis, a critical stress criterion is proposed above which failure is brittle. The evolution of the ductile to brittle transition temperature that followed from the use of this hypothesis and a model for the polymer yield stress agrees qualitatively with the experimental data. A minor increase in transition temperature was observed for the water pipe grade with aging. Applying the same hypothesis to a uPVC gas pipe grade shows a more pronounced influence of physical aging.

Modeling of Stress Development During Thermal Damage Healing in Fiber-reinforced Composite Materials Containing Embedded Shape Memory Alloy Wires:

Fiber-reinforced composite materials are susceptible to damage development through matrix cracking and delamination. This article concerns the use of shape memory alloy (SMA) wires embedded in a composite material to support healing of damage through a local heat treatment. The composite material contains a thermoplastic matrix that allows healing at elevated temperatures. The woven in SMA wires, oriented in the out-of-plane direction of the composite material, are used to close the delamination upon heating. Several case studies were performed. The influence of the SMA wire fraction, the degree of prestraining of the SMA wires, the glass transition temperature of the amorphous thermoplastic matrix, and the healing temperature have been considered. The delamination is compacted while heating up to the healing temperature. The thermal expansion coefficient of the thermoplastic matrix is much larger than that of the SMA wires causing a compressive thermal stress in the composite material upon heating. Prestraining of the SMA wires is not a priori required to obtain a compressive stress at the delamination interfaces at the healing temperature. After cooling to room temperature residual stresses occur in the composite material and SMA wires if the SMA wire composition at the start of the heat treatment differs from that at the end. The conditions under which no residual stresses develop have been determined. SMA wire fractions have been calculated to achieve a preset stress level in the composite material at the healing temperature and a stress-free state after healing. Finally, the use of high strength wires to replace SMA wires has been considered and shown to be a worthwhile alternative.

Vibration Based Structural Health Monitoring and the Modal Strain Energy Damage Index Algorithm Applied to a Composite T-Beam

A Finite Element based numerical model for a vibration based damage identification method for a thin-walled slender composite structure is discussed in this chapter. The linear dynamic response of an intact and a locally delaminated 16-layer unidirectional carbon fibre PEKK reinforced T-beam is analysed. The capabilities of the modal strain energy damage index algorithm to detect and localize a delamination is assessed. Both bending and torsion modes of the structure are used in the algorithm. Both an experimental set-up and a numerical model are discussed. Measurements are performed on an intact and an artificially delaminated structure, using a laser-vibro measuring system to determine the response to a force excitation. A commercially available Finite Element package is employed for the numerical model. The aim of the numerical model is to perform a parametric study. The study is preceded by an experimental verification of the numerical model. Subsequently, it is used to analyse the effect of the size and location of a delamination, as well as the number of data points employed, on the damage index.

On crystallisation and fracture toughness of poly(phenylene sulphide) under tape placement conditions

Abstract Fibre reinforced thermoplastic tapes are subjected to high heating and cooling rates during the tape placement process. Such high cooling rates can significantly inhibit the crystallisation of the thermoplastic polymer and thereby affect its mechanical properties, such as strength or toughness. In the present work, the crystallisation of poly(phenylene sulphide) (PPS) subjected to high cooling rates was investigated using a fast scanning calorimeter. The PPS was found to be unable to crystallise when subjected to cooling rates higher than 20°C s−1. The influence of the degree of crystallinity on fracture toughness was investigated using an essential work of fracture approach. The amount of plastic work during the fracture process was found to decrease after moderate annealing.

CORD, A Novel Numerical Mechanical Model for CICCs

The strain state of the superconducting Nb3Sn strands in multi-stage twisted ITER Cable-In-Conduit Conductors (CICCs) strongly determines the transport properties. For an accurate prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the stress and strain distribution along all individual strands is imperative. Also during the cabling process, the axial stress of the individual strands must be well controlled to avoid kinks, in particular when mixing different strands, e.g., Nb3Sn and copper strands. A mechanical model for a superconducting cable (CORD) was developed, which can predict the strain and stress states of all single strands including interstrand contact force and the associated deformation. The simulation results are not only important for analysis but can be used for optimization of cable manufacturing and conductor design optimization. We discuss the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties.

Optimization of composite panels using neural networks and genetic algorithms

The objective of this paper is to present first results of a running study on optimization of aircraft components (composite panels of a typical vertical tail plane) by using Genetic Algorithms (GA) and Neural Networks (NN). The panels considered are standardized to some extent but still there is a wide scope of discrete and continuous design variables that can be adjusted to increase performance or reduce structural weight. A NN is trained for every panel configuration using a backpropagation algorithm with data sets taken from finite element analyses spread randomly over the design space. The trained network is then used to predict the values of the constraint functions (strain and buckling multipliers). The approach is formulated in this manner to maintain maximum flexibility regarding the implementation of new variables or models and with the prospect of optimizing the assembly as a whole. Results show that in design problems with high dimensionality the approach becomes more attractive, especially when the optimization has to be run repeatedly for panels under different loading/sizing conditions. The optimization algorithm has proven to be robust though dependent on the smoothness of the network output function. A modified method that feeds back the found optima is proposed to improve accuracy of the NN and decrease preparation time.

CORD, A Novel Numerical Mechanical Model for Nb3Sn CICCs

The strain state of the superconducting Nb3Sn strands in multi-stage twisted ITER Cable-In-Conduit Conductors (CICCs) strongly determines the transport properties. For an accurate prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the stress and strain distribution along all individual strands is imperative. Also during the cabling process, the axial stress of the individual strands must be well controlled to avoid kinks, in particular when mixing different strands, e.g., Nb3Sn and copper strands. A mechanical model for a superconducting cable (CORD) was developed, which can predict the strain and stress states of all single strands including interstrand contact force and the associated deformation. The simulation results are not only important for analysis but can be used for optimization of cable manufacturing and conductor design optimization. We discuss the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties.