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Die Festigkeitssteigerung von Betonen unter hohen Belastungsgeschwindigkeiten ist seit über 100 Jahren im Fokus der Forschung. Bisher konnten bereits eine Vielzahl von möglichen Er-klärungen dieser Festigkeitssteigerung benannt werden, wobei die Heterogenität des Betons im Allgemeinen als eine wesentliche Ursache angenommen werden kann. Die Heterogenität des Betons resultiert aus den im Zementstein eingebetteten Zuschlägen, aber auch durch da-rin eingeschlossene Luft- und Wasserporen, welche Hauptgegenstand dieser Arbeit sind. Es wurde untersucht, inwieweit vorhandenes Porenwasser die Festigkeitssteigerung von Be-ton unter hohen Belastungsgeschwindigkeiten beeinflusst. Auf Basis von experimentellen Ver-suchen im Split-HOPKINSON-Bar an verschiedenen Normalbetonen konnte gezeigt werden, dass vorhandenes Porenwasser die statische und dynamische Betondruckfestigkeit in gleicher Weise signifikant reduziert. Diese Abnahme der Betondruckfestigkeit kann als unabhängig von der Belastungsgeschwindigkeit angesehen werden. Des Weiteren wurden numerische Untersuchungen zum Einfluss von Poren auf die Ausbrei-tung von Belastungswellen durchgeführt. Dabei rückten sowohl das globale Wellenausbrei-tungsverhalten in einer porendurchsetzten Struktur als auch die lokale Wellenbrechung an ei-ner Einzelpore, sowie das Porenmedium (Luft oder Wasser) in den Fokus. Es konnte gezeigt werden, dass vorhandene Poren die Wellenausbreitungsgeschwindigkeit reduzieren und dass das Verhältnis aus Porengröße zur Belastungswellenlänge einen wesentlichen Einflussfaktor für die transiente Spannungsverteilung im Porenbereich darstellt. Insbesondere konnte mit ab-nehmender Belastungswellenlänge eine Reduzierung der örtlichen Kerbspannungen am Po-renrand beobachtet werden, woraus festigkeitssteigernde Effekte resultieren können.

The strain rate dependent behaviour of fine grained soils is an important aspect of geotechnical engineering. During dynamic or rapid events such as earthquakes and rapid pile testing, a fine grained soil will display significantly different behaviour than may be observed over the long life span of a structure. There is currently little understanding of the factors which influence the behaviour of fine grained soils during dynamic events (extremely high strain rates), making their response difficult to predict. This research investigates the behaviour of fine grained soils subjected to a wide range of constant strain rates in monotonic triaxial compression testing. Each test is conducted under drained conditions to observe the behaviour of soils as they transition from a drained response at lower strain rates, through to an undrained or viscous response at higher strain rate tests. Where the response of soils is drained or partially drained, higher strain rate tests measure a decrease in strength. The point of transition from partially drained to undrained behaviour corresponds to the lowest strain rate dependent strength. Further tests at higher strain rates measure consistently greater strength. The strain rate dependence of three fine grained soils is investigated, enabling a comparison of strain rate effects with soil index properties. The influence of initial state on the strain rate dependence of these Kaolin based model soils is also evaluated. The drained to partially drained response of the soils to strain rate increase is controlled by the coefficient of consolidation. Tests at high strain rates show the undrained or viscous strain rate effect on strength is related to liquidity index. Local strain instrumentation allowed comparison of strain rate effects on small strain stiffness. At higher strain rate the soils display increasingly linear behaviour. At non-linear elastic strains, liquidity index appears to control the magnitude of the strain rate effects on stiffness.

The bond between concrete and reinforcing steel is fundamental to the load bearing capacity of reinforced concrete structures. Several experimental studies indicate strength or rather resistance enhancements coming with increasingly dynamic loading. The phenomenon is known as strain or loading rate effect and its causes are still not fully clarified. The work presented herein provides a numerical view of the bond of reinforcement in concrete and investigates its loading rate dependent behaviour. Finite element analyses focusing on structural and inertia effects are carried out. Modelling is conducted at the rib scale, where bond is predominately controlled by mechanical interaction. In the first step, the model is developed and calibrated. Its quality, credibility, and limitations are assessed by a series of numerical case studies and the results are compared with available experimental data. Numerical parametric studies follow. The loading rate dependence of bond is featured, loading rate dependent characteristics are identified, and conclusions on causes of the phenomenon drawn. It is shown that structural effects are strongly involved and the same holds for hydrostatic pressure stress states and inertia effects. The thesis concludes in reviewing currently available methods for incorporating the results into large-scale simulations and highlighting further investigations and developments that are necessary in order to design dynamic loading-resistant structures in the future.

The linear instability of laterally diverging/converging flows is an important aspect towards understanding the laminar-transition process in many viscous flows. In this work the linear instability of constant pressure laterally diverging/converging flow has been investigated. The laminar velocity field for laterally diverging/converging flows, under the source/sink approximation, has been reduced to two-dimensional flows. This reduction is alternative to the Mangier transformation used earlier. For a constant pressure laterally strained flow, the laminar velocity is found to be governed by the Blasius equation for flow over a flat plate. The non-parallel linear instability of constant pressure laterally strained flows has been examined. The instability equation is found to be same as that for the Blasius flow. This implies that the stability is same as that for the Blasius flow. A lateral divergence/convergence is shown to alter the Reynolds number from that in a two-dimensional flow. The instability of a laterally converging/diverging flow thus can be obtained from the available results for the Blasius flow by scaling the Reynolds numbers. This leads to the result that while a diverging flow is more unstable than the Blasius flow, a converging flow is more stable. Some additional relevant results are also presented.

Le bois est un matériau cellulaire naturel et excellent absorbeur d’énergie. Employé au sein de structures du type limiteur d’impact, il subit de nombreux phénomènes lors d’un cas de chute. Une large campagne expérimentale est réalisée afin d’analyser les réponses en compression du hêtre et de l’épicéa, en fonction de la direction de sollicitation, de la vitesse de déformation pour la plage [0.001-600] s−1 et de deux types de restrictions latérales qualifiées d’extrêmes. La direction longitudinale se révèle la plus sensible à la vitesse ainsi qu’au type de restrictions latérales et les conséquences sur la capacité d’absorption d’énergie du bois sont alors significatives. Par ailleurs, les protocoles développés ont vocation à être déclinés pour un large panel d’essences aux propriétés mécaniques variées. Un modèle matériau élastoplastique, isotrope transverse et sensible à la vitesse de déformation est élaboré à l’aide des techniques multi-échelles et de la micromécanique. Les propriétés élastiques macroscopiques sont estimées à l’aide du schéma d’homogénéisation de Mori-Tanaka à partir de données issues de la microstructure. Un critère de type Gurson étendu reposant sur l’approche micromécanique de l’endommagement ductile est employé pour retranscrire le comportement non linéaire, la densification et le caractère compressible du bois. Des paramètres de dégradation découplés du critère sont appliqués selon la direction longitudinale. La modélisation proposée repose sur une description simplifiée du bois et les résultats numériques associés illustrent la bonne capacité du modèle à reproduire les différentes réponses observées lors d’un cas de chute. / Wood is a natural cellular material, which is widely and advantageously used as shock absorber for the transport of radioactive materials. Accident situations are evaluated based on the 9 m drop test, which allows us to observe the complex crushing behavior of wood. A compressive experimental study is conducted on spruce and beech wood species over a large range of strain rates (from 0.001 to 600 s−1) to investigate the effect of the loading direction and of two extreme lateral confinements. The longitudinal direction is the most sensitive to the effect of strain rate and of lateral confinements which have significant consequences on the energy absorption. Besides, the experimental investigation can be adapted to various wood species with very different mechanical properties. A strain rate dependent elastoplastic model with transversal isotropy is developed using multi-scale and micromechanics techniques. The elastic macroscopic properties of wood are estimated with a Mori-Tanaka scheme and information extracted from the microstructure. The Gurson type criterion based on the micromechanical approach of the ductile damage is used in order to describe the non linear behavior of wood, its densification regime and its compressibility as well. Additionally, uncoupled degradation parameters are applied to reproduce the failure mechanisms involved in the longitudinal response. A simplified description of wood is used within the modeling and the numerical results exhibit the good ability of the model to reproduce the various wood responses during an accident situation.