Oliver Millon’s research while affiliated with Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut and other places

The present paper describes essential results of comprehensive experimental investigations of the analysis of rc‐slabs under impact loads. The presented work was done in collaboration between the Fraunhofer Institute for High‐Speed Dynamics – Ernst‐Mach‐Institut (EMI) and the Technical University of Dresden (TUD) in the national research project “impact on concrete structures – experiment and simulation” sponsored by the DFG (Deutsche Forschungsgemeinschaft). In a comprehensive test series the loading of a so‐called hard impact – that means the kinetic energy is almost completely introduced in the structure – was studied for different velocity ranges (4.2–6.0 m/s; 44–67 m/s and 200–274 m/s). The resulting damage phenomena and the dynamic structural response were measured by extensive instrumentation (strain gauges, pressure sensors, laser and high‐speed cameras). The received data for low and high strain rates serves for the general understanding of the relations between local transient loading and global structural response. It is the basis for the adaption of existing material models, the development of engineering models and the validation of numerical calculations.

The determination of the blast protection level of civil engineering buildings components against explosive effects represents a design topic of crucial importance, in current practice. However, some key aspects of blast resistant structures design have been only marginally considered in the last decade, and currently still require appropriate regulations. This is especially true in the case of glass windows and facades, where the intrinsic material brittleness is the major influencing parameter for blast-resistant assemblies. While blast assessment of buildings and systems is usually achieved by means of experimental investigations, as well as Finite-Element numerical simulations, general regulations and guidelines are currently missing. In this regard, the European Reference Network for Critical Infrastructure Protection - Task Group (ERNCIP-TG) “Resistance of Structures to Explosion Effects” attempts to develop guidelines and recommendations aimed to harmonise test procedures in experimental testing of glass windows under blast, as well as standardized approaches for their vulnerability assessment via Finite Element numerical modelling. In this paper, major ERNCIP-TG outcomes and next challenges are briefly summarized.

In multiple engineering fields such as rock drilling or building constructions or extreme events like earthquakes or impacts, the dynamic properties of rock play an important role. A way to model these events and define measures to minimize the damage derived from these events is created by means of numerical analysis. Hence, the knowledge of the dynamic material behavior is essential for studying the effects of such a loading scenario. Solid geological materials, from the family of the sedimentary rocks, have been analyzed under quasi-static loads. However, there is a lack of knowledge when high strain rate loadings are involved. Within this context, the paper focuses on the experimental characterization of two sedimentary rocks, sandstone and limestone, under impact loading using the Hopkinson-Bar spallation and compression tests. The analysis encompasses the determination of the tensile and compressive properties as well as the comparison between the quasi-static and dynamic behavior (dynamic increase factors). The paper fills the gap of information existing about dynamic behavior of sedimentary rocks under strain rates between 100 and 5.2 × 102 s−1. Furthermore, the fragmentation under different strain rates is investigated and conclusions with respect to energy absorption capacity are drawn.

Das Verständnis von Impaktvorgängen auf Stahlbetonstrukturen ist von grundlegender Bedeutung für die Auslegung von Bauwerken in Hinblick auf die Bewertung außergewöhnlicher Belastungssituationen, wie beispielsweise Steinschlag oder aber Fahrzeug‐ oder Flugzeuganprall. Bei hohen Verzerrungsraten, die sich bei derartigen Belastungen im Bauteil einstellen, wird ein anderes Material‐ und Werkstoffverhalten beobachtet als unter quasi‐statischen Einwirkungen. Dieses äußert sich zum Beispiel durch eine Zunahme der Festigkeit bei steigender Verzerrungsrate. Daher ist eine Analyse und Bewertung des Tragverhaltens von Bauteilen unter derartigen Belastungszuständen geboten, um das Tragverhalten auf Material‐ und Bauteilebene zu ermitteln und Tragreserven erkennen zu können. Eine entsprechende Methodik und der Einsatz geeigneter Messtechnik sind dafür essenzielle Voraussetzungen. Der vorliegende Aufsatz richtet seinen Fokus auf die messtechnischen Methoden und die Herausforderungen bei schnell ablaufenden Belastungsvorgängen. Am Beispiel der experimentellen Untersuchung von Stahlbetonplatten unter Impakt, bei denen Belastungszeiten von wenigen μs und ms auftreten, werden der Versuchsaufbau und die Mess‐ und Analysesysteme in Hinblick auf die Messmethoden und die erzielten Messergebnisse beschrieben. Die Ermittlung von Zeitverläufen bzgl. der Impaktgeschwindigkeit, der Reaktionskräfte, des Verformungsverhaltens sowie auch der Bestimmung von Schallwellengeschwindigkeiten innerhalb des Bauteils erfordert eine entsprechend hohe Abtastrate bei der Signalaufzeichnung. Diese Ergebnisse ermöglichen letztlich eine ganzheitliche Bewertung des Strukturverhaltens und der Schädigungsphänomene.

The material behavior under dynamic processes, such as ballistic impacts, demands a deep knowledge of high-rate testing techniques, dynamic material response and dynamic material models. In this field, ultra-high molecular weight polyethylene (UHMW-PE) composites have been proven to be extremely effective against such threats. During a ballistic impact, a transverse shock wave travels through the material layers. When the shock wave is reflected at the rear face of the component, tensile stresses are created through the material thickness. This dynamic sequence produces a damage mechanism which combines shearing within the top plies of the material, and bulging and delamination at the rear face of the component. The number of scientific works related to the dynamic characterization of such composites have increased within the last years, including experimental programs, material models, and hydrocode simulations. Nevertheless, there is still a lack of information on the dynamic behavior of such composites under out-of-plane shear loading. An experimental way to characterize the shear strength of laminar material is by using the Split Hopkinson Bar (SHB) device in the so-called shear punch tests. The mechanism is simple – the SHB device is modified by including a sample holder which enables the material to deform by shearing and not by bending. Several authors have used this technique satisfactorily to characterize brittle solids, metals and some epoxy and tungsten fiber composites, but still, none of them have proportionated data from UHMW-PE composites. This work presents the experimental characterization of out-of-plane shear strength of Dyneema® HB26. Quasi-static and dynamic punching tests were performed in laminar specimens using a UTS and a SHB device. Furthermore, an optimization of the experimental variables affecting the failure mode is also included in this work. The results are very promising, filling the existing gap in the investigation of the dynamic behavior of such a material.

The need for the assessment of structures subjected to natural and man-made hazard scenarios is steadily increasing. The analysis of a building structure with regards to potential progressive collapse as the result of a single hazardous event requires the realistic assessment of the residual capacity of structural members after the initial event. Common current building codes provide little guidance in the establishment of relevant loading scenarios and the assessment of structural members subjected to extreme loads. Using extreme loading from impact or close-in detona-tions as an example, a design path is outlined describing (1) the establishment of relevant hazard scenarios based on risk analysis, (2) the analysis of structural members subjected to high speed dynamic loading using hydrocodes, and (3) employing an efficient rigid-body spring model to analyze the entire structure subjected to potential progressive collapse. After initial member failure, the utilization of the remaining structural elements as established under regular dead and live loads is compared to the newly required capacity after the event. The degree of utilization of the remain-ing structural members after an event is amplified by additional loads redistributed from the failed member(s) and the damage caused by the initial event.

The determination of the blast protection level of laminated glass windows and façades is of crucial importance, and it is normally done by using experimental investigations. In recent years numerical methods have become much more powerful also with respect to this kind of application. This report attempts to give a first idea of a possible standardisation concerning such numerical simulations. Attention is drawn to the representation of the blast loading and of the behaviour of the material of the mentioned products, to the geometrical meshing, as well as to the modelling of the connections of the glass components to the main structure. The need to validate the numerical models against reliable experimental data, some of which are indicated, is underlined.

The article at hand describes the behavior of high-strength and normal-strength strain-hardening cement-based composites (SHCCs) made of fine-grained matrix and high-density polyethylene fibers under quasi-static and impact tensile loading. The dynamic tension testing of unnotched and notched cylinders was performed using the Hopkinson bar at strain rates of around 150 s− 1. The responses of the materials under dynamic and quasi-static tensile loading were compared to the corresponding results for normal-strength SHCC made of polyvinyl-alcohol fibers as obtained in previous investigations. To explain the pronounced differences in rate effects on the material performance of various SHCC compositions, cracking pattern and fracture surface conditions were studied. Additionally, strain rate dependent changes in the mechanical behavior of individual fibers and in the fiber–matrix interfacial properties were deduced from single-fiber tension tests and fiber pullout tests, respectively. Altogether, the results obtained provide clear indications as to the decisive parameters for a purposeful material design of impact resistant types of SHCC for use in structural elements or protective overlays.