Klaus Thoma

Engineering and Numerical Tools for Explosion Protection of Reinforced Concrete

A range of prediction methods is presented to assist damage analysis and design of building components against explosion effects. The scaled distance can give a first indication if global structural modes, e.g. bending, or local failure modes such as breaching or shear failure have to be considered. Engineering tools are helpful for quick prediction and design of buildings and facilities. They can analyze simple geometries, also as part of a complex construction, if the failure mechanisms are well understood. Two distinct examples of such PC tools for different loading regimes are highlighted and explained: a physically based single-degree-of-freedom analysis program (EMI-BAUEX), using similarity methods for transition from actually tested to arbitrary component geometries and the empirical analysis tool XPLOSIM for breaching predictions. Far more powerful, versatile but also expensive in terms of expertise and time requirements are hydrocode simulations with appropriate material models. Comparison of both hydrocode and engineering tool simulations to experimental results helps to understand the predictive capabilities. State-of-the-art analysis demands customized and often integrated use of such a range of methods in order to predict effectively and reliably what is expected to happen in case of an explosive event.

Effect of high explosive detonations on concrete structures

A numerical method to calculate the damage caused to concrete structures by surface charges is presented. The effect of the detonation on the concrete has been modelled in the case of high explosives, positioned and detonated on the surface of a concrete structure.

In situ flash x-ray high-speed computed tomography for the quantitative analysis of highly dynamic processes

The in situ investigation of dynamic events, ranging from car crash to ballistics, often is key to the understanding of dynamic material behavior. In many cases the important processes and interactions happen on the scale of milli- to microseconds at speeds of 1000 m s−1 or more. Often, 3D information is necessary to fully capture and analyze all relevant effects. High-speed 3D-visualization techniques are thus required for the in situ analysis. 3D-capable optical high-speed methods often are impaired by luminous effects and dust, while flash x-ray based methods usually deliver only 2D data. In this paper, a novel 3D-capable flash x-ray based method, in situ flash x-ray high-speed computed tomography is presented. The method is capable of producing 3D reconstructions of high-speed processes based on an undersampled dataset consisting of only a few (typically 3 to 6) x-ray projections. The major challenges are identified, discussed and the chosen solution outlined. The application is illustrated with an exemplary application of a 1000 m s−1 high-speed impact event on the scale of microseconds. A quantitative analysis of the in situ measurement of the material fragments with a 3D reconstruction with 1 mm voxel size is presented and the results are discussed. The results show that the HSCT method allows gaining valuable visual and quantitative mechanical information for the understanding and interpretation of high-speed events.

Numerical and experimental analysis of the compact tension test for a group of modified epoxy resins

Abstract A three-dimensional finite element analysis based on a general material model implemented in the DYNA3D code was carried out in order to simulate crack initiation propagation in a typical neat resin fracture toughness test specimen (CT-specimen). Additionally, the effect of crack initiation and the dynamics of crack propagation was investigated. The simulation of the fracture process in an isotropic neat resin is the baseline for the next step: the simulation of the crack propagation in a continuous fibre reinforced composite. The crack initiation and crack propagation were measured in the compact tension test for the different epoxy resin formulations. The calculated stress distribution was verified by comparing the calculated stress distribution with a photoelastically measured distribution. In the case of a continuous crack propagation, simulation and experiment were in good agreement; in the case of a stick-slip crack propagation, the model would have to be extended to include crack tip blunting effects.

Numerical simulation of a high velocity impact on fiber reinforced materials

Whereas the calculation of a high velocity impact on isotropical materials can be done on a routine basis, the simulation of the impact and penetration process into nonisotropical materials such as reinforced concrete or fiber reinforced materials still is a research task. We present the calculation of an impact of a metallic fragment on a modern protective wall structure. Such lightweight protective walls typically consist of two layers, a first outer layer made out of a material with high hardness and a backing layer. The materials for the backing layer are preferably fiber reinforced materials. Such types of walls offer a protection against fragments in a wide velocity range. For our calculations we used a non-linear finite element Lagrange code with explicit time integration. To be able to simulate the high velocity penetration process with a continuous erosion of the impacting metallic fragment, we used our newly developed contact algorithm with eroding surfaces. This contact algorithm is vectorized to a high degree and especially robust as it was developed to work for a wide range of contact-impact problems. To model the behavior of the fiber reinforced material under the highly dynamic loads, we present a material model which initially was developed to calculate the crash behavior (automotive applications) of modern high strength fiber-matrix systems. The model can describe the failure and the postfailure behavior up to complete material crushing. A detailed simulation shows the impact of a metallic fragment with a velocity of 750 m s−1 on a protective wall with two layers, the deformation and erosion of fragment and wall material and the failure of the fiber reinforced material.

Dynamic temperature measurements on a thermally activated self-healing ionomer

An ionomer, which is able to self-heal the damage after ballistic impact of a projectile, was studied using dynamic puncture tests. A temperature increase in this polymer is a fundamental part of the self-healing process. Therefore, the temperature during the puncture tests was measured using embedded thermocouples. These adiabatic temperature measurements allow for some first conclusions about the processes that are involved in the heating of the material during an impact process.

In situ measurements of impact-induced pressure waves in sandstone targets

In the present study we introduce an innovative method for the measurement of impact-induced pressure waves within geological materials. Impact experiments on dry and water-saturated sandstone targets were conducted at a velocity of 4600 m/s using 12 mm steel projectiles to investigate amplitudes, decay behavior, and speed of the waves propagating through the target material. For this purpose a special kind of piezoresistive sensor capable of recording transient stress pulses within solid brittle materials was developed and calibrated using a Split-Hopkinson pressure bar. Experimental impact parameters (projectile size and speed) were kept constant and yielded reproducible signal curves in terms of rise time and peak amplitudes. Pressure amplitudes decreased by 3 orders of magnitude within the first 250 mm (i.e., 42 projectile radii). The attenuation for water-saturated sandstone is higher compared to dry sandstone which is attributed to dissipation effects caused by relative motion between bulk material and interstitial water. The proportion of the impact energy radiated as seismic energy (seismic efficiency) is in the order of 10−3. The present study shows the feasibility of real-time measurements of waves caused by hypervelocity impacts on geological materials. Experiments of this kind lead to a better understanding of the processes in the crater subsurface during a hypervelocity impact.

Novel High-g Accelerometer Geometry Requiring 90 Degree Contacting Techniques

This paper presents a novel silicon MEMS high-g accelerometer and its package solutions. It exhibits superior performance compared with commercially available transducers, but requires 90 degree contacting. It has a resonant frequency of more than 2.5 MHz, while having a sensitivity of about 0.2 μV/Vexc./g, which means a performance increase of about one magnitude over existing similar sensors. These high values are due to its unusual geometry, which causes the sensing direction to lie in the wafer plane. This means that the sensor can only be used to its full abilities, when 90 degree contacting is applied. Three solutions are presented that solve this problem on different levels, i.e. on cable, package and die level. They are based upon a machined bulk alumina ceramic, a thick nine layer LTCC ceramic and a non-contact direct write technique called aerosol printing, respectively. The latter, which works at the lowest, i.e. die, level, promises the best characteristics regarding size, cost and measurement...

Design of a 1D and 3D monolithically integrated piezoresistive MEMS high-g accelerometer

This paper reports on the design of a new monolithically integrated 3-axis high-g accelerometer. It is based on a 1D-sensor which exhibits a sensitivity of 0.65 μV/V/g and a resonant frequency of 1.5 MHz. Both figures are higher than for any piezoresistive high-g accelerometer in the literature or on the market and are due to an unconventional spring-mass-system geometry. The 3-axis accelerometer incorporates the 1D- design to form a single-crystal silicon 3D-sensor.

A Combined Experimental/Computational Approach for Assessing the High Strain Rate Response of High Explosive Simulants and Other Viscoelastic Particulate Composite Materials

The quasistatic and dynamic mechanical properties of a viscoelastic particulate composite employed as a surrogate, cast‐cure high explosive were determined from uniaxial compression experiments at strain rates up to 107 sec−1. The results from these experiments were used to obtain parameters for a non‐linear viscoelastic material model. The viscoelasticity described by the macroscopic material model introduced in this paper affects not only the deviatoric components of stress and strain but the volumetric components as well. The material description is adequate for reproducing experimentally observed responses at loading rates ranging from quasistatic to shock levels with a single set of material parameters. Parameters for an HTPB‐sugar composite are provided.