Publications Search Results
Now showing 1 - 10 of 285
PublicationNumerische Simulation der Verbrennung von Ersatzbrennstoffen in industriellen Feuerungen( 2013)
;Arnold, W. ;Wirtz, S. ;Scherer, V.Krüger, B.
PublicationMicrostructural metrics and damage quantification in energetic materials( 2013)
;Jordan, J.L. ;Krawietz, T.R. ;Sunny, G. ;Arnold, W. ;Salk, M.Spowart, J.E.Polymer bonded explosives (PBXs) are complex composites consisting of explosive crystals in a polymeric binder. The mesostructure of these explosives greatly affects the initiation and sensitivity of the material. As the materials are prepared in high shear mixers, the initial particles can be damaged and the resulting microstructure is not simply additive from the constituents. Additionally, mechanical loading, such as occurs in a split Hopkinson pressure bar or shock loading experiment, can induce damage, which can lead to increased sensitivity. Damage may be defined as increased numbers of hot spots, which may be represented microstructurally as increased particle-particle contacts, binder debonding, and creation of new void spaces. These microstructural features may be identified using optical microscopy and X-Ray Computed Microtomography (XCMT). Optical microscopy provides only a two dimensional representation of the material, while XCMT provides three dimensional data. However, the limited contrast between explosive crystals and binder due to their similar densities can make it difficult to perform quantitative characterization on XCMT images. In this study, "as mixed" and explosively loaded samples are compared using quantitative techniques to identify correlations between microstructural features - Including convexity by area and perimeter, grain size distribution, and minimum nearest neighbor distance - and measured run-to-detonation using a custom gap test.
PublicationMicrostructure of the PBX KS32 after mechanical loading investigated by x-ray diffraction and DMA( 2013)
;Herrmann, M. ;Bohn, M.A. ;Krause, H. ;Koch, M.Arnold, W.Plastic bonded explosives are highly filled polymers containing up to 90% high energy crystalline solid loads. Different non-destructive X-ray diffraction techniques were applied in order to characterize the ingredients of PBXs and their behavior in shock-loading tests. The investigations revealed damages to embedded coarse HMX crystals such as deformation twinning and crystal cleavage, and a change in the micro structure of the binder HTPB; the latter was verified by advanced DMA investigations. The investigations are part of a bilateral cooperation and a project agreement on Insensitive High Explosives for High Speed Penetrators (IHE-HSP) between the USA and Germany.
PublicationMicrostructure of the plastic bonded explosive KS32( 2012)
;Herrmann, M. ;Kempa, P.B. ;Förter-Barth, U.Arnold, W.
PublicationTomographic reconstruction of defects in composite plates using genetic algorithms with cluster analysis( 2011)
;Kishore, N.N. ;Munshi, P. ;Ranamale, M.A. ;Ramakrishna, V.V.Arnold, W.Tomographic reconstructions of defects in composite dates is a complex procedure, as the elastic properties of the composites are direction-dependent also leading to the directional dependence of the wave velocities. Furthermore, at the interface of inhomogeneties a wave undergoes refraction, mode conversion, and beam skewing. A tomographic method for composite materials will have to deal with multiple elastic properties and ray bending. In the present approach, the genetic algorithm (GA), being an efficient optimization method, is used as a tool for reconstruction. The objective function of GA consists of three terms, viz. time-of-flight difference between the projection data and the date generated by the GA, the number of clusters in that data set, and the number of violation of bounds on variables. The laminate is considered thin, and Lamb waves are used as the source of radiation. A ray-tracing algorithm is used to determine the ray-paths, and an edge-detection algorithm is used to find out the boundary of the insert from the data created by the GA as it proceeds. This approach is successfully applied to simulated data sets, and the results are found to be quite encouraging.
PublicationSize retrieval of defects in composite material with lockin thermography( 2010)
;Zöcke, C.M. ;Langmeier, A.Arnold, W.The article deals with the retrieval of the depths and sizes of defects situated in carbon fibers reinforced polymer material from optical lockin thermography phase images. A model that describes the images formation process in anisotropic, homogeneous material is presented. It is used to retrieve the depth and shape of the defects.
PublicationTomographic reconstruction of elastic constants in composite materials using numerical and experimental laser ultrasonic data( 2010)
;Rathore, S.K. ;Kishore, N.N. ;Munshi, P.Arnold, W.Nondestructive characterization of composite materials has been attempted using the concept of ultrasonic tomography. A new method, Composite-ART (C-ART) is proposed for measuring the 2-d distribution of elastic constants. This leads to a nonlinear relation between the time-of-flight and the field variables. The solution approach employs a nonlinear least-square minimization with appropriate constraints to attain a reliable solution. The method turns out be a multiple parameter reconstruction as the elastic constants characterize a pixel. Extensive studies using simulated data are carried out to establish the robustness of the method. These studies provide the guidelines with regards to choice of the relaxation parameter and an initial guess as the controlling parameters for the reconstruction of a cross-section. The sensitivity of the controlling parameters towards the solution behavior is also discussed in detail. The experimental studies were done using laser-based ultrasound as the source radiation allowing one to exploit the point source and point detector approximation. Unidirectional composites with defects were scanned and good quality reconstructions are obtained. It is observed that a low relaxation parameter helps in presence of noise due to measurement error.
PublicationReconstruction of the defect shape from lock-in thermography phase images( 2009)
;Zöcke, C. ;Langmeier, A. ;Stößel, R.Arnold, W.We present a method for reconstructing the shape of buried planar defects supposed to be infinitely thick front optical lock-in thermography phase images. Several image processing algorithms are combined, to extract quantitative defect information from a series of phase images (phasegram) in a semi-automatic way. It is shown, that the depth and the shape of a planar defect can be retrieved.
PublicationSimulation of vibrational resonances of stiff AFM cantilevers by finite element methods( 2009)
;Espinoza-Beltrán, F.J. ;Geng, K. ;Munoz Saldana, J. ;Rabe, U. ;Hirsekorn, S.Arnold, W.We report on the simulation of vibrational resonances of stiff atomic force microscope cantilevers made of silicon by finite element methods (FEM) for application in contact-resonance spectroscopy. The FEM model considers the cubic symmetry of silicon single crystals and the geometrical shape of the cantilevers with a trapezoidal cross section and a triangular free end. Using a two-step iterative procedure, we fitted our FEM model to the experiment. In a first step, we used the measured resonant frequencies of an individual cantilever to fit the geometrical dimensions of the cantilever beam model. In a second step, we measured the resonant frequencies of the same cantilever in contact with a sample and determined the out-of-plane and in-plane tip-sample contact stiffness values by a fitting procedure. The FEM model also allows precise calculation of the spring constant of the cantilever, and consequently calculation of the force in contact. Finally, we compared the contact stiffness values with those predicted by contact mechanics models