Frank Balle

Analysis of fatigue properties and failure mechanisms of Ti6Al4V in the very high cycle fatigue regime using ultrasonic technology and 3D laser scanning vibrometry.

Accelerated fatigue tests with Ti6Al4V were carried out using a 20kHz ultrasonic testing facility to investigate the cyclic deformation behavior in the Very High Cycle Fatigue (VHCF) regime in detail. Beside parameters like the ultrasonic generator power and the displacement of the specimen, a 3D laser scanning vibrometer was used to characterize the oscillation and fatigue behavior of the Ti-alloy. The course of the S-N(f) curve at the stress ratio R=-1 shows a significant decrease of the bearable stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 10⁷ cycles. Microscopic investigations of the distribution of the α- and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. High resolution vibrometry was used to visualize the eigenmode of the designed VHCF-specimen at 20 kHz in the initial state and to indicate local changes in the eigenmodes as a result of progressing fatigue damage. Non-contact strain measurements were realized and used to determine the stress amplitude. The determined stress amplitudes were correlated with strain gauge measurements and finite element analysis.

Fügen artfremder Werkstoffe — Ultraschallschweißen von Kohlenstofffasertextilien mit Metallen

Kurzfassung Das Ultraschallschweißen wird in vielen Bereichen der kunststoff- und metallverarbeitenden Industrie erfolgreich eingesetzt. Insbesondere Bauteile aus artgleichen Werkstoffen werden mit diesem Pressschweißverfahren in großen Stückzahlen gefertigt. Das Kunststoff-Ultraschallschweißen wird zum Fügen von thermoplastischen Werkstoffen verwendet. Die Metallschweißvariante des Ultraschallschweißens wird beispielsweise zur Herstellung von Heizungsrohren oder Litzen-Steckerverbindungen eingesetzt. Seit einigen Jahren können mit dem Metallschweißverfahren auch artfremde Werkstoffpaarungen zwischen Glas bzw. Keramik und verschiedenen Metallen sowie deren Legierungen hergestellt werden. Aufbauend auf diesen Erfahrungen werden am Lehrstuhl für Werkstoffkunde der TU Kaiserslautern auch Ultraschallschweißungen zwischen Glasfaser- sowie Kohlenstofffasertextilien mit metallischen Werkstoffen erfolgreich durchgeführt.

Nonlinear ultrasonic testing of carbon fibre reinforced plastics in the very high cycle fatigue regime

Carbon fibre reinforced plastics (CFRP) are applied as a light-weight construction material. In their lifespan in service CFRP components are exposed to variable loads, which can amount to 1011 load cycles. It is therefore important to examine the fatigue of CFRP materials in the very high cycle regime (more than 107 loading cycles). In a joint project a three point bending ultrasonic fatigue testing system was developed at WKK. A high amplitude sinusoidal vibration is added to a static load. The operating frequency of the system f = 20 kHz is used for sample load case and at the same time as input for online ultrasonic monitoring of the fatigue process. CFRP plate material with a thickness of 4 mm was investigated. During the fatigue process the vibrations of CFRP samples were measured with a laser vibrometer and a microphone. The analysis methods fast Fourier, short time Fourier, and Hilbert Huang transformation were used to evaluate the linear and nonlinear spectral content of the signals. Increase of ...

Computation and Visualization of Local Deformation for Multiphase Metallic Materials by Infimal Convolution of TV-Type Functionals

Estimating the local strain tensor from a sequence of microstructural images, realized during a tensile test of an engineering material, is a challenging problem. In this paper we propose to compute the strain tensor from image sequences acquired during tensile tests with increasing forces in horizontal direction by a variational optical flow model. To separate the global displacement during insitu tensile testing, which can be roughly approximated by a plane, from the local displacement we use an infimal convolution regularization consisting of first and second order terms. We apply a primal-dual method to find a minimizer of the energy function. This approach has the advantage that the strain tensor is directly computed within the algorithm and no additional derivative of the displacement must be computed. The algorithm is equipped with a coarse-to-fine strategy to cope with larger displacements and an adaptive parameter choice. Numerical examples with simulated and experimental data demonstrate the advantageous performance of our algorithm.

Notch Effects in High Cycle Fatigue of Ti-6Al-4V

In this study, a series of experimental studies was conducted to investigate the fatigue behavior of Ti-6Al-4V alloy at room temperature. Specifically, by inspecting the cylindrical specimens with a circumferential notch of different depths (20-200µm) and notch root radii (20-100µm), the notch effect was systematically investigated with tension-compression fatigue tests (R = –1). To quantify the effects of small notch, the -parameter model was adopted and its applicability for Ti-6Al-4V alloy was examined. Finally, the fatigue characteristics are discussed in conjunction with the behavior of small fatigue cracks at notches.

Innovative Ultrasonic Testing Facility for Fatigue Experiments in the VHCF Regime

Abstract At the Institute of Materials Science and Engineering of the University Kaiserslautern an ultrasonic testing facility was developed to perform fatigue tests in the VHCF regime. The individual design of the ultrasonic testing facility allows to control the process parameters and to measure relevant data with a frequency of up to 500 kHz in reasonable time. This permits a high-resolution measurement of the ultrasonic oscillation and the dissipated energy as well as a determination of microstructural reactions inside the specimens. With the described ultrasonic testing facility constant amplitude tests and load increase tests were carried out with the alloy Ti6Al4V. The results show that the innovative test and measuring techniques are best suited to describe the fatigue behaviour of Ti6Al4V. Additionally, for the first time high resolution 3D scanning laser vibrometry were carried out to determine the oscillation behaviour of specimens under ultrasonic cyclic load.

Mechanische Charakterisierung ultraschallgeschweißter Aluminium/CFK-Verbunde bei automobilrelevanten Temperaturen und Prüfgeschwindigkeiten

Kurzfassung Aktuelle Untersuchungen an der TU Kaiserslautern beschäftigen sich im Rahmen der DFG-Forschergruppe 524 mit dem Fügen hybrider Strukturen aus C-Faser-Kunststoff-Verbunden und Leichtmetallen. Am Lehrstuhl für Werkstoffkunde (WKK) wurde hierzu erstmals die Metall-Ultraschallschweißtechnik genutzt. An den erzeugten Verschweißungen wurden umfangreiche Untersuchungen zum mechanischen Verhalten der Leichtbauwerkstoffverbunde durchgeführt. Dabei wurden insbesondere die erreichbaren Festigkeiten bei unterschiedlichen Beanspruchungsarten (Quer-Druck und Schub) in Abhängigkeit von der Temperatur- und Prüfgeschwindigkeit untersucht. Am Institut für Verbundwerkstoffe GmbH (IVW) wurden quasistatische Versuche bei automobiltypischen Betriebstemperaturen sowie Hochgeschwindigkeitsversuche durchgeführt, um das Verformungsverhalten der geschweißten Leichtbaustrukturen detailliert zu beschreiben.

Strain analysis by a total generalized variation regularized optical flow model

ABSTRACT In this paper, we deal with the problem of estimating the local strain tensor from a sequence of micro-structural images realized during deformation tests of engineering materials. Since the strain tensor is defined via the Jacobian of the displacement field, we propose to compute the displacement field by a variational model which takes care of properties of the Jacobian of the displacement. In particular, we are interested in areas of high strain. The data term of our variational model relies on the brightness invariance property of the image sequence. As prior we choose the second order total generalized variation of the displacement field. This splits the Jacobian into a smooth and a non-smooth part. The latter reflects the material cracks. An additional constraint is incorporated to handle physical properties of the non-smooth part for tensile tests. We prove that the resulting convex model has a minimizer and show how a primal-dual method can be applied to find a minimizer. The corresponding algorithm has the advantage that the strain tensor is directly computed within the iteration process. It is further equipped with a coarse-to-fine strategy to cope with larger displacements. Numerical examples with simulated and experimental data demonstrate the very good performance of our algorithm. In comparison to state-of-the-art engineering software, our method can resolve local phenomena much better.

Ultrasonic Fatigue of CFRP - Experimental Principle, Damage Analysis and Very High Cycle Fatigue Properties

Structural aircraft components are often subjected to more than 108 loading cycles during their service life. Therefore the increasing use of carbon fiber reinforced polymers (CFRP) as primary lightweight structural materials leads to the demand of a precise knowledge of the fatigue behavior and the corresponding failure mechanisms in the very high cycle fatigue (VHCF) range. To realise fatigue investigations for more than 108 loading cycles in an economic reasonable time a novel ultrasonic fatigue testing facility (UTF) for cyclic three-point bending was developed and patented. To avoid critical internal heating due to viscoelastic damping and internal friction, the fatigue testing at 20 kHz is performed in resonance as well as in pulse-pause control resulting in an effective testing frequency of ~1 kHz and the capability of performing 109 loading cycles in less than twelve days. The fatigue behavior of carbon fiber twill 2/2 fabric reinforced polyphenylene sulfide (CF-PPS) and carbon fiber 4-H satin fabric reinforced epoxy resin (CF-EP) was investigated. To study the induced fatigue damage of CF-PPS and CF-EP in the VHCF regime in detail, the fatigue mechanisms and damage development were characterized by light optical and SEM investigations during interruptions of constant amplitude tests (CAT). Lifetime-oriented investigations showed a significant decrease of the bearable stress amplitudes of CF-PPS and CFEP in the range between 106 to 109 loading cycles. The ultrasonically fatigued thermoset matrix composite showed a significantly different VHCF behavior in comparison to the investigated thermoplastic matrix composite: No fiber-matrix debonding or transversal cracks were present on the specimen edges, but a sudden specimen failure along with carbon fiber breakage have been observed. The fatigue shear strength at 109 cycles for CF-PPS could be determined to τa, 13 = 4.2 MPa and to τa, 13 = 15.8 MPa for the thermoset material CF-EP.

Ultrasonic Torsion Welding of Aging Resistant Al/CFRP Joints - Concepts, Mechanical and Microstructural Properties

Multi-material-design offers high potential for weight saving and optimization of engineering structures but inherits challenges as well, especially robust joining methods and long-term properties of hybrid structures. The application of joining techniques like ultrasonic welding allows a very efficient design of multi-material-components to enable further use of material specific advantages and are superior concerning mechanical properties.The Institute of Materials Science and Engineering of the University of Kaiserslautern (WKK) has a long-time experience on ultrasonic welding of dissimilar materials, for example different kinds of CFRP, light metals, steels or even glasses and ceramics. The mechanical properties are mostly optimized by using ideal process parameters, determined through statistical test planning methods.This gained knowledge is now to be transferred to application in aviation industry in cooperation with CTC GmbH and Airbus Operations GmbH. Therefore aircraft-related materials are joined by ultrasonic welding. The applied process parameters are recorded and analyzed in detail to be interlinked with the resulting mechanical properties of the hybrid joints. Aircraft derived multi-material demonstrators will be designed, manufactured and characterized with respect to their monotonic and fatigue properties as well as their resistance to aging.