Jan Kober

Jan Kober
Institute of Thermomechanics, The Czech Academy of Sciences, Prague, Czech Republic · Impact and Waves in Solids

PhD in Material Science

About

18
Publications
2,603
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53
Citations
Additional affiliations
September 2004 - present
Czech Technical University in Prague
Position
  • PhD Student

Publications

Publications (18)
Article
This paper studies theoretical results of a nonlinear ultrasonic method based on interaction of two elastic waves of different frequencies. A virtual Nonlinear Wave Modulation Spectroscopy experiment is performed in the vicinity of a crack described by a model combining classical and hysteretic nonlinearity. Quasistatic response to two frequency ex...
Article
Full-text available
In this letter, fundamentals of transferring a time reversal experiment between similar objects are discussed. The time reversal experiment consists of two steps: forward propagation, when a source excites the medium and a complex wave field is created, and back propagation, resulting in time reversal focusing. Here the procedure of performing the...
Conference Paper
Full-text available
Time reversal (TR) ultrasonic signal processing is now broadly used in a variety of applications, and also in NDE/NDT field. TR processing is used e.g. for S/N ratio enhancement, reciprocal transducer calibration, location, identification, and reconstruction of unknown sources, etc. TR procedure in conjunction with nonlinear elastic wave spectrosco...
Article
Full-text available
Selective electron beam melting (SEBM) is an additive manufacturing process for the production of complex metallic parts. To optimize their mechanical properties, pores and fusion defects, since not completely avoidable, have to be detected and evaluated. The generated microstructure may contain not only voids, but also weak bonds, sharp interfaces...
Article
By measuring the elastic slow dynamics behavior of various consolidated granular materials and metallic alloys, we show that the evolution of the wave velocity as a function of time can be reproduced by using a continuous relaxation-time spectrum, whose main features can be extracted from the experiments. Here we propose to adopt a Weibull distribu...
Article
Elastic slow dynamics, consisting in a reversible softening of materials when an external strain is applied, was experimentally observed in polycrystalline metals and presents analogies with the same phenomenon more widely observed in consolidated granular media. Since the effect is extremely small in metals, precise experimental techniques are nee...
Article
Two-dimensional wave propagation in an aluminium strip is examined experimentally, analytically, and numerically in this paper. The high-frequency pulse is generated by a piezoelectric transducer, and the velocity component normal to the strip edge is measured by the non-contact vibrometer system. In-plane waves are then investigated under plane st...
Article
The Hopkinson split pressure bar (HSPB) was used for the testing of three polymers at strain rates between 102 to 103 s-1. Higher strain rates were achieved using the direct Hopkinson test. Experimental data were evaluated in time as well as in the frequency domain. A more detailed analysis in the frequency domain showed that the description of tes...
Article
The behavior of three copolymers high density polyethylene at high rate loading was investigated. Namely, one unimodal type (HDPE-1) and two bimodal type. Structure of PE samples was characterized by 13C NMR (Carbon-13 nuclear magnetic resonance). The Taylor impact test was used. Specimens in form of a cylinder (14 mm in diameter and 50 mm in lengt...
Research
Full-text available
Achieving the classification of hand movements using non-invasive sensing technics is a very valuable task, which can be used as very effective human-machine interface. A few technologies such as Optical Motion Capture or Surface Electromyography (sEMG) are already in use in fields like cinematography, muscle rehabilitation or prosthesis control. H...
Article
Full-text available
Current metal additive manufacturing (AM) technologies imply complicated processes, with many variable parameters determining their performance and the quality of the resulting product. Process optimisation requires feedback from nondestructive evaluation (NDE) of the additively manufactured parts. The presence of a complicated internal structure a...
Article
Conventional methods of calibration of acoustic emission transducers require an already calibrated reference transducer and/or special test specimen. Moreover the calibration results are not directly transferrable to practice, since they are dependent on bonding conditions and mechanical properties of the medium. In this paper a reference-free in s...
Conference Paper
Full-text available
Concrete and reinforced concrete parts of nuclear power plants (NPP) like concrete cavity of reactor pressure vessel (RPV) are during their long term service exposed to temperature and radiation effects, which cause progressive concrete aging and degradation. Contrary to metalic NPP parts, no effective nondestructive inspection procedure is there u...
Conference Paper
Full-text available
The question about the nature of detected AE sources in a monitored structure is as old as AE method. Reliable identification and classification of already localized AE sources is one of the most important and also most difficult problems in AE monitoring. In this paper we discuss relatively new methods of precise AE source localization and identif...
Article
Full-text available
The blade of the third turbine wheel (3TW) placed on the third low-pressure part (3LP) of the TG 1000 MW turbine broke during the start of turbine in October 2008. The low-pressure part blades of the third wheels are made from modified 12 % Cr martensitic steel AK1 TD.9, and the blades of the fourth wheels are made from X2CrNiMo13-4 steel. The stud...
Article
Full-text available
Nonlinear Wave Modulation Spectroscopy (NWMS) is one of Nonlinear Elastic Wave Spectroscopy (NEWS) methods proven as very sensitive to the variety of material damage ranging from distributed microdamage to single cracks. The experimental procedure consists of simultaneous excitation of a specimen by two harmonic waves with frequencies f 0 resp. f 1...
Article
Full-text available
Nondestructive evaluation of materials using nonlinear ultrasonic spectroscopy (NEWS) was performed on flat samples of Mg alloys. Initial state of variously prepared alloys specimens was tested by two-frequency NEWS method -Nonlinear Wave Modulation Spectroscopy (NWMS). Three piezoelectric transducers were attached to a specimen. Two of them transm...
Article
Full-text available
Significant amount of corrosion damage was revealed during recent complete inspections of airplane structures after years of service. The corrosion damage was located inside airplane's hull and remained undetected until reaching hull's outer surface. Therefore a demand for means of timely detection and non-destructive evaluation of corrosion damage...

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