Joana Rebelo Kornmeier

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• Dr
• Staff Scientist at Technical University of Munich

Two high-power lithium-ion batteries of 26650-type adopting LiFePO 4 |C cell chemistry are non-invasively characterized using electrochemical, thermodynamical, and diffraction-based methods. Their discharging and charging behavior are probed using incremental capacity analysis and differential voltage analysis normalized to the state-of-charge. Dif...

Wear is a prevalent issue across various industries. Spherical fused tungsten carbide (sFTC) reinforced nickel-aluminum bronze (NAB) matrix composite surface deposits have shown remarkable potential in mitigating wear by approximately 80%. However, the performance of these sFTC/NAB composite surface deposits is determined by their residual stress s...

An accurate and precise set of experimental data is essential for the characterization of material behavior across different length scales and for calibrating respective constitutive models. An independent validation experiment, the MUC-Test, is presented to validate such constitutive models-the proposed benchmark problem. The unique benchmark data...

With neutron diffraction, the local stress and texture of metallic components can be analyzed non-destructively. For both, highly accurate positioning of the sample is essential, requiring the measurement at the same sample location from different directions. Current sample-positioning systems in neutron diffraction instruments combine XYZ tables a...

The onset of plastic deformation is an important parameter for an accurate description of the flow curve and the Young’s modulus. Determining the actual physical start of flow is already experimentally challenging for classic sheet metal materials. In addition to the experimental challenge, the onset of flow depends on numerous parameters such as s...

Structural and spatial aspects of cell degradation are studied using a combination of diffraction-and imaging-based tools applying laboratory X-rays, neutron scattering and synchrotron radiation with electrochemical and thermal characterization. Experimental characterization is carried out on cylindrical cells of 21700-type, where four regimes of c...

Verschleißschutzschichten aus MMC-Werkstoffen (engl.: metal matrix composites, dt.: Metallmatrix-Verbundwerk- stoffe ) können die Standzeiten abrasiv hochbelasteter Werkzeuge deutlich erhöhen. Während der Herstellung der MMC-Oberflächen mittels Laserstrahl-Dispergieren werden jedoch thermisch induzierte Eigenspannungen in die Randschicht eingebrach...

Abstract Tungsten carbide particles reinforced metal matrix composite (MMC) coatings can significantly improve surface wear resistance owing to their increased surface hardness. However, the presence of macro- and micro-residual stresses in MMC coatings can have detrimental effects, such as reducing service life. In this study, neutron diffraction...

In the present work, ring-shaped samples made from steel 20MnCr5 were low-pressure carburized (LPC) and subsequently hardened by gas quenching (case-hardened). This results in a near-surface gradient in chemical composition, microstructure- and hardness distribution, as well as a three-dimensional residual stress (RS) distribution, which was invest...

Laser melt injection (LMI) is used to improve the wear resistance of highly loaded metal surfaces such as copper-based alloys for injection molds or casting tools. Therefore, a metal matrix composite (MMC) is formed by the injection of hard particles. When using copper as substrate material a laser with 515 nm wavelength is beneficial, due to its h...

Modeling the elastic behavior of dual-phase steels is complex due to the strain dependency of Young's modulus and high elastic nonlinearity. Since it is assumed that reasons for this are to be found in microstructural behavior, microscopic in-situ analysis are necessary, but due to the overlap of the martensite and ferrite peaks, the evaluation of...

The cooling of the material during elastic tensile loading is well known as the thermoelastic effect. It is already known that the temperature minimum at the elastic-plastic transition can be used for the determination of the onset of yielding. Conceivable parameters for this have already been presented and investigated. Within this study factors i...

The thermoelastic effect indicates the dependency of temperature and volume change in the material and, due to the heat released during plastic deformation, a temperature minimum occurs in the region of the onset of yielding. An experimental setup is presented for the microscopic analysis of the thermoelastic effect, which allows high precision mea...

The effect of the heat treatment on the residual stresses of welded cladded steel samples is analyzed in this study. The residual stresses across the plate’s square sections were determined using complementary methods; applying diffraction with neutron radiation and mechanically using the contour method. The analysis of the large coarse grain auste...

Experimental analyses of depth distributions of phase-specific residual stresses after deep rolling were carried out by means of laboratory X-ray diffraction and neutron diffraction for the two duplex steels X2CrNiMoN22-5-3 and X3CrNiMoN27-5-2, which differ significantly in their ferrite to austenite ratios. The aim of the investigation was to eluc...

Positional accuracy is an important parameter in residual stress investigations with neutron diffraction, considering that precise measurements of strains at the same localised position along a number of sample orientations are required, including investigations of complete complex shaped engineering components. This study reports the development o...

Neutron diffraction strain measurements with high spatial resolution were carried out on a dissimilar metal weld (DMW). Residual stress were calculated and compared using three different approaches for reference value (d0). The results underline the sensitivity of the resulting residual stress profiles on the d0 used, particularly in the welded reg...

Arc welding generally introduces undesired local residual stress states on engineering components hindering high quality performance in service. Common procedures to reduce the tensile residual stresses are post heat treatments or mechanical surface treatments like hammering or shot peening. Assessment of residual stress profiles of post weld treat...

Neutron diffraction is a powerful non-destructive volumetric evaluation method for the analysis of the internal stress state in components processed by Laser Powder Bed Fusion (LPBF). High cooling rates and heterogeneous distribution of temperature during additive manufacturing lead to large residual stress fields. Residual stresses developed durin...

Machining part distortion is due to residual stresses induced by previous manufacturing processes. This study aims to evaluate the influence of machining conditions on AISI 316L plate distortion. Therefore, a special experimental device with force sensors integrated in the clamping system and numerical model of distortion were developed. Residual s...

The High Frequency Mechanical Impact (HFMI) process is a comparably new, manually-driven machine hammer peening (MHP) process applied to increase the fatigue performance of metallic components. The HFMI-process is characterized by a large number of impacts, also when the tool was used with a high feed rate. This leads to a fast saturation and highl...

Three rectilinear blocks of the aluminium alloy 7449 were characterised using neutron and X-ray diffraction. One block was heat treated normally and two blocks were subject to uphill quenching from −196°C to 100°C. Boiling water and steam were used to rapidly increase the temperature of the blocks to reverse the thermal gradients introduced by cold...

Diffraction is a powerful tool for analyzing residual as well as applied stresses. Both x ray and neutron diffraction can be used for this purpose. The interest in neutron stress analysis stems from the high penetrating power of neutrons when compared to laboratory x ray sources, i.e. several cm instead of a few tens of μm. This contribution will h...

A method for near-surface strain scanning measurements using neutron diffraction with large gauge volumes is described. The approach corrects for pseudo (spurious) strains related to partially illuminated gauge volumes and enables deconvolution of the large gauge volume measurements into the assumed real profile. It was validated against X-ray and...

Due to the thermal cycle during welding, plastic deformation can occur in the heataffected zone. After cooling, the yield stress can be locally increased due to the hardening effect and thus permits higher residual stresses in these areas. Therefore, a precise description of the hardening behavior in welding simulations is indispensable for reliabl...

Low transformation temperature (LTT) weld filler materials offer an attractive alternative to cost intensive postweld treatments as they can mitigate detrimental welding residual stresses during the welding process. Compared to conventional weld filler materials, LTT alloys are characterized by a delayed martensite transformation at low temperature...

A semi-empirical method for treatment of spatial resolution effects in neutron strain scanning is proposed. The method is based on the description of neutron sampling volume as a Gaussian function perturbed by sample boundaries and other material heterogeneities. This approach results in a simple analytical model with a small number of free paramet...

Yttria stabilized zirconia (YSZ) is often applied as thermal barrier coating on metal parts as e.g. turbine blades made of nickel base super alloys. The coating process in combination with the preconditioning of the substrate material induces characteristic residual stress distributions in the coating system consisting of topcoat, bondcoat and the...

In this study, lithium gradients forming in the graphite anode of a commercial 18650-type lithium-ion battery during discharge and the associated relaxation processes after discharge were monitored by neutron diffraction. The experiments reveal the coexistence of several Li1-xC6 phases with different lithium contents during discharge, which can be...

The determination of residual stress in engineering materials with large grains is a challenge when it comes to using diffraction techniques. Not only are the accuracies of the residual stresses themselves important but also the accurate evaluation of their uncertainties. An austenitic steel three-pass slot weld (NeT- TG4) with varying grain size h...

The high flux neutron diffractometer STRESSSPEC at FRM II, Garching Germany, offers a flexible instrument setup suitable for fast and surface residual strain measurements. Likewise bulk, local or gradient texture analyses are feasible. Here improvements of the hardware (detector, slits) of the instrument as well as developments on methods for resid...

The accurate determination of strain during measurement using neutron diffraction depends on many factors. The statistical uncertainty of the diffraction data is not always the most important contributor to the total uncertainty in the measured strain. Other contributors, such as sample positioning, size and shape of the sampling (gauge) volume and...

Numerous experimental works have been devoted to studying the influence of different shot peening parameters on the surface material conditions in different metals in the last decades. Most of the research work has been focused on experimental determination of the residual stress and its depth profile by means of X-ray diffraction and corresponding...

With respect to residual stress analysis the inner gearing of an automotive sliding collar is a component with a challenging, complex geometry. The accessibility of the tooth root does not exist for conventional measuring approaches. However, the process steps like e.g. broaching and case hardening induce characteristic residual stress distribution...

In this study local texture of process vessels made of carbon steel cladded by protective layers of stainless steel by submerged arc welding (SAW) were investigated by neutron diffraction using the diffractometer STRESSSPEC at FRM 2 (Garching, Germany). Different samples were prepared: as welded and as welded plus relevant industrial heat treatment...

This work is on the interaction of residual stresses and fatigue cracks in welded steel plates. A longitudinal dummy weld was used to create high tensile residual stresses in axial loading direction. The fatigue crack was trained to start at an artificial notch and propagated through a field of high tensile residual stresses. The fatigue crack prop...

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. STRESS-SPEC, the materials science diffractometer, which is jointly operated by the Technische Uni...

Laser Shock Peening is a fatigue enhancement treatment using laser energy to induce compressive Residual Stresses (RS) in the outer layers of metallic components. This work describes the variations of introduced RS-field with peen size and coverage for thin metal samples treated with under-water-LSP. The specimens under investigation were of alumin...

Thermal cycles during laser cladding can alter mechanical properties of the original part significantly. In case of cyclically loaded parts residual stresses are suspected to be a property strongly determining fatigue life. Therefore, in this paper the influence of processing and post-processing strategies on resulting residual stresses are determi...

Lithium plating in commercial LiNi1/3Mn1/3Co1/3O2/graphite cells at sub-ambient temperatures is studied by neutron diffraction at Stress-Spec, MLZ. Li plating uses part of the active lithium in the cell and competes with the intercalation of lithium into graphite. As a result, the degree of graphite lithiation during and after charge is lower. Comp...

Weld filler alloys that exploit transformation plasticity through low austenite to martensite transformation temperatures offer an effective method of reducing residual stresses in strong steel welds. However, in multipass welds, the heat input from later weld passes may be insufficient to retransform prior welding passes, leading to the accumulati...

Transformation plasticity can be utilised to control residual stresses in steel welds. This requires special filler alloys that transform at a sufficiently low temperature to compensate for accumulated thermal contraction strains. However, the welding parameters needed to optimise the effect in multipass joints have yet to be established. This topi...

Friction Stir Processing (FSP) is a relatively new post-processing technique. Fatigue strength of MIG fillet welds of aluminium alloys can be substantially improved using FSP. Beyond other properties intrinsically tied to fatigue life, especially attention should be paid to the effect of the final residual stress state. In this study the residual s...

Precise determination of diffraction peak positions is of particular importance for the evaluation of residual strains. Neutrons are commonly used to probe residual strains from material volumes in depths of several millimetres under the sample surface. However, neutron strain analyses are critical for the near surface region. When scanning close t...

Neutron strain diffractometers usually use slits to define a gauge volume within engineering samples. In this study a multi-channel parabolic neutron guide was developed to be used instead of the primary slit to minimise the loss of intensity and vertical definition of the gauge volume when using slits placed far away from the measurement position...

In machining, due to the tool vibration the forces, temperatures and stresses vary during the cutting process, so as the residual stresses over the machined surface. This variation of the residual stress has never been measured and, therefore, imposes new experimental challenges. The objective of this work is to correlate the variation of the resid...

Precise determination of diffraction peak positions is of particular importance for the evaluation of residual strains. Neutrons are commonly used to probe residual strains from material volumes in depths of several millimetres under the sample surface. However, neutron strain analyses are critical for the near surface region. When scanning close t...

At the German neutron source Heinz Maier-Leibnitz (FRM II) the diffractometer STRESS-SPEC is the dedicated instrument to tackle problems in the fi eld of engineering and applied materials science. Modern engineering science spans a wide field of scientific areas including the analysis of residual stresses, characterisation of textures and phase ana...

The neutron diffraction is a non-destructive technique, particularly suitable for the analysis of residual stress fields in welds. The technique is used in this article to study ferritic samples, coated by submerged arc welding using stainless steel filler metals. This procedure is often used for manufacturing process equipment for chemical and nuc...

Residual strains measured by neutron diffraction near sample boundaries can be biased by the surface effect as a result of incomplete filling of the instrumental gauge volume. This effect is manifested as anomalous shifts of diffraction lines, which can be falsely interpreted as a lattice strain unless appropriate data corrections are made. A new a...

Residual stresses resulting from circumferential laser cladding of fatigue test specimen of austenitic steel X5CrNi18-10 and heat treatable steel 42CrMo4 with Stellite 21 are evaluated by neutron diffraction. Below the interface of cladding and base material the former shows compressive residual stresses, the latter shows tensile residual stresses....

In multi-pass welds, the development of residual stress generally depends on the response of the weld metal, heat-affected zone (HAZ) and nearby parent material to complex thermo-mechanical cycles. Here, the evolution of local material properties and residual stress was investigated for each of these zones during the manufacture of a three-pass gro...

In response to the development of new materials and the application of materials and components in advanced technologies, non-destructive measurement methods of textures and residual stresses have gained worldwide significance in recent years. The materials science neutron diffractometer STRESS-SPEC at FRM II (Garching, Germany) is designed to be a...

Neutron strain measurements are critical at the surface. When scanning close to a sample surface, aberration peak shifts arise due to geometrical and divergence effects. These aberration peak shifts can be of the same order as the peak shifts related to residual strains. In this study it will be demonstrated that by optimizing the horizontal bendin...

The influence of the heat treatment on the residual stress fields of weld cladded samples is discussed in this paper. The samples were elaborated from carbon steel plates, cladded in one of the faces with stainless steel filler metals by submerged arc welding. After the cladding process some of the samples were submitted to heat treatments with dif...

For non destructive stress analysis of surface treated steel samples the application of laboratory X rays or high energy synchrotron radiation in reflection mode covers the region from some micrometers up to a depth of about 150 - 200 µm. To access depth regions deeper than 200 µm the incremental layer removal technique in combination with the repe...

The determination of strain from neutron diffraction data is normally based upon the fit of a Gaussian function to a Bragg reflection. The error in the fit is assumed to be that based on ‘counting statistics’ and this error propagates through the analyses until the final stress evaluation. This relies on there being a big enough number of diffracti...

The measurement and analysis of residual stresses has gained significant importance over the past couple of years due to the increasing demands in improving the properties of new engineering materials and components. The ability to measure these residual stresses accurately will thus lead to the manufacture of stronger, lighter, and cheaper compone...

Carbon fibre ceramic matrix composite materials, C/SiC, are excellent candidates as lightweight structural materials for high performance hot structures such as in aerospace applications. Satellite nozzles are manufactured from C/SiC, using, for instance, the Liquid Polymer Infiltration (LPI) process. In this article the applicability of different...

In response to the development of new material and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. In order to cater for the development of these analytical techniques the Materials Science Diffract...

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by mea...

During electro discharge machining (EDM) a characteristic residual stress profile is formed. The process is essentially thermal and leads to tensile residual stresses at the surface. The profile form is independent from the discharge energy. However, the profile depth, the maximum value of the tensile residual stresses and the depth where the maxim...

An experimental study on the effect of electro-discharge machining (EDM) parameters on material removal rate (MRR) and surface quality, when machining high strength copper–beryllium alloys is presented. Processing parameters for rough, finishing and micro-finishing or polishing regimes were analysed, as well as the effect of the introduction of pla...

Electro-discharge machining (EDM) is essentially a thermal process with a complex metal-removal mechanism, involving the formation of a plasma channel between the tool and workpiece electrodes, resulting in metallurgical transformations, residual tensile stresses and cracking. These properties determine the operational behaviour of the material and...