Christoph Hubertus Wölfle’s research while affiliated with Technical University of Munich and other places

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Publications (9)


Sketch of light-induced deformation mechanisms
a azo-SEBS film deposited on a PDMS layer, (b) self-standing azo-SEBS membrane; (c) illustrative deformation of azopolymer microdomains upon linearly polarized laser irradiation, leading to overall stretching of the azo-SEBS layer on PDMS along the polarization direction; (d) inhomogeneous stretching of the free-standing azo-SEBS membrane and corresponding bending caused by the gradient of absorbed light through the membrane. Both SEBS and PDMS are translucent/transparent materials in the real world.
Analysis of 2D deformations
a Confocal microscope transmission image of the 33.8 × 33.8 µm² area decorated with 1 µm spaced, 1 µm wide pillars, before illumination. b Fourier transforms the image of a, with detected peak locations (pink squares). Inset: 2D Gaussian fitting of exact peak position. Shift of peaks in Fourier space after irradiation with (c) linear polarization and after irradiation with (d) linear and subsequent circular polarization (illumination cycle). Red arrows: data, blue arrows: fit. Arrow magnification: x45. e Extracted fit parameters for 20 illumination cycles: relative horizontal elongation strain ε1 per X-POL step (blue circles), absolute value of relative vertical compression strain |ε2| = −ε2 per X-POL step (red asterisks), relative area expansion δAcycle per full illumination cycle (X-POL + CIRC-POL) (green triangles). Dashed green line: asymptotic behaviour. Solid blue line: zero deformation limit. Short/tall x-axis ticks: X-pol step/two-step illumination cycle. Error bars: Sample standard deviation (s.d.) of 3 acquisitions on distinct areas. Inset: Confocal image of the sample’s fine structure (bulk) showing the size of the azopolymer domains. Inset scale bar: 3 µm.
Finite element model
Logarithmic normal strain in x-direction εxxH\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varepsilon }_{{xx}}^{{{\mbox{H}}}}$$\end{document} (a) and first-order in-plane expansion εxxH\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varepsilon }_{{xx}}^{{{\mbox{H}}}}$$\end{document} + εyyH\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varepsilon }_{{yy}}^{{{\mbox{H}}}}$$\end{document} (b) around the azo-domains stretched along the x-axis using Flin\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{{{{{\bf{F}}}}}}}^{{{{{{\bf{lin}}}}}}}$$\end{document} to model the illumination-induced eigenstrain of the azo-domains associated to linear x-polarization. Solid and dashed white lines provide a guide to the eye for identifying compression/expansion zones, respectively. The shear component and logarithmic normal strain in y-direction are shown in Supplementary Fig. 6b, c. c Normal strain transmission factor Tiitype\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{{{{{{\rm{ii}}}}}}}^{{{{{{\rm{type}}}}}}}$$\end{document} as a function of imposed azo-domain strain εiiaz\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varepsilon }_{{{{{{\rm{ii}}}}}}}^{{{{{{\rm{az}}}}}}}$$\end{document} for each type of deformation (type = {lin, circ}) and principal axis (i = {x,y}). Error bars: sample standard deviation from 9 simulations with different randomly generated azo-domain arrangements. For simplicity of representation Txxcirc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{{xx}}^{{{{{{\rm{circ}}}}}}}$$\end{document} and Tyycirc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{{yy}}^{{{{{{\rm{circ}}}}}}}$$\end{document} are plotted with the same marker type due to their substantial overlap.
Influence of illumination conditions on macroscopic strain
Plot of asymptotic strain parameters ε1,∞ (a) and ε2,∞ (b) respectively as a function of applied dose (intensity times the exposure time) per illumination step. The asymptotic values are based on acquisitions whose graphs are shown in Supplementary Fig. 8. Error bars are three times the standard error of the mean (s.e.m.), as detailed in the Methods section.
Influence of sample fine structure
Plot of fitted parameters for multiple actuation cycles for Sample 2 (a) and Sample 4 on top of large aggregates (b) and in between aggregates (c). Relative horizontal elongation strain ε1 per X-POL step (blue circles), absolute value of relative vertical compression strain |ε2| = −ε2 per X-POL step (red asterisks), relative area expansion δAcycle per full illumination cycle (X-POL + CIRC-POL) (green triangles). Dashed green line: asymptotic behaviour. Solid blue line: zero deformation limit. Short/tall x-axis ticks: X-pol step/Two-step illumination cycle. Error bars: s.d. of 3 acquisitions on distinct areas. Insets: confocal images of samples’ structures and aggregate sizes. Inset scale bars: (a) 3 μm, (b, c) 75 μm. White frames on insets in (b,c) show a typical illumination area.

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Polarization-driven reversible actuation in a photo-responsive polymer composite
  • Article
  • Full-text available

October 2023

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151 Reads

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7 Citations

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Niccolò Marcucci

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Christoph Hubertus Wölfle

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[...]

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Light-responsive polymers and especially amorphous azopolymers with intrinsic anisotropic and polarization-dependent deformation photo-response hold great promises for remotely controlled, tunable devices. However, dynamic control requires reversibility characteristics far beyond what is currently obtainable via plastic deformation of such polymers. Here, we embed azopolymer microparticles in a rubbery elastic matrix at high density. In the resulting composite, cumulative deformations are replaced by reversible shape switching – with two reversible degrees of freedom defined uniquely by the writing beam polarization. We quantify the locally induced strains, including small creeping losses, directly by means of a deformation tracking algorithm acting on microscope images of planar substrates. Further, we introduce free-standing 3D actuators able to smoothly undergo multiple configurational changes, including twisting, roll-in, grabbing-like actuation, and even continuous, pivot-less shape rotation, all dictated by a single wavelength laser beam with controlled polarization.

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On the Applicability of Approximate Rolling and Sliding Contact Algorithms in Anisothermal Problems with Thermal Softening

February 2022

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47 Reads

Applied Sciences

The residual stress approximation methods formulated by McDowell and Moyar, Jiang and Sehitoglu, and McDowell for rolling and sliding contact problems are reconsidered in the context of single anisothermal loading cycles and isotropic hardening. A consistent extention to incorporate thermal softening is developed and the generalized thermoelastoplastic algorithms are cast into a proper predictor–corrector formulation. Detailed explicit and implicit numerical integration strategies are presented and validated using specifically designed finite element models that conform to the underlying mechanical assumptions. Then, the applicability of the approximate algorithms to anisothermal problems with isotropic hardening and thermal softening is analyzed by assuming a rate-independent Johnson–Cook-type yield stress model and by comparing the obtained transient and residual stresses to results from full-scale finite element half-space models under varying loading and strain-hardening intensities. An in-depth, comparative discussion on the adequacy of the algorithms in conjunction with the justification of their respective mechanical simplifications follows. Sufficiently strong strain hardening is found to be a prerequisite for accurate predictions, and Jiang and Sehitoglu’s approach is deemed to be preferable for the considered type of problem. The conclusions drawn from the investigations are discussed in the context of common applications with particular emphasis on manufacturing process modeling and the corresponding guidelines are proposed for such use cases.


An Approach to Predict Geometrically and Thermo-Mechanically Induced Stress Concentrations in Ribbed Reinforcing Bars

February 2022

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187 Reads

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5 Citations

Metals

Ribbed reinforcing steel bars (rebars) are used for the reinforcement of concrete structures. In service, they are subjected to cyclic loading. Several studies addressing the relationship between rib geometry, stresses at the rebar surface induced by service loads and the rebar fatigue performance can be found in literature. However, the rebar’s fatigue performance is also influenced by residual stresses originating from the manufacturing process. In this contribution, a modeling approach is proposed to examine geometrically and thermo-mechanically induced stress concentrations in ribbed reinforcing bars made of the steel grade B500B. A linear-elastic load stress analysis and a thermo-mechanical analysis of the manufacturing process are conducted. The results are discussed and compared to literature results. In case of the load stress analysis, the results agree well with findings reported in literature and extend the current state of knowledge for B500B rebars with small diameters. In case of the thermo-mechanical analysis, compressive residual stresses at the rebar surface between two ribs and tensile residual stresses in the longitudinal direction at the tip of the ribs can be reported.


An implicit integration scheme with consistent tangent modulus for Leblond’s model of transformation-induced plasticity in steels

January 2022

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202 Reads

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1 Citation

Continuum Mechanics and Thermodynamics

Thermomechanical treatments involving solid-state phase transformations play an important role for the manufacturing of functional and reliable components in many engineering applications. Accordingly, numerical investigation and optimization of such processes require considering thermoelastoplasticity under the influence of ongoing transformations and in particular the impact of transformation-induced plasticity (TRIP). While a number of elaborate plasticity models have been proposed for the description of TRIP, none of them seem to have received much prevalence in applications due to their complexity or hard to determine model parameters. Instead, the overwhelming majority of applied research either relies on simplistic formulations dating back to early phenomenological approaches or neglects TRIP altogether. In this work, we therefore provide an accessible, straightforward and easy-to-implement solution scheme for the TRIP model proposed by Leblond et al. which, despite being widely recognized, is hardly ever employed in full form. Specifically, we employ implicit backward-Euler integration and an elastic–plastic operator split approach to update the stresses in order to obtain a simple and concise algorithm for which we then derive the corresponding consistent tangent modulus. Furthermore, the work contains an application of the solution scheme to a symmetrically cooled plate and an in-depth discussion of the influence of TRIP by means of this tractable numerical example. Specifically, we highlight the discrepancies arising in transient and residual stresses and strains compared to the conventional J2J_2 J 2 -plasticity approach where the phase transformation is accounted for merely by adapting the yield strength of the compound.


The influences of process parameters on the thermo-mechanical workpiece load and the sub-surface residual stresses during peripheral milling of Ti-6Al-4V

September 2021

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21 Reads

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11 Citations

Procedia CIRP

The residual stress state of the sub-surface zone is a significant influencing factor, which determines the strength, lifetime and reliability of a component after machining. A reliable adjustment of this surface characteristic during peripheral milling is currently not possible. It is desired to control the milling process using a model-based approach in order to generate defined residual stresses in titanium components. This paper focuses on the determination of the thermo-mechanical load leading to the residual stress state during peripheral milling of Ti-6Al-4V. A newly developed sensory tool holder for the in-process measurement of the temperatures in the process zone was used. The influence of the process parameters cutting speed, feed per tooth, and radial depth of cut on the residual stress condition is shown. In addition, the varying microgeometry of the tool due to tool wear is taken into account as an observable disturbance variable.


Towards real-time prediction of residual stresses induced by peripheral milling of Ti–6Al–4V

July 2021

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218 Reads

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7 Citations

Continuum Mechanics and Thermodynamics

Residual stresses introduced into components in the course of manufacturing processes may considerably impair fatigue life and therefore the operational reliability and safety of the final product. Particularly in critical applications in the aerospace industry, where peripheral milling is a common surface finishing operation for components made from the titanium alloy Ti–6Al–4V, it is desirable to control the introduced residual stresses in the process while accounting for disturbance quantities such as tool wear. To this end, we propose a numerical scheme for the prediction of milling induced residual stresses that provides sufficient efficiency for real-time application. The scheme is based on a two-dimensional model where semi-analytical approaches from contact theory and thermoelasticity are combined with an approximate elasto-plastic solution technique based on an algorithm established in rolling contact mechanics to achieve the required performance in the plastic domain. Following the derivation of the numerical solution strategy, we turn our attention to the peripheral milling process under consideration and present predictions for the induced residual stresses along with a discussion of the general modeling approach, the major influencing factors on the predictions as well as efficiency aspects.


Parameter Identification for Thermo-Mechanical Constitutive Modeling to Describe Process-Induced Residual Stresses and Phase Transformations in Low-Carbon Steels

January 2021

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1,227 Reads

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6 Citations

Applied Sciences

Reinforcing steel bars (rebars) are widely manufactured using the Tempcore™ process. Several studies have been conducted analyzing the effect of the heat treatment route on the strength and corrosion resistance of rebars, but knowledge of its effects on the residual stresses of the finished product are largely lacking. This paper presents experimental investigations to identify the material parameters necessary to simulate the Tempcore™ process using thermo-elasto-plastic constitutive modeling in order to study the generation of residual stresses during the manufacturing process. Mechanical parameters such as yield strength at elevated temperatures and elastic constants were determined experimentally. A continuous cooling transformation diagram needed to model the phase transformations was also identified and is presented here. Residual stress distributions in the surface region of the rebar were characterized using X-ray diffraction. Further characterizations of microstructure, chemical composition, and hardness were carried out. The constitutive modeling approach for the numerical simulation is briefly described for which the experimentally determined parameters are required as input.


The influence of the process parameters on the surface integrity during peripheral milling of Ti-6Al-4V

November 2020

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60 Reads

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10 Citations

tm - Technisches Messen

The titanium alloy Ti-6Al-4V represents a significant metal portion of state-of-the-art aircraft structural and engine components. When critical structural components in the aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of machined surfaces. The residual stresses and the surface alteration induced by machining titanium alloys are critical due to safety and sustainability issues. In this paper, a series of end milling experiments was conducted to comprehensively characterize the surface integrity at various milling conditions. The experimental results have shown that the surface roughness value increases with the feed and the cutting velocity. However, the residual stress state in the surface layer zone is influenced by the variation of the process control variables. Here, compressive residual stresses occur both in cutting and in feed direction. In addition, a new type of sensory tool holder is presented, which should enable the indirect measurement of residual stresses during the milling process.

Citations (7)


... However, only void fractions between 70% and 98% were considered. In [21], likewise, the effective electrical conductivities of Gyroid and Schwarz-P structures for void fractions between 63% and 88% were investigated. Catchpole-Smith et al. [22] used a test rig to characterize the thermal conduction of three isotropic TPMS architectures (Gyroid, Schwarz-D, and Schwarz-P) for void fractions between 50% and 80%. ...

Reference:

Electrical Tortuosities of Porous Structures Based on Triply Periodic Minimal Surfaces and Honeycombs for Power-to-Heat Systems
On the porosity-dependent permeability and conductivity of triply periodic minimal surface based porous media
  • Citing Article
  • September 2023

Journal of Materials Research and Technology

... At the same time, the martensitic rim is reheated and tempered by the heat from the core. By adjusting the heat-treatment parameters, e.g., by varying the quenching time, the TempCore TM process allows the production of rebars of different strengths and ductilities [3][4][5]. ...

An Approach to Predict Geometrically and Thermo-Mechanically Induced Stress Concentrations in Ribbed Reinforcing Bars

Metals

... Several studies addressing the relationship between rib geometry, stresses at the re- 38 bar surface induced by service loads and the rebar fatigue performance were carried 39 out [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The rib volume and the foot radius of the transverse rib were identified to 40 have the main impact on the rebar fatigue performance. 41 However, the rebar's fatigue performance is influenced also by residual stresses originat- 42 ing from the manufacturing process due to the asynchronous shrinkage of the surface 43 and the core regions as well as the transformation strain accompanying the decomposi- 44 tion of austenite [25]. ...

An implicit integration scheme with consistent tangent modulus for Leblond’s model of transformation-induced plasticity in steels

Continuum Mechanics and Thermodynamics

... The simulative investigations showed a strong influence of the cutting speed v c and the chip thickness h on the properties of the sub-surface. However, varying the cutting-edge radius r β did not show any significant influence, which is contrary to the majority of other findings across the literature [21][22][23]. ...

The influences of process parameters on the thermo-mechanical workpiece load and the sub-surface residual stresses during peripheral milling of Ti-6Al-4V
  • Citing Article
  • September 2021

Procedia CIRP

... The surface layer of the rebar is transformed from austenite to martensite [19]. The depth of formed martensite layer is controlled to get specific mechanical properties [20]. In the subsequent step, the rebar leaves the quenching box with a temperature gradient between its surface layer and the core. ...

Parameter Identification for Thermo-Mechanical Constitutive Modeling to Describe Process-Induced Residual Stresses and Phase Transformations in Low-Carbon Steels

Applied Sciences

... The dimensions of the workpiece were 100 mm × 100 mm × 11 mm. The methodology for conducting the residual stress measurements ist detailed in [49], where X-ray diffraction (XRD) was employed, an thin layers were removed from the component Fig. 5 a Influence of different cutting-edge radii r β on the measured process forces (F c and F f ) and b change in surface residual stresses depending on the cutting-edge radius r β surface by electrochemical etching to determine the residual stress depth curves. The stress components were measured in the feed direction σ f and in the axial direction σ a . ...

The influence of the process parameters on the surface integrity during peripheral milling of Ti-6Al-4V
  • Citing Article
  • November 2020

tm - Technisches Messen

... Ti-6246 alloy is an alpha-beta titanium alloy offering very high mechanical strength with good retention up to 450 • C. The alloy is heat-treatable and deep-harden-able. Ti-6246 alloy is mainly used in the manufacture of compressor discs, blades and other parts of aero-engines [3][4][5]. Due to the inhomogeneous microstructure of the material, in the random distribution of material defects and the uncertainty in processing and manufacturing there is usually a certain dispersion in fatigue life for the same loading conditions. The use of different fatigue models to describe the stress/strain-life behavior also brings with it a certain dispersion of models, and the uncertainty of the random distribution of material defects usually leads to a certain dispersion of fatigue life under the same loading conditions. ...

Towards real-time prediction of residual stresses induced by peripheral milling of Ti–6Al–4V

Continuum Mechanics and Thermodynamics