Showing papers by "Jens Gibmeier published in 2017"

Incremental Hole Drilling for Residual Stress Analysis of Thin Walled Components with Regard to Plasticity Effects

Abstract: The incremental hole-drilling method is widely used in residual stress depth distribution analysis. However, two specific difficulties with the generalization of the incremental method exist, including the consideration of the sample thickness and residual stress states close to the local material’s yield strength. The stress concentration effect of the hole can lead to plastic deformation in the vicinity of the hole, which results in an overestimation of residual stresses. Typically, the effect of the component’s thickness and the plasticity effects are analyzed separately and correction approaches are proposed. In the current paper, we analyze the combined effects of plasticity and thickness on residual stress analysis using the incremental hole-drilling method. A systematic study was performed on steel samples with (i) isotropic and (ii) anisotropic elastic and elasto-plastic material behavior with varying thicknesses ranging between 1 mm and 4 mm. Electronic speckle pattern interferometry (ESPI) hole-drilling experiments were conducted on beam samples loaded using a 4-point bending fixture. Finite element simulations were conducted to gain insight into the effects of incremental hole-drilling. The results indicate that reducing the component’s thickness increases the plastic deformation in the vicinity of the hole and results in significant stress deviations. Thin components bend during hole-drilling as a result of the loss of stiffness, which amplifies the plasticity effect.

Residual stresses of LTT welds in large-scale components

Abstract: Residual stresses of welds become more and more important influencing cold cracking as well as the fatigue life of welded components. Low transformation temperature (LTT) filler materials offer the opportunity to alter the residual stresses already during the welding process by means of adjusted martensite phase transformation temperature (M S). In the current paper, welding residual stresses are studied putting the focus on M S while joining heavy steel sections with a thickness of 20 and 25 mm, respectively. The residual stress state was determined at the top surface using X-ray diffraction as well as in the bulk by neutron diffraction. The results compare the residual stresses present in a conventional weld and LTT welds when multi-pass welding of large-scale components was applied. Repeated phase transformation in the case of the LTT weld is more vital for the residual stresses present in the real-life-like joints. This accounts for the top surface in longitudinal direction but is most pronounced for the bulk of the welds. Detrimental tensile residual stresses are mainly reduced in the bulk in comparison to a conventional filler wire even in multi-pass welds of thick steel sections.

Influence of shot peening on the mechanical properties of bulk amorphous Vitreloy 105

Abstract: The quasi-static and cyclic properties of bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 alloy (Vitreloy 105) were investigated under three-point bending conditions for two different shot-peened surface states. Residual stress analysis and nanoindentation measurements revealed the presence of compressive residual stresses and an enhanced hardness in the near surface layer after shot peening. Further investigations of the longitudinal cross-sections of the mechanically tested specimens by optical and scanning electron microscopy showed small cracks propagating along shear bands in the vicinity of the fracture surface. The results are in accordance with the improved plasticity of the shot-peened states under quasi-static loading conditions compared to the as-cast reference state. All mechanical testing was carried out with the aim to find a material’s state with improved mechanical properties with a special focus on the improvement of the fatigue lifetime and the endurance limit of Vitreloy 105 bulk metallic...

In Situ EDXRD Study of MAG-Welding Using LTT Weld Filler Materials under Structural Restraint

Abstract: Welding using low transformation temperature (LTT) filler materials is an innovative approach to mitigate detrimental welding residual stresses without cost-intensive post weldtreatments. Due to the local Generation of compressive residual stresses in the weld line by means of a delayed martensite transformation a significant enhancement of the cold cracking resistance of highly stressed welded components can be expected. For the effective usage of These materials a deeper understanding of the microstructural evolution inside the weld material is necessary to determine the complex processes that cause the residual stress formation during welding. Solid-state phase transformation kinetics and the evolution of strain in LTT weld filler materials are monitored in-situ at the instrument ID15A at the ESRF in Grenoble, France. The transferability to real components is implemented by using a realistic MAG welding process under consideration of structural restraint. During welding of multilayer joints, the phase Transformation and phase specific strain evolution of each individual layer is investigated in transmission geometry by means of energy-dispersive X-ray diffraction EDXRD using high energy synchrotron Radiation with a counting rate of 2.5 Hz. The measurement results of a 10% Cr / 10% Ni LTT weld filler are compared to data monitored for the conventional weld filler material G89. The in-situ data clearly indicate a strong effect on the local strain evolution and the formation of compressive strain. This results from the restraint volume expansion during the postponed austenite to martensite transformation of the LTT weld filler, which counteracts the thermal shrinkage. In contrast, for the conventional weld filler material the thermal contraction strains lead to tensile residual strain during welding. Furthermore, the results of in-situ observation during welding Show that the transformation kinetic is dependent on the welding sequence.

Residual Stress States After Piezo Peening Treatment at Cryogenic and Elevated Temperatures Predicted by FEM Using Suitable Material Models

Abstract: Piezo peening is a recently developed mechanical surface treatment and belongs to machine hammer peening technologies. It has proven suitable to generate a wide range of compressive residual stress profiles and penetration depths depending on the parameters chosen for the process. By this means, greatly enhanced fatigue behavior could be achieved. In this study, the residual stress states after modified piezo peening treatments were determined experimentally and by 3D finite element (FE) simulation. Low alloy steel AISI 4140 was treated at ambient, cryogenic and elevated temperatures. Residual stresses were determined experimentally using the sin(ψ) method combined with subsequent electrolytic surface layer removal. The FE simulation makes use of a material model, which is capable of describing strain-rate and temperature dependent material behavior as well as the Bauschinger effect and allows for the emulation of surface layer removal for proper residual stress determination. Thus, the applicability of appropriate material modeling to predict experimentally determined residual stress profiles could be demonstrated. Introduction Due to the generation of smooth surfaces together with compressive residual stresses and work hardening, machine hammer peening (MHP) has become a crucial process step, e.g. in the fabrication of molds and dies. Mostly utilizing electromagnetic, pneumatic and hydraulic transducers, today’s MHP processes allow for the generation of specific surface characteristics [1]. A recently developed MHP technology utilizing a piezo-electric power transducer is piezo peening [2]. It has been applied to the quenched and tempered steel AISI4140, where the fatigue strength could be greatly improved. This was found to be mainly due to the introduction of nearsurface compressive residual stress fields. It was shown that residual stress profiles can widely be varied depending on the applied process parameters [2]. During the last decades, computational mechanics such as the finite element (FE) method has been applied extensively to understand process-property-relationships. Therefore an approach towards the FE simulation of piezo peening has been presented in a recent publication [3], showing good agreement between numerical and experimental results. Since strain-rate and temperature dependent material modeling has been applied in simulation, it is particularly interesting to investigate the influences of temperature variation upon flow stresses and residual stress profiles. On the one hand, potential influences of cryogenic and elevated temperatures on the residual stress profiles after piezo peening are explored experimentally, since the effect of temperature on residual stress generation has not yet been investigated for this process. Furthermore, tendencies regarding residual stress maxima and penetration depths in experiment and simulation are compared, such that the obtained results serve as validation for the applied material model. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 175-180 doi: http://dx.doi.org/10.21741/9781945291173-30 176 Process Description and Experimental Setup A schematic drawing of the utilized piezo peening device [2] is shown in Fig. 1 (left). Specimens are mounted to a linear x-y-slide to be peened by the spherical hammer head. The latter is driven by the piezo actuator with a specific frequency (f), stepover distance (s) and stroke (h), generating “impulsive regular” [4] deformation of the specimen surface. By means of the x-y-slide, the work piece surface can be treated using different patterns, such as meanders. The soft bearing on top is used to control the contact properties. The hammer head is lifted off the specimen surface after each stroke. In Fig. 1 (right), the experimental setup for piezo peening at cryogenic temperatures is shown. Specimens were cooled down to -180 °C using liquid N2 flowing through a brass block specimen holder. For piezo peening at elevated temperatures, the same block was heated by heating rods, thus achieving temperatures of +200 °C. In each case a two-point temperature controller was used. Figure 1: Piezo peening process (schematic, left) [2] and test bench for cryogenic peening at KIT (right) Low alloy steel AISI 4140 with hardness of 430 HV1 was used for the investigations. The chemical composition is shown in Table 1. The material was austenitized at 850 °C for 20 minutes, oil-quenched and then tempered at 450 °C for 120 minutes. Afterwards, it was furnace-cooled to room temperature. Table 1: Chemical composition of AISI 4140 Chemical composition (wt.-%)

Study of stability of microstructure and residual strain after thermal loading of plasma sprayed YSZ by through surface neutron scanning

Abstract: 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 substrate material. Knowledge about the residual stress depth distribution in the coating and at the interfaces down to the substrate material is essential for the assessment of the mechanical integrity and the reliability of the coating. In this regard the stability of the microstructure and the residual stresses is of particular interest; hence this forms the scope of our investigations. Yttria (8 wt.%) stabilized zirconia with a NiCoCrAlY bondcoat was deposited by atmospheric plasma spraying (APS) at different spray conditions on a nickel base super alloy substrate material. The coatings were subjected to different heat-treatment processes, i.e. static aging and cyclic thermal loadings. Through surface scanning using neutron diffraction was carried out for the as sprayed condition and for the thermally loaded samples. Based on the measured diffraction data the stability of the microstructure (phases) and the residual strain/stresses through the depths of the coating system were assessed.

Residual Stress Depth Distributions for Atmospheric Plasma Sprayed MnCo1.9Fe0.1O4 Spinel Layers on Crofer Steel Substrate

Abstract: In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the use of ferritic stainless steel can lead to degradation in cell performance due to chromium migration into the cells at the cathode side [1]. Application of a coating on the ferritic stainless steel interconnect is one option to prevent Cr outward migration through the coating. MnCo1.9Fe0.1O4 (in the following designated as MCF) spinels act as a diffusion barrier and retain high conductivity during operation [2]. Knowledge about the residual stress depth distribution throughout the complete APS coating system is important and can help to optimize the coating process. This implicitly requires reliable residual stress analysis in the coating, the interface region and in the substrate.For residual stress analysis on these specific layered systems diffraction based analysis methods (XRD) using laboratory X-ray sources can only by applied at the very surface. For larger depths sublayer removal is necessary to gain reliable residual stress data. The established method for sublayer removal is electrochemical etching, which fails, since the spinel layer is inert. However, a mechanical layer removal will affect the local residual stress distribution.As an alternative, mechanical residual stress analyses techniques can be applied. Recently, we established an approach to analyse residual stress depth distributions in thick film systems by means of the incremental hole drilling method [5, 6]. In this project, we refined our approach for the application on MCF coatings with a layer thickness between 60 – 125 μm.

Influence of Heat Control on Residual Stresses in Low Transformation Temperature (LTT) Large Scale Welds

Abstract: The current paper presents residual stress analyses of large scale LTT (Low Transformation Temperature) welds. LTT filler materials are specially designed for residual stress engineering by means of an adjusted martensite phase transformation. Controlling the level of mostly detrimental residual stresses already during the welding process would be highly attractive as time and cost consuming post processing may be prevented. In large scale welds the residual stress state is influenced by the heat control (e.g. interpass temperature) during welding. Therefore, welding residual stresses are studied here putting the focus on the influence of welding process parameters while joining heavy steel sections with a thickness of 25 mm. The residual stress state was determined at the top surface using X-ray diffraction as well as in the bulk by neutron diffraction. The results show that control of the interpass temperature is vital for the residual stresses present in the joints. This accounts for the top surface but is most pronounced for the bulk of the welds. While high interpass temperatures are appropriate to induce compressive residual stresses in the weld metal, low interpass temperatures favor unwanted tensile residual stresses instead.

Local Residual Stress Analysis on Deep Drawn Cups by Means of the Incremental Hole-Drilling Method

Abstract: In addition to residual stresses sheet metal forming induces characteristic crystallographic texture, hence, the material behavior is anisotropic. In general, the standard evaluation procedures of residual stress analysis techniques are limited to isotropic material states. In the present paper deep drawn steel cups of dual phase steel DP600 are analyzed by using a recently proposed calibration approach for residual stress analysis by means of the incremental hole-drilling method for highly textured material states. It is based on the differential method, which is enhanced with four case specific calibration functions. The multiple case specific calibration functions are determined by means of finite element simulations using the orientation distribution function (ODF) in combination with Hill’s assumption and single crystal elastic constants of iron to calculate the effective elasticity tensor to account for elastic anisotropy. Supplementary, the deep drawing process is simulated using a finite element model based on the Hill48 yield criterion. Finally, the comparison shows that the numerical results are in satisfactory agreement to the experimental data. Introduction Standard methods of residual stress measurement techniques are restricted to isotropic material states. However, forming processes like e.g. rolling or deep drawing cause preferred orientations of the grains due to the limited possibilities of gliding. Crystallographic textures oftentimes result in anisotropic material behavior. The standard approach of X-ray diffraction stress analysis according to the sinψ-method [1] is no longer applicable in case of textured material states, since the 2θ-sinψ distributions are strongly nonlinear. A remedy is the application of special measurement strategies like e.g. the crystallite group method [2] and stress factors [3], where the texture of the sample must be known a priori. These measurement strategies are elaborate and time-consuming and the knowledge of the stress free lattice parameter D0 is required. Furthermore, formed components often obtain complex geometries and can be large (e.g. A-/B-/C-pillar, cowl). Shadowing effects can occur due to the complex shape of the sample. Owing to a limited installation space of the X-ray diffractometer the samples have to be cut and stress redistributions must be considered during the residual stress calculation. Since the penetration depth of conventionally generated X-rays is limited to a few microns, layers of the material must be removed by means of e.g. electrochemical polishing to determine residual stress depth distributions. The incremental hole-drilling method has great potential, since it is versatile and fast compared to X-ray diffraction. Standard stress calculation methods (e.g. integral method [4], differential method [5]) use calibration data, which is based on isotropic material behavior. Significant errors in stress calculation can occur, if conventional calibration data is applied to residual stress analysis on Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 187-192 doi: http://dx.doi.org/10.21741/9781945291173-32 188 strongly textured materials [6]. Thus, we proposed a new calibration approach based on the differential method [7]. Four case-specific calibration functions must be determined numerically to account for the anisotropic elastic material properties. The elastic constants of the textured sample must be known beforehand for the calibration. A model assumption considering the interactions of the grain boundaries e.g. Voigt, Reuss, Eshelby/Kröner or Hill [8] can be used to calculate the effective elasticity tensor by means of the orientation distribution function (ODF) and single crystal elastic constants. In the present study residual stress depth distributions are determined at different positions of a deep drawn cylindrical cup made of dual phase steel DP600. They are compared to results obtained from finite element (FE) simulations of the deep drawing process. Experimental investigation A deep drawn steel cup with a nominal diameter of 100 mm and a drawing ratio of 1.8, which is the relationship between the blank diameter prior to the drawing operation and the punch diameter, was analyzed. Here, a cold rolled dual phase steel sheet DP600 (ferritic-pearlitic microstructure, model sample taken out during the rolling process prior to the final heat treatment) with a thickness of 1 mm was deep drawn with a blank holder force of 180 kN. The deep drawing operation leads to characteristic earing at the edge of the cup caused by the planar anisotropy of the material. Three different measuring locations were defined as can be seen in Fig. 1. A measuring location below an ear (path I) and one below a trough (path II) are located at the half of the maximum cup height. The rolling direction (RD) of the steel sheet corresponds to path II. The third measuring position is located in RD at the radius between cup wall and bottom. Stress components in drawing direction (DD) and tangential direction (TD) were determined. The chemical composition of the dual phase steel is shown in Table 1. Table 1: Chemical composition of DP600 in weight-% C Si Mn P S B Al Cr Mo Ni 0.093 0.29 1.663 0.011 <0.001 0.0002 0.042 0.341 0.007 0.047 Fig. 1: Definition of measuring locations ́ear ́, ́trough ́ and ́radius ́ and directions. X-ray diffraction texture analyses were carried out using a diffractometer of type XRD 3003 PTS from Seifert. A pin hole collimator with a nominal diameter of 1 mm and CoKα radiation was used. Incomplete pole figures for the lattice planes of type {110}, {200}, {211} and {220} were measured using a β-range of 0°..65° and a α-range of -170°..170° each in a step size of 5°. Fig. 2 illustrates the pole figures {100}, {110} and {111}, which were recalculated from the ODF. The rolling direction of the cold rolled steel sheet prior to the deep drawing operation is still visible for the ́trough ́ and ́radius ́ location and points into the drawing direction. In contrast, the maximum intensity in the pole figures at the measuring position below the ear is turned 45° with respect to the drawing direction. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 187-192 doi: http://dx.doi.org/10.21741/9781945291173-32 189 The ODF, Hill’s model assumption [8] and the single crystal elastic constants of iron (C11 = 230 GPa, C12 = 135 GPa and C44 = 117 GPa [9]) were used to determine the elasticity tensor Cijkl. The crystallographic texture leads to a Young’s modulus ratio Emax/Emin within the measuring plane of 1.09 in case of the trough and the radius and 1.03 in case of the ear. Fig. 2: Pole figures of type {100}, {110} and {111} for the three measuring locations ́ear ́, ́trough ́ and ́radius ́. A hole-drilling device RS200 from Vishay Measurement Group was used for the hole-drilling experiments. TiN coated end mills with a nominal diameter of 0.8 mm were used. Accordingly, strain gage rosettes of type EA-11-031RE-120 from Vishay Measurement Group were applied on the sample. The residual stress depth profiles were calculated using the new calibration approach based on the differential method and multiple case-specific calibration functions considering the elastic anisotropy, which we recently proposed in [7]. The following steps have to be conducted: (i) Texture measurement and determination of the ODF (ii) Calculation of the elasticity tensor Cijkl (iii) Determination of four case-specific calibration functions (two FE simulations required) (iv) Residual stress calculation Basically, the residual stress components in the two perpendicular directions of the rosette can be determined with this approach. The FE model shown in [7] was used for the determination of the case-specific calibration functions. The elastic anisotropy was considered by means of the elasticity tensor. Additionally, the FE model accounts for the small component’s thickness. An external load was applied to the outside surfaces of the model to induce an in-plane calibration stress. Furthermore, the direction of calibration stress was aligned to the orientation of the elastic constants in drawing Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 187-192 doi: http://dx.doi.org/10.21741/9781945291173-32 190 and tangential direction of the cup. Drilling of the hole was realized by a stepwise deletion of the elements. An integral strain relaxation over the strain gage area was calculated for each drilling step. Complementary, XRD stress analyses by means of the sinψ-method were carried out. Due to the limited installation space of the stationary diffractometer the sample must be cut, which causes complex redistributions of the original residual stresses. Furthermore, non-linear 2θ-sinψ distributions were obtained. Thus, by this means no reliable determination of the residual stress state on the analyzed cup was suitable. Finite element simulation of the deep drawing process The numerical simulation of the deep drawing process was performed using the finite element software package Abaqus. First, the forming of the cylindrical cup was simulated with an explicit time integration scheme. An elasto-plastic model with isotropic hardening was used to describe the material behavior. The comparison between the punch forces measured during the forming process and predicted by the simulation was used to adjust the flow curve of the material for high strains. In order to account for the plastic anisotropy of the sheet, the Hill48 [10] yield function was applied. The anisotropy parameters were determined to give the best fit to both the Lankford parameters (rvalues) and yield stresses evaluated from tensile tests in three directions (0°, 45° and 90° to rolling direction). Due to the material and specimen symmetry, only one quarter of the blank was mod

Consideration of Tool Chamfer for Realistic Application of the Incremental Hole-Drilling Method

Abstract: The incremental hole-drilling method is well-established in residual stress analysis. A small blind hole is drilled incrementally and the strain relief is measured on the sample’s surface. In order to calculate residual stresses from the measured strain calibration data is necessary. Typically, the calibration data is determined numerically and is based on the assumption of an ideal cylindrical blind hole. However, widely used six-blade milling bits have rather large chamfers at the cutting edges, which result in hole geometries that clearly differ from the ideal cylindrical blind hole. Especially in the first drilling increments a significant impact on the calibration data by the real hole geometry is expected. In this work, a numerical model is set up that allows for consideration of tool chamfers. A systematic finite element study is carried out to analyze the effect on relieved strains. Calibration data is computed for the ideal blind hole as well as for the realistic hole geometry. Finally, numerical results are compared with experimental results gained by defined uniaxial loading experiments. The results clearly indicate a significant impact of the tool chamfer geometry for strain relief and stress data close to the surface. Hence, based on the results it is highly recommended to consider the real tool geometry to provide accurate stress evaluation by means of the incremental hole-drilling method in particular for the first drilling increments. Introduction The incremental hole-drilling method is a widely used mechanical method for residual stress depth profiles analysis due to its simple instrumentation and fast execution. Since J. Mathar [1] first proposed the test method in 1933 it has been under constant development. State of the art is incremental drilling of blind holes of small diameters in order to determine local stress gradients. The method is based on redistribution of residual stresses due to local material removal. Entailed strain relaxations can be measured on the surface area around the hole e.g. by means of strain gages. Due to the fact that strains are only partly released when introducing a blind hole, calibration data is needed to evaluate the residual stress profile from measured strains. Usually, finite element (FE) simulations are used to calculate these calibration data. In commercially available evaluation routines generally an ideal cylindrical blind hole is considered. Flaman [2] showed that the experimental setup of high speed drilling in combination with inverted cone end mills lead to the best approximation of the ideal cylindrical blind holes, while inducing a negligible amount of machining stresses. The widely used six-blade tungsten carbide milling bits with nominal diameters of 0.8 or 1.6 mm, that are used in commercially available pneumatic high speed drilling devices, have large chamfers at the cutting edges, which result in hole geometries that differ from the ideal cylindrical blind hole. Two shortcomings arise out of this geometrical deviation (see also Fig. 1): • a remaining bottom fillet for each single drilling increment • an increasing hole diameter for the first drilling increments Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 473-478 doi: http://dx.doi.org/10.21741/9781945291173-80 474 In Scafidi, et al. [3] the first impact, i.e. the bottom fillet, was investigated by considering a bottom fillet radius in numerical simulations. It was figured out, that deviations of strain relaxations can cause erroneous results in case of large bottom fillet radii. In Nau, et al. [4] different kinds of end mills and their influence on the occurring hole geometries for a nominal hole diameter of 1.6 mm were studied. A numeric model was set up considering the effect of increasing hole diameters in the first drilling steps by using the mean hole diameter over the actual removed depth increment. The conclusion of both studies was that the consideration of more realistic tool geometries for calibration results in more meaningful stress data. However, in [3] and [4] only the general effects were discussed without taken into account the real chamfered cutting edge of the most often applied end mills. Furthermore, up to now only chamfered tools with a nominal diameter of 1.6 mm were considered. In our project we have the inherent necessity to apply small hole diameters using a nominal tool diameter of 0.8 mm for residual stress depth distributions showing a relative steep gradient close to the surface. Hence, in this work a FE model is set up for a more accurate simulation of the hole geometry by considering the real chamfered edges using a conventional tapered end mill with 0.8 mm diameter. Thus, both shortcomings of the non-ideal cylindrical hole geometry are taken into account (remaining bottom fillet and unsteady hole diameter). The FE model can be used to calculate chamferconsidering calibration data for reliable stress evaluations using the differential method [5]. Finally, the numerical results are validated by experimental findings of a defined 4-point bending test. Finite element simulation Finite element (FE) model. The 3D FE model was defined in ABAQUS and consists of 750,000 elements of type C3D8R and C3D6. This hybrid element model is needed to take the chamfer geometry into account. An inverted cone tungsten carbide six-blade end mill of 0.8 mm diameter with a chamfer height of 0.06 mm at an angle of 45° was assumed for the numeric model (Komet, Gebr. Brasseler GmbH & Co. KG). Fig.1 shows the cutting edges of the end mill and the cross sections of a steel sample with two drilled blind holes of different depths. On the right hand side a side view of the FE model is presented. It can be seen that it is in good agreement with the actual blind hole geometry, even for a small drilling depth of 40 μm. Due to the symmetry of the calculated problem only a quarter model was used. A full view of the model is shown in Fig. 2. The drilling process is simulated by stepwise removing the elements in the region of the hole (drilling increments are highlighted in Fig. 2). For each drilling step the released strains on the surface area around the hole were calculated and averaged at element surface nodes. In a post processing step, the strains were integrated and averaged over three virtual strain gage areas at the positions 0°, 45° and 90° leading to three single strain values ε0°, ε45°, ε90°, comparable to those, gained in experimental studies. Fig. 1: Six-blade TiN coated tungsten carbide end mill with nominal diameter of 0.8 mm (1), drilled holes of 40 μm depth (2) and 140 μm depth (3), cross section of drilled sample (a), side view of FE model (b). 200 μm 1