Mean Strain Rate Research Articles

Enzymatic saccharification of cellulose: a study of mixing and agitation in an oscillatory baffled reactor and a stirred tank reactor

Cellulose saccharification has been shown to be a function of agitation. Here, the effect of agitation by oscillatory mixing in an oscillatory baffled reactor (OBR) has been assessed and contrasted with a stirred tank reactor (STR). After 168 h of saccharification at 200 Wm−3, 91% conversion of the cellulose (∼25 g/L glucose) was observed in the OBR, as against 74% conversion (∼21 g/L glucose) in the STR. At 120 Wm−3 in both systems, the conversion in the OBR was 69% (∼19 g/L glucose) within the first 24 h, and 88% conversion (24 g/L glucose) after 168 h. The STR yielded 55% (15.3 g/L glucose) and 67% (∼18.6 g/L glucose) within the same time scale respectively, differences of 14 and 21% respectively. At equivalent power density of 10 Wm−3, the two reactors exhibited the same mean strain rates of 6.65 s−1, but as the power densities increased the mean strain rates in the STR became significantly higher than that in the OBR. This observation could be partly responsible for the seemingly better saccharification observed in the OBR. Although agitation is essential for optimum saccharification, the nature of the agitation is perhaps a more important factor owing to shear stresses on enzymes.

Turbulence intensity in wall-bounded and wall-free flows

Turbulence intensity variations in the outer region of turbulent shear flows are considered, in the context of the diagnostic plot first introduced by Alfredsson et al. (Phys. Fluids, vol. 23, 2011, 041702) and for both (smooth and rough) wall-bounded flows and classical free shear flows. With $U$ defined as the mean velocity within the flow, $U_{e}$ as a suitable reference velocity and $u^{\prime }$ as the root mean square of the fluctuating velocity, it is demonstrated that, for wall flows, the attached eddy hypothesis yields a closely linear diagnostic plot ($u^{\prime }/U$ versus $U/U_{e}$) over a certain Reynolds number range, explaining why the relation seems to work well for both boundary layers and channels despite its lack of any physical basis (Castro et al., J. Fluid Mech., vol. 727, 2013, pp. 119–131). It is shown that mixing layers, jets and wakes also exhibit linear variations of $u^{\prime }/U$ versus $U/U_{e}$ over much of the flows (starting roughly from where the turbulence production is a maximum), with slopes of these variations determined by the total mean strain rate, characterised by Townsend’s flow constant $R_{s}$. The diagnostic plot thus has a wider range of applicability than might have been anticipated.

Rotation invariant constitutive relation for Reynolds stress structure parameter

A new Reynolds stress constitutive formula is constructed using the first-order statistics of turbulent fluctuations instead of the mean strain rate. It includes zero empirical coefficients. The formula is validated with the direet numerieal simulation (DNS) data of turbulent channel flow at Reτ=180. The Reynolds stresses given by the proposed formula agree very well with the DNS results. The good agreement persists even after the multi-angle rotation of the coordinate system, indicating the rotation invariance of the formula. The autocorrelation of the fluctuating velocity rather than the mean strain rate is close to the essence of the Reynolds stress.

Water-Channel Study of Flow and Turbulence Past a Two-Dimensional Array of Obstacles

A neutral boundary layer was generated in the laboratory to analyze the mean velocity field and the turbulence field within and above an array of two-dimensional obstacles simulating an urban canopy. Different geometrical configurations were considered in order to investigate the main characteristics of the flow as a function of the aspect ratio (AR) of the canopy. To this end, a summary of the two-dimensional fields of the fundamental turbulence parameters is given for AR ranging from 1 to 2. The results show that the flow field depends strongly on AR only within the canyon, while the outer flow seems to be less sensitive to this parameter. This is not true for the vertical momentum flux, which is one of the parameters most affected by AR, both within and outside the canyon. The experiments also indicate that, when (i.e. the skimming flow regime), the roughness sub-layer extends up to a height equal to 1.25 times the height of the obstacles (H), surmounted by an inertial sub-layer that extends up to 2.7 H. In contrast, for (i.e. the wake-interference regime) the inertial sub-layer is not present. This has significant implications when using similarity laws for deriving wind and turbulence profiles in canopy flows. Furthermore, two estimations of the viscous dissipation rate of turbulent kinetic energy of the flow are given. The first one is based on the fluctuating strain rate tensor, while the second is related to the mean strain rate tensor. It is shown that the two expressions give similar results, but the former is more complicated, suggesting that the latter might be used in numerical models with a certain degree of reliability. Finally, the data presented can also be used as a dataset for the validation of numerical models.

Research of Formation Mechanics on Nanostructured Chips by Multi-Deformations Based on Finite Element Method

Formation of chip is a typical severe plastic deformation progress in machining which is only single deformation stage. The rake angle of tool is governing parameter to create large strain imposed in the chip. Effect of rake angle and deformation times on effective strain, mean strain, strain variety and strain rate imposed in the chip are researched respectively. The result of simulation have shown that the chip with large strain and better uniform of strain along the longitudinal section of chip can be produced with negative rake angle at some lower cutting velocity by multi-deformations in large strain machining.

On the application of a new formulation of nonlinear eddy viscosity based on anisotropy to numerical ocean models

The present paper explores the derivation of an alternative nonlinear eddy viscosity formulation based on Reynolds stress anisotropy and its implementation to numerical ocean models. This formulation takes into account the vorticity in addition to the mean strain rate. The proposed formulation does not include the stability function method which is a common approach in eddy viscosity calculations used in the present state-of-the-art numerical ocean models. Instead, it depends on the second invariant of anisotropy. Initially, the performance of the formulation is checked through a simple channel flow simulation. Consequently for model calibration, an idealised experiment of mixed-layer entrainment into stably stratified flow has been simulated and compared to empirical data. For sensitivity studies related to shear and stable stratification, concept of steady-state Richardson number is applied for homogeneous shear layer. Finally, the performance of the new formulation is tested by implementing it into one-dimensional General Ocean Turbulence Model. Furthermore, a realistic oceanic test case of a storm has been investigated considering different physical processes for the Fladenground Experiment (FLEX’ 76) in the northern North Sea and the results have been compared to the measured data. The main results signify that the overall performance of the nonlinear eddy viscosity model with a different value of steady-state Richardson number is as good as the Mellor–Yamada model in terms of predictability, and the eddy viscosity and diffusivity profiles follow the principle of law of the wall. Additionally, the present formulation does not require computing the stability functions and the ease of implementation into numerical ocean models gives the present formulation an upper hand over the existing formulations in the field of turbulence modelling in oceanography.

New three-dimensional strain-rate potentials for isotropic porous metals: Role of the plastic flow of the matrix

At present, modeling of the plastic response of porous solids is done using stress-based plastic potentials. To gain understanding of the combined effects of all invariants for general three-dimensional loadings, a strain-rate based approach appears more appropriate. In this paper, for the first time strain rate-based potentials for porous solids with Tresca and von Mises, matrices are obtained. The dilatational response is investigated for general 3-D conditions for both compressive and tensile states using rigorous upscaling methods. It is demonstrated that the presence of voids induces dependence on all invariants, the noteworthy result being the key role played by the plastic flow of the matrix on the dilatational response. If the matrix obeys the von Mises criterion, the shape of the cross-sections of the porous solid with the octahedral plane deviates slightly from a circle, and changes very little as the absolute value of the mean strain rate increases. However, if the matrix behavior is described by Tresca’s criterion, the shape of the cross-sections evolves from a regular hexagon to a smooth triangle with rounded corners. Furthermore, it is revealed that the couplings between invariants are very specific and depend strongly on the particularities of the plastic flow of the matrix.

Evaluation of left ventricle functions by tissue Doppler, strain, and strain rate echocardiography in patients with primary hyperparathyroidism

Cardiovascular morbidity and mortality are increased in patients with primary hyperparathyroidism (PHPT). We aimed to evaluate left ventricle systolic and diastolic functions with tissue Doppler imaging (TDI) and strain and strain rate echocardiography in patients with PHPT. Thirty-one patients with PHPT and 29 healthy controls were evaluated with conventional and pulse Doppler echocardiography, TDI and strain and strain rate echocardiography. Myocardial performance index (MPI) was calculated. Strain and peak systolic strain rate in mid and basal segments of lateral, anterior, inferior, and septal walls of left ventricle were determined. TDI showed similar late diastolic myocardial peak velocity in two groups. Peak systolic mitral annular velocity, early diastolic myocardial peak velocity, and ratio of early to late diastolic myocardial peak velocity were lower in PHPT patients (p = 0.01, p < 0.001 and p < 0.001, respectively). MPI calculated by TDI was 0.53 ± 0.15 in PHPT group and 0.44 ± 0.09 in control group (p = 0.013). Strain values were lower in mid and basal segments of septum, lateral and anterior walls, and basal segment of inferior wall in PHPT patients. Mean systolic strain was -20.88 ± 2.30 and -24.25 ± 2.13 in PHPT patients and control group, respectively (p < 0.001). Mean strain rate was lower in PHPT patients compared to control group (-1.38 ± 0.19 vs -1.57 ± 0.25) (p = 0.002). Patients with PHPT, but no cardiac symptoms or documented cardiovascular disease, have subclinical systolic and diastolic myocardial dysfunction. Evaluation of these patients with TDI and S and Sr echocardiography in addition to conventional echocardiography might be valuable to detect subclinical cardiac involvement.

On Modeling Plasticity-damage Couplings in Polycrystalline Materials

At present, modeling of the plastic response of porous solids is done using stress-based plastic potentials. To gain understanding of the combined effects of all invariants for general three-dimensional loadings, a strain-rate based approach appears more appropriate. In this paper, for the first time strain rate-based potentials for porous solids with Tresca and von Mises matrices are obtained. The dilatational response is investigated for general 3-D conditions for both compressive and tensile states using rigorous upscaling methods. It is demonstrated that the presence of voids induces dependence on all invariants, the noteworthy result being the key role played by the plastic flow of the matrix on the dilatational response. If the matrix obeys the von Mises criterion, the shape of the cross-sections of the porous solid with the octahedral plane deviates slightly from a circle, and changes very little as the absolute value of the mean strain rate increases. However, if the matrix behavior is described by Tresca's criterion, the shape of the cross-sections evolves from a regular hexagon to a smooth triangle with rounded corners. Furthermore, it is revealed that the couplings between invariants are very specific and depend strongly on the particularities of the plastic flow of the matrix.

Evaluation of the function and synchronization of left atrial in chronic heart failure patients by two dimensional-speckle tracking imaging and real-time three-dimensional echocardiography

Objective To evaluate the function and synchronization of left atrial in patients with chronic heart failure(CHF) by two dimensional-speckle tracking imaging (2D-STI) and real-time threedimensional echocardiography(RT-3DE).Methods Thirty-two healthy controls and twenty-nine patients with CHF were studied.The mean strain and strain rate during systole,early and late of diastole(mSs,mSe and mSa,mSRs,mSRe and mSRa) were measured at each left atrial wall by 2D-STI.From the volume-time curve of RT-3DE workstation,the maximum volume (LAVmax),the minimum volume(LAVmin),the volume before contraction(LAVp) of left atrial were acquired.Accordingly,the total,passive and active ejection fraction could be calculated.The synchronization parameters derived from 2D-STI and RT-3DE were the dispersion of time to peak strain and time to the minimum volume (T-SD and Tmsv-16-SD).The index of volume and time were corrected by the body surface area and interval of R-R,respectively.The correlation were analysed betwee them.Results () Comparing between two groups,the mSs,mSe and mSa were shortened and the mSRs,mSRe and mSRa were slower in CHF group (P ＜ 0.05).② All the index of volume were gradually increasing from control to CHF group,but the LATEF,LAPEF and LAAEF were just the reverse,with all parameters had significant difference between two groups (P ＜0.05).③ The synchronization parameters were significantly higher from control to CHF group by the two kinds of skills (P ＜0.05).Comparing with 2D-STI,the indexes of synchronization were bigger and the asynchronization rate of CHF group were higher by RT-3DE.④The indexes of synchronization had a negative correlation with the strain rate and the ejection fraction,but had a positive correlation with the index of volume.Conclusions The function of left atrial are reduced in patients of CHF,there are asynchronization.2D-STI and RT 3DE may paly important roles in the evaluation of function and synchronization of left atrial.The relevance ratio of the left atrial asynchronization are higher from the RT-3DE than 2D-STI. Key words: Echocardiography, real-time three-dimensional; Heart failure, congestive; Atrial function,left ; Speckle tracking imaging

Right ventricular involvement in anterior myocardial infarction: a tissue Doppler-derived strain and strain rate study

OBJECTIVE:Strain and strain rate imaging is currently the most popular echocardiographic technique that reveals subclinical myocardial damage. There are currently no available data on this imaging method with regard to assessing right ventricular involvement in anterior myocardial infarction. Therefore, we aimed to evaluate right ventricular regional functions using a derived strain and strain rate imaging tissue Doppler method in patients who were successfully treated for their first anterior myocardial infarction.METHODS:The patient group was composed of 44 patients who had experienced their first anterior myocardial infarction and had undergone successful percutaneous coronary intervention. Twenty patients were selected for the control group. The right ventricular myocardial samplings were performed in three regions: the basal, mid, and apical segments of the lateral wall. The individual myocardial velocity, strain, and strain rate values of each basal, mid, and apical segment were obtained.RESULTS:The right ventricular myocardial velocities of the patient group were significantly decreased with respect to all three velocities in the control group. The strain and strain rate values of the right mid and apical ventricular segments in the patient group were significantly lower than those of the control group (excluding the right ventricular basal strain and strain rate). In addition, changes in the right ventricular mean strain and strain rate values were significant.CONCLUSION:Right ventricular involvement following anterior myocardial infarction can be assessed using tissue Doppler based strain and strain rate

Longitudinal strain and strain rate by two-dimensional speckle tracking in non-sedated healthy cats

Two dimensional speckle tracking (2D-ST) is a relatively new tool that has been used in veterinary medicine to quantify myocardial function. However, there is little information about the use of this tool in healthy cats. Thus, the aim of this study was to evaluate left ventricular longitudinal strain and strain rate in non-sedated healthy cats, as well as longitudinal velocity and displacement, by means of 2D-ST echocardiography. Thirty non-sedated healthy cats were examined and 2D images were obtained for 2D-ST analyzes.The global mean strain and strain rate was −15.65±5.46% and −1.80±0.59 s−1, respectively. Velocity and displacement were 1.41±0.87cm/s and 1.27±0.80mm, respectively. Thus, 2D-ST demonstrated to be feasible for measuring left ventricular longitudinal strain, strain rate, velocity and displacement in cats and preliminary reference values for non-sedated healthy cats can be established from the results of this study.

Doppler-derived strain and strain rate imaging assessment of right ventricular systolic function in adults late after tetralogy of Fallot repair: an observational study

Tetralogy of Fallot (TOF) is the most common form of cyanotic congenital heart disease. Today, we are faced with an increasing number of patients with residual pulmonary regurgitation (PR) late after TOF repair. The right ventricular (RV) volumes and function are among the most important factors influencing clinical decision-making. Cardiac magnetic resonance (CMR) is the gold standard method for the quantitative assessment of the RV function; it is, however, expensive for routine clinical follow-up and sometimes is contraindicated. We sought to evaluate the RV systolic function via CMR and compare it with Doppler-derived strain(S) and strain rate (SR) imaging in patients with repaired TOF. In an observational cross-sectional study, 70 patients (22 women, mean age=22±4.9 years) late after TOF repair with severe PR were evaluated. Peak systolic strain and SR in the basal, mid, and apical segments of RV free wall (RVFW) were measured and compared with the RV function measured in the short-axis cine MR. Associations between RVEF and S/SR, investigated by ordinal logistic regression models. Significant association was observed between RV function and mean S of all the three segments of the RVFW segments [OR (CI95%): 1.17 (1.05-1.31)]. Association between RV function and mean SR of all the three segments of the RVFW segments was borderline significant [OR (CI95%): 1.7 (0.97-2.93)]. There was a significant correlation between the Doppler-derived mean strain of RVFW and the RV function measured by CMR in adults late after TOF repair. These quantitative methods improved the assessment of the RV function and served as an additional method to follow up patients with contraindications to CMR.

A geometric average interpretation on the nonlinear oscillatory shear

Nonlinear oscillation shear has become an important method to study the complex fluids. However, choosing the suitable material functions is not as simple as that in linear viscoelasticity. A framework is suggested in this work to account for the stress–strain and stress–strain rate relationship based on the concept of mean stress and mean strain (rate) in the Lissajous curves. The applications of such framework in an imposed oscillatory shear strain and an imposed oscillatory shear stress are clearly demonstrated. The intracycle nonlinear modulus and viscosity are defined from the slope of the stress–strain curve and the stress–strain rate curve, respectively. The intercycle nonlinear behaviors are obtained from the strain (rate) amplitude dependence of the zero mean strain (rate) modulus or viscosity. We justify the strain-hardening/softening and shear-thickening/thinning behaviors of nonlinear moduli and viscosities by using typical constitutive models like Bingham model and the Maxwell model. It is found that the modulus (viscosity) defined from the stress–mean strain (rate) curves is the most physical reasonable quantity. In addition, we apply the new analysis method to two yield stress fluids, which reveals critical balance between aging and shear rejuvenation.

Decomposition of the turbulent kinetic energy field into regions of compressive and extensive strain

Based on direct numerical simulations of homogeneous shear turbulence, homogeneous isotropic decaying turbulence and a turbulent channel flow, the scaling of the two-point velocity difference along gradient trajectories 〈Δun|s〉 as well as between the extreme points of the instantaneous turbulent kinetic energy field k is studied. In the first step, we examine the linear 〈Δun|s〉∝s·a∞ scaling, where s denotes the separation arclength along a gradient trajectory and a∞ is the asymptotic value of the conditional mean strain rate of large dissipation elements. Then, we investigate the scaling of the velocity difference between scalar extreme points 〈Δun|l〉 as well as the probability density function of the Euclidean distance P(l) between them, conditioned on compressive and extensive strain regions. We observe that while the overall velocity difference along gradient trajectories and between diffusively connected scalar extreme points exhibits linear scaling behaviour 〈Δun|l〉∝l, the conditional velocity differences of extensive 〈Δun|l+〉 and compressive regions 〈Δun|l−〉 are in contrast to the K41 theory proportional to l2/3. The scaling exponent of the overall comes to one part from the purely extensive (compressive) 〈Δun|l+〉 (〈Δun|l−〉), while the second contribution is due to the difference in weighting the different regions, thus involving the conditioned pdfs P(l+) and P(l−). We find that the latter relation scales with l1/3. The decomposition of 〈Δun|l〉 into two contributions scaling with l2/3 and l1/3, respectively, hence yields an alternative explanation for the observed linear regime.

Joint statistics and conditional mean strain rates of streamline segments

Based on four different direct numerical simulations of turbulent flows with Taylor-based Reynolds numbers ranging from Reλ = 50 to 300 among which are two homogeneous isotropic decaying, one forced and one homogeneous shear flow, streamlines are identified and the obtained space curves are parameterized with the pseudo-time as well as the arclength. Based on local extrema of the absolute value of the velocity along the streamlines, the latter are partitioned into segments following Wang (2010 J. Fluid Mech.648 183–203). Streamline segments are then statistically analyzed based on both parameterizations using the joint probability density function of the pseudo-time lag τ (arclength l, respectively) between and the velocity difference Δu at the extrema: P(τ,Δu), (P(l,Δu)). We distinguish positive and negative streamline segments depending on the sign of the velocity difference Δu. Differences as well as similarities in the statistical description for both parameterizations are discussed. In particular, it turns out that the normalized probability distribution functions (pdfs) (of both parameterizations) of the length of positive, negative and all segments assume a universal shape for all Reynolds numbers and flow types and are well described by a model derived in Schaefer P et al (2012 Phys. Fluids24 045104). Particular attention is given to the conditional mean velocity difference at the ending points of the segments, which can be understood as a first-order structure function in the context of streamline segment analysis. It determines to a large extent the stretching (compression) of positive (negative) streamline segments and corresponds to the convective velocity in phase space in the transport model equation for the pdf. While based on the random sweeping hypothesis a scaling is found for the parameterization based on the pseudo-time, the parameterization with the arclength l yields a much larger than expected l1/3 scaling. A theoretical indication for this finding is given and a scaling of 〈|Δu∥l〉∝l2/3 is found from the DNS.

Influence of Calculation Method on Value of Activation Energy in Hot Forming

AbstractInfluence of the methodology of mathematical processing of experimental data on the calculated value of the activation energy in hot forming was evaluated for three very different alloys: iron aluminide, low-alloyed steel and magnesium alloy. Application of the mean strain rates is appropriate but calculations with use of nominal strain rates of the plastometric tests do not give significantly different results. Calculations based on the processing of the simplified power dependence between strain rate and peak stress can give the results differing very substantially from those that were gained by the solution of the complex hyperbolic relationship. This has negative impacts, particularly at prediction of the maximum flow stress in the broad range of forming conditions, represented by the Zener-Hollomon parameter.

An explicit algebraic Reynolds-stress and scalar-flux model for stably stratified flows

AbstractThis work describes the derivation of an algebraic model for the Reynolds stresses and turbulent heat flux in stably stratified turbulent flows, which are mutually coupled for this type of flow. For general two-dimensional mean flows, we present a correct way of expressing the Reynolds-stress anisotropy and the (normalized) turbulent heat flux as tensorial combinations of the mean strain rate, the mean rotation rate, the mean temperature gradient and gravity. A system of linear equations is derived for the coefficients in these expansions, which can easily be solved with computer algebra software for a specific choice of the model constants. The general model is simplified in the case of parallel mean shear flows where the temperature gradient is aligned with gravity. For this case, fully explicit and coupled expressions for the Reynolds-stress tensor and heat-flux vector are given. A self-consistent derivation of this model would, however, require finding a root of a polynomial equation of sixth-order, for which no simple analytical expression exists. Therefore, the nonlinear part of the algebraic equations is modelled through an approximation that is close to the consistent formulation. By using the framework of a$K\text{{\ndash}} \omega $model (where$K$is turbulent kinetic energy and$\omega $an inverse time scale) and, where needed, near-wall corrections, the model is applied to homogeneous shear flow and turbulent channel flow, both with stable stratification. For the case of homogeneous shear flow, the model predicts a critical Richardson number of 0.25 above which the turbulent kinetic energy decays to zero. The channel-flow results agree well with DNS data. Furthermore, the model is shown to be robust and approximately self-consistent. It also fulfils the requirements of realizability.

난류 예혼합 화염 선단부의 통계적 특성에 관한 수치적 연구

Leading edge statistics are obtained by direct numerical simulation(DNS) of freely propagating incompressible and stagnating compressible turbulent premixed flames. Conditional averages of velocities in terms of reaction progress variable, c, and local flame surface density, <TEX>${\sum}^{\prime}_f$</TEX>, are defined and compared through the flame brush. It holds asymptotically that <TEX>$</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_f=</TEX><TEX><</TEX><TEX>S_d</TEX><TEX>></TEX><TEX>_f$</TEX> and <TEX>$</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_u-</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_b=D_t/L_w$</TEX> with the characteristic length scale of <TEX>$\bar{c}$</TEX> variation, <TEX>$L_w$</TEX>. It also holds that <TEX>$</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_b=</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_f$</TEX> for a freely propagating flame under no mean strain rate. The turbulent burning velocity, <TEX>$S_T$</TEX>, is determined by the conditional statistics at the leading edge under large activation energy.

Study of hot deformation behaviour of 2205 duplex stainless steel through hot tension tests

The hot working behaviour of duplex stainless steels has been studied in the literature mainly through hot torsion or hot compression tests. The aim of this paper is to investigate the hot deformation behaviour of a duplex stainless steel type 2205 (F51) through hot tension tests, which are easier to carry out and can offer some additional information about the maximum sustainable strain in tension before starting of irreversible damage. In fact, even under the uniaxial compressive action of a forging press, tensile stresses may develop in some zones of the forged product. These zones become most critical and the knowledge of the limiting conditions are important. Three different test temperatures were selected, namely 900, 1000, and 1100 °C. The hot working behaviour was characterised by fitting the mean flow stress, strain rate, and temperature relationship with the hyperbolic sine function defined by Sellars and Tegart. An activation energy Q equal to 430 kJ/mol was obtained for plastic straining at high temperature. It corresponds to values, obtained with hot torsion or hot compression tests, reported in the literature where a large variation in activation energy is also found for duplex stainless steels. It was explained by considering the intrinsic two-phase nature of the investigated steel. The ductility in the different hot working conditions was characterised by the true strain at the onset of damage formation under prevailing tensile stress conditions. The role of the microstructure as well as of damage formation during deformation on the shape of the flow stress curves was analyzed.