In-situ analysis of the elastic-plastic characteristics of high strength dual-phase steel

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 diffraction profiles is highly complex. Within this work, CR590Y980T (DP1000) is investigated in a continuous cyclic tensile and tension-compression test under synchrotron radiation at High Energy Material Science beamline P07 in Petra III, DESY. On basis of additional EBSD measurements, an evaluation approach is shown to analyze the dual-phase diffraction profiles in such a way that martensite and ferrite can be separated for three lattice planes. The origin of the specific elastic-plastic behavior of dual-phase steels in terms of onset of yielding, anelasticity or early re-yielding is analyzed on the basis of lattice strains and interphase stresses. For this, the time-synchronously measured micro data is correlated with the macro stress-strain relationship and thermoelastic effect. The results help to better understand strain-dependent elastic-plastic behavior of DP steels on a micro level and provide great potential to improve characterization and modeling in terms of springback prediction

A review of the effects of cyclic contact loading on fretting fatigue behaviour

A damage phenomenon called fretting fatigue frequently takes place when two contact bodies are clamped together under a normal contact load along with a small-scale oscillatory motion due to cyclic loading. In contrast to the constant contact loading, less attention has been paid to variable contact loading which was technically reviewed in this study. Emphasis was placed on the efforts made over the past decade and the future challenges including nonlinear effects of contact loads, friction, frequency, slip amplitude, wear, and contact mechanic are discussed extensively. It was revealed a need for new fatigue and contact mechanics models by identifying the aforementioned missing parameters

Plasticity evolution of an aluminum-magnesium alloy under abrupt strain path changes

During the forming and manufacturing of engineering materials, plasticity behavior could be evolving significantly due to complex deformation history. Therefore, this study aims to characterize the plasticity evolution of an aluminum-magnesium alloy under simple monotonic and non-monotonic loading with abrupt strain path changes. Instead of focusing only on one single stress state in the first-step loading for most of the studies in the literature, the current non-monotonic strain path testing program investigates three stress states – uniaxial, plane-strain, and biaxial tension – in the first-step loading and combines them with a second-step uniaxial loading along and orthogonal to the initial loading direction. This combination generates non-monotonic stress–strain data in a quite large and distributed spectrum in terms of the Schmitt parameter. It is found that the aluminum-magnesium alloy shows a unique phenomenon with a lower yield strength at reloading compared to monotonic cases coupled with a steady increase of stress overshooting the monotonic one at large strains. This increase of stress as well as the strain hardening rate lasts till the uniform strain and is therefore referred to as permanent hardening. The comprehensive non-monotonic behavior delivered by the new experimental program in this study could further assist the development of material models and an in-depth understanding of the underlying mechanisms

Elastic behaviour characterisation of TRIP 700 steel by means of loading-unloading tests

The elastic behaviour of TRIP 700 steel under plastic deformation is analysed. The analysis is carried out by means of classical tensile test and loading–unloading cyclic tests. These tests have been performed using high deformation strain gages, which enable an accurate and continuous measurement of strain. An elastic modulus reduction of 20% is observed for 12% plastic deformation. Furthermore, non-linear unloading and loading paths have been found in this work. This is an important difference with respect to other authors and opens new possibilities for the development of new material models to improve the prediction of the post-forming springback of industrial parts, which is an important issue for the automotive industry

Initiation of dynamic recrystallization of as-cast N08028 alloy

The use of high nickel content austenitic stainless steels (SASS) has significantly increased in the last decade. The corrosion and high fatigue resistance of these materials make them suitable for manufacturing oil country tubular goods (OCTG). SASS are processing by forging from casting conditions. Dynamic recovery (DRV) and recrystallization (DRX) of as-cast super austenitic stainless steel, N08028 Alloy, is investigated to study the refining effect from the as-cast grain structure to fully recrystallized austenite due to hot deformation. Both the critical stress and strain for the initiation of DRX are determined using the flow curves. To perform this analysis, hot compression tests are performed at temperatures between 900°C and 1250°C, and strain rates between 0.1 s-1 and 10 s-1, up to 0,8 final strain using a Gleeble®3800 thermomechanical simulator. Subsequently, the Johnson-Avrami-Mehl-Kolmogorow (JMAK) model is used to numerically fit the flow curves and consequently determine the critical strain. No critical points are seen for temperatures under 1100°C. Above this temperature, the JMAK model proves to be valid in all studied strain rates

Post-forming, electro-plastic effect internal stress reduction in AA5754 aluminium alloy

Aluminium alloys are one of the most efficient materials for weight reduction in the car industry. However, the favourable performance of alloys conflicts with the difficulties of transforming these materials through plastic deformation processes such as stamping. Among the different issues, they are characterised by a high springback effect. Numerous authors have explored the use of the electro-plastic effect (EPE) to mitigate internal stresses and the resultant springback. However, up-scaling existing laboratory solutions to an industrial framework is a critical challenge. Therefore, in this work, the post-forming electro-plastic effect (PFEPE) is explored in AA5754 material. Stress-relaxation + PFEPE experiments were conducted using different electric pulse charge passing though the sample, and, apart from the impact on residual stresses, the potential occurrence of recrystallisation was evaluated. The results indicate that a range of pulses exists (>2000 A·ms/mm2 and <4500 A ms/mm2) in which a 10–30% reduction in stresses can be achieved without critically impacting the material's mechanical performance

Strain path's influence on the elastic behaviour of theTRIP 700 steel

This paper deals with the analysis of thestrain path's influence on the elastic behaviour of TRIP700 steel; it aims to validate the cyclic testing method to characterise inelastic behaviour of advanced high strength steels (AHSS). Different cyclic tests are done, where the strain path is changed from test to test. Large deformation strain gages are used to determine the inelastic behaviour of the specimens at macro-level. At a lower scale, stress measurements are carried out using the XRD technique during an in-situ tensile test: ferrite and austenite phases’stresses are measured before unloading and after loading again to study the strain path's influence. By means of this work it is confirmed that the elastic strain path has no influence on the unloading–loading of this TRIP steel. These results prove that conventional loading–unloading cyclic testing is a valid methodology for a detailed characterisation of the elastic modulus and reliable numerical modelling of springback

Hardening prediction of diverse materials using the Digital Image Correlation technique

In recent years, due to the introduction of higher resistance materials in the automotive sector, sheet metal-forming tool-makers have been forced to deal with more challenging process designs. Therefore, the optimisation of the manufacturing process has become a key factor in obtaining a part which fits the required tolerances, and the finite element method (FEM) is the most widely used technique to speed up that optimisation time. However, to obtain a numerical result as close as possible to those of industrial conditions, the FEM software inputs must be highly accurate. The present work is focused on the hardening extension of the currently available reduced-formability materials, as it is a key factor in the correct prediction of the stress state and hence, of the springback during a sheet metal-forming process. The objective in this work was the selection of the most appropriate hardening model to extend the flow curve beyond the necking limit for a wide variety of material families currently utilised in the industrial environment. To carry out that analysis, a digital image correlation (DIC) technique was utilised during conventional tensile tests to extend the experimental flow curves of the analysed materials. Commonly used hardening models were fitted to the experimental tensile flow curves with the aim of selecting the model that best predicts the hardening behaviour of each analysed material family. The results showed that the DIC technique was valid for the extension of the hardening curve of the analysed materials and for the final selection of the most suitable hardening model for each analysed material family

Contact pressure and sliding velocity ranges in sheet metal forming simulations

In the last few years many efforts have been carried out in order to better understand what the real contact between material and tools is. Based on the better understanding new friction models have been developed which have allowed process designers to improve numerical results in terms of component viability and geometrical accuracy. The new models define the coefficient of friction depending on different process parameters such as the contact pressure, the sliding velocity, the material strain, and the tool temperature. Many examples of the improvements achieved, both at laboratory scale and at industrial scale, can be found in the recent literature. However, in each of the examples found in the literature, different ranges of the variables affecting the coefficient of friction are covered depending on the component analysed and the material used to produce such component. The present work statistically analyses the contact pressure and sliding velocity ranges achieved during numerical simulation (FEM) of sheet metal forming processes. Nineteen different industrial components representing a high variety of shapes have been studied to cover a wide range of casuistic. The contact pressure and sliding velocity corresponding to typical areas of the tooling have been analysed though numerical simulation in each case. This study identifies the ranges of contact-pressure and sliding velocities occurring in sheet metal forming aimed to set the characterization range for future friction studies