The hardening in alloys and composites and its examination with a diffraction and self-consistent model

The paper presents the results of diffraction stress measurement in Al/SiC composite and in 2124T6 aluminum alloy during the in situ tensile test. The main aim of the work is to observe the stress values for different stages of tensile test for the composite after applying two types of thermal treatment and for the alloy used as a matrix in this composite, to identify the type of hardening process. The experimental results were compared against the calculations results obtained from the self-consistent model developed by Baczmański [1] - [3] to gain the information about the micromechanical properties (critical resolved shear stress τ$_{cr}$ and hardening parameter H) of the examined materials. This comparison allowed researchers to determine the role of reinforcement in the composite as well as the impact of the heat treatment on the hardening of the material

Stress distribution correlated with damage in duplex stainless steel studied by synchrotron diffraction during plastic necking

The goal of this work was the determination of lattice strains distribution in two phases of duplex steel during plastic necking. Subsequently, the stress heterogeneity in the neck was studied in order to determine the reason for the damage initiation and to verify the hypothesis that the damage begins in the ferritic phase. To do this, X-ray synchrotron radiation was used to scan the ‘in situ’ variation of the interplanar spacings along the necking zone for samples subjected to tensile loading. A self-consistent model and FEM simulation were applied for the experimental data interpretation. It was found that for advanced necking the phase lattice strains, especially those measured at some distance from the neck centre, show a large inversion of the loads localised in both phases compared to the undamaged state (the lattice strains in the ferrite become smaller than in the austenite). This effect indicates stress relaxation in the ferrite which is connected with the damage phenomenon. Correlation of the experimental results with the modelling shows that the value of von Mises stress is responsible for the initiation of the ferritic phase softening

Elastoplastic deformation and damage process in duplex stainless steels studied using synchrotron and neutron diffractions in comparison with a self-consistent model

In situ time of flight neutron diffraction and X-ray synchrotron diffraction methods were applied to measure lattice strains in duplex steels during a tensile test. The experimental results were used to study slips on crystallographic planes and the mechanical effects of damage occurring during plastic deformation. For this purpose the prediction of an elastoplastic self-consistent model was compared with the experimental data. The used methodology allowed to determine the elastic limits and parameters describing work hardening in both phases of studied polycrystalline materials. In the second part of this work the developed elastoplastic model was applied to study damage occurring in the ferritic phase. The theoretical results showed a significant reduction of stresses localized in the damaged phase (ferrite) and confirmed the evolution of the lattice strains measured in the ferritic and austenitic phases

Methods for Different Orders Stresses Estimation with Diffraction Methods

The publication describes how diffraction methods and mathematical bases can be used for measurement of various types of stresses in single-phase and multiphase materials. Firstly, the paper defines the stresses and classifies them from the scale of their interactions point of view. Subsequently, the phenomenon of radiation diffraction on the crystalline lattice is presented including formulas describing this phenomenon and the dependencies enabling stress measurements. The key part of the paper is the description of one of the second order stress estimation methods based on diffraction data and a selfconsistent model

Measurements Parameters Optimisation for X-Ray Diffractometry Measurements of Stress State Around the Rivets

X-ray diffractometry is one of the basic methods of stress measurement. This method was used to measure stress distributions around rivets as described further in this paper. There were two types of riveted samples, six types of samples made of rivet wire (after different types of treatment) and a aluminium sheet sample with three measurement areas: plate with both cladding and anodized layer, plate after removing the anodized layer and plate after removing both cladding and anodized layer. Riveted samples were prepared to measure the stress distribution around the rivets and the samples of wire and the plate with three areas were prepared to check the effect of different types of treatment on stress state

Residual stresses measurements with X-ray diffractometry on aluminum specimens - determination of the most suitable parameters of measurement

The work was done as apart of the IMPERJA Eureka Project. The goal of the IMPERJA project is to increase the fatigue life of riveted joints, which will lead to an increase of the aircraft service life, a smaller number of inspections and lower operation costs of an aircraft. The consortium intends to meet this goal by investigating and improving the riveting process as well as improving the prediction methods for fatigue life. Riveting is the most commonly used method of joining sheet metal components of the aircraft structure. Typically, the number of rivets ranges from several thousands to some millions in a single aircraft depending on the specific aircraft type and size. The riveted joints are critical areas of the aircraft structure due to severe stress concentrations and effects such as fretting and secondary bending. Therefore the fatigue crack initiation will start at the rivets holes. Fatigue crack initiation usually occurs at a number of rivet holes (multiple site damage), which may lead to widespread fatigue damage and reduced residual strength. Although the literature on the fatigue behaviour of riveted joints is quite abundant, many aspects are still not sufficiently understood and investigated and, therefore, they require a further study. The work contains the results of stress measurements obtained with X-ray diffractometer. The aim of the work was to determine the stress values after different kinds of treatment, to check what are the limits of the x-ray measurement for aluminum alloys and to obtain the most suitable measurement parameters for this kind of alloy. There were 5 kinds of specimens: -specimen no. l - technically pure aluminum, specimen annealed in temperature 300° C for l hour, -specimen no. 2 - technically pure aluminum, raw state without any additional treatment, -    -specimen no. 3 - technically pure aluminum, squeezed perpendicularly to the axis direction, force: l00 kN, longitudinal intersection, specimen no. 4 - technically pure aluminum, squeezed perpendicularly to the axis direction, force: 100 kN, transverse intersection, specimen no. 5 - PA24 alloy, 05 bar, squeezed along the axis of the rod, force: l3,9 kN, longitudinal intersection. The second part of the work contains the measurements of the stress distribution around the rivets. The specimen prepared to realize this kind of measurements had four areas. The rivets on every area were riveted with the different riveting force: 1.2 kN; 1.4 kN; 1.5 kN and 1.55 kN

X-ray stress measurements in the institute of aviation possibilities and examples

From the point of view of the airplane construction, its fatigue lifetime and exploitation process, the stress states and levels are of crucial importance. The most appropriate experimental methods to determine stress values are diffraction methods with different radiation type employed. These methods allow the determination of the elastic lattice deformation and distortion (effectively the stress state) from the displacement and broadening of the diffraction peak. Diffraction methods are widely known as the experimental methods for determining not only the stress values but also the elastic properties of polycrystalline materials (also of all alloys types used in the aerospace industry). The advantages of diffraction experiments result from their non-destructive character and the possibility to obtain absolute values of stresses in different phases of each type of crystal material (the measurements are performed selectively only for crystallites contributing to the measured diffraction peak, i.e. for the grains having lattice orientations for which the Bragg condition is fulfilled). In the frame of this work, the laboratory possibilities of the Institute of Aviation in this area are presented as well as the exemplary results of stress measurements performed there

Micromechanical Properties and Stress Measurements with Diffraction Methods

Diffraction methods are commonly used for the determination of the elastic lattice deformation and distortion from the displacement and broadening of the diffraction peak. These methods enable researchers to measure stresses and elastic properties of polycrystalline materials. The main advantages of diffraction methods are their non-destructive character and the possibility of macrostress and microstress analysis for multiphase and anisotropic materials. Measurements are performed selectively only for crystallites contributing to the measured diffraction peak, i.e. for the grains having lattice orientations for which the Bragg condition is satisfied. When several phases are present in the sample, measurements of separate diffraction peaks allow for the behaviour of each phase to be investigated independently. This method can be applied without any limitations to flat specimens. Numerical calculations of residual stresses around the rivets imply a very high stress gradientin the case of tangential stresses as well in the case of radial stresses. Attempting to verify these predictions, the residual stress measurements with an X-ray diffractometer were performed on riveted samples after the riveting process. In addition, complementary measurements of strain values with strain gauges during the riveting process were performed as well as the finite elements modelling. The aim of these measurements was to determine the stress values around the rivets and to compare results obtained with different techniques. On the other hand, the multi-scale crystallographic model of elastoplastic deformation is very convenient for the study of elastoplastic properties in microscopic and macroscopic scales. Comparison of experimental data with model predictions allows us to understand the physical phenomena that occur during a sample's deformation at the level of polycrystalline grains. Moreover, the micro and macro parameters of elastoplastic deformation can be experimentally established. It should be stated that the characterisation of the residual stress field and elastic properties is important in the study of the mechanical behaviour of polycrystalline materials, including plasticity and damage phenomena. In this work, a new analysis method of neutron diffraction results obtained during in-situ tensile load is proposed and tested. The methodology is based on the measurements of lattice strains during in-situ tensile testing for several hkl reflections and for different orientations of the sample with respect to the scattering vector. As the result, the full stress tensor for preferred texture orientations in function of the applied stress can be determined using the crystallite group method. The experimental data are presented and compared with the self-consistent model calculations performed for groups of grains selected by different hkl reflections

Micromechanical Properties and Stress Measurements with Diffraction Methods

Diffraction methods are commonly used for the determination of the elastic lattice deformation and distortion from the displacement and broadening of the diffraction peak. These methods enable researchers to measure stresses and elastic properties of polycrystalline materials. The main advantages of diffraction methods are their non-destructive character and the possibility of macrostress and microstress analysis for multiphase and anisotropic materials. Measurements are performed selectively only for crystallites contributing to the measured diffraction peak, i.e. for the grains having lattice orientations for which the Bragg condition is satisfied. When several phases are present in the sample, measurements of separate diffraction peaks allow for the behaviour of each phase to be investigated independently. This method can be applied without any limitations to flat specimens. Numerical calculations of residual stresses around the rivets imply a very high stress gradientin the case of tangential stresses as well in the case of radial stresses. Attempting to verify these predictions, the residual stress measurements with an X-ray diffractometer were performed on riveted samples after the riveting process. In addition, complementary measurements of strain values with strain gauges during the riveting process were performed as well as the finite elements modelling. The aim of these measurements was to determine the stress values around the rivets and to compare results obtained with different techniques. On the other hand, the multi-scale crystallographic model of elastoplastic deformation is very convenient for the study of elastoplastic properties in microscopic and macroscopic scales. Comparison of experimental data with model predictions allows us to understand the physical phenomena that occur during a sample's deformation at the level of polycrystalline grains. Moreover, the micro and macro parameters of elastoplastic deformation can be experimentally established. It should be stated that the characterisation of the residual stress field and elastic properties is important in the study of the mechanical behaviour of polycrystalline materials, including plasticity and damage phenomena. In this work, a new analysis method of neutron diffraction results obtained during in-situ tensile load is proposed and tested. The methodology is based on the measurements of lattice strains during in-situ tensile testing for several hkl reflections and for different orientations of the sample with respect to the scattering vector. As the result, the full stress tensor for preferred texture orientations in function of the applied stress can be determined using the crystallite group method. The experimental data are presented and compared with the self-consistent model calculations performed for groups of grains selected by different hkl reflections

X-Ray Diffraction Measurements for Riveted Joints. The Application of a Novel Methodology

The X-ray diffraction method is the best, widely available, non-destructive measurement method used to determine the residual and load stresses in crystalline materials. This method can be applied without any limitations to flat specimens. Depending on the equipment geometry, the type of material and geometry of the specimen, there are many limitations, restrictions and recommendations which have to be fulfilled to obtain reliable results. This was the reason for working out a methodology for X-ray diffraction stress measurements for riveted specimens.The first case to analyze is the necessity of choosing an X-ray tube suitable for the specimen material which will give the diffraction peaks in the range of 2Θ angles between 120° and 180°. Afterwards it is crucial to make the best selection of Bragg's angle 2Θ. In the vast majority of cases the best selection is the possibly biggest 2Θ angle because of the best accuracy of the measurement. However, for example for aluminum alloys (for CrKα radiation), this choice is not so obvious. It is much more convenient to perform measurements not for the highest diffraction angle. The best selection in this case is 2Θ=139,3°, and not 156,7°. Other selections which are necessary to be made before measurements are the collimator diameter, time of exposure, ψ tilts and φ oscillations. The proper selection of these parameters is crucial for the fast and efficient performing of measurements and for obtaining reliable results. Before performing the measurement, especially in the case of the specimen with complicated geometry (for example in the case of riveted specimens made of aluminum alloys), it is necessary to analyze the results obtained paying special attention to the possibility of the appearing of the rivet head/driven rivet head shadow during the measurement. The work describes differences between the X-ray stress measurement results obtained without any interference and the results received after eliminating the selected diffraction peaks for which the shadow of rivet head/driven rivet head has appeared