Thilo F. Morgeneyer

Synchrotron and neutron laminography for three-dimensional imaging of devices and flat material specimens

Abstract Computed laminography has been introduced to synchrotron and neutron imaging set-ups to complement computed tomography for three-dimensional imaging of laterally extended (i. e. plate-like) specimens. The wide application field of computed laminography due to different contrast modes (X-ray or neutron absorption and X-ray phase contrast) and spatial resolutions ranging from some 100 down to approximately 0.5 μm is demonstrated. Selected examples from device inspection and from materials science are reported. They outline the interest of the method for non-destructive and in-situ measurements of regions of interest in large planar specimens where engineering-relevant boundary conditions have to be met. With a materials science background, the in-situ investigation of crack propagation in aluminium sheets and carbon-fibre composite panels under mechanical loading is reported.

Effect of joint line remnant on fatigue lifetime of friction stir welded Al–Cu–Li alloy

Abstract The effect of joint line remnant (JLR) on the fatigue lifetime of friction stir welds of a 2198 Al alloy in T851 condition has been assessed experimentally. The base material, sound welds (welded in one sheet) and welds with JLR (produced via welding of two sheets with a natural oxide layer) have been investigated. A strong decrease in microhardness is found for the weak weld zone that is consistent with the reduction in tensile properties compared to the base material: 45% in yield strength and 22% in ultimate tensile strength. The fatigue strengths of sound and JLR bearing welds at 100 000 cycles (R=0·1) are reduced by 10 and 15% respectively compared to the base material. Only for one-third of the JLR bearing specimens, the fatigue crack initiated in regions close to the JLR and this without reduction in fatigue lifetime.

Three-dimensional characterization of fatigue-relevant intermetallic particles in high-strength aluminium alloys using synchrotron X-ray nanotomography

Second-phase particles and small porosities are known to favour fatigue crack initiation in high-strength aluminium alloys 2050-T8 and 7050-T7451. Using high-resolution X-ray tomography (320 nm voxel size), with Paganin reconstruction algorithms, the probability that large clusters of particles contain porosities could be measured for the first time in 3D, as well as precise 3D size distributions. Additional holotomography imaging provided improved spatial resolution (50 nm voxel size), allowing to estimate the probability of finding cracked particles in the as-received material state. The extremely precise 3D shape (including cracks) as well as local chemistry of the particles has been determined. This experiment enabled unprecedented 3D identification of detrimental stress risers relevant for fatigue in as-received aluminium alloys.

Three-Dimensional Damage Evolution Measurement in EB-PVD TBCs Using Synchrotron Laminography

This study provides the first results of three dimensional microstructural evolution analysis of a typical EB-PVD thermal barrier coating subjected to thermal cycling. Results have been obtained using the technique of synchrotron-radiation computed laminography (SRCL), enabling the scanning of samples that are thin but extended in two dimensions. A correlation with quantitative measurements of phase transformation from β-NiAl to γ’-Ni3Al was achieved from 3D images and 2D cross-section analysis. Morphological features of phase transformation and damaged areas within the bond-coat layer obtained by SRCL are discussed.

EXPERIMENTAL ANALYSIS OF TOUGHNESS IN 6156 AL-ALLOY SHEET FOR AEROSPACE APPLICATIONS

Analysis of toughness in 6156 Al-Mg-Si-Cu sheet has been performed using enhanced Kahn tear tests on samples quenched at different rates, whilst microstructures of the samples have been assessed using differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. Crack initiation energies were unaffected by changing water quench temperature from 20°C to 60°C, however a significant reduction was evident on air cooling. Crack propagation resistance was reduced for both 60°C water quenched and air cooled materials. The failure morphology of the air cooled material appears consistent with classical intergranular ductile failure. Coarse voiding and shear decohesion was prevalent in the 20°C water quenched material, whilst the 60°C water quenched material showed a mixture of transgranular and intergranular fracture modes. Changes in microstructure and precipitation behaviour resulting from reduced quenching rate were identified and related to the observed fracture behaviour, particularly in terms of precipitate free zone formation and the simultaneous presence of coarse particles at grain boundaries.

Evaluation of measurement uncertainties of digital volume correlation applied to laminography data

The effect of different data acquisition parameters on the displacement and strain uncertainties of kinematic fields is assessed via digital volume correlation applied to three-dimensional data sets imaged by synchrotron laminography. The measurement uncertainty is estimated for (1) different materials and associated varying contrast; (2) repeated scans, that is, uncertainty due to the imaging system and three-dimensional reconstruction; (3) rigid body motions between two subsequent scans; (4) rescaling of the gray level histogram during 32-bit floating point to 8-bit integer data conversion; (5) changes in the beam properties, that is, use of a monochromator or an undulator; and (6) changes in camera/detector characteristics. It is found that the amount of image contrast is not the only parameter that controls uncertainties for different materials. Furthermore, the rigid body motion procedure should be preferred over repeated scans as it provides more conservative measurement uncertainty values. The applied 32- to 8-bit conversion procedure, beam tuning and detector characteristics hardly affect the measurement uncertainty.

Interaction of the Portevin–Le Chatelier phenomenon with ductile fracture of a thin aluminum CT specimen: experiments and simulations

An attempt is made here to capture numerically slant ductile fracture and its early slant strain precursors via combining a dynamic strain aging (DSA) model with ductile damage models. In recent experimental studies it has been shown that in an AA2XXX alloy strain localization in slant bands preceded the onset of damage, originating slant fracture ahead of a notch. Here tensile tests are performed at different strain rates revealing some negative strain rate sensitivity which is an indication of DSA effect for AA2198-T8. A McCormick-type DSA model in conjunction with a Rousselier damage model, a reduced polycrystalline plasticity model and a Coulomb fracture criterion for slip systems have been used. Full 3D finite element simulations using this model and typical parameters for aluminum alloys capture the early strain localization in slanted bands, their intermittent activity and the final slant fracture. Prior simulation results without the DSA model and others using the von Mises plasticity or the GTN model did not capture the early slant strain localization thereby suggesting that DSA may well be the physical origin of the early slant strain localization and final slant fracture phenomena in this alloy.

Experimental-Numerical Validation Framework for Micromechanical Simulations

A combined experimental-numerical framework is presented in order to validate computations at the microscale. It is illustrated for a flat specimen with two holes, which is made of cast iron and imaged via in situ synchrotron laminography at micrometer resolution during a tensile test. The region in the reconstructed volume between the two holes is analyzed via Digital Volume Correlation (DVC) to measure displacement fields. Finite Element (FE) simulations, whose mesh is made consistent with the studied material microstructure, are driven by measured Dirichlet boundary conditions. Damage levels and gray level residuals for DVC measurements and FE simulations are assessed for validation purposes.

Multiaxial stress state assessed by 3D X-Ray tomography on semi-crystalline polymers

This work aims at linking the microstructural evolution of semi-crystalline polymers to the macroscopic material behaviour under multiaxial stress state. Tensile tests on notched round bars, interrupted after different stages of deformation before failure, were supposed to have undergone various states of stress in the vicinity of the net cross-section. They were examined using synchrotron radiation tomography. A sample obtained from a test stopped at the end of stress softening stage showed elongated axi-symmetric columns of voids separated by thin ligaments of material. This special morphology allowed investigating the distributions of voids in terms of both volume fraction and orientation. This distribution was compared with the theoretical multiaxial stress/strain field. The combination of the tomographic images analyses and the continuum mechanics approach results in a determination of the principal mechanical parameter driving voids evolution.

In-situ synchrotron-radiation computed laminography observation of ductile fracture

Synchrotron-radiation computed laminography (SRCL) allows for imaging at high resolution (~ 1 µm) and in three dimensions objects that are thin (~ 1 mm) but extended laterally in two dimensions. This represents a major advantage over computed tomography in terms of loading conditions that can typically only investigate samples elongated in one direction. Here SRCL is used to observe ductile crack initiation and propagation in high strength aluminium alloy sheet for aerospace applications. Several load steps are applied and permit us to follow the changes of damage and crack path. An attempt is made to measure strains via a digital volume correlation technique.