R. Pippan

Deformation behaviour of closed-cell aluminium foams in tension

Abstract The mechanical behaviour of commercially available ALPORAS aluminium foam with two different densities was studied under tension loading. In addition to the common stress–strain measurements, local deformation, notch-opening displacement and damage evolution were determined. The deformation characteristics deviated from those observed in aluminium foams under compression. No deformation bands or plastic instabilities could be observed in tension, which are very frequent in compression of metallic foams. Four regimes were evident in the stress–strain curves and deformation maps: the linear elastic regime, the plastic regime with no significant crack initiation and propagation, the regime of formation of a fracture process zone and, finally, the regime of fracture, where a main crack propagates through the specimen and leads to failure. The fracture strain was only a few per cent, with the higher-density foam showing a larger fracture strain, and the plastic Poissons ratio was about 0.35. The notched specimens showed increasing fracture strengths in terms of the net section stress with increasing notch depth. It is suggested that a change in stress state, caused by a non-vanishing Poissons ratio, in front of the notch tip creates an increase of the fracture strength similar to the behaviour in ductile bulk metals.

Fracture behaviour and fracture toughness of ductile closed-cell metallic foams

Standard fracture mechanics tests were carried out on two different types of aluminium foam, ALPORAS® foams and ALULIGHT® foams, with a variety of densities. Standard fracture toughness tests on compact tension (CT) specimens with widths from 50 mm to 300 mm and in situ tests in the scanning electron microscope were performed. Fracture toughness values in terms of the critical stress intensity factor, KIC, the critical J-integral, JIC, and the critical crack-tip opening displacement, COD5,i, were determined. To identify the fracture process, local deformation measurements were performed on the foam surfaces with a digital image processing system. From the deformation measurements it is evident that the deformation is strongly localised on different length scales. A relatively large fracture process zone, 6–8 cells in height, is developed, where only few of them are heavily deformed. On the cell wall level the deformation is again strongly localised to the thinnest parts of the cell wall, where cracks initiate and propagate. The crack propagates through the foam, building many secondary cracks and crack bridges. The comparison of K vs. Δa (crack extension), J vs. Δa and COD vs. Δa with the current fracture processes at the crack tip and the load–displacement response reveals that COD gives the most reliable measured values to characterise the fracture toughness.

Disorientations as a function of distance: a new procedure to analyze local orientation data

A new procedure for analyzing local orientation data has been developed to investigate the relation between orientations and data point distances. The key idea is to determine the distribution of the disorientation angle as a function of the distance between the data points. The evolution of the disorientation angle distribution with increasing data point distance provides quantitative information about average local variation of the crystal orientation which can be used to classify structural elements, to determine the average structural size, and allows the quantification (crystallographically) of the boundaries between adjacent structural elements. The potential of this analyzing procedure is demonstrated by investigating the orientation data of Electron Backscattering Diffraction (EBSD)-measurements performed on severe deformed copper samples. The benefits in comparison to other commonly used methods to quantify local disorientations are discussed.

The deformation-induced zone below large and shallow nanoindentations: A comparative study using EBSD and TEM

A comparison is made between the deformation-induced zone beneath nanoindentations obtained by Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Since there are resolutional limitations associated with EBSD, especially at very small scan sizes, it is not known how accurately the deformed volume beneath the imprints can be characterized. To aid in answering this question, cross-sectional EBSD and TEM samples of nanoindentations were fabricated by means of a Focused Ion Beam (FIB) workstation, analyzed, and subsequently compared with each other. For large indentations as well as for shallow ones, agreement of the determined zones was found. The results of the EBSD and TEM experiments were also used to characterize the deformed volumes. In the EBSD maps of large indentations, strongly confined deformation patterns were found, while for the shallow indentations the observed patterns are more diffuse. The TEM micrographs and the Selected-Area Electron Diffraction (SAED) support these facts and give insight into the dislocation structure of the deformation zone.

Ultra-strong and damage tolerant metallic bulk materials: A lesson from nanostructured pearlitic steel wires

Structural materials used for safety critical applications require high strength and simultaneously high resistance against crack growth, referred to as damage tolerance. However, the two properties typically exclude each other and research efforts towards ever stronger materials are hampered by drastic loss of fracture resistance. Therefore, future development of novel ultra-strong bulk materials requires a fundamental understanding of the toughness determining mechanisms. As model material we use today’s strongest metallic bulk material, namely, a nanostructured pearlitic steel wire, and measured the fracture toughness on micron-sized specimens in different crack growth directions and found an unexpected strong anisotropy in the fracture resistance. Along the wire axis the material reveals ultra-high strength combined with so far unprecedented damage tolerance. We attribute this excellent property combination to the anisotropy in the fracture toughness inducing a high propensity for micro-crack formation parallel to the wire axis. This effect causes a local crack tip stress relaxation and enables the high fracture toughness without being detrimental to the material’s strength.

Effects of microstructure and crystallography on crack path and intrinsic resistance to shear-mode fatigue crack growth

ABSTRACT. The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations.

Intrinsic Behaviour of Mode II and Mode III Long Fatigue Cracks in Zirconium and the Ti-6Al-4V Alloy

This work is focused on experimental study of micromechanisms of mode II and mode III fatigue cracks in metallic materials in the near-threshold regime. The resistance to fatigue crack growth can be divided to an intrinsic component (ahead of the crack tip) and an extrinsic component (shielding, closure), which is significantly higher than the intrinsic one. Fracture surfaces from the Ti6Al4V alloy and pure zirconium were observed in three dimensions. Experiments were conducted using a special device for simultaneous crack loading in modes II and III. Additionally, pure mode II and pure mode III experiments were done using CTS and torsion specimens, respectively. At the beginning of all experiments, crack closure was eliminated due to precracks generated under cyclic compressive loading. A common mechanism of local mode II advances was observed in both modes II and III. The results were similar to those of pure titanium. The hcp metals exhibit a transition behaviour between materials with coplanar shear-mode crack propagation and materials with a high tendency to deflect to the opening mode I.

Load history effects on fatigue crack propagation: Its effect on the R-curve for threshold

The study deals with the effect of load history during threshold and fatigue crack propagation measurements in the near threshold regime. The compression pre cracking and the stepwise increasing load amplitude test was applied. The effects of pre-cracking as well as the effect of compression and tension overloads after the pre-cracking on the long crack threshold and on the R-curve of the threshold of stress intensity range are analysed.

Thermal Behavior of Nickel Deformed to Ultra-High Strain by High Pressure Torsion

Polycrystalline Ni (99.5 %) has been deformed to an ultra-high strain of εvM=100 (εvM, von Mises strain) by high pressure torsion (HPT) at room temperature. The deformed sample is nanostructured with an average boundary spacing of 90 nm, a high density of dislocations of >1015m-2 and a large fraction of high angle boundaries (>15o) 68% as determined by transmission electron microscopy and 80% as determined by electron backscatter diffraction. The thermal behavior of this nanostructued sample has been investigated by isochronal annealing for 1h at temperatures from 100 to 600°C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness have been determined. Based on microstructural parameters the stored energy in the deformed state has been estimated to be 24 MPa. The isochronal annealing leads to a hardness drop in three stages: a relatively small decrease at low temperatures (recovery) followed by a rapid decrease at intermediate temperatures (discontinuous recrystallization) and a slow decrease at high temperatures (grain growth). Due to the presence of a small amount of impurity elements, the recovery and recrystallization are strongly retarded in comparison with Ni of high purity (99.967%). This finding emphasizes the importance of alloying in delaying the process of recovery and recrystallization, which enables a tailoring of the microstructure and properties through an optimized annealing treatment.

A Discussion of the Applicability of ΔK-Values to Explain the Fatigue Crack Growth Behaviour of Short Cracks

In a joint research project of AIRBUS Deutschland in Hamburg and the Institute of Materials Research of the German Aerospace Centre (DLR) in Koln short corner cracks emanating from holes were investigated.