Hans Albert Richard

Highly Anisotropic Steel Processed by Selective Laser Melting

For additive manufacturing of metals, selective laser melting can be employed. The microstructure evolution is directly influenced by processing parameters. Employing a high energy laser system, samples made from austenitic stainless steel were manufactured. The microstructure obtained is characterized by an extremely high degree of anisotropy featuring coarse elongated grains and a 〈001〉 texture alongside the build direction during processing. Eventually, the anisotropy of the microstructure drastically affects the monotonic properties of the current material.

A new criterion for the prediction of crack development in multiaxially loaded structures

In many cases the lifetime of technical structures and components is depending on the behaviour of cracks. Due to the complex geometry and loading situation in real-world structures cracks are often subjected to a superposition of normal, in-plane and out-of-plane loading. In this paper a new criterion for 3D crack growth under multiaxial loading, that means superposition of the fracture modes Mode I, II and III, is described. The criterion allows the prediction of three-dimensional crack surfaces advancing from arbitrary 3D crack fronts with the help of the two deflection angles φ0 and ψ0. The underlying theory for the development of this new criterion is described in detail.

INVESTIGATION OF MIXED MODE I/II INTERLAMINAR FRACTURE TOUGHNESS OF LAMINATED COMPOSITES BY USING A CTS TYPE SPECIMEN

Abstract The interlaminar fracture behavior of a unidirectionally glass fiber reinforced composite under the full range of in-plane loading conditions has been investigated. Loading conditions from pure mode I through various mixed mode I/II ratios up to pure mode II have been generated by the aid of the proposed compound version of the CTS (compact tension shear) specimen. From the experimentally measured critical loads, the mode I, mode II and the various mixed mode I/II critical energy release rates at crack initiation have been determined by the aid of the finite element method and the modified virtual crack closure integral method. Based on these results the parameters for a fracture criterion for the composite under consideration have been determined.

Fracture analyses and experimental results of crack growth under general mixed mode loading conditions

Abstract In this paper detailed results of 3D finite element (FE) and mixed mode analyses of different fracture specimens are presented and discussed. Special interest is taken in 3D and mode coupling effects to be found in strain energy release rate (SERR) results along crack fronts, in particular adjacent to corners, where a crack front intersects a free surface of a specimen. It will be shown that these effects stay small if they are related to Poisson’s ratio but that they can also be considerably pronounced if they are related to the global deformation behaviour of the specimen. The computational fracture analysis is based on the calculation of separated energy release rates (SERRs) by the aid of the modified virtual crack closure integral (MVCCI)-method in order to calculate the local SERR-distributions along the crack front. Furthermore some qualitative experimental results will show the influence of these variable mixed mode I, II and III loading conditions along the crack front on crack initiation and on the further development of 3D crack growth in the specimens.

Lifetime predictions for real loading situations: concepts and experimental results of fatigue crack growth

In this paper experimental results for real fatigue loading situations and loading changes are presented. It can be shown that the retardation effect depends on many factors, e.g. the overload ratio or the loading direction. Also, the lifetime is affected by the reconstruction of counting methods. In addition the results of simulations with the program code NASGRO are presented and verified by appropriate experiments.

Numerical and experimental analysis of residual stresses for fatigue crack growth

In this paper computational and experimental results are presented concerning residual stress effects on fatigue crack growth in a Compact Tension Shear (CTS) specimen under cyclic mode I loading. For a crack of constant length it is found that hardly any compressive residual stresses or crack closure effects are generated along the crack surfaces behind the crack tip through the considered cyclic mode I loading with a load ratio of R=0.1. Only if fatigue crack growth is modelled during the simulation of the cyclic loading process these well-known effects are found. On the other hand it is shown that they have hardly any influence on the residual stresses ahead of the crack tip and thus on further fatigue crack growth. For all cases considered the computational finite element results agree well with the experimental findings obtained through X-ray diffraction techniques.

Theoretical and experimental study of superimposed fracture modes I, II and III

Abstract The authors wish to present an all-fracture mode specimen with which it is possible to conduct fracture mechanics tests for pure mode I, pure mode II, pure mode III, as well as for all possible combinations of the above-mentioned. By means of a finite element analysis of this specimen, the stress intensity factors K I , K II , and K III were computed. It was discovered that K II and K III are coupled for in-plane shear and anti-plane shear loading, i.e. a mixed state occurs locally. The integral mean along the crack front yields however only to a K II factor for in-plane shear and to a K III factor for anti-plane shear loading. Fracture experiments under mixed-mode loading, using this new specimen, demonstrate the influence of the loading type on the orientation and on the structure of the fracture surface.

The behaviour of cracks in elastic-plastic materials under plane normal and shear loadings

Cracks in structures are often subjected to complex loading conditions. The direction of the crack extension depends on the normal and the shear components of the load. This paper is based on the kinking behaviour of cracks taking elastic-plastic behaviour of materials into account. The J-integral and the mixed-mode components JI and JII were determined after having performed several finite element analyses for different loading conditions. The path independence of J, JI and JII is investigated for both, the line integral proposed by Rice and the volume integral proposed by deLorenzi. For correctly determined crack deflection angles the JII-component vanishes when a FE-model with a kinked crack is considered. Hence, cracks propagate perpendicularly to the local mode I load.

Finite-Elemente-Simulation im Vergleich zur Realität

Kurzfassung Im vorliegenden Beitrag werden spannungsanalytische und bruchmechanische Untersuchungen zum ICE-Radreifenbruch beschrieben. Das Risswachstum beim ICE-Rad begann am Innenrand des Radreifens. Der Riss wuchs zunächst mehr in die Tiefe des Radreifens, um sich später in halbelliptischer Form auszubreiten. Erst als der Radreifenquerschnitt zu etwa 80% durch das Risswachstum geschädigt war, trat der Bruch des Radreifens ein. Die Bruchfläche weist Farbeffekte und Bruchflächenstrukturen auf, die auf ein sehr diskontinuierliches Risswachstum hindeuten. Es wird aufgezeigt, dass sich das Risswachstum mittels Finite-Elemente-Simulationen und experimenteller Untersuchungen erklären lässt.

Comparison of Computational Crack Path Predictions with Experimental Findings for a Quarter-Circular Surface Crack in a Shaft under Torsion

In this paper the rather complex 3D fatigue crack growth behaviour in a shaft with a quarter-circular crack under torsion is investigated by the aid of the programme ADAPCRACK3D and by application of a recently developed 3D fracture criterion. It will be shown that the computationally simulated results of fatigue crack growth in the FE-model of the shaft are in good agreement with experimental findings for the development of two anti-symmetric cracks, which originate from the two crack front corner points, that is where the crack front intersects the free surface of the cylindrical laboratory test-specimens. Consequently, also for this case with a rather complex 3D crack growth of two anti-symmetric cracks, the functionality of the ADAPCRACK3Dprogramme and the validity of the proposed 3D fracture criterion can be stated.