Volume 97, Issue 12

The established K -concept with the Paris law as the central equation often used for life-time prediction and dimensioning does not fit with the micromechanical crack tip opening displacement (CTOD) concept. While the K -concept is based on continuum mechanics, the CTOD is influenced by the local microstructure. Consequently, the CTOD-concept is checked in situ in the scanning electron microscope and is found to be the appropriate micromechanical description of the crack propagation mechanism. However, it is shown that during low cycle fatigue of turbine materials, even under extreme conditions short cracks behave less harmfully than expected. Additionally a new technique of artificial crack initiation by operating with a focused ion beam is used to investigate special crack problems such as the interaction of cracks with microstructural barriers in detail, which was not previously possible in this systematic way.

The development of residual stresses and strains in the matrix material of the extruded aluminium alloy EN AW-6061-T6 both with no reinforcement and also reinforced with 15vol.% and 22vol.% α-alumina particles was investigated in thermal cycling tests and additionally by finite-element-simulation for the non-reinforced material. For this purpose cylindrical specimens were heated by a 3kW diode laser and cooled by a water quench. Residual stresses in the surface area of the laser beam and temperature distribution inside the specimen were measured. The results were used to validate the finite-element model for calculating stress and strain distributions inside the specimens.

To understand the damage behavior of mechanically deformed metallic materials in more detail, the kind of defect and its concentration have to be known. In addition, the kinetics of decomposition and of precipitation hardening are influenced by the presence of defects and the corresponding concentration. Consequently, an analysis of dominating defects would be helpful. Compared with well known techniques, positron annihilation spectroscopy offers the opportunity for lifetime measurements that are characteristic for special kinds of defects, e.g. dislocations, small vacancy clusters and micro-voids. To detect the spatial distribution and to determine the concentration of defects, the low energy pulsed positron beam technique can be used. This technique, in combination with the scanning positron microscope, opens a broad field of applications for defect analysis in metallic and ceramic materials.

The TRIP-effect in metastable austenitic steels is caused by a moderate local martensitic transformation, influencing both the strain hardening and the damage behaviour. The main conditions of a marked TRIP-effect are a low flow stress in the undeformed state, a high strain hardening exponent, a continuous martensitic transformation up to about 20% and a high resistance to damage (e.g. crack formation and propagation). The martensite transformation starts in glide or shear bands and their intersections even at temperatures clearly above the conventional M d -temperatures, reducing the local stress concentrations and so preventing the onset of damage. Furthermore, high martensite content can be detected along the fracture path impeding the crack propagation by the transformation. To demonstrate the influence of damage on the TRIP-effect the deviations from the so-called Considere-criterion for maximum uniform elongation can be used.

Mechanical stress–strain hysteresis, temperature and magnetic measurements were performed for the detailed characterisation of the cyclic deformation behaviour of metastable austenitic steels with particular attention to plasticity-induced martensite formation. Specimens of AISI 304, AISI 321 and AISI 348 stainless steels were investigated under plastic strain control at ambient temperature. On the basis of comprehensive experimental data, a model was developed for the description of the plasticity-induced martensite formation in metastable austenites under cyclic loading. The process of martensite formation was described as a function of the cumulative plastic strain and the cumulative strain energy density.

The present study investigates crystal orientations and residual stresses in TiN films deposited by arc ion plating. TiN films approximately 2.0 μm thick were deposited on a steel substrate. With a bias voltage of 0 V, the TiN film exhibited strong {110} texture, whereas the dominant orientation of the film deposited at −100V was {111}. TiN films had very high compressive residual stresses: −8.6 GPa in the {110} textured film and −10.0 GPa in the {111} textured film. These residual stresses decreased with increasing annealing temperature and the reduction rate was greater for the {111} than for the {100} film. The behavior of residual stresses in the {111} and {110} textured layers of {111}/{110} textured double-layer film was identical to that for single-layer films.

Data of flow stresses σ for pure Cu with ultrafine grains with grain size d ≍ 0:35 μm produced by severe plastic deformation and grains of conventional size obtained in the range of homologous temperatures T hom from 0.22 (room temperature) to 0.33 are compared to data for hardnesses H for Cu of various grain structures from single crystalline to d = 0:01 μm measured by nanoindentation at room temperature. The two sets of data appear to be consistent when σ ≍ H =3. At room temperature the Hall – Petch relation holds, i. e., the flow stress increases monotonically with decreasing grain size by Δ σ ∝ d −0.5 . At elevated T hom the saturated flow stress decreases when the grains become ultrafine. The transition from hardening to softening by grain boundaries in the saturation stage is discussed on the basis of a simple statistical dislocation model considering the influence of grain boundaries on the balance between generation and annihilation of dislocations.

Nanocomposites made of polymer-derived Si–C–N ceramic reinforced by single-wall carbon nanotubes (SWCNTs) were prepared for the first time. The synthesis procedure involved ultrasonic dispersion of the nanotubes into a liquid polysilazane precursor polymer, followed by cross-linking and thermolysis. With the aid of nanoindentation testing, dependence of the mechanical properties of the composites on the concentration and agglomeration state of SWCNTs, was studied. The nanotube-filled composites showed improved mechanical performance, as reflected by an increase in Young's modulus which was found to be correlated with the microstructure of the composites, in particular the degree of dispersion of the nanotubes inside the matrix, whereas the hardness is hardly affected.

Shot peening is a common procedure for improving the static and cyclic strength of metal components by a combination of work hardening and the introduction of compressive stresses into the surface region. Recent investigations showed that high compressive stresses of more than 1 GPa can also be introduced in brittle ceramics such as alumina and silicon nitride under specific shot peening conditions. These stresses dramatically increase the near-surface strength. This paper compiles the current state of the art of shot peening for silicon nitride ceramics. Detailed information about the influence of shot peening on the residual stress state, dislocation density and damage are given. The effect of the residual stresses on the static and cyclic nearsurface strength is quantified.

Mechanical properties of a T6-AlSi6Cu4 cylinder-head alloy are investigated at 4 stress levels up to 545 K through tensile and compressive test to assess creep damage. The effect of over-ageing for 200 h at comparable temperatures was studied through RT hardness tests. Creep behaviour was characterised by a minimum creep rate and an extended steady state was not observed. No threshold stresses were apparent from minimum creep rate vs. stress plots at any of the temperatures studied here. Apparent stress exponents were both temperature and stress dependent and the apparent creep activation energy was 2–3 times the lattice selfdiffusion activation energy for aluminium. The Norton plot was best described by an exponential relation. The total strain to failure exhibited a maximum for a particular applied stress and decreases for both higher and lower applied stress. Creep life is well described by the Monkman– Grant relation at the highest temperature, but exhibits deviations from the idealised behaviour at the lower test temperatures.

Bending rotation fatigue experiments are a standard evaluation procedure for pseudo-elastic NiTi wires used in the medical device industry. However, the non-linear mechanical behavior and the possible occurrence of localized deformation during bending complicate the physical interpretation of fatigue life data. In the present study, some related experimental and theoretical aspects are addressed: Optical measurements are used to characterize the local deformation of the wire specimens in a typical experimental setup, and a simple mechanical model of stresses and strains associated with bending rotation is employed to rationalize the impact of several experimental parameters on fatigue life data. Dissipated energies per cycle and unit length are calculated. How dissipation leads to an increase of both specimen temperature and maximum stresses and, therefore, considerably decreases fatigue lives is demonstrated.

Many investigations have revealed the beneficial effect of mechanical surface hardening in combination with induced compressive residual stresses leading to improved fatigue strength. The benefit will be higher, the higher the quality of the weld is. On the other hand, methods like shot peening are useless if the weld toe profile is poor and the connected notch effect strong. Increased fatigue strength is then only possible after a post weld treatment which produces a smoother shape of the weld toe. Experiments reveal that methods which aim to increase the intensity of the penetration depth do not necessarily produce the best fatigue performance. High intensities of the mechanical surface treatments, especially in ductile aluminium alloys, may produce new geometrical defects which may work against the benefit of the deeper penetration depth of cold hardening and compressive residual stresses. A systematic evaluation of the results of improvement methods is possible with the help of the local fatigue strength concept, which compares the local strength properties and the local load stresses.

Processing of long-fibre reinforced thermoplastics with a discontinuous reinforcement of long glass fibres, especially by direct-processing, leads to inhomogeneous and anisotropic properties. Because of this, locally resolved characterisation of the material properties is necessary. Non-contact testing methods are required to derive these material properties. Locally resolving hysteresis measurement, full-field strain measurement techniques and thermoelastic stress analysis (TSA) are applied to an incremental step test and to a fatigue test. It is shown that there is good correspondence between the local TSA-signals and the locally measured values of the major strain. This correlation is valid not only for lower linear elastic load levels, but also up to nonlinear-viscoelastic levels. A model is presented to describe the correlation between the TSA-signal and the measured major strain.

The spectral selectivity of Cu–Al 2 O 3 cermet solar absorbers was studied theoretically and experimentally in different layer designs. The results showed clearly that, by increasing the substrate temperature during film deposition, the desired random roughness features will be created on the surface of the cermet, which leads to higher selectivity. Theoretical simulation showed that a single cermet structure with the assistance of an anti-reflection layer, results in the best optical properties. Instead of such a structure, which is sensitive to fine thickness control, it was tried experimentally to increase the selectivity of a single cermet structure as much as possible using a roughness template layer. Surface roughening using such a layer design increased the solar absorptance from 0.840 to 0.956 while keeping the thermal emittance almost unchanged at about 0.028.

The aim of this study is to clarify the role of the microstructure in damage evolution. The influence of the transformation induced plasticity effect on the crack initiation and the impact of the different phases in development of cracks are important factors that control the fracture mechanisms. The fracture mechanisms of multiphase steels have been investigated on the basis of extensive light optical microscopy and scanning electron microscopy investigations. On the micro-scale, two failure modes appear: cleavage and ductile fracture, depending on the stress–strain conditions, the internal cleanness, the volume fraction of the retained austenite and the position of the austenite and the martensite grains. If a martensite or an austenite grain fails inside a bainitic island, the crack develops rapidly leading to cleavage fracture. If the failure starts inside the ferritic matrix due to void initiation at hard phases, the emerged void causes ductile damage to the surrounding ferrite.

Sophisticated serial products require precise plastic parts that can be effectively manufactured employing the injection moulding process. However, the cooling of solid and molten material at the same time leads to high shrinkage, warpage and residual stresses. These processing characteristics determine the limits of dimensional stability that can be achieved with conventional injection moulding. A novel processing approach, in which the solidification is accomplished by pressure loading only, makes sure that these difficulties can be overcome. With this approach solidification takes place over the entire moulding at the same time. Investigations have been carried out into the viscosity, compressibility and thermal expansion of amorphous thermoplastics, which allowed evaluation of the physical pre-conditions for the novel moulding technique.