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Die „auffälligen großen Streifen“ sind von echten Schwingstreifen nur sehr schwer zu trennen, weil sie parallel zu den Schwingstreifen verlaufen und meistens ebenfalls nicht sehr groß sind. Die im Labor getesteten Proben zeigten, dass diese großen Streifen zusammen mit Verreibungen auftraten und „tire tracks“ („Reifenspuren“) verursachten. Dies wurde durch eine hochauflösende REM-Untersuchung bestätigt, die „tire tracks“ auf den „großen Streifen“ zeigte. Ein weiterer Hinweis ist die Änderung des R-Werts von -1 auf 0,1, was zu einer Bruchfläche ohne große Streifen und „tire tracks“ führt.
In this study, the influence of aging and mean stress on fatigue of the aluminium-alloy EN AW-2618A is investigated. Therefore axial fatigue tests are carried out on smooth specimens. The experiments show that the fatigue life decreases with increasing mean stress. Furthermore, the tests with the overaged specimens demonstrate that the number of cycles to failure is decreasing with increasing aging time.
The fatigue assessment of welded joints requires several input data, which can be subdivided into three categories: geometry, material and loading. The number of input data depends essentially on the complexity of the models employed and on the level of accuracy of the analysis. It is common practice to use safety factors in design to account for the scatter of the input parameters. Nevertheless, overly-conservative factors lead often to unrealistic estimations of fatigue life. This work presents a fracture mechanics-based model for the structural integrity assessment of welded joints under constant amplitude fatigue loading, in which the local geometry at the weld toe and the fatigue crack growth properties are considered statistically distributed. The approach is validated against a large number of experimental data.
Structural alloys are largely employed in key industrial sectors and their demand is predicted to rise rapidly for the next decades. Most of these materials require a large amount of energy for extraction and manufacturing, which causes the emission of greenhouse gases and other pollutants. Therefore, strategies for improving the sustainability of structural metallic alloys are urgently needed.
Additive Manufacturing (AM), in particular Laser Powder Bed Fusion (PBF-LB/M), aims to be a sustainable manufacturing process, as it allows the build-up of complex geometry in near net-shape from 3D models, while minimizing material waste and the energy required for the process and post-process treatments.
Nevertheless, the application of additively manufactured parts in structural safety-relevant applications is still hindered by the poor fatigue performance. The cause of this has been mainly attributed to the presence of manufacturing defects and surface roughness. Therefore, a huge effort has been made to optimize the process parameters and to introduce post-process treatments to minimize the defect content. However, material flaws cannot be fully eliminated, but these can be considered in a damage tolerance framework for the prediction of the fatigue performance of additively manufactured metallic materials, which is essential for part design and qualification.
This work aims at presenting different modelling strategies for the prediction of the fatigue limit of AM metals. Simple empirical models and more complex models based on fatigue short crack propagation are proposed. The investigated material is an AlSi10Mg alloy fabricated by PBF-LB/M and subjected to two different low-temperature heat-treatments (265°C for 1 h and 300°C for 2h). The results show that the models can provide good approximation of the fatigue limits and help in the interpretation of the scatter of fatigue data.
Laser Powder Bed Fusion (L-PBF) allow the fabrication of lightweight near net shape AlSi10Mg components attractive to the aerospace, automotive, biomedical and military industries. During the build-up process, high cooling rates occur. Thus, L-PBF AlSi10Mg alloys exhibit a Si-nanostructure in the as-built condition, which leads to superior mechanical properties compared to conventional cast materials. At the same time, such high thermal gradients generally involve a deleterious residual stress (RS) state that needs to be assessed during the design process, before placing a component in service. To this purpose post-process heat treatments are commonly performed to relieve detrimental RS. In this contribution two low-temperature stress-relief heat treatments (SRHT) are studied and compared with the as-built state: a SRHT at 265°C for 1 hour and a SRHT at 300°C for 2 hours. At these temperatures microstructural changes occur. In the as-built state, Si atoms are supersaturated in the α-aluminium matrix, which is enveloped by a eutectic Si-network. At 265°C the Si precipitation from the matrix to the pre-existing network is triggered. Thereafter, above 295°C the fragmentation and spheroidization of the Si branches takes place, presumably by Al–Si interdiffusion. After 2 hours the original eutectic network is completely replaced by uniformly distributed blocky particles. The effect of the heat and the microstructure modification on the RS state and the fatigue properties is investigated. Energy dispersive x-ray and neutron diffraction are combined to investigate the near-surface and bulk RS state of a L-PBF AlSi10Mg material. Differences in the endurance limit are evaluated experimentally by high cycle fatigue (HCF) tests and cyclic R-curve determination.
In recent years the German highway network has seen an increase in the occurrence of damage in concrete road surfaces which can be attributed to the alkali-silica reaction (ASR). In view of the often drastic reduction in life expectancy experienced by concrete pavements due to ASR, research activity in this field has notably increased. Until now the main research focus has been, alongside preventative measures in concrete technology, the development of performance-oriented testing procedures for ASR prevention. This included more specifically the accelerated simulation of climatic effects and external alkali supply on concrete pavements. The effects of pre-damage from the additionally interacting cyclic traffic loading had previously not been taken into consideration.
Since 2011, a five-partner research group from the German Research Foundation has been pursuing research on the effects of simultaneous cyclic loading and external alkali supply on the destructive ASR in concrete pavements. The depiction of the myriad degradation and transport processes necessary for an understanding of these effects requires a close interaction between experiments and their multi-scale modelling. This contribution aims to focus on the aforementioned experiments by means of innovative testing techniques. The research is founded on a series of cyclic fatigue tests performed on large-format beams, both with and without previous application of a NaCl solution, with simultaneous tracking of crack development. Thereafter, smaller test specimens were extracted from the pre-damaged beams for further experiments. These included the spatial visualization and quantification of fatigue-induced cracks with X-ray 3D-computed tomography. Additionally, the effects of fatigue-induced cracks on moisture and alkali transport were investigated using 3D-CT and Laser-Induced Breakdown Spectroscopy (LIBS). Subsequent storage of the small-format test specimens, with and without cyclic pre-damage, in an ASR-conducive environment was then able to shed light on the influence of fatigue-induced pre-damage on the ASR.
Carbon Fibre Reinforced Plastics (CFRP) are more and more used in modern civil aircrafts. These days the whole fuselage is made of this material (B787; A350). Due to strict certification standards the normal in-service loading gives a low stress level compared to the static and even the fatigue strength of the material. Hence CFRP are assumed to have an infinite life. To evaluate this assumption, fatigue tests on CFRP-specimens were performed up to 108 load cycles and the first inter-fibre failure was evaluated non-destructively by accompanying X-ray-refraction topography.
A tensile testing machine was integrated in a small angle X-ray scattering (SAXS) setup. X-ray refraction topography was performed while the CFRP-samples were tensile loaded. This non-destructive technique enables the detection of micro-cracking and inter-fibre failure especially for CFRP. For Glass Fibre Reinforced Plastic (GFRP) X-ray refraction and in-situ loading has already been successfully used. The increase of inner surfaces due to inter fibre failure was measured as a function of the stress state. Fatigue tests were performed at and below the limit of inter-fibre failure strength.
State of the art is to assume the failure of the samples under cyclic loading as the fatigue life. Accompanying non-destructive X-ray refraction measurements reflects the damage state and enables to trace its evolution even if the total failure of the specimens does not occur. This investigation technique is of high interest to give the engineer a design value of infinite life which is practically often reached due to knock down factors of certification standards. Finally the infinite life was found for cyclic fatigue loaded CFRP-samples even under high inter fibre transverse and shear loading investigated up to 108 load cycles.
In this study, quasi-static material tests on unidirectional and multi-angle glass fiber reinforced epoxy laminates at ambient temperatures from -213 K to 353 K (-60 °C to +80 °C) are performed. In addition, neat resin is investigated under tension and compression loads at different temperatures. The coefficient of thermal expansion is determined for neat resin and unidirectional reinforced specimens. The dependence of the resin’s thermomechanical properties on the ambient temperature is shown. The point of damage onset at which first cracks appear within the matrix under quasi-static loading is investigated by means of optical grey scale analysis. The correlation between damage onset and effective matrix stress at different ambient temperatures is identified. An approach for the calculation of thermomechanical loads and the prediction of the damage onset by means of inverse calculations is presented. The impact of the strain blocking effect of the matrix is considered as well as residual thermal stresses due to curing and resin shrinkage.
As subcritical crack growth (SCCG) can reduce tensile strength of glasses by many orders of magni-tude, the potential for improvement of fatigue behaviour is most intriguing in developing ultra-strong glasses. An essential bottleneck is the basic understanding of the numerous interplaying pressure-, temperature- and water-affected relaxation phenomena at the crack tip and related toughening strat-egies. Therefore, the present project aims to advance the basic understanding of structural relaxa-tion effects and local properties caused by increased water concentration and tensile stresses at the crack tip as they are a key for structural toughening designs to develop SCCG-free glasses and glass surfaces.
Our first studies give clear evidence that glass structure and dynamics is strongly modified upon hy-dration of glasses. These changes are highly related to the nature of network formers but are affect-ed as well by the counter ions (network modifier). Results of the 1st project part suggest that struc-tural relaxation below glass transition temperature, i.e. overlapping of short-range (beta) and long-range (alpha) interactions can contribute to SCCG in water-free environments and that structurally dissolved water in the glasses can have decisive impact on this effect.
In the 2nd project stage specific glasses compositions will be investigated to gain an improved un-derstanding on the relation of sub-Tg relaxation and inert SCCG as well as to shed light to the relat-ed effects of dissolved water and its speciation. These glasses cover a broader range of different glass topologies and binding partners, whereby the coupling of alpha and beta relaxations is varied systematically by alkali-, alkaline earth ions and water species concentrations. Preparation of hy-drous glasses (up to 8 wt% water) will be performed by high pressure syntheses. Structure will be resolved by NMR, Raman and IR spectroscopy while structural relaxation is accessed in the temper-ature and frequency domain using dynamic mechanical spectroscopy and ultrasonic damping. We will focus on measurements of inert SCCG (region III) conducting experiments in vacuum and dry gas atmospheres using indentation techniques and stressing of glass specimens in DCB geometry.
Experimental data on SCCG will be provided to SPP groups, which deals with fatigue in metallic glasses and vice versa we will test theoretical predictions of ab-initio simulations of partner within SPP 1594 in order to quantify the effect of water on the crack tip. In summa topological factors con-trolling the subcritical crack growth with respect to water will be identified from which structural toughening designs for highly fatigue resistant-glasses can be derived.
Fatigue induced degradation in pavement concrete and its effect on the alkali-silica-reaction
(2018)
In recent years the German motorway network has seen an increase in the occurrence of damage to concrete road surfaces that can be attributed to the alkali-silica reaction (ASR). In view of the often drastically reduced service life of road surfaces due to ASR, research activity in this field has notably increased. Alongside preventative measures in concrete technology i.e. the usage of low-alkali cements, the main research focus up to now has been on the development of performance-oriented testing procedures for ASR prevention. The effects of mechanicallyinduced damage resulting from cyclic traffic and climatic loading have previously not been taken into consideration.
The assessment of the myriad degradation and transport processes necessary for an understanding of these effects requires close interaction between experiments and corresponding multi-scale models. This contribution is focused on the experiments utilizing innovative testing techniques. The research is founded on a series of fatigue tests performed on large-scale beams with simultaneous tracking of the degradation process using non-destructive evaluation methods. Subsequently, smaller test specimens were extracted from the predamaged beams for further experiments. These experiments included investigation of the influence of fatigue-induced cracks on moisture and alkali transport. Subsequent storage of the small-scale test specimens, with and without cyclic pre-damage, in an ASR-conducive environment then provided initial findings on the influence of fatigue-induced degradation on the ASR.
The results show that the degradation process can be well observed with the test Setup developed for the fatigue experiments. Further, a small but noticeable increase of mass Transport due to fatigue-induced degradation was measured, which correspondingly indicated an increased ASR damage potential.