Multi-challenge aspects in fatigue due to the combined occurrence of multiaxiality, variable amplitude loading, and size effects

The three major issues which contribute to uncertainties in fatigue life estimations are the limited transferability of material data obtained in a laboratory for describing the fatigue behavior of components, the lack of damage accumulation rules to realistically take into account the effects of various load sequences, and the uncertainty in the assessment of multiaxial stress states. The three issues contribute to life prediction errors in a balanced way. Some modeling effort was spent in joining solution proposals in a unifying short crack model. The actual state is presented in the paper together with some ideas for further improvement as well as simplification for promoting the model’s acceptance in practical applications

Fatigue of weld ends under combined in- and out-of-phase multiaxial loading

Weld start and end points are fatigue failure sensitive locations. Their fatigue behaviour especially in thin sheet structures under multiaxial load conditions is not sufficiently explored so far. Therefore, a research project was initiated to increase the knowledge concerning this topic, which is of special interest in the automotive industry. In the present study, fatigue tests on welded joints were conducted. In the numerical part of the study, notch stresses were calculated with an idealised weld end model. A numerical method which combines the geometrical and statistical size effect to an integrated approach was used, in order to consider the size effects

Sharp three-dimensional notches under combined nominal normal and shear fatigue loading

Many engineering structures are exposed to non-proportional fatigue loading. However, at the critical location of crack initiation, often only one of the load sequences dominates – the individual load cases result in different crack initiation sites. Based on engineering judgment a simplified local uniaxial or proportional loading situation may be assessed. In general, the critical location must be identified by calculating fatigue lives for a variety of locations. The position of the critical location depends on the hypothesis applied for life calculation – besides loading, geometry, material and many other influence factors. For sharp three-dimensional notches the region for the search of the critical location may be restricted considerably. Weld start and stop points are an industrial example for such sharp notches. They are the object of the present investigation. They are exposed to nominal normal and shear loading. The individual hot spots for crack initiation lie necessarily close together. A strong interaction of the loading cases for both proportional and non-proportional loading was experimentally observed. In the numerical investigation notch stresses were calculated using an idealised weld end model. Based on the critical plane approach according to Findley, numerical interaction lines were produced

Measurement and simulation of crack growth rate and direction under non-proportional loadings

A series of fatigue experiments on thin-walled tubes under tension and torsion, the experimentalresults – crack path and crack growth life – are measured and compared. It is observed that the cracks follow acurvature from a tensile to a shear dominated growth with increasing crack length. The results are enforced bythe high amplitudes applied to the specimens causing large cyclic plastic deformations and crack growth rates in the order of 10-3 mm/cycle. The non-linear nature of the cyclic deformation has been taken into account byapplying a cyclic plasticity model, and plasticity-induced crack closure is captured by a contact formulation.Already for the uniaxial reference case the current limitations in modelling plasticity induced crack closure – aprerequisite for achieving realistic simulation results – have become obvious. Measurements have shown thatfriction and roughness induced closure processes come up, especially for non-planar crack surfaces, challenge to be met in the future

Growth of long fatigue cracks under non-proportional loadings – experiment and simulation

An experimental campaign was carried out on thin-walled tubes under tension and torsion. The results from experiments are measured and compared. It is observed that cracks follow a shear-dominated growth pattern with increasing crack length, instead of a tension-dominated one. The experiments are performed with high amplitudes applied to the specimens, resulting in large cyclic plastic deformations and crack growth rates up to 10-3 mm/cycle. Stress intensity factors were calculated for the proportional loading case

Postfledging Survival and Movements of Willow and Dusky Flycatchers in the Central Sierra Nevada

Understanding factors limiting population growth is critical for species exhibiting declining populations. Reproductive success has an important effect on population dynamics; however, our ability to accurately estimate productivity is limited. Studies on avian breeding biology have focused on nest survival; however, surviving to fledging does not ensure survival to the end of the breeding season. Furthermore, our understanding of habitat selection by birds based on the nesting cycle may not adequately represent the breeding habitat requirements because habitat use often changes after the young leave the nest. My goal was to examine the postfledging dependence period of two flycatcher species in the central Sierra Nevada: the California state endangered willow flycatcher (Empidonax traillii) and the dusky flycatcher (E. oberholseri). My focus was to estimate fledgling survival and examine factors that influence survival, evaluate postfledging movements and habitat use, and estimate post-breeding home range sizes of postfledging flycatchers. I monitored nests of both flycatcher species, individually color banded nestlings, and observed family groups daily once the young fledged. Flycatcher fledgling survival ranged from 46 percent to 76 percent and varied by year and species. Survival was lowest during the first week of the postfledging dependence period for both species. Fledgling flycatchers moved on average ~45m per day during the dependence period. I detected family groups in the natal meadows from 13 to 33 days. I detected willow flycatchers in riparian shrub vegetation 94 percent of the time, with the remaining detections being along the upland forest edge. Dusky flycatchers were more likely to use upland forest vegetation after leaving the nest, as I detected them in riparian shrub vegetation 70 percent of the time. For both years combined, mean 95 percwnt home range sizes were 1.80 ± 1.44 ha for willow flycatchers and 1.82 ± 1.70 ha for dusky flycatchers. Mean 50 percent core areas were 0.33 ± 0.27 ha for willow flycatchers and 0.38 ± 0.44 ha for dusky flycatchers. My results suggest that using fledgling survival throughout the dependence period to assess reproductive output is more accurate than using nesting data alone. Furthermore, postfledging family groups used a larger area of habitat than what is typically estimated from territory mapping singing males. Future research should continue to stress the importance of gaining knowledge about the postfledging period, especially for species with declining populations

The non-proportionality of local stress paths in engineering applications

A scalar measure, which describes the non-proportionality of local stress paths in engineering applications, is introduced. For this purpose the moment of inertia approach by Meggiolaro is modified in a way that the stress time history is evaluated in a tresca-stress-space. This modification makes the non-proportionality factor invariant with respect to the coordinate system. An optimization procedure is implemented to derive a test set-up for new component tests with 2 load channels. The aim of the planned tests is to get a high non-proportionality at the potential crack initiation site. It is not possible to obtain a high non-proportionality factor at the failure location without selective weakening of the component (housing of a rear axle steering). Therefore specific areas of the structure are cut out and the optimization procedure is repeated. As a result of the optimization a test set-up with high local non-proportionality at the potential crack initiation site is achieved for the weakened structure. Another set-up with slightly less non-proportionality but with a very localized damage is derived. This set-up is preferred, because of the robustness in the physical test

Measurement and simulation of strain fields around crack tips under mixed-mode fatigue loading

For various configurations examined in a series of fatigue experiments on thin-walled tubes undertension and torsion, the experimental results – crack path, crack growth life, near crack tip deformations, andcrack closure – are measured and compared to the results of some numerical simulations. Partially automatedoptical inspection and the digital image correlation technique were used in the experimental investigation. In thefinite element analyses, an actual geometry of a cracked structure has been modeled. The mechanical materialbehavior has been assumed as linear elastic in a first approach for calculating stress intensity factors. The nonlinearnature of the cyclic deformation has been taken into account by applying a cyclic plasticity model, andplasticity-induced crack closure is captured by a contact formulation

Characterisation of crack tip fields—CCTF5

Single parameter characterisation of the crack/notch tip field using fracture mechanics parameters like K, J, or CTOD has been extremely powerful in advancing predictive technologies for critical or subcritical crack growth. It has also become clear over the last decades that single parameter approaches have limitations particularly in dealing with crack growth phenomena arising from crack tip shielding, often resulting from the plastic enclave surrounding a crack. Influences of this enclave on the crack tip stress field ahead of the crack are maximised during cyclic loading. In the case of a parameter like the stress intensity factor that characterises the crack tip field via an elastic approximation, it is not surprising that any set of plasticity-induced circumstances that perturb the size of the plastic enclave and its associated strain field leads to predictive difficulties. Over the last 40 years, notable areas of activity related to such difficulties include short cracks, plasticity-induced closure, variable amplitude, multiaxial loading, and notch effects