Martin Alexander Eder

Measurement of local relative displacements in large structures

This article presents a novel measurement technique to measure local relative displacements between parts of large-scale structures. The measured deformations can be of significant importance for fracture analyses in many different types of structures in general and for adhesive connections in particular. The measurement of small local relative displacements in structures subjected to large global deformations is complex and hardly feasible with conventional measurement methods. Therefore, a small displacement measurement system has been devised. The small displacement measurement system is based on stereo photogrammetry and capable of measuring three-dimensional local displacements with a high degree of accuracy. In this article, the technique is used to measure local deformations in the vicinity of the adhesive trailing edge joint of a wind turbine rotor blade. The small displacement measurement system results correspond well with another independent measurement method.

Fracture mechanics approach to optimize inspection planning of offshore welds for wind turbines

In the present work, fracture mechanics-based concepts are introduced into a fatigue life prediction framework used to optimise inspection planning of offshore welds for wind turbines. Offshore welds are typically subject to fatigue loading conditions in corrosive environments which can lead to accelerated crack growth, detrimentally affecting the structural integrity. The offshore wind industry commonly applies inspection/repair procedures in conjunction with corrosion protection systems in order to prolong the lifetime of offshore welds. The research conducted through this work addresses three important issues in the offshore wind industry, namely, the optimal inspection interval, cost of maintenance as well as the impact of protection system failure on the maintenance planning. This thesis is grossly divided into three different studies which encompass a stress-based fatigue approach, a fracture mechanics-based fatigue approach and a probability theory-based approach. Aforementioned fields are incorporated into inspection planning simulations, all of which are shown to provide meaningful predictions for the industry. Furthermore, the current research entails innovative approaches and methods used to simulate fatigue crack propagation in welded steel components with the following common characteristics: the Multiple-Site Damage and Residual Stresses. The research results presented herein show a strong dependency of the maintenance planning on the prevailing environmental conditions. Moreover, the parametric studies conducted suggest that the total maintenance costs can significantly vary depending on the adopted inspection planning strategy. The research results presented in this thesis can aid the development of cost effective solutions in the off-shore maintenance framework.

Effects of Coatings on the High-Cycle Fatigue Life of Threaded Steel Samples

In this work, high-cycle fatigue is studied for threaded cylindrical high-strength steel samples coated using three different industrial processes: black oxidation, normal-temperature galvanization and high-temperature galvanization. The fatigue performance in air is compared with that of uncoated samples. Microstructural characterization revealed the abundant presence of small cracks in the zinc coating partially penetrating into the steel. This is consistent with the observation of multiple crack initiation sites along the thread in the galvanized samples, which led to crescent type fracture surfaces governed by circumferential growth. In contrast, the black oxidized and uncoated samples exhibited a semicircular segment type fracture surface governed by single-sided growth with a significantly longer fatigue life. Numerical fatigue life prediction based on an extended Paris-law formulation has been conducted on two different fracture cases: 2D axisymmetric multisided crack growth and 3D single-sided crack growth. The results of this upper-bound and lower-bound approach are in good agreement with experimental data and can potentially be used to predict the lifetime of bolted components.

An Improved Sub-component Fatigue Testing Method for Material Characterization

In this paper an improved sub-component fatigue testing method is proposed, in which structural optimization is used to obtain specimens in which fatigue failure is precipitated in the designated area away from the boundaries, i.e., load application and fixture points. This is achieved by optimizing the nonlinear beam taper and the dynamic excitation. The major requirement for accurate material characterization through sub-component tests concerns unbiased stress states in the gauge section. However, empiricism shows that many sub-component high-cycle fatigue testing methods suffer from failure in the boundaries rather than the gauge section, which causes bias. The common practice for reinforcing those regions only shifts the issue into new areas of local discontinuities where failure is still caused remotely from the gauge section. An experimental proof of concept demonstrates that optimization of the beam taper can be used to obtain unbiased fatigue test data.