The final conference program can be downloaded here.
The Book of Abstracts can be downloaded here.
The received papers can be downloaded from the links below:
Prof. Bent F. Sørensen
Head of Composites and Materials Mechanics - DTU
MICROMECHANICAL MODEL FOR THE PREDICTION OF FATIGUE LIFE OF UNIDIRECTIONAL FIBER COMPOSITES
Recent X-ray computed tomography (XCT) studies have uncovered new details of how fatigue damage initiate and grow in unidirectional fiber composites. Fibre failure is found to occur for the next fiber, essentially creating a damage front, where on one side the fibers are unbroken and on the other side all fibers are broken. The XCT investigation shows that fiber do not fail co-planar but a short distance (in the fiber direction) from each other. Other experimental studies have shown that fiber/matrix debonding takes place in connection with a fiber break and that the debond length is increases under cyclic loading. Experiments using Raman spectroscopy have shown that the neighbor fiber experiences a stress concentration at the locus of fiber failure and at the location of the debond crack tips.
A micromechanical model is proposed on the basis of the experimental observations. The failure of the next fibre is attributed to the stress induced from the stress field of the debond crack tip of the broken fiber. The “delay” in terms of load cycles between a fiber failure and the failure of the neighboring fiber is attributed to progressive debonding taking place along the fiber/matrix interface caused by a decrease in the frictional sliding shear stress.
The micromechanical model enables the prediction of the fatigue life as a function of microstructural parameters, such as debond energy, interfacial friction sliding shear stress and fiber volume fraction. Model predictions will be compared with experimental results.
Prof. Bent F. Sørensen is head of the section "Composites and Materials Mechanics at the Department of Wind and Energy Systems, Technical University of Denmark (DTU). He got his MSc in 1988, his PhD in 1993 and obtained a Dr. Techn. degree in 2010, all from DTU. He worked at the Materials Research Department of the former Risø National Laboratory from 1988 and transferred to the new-established DTU Wind and Energy Systems Department from 2012. His major research area is fatigue, fracture and cohesive laws of composite materials and layered materials, dealing in particularly with theoretical and experimental research related to mechanical properties of interfaces at the micro- and macroscale.
Dr. Alexander Krimmer
Senior Engineer - Composite Materials and Structures at TPI Composites Germany GmbH
PREDICTION OF DAMAGE INITIATION FOR FULL SCALE FATIGUE TESTING OF A WIND ROTOR BLADE STRUCTURE
Today, rotor blades for wind energy converters are subjected to full scale fatigue tests for structural verification. These are usually interpreted to reflect the rotor blade’s service life. This is a misinterpretation since complex loading combinations cannot be covered when either carrying out separate edgewise and flatwise tests or even do combined biaxial testing. At the same time the blade test is a significant investment that is not yet used to its full potential. This work outlines a method to predict fatigue damage initiation for rotor blade structures under testing loads. Further, it provides a method of interpretation to derive a big number of data points for validation of model assumptions for fatigue life prediction of rotor blade structures. This is enabled by using the rotor blade structural model as transferring function from global loads into ply stresses. When now the structure is subdivided into shell elements, each of the elements shows an individual 3D stress state and is subjected to the same load cycle number. The applied fatigue life prediction model then predicts the load cycle number at damage initiation which is validated using the full-scale blade test. During fatigue testing, different inspection load cycle numbers must be specified. When inspected at these load cycle numbers, different states from “undamaged” over “damage initiated” showing low crack density up to “severe damage” meaning high crack density in the elements can be observed. These observations build up a damage-heat-map for elements that develops as a function of load cycle number. This is used for the validation of the fatigue life prediction model. Subsequently, when validated, the model can be used for prediction of the fatigue behavior throughout service life under fatigue design loads.
Dr. Alexander Krimmer specialized into Fiber Reinforced Plastics (FRP) during his studies on aeronautical engineering at the Technical University Berlin. After joining EUROS company as a structural designer of rotor blades for wind energy converters, in parallel he was working as composites material engineer and on his Phd-thesis. From 2010 on he worked as materials engineer, providing consulting to structural engineers. After finishing his Phd-thesis in 2013 he joined the TU Berlin as a visiting professor in composite lightweight design. In 2017 he switched back to EUROS company acting as an external lecturer in composite lightweight design at the TU Berlin. Today, Alexander is working as Senior Engineer Composite Materials and Structures for TPI Composites.