TY - BOOK

T1 - Fatigue of continuously fibre reinforced composites - Engineering approaches to fatigue-life prediction

AU - Brunbauer, Julia

N1 - no embargo

PY - 2015

Y1 - 2015

N2 - The climate change, which has been significantly accelerated by the high amount of carbon dioxide emissions in the last decades, is one of the great challenges of the 21st century. Possibilities to meet the aspired aims and reduce the produced emissions are e.g. the increase of renewable energy fraction of the totally consumed energy or the conversion of big emission causers such as transportation. However, to make new airplanes or new drive concepts in cars as efficient as possible the reduction of the total weight is of essential importance. Carbon fibre reinforced plastics offer great possibilities in these new applications due to their outstanding mechanical properties in relation to weight. If composite materials are intended to be applied in highly-stressed applications, the assurance of mechanical properties and mechanical behaviour is of tremendous importance. This includes not only the experimental testing of the material but also predictive calculations. The high performance structures in which composites are most superior are those in which fatigue is likely to occur. Consequently, for life-time prediction it is not only important to define the mechanical loads which can be applied to structural parts but also to answer the question of how long the material will last under the contemplated conditions. The objective of this thesis was to investigate possibilities for application-oriented fatigue-life prediction approaches for continuously carbon fibre reinforced composites. In order to be able to build predictions on sufficient experimental data, in preliminary tests different strain measurement techniques and the effects of experimental test parameters on the measured material properties were assessed. Test set ups for the subsequent material characterisation were defined. Furthermore, it was important to study the unique fatigue-induced damage parameters in detail. Apart from the significant influence of anisotropy and mean stress on the mechanical properties and the damage mechanisms, the effect of fibre volume content was investigated. To be able to consider the strain-rate dependent material behaviour measured in the mechanical tests, a new experimental test procedure was developed. By implementing this procedure in fatigue tests, a comprehensive mechanical data base reflecting fatigue stiffnesses and strengths of carbon/epoxy materials with different stacking sequences and tested at various angles was created. Two different approaches towards fatigue-life prediction were pursued in this thesis. Both approaches might be considered as engineering approaches, since micro-mechanical effects and damage mechanisms are not taken into account explicitly but are already included in the experimentally measured input parameter. First, the classical laminate theory, which was initially developed for quasi-static loads, was adapted for fatigue loads. By implementing the strain-rate independent stiffness properties of unidirectional carbon/epoxy laminates, the fatigue-induced stiffness decreases of multidirectional carbon/epoxy composites were calculated on different load levels. Result proved the good correlation between calculation and experiment. Second, a newly adapted strength-based approach known from classical metal fatigue implemented in a commercially available software tools was used. The advantage of software tools, which are in use for many years now, is the possibility of including load time history or complicated geometries by finite element analysis. The S-N curves of unidirectional laminates reflecting the anisotropy and the effect of fibre volume content were evaluated at different mechanical mean stresses. This input parameter set was used to calculate the fatigues strength of a unidirectional and a multidirectional composite. Predictions showed good correlations with the experimental results. In addition, the necessity of further develo

AB - The climate change, which has been significantly accelerated by the high amount of carbon dioxide emissions in the last decades, is one of the great challenges of the 21st century. Possibilities to meet the aspired aims and reduce the produced emissions are e.g. the increase of renewable energy fraction of the totally consumed energy or the conversion of big emission causers such as transportation. However, to make new airplanes or new drive concepts in cars as efficient as possible the reduction of the total weight is of essential importance. Carbon fibre reinforced plastics offer great possibilities in these new applications due to their outstanding mechanical properties in relation to weight. If composite materials are intended to be applied in highly-stressed applications, the assurance of mechanical properties and mechanical behaviour is of tremendous importance. This includes not only the experimental testing of the material but also predictive calculations. The high performance structures in which composites are most superior are those in which fatigue is likely to occur. Consequently, for life-time prediction it is not only important to define the mechanical loads which can be applied to structural parts but also to answer the question of how long the material will last under the contemplated conditions. The objective of this thesis was to investigate possibilities for application-oriented fatigue-life prediction approaches for continuously carbon fibre reinforced composites. In order to be able to build predictions on sufficient experimental data, in preliminary tests different strain measurement techniques and the effects of experimental test parameters on the measured material properties were assessed. Test set ups for the subsequent material characterisation were defined. Furthermore, it was important to study the unique fatigue-induced damage parameters in detail. Apart from the significant influence of anisotropy and mean stress on the mechanical properties and the damage mechanisms, the effect of fibre volume content was investigated. To be able to consider the strain-rate dependent material behaviour measured in the mechanical tests, a new experimental test procedure was developed. By implementing this procedure in fatigue tests, a comprehensive mechanical data base reflecting fatigue stiffnesses and strengths of carbon/epoxy materials with different stacking sequences and tested at various angles was created. Two different approaches towards fatigue-life prediction were pursued in this thesis. Both approaches might be considered as engineering approaches, since micro-mechanical effects and damage mechanisms are not taken into account explicitly but are already included in the experimentally measured input parameter. First, the classical laminate theory, which was initially developed for quasi-static loads, was adapted for fatigue loads. By implementing the strain-rate independent stiffness properties of unidirectional carbon/epoxy laminates, the fatigue-induced stiffness decreases of multidirectional carbon/epoxy composites were calculated on different load levels. Result proved the good correlation between calculation and experiment. Second, a newly adapted strength-based approach known from classical metal fatigue implemented in a commercially available software tools was used. The advantage of software tools, which are in use for many years now, is the possibility of including load time history or complicated geometries by finite element analysis. The S-N curves of unidirectional laminates reflecting the anisotropy and the effect of fibre volume content were evaluated at different mechanical mean stresses. This input parameter set was used to calculate the fatigues strength of a unidirectional and a multidirectional composite. Predictions showed good correlations with the experimental results. In addition, the necessity of further develo

KW - Fatigue

KW - Carbon fibre reinforced Plastics

KW - mechanical testing

KW - fatigue damage mechanisms

KW - fatigue-life prediction

KW - Laminate mechanics

KW - Ermüdung

KW - kohlenstofffaserverstärkte Kunststoffe

KW - mechanische Prüfung

KW - Schädigungsmechanismen

KW - Lebensdauervorhersage

KW - Laminatmechanik

M3 - Doctoral Thesis

ER -