Steffen Stelzer

Stiffness Based Fatigue Characterisation of CFRP

This paper investigates the possibility of determining damage under fatigue loading in carbon fibre reinforced plastics (CFRP) by using mechanical stiffness. Therefore, stress-strain-hysteresis recorded in fatigue tests under sinusoidal loads are used for moduli calculation. Additionally, a new method for stiffness evaluation called cyclic tensile tests is presented. Its results are compared to results from hysteresis analysis and to actual damage mechanisms monitored non-destructively with thermography analysis.

Composite-Composite Joining with through the Thickness Reinforcements for Enhanced Damage Tolerance

A novel composite-composite joining technology based on metal pins oriented in through thickness direction of the composites is presented. A defined pin geometry, which is capable of establishing a through-thickness form-fit connection between composites and the metal reinforcement, is created on thin metal sheets in an automated pin production process. Based on numerical simulations of the fracture of unreinforced single lap shear (SLS) composite specimens, optimum locations for the pin reinforcement were found. Tests on reinforced SLS specimens proved that an enhanced damage tolerance can be achieved by the use of cold metal transfer welded pins (CMT pins) as through-the-thickness reinforcement of the joint area. This paper investigates the mechanisms responsible for the load transfer and failure of such through-the-thickness reinforced composite-composite joints during monotonic loading.

Fracture mechanical characterization of mica-filled epoxy glass composites under monotonic and cyclic loading

Stator insulations comprised of mica-filled epoxy glass composite materials are of paramount importance for the reliability of high-voltage rotating machines. The present work deals with the fracture mechanical characterization of the winding insulation under conditions of monotonic and cyclic loading. The identification and quantification of the weak interfaces in the material that will most likely result in the initiation and propagation of defects are investigated in detail. Material specimens are processed from insulation tapes, and tests are conducted under mode I, mode II and mixed mode (Fixed Ratio Mixed Mode) loading to characterize the weak interface. The influence of resin content on the strength of the weak interface is also investigated. The results give an indication of the delamination mechanism and a measure of the critical energy release rate in the insulation materials.

HybridRTM - Quality Controlled Manufacturing of Hybrid Material Composites through Resin Transfer Moulding

HybridRTM terms a publicly funded project, which aims at the development of a processing technique for manufacturing of light weight structural components from hybrid materials. In particular, components involving metal as well as fibre-reinforced polymer composite materials are manufactured in a single processing step by means of the resin transfer moulding (RTM) technique. Project activities include material development and characterization, modelling of thermally induced residual stresses, process simulation, mould development as well as model-based process control in order to ensure consistently high component quality. This paper outlines the fundamental idea of the project and summarizes the most important results gained during the first two years of project activities.

Fatigue Delamination Growth in CFRP Composites: From Pure Mode I and Mode II to Mixed Mode I/II

For composite design, it is desirable to have data covering the failure envelope from Mode I to Mode II. The existing standard procedures for quasi-static testing (ISO 15024 for Mode I and ISO DIS 15114 for Mode II) have recently been shown to be adaptable for the respective fatigue tests under displacement control. The Calibrated End-Loaded Split (C-ELS) test set-up developed for Mode II further allows performing a Fixed-Ratio Mixed Mode I/II (FRMM) test by simply inverting the loading direction compared to mode II.