Florian Arbeiter

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.

Numerical Modelling of Cylindrical Specimen under Mixed-Mode Loading Conditions

Mixed mode loaded fatigue specimens are not used very frequently. The main reason is that there are still very few qualitative results, which would help with better understanding of how the mode II and mode III are affecting the overall crack propagation. On the other hand, there is considerable number of parts loaded in mixed mode. Studying fatigue failure of roller bearing elements made of polymer material was a motivation to design a new experimental specimen for study of fatigue behaviour of the material loaded in mixed mode. The contribution presents developed FEM model of such specimen with focus on determination of fracture mechanics parameters of propagating cracks.

Fracture Mechanics Methods to Assess the Lifetime of Thermoplastic Pipes

When performing lifetime estimations using extrapolation concepts, it is vital to estimate the uncertainties which always accompany accelerated testing methods. Uncertainties may arise from deviating parameters such as changes in environmental conditions, temperature, different loading ratios, chemicals such as stress cracking agents, etc. Only when these influences are known it is justifiable to go into lifetime calculations. Own studies showed, that fracture mechanics extrapolation concepts for accelerated prediction of PE pressure pipes using short-time fatigue tests provide valid results when compared to pre-notched internal pipe pressure tests. The use of a cracked round bar (CRB) specimen for linear elastic fracture mechanic (LEFM) tests improves the results compared to classical compact tension (CT) specimens, which tend to overestimate lifetimes. This can mainly be attributed to bigger plastic zone sizes which restrict slow crack growth (SCG) in CT specimens. Another advantage is the similarity of constraint and K I-development between a pipe and CRB specimens. Summarising, the extrapolation concept using short-term fatigue tests on CRB specimens provides a valuable and valid tool to perform lifetime estimations for pipe systems made from high-density polyethylene (PE-HD) pipe materials. Further steps in the development of the approach using cyclic CRB Tests are currently under evaluation. For example, the implementation of influences due to media is an important addition, to be able to cover the area of media and crude oil transportation. The impact of crack growth initiation is also a topic which has yet to be addressed. So far it has often been neglected in lifetime estimations due to complex testing procedures. Also the use of the cyclic CRB Test for different polymeric pipe materials is currently examined. Seeing that only about a third of all thermoplastic pipes is produced from PE-HD material this is a logical next step. Besides lifetime estimation, the use of the cyclic CRB Tests at R = 0.1 is also discussed for ISO-standardisation for material quality control. Good correlations with established methods support its claim as a precise and fast ranking tool for PE-HD pipe grades.

The Effect of Soil Load on Fracture Behaviour of Three-Layer Polymer Pipe for Non-Pressurised Applications

Fracture behaviour of a three-layer polymer pipe subjected to nonhomogenous distribution of external pressure induced by soil embedding is studied in this paper. Both long term and additional short term loading is considered. Such loading induces tensile stresses in the inner pipe wall which can lead to crack initiation and further slow crack propagation. The material interface between a protective layer and the base pipe can contribute to crack deceleration and can prolong the residual lifetime of the pipe. The paper presents three-dimensional numerical analysis of a commercial three-layer pipe containing an internal semi-elliptical crack. The effect of soil load on the fracture behaviour of the cracked pipe is quantified and discussed.