Official regulations concerning greenhouse gases and energy efficiency force the transport industry to lightweight actions. Consequently, the focus of component manufacturers is shifting more and more to lightweight materials, like polymers and therefore a deep knowledge about their load capacity for in different uses. To design the material, a number of publications and models have been published in the past. The main focus of these publications is on the behaviour of so-called construction polymers. Since nowadays also standard polymers are considered for structural composites, this thesis deals with the investigation of short-glass-fibre reinforced polypropylene. The investigated factors are fibre orientation and fibre content, mean stress, temperature as well as notch effect. Due to the applications of polypropylene mainly above the glass transition temperature, the focus is on visco-elastic behaviour. For this, injection moulded specimens and specimens extracted from plates have been tested. To investigate the influence factors, quasi -static (tensile tests), static (creep tests) and cyclic tests (fatigue tests) have been performed. Additionally, tests including a combination of cyclic loads and static loads (hereafter named as combined tests) have been performed, to get the influence of the visco-elastic behaviour on the lifetime. Thereby, the constant load levels are set to the maximum, minimum or mean stress of the cyclic loading. The constant load times have been varied between 0% (pure cyclic load), 25%, 50% and 100% (pure creep) according to the total test time. The results from static and cyclic tests show an influence of the fibre content on fibre orientation. There is a difference of the tensile strength of about 40% along with the plate for polypropylene with a fibre content of 50%. Polypropylene with a fibre content of 40% doesn't show this effect. Further there is an influence of fibre orientation on the notch effect. So, there is a more pronounced notch support effect with longitudinally oriented specimens compared to transversal oriented specimens. This effect reinforces with smaller notch radii. The combined tests with a constant load at maximum stress, shows a reduction of up to 90% in bearable load cycles, compared to pure cyclic loading. So, constant loads have an effect on time to failure, as well as the number of bearable cycles. Since this effect is more pronounced for time to failure, this should be considered in lifetime prediction. This is done by using time to failure curves, (supplementary to S/N curves), isochronous Haigh diagrams and a newly developed classification method. The tests show that the creep strength should be taken as the upper limit for the mean stress. A method developed in this thesis makes it possible to consider static loads, depending on the load duration and load level in lifetime prediction. The developed, time dependent classification offers the possibility to classify constant loads and strain rates of stochastic loads in the lifetime prediction.
|Translated title of the contribution
|Consideration of viscoelasticity in the fatigue strength analysis of short-fibre reinforced plastics
|Published - 2021
Bibliographical noteembargoed until 26-08-2026
- short-fibre reinforced polymers
- fatigue tests
- operational strength