Fundamental study on environmental stress cracking effects in polymer nanocomposites
Final Report Abstract
The joint project of the Chair of “Composite Engineering” in Kaiserslautern and the Chair of “Polymer Engineering” in Bayreuth aimed to determine possible relationships of static and dynamic ESC investigations. Furthermore, the effect of nanoparticles on the ESC resistance was investigated. This was realized by using different PMMA materials (8N, 8H) and nano-scaled fillers (1D (Halloysite nanotubes), 2D (layered silicate), and 3D (nano SiO2 particles)) varying from 0.5 vol.-% to 4 vol.-%. In addition, the SCA were chosen by means of HSP (DI, EG, NH). The compounded and injection molded PMMA nanocomposites were first characterized in terms of their thermal, optical and mechanical properties. It was shown that the addition of nanofillers led to: a) no significant change in the glass transition temperature and molecular weight, b) a large change in optical qualities depending on the geometry and amount of the nanofillers. The 3D nano silica was observed to retain the best qualities, c) led to a degradation of the mechanical performance (tensile properties, charpy properties, fracture toughness). ESC behavior in both investigated systems have shown a general decrease in K1c with the addition of nanofillers. This behavior was independent from the filler geometry. Furthermore, the ESC susceptibility in combination with the used SCA was ranked as follows K1c, NH < K1c EG < K1c DI. The application of a static loading method interestingly shows a different fracture behavior when compared to the dynamically loaded samples. With the static method a discontinuous crack propagation followed by catastrophic failure was observed in all nanocomposite-SCA combinations. This discontinuous crack propagation results in a fracture surface, characterized by several resting lines. In da/dN investigations however, these resting lines were only observed during the first phase of the experiment. A comparison between the resulting dynamic and static K1c values reveal a similar trend in DI water. To obtain a first impression of the ESC behavior, static investigation can be conducted due to the fact, that the dynamic approach is usually coupled with technical complexity and expensive equipment. Nevertheless, an accurate model for the transfer of results is not possible due to the different macroscopic fracture mechanisms. Previous experiments in Polycarbonate have shown similar fracture surfaces despite of loading types and thus exhibit a more suitable behavior in regard to transferability. This means, that next to the loading type, the material is also crucial regarding a possible model transfer. For future attempts it has to be considered, that the choice of material and a similar fracture behavior independent from loading type may be a requirement for a possible successful model development. Nevertheless, the investigation of the influence of media on the crack growth behavior will be more important independent of static or dynamic loading. ESC is still one of the most prominent failure mechanisms for polymer components and most common reasons of lifetime shortening for plastic components. The high sensitivity of plastics in regard to environmental influences has always meant that plastics as a material have been viewed very critically in outdoor applications and medical applications, where influence of media plays the key role of the material behavior. Depending on the mechanical stress, which is induced to the plastic, the dynamic and static mechanical behavior plays an important role. Therefore, a more deep insight of the predication of the behavior during media contact and interaction and transfer from dynamic to static is on very interest. Here the Polymers, the SCA and other environmental conditions can be varied.
Publications
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Influence of Sample Wetting Method on ESC-Behavior of PMMA under Dynamic Fatigue Crack Propagation. Materials, 15(12), 4114.
Bubmann, Tobias; Shi, Simon; Brueckner, Alexander; Menzel, Teresa; Ruckdäschel, Holger; Schlarb, Alois K. & Altstädt, Volker
