Influence of matrix properties on fatigue behavior of fiber reinforced polymers
Final Report Abstract
A constituent-based analysis of the fatigue performance and damage initiation of unidirectional reinforced composites is an approach drastically limiting the number of variables to essentially three. The basis for a constituent based description of the fatigue performance is a detailed understanding of the role of each of the constituents. This project aimed at filling the gap identified for the matrix polymer by a systematic study of loading scenarios with the matrix polymer taking a prominent role either by initiating damage or by participating in the load redistribution within the laminate. For this purpose transverse tension-tension, on-axis compression-compression loading and shear loading were investigated. A basic prerequisite for a systematic study is a modification of the polymer properties. This was achieved by irradiation treatment. The method is applicable to both neat polymer specimens and laminates alike and keeps most other properties unaffected. At the beginning of the project, six polymers have been identified as potential candidate materials susceptible to irradiation-induced changes: three polymers based on cyanate ester resin, one bisphenol-A based epoxy resin, a polycarbonate and one thermoplastic polyurethane. Irradiation induced changes are hard to predict and therefore an initial screening helped for the selection of two polymers. In a second experimental series, laminates reinforced with carbon and glass fibers are studied along with additional experiments on the neat polymer specimens. Both, laminate and neat polymer specimens were fatigue tested. In addition, the polymers were characterized by monotonic properties including the pressure dependence, transient properties, irradiation induced relaxation and fatigue crack resistance. A newly developed mechanism allowing the observation of the specimen’s edge during fatigue testing allowed the evaluation of strains within the transverse ply during loading. From these observations and additional experimental investigations, several aspects could be identified as main factors for damage initiation: fiber-matrix adhesion, the geometric fiber arrangement within the plies and auxiliary elements like stitching fibers. These factors mainly dictate the failure location. The number of cycles for crack initiation and subsequently the resulting crack density after a fixed number of cycles could be shown to depend on a combination of residual stresses, fracture toughness and the number of cycles for crack initiation within the matrix polymer. A mesoscopic FE-model accounting for the inhomogeneous fiber distribution and the polymer relaxation could show that locally different stress relaxation can lead to a more homogenous stress state and unload fiber rich regions. In addition, it could also be shown that the time-dependent matrix properties directly relate to the on-axis compressive fatigue strength.
Publications
-
“Irradiation induced crosslinking in thermoplastic polyurethanes for structural applications,” presented at the 23. Symposium Verbundwerkstoffe und Werkstoffverbunde, Leoben, Austria, Jun. 2022.
A. Baumann & J. Hausmann
-
“Correlating composite fatigue to its matrix properties,” presented at the Fatigue 2022+1, Hiroshima, Japan, Nov. 2023.
A. Baumann & J. Hausmann
-
“Effect of high energy radiation on technical polymers,” presented at the Proceedings 60th ISC, Ilmenau: ilmedia, 2023.
A. Baumann & J. Hausmann
-
In Situ Microscopy of Fatigue-Loaded Embedded Transverse Layers of Cross-Ply Laminates: The Role of an Inhomogeneous Fiber Distribution. Journal of Composites Science, 8(9), 366.
Baumann, Andreas; Duhovic, Miro & Hausmann, Joachim
-
Effect of high-energy radiation on the relaxation of residual stresses in polycarbonate and epoxy resin by stress optics. Radiation Physics and Chemistry, 226, 112236.
Baumann, Andreas & Hausmann, Joachim
