Structural composite fatigue life prediction based on dynamical mechanical analysid

Author(s)

Thatiane Brocks

• Document type: Doctoral Thesis
• Press: Guaratinguetá. 2016-01-26.
• Institution: Universidade Estadual Paulista (Unesp). Faculdade de Engenharia. Guaratinguetá
• Defense date: 2015-12-18
• Advisor: Maria Odila Hilário Cioffi; Herman Jacobus Cornelis Voorwald

Abstract

The knowledge of composite properties, especially in the structural application case, have been considered extremely necessary in order to support the transportation industry into apply these materials. It is widely known that 90% of failures can be attributed to cyclic loading, so the study of fatigue effects on composites behavior and failure mechanisms become extremely necessary. Recent studies have shown that mechanical and thermal analysis techniques association is very usefull in order to determinate mechanical behavior of polymeric composites since the polymeric chain relaxation effects over that are also considerate. The dynamical mechanical analyzes (DMA) is the most sensitive technique for detection of molecular motion caused by temperature and/or frequency incidence over polymers, reason that classify the DMA as ideal to get results in order to relate with fatigue results in this study. The fatigue-DMA results correlation is not very explored because the difficulties involved in associates viscoelastic and mechanical properties, so, the originality of this study is define a correlation between fatigue and DMA results able to contribute with a carbon/epoxy composite fatigue life prediction. This work development has consisted initially in the composite laminates manufactured by RTM (resin transfer molding) process, using a one-part aeronautical epoxy resin (Prism EP2400 - Cytec) as matrix and a quadriaxial (0°/+45°/-45°/90°) carbon non crimp fabric (NCF) as reinforcement, stacked to reach a fiber volume fraction up to 50%. The manufactured composite was characterized regarding impregnation quality and thermal stability and using the DMA (dynamical mechanical analyses) in a isothermal form between -70 °C and 220 °C, at each 10 °C, for frequencies of 0,01; 0,05; 0,2; 0,5; 1; 5; 28; 40 and 100 Hz. Results obtained were used to plot the TTS (time-temperature superposition) curves for E’, E” and Tanδ for both, matrix and composite. These data were used to plot the interfacial strength TTS curve for three references temperatures: 0 °C, room temperature (~25 °C) and 80 °C. The composite was also tested under fatigue in the same three references temperature. During the fatigue tests the Young´s Modulus decreasing were monitored, allowing determine the delamination starts for each load applied on each test temperature. Results reveals that the composite fatigue resistance decreases with the temperature rise due to matrix ductility increasing which provides the cracks onset, followed by delamination process. Quasi-static flexural tests were carried out after fatigue tests, since the failure criterion adopted did not allow the samples failure. This fact allowed the residual flexural resistance ascertainment after fatigue. Fractures were observed using a scanning electronic microscope (SEM) and showed that highest temperatures induces highest deformations in the matrix, corroborating with the fatigue test results. Among the DMA results, the E’ TTS curve have showed the best correlation with the composite fatigue behavior due to the delamination resistance tendency showed and due to this it was the dynamical mechanical parameter choose to be related with the composite fatigue life. Thus, this work proposes a relation between E’ and the composite fatigue life as a time function for all of the three temperatures as a prediction fatigue life method. (AU)

• FAPESP's process: 11/11311-1 - Structural composites fatigue life prediction based on dynamic mechanical analysis
• Grantee: Thatiane Brocks
• Support Opportunities: Scholarships in Brazil - Doctorate