Session: 03-07-01 Materials for Extreme Environment I

Paper Number: 107323

107323 - Flexural Fatigue Behavior of Sea Water Conditioned Carbon/epoxy-Nanoclay Composites 

The choice of fiber reinforced plastic (FRP) composites for marine structures over metals is driven by considerations of lowering the overall weight, the possibility to manufacture complex shapes without expensive tooling with a smaller number of parts, excellent fatigue and corrosion resistance properties. Researchers have noted that carbon fiber reinforced plastic (CFRP) composites show excellent fatigue performance in tensile loading where fiber are more dominant in withstanding and transferring the applied load. However, the challenges for composite materials used in marine environment include the long exposure time to moisture, temperature, numerous ionic species as well as the microorganisms. In transverse loading conditions, interlaminar characteristics directly control the fatigue performance of the laminated composites. Therefore, study of fatigue behavior in flexural loading with detailed life analysis is important to accurately understand the transverse fatigue behavior of the CFRPs. Under transverse loading, cracks initiate in the matrix phase in between the fibers, leading to fiber-matrix debonding and delaminations resulting in overall degradation of stiffness and load bearing capability of the composites. Researchers have been investigating ways to achieve improved transverse properties of the CFRPs by improving the fracture toughness of the matrix and achieving stronger fiber-matrix interfacial bonding. Addition of nanoparticles in the epoxy matrix has been considered to be a promising and potential way to achieve improved transverse mechanical properties of CFRPs. Incorporation of nanoparticles increase the fracture energy of the epoxy matrix by means of crack deflection and reducing stress concentration at the crack tip. In addition, surface modified nanoparticles due to their active functional groups and outstanding surface to volume ratio increase fiber-matrix interfacial bonding.

In this research work, main focus was on investigating the static and fatigue behavior of CFRP composites subject to seawater and cold temperature conditioning by incorporating nanoclay, which has demonstrated better durability. The flexural fatigue behavior of CFRPs with respect to stiffness degradation, fatigue life prediction and 3D damage mode with addition of graphene and nanoclay as matrix filler is investigated. Three-point static flexural tests were conducted to measure the elastic modulus and failure strength/deflection of the composites. Load-controlled flexural fatigue tests in the three-point bend mode were performed at different stress levels; 0.9, 0.8, 0.75 and 0.7 times the static strength of the corresponding samples (baseline neat, and nanophased), to determine the fatigue life and stiffness degradation, respectively. Fatigue life was analyzed and predicted as a function of failure probability using Weibull distribution function  and Kolmogorov-Smirnov test. Finally, to investigate the fatigue failure modes optical microscopy (OM) and scanning electron microscopic (SEM) analyses were performed. The fatigue life was reliably predicted by means of combined Weibull and Sigmoidal model. It was found that nanoclay added samples exhibited more than 327% improvement in the mean fatigue life and more than 352% improvement in the predicted fatigue life (P_f = 0.5), compared to the control samples. At 0.7 stress level, all the nanoclay added samples passed the ‘run-out’ fatigue criteria of one million cycles, whereas majority of the control samples failed much earlier. Stiffness degradation model and static test after a pre-determined number of fatigue cycles indicated that addition of nanoclay significantly improves the residual fatigue properties of the CFRPs. Conditioning (6 months seawater at ambient and -20°C temperatures) effect on the fatigue performance of the CFRP samples showed that both control and nanoparticles added samples demonstrated lower fatigue life in comparison with corresponding unconditioned samples.

Presenting Author: Mahesh Hosur Texas A&M University-Kingsville

Presenting Author Biography: Mahesh Hosur received his education from India with a Bachelor of Engineering (B.E.) degree in Civil Engineering from Karnataka University (1985), Master of Technology (M. Tech.) degree in Aeronautical Engineering from Indian Institute of technology, Bombay (1990), and Doctor of Philosophy (Ph.D.) in Aerospace Engineering from Indian Institute of Science, Bangalore (1996). He worked as Scientist for one year in Aeronautical Development Agency before coming to the USA. After serving Tuskegee University for 21 years, he joined Texas A&M University-Kingsville (TAMUK) in his current position in August 2018. His teaching and research interests are in advanced composite materials, nondestructive evaluation, experimental and structural mechanics. Over last 25 years, He has led research efforts of nearly $36.5 M as Principal Investigator and over $40 M as Co-Principal Investigator. He has graduated 12 Ph.D. and 37 M.S. students and advised over 50 undergraduate students besides mentoring junior faculty members. He has authored or coauthored 4 books, 7 book chapters, 125 refereed articles in journals and over 220 refereed articles in conference proceedings besides numerous technical reports. He has received honors which include recognition as a Fellow of American Society for Mechanical engineers, Faculty Achievement Award at Tuskegee University and Russell Brown Award from Tuskegee University Sigma Xi Chapter.

Authors:

Md. Sarower Tareq Michigan State University
Mahesh Hosur Texas A&M University-Kingsville
Shaik Zainuddin Tuskegee University