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Abstract
Over the years, unplasticised PVC (PVC-U) has been extensively used in the manufacturing of plastic pipes. However, in spite of offering many advantages, PVC-U pipes have been found to fracture in a brittle manner. A significant improvement in the fracture toughness properties of PVC-U pipes has been achieved by the addition of impact modifiers. This increased the use of the modified PVC materials (PVC-M), particularly in pressurized water applications. Nevertheless, since the pipes are subjected to cyclic loads during service, a potential failure of the pipes appears due to fatigue fracture of the pipe material. Therefore, in designing pipes with improved durability, it is imperative to understand the response of PVC and modified PVC to cyclic loading. Since the PVC pipes are used for transporting water, they may behave in a different manner in water and in air environments. Thus, the effect of the environment on the fatigue behavior of PVC-U and PVC-M also needs to be investigated.
Modification of PVC-U with nanoparticles (i.e., CaCO3) in addition to the impact modifiers has also shown to provide promising property improvements with regards to various applications. Furthermore, improving the toughness with the addition of nanoparticles under static loading has been well researched; however, the effect of cyclic loading on fatigue properties has received limited attention.
This study was undertaken to address the issues of fracture and fatigue of PVC-U and toughened variations. In this work, fatigue crack growth rates studies were conducted for two sets of materials: (1) PVC-U (monolithic) and PVC-M in air and water environments, and (2) PVC-U and PVC-CaCO3 nanocomposites in air. The PVC-M comprised of 6 pphr of chlorinated polyethylene (CPE) as an impact modifier. The fatigue tests were conducted at different frequencies (1, 7 and 20 Hz) and different R-ratios (0.1, 0.2 and 0.6), and the cyclic fatigue threshold values (ΔKth) of both PVC materials were determined at the respective frequencies. For the nanocomposite samples, the CaCO3 content was varied from 3 pphr to 20 pphr, and similar fatigue tests with similar parameters were conducted. In addition, the effect of a titanate coupling agent with 0.6 pphr on the fatigue behavior of nanocomposites was also evaluated. Crack growth mechanisms related to their morphologies were examined with light optical and scanning electron microscopes.
The frequency was found to considerably affect the fatigue crack growth rate and the fatigue threshold (ΔKth) value in both materials, regardless of the testing environment. A slight difference in the crack growth rates of PVC-M and PVC-U was observed at lower stress intensity factor amplitudes (below ΔK=1MPa.m1/2) which is associated with the presence of CPE particles. It was also found that the fatigue resistance in water is higher than in air. However, the benefits seen in water deteriorated under conditions of higher stress intensity factor amplitude (ΔK) and frequency.
Fractographic surface analysis revealed that the basic fatigue fracture mechanism of PVC is unchanged even in the presence of CPE; the formation of craze structures. The absorption of water into the PVC matrix was evident in a water environment which leads to the formation of nodular and plasticized structures at low and high ΔK. Nevertheless, in an air environment, formations of those structures were absent. The formation of a stretch zone in PVC-M at a high R-ratio resulted in an insignificant increment in the crack growth rates at all respective R-ratios.
The trend of fatigue behaviour of PVC is consistent even in samples that had been modified with nano-CaCO3 particles, i.e. no significant deterioration in the fatigue resistance was observed compared to PVC-U. Similar results were obtained in the PVC nanocomposites that added with coupling agent. Variation in the frequency and the R-ratio levels was found to result in lower crack growth rates at higher frequencies and higher crack growth rates at higher R-ratios. Fracture surface observations showed that the fatigue fracture in the PVC nanocomposites occurred through different mechanisms; that are, through particle debonding and ligament yielding of the PVC matrix.
Analysis of the crack advance mechanism in all samples (PVC-U, PVC-M, PVC nanocomposites) by microscopic observations at the crack tip process zone indicated that shear bands were not formed. Therefore, it is emphasized that craze formation and failure is the primary fatigue facture mechanism in PVC pipes, irrespective of the nature of addition phases to enhance toughening.