The investigation of the stress cracking behavior of PBT by acoustic emission

Abstract

Despite being a major cause of failure of polymer products under service, environmental stress cracking (ESC) is still a phenomenon poorly investigated and, as consequence, not fully understood. This article aims to contribute to this field in a study of the stress cracking behavior of polybutylene terephthalate (PBT) exposed to ethanol (a common fluid, used in several daily situations), by means of two types of mechanical tests (tensile and stress relaxation). During mechanical testing the technique of acoustic emission (AE) was applied to identify certain mechanisms of internal failure.As a non-destructive technique, AE is widely used with metals to monitor the integrity of components but it is very scarcely applied to polymers. The article, therefore, innovates by using AE as a tool to better understand the ESC behavior of an engineering polymer produced under different conditions. The PBT samples were prepared by injection molding, using different mold temperatures (20 C, 40 C, and 80 C), resulting in different crystallinities. The results showed that the tensile properties were highly dependent on the crosshead speed and that ethanol affected the mechanical performance of the materials, anticipating the necking, but its evaporation during the experiment reduced the actual temperature on the sample bar. This latter effect caused an unexpected behavior, with an increase in tensile strength and reduction of stress relaxation rate, the opposite trends that would be expected from a stress cracking agent. The crystallinity of PBT had some influence on the results, with the more crystalline samples being more resistant to ESC, as observed by both stress relaxation and AE analyses. The technique of AE showed to be a very useful tool to understand the mechanical behavior and to differentiate the various testing conditions, with the detection and quantification of hits and released energy, related to the several stages of deformation and failure, including the formation of shear bands, nucleation and growth of crazes and cracks, and so forth. The fracture surface analyses by scanning electron microscopy were very consistent with the AE experiments, in which rough topography were related to high acoustic activities.

Embrittlement of polybutylene terephthalate induced by injection molding

“…One can also note from Fig. 2 b that the possible degradation induced by injection process does not significantly affect neither the Young's modulus nor the yield stress (NB: consistently with other recent examples illustrating the effect of crystallinity on PBT storage [16] or modulus average molar mass and crystalline ratio on POM yield stress [17] ).…”

Aminolysis of polyesters for cracking and structure clarifying: A review

“…In the subject of degraded polyester morphology determination, 110–145 several analytical techniques have been used by researchers to reveal the elements of the changes. These techniques include microscopy, gel permeation chromatography, X‐ray diffraction, spectroscopy, and so on.…”

“…Stress‐cracking behavior of PBT exposed to ethanol during tensile and stress relaxation test was examined and internal failure was determined. The tensile properties were dependent on the crosshead speed, ethanol evaporation, and the crystallinity of samples 133 . In a study of crack growth in PLA, in hydrolysis reaction, it is concluded that the crack velocity was insensitive to load but was sensitive to humidity and pH of the medium 134 .…”

“…The tensile properties were dependent on the crosshead speed, ethanol evaporation, and the crystallinity of samples. 133 In a study of crack growth in PLA, in hydrolysis reaction, it is concluded that the crack velocity was insensitive to load but was sensitive to humidity and pH of the medium. 134 The effect of NaOH solution as a stress-cracking agent on the properties of PET and PET/ZnO composites was evaluated, and the solutions of NaOH were applied during tensile testing.…”

Influence of Sample Wetting Method on ESC-Behavior of PMMA under Dynamic Fatigue Crack Propagation

“…The investigation of the ESC behavior under mechanical load can be carried out under static or dynamic load. Mechanical tests of thermoplastics under static loading, either tensile, bending, or creep tests, are widely known in the literature [11][12][13][14][15][16][17][18][19][20]. These investigations were either carried out on specimens previously immersed in the SCA [20] or during in situ wetting [10,17,18].…”