Failure analysis of aging in polyoxymethylene fuel valves using fractography and thermal-FTIR analysis. (September 2017)

• Record Type: Journal Article
• Authors: Aronson, Ayal; Tartakovsky, Konstantin; Falkovich, Reuven; Rabaev, Moshe; Gottlieb, Moshe; Kozlovsky, Alex; Levi, Ofer
• Is Part Of: Engineering failure analysis. Volume 79(2017)
• Page Start: 988
• Page End: 998
• Publication Date: 2017-09
• Subjects: TGA -- FTIR -- Polyoxymethylene -- Aging -- Degradation -- Fractography -- Thermal analysis; System failures (Engineering) -- Periodicals; Fracture mechanics -- Periodicals; Reliability (Engineering) -- Periodicals; Pannes -- Périodiques; Rupture, Mécanique de la -- Périodiques; Fiabilité -- Périodiques; Fracture mechanics; Reliability (Engineering); System failures (Engineering); Periodicals; Electronic journals; 620.112
• DOI: 10.1016/j.engfailanal.2017.06.038
• Languages: English
• ISSNs: 1350-6307

Abstract

Abstract: Polyoxymethylene (POM), a thermoplastic polymer used in precision parts, is popular for its high stiffness, low friction, and resistance to various working fluids. The polymer also presents unique chemical degradation and aging mechanisms. An investigation into commercial POM-made fuel valves (commercial, heat-stabilized copolymer), which failed in similar ways after over a decade of service in an aviation system, is presented and compared to an unused valve of the same type and material, with failures induced by overload. A comparison between fracture surfaces imaged by SEM microscopy, of POM failure as a result of overload or as result of ten years aging is shown for the first time. No definitive differences were found between aged and new POM material based on Differential Scanning Calorimetry (DSC) and direct Fourier Transfer Infra-Red (FTIR) measurements. However, a significant difference in decomposition profiles was found according to Thermogravimetric Analysis coupled with FTIR (TGA-FTIR), accompanied by release of formaldehyde, indicative of depolymerization in the decade-aged POM. Other mechanisms, such as overpressure failure, effects of exposure to gasoline, increased crystallinity, intermolecular rearrangements or internal hydrolysis were refuted. The failure mechanism is attributed to aging depolymerization in the bulk, accompanied by static stress. Highlights: Failure investigation of POM-made avgas after over a decade of service was provided. The failure mechanism was attributed to depolymerization in the bulk material. A significant difference in decomposition profiles was found by TGA-FTIR. No definitive differences were found by DSC and direct FTIR measurements.