Abstract
In continuous fiber-reinforced thermoplastics, the macroscopic failure mode in transverse long-term failure is dominated by a brittle crack-growth mechanism. Neat thermoplastic matrices, on the other hand, generally display also a plasticity-controlled mechanism in long-term loading at elevated stress levels and/or temperature. This failure mechanism requires a different approach to lifetime prediction than crack growth; hence, it is important to identify it in the long-term performance of composites. In this study, we demonstrate the presence of the plasticity-controlled failure mechanism in long-term failure of transversely loaded unidirectional (UD) thermoplastic composites made of glass/iPP, carbon/PEEK and carbon/PEKK. The main method used is to compare the lifetime in cyclic loading to that in static loading at the same level of maximum stress, where an increase in lifetime is characteristic for plasticity controlled failure, and, vice versa, a decrease is indicative for fatigue crack growth. In addition, the applicability of a lifetime prediction method common to plasticity-controlled failure of neat thermoplastics is evaluated for the composites investigated. The results of this study indicate that the plasticity-controlled failure was present in composites, although the extent to which the effects are present varied depending on the materials investigated. Glass/iPP showed the most explicit evidence of the plasticity-controlled failure over the entire load range experimentally covered. Its long-term failure was delayed with a decrease in the stress ratio and lifetime was predicted well using the principles of plasticity-controlled failure.
Highlights
Thermoplastic composites are often used in engineering applications where they need to satisfy long lifetime requirements under creep and fatigue loading
First, the applicability of the lifetime prediction method for the plasticity-controlled failure of neat thermoplastics to the composites will be discussed. This will require a comparison of the short- and long-term behavior of the composites
In the “Background” section, we introduced a lifetime prediction methodology for plasticity-controlled failure which was based on identical yield conditions in constant-strain-rate and creep tests
Summary
Thermoplastic composites are often used in engineering applications where they need to satisfy long lifetime requirements under creep and fatigue loading. Lifetime prediction methods can be used that are based on time-temperature superposition, time-stress superposition, and rate theory, where temperature and/or stress are used to accelerate the failure.[3,4,5] In long-term cyclic loading, the estimation of the lifetime of composite laminates is a little more challenging. Where e_ 0 is a rate factor, DU is the activation energy of the molecular relaxation mechanism, và is the activation volume, T is the absolute temperature, k is the Boltzmann’s constant and R is the universal gas constant. Equation (1) accounts for multiple factors that affect the development of the plastic flow rate. The Arrhenius term and the sine hyperbolic part cover the temperature and stress dependence of the plastic flow rate, respectively. Equation (1) can be rearranged to obtain the stress r at yield in a constant-strain-rate test in terms of the applied strain rate e_ :
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
