Long‐term failure of transversely loaded glass/iPP

Abstract

Herein, temperature‐dependent long‐term behavior of polypropylene and its transversely loaded unidirectional glass fiber reinforced composite is investigated and a lifetime prediction method is proposed, which is based on the observed long‐term failure mechanisms. Furthermore, the effect of cooling rate during processing on the time‐dependent behavior is addressed. The composite is revealed to exhibit multiple molecular deformation mechanisms, similar to neat polypropylene, which is modeled using the Ree–Eyring approach. Failure kinetics under constant‐strain‐rate and creep tests are found to be identical and switching from creep to cyclic loading decelerates the failure, which are signs of plasticity‐controlled failure. Hence, lifetime is predicted well by using a lifetime prediction methodology for the plasticity‐controlled failure which combines the Ree–Eyring approach and the concept of critical strain. A change in the cooling rate alters the deformation and failure kinetics: lower cooling rates promote embrittlement.

Time‐dependent failure of off‐axis loaded unidirectional glass/iPP composites

“…Similar to the data at = 90°, the application of cyclic loading at = 45° also results in a longer lifetime at the same maximum stress compared to creep loading. A longer lifetime under fatigue loading was associated with the plasticity‐controlled failure following the previous research of Kanters et al 12 on short fiber‐reinforced composites and our work on transversely loaded UD continuous fiber‐reinforced composites presented in References 1 and 10. Hence, this observation shows that plasticity‐controlled failure dominates the failure of off‐axis loaded glass/iPP over a large time range, in addition to the previous observations on the identical failure kinetics in constant‐strain‐rate and creep tests and the applicability of the concept of critical strain.…”

“…Observing that creep stress dependence of the plastic flow rate and strain‐rate‐dependence of the tensile strength were identical, Bauwens‐Crowet 38 postulated that the steady‐state reached in creep tests of glassy polymers during plastic flow is identical to the steady‐state at yield in constant‐strain‐rate tests. We showed in References 1 and 10 that the same condition holds also for glass/iPP loaded at = 90°. To investigate if constant‐strain‐rate and creep tests are related also at the other off‐axis angles, stress‐dependent plastic flow rates obtained from the Sherby‐Dorn plots for = 10°, 30°, and 45° are superimposed on the plot of the tensile strength versus logarithm of the applied strain rate, as presented in Figure 9a.…”

“…This trend is plotted in Figure 8b, which shows that the dependence of the critical strain on the off-axis angle can be described by a linear relationship for the range of off-axis angle we are interested in. Hence, for the lifetime prediction, the fit F I G U R E 6 Orientation-and rate-dependent tensile strength for the UD continuous fiber-reinforced composite, neat iPP, 10 and long fiber-reinforced composite (LFC). 4 The LFC and neat iPP data are reproduced from the References 4 and 10, respectively [Color figure can be viewed at wileyonlinelibrary.com] shown by the line in Figure 8b is used to describe the relationship between the critical strain and the off-axis angle.…”

“…[5][6][7][8][9] Hence, one of the main challenges is the development of predictive models capturing their timedependent behavior, which is crucial to exploiting the full potential of these materials. Since the time-dependent behavior of the neat matrix plays a significant role in the off-axis failure, 1,4,10 a discussion of the time-dependent failure of the neat thermoplastics is important to understand the off-axis time-dependent behavior of composites.…”

“…12 If, on the other hand, crack growth is the dominant mechanism, the cyclic component will have a strong accelerating effect on failure. 12 In previous work, 1,10 this method was employed to identify the plasticity-controlled failure regime in off-axis loaded continuous fiber reinforced thermoplastics. In the specific case of glass/iPP, failure was demonstrated to be extensively dominated by plasticity.…”