Effect of Cycling Frequency and Self-Heating on Fatigue Behavior of Reinforced and Unreinforced Thermoplastic Polymers Seyyedvahid Mortazavian, 1 Ali Fatemi, 1 Stephen R. Mellott, 1 Abolhassan Khosrovaneh 2 1 Mechanical, Industrial and Manufacturing Engineering Department, The University of Toledo, Toledo, Ohio 43606 2 General Motors, Warren, Michigan 48093 An experimental study was conducted to evaluate the effect of frequency and self-heating on fatigue behavior of two unreinforced and two short glass fiber reinforced thermo- plastic polymers. Load-controlled fatigue tests were con- ducted under fully reversed (R 521) and R 5 0.1 conditions with specimens loaded in either longitudinal or transverse direction to the mold flow direction. Effect of frequency on fatigue life was evaluated at 23 and 1258C and for a range of frequencies between 0.063 and 20 Hz. Incremental step fre- quency tests were also performed at different stress ratios and stress levels. Surface temperature rise was found to be material, frequency, and stress level dependent. Three energy-based models were applied to the incremental step frequency data and relationships were developed for each material to estimate surface temperature rise as a function of test frequency and stress level. Relationships were also developed to assess critical frequency for the unrein- forced thermoplastics at a given stress level above which surface temperature does not stabilize. POLYM. COMPOS., 00:000–000, 2015. V C 2015 Society of Plastics Engineers INTRODUCTION The usage of thermoplastics is rapidly growing in many appli- cations due to their low weight and cost and ease of manufactur- ing and reprocessability. In such applications, components made of thermoplastics are typically under cyclic loading and, there- fore, prone to fatigue failure. An aspect of thermoplastics which distinguishes them from metallic alloys is their time dependent properties as well as their relatively low melting temperatures. As a result, frequency by which a load cycle is applied is a deter- mining factor in fatigue behavior of this class of materials. The enclosed hysteresis area in each load cycle represents energy loss per unit volume. At relatively low frequencies, a greater time is given to entangled thermoplastic polymer chains to disentangle and align into the load direction. Therefore, at low frequencies, thermoplastics typically indicate a lower stiffness and a higher degree of energy dissipation per cycle. At higher frequencies, the dissipated energy reduces and a higher stiffness is observed. However, due to a low thermal conductivity of ther- moplastics, the generated heat due to energy dissipation at higher frequencies results in self-heating and thermal degradation. The variations of stiffness and energy dissipation with frequency cause fatigue life to non-linearly change with applied frequency [1, 2]. Due to the broad variations in polymer chain structures, the effect of frequency is highly material dependent. Presence of short glass fibers can increase strength, stiff- ness, and thermal stability of thermoplastics. However, fric- tional heating between fiber and matrix and a higher intensity of stress near fiber ends may increase the degree of self- heating in reinforced thermoplastics. Fatigue life predictions for actual service load histories are typically based on constant amplitude fatigue data, which can be significantly affected by cyclic frequency in thermoplastics. Therefore, an appropriate frequency needs to be chosen for obtaining constant amplitude fatigue properties of thermoplastics. Depending on the loading mode, stress level, and material characteristics, frequency influ- ences the cyclic behavior of thermoplastics to various degrees. A number of studies have investigated the effect of cyclic fre- quency on reinforced and unreinforced thermoplastics, as pre- sented in a recent review by Mortazavian and Fatemi [3]. At a cyclic frequency of 10 Hz under fully reversed cyclic bending condition, a transition from glassy state to rubbery state occurred due to temperature rise in short glass fiber reinforced polyamide-6.6 [4]. This transition resulted in ther- mal failure along with mechanical failure. In a study con- ducted by Esmaeillou et al. [5] on fatigue behavior of short glass fiber reinforced polyamide-6.6, temperature rise due to self-heating was higher in axial load-controlled fatigue tests than in flexural strain-controlled fatigue tests. In axial load- controlled fatigue tests, the same fatigue lives were achieved at frequencies of 10 and 20 Hz in LCF, while at HCF a higher fatigue life was observed at frequency of 10 Hz com- pared with 20 Hz. In flexural strain-controlled fatigue tests, no difference was observed between stress-life curves under Correspondence to: Seyyedvahid Mortazavian; e-mail: seyyedvahid.mortaza- vian@rockets.utoledo.edu Contract grant sponsor: General Motors. DOI 10.1002/pen.24124 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Society of Plastics Engineers POLYMER COMPOSITES—2015