  The work deals with the fatigue lifetime estimation of Short Fiber Reinforced
Thermoplastics (SFRP), with a focus on conjugated effects of thermal aging. Two
materials containing 35% (V35) and 50% (V50) weight ratio of short glass fibers
were aged for 500h at 200{\textdegree}C in air and compared to the same
materials in a Dry-As-Molded (DAM) state. Monotonic and fatigue tests were
performed in samples machined out of injected plates and cut along three
different orientations to the injection one (0, 45, 90{\textdegree}) in order
to capture the anisotropy of the skin-core microstructure, classical in
injected SFRP. Monotonic tensile tests evidenced the stiffening and
embrittlement of the Polyamide matrix reported in the literature, nevertheless
with an acuity depending on the matrix ratio and fiber orientation.
Stress-controlled fatigue tests were performed at constant amplitude, frequency
(10Hz), temperature (200{\textdegree}C) and stress ratio R=0.1. The fatigue
curves of V35 are more affected by aging than those of the V50 material. The
combined results from the mean strain evolutions and SEM observations confirmed
that the initiation approach for fatigue lifetime estimation was still valid in
aged composites. Several fatigue criteria, among the most recently reported in
the literature, were evaluated from this database. The effect of aging on the
cyclic evolution of the different Fatigue Indicator Parameters (FIP) involved
in the criteria was analyzed preliminarily. For the present fatigue conditions,
the best criteria for unaged materials (i.e cyclic creep energy-based ones)
were shown to still be the best for aged materials. Finally, the ability to
predict fatigue lifetime for aged composites from the identification of the
fatigue criterion in the unaged state was evaluated.
