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In a previously reported study, it was found that the fatigue crack growth rate actually accelerated after a tensile overload under a negative value of baseline stress ratio R. This type of crack propagation behavior is related to the change in the compression residual stress to tensile residual stress distributed in the vicinity of the crack tip upon unloading to minimum load after an overload. In the present investigation, focused on the negative stress ratio, it was found that the deceleration and acceleration of crack propagation were associated with crack opening displacement at the overload point and the conditions of applied baseline cyclic stress. The fatigue crack growth rate decelerated after being overloaded at a positive R value. However, that rate accelerated rather than decelerated in some stress conditions at a negative R value. Therefore, the residual fatigue life after overloading became longer or shorter, depending on the overload level and baseline stress conditions.

Eco-friendly composite made of Timoho Fiber (TF) continuously developed to get the best performance to replace plastic-based synthetic fibers. This study focuses on investigating physical characteristics, mechanical properties, thermal analysis, and the morphology of TF-reinforced polyester composites by adding organic (egg shell powder-ESP) and inorganic (aluminum powder-AP) fillers. Hot press method was used in the composite fabrication with considered volume fraction of TF, organic, and inorganic fillers. The results showed that the density of TF-polyester composites decreases with the increasing volume fraction of the fibers. For additional fillers, it was shown that AP was more effective to be used to improve density than ESP. The tensile and impact strength of the composite increased with increasing TF volume. However, the addition of ESP and AP fillers into the composite caused different mechanical characteristics. Filler addition increased the elasticity modulus, toughness, thermal resistance increased, while the tensile strength decreased. ESP and AP fillers provided the best thermal resistance due to the relatively high thermal conductivity of ±1700 C compared to composites without fillers and amorphous ESP fillers. SEM observation supported the analysis of TF-polyester composite mechanical characteristics.

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A study of the effect of an overload on fatigue crack propagation behavior has been carried out at baseline R-values of 0, -1, and -1.5. The usual delayed retardation event was observed at R = 0. At R = -1 the degree of retardation was reduced. At R = -1.5 the fatigue crack growth rate was observed to accelerate after the overload. These behaviors were related to the extent of crack closure, which existed following the overload. The rate of fatigue crack propagation for all loading conditions was correlated with the parameter AK e tt -AKeaii, where AK e ffth is the effective value of the stress intensity factor range at threshold.