A novel fatigue life prediction methodology based on energy dissipation in viscoelastic materials

This paper introduces a new fatigue life prediction methodology for viscoelastic materials in the tension-tension fatigue loading region. The model was established based on the total amount of energy dissipated during fatigue loading, and offers two main advantages with respect to existing models in the literature, i.e. it considers the creep effect on fatigue behavior and requires less input data. The model was applied to three different materials an angle-ply glass/epoxy fiber-reinforced polymer composite, a cross-ply glass/epoxy fiber-reinforced polymer composite, and an epoxy adhesive - to cover a wide range of structural viscoelastic materials used in the industry. It was observed that the model predicted the fatigue life of the studied materials well at different stress ratios including those close to 1.0 where the creep effect was considerable. The model was used to plot constant life diagrams (CLDs) by considering the cyclic-creep interaction to counter the lack of accuracy of existing models at high stress ratios. A new definition for the cyclic-creep interaction was also proposed, which suggests that the participation of the creep and cyclic parts in the cyclic-creep interaction is equal to the total amount of energy dissipated by each. Accordingly, the proposed model was employed to simulate cyclic-creep interaction and determine the cyclic- and creep-dominated regions in CLDs

Creep effects on tension-tension fatigue behavior of angle-ply GFRP composite laminates

Angle-ply (±45)2S glass/epoxy composite specimens have been subjected to pure creep and tension-tension constant amplitude fatigue loading interrupted at max by creep intervals lasting for 2 or 48 hours in order to examine the effects of creep loading on the fatigue response and vice versa. The specimens’ behavior and damage status were continuously monitored during the experiments; strains were measured by a video extensometer, the self-generated temperature on the specimens’ surface was recorded by an infrared camera, while a digital camera with sufficient backlighting was used in order to capture the damage development in the translucent specimens throughout the experiment. Post-mortem photos were taken by a digital microscope for the analysis of the fracture surfaces. In comparison to continuous fatigue, applying the creep-fatigue loading pattern with a 2-h creep time at low stress levels had no effect on fatigue life. However, as the stress level increased, specimen stiffening occurred during creep loading because of the glass fiber realignment, which also decreased the internal friction, hysteresis loop area, and self-generated temperature, thus prolonging the fatigue life. The restoring of fatigue stiffness was greater at a creep time of 48h due to more creep strain, which led to more fiber realignment. However, the higher creep strain at high stress levels caused more creep damage and thus resulted in a shorter fatigue life. In addition, it was observed that the fatigue damage accelerated creep deformation

Modeling of fatigue behavior based on interaction between time- and cyclic-dependent mechanical properties

A general constitutive equation was established using the theory of viscoelasticity in order to consider the interaction of the time- and cyclic-dependent mechanical properties of laminated composites. This equation was solved for two specific loading patterns, (1) stress unloading to zero stress level (recovery solution), and (2) load control sinusoidal loading (fatigue solution), and was subsequently imported to model tensile-tensile interrupted fatigue experiments (including recovery phases) of +/- 45 degrees angle-ply glass/epoxy composite laminates at different stress levels. The viscoelastic parameters in the recovery solution were calibrated at different percentages of fatigue lifetime using the experimental recovery results. The estimated viscoelastic parameters were then imported into the fatigue solution to predict the fatigue stiffness, hysteresis loop area, cyclic creep, storage and loss moduli as well as tan(delta) under cyclic loading. The theoretical predictions compared well to the experimental data

Stress ratio effect on tension-tension fatigue behavior of angle-ply GFRP laminates

The effect of the stress ratio (R = sigma(min)/sigma(max)) on the fatigue behavior of (+/- 45)(2s) angle-ply glass/epoxy composite laminates was investigated by comparing their mechanical, thermal, and optical properties under the stress ratio of 0.5 with previous fatigue results obtained under the stress ratio of 0.1. When the stress ratio was increased from 0.1 to 0.5, the fatigue life was enhanced at the same sigma(max) and the slope of S-N curve decreased, exhibiting more scattered responses. In addition, as the stress ratio increased, the fatigue damage was distributed more uniformly with a lower self-generated temperature at the same sigma(max). At R = 0.5, fiber realignment, due to cyclic creep, increased the fatigue stiffness, thus compensating the decreasing effect of fatigue damage. At high stress levels, the stiffening effect dominated the stiffness evolution, resulting in greater fatigue stiffness with an increasing number of fatigue cycles; however, at low stress levels the degrading effect due to fatigue damage was prevalent. The stiffening effect led to smaller hysteresis loop areas at the beginning of the fatigue experiments, which subsequently stabilized for the greater part of the specimen fatigue life, followed by a slight increase before failure. At R = 0.1, the fatigue stiffness decreased further and the hysteresis loop area became larger at all stress levels at the same sigma(max), since the stiffening effect was less and fatigue damage more severe