PSI - Issue 47

Artur Mugatarov et al. / Procedia Structural Integrity 47 (2023) 654–659 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

655

2

Nomenclature α , β

beta-function parameters

B ( α , β ) B n ( α , β ) GFRP

beta-function

incomplete beta-function glass fiber reinforced plastic fatigue sensitivity coefficient Weibull law parameters preliminary cyclic exposure number of preloading cycles Mao and Mahadevan model parameters

K B κ , λ

m 1 , m 2 , q

n N

fatigue life

N 0

maximum stress value during the cycle

σ max

ultimate strength

σ u

ultimate strength of non-damaged material

σ u 0 ω B ω′ B

damage value

derivative of damage value function

Previously authors proposed to plot the fatigue sensitivity curves as the dependence of fatigue sensitivity coefficient K B on preliminary cyclic exposure n (Wil ’ deman et al. (2018)). These curves can be divided into three stages. The first of them, called initiation stage, and the last one, called aggravation stage, are characterized by fast mechanical properties reduction and high value of accumulated defects. The second one, the stabilization stage, takes place when fatigue sensitivity coefficient decrease slowly, and damage accumulation rate is low. Boundaries for these stages can be defined using the points where damage accumulation rate is equal to 0.3. Such division is conditional and may vary depending on the material (and its class). Various fatigue sensitivity curves approximations were represented in literature. One of the most simple, convenient and widely-used models is based on two power functions usage (Mao and Mahadevan (2001)). This approximation requires three parameters definition. Staroverov et al. (2023) proposed two models based on cumulative distribution functions: Weibull law and beta-distribution, each of them requires only two parameters definition. All of the model parameters depend on loading conditions, temperature, etc. In this work, using various models, the processing experimental data on the GFRP strength reduction during cyclic loading under various conditions is carried out. Influence of maximum stress value during the cycle, test temperature and composite layout is investigated. Approximation parameters are calculated, high descriptive ability of the models is demonstrated. 2. Material and methods 2.1. Experimental procedure Fiber-glass laminate specimens with layouts [0/90] n , [±45] n and [0/±30/90/±60] n were used in the experimental studies. The test method is based on the existing standards of quasi-static and fatigue tension of polymer composite materials under normal and elevated temperatures. Nominal values of ultimate strength σ u 0 were taken from quasi static uniaxial tension tests (ASTM D3039). The maximum number of cycles to failure N 0 was found for uniaxial cyclic tension at the various maximum stress value σ max , the asymmetry coefficient R =0.1, and the frequency ν =20 Hz (ASTM D3479). Three specimens in each group were tested for quasi-static and cyclic tension. The other specimens were exposed to preliminary cyclic loading and then statically tested. Preliminary cyclic exposure was implemented within 0.1 to 0.9 nominal fatigue life N 0 . The test method is shown in Fig. 1.