EFFECT OF SHEAR STRESS ON FATIGUE LIFE OF COMPOSITE LAMINATES G. - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS EFFECT OF SHEAR STRESS ON FATIGUE LIFE OF COMPOSITE LAMINATES G. Mustafa 1 *, S.K. Ha 2, Y. Haung 2 , K. Jin 2 1 Design Department, AE Design, Pakistan, 2 Department of Mechanical Engineering, Hanyang University, Ansan, Korea *Corresponding author(enginer315@gmail.com) Keywords : Composite laminates, micromechanics of failure (MMF), fatigue life, constituents, stress ratio, damage parameter Abstract presented for the case of wind turbine rotor blade composite laminates. An initial estimate on the The effect of complex stress state is not properly effect of neglecting shear stresses in fatigue life taken into account for determining the fatigue life of calculations is provided based on predictions. It is thin-walled structures made of composite materials concluded that in wind turbine blade GFRP like wind turbine blade. Fatigue life predictions, as laminates, shear stresses have an important per state-of-the-art design codes like IEC 61400-1 contribution in reducing fatigue life. and GERMANISCHER LlOYD (Part 1), account 1 Introduction only for normal stresses, neglecting the contribution Fiber reinforced laminated composites have been of shear stresses. This is due to either misconception or lack of experimental data and theoretical models. used in many structural applications because of their superior specific properties compared with metals. Many mechanical tests are required to fully Typical modern composite structures include characterize the composite laminates made of various materials and having different layup aeronautical vehicles, ships, wind turbine blades, flywheels, pressure vessels, helipads, Clock Tower sequences under many load combinations of in- in KSA, and sporting goods. Fiber reinforced plane stress tensor components. A micromechanics composites consist of fibers and visoelastic matrix. based fatigue life prediction of composite laminates With the increase of loads and changes in under multi-axial loading was developed that can environmental conditions, the damage grows and take care of complex stress state. In order to reduce number of tests, a methodology was presented in this progresses until ultimate failure of structures. The stress state in composite structural elements, either paper to predict fatigue life of composite laminates in the form of thin or moderately thick shell based on fatigue life of constituents, i.e. the fiber, matrix and interface, using micromechanics of construction, can be assumed plane, i.e. composed of two normal components and an in-plane shear failure (MMF). For matrix, the equivalent stress component of the stress tensor. The composite model which is generally used for isotropic materials structures should be stiff enough to resist was employed to take care of multi-axial fatigue deformations and strong enough for long term loading. For fiber, a maximum stress model considering only stress along fiber direction was operation in service under particular stress state. Fatigue failure of composite materials has been used. Critical plane model was introduced for the widely discussed in the literature by the researchers interface of the fiber and matrix. The modified Goodman approach was utilized to take into account during the past two decades [1-5]. Hashin and Rotem [3] presented a simple fatigue the mean stress effect. In order to validate the failure criterion expressed in terms of S-N curves proposed methodology, the fatigue life of three obtained by uniaxial cyclic testing of unidirectional different GFRP laminates, UDT [90°], BX [±45°]S specimens. and TX [0°2/±45°]S, was examined experimentally. Energy-based criteria incorporate both stresses and The predictions are compared with the experimental data, and are shown in good agreement. A strains. In this approach, the damage is related to comprehensive implementation example is also

EFFECT OF SHEAR STRESS ON FATIGUE LIFE OF COMPOSITE LAMINATES input energy which cannot give any information stresses in fiber, matrix, and interface. These six about failure mechanism [6, 7]. components of stresses, varying with time, depend Several researchers [8, 9] have investigated fatigue on micro model. From these stress components, damage response of polymer composites based on effective stress is calculated which, also, is a reduction of composite stiffness as the number of function of time. The Modified Goodman diagram is fatigue cycles increased. Mayes and Hansen [10] used to take care of mean stress effects. For particular mean and amplitude of effective stress proposed a muti-continuum theory (MCT) in which phases averaging of micro-level stresses is applied to cycles, damage index is calculated. Finally, Linear each constituent. This micromechanical approach Damage Accumulation rule, i.e., Miner's Rule is effectively includes changes in constituent level used to give fatigue damage for all varying stresses properties. However, since they used volume with different means and amplitudes. averaged micro stresses, it is difficult to distinguish 2.2 Macro and Micro Stresses failure location either in fiber or matrix or interface and, also, incorporating damage degradation in Composite laminates are subjected to time vary ing load. These causes in-plane loads and in-plane m fatigue analysis. oments, expressed also as function of time, in lamina Due to the large variety of laminates resulting from tes. Then using FEM or CLT, on-axis macro stresses numerous materials, lay-ups and stacking sequences, are calculated on each ply. Micro stresses in fiber, and loading conditions, it is uneconomic to determine fatigue life curves for any degree of matrix, and interface are calculated from on-axis ply macro stresses using Micro-mechanics. For this, diff generality by experiments only. Most of fatigue life erent micro unit cell models are available. Unidirecti assessment methods used to estimate the experimental results was empirically approximated onal (UD) composite ply micro model depends on fi ber arrangement. It consists of square array (SQR) or by phenomenological theories which cannot prove hexagonal array (HEX). These unit cell models can its absolute validity and universal generality. simulate the behavior of entire UD [11, 12]. There is In this paper, a general methodology based on another choice for micro model based on Multi-Con micromechanics of failure (MMF) approach is tinuum Theory (MCT) that defines stress and strain s presented for fatigue life prediction of composite laminates. The methodology is based on fatigue life imply as average over unit cell model [13]. Micro str esses at any arbitrary point within fiber and matrix c of constituents, i.e. the fiber, matrix and interface. A an be obtained using these models which vary over f wide range of experimental fatigue data of composite laminates are used to validate the ( f m i , , ) M iber and matrix. ij , stress amplification factor ( proposed methodology. The predictions based on the SAF) gives relation between macro-micro stresses. MMF based fatigue model agree very well with the ( f m i , , ) A experimental results. j , stress amplification factor for temperature in 2 Theory and Methodology Macro Stress Analysis 2.1 Micromechanics of Failure (MMF) Approach Composite Laminate Ply structures ∆ The composite structures are made from composite T Loads & F σ Temperatures FEM CLT laminates. The Fig. 1 shows the Micromechanical N M Multi-axial Random approach to evaluate fatigue life of composite Fatigue loading time time structure. This approach is divided into two parts, Micro Fatigue Analysis namely Macro Stress Analysis and Micro Fatigue Micro model Micro stresses S-N curve Predict life of each constituent σ σ i a Analysis. In Macro Stress Analysis, macroscopic i , Matrix τ Interface σ i m σ m S σ n , = ∑ n stresses are calculated on the composite laminates i time D Fiber σ σ N under varying external mechanical and thermal loads eff a , eff σ f i N σ using finite element method (FEM). In addition, on- eff m , time axis macro ply stresses are calculated using Classical Fig. 1 MMF based fatigue life prediction procedure Laminate Theory (CLT) or FEM. In Micro Fatigue for composite laminates Analysis, micro model is employed to calculate