Multiscale-based Fatigue Life Prediction of Composite Wind Turbine Blades
- Multiscale-based Fatigue Life Prediction of Composite Wind Turbine Blades
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- The fatigue life evaluation of the wind blade has been considered as one of the most critical design requirements, and various fatigue life prediction methodologies were therefore investigated in detail. Traditionally, a laminate-based fatigue life prediction method is used, as recommended by the Germanischer-Lloyd (GL) regulations. However, the GL method only considers the longitudinal stress in a laminate level and is unable to fully explain the fatigue behavior of a composite structure. To overcome this issue, the multiscale-based life prediction methodologies devel-oped previously, were employed and compared with the GL predictions in this study. The theory called micromechanics of failure (MMF) was used for the strength prediction and the theory called micromechanics of fatigue (MMFatigue) was applied to the fatigue life evaluation.
The multiscale methodology is based on three constituents of the composite materials, i.e., fiber, matrix, and interface. It is well known that the interface plays an important role the compo-sites since it contributes to transfer the loads from the fiber to the matrix. The interface is also critically important to the adhesively bonded joints which are increasingly popular alternatives to mechanical joints in aerospace engineering applications and provide many advantages over conventional mechanical joints. The conventional theories for strength and life evaluations of the interface need complicated costly stress analyses and tremendous of tests to generate material database for different interfacial failure modes. In this research, the physical-based models are developed to effectively evaluate the mechanical behaviors of the interface.
In the beginning, the newly developed interface model called Chain Interphase Model (CIM) was introduced. The CIM is based on the modeling of the polymer chains to simulate the mechanical behaviors of the interface. The chains can be generated based on different probabilis-tic distributions. The stress response of single chain is calculated, and the failure of each chain is obtained based the static and fatigue models. The failure of interface is then predicted based on the progressive failure of the chains. A Microsoft Excel-based tool was developed for the CIM. By using the CIM tool, the force-displacement curves can be generated from the static analysis, and the stress-number of cycles to failure (S-N) curves can be obtained from the fatigue analysis. The interface constant life diagram (CLD) can be generated based on the generated S-N curves. The CIM is also applicable to generate the virtual test data to support the existing theories. The Chain Adhesive Model (CAM) for simulating the adhesive was then introduced. The randomly distribut-ed chains are generated in the CAM. CAM also utilizes the progressive failure of each chain to predict the strength and life of the whole model. The CAM simulation is performed using the commercial Finite Element Method (FEM) tool Abaqus.
Four different types of experiments were then carried out to study the adhesive bonding behavior. Both static and fatigue tests were performed for each case. The bulk adhesive test was carried out to evaluate the static and fatigue performance of the pure adhesive. The lap-shear test was performed the understand mechanical behavior of the adhesive system. The mode I and mode II tests were carried out to measure the fracture toughness. In the case of static tests, three different types of curing conditions were considered to observe the process-driven bonding proper-ties. The FEM simulations using the cohesive element were also performed to compare with the experimental results.
The chain properties needed for the CIM and CAM were then determined based on test da-ta. The parametric studies were carried out to observe the effect of chain parameters on the inter-face, and the detailed procedures of chain property determination were described. The properties for CAM were determined from the test data of bulk adhesive first, and the properties for CIM were then obtained based on the lap-shear test results. The chain stiffness and static properties were decided using the stress-strain curves, and the chain fatigue properties were determined based on the S-N curves measured from the test.
The CIM was applied to simulate the interface failure. The static failure envelope was plot-ted to describe the interfacial static failure. The CIM-based failure envelop was also compared with the existing theories. The fracture toughness for the interfacial static failure criteria was simu-lated using CIM. The predicted fracture toughness was compared with the test results. The mixed mode fatigue life predictions of interface were performed using CIM. The S-N curves for different combinations of fatigue loads were generated considering different stress ratios.
The multiscale-based strength and life predictions of the multi-axial laminates were per-formed using the MMF and MMFatigue. Typical laminates used in the composite layup of wind turbine blades were considered. The static tests were performed to measure the static tensile and compressive strengths, and the tension-tension fatigue tests under cyclic fatigue loads with a stress ratio of 0.1 were carried out to measure the S-N curves. The predictions were in an acceptable agreement with the test results. The effect of shallow angles on the static strength and fatigue life of multi-directional laminates were also observed.
Finally, the fatigue life prediction of two different large-scale wind blades based on multiscale approach was presented. Four different types of fatigue loads were applied separately. The effects of shallow angled skins on the fatigue life of the 2MW blade were observed. Both the MMFatigue and GL results showed that a shallow angle improves the blade fatigue life significantly. The fatigue life evaluation of a 5MW blade was also carried out based on the MMFatigue. The fatigue damage of fiber, matrix, and interface was calculated separately, and the blade fatigue life was estimated based on the most critical constituent fatigue damage.
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- GRADUATE SCHOOL[S](대학원) > MECHANICAL ENGINEERING(기계공학과) > Theses (Ph.D.)
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