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Defects that occur during the manufacturing process or operation of a wind turbine blade have a great influence on its life and safety. Typically, defects such as delamination, pore, wrinkle and matrix crack are found in a blade. In this study, the detectability of the pores, a type of defect that frequently occur during manufacturing, was examined from the full field strain distribution determined with the image correlation technique. Pore defects were artificially introduced in four-ply laminated GFRP composites with fiber direction. The artificial pores were introduced in consideration of their size and location. Three different-sized pores with diameter of 1, 2 and 3 mm were located on the top and bottom surface and embedded. By applying static loads of 0-200 MPa, the strain distributions over the specimen with the pore defects were determined using image correlation technique. It was found the pores with diameter exceeding 2 mm can be detected in diameter.

In this study, the temperature-dependency of the tensile properties of a glass fiber reinforced plastic (GFRP) used in wind turbine blades was examined. The tensile strength, elastic modulus, and Poisson's ratio of the tensile specimen manufactured from uniaxial () and triaxial () laminate composite plates were measured at four different testing temperatures-room temperature, , , and . It was found that the tensile strengths and elastic moduli of the uniaxial laminates were greater than those of the triaxial laminates over the testing temperature range. The tensile strength of the two laminates was significantly dependent on the testing temperature, while the dependency of the elastic modulus on the temperature was insignificant. Furthermore, it could be considered that the Poisson's ratio changed slightly with a change in the testing temperature.

Key Words: Wind Turbine Blade(풍력블레이드), Dual-Axis Loads(이축하중), Puck Failure Criterion(Puck파손지수), Composite Laminate(복합적층), Static Behaviors(정적거동), Inter-fiber FailureCriterion(모재파손지수), Fiber Failure Criterion(섬유파손지수)초록: 블레이드의성능평가를위하여실제사용환경과근접한하중모사에따른이축하중의필요성이제기되고있으며, 본본문에서는이러한이축하중에따른블레이드의거동을해석하였다. 100kW급풍력블레이드를대상으로하였으며, ANSYS를사용하여정적거동을분석하였다. 정적거동은파손해석과좌굴거동으로분석하였으며, 파손평가는Puck이제안한파손방정식을이용하여섬유파손과섬유간파손기준을검토하였다. 이축하중의하중비가증가함에따라루트부단면이변하는후연부와루트에서Z+ 3300~3600인지점에서응력이상대적으로크게나타났다. 또한이축하중비가증가함에따라블레이드좌굴지점이루트부쪽으로이동되고있음을확인하였다. 따라서블레이드의사용신뢰성을검증하기위해서는이축하중에의한시험이요구되고있음을본해석을통해서확인하였다.Abstract: For the assessment of the performance of a wind-turbine blade, a simulated loading test may be required. Inthis study, the blade behavior was investigated through numerical analysis using a dual-axis loading test, closelysimulating the real operation conditions. The blade structure for the 100-kW-class wind-turbine system was modeledusing the finite element (FE) program ANSYS. The failure criteria and buckling analysis under dual-axis loading wereexamined. The failure analysis, including fiber failure and inter-fiber failure, was performed with Puck's failure criterion.As the dual-axis load ratio increases, the relatively increased stress occurs at the trailing edge and skin surface 3300–3600 mm away from the root. Furthermore, it is revealed that increasing the dual-axis load ratio makes the locationthat is weakest against buckling move toward the root part. Thus, it is seen that the dual-axis load test may be anessential requirement for the verification of blade performance.

Using three-dimensional (3-D) FE analyses, this paper provides a method for analyzing the effects on stresses and strains produced by angular and concentric misalignment of a test specimen for a torsion test. To quantitativele compare of the FE results, the average bending strain for the angular, concentric, and combined misalignment was proposed. To verify the effects of axial misalignment of the test specimens, we used both circular and tubular specimens. From the FE results, we proposed general predictions for the effects caused by the various types of axial misalignment and its direction. In addition, we confirmed the effect of initial yielding moment based on the initial yielding condition for axial misalignment of specimens in torsion tests.

Estimation of tensile properties measurement uncertainty of material was carried out. Sources of uncertainty affecting the measurement of tensile properties were classified and analyzed. The models for uncertainty evaluation of measurands to be determined from tensile test, such as elastic modulus, yield strength and tensile strength, were suggested and derived from the mathematical relations, corresponding to the respective measurands, and the measuring quantities by calculating each sensitivity coefficient of the quantities. Based on these models, the uncertainty of the tensile properties was evaluated from the experimental data of SUS316LN determined according to ISO 6892.

Fatigue limit of the copper film coated by Sn was estimated using Goodman diagram and Gerber diagram. To obtain the high cycle fatigue life curve, S-N curve, of the film, the high cycle fatigue test was carried out by applying the constant amplitude load to the film specimen with three different stress ratio of 0.05, 0.3 and 0.5 and the frequency of 40 Hz at room temperature in air. The free-standing film specimen 15.26 thick was fabricated by etching process. The fatigue limits and S-N curves at the respective stress ratios were determined from the experimental works. It was shown that the S-N curves were dependent on the stress ratio and the fatigue limit was increased with decreasing the stress ratio. The dependency of the fatigue behavior was presented in empirical relationship. Using these relationships, the fatigue limit was predicted.

Fatigue crack initiation around a hole subjected to biaxial fatigue loads with a phase difference was investigated. Axial and torsional biaxial fatigue loads with different phase differences and biaxiality of 1/√3 were applied to thin-walled tubular specimens. Five phase differences of 0, 45, 90, 145 and 180 degrees were selected. Directions of the fatigue crack initiation around the hole were found to approach to the circumferential direction of the specimen with increment of the phase difference for fatigue tests with phase differences less than 90. Whereas directions for tests with phase differences greater than 90 got away from the circumferential direction and those were symmetric to the directions for tests with phase difference less than 90. . Furthermore, it was shown that the fatigue initiation life decreased with increment of phase difference for fatigue tests with phase differences less than 90, but it increased for tests with phase difference greater than 90. The crack initiation direction can be successfully explained by using the direction of the maximum tangential stress range obtained around the hole and at far-field.

Governing parameters for determination of the location of crack initiation and direction of crack initiation were investigated by performing fretting fatigue tests and analysis on Al 2024-T351. Fatigue tests were carried out using biaxial fatigue machine. It was shown that the dominant fatigue crack tended to initiate at the outer edge of one of the four bridge pads, growing at an angle beneath a pad, before turning perpendicular to the orientation of the axial load. Distribution of stresses generated during fretting fatigue loading along the interface was calculated by elastic FE simulation. It can be known that the location of crack initiation can be predicted by using the maximum tangential stress range. Futhermore, the crack initiation direction can also be predicted by a maximum tangential stress range.