James A. Sherwood

Modeling of Friction and Shear in Thermostamping of Composites - Part I

The effects of processing parameters on the friction coefficient between commingled glass-polypropylene plain-weave fabric composites (Twintex) and the steel tool during thermostamping processes are under investigation. This work focuses on the effect of fiber orientation, fabric velocity, normal force, and resin viscosity (through variations in tool and fabric temperatures) under conditions similar to those in the thermostamping processes. In comparison to the experiments conducted to date, velocity, normal force, and tool temperature have the greatest effect on the friction coefficient. The effect of tool temperature on the friction coefficient dominated the effect of initial fabric temperature on the friction coefficient. Based on the effect of these parameters, a phenomenological model has been incorporated into ABAQUS/Standard as a user-supplied friction subroutine. This model was first used in a finite element model of the friction test. A good agreement was found between the experimentally measured friction force and the numerically calculated one.

Inspection and monitoring of wind turbine blade-embedded wave defects during fatigue testing

The research presented in this article focuses on a 9-m CX-100 wind turbine blade, designed by a team led by Sandia National Laboratories and manufactured by TPI Composites Inc. The key difference between the 9-m blade and baseline CX-100 blades is that this blade contains fabric wave defects of controlled geometry inserted at specified locations along the blade length. The defect blade was tested at the National Wind Technology Center at the National Renewable Energy Laboratory using a schedule of cycles at increasing load level until failure was detected. Researchers used digital image correlation, shearography, acoustic emission, fiber-optic strain sensing, thermal imaging, and piezoelectric sensing as structural health monitoring techniques. This article provides a comparison of the sensing results of these different structural health monitoring approaches to detect the defects and track the resultant damage from the initial fatigue cycle to final failure.

Damage detection and full surface characterization of a wind turbine blade using three-dimensional digital image correlation

The increasing demand for wind power has led to a significant increase in the number and size of wind turbine blades manufactured globally. As the number and physical size of turbines deployed grow, the probability of manufacturing defects being present in composite turbine blades also increases. As capital blade costs and operational and maintenance expenses increase in ever larger turbine systems, the need for inspection of the structural health of large-scale turbine blades during operation critically increases. One method for locating and quantifying manufacturing defects, while also allowing for the in situ measurement of the structural health of blades, is monitoring the full-field deformation and strain of a blade. In a demonstration of this methodology, static tests were performed on a Sandia National Laboratories CX-100 9-m composite turbine blade to extract full-field displacement and strain measurements. Three-dimensional digital image correlation was used. Measurements were taken at previously identified damaged areas near the blade root, along the high- and low-pressure surfaces. The results indicate that the measurement approach can clearly identify failure locations and discontinuities in the blade curvature under load. Postprocessing of the data, using a stitching technique of digital image correlation snapshots taken along the length of the blade, allows observation of the shape and curvature of the entire blade. The experiment demonstrates the feasibility of the approach and reveals that the technique can be readily scaled to accommodate utility-scale blades. As long as a trackable pattern is applied to the surface of the blade, measurements can be made in situ when a blade is on a manufacturing floor, installed in a test fixture, or installed on a rotating turbine. The results demonstrate the potential of the optical measurement technique for use in the wind industry.

Correlation of Superpave G*/Sin δ with rutting test results from accelerated Loading Facility

In 1992, FHWA initiated a Superpave validation study by utilizing the Accelerated Loading Facility (ALF) at the Turner-Fairbank Highway Research Center in McLean, Virginia. The study focused on the validation of the concepts, tests, and predictive models underlying the Superpave binder specifications and mixture analysis system. Twelve full-scale pavement lanes with 48 test sites were constructed at the FHWA Pavement Testing Facility in 1993. Pavement testing with the ALF started in late spring of 1994. The results of accelerated full-scale pavement tests in conjunction with extensive laboratory tests will be used to validate the Superpave binder parameters for rutting and fatigue cracking. Presented in this paper are the results of rutting tests and some of the data analysis completed through June 1997.

Micromechanical modeling of damage growth in titanium based metal-matrix composites

Abstract The thermomechanical behavior of continuous-fiber reinforced titanium based metal-matrix composites (MMC) is studied using the finite element method. Fiber and interface failures are modeled as discrete damage. The evolution of matrix failure is considered as continuum damage. A thermovis-coplastic unified state variable constitutive theory is employed to capture inelastic and strain-rate sensitive behavior in the Timetal®-21 s matrix. The SCS-6 fibers are modeled as thermoplastic and can fail at some prescribed plastic strain to failure. The effects of residual stresses generated during the consolidation process on the tensile response of the composites are considered. Unidirectional and cross-ply geometries are studied. Differences between the tensile responses in composites with perfectly bonded, weakly bonded and completely debonded fiber/matrix interfaces are discussed.

Similitude Analysis of Composite I-Beams with Application to Subcomponent Testing of Wind Turbine Blades

The mechanical behavior of new materials for wind turbine blades is initially characterized by using coupon testing. If the results of the coupon testing look promising, then the materials are incorporated into a blade and certified through a full-scale blade test. The coupon testing is not always representative of performance of the new materials, and full-scale-blade testing is time consuming and very expensive—on the order of several hundred thousand dollars. To bridge the large gap between coupon testing and a full-scale test, subcomponent testing is proposed as a cost-effective alternative. To design a meaningful scaled-down subcomponent emulating the structural conditions experienced in the full-scale component, it is proposed that similitude theory can be applied to a scaled replica of the I-beam structure of a wind turbine blade involving spar caps and the shear web. In the current research, the governing equations for the bending of a simply-supported shear deformable thin-walled composite I-beam are analyzed to derive the scaling laws. Fidelity of the derived scaling laws is then examined as a model-validation criterion by mapping the maximum deflection of variant subcomponents to the full-scale composite I-beam. The combined effects of the size of the subcomponents and the ply stack-up schemes on the fidelity of the scaling laws are then investigated through complete and partial similarity conditions. According to the results, preserving the aspect ratio plays a critical role in successful prediction of the maximum bending of the full-scale I-beam. Also, subcomponents with a modified ply stack-up could be found with good accuracy in maximum bending prediction using the derived scaling laws.

Predicting the Vibration Response in Subcomponent Testing of Wind Turbine Blades

Currently new wind turbine blade materials are certified by starting with coupon testing for initial strength and fatigue analysis, followed by full-scale blade testing as a final quality control to assess material characteristics. Subcomponent testing has been proposed as a supplement to the structural analysis and material characterization, bridging the gap between coupon and full-scale tests. In this study, similitude theory is applied to a simply-supported rectangular plate that is representative of a wind turbine blade spar cap with the goal of designing a validated scaled-down subcomponent. The vibration of a specially orthotropic rectangular laminated plate is analyzed to extract the scaling laws based on direct use of the field equations. The accuracy of the derived scaling laws is analyzed as a model validation criteria by mapping the first natural frequency of the variant subcomponents to the full-scale plate. The effect of the ply stack up scheme and size of the subcomponents in predicting accuracy of the scaling laws are then investigated by applying partial and complete similarity conditions. According to the results, subcomponents with modified ply stack up could be found that have a good accuracy in predicting the first natural frequency of the full-scale plate. However, picking an appropriate aspect ratio is critical to the success of the prediction of full scale plate response as shown in the cases studied.

Full-field inspection of a wind turbine blade using three-dimensional digital image correlation

Increasing demand and deployment of wind power has led to a significant increase in the number of wind-turbine blades manufactured globally. As the physical size and number of turbines deployed grows, the probability of manufacturing defects being present in composite turbine blade fleets also increases. As both capital blade costs, and operational and maintenance costs, increase for larger turbine systems the need for large-scale inspection and monitoring of the state of structural health of turbine blades during manufacturing and operation critically increase. One method for locating and quantifying manufacturing defects, while also allowing for the in-situ measurement of the structural health of blades, is through the observation of the full-field state of deformation and strain of the blade. Static tests were performed on a nine-meter CX-100 composite turbine blade to extract full-field displacement and strain measurements using threedimensional digital image correlation (3D DIC). Measurements were taken at several angles near the blade root, including along the high-pressure surface, low-pressure surface, and along the trailing edge of the blade. The overall results indicate that the measurement approach can clearly identify failure locations and discontinuities in the blade curvature under load. Post-processing of the data using a stitching technique enables the shape and curvature of the entire blade to be observed for a large-scale wind turbine blade for the first time. The experiment demonstrates the feasibility of the approach and reveals that the technique readily can be scaled up to accommodate utility-scale blades. As long as a trackable pattern is applied to the surface of the blade, measurements can be made in-situ when a blade is on a manufacturing floor, installed in a test fixture, or installed on a rotating turbine. The results demonstrate the great potential of the optical measurement technique and its capability for use in the wind industry for large-area inspection.

A material model for woven commingled glass-polypropylene composite fabrics using a hybrid finite element approach

Woven fabric composites made from commingled fibreglass and polypropylene fibres have the potential to make lightweight structural parts to replace metal structures in automobiles when competitively produced using thermostamping techniques. To expedite the acceptance of such composites, a credible design tool needs to be available to study the associated manufacturing process. For such a numerical method, a material model is required. Because the tows of the fabric are essentially inextensible, shear is the major deformation mode for assuming the stamped shape. The shear mechanical behaviour of fabric is obtained from a shear-frame test. A finite element model using a combination of truss and 2D solid elements is proposed. The truss elements represent the tows and use a nonlinear material model for capturing the increasing stiffness of the tows with increasing tensile strain. The 2D Solid elements capture the interaction between tows and the viscosity of the resin. The bias-extension test is a valuable test for investigating the shear performance of woven fabric composites. Using the finite element method, the bias-extension test is modelled, analysed and compared experimentally to verify the credibility of the woven-fabric material model.

Design of Scaled-Down Composite I-Beams for Dynamic Characterization in Subcomponent Testing of a Wind Turbine Blade

Blade certification in the wind industry starts with coupon testing of materials and eventually culminates with full-scale blade testing. Coupon testing is not always representative of the materials’ performance and full-scale testing is expensive and time consuming. Subcomponent testing can bridge this gap and increase the assurance of blade manufacturers for introducing new materials and designs into wind industry. In this study, similitude theory is applied to the I-beam structure of a utility-scale wind turbine blade to design scaled down models that emulate the dynamic characteristics of the full-scale I-beam. The governing equations of motion for vibration of a thin walled laminated I-beam are analyzed to derive the scaling laws. Derived scaling laws are used as a design criterion to develop models that can accurately predict the fundamental frequency of the full-scale I-beam. Both complete and partial similarity cases are investigated. The distorted layup scaling technique is introduced as a novel approach to design scaled down composite models with totally different layups than the full-scale component. According to the results, depending on the desired size of the scaled models and ply scheme of the full-scale I-beam, models could be found with very good accuracy in predicting the fundamental frequency of the full-scale I-beam using derived scaling laws.