Julie Chen

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.

Low-cost rapid miniature optical pressure sensors for blast wave measurements

This paper presents an optical pressure sensor based on a Fabry-Perot (FP) interferometer formed by a 45° angle polished single mode fiber and an external silicon nitride diaphragm. The sensor is comprised of two V-shape grooves with different widths on a silicon chip, a silicon nitride diaphragm released on the surface of the wider V-groove, and a 45° angle polished single mode fiber. The sensor is especially suitable for blast wave measurements: its compact structure ensures a high spatial resolution; its thin diaphragm based design and the optical demodulation scheme allow a fast response to the rapid changing signals experienced during blast events. The sensor shows linearity with the correlation coefficient of 0.9999 as well as a hysteresis of less than 0.3%. The shock tube test demonstrated that the sensor has a rise time of less than 2 µs from 0 kPa to 140 kPa.

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.

An ultra-fast fiber optic pressure sensor for blast event measurements

Soldiers who are exposed to explosions are at risk of suffering traumatic brain injury (TBI). Since the causal relationship between a blast and TBI is poorly understood, it is critical to have sensors that can accurately quantify the blast dynamics and resulting wave propagation through a helmet and skull that are imparted onto and inside the brain. To help quantify the cause of TBI, it is important to record transient pressure data during a blast event. However, very few sensors feature the capabilities of tracking the dynamic pressure transients due to the rapid change of the pressure during blast events, while not interfering with the physical material layers or wave propagation. In order to measure the pressure transients efficiently, a pressure sensor should have a high resonant frequency and a high spatial resolution. This paper describes an ultra-fast fiber optic pressure sensor based on the Fabry–Perot principle for the application of measuring the rapid pressure changes in a blast event. A shock tube experiment performed in US Army Natick Soldier Research, Development and Engineering Center has demonstrated that the resonant frequency of the sensor is 4.12 MHz, which is relatively close to the designed theoretical value of 4.113 MHz. Moreover, the experiment illustrated that the sensor has a rise time of 120 ns, which demonstrates that the sensor is capable of observing the dynamics of the pressure transient during a blast event.

Ultrafast Fabry–Perot fiber-optic pressure sensors for multimedia blast event measurements

A shock wave (SW) is characterized as a large pressure fluctuation that typically lasts only a few milliseconds. On the battlefield, SWs pose a serious threat to soldiers who are exposed to explosions, which may lead to blast-induced traumatic brain injuries. SWs can also be used beneficially and have been applied to a variety of medical treatments due to their unique interaction with tissues and cells. Consequently, it is important to have sensors that can quantify SW dynamics in order to better understand the physical interaction between body tissue and the incident acoustic wave. In this paper, the ultrafast fiber-optic sensor based on the Fabry-Perot interferometric principle was designed and four such sensors were fabricated to quantify a blast event within different media, simultaneously. The compact design of the fiber-optic sensor allows for a high degree of spatial resolution when capturing the wavefront of the traveling SW. Several blast event experiments were conducted within different media (e.g., air, rubber membrane, and water) to evaluate the sensors performance. This research revealed valuable knowledge for further study of SW behavior and SW-related applications.

Sheet forming analysis of woven FRT composites using the picture-frame shear test and the nonorthogonal constitutive equation

This study investigates the validity of a nonorthogonal constitutive equation that has been developed for simulating the deformation behaviour of woven fabric thermoplastic (FRT) composites. The model incorporates shear material properties measured from picture-frame shear testing into the constitutive model. Two different types of single layer materials were investigated; a commingled woven fabric (preform) and a consolidated form of the preform. Picture-frame testing at elevated temperatures was conducted on the consolidated samples and experimental test data was used to determine the relationship between shear stress and shear strain. Experimental validation of the model was conducted utilising a stamp thermo-hydroforming press without the use of the counteracting fluid. To address an important issue in utilising picture-frame shear testing, a virtual test was performed with experimentally determined data in an implicit finite element solver. The stamping forming simulation was also performed by the constitutive equation with the shear material properties, showing good agreement with experimental results.

Sensing performance of electrically conductive fabrics and suspension lines for parachute systems

The electronic sensing capabilities of parachute fabrics and suspension lines coated with conducting polymers, single-walled carbon nanotubes, and their composites are described. A new synthetic method is described to obtain a thin, strongly adhering coating of conducting polymers on commercial parachute fabrics and suspension lines using oligoanilines as an undercoating. The results indicate that both materials have a sensing ability; however, the coated suspension lines show superior performance compared to the coated parachute fabrics.