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Miamisburg, OH, United States

Gao Y.,Illinois Institute of Technology | Zhou Y.,Illinois Institute of Technology | Wu B.,Illinois Institute of Technology | Tao S.,Illinois Institute of Technology | And 2 more authors.
Journal of Manufacturing Science and Engineering, Transactions of the ASME

Silicon carbide, due to its unique properties, has many promising applications in optics, electronics, and other areas. However, it is difficult to micromachine using mechanical approaches due to its brittleness and high hardness. Laser ablation can potentially provide a good solution for silicon carbide micromachining. However, previous studies of silicon carbide ablation by nanosecond laser pulses at infrared wavelengths are very limited on material removal mechanism, and the mechanism has not been well understood. In this paper, experimental study is performed for silicon carbide ablation by 1064 nm and 200 ns laser pulses through both nanosecond time-resolved in situ observation and laser-ablated workpiece characterization. This study shows that the material removal mechanism is surface vaporization, followed by liquid ejection (which becomes clearly observable at around 1 μs after the laser pulse starts). It has been found that the liquid ejection is very unlikely due to phase explosion. This study also shows that the radiation intensity of laser-induced plasma during silicon carbide ablation does not have a uniform spatial distribution, and the distribution also changes very obviously when the laser pulse ends. © 2011 American Society of Mechanical Engineers. Source

Nalladega V.,University of Dayton | Na J.K.,University of Dayton | Druffner C.,Mound Laser and Photonics Center Inc.
AIP Conference Proceedings

Interdigital transducers (IDT) generate and receive ultrasonic surface waves without the complexity involved with secondary devices such as angled wedges or combs. The IDT sensors have been successfully applied for the NDE of homogeneous materials like metals in order to detect cracks and de-bond. However, these transducers have not been yet adapted for complex and anisotropic materials like fiber-reinforced composites. This work presents the possibility of using IDT sensors for monitoring structural damages in wind turbine blades, typically made of fiberglass composites. IDT sensors with a range of operating frequency between 250 kHz and 1 MHz are initially tested on representative composite test panels for ultrasonic surface wave properties including beam spread, propagation distance and effect of material's anisotropy. Based on these results, an optimum frequency range for the IDT sensor is found to be 250-500 kHz. Subsequently, IDT sensors with operating frequency 500 kHz are used to detect and quantify artificial defects created in the composite test samples. Discussions are made on the interaction of ultrasonic fields with these defects along with the effects of fiber directionality and composite layer stacking. © 2011 American Institute of Physics. Source

Druffner C.,Mound Laser and Photonics Center Inc. | Nalladega V.,University of Dayton | Na J.K.,University of Dayton
AIP Conference Proceedings

To increase the power generating capacity of a wind turbine composite turbine blade manufacturers have been increasing the size of blades. Current utility-scale windmills are equipped with blades ranging from 40 m (130 ft) to 90 m (300 ft) in their sweep diameter. The increased blade size brings greater structural and safety demands. Recent blade recalls and field failures highlights the market need for sensors capable of part quality inspections on manufacturing line and for structural health monitoring (SHM) of the composites in service. An ultrasonic surface wave sensor technology based on interdigitization transduction (IDT) has been developed that can inspect and detect defects in the composite blades. The current work covers the design, fabrication, and characterization of the IDT sensors. The sensor characterization, coverage area, and detection capability for a variety of defects such as impact, cracking and delamination will be discussed. © 2011 American Institute of Physics. Source

Na J.K.,University of Dayton | Nalladega V.,University of Dayton | Druffner C.,Mound Laser and Photonics Center Inc.
Proceedings of SPIE - The International Society for Optical Engineering

Polyester resin based glass fiber reinforced composite panels obtained from a local windmill turbine blade part manufacturing company are used to evaluate the performance of inter-digital transducer (IDT) surface wave transducers. Interaction of surface waves with fiberglass layers is addressed in this work. Additionally, artificially created flaws such as cracks, impact damage and delamination are also studied in terms of amplitude changes in order to attempt to quantify the size, location and severity of damage in the test panels. As a potential application to the structural health monitoring (SHM) of windmill turbine blades, the coverage distance within the width of the sound field is estimated to be over 80 cm when a set of IDT sensors consisted of one transmitter and two receivers in a pitch-catch mode. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE). Source

Tao S.,Illinois Institute of Technology | Jacobsen R.L.,Mound Laser and Photonics Center Inc. | Wu B.,Illinois Institute of Technology
Applied Physics Letters

Investigations have been performed on the physical mechanisms of picosecond laser ablation of silicon carbide at 355 and 1064 nm, which have not been well understood yet. The study shows that the low-fluence ablation rates are close for 355 and 1064 nm, and the dominant material removal mechanism should be surface evaporation. At fluences above ∼2J/ cm2, the ablation rate increases very quickly for 355 nm, and the associated dominant mechanism is very likely to be critical point phase separation. For 1064 nm, the ablation rate variation with fluence above ∼2 J/ cm2 follows the same trend as that for low fluences, and the mechanism should remain as surface evaporation. © 2010 American Institute of Physics. Source

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