Shridhar Yarlagadda

A study on the induction heating of conductive fiber reinforced composites

A unified approach, considering three possible heating mechanisms: fiber (Joule losses) and fiber crossover junction (dielectric hysteresis and contact resistance), to identify dominant heating mechanisms during induction processing of conductive fiber reinforced composites is presented. Non-dimensional parameters were proposed to identify the relationships between heating mechanisms and process and material parameters. Parametric studies showed that junction heating mechanisms dominate fiber heating for the material systems considered, with the exception of relatively low contact resistance (< 10 3). Results for dielectric hysteresis and low contact resistance were consistent with individual models in the literature. A design map relating the three mechanisms is presented that can help identify the dominant heating mechanism, given the properties of the composite.

Resistive Susceptor Design for Uniform Heating during Induction Bonding of Composites

A novel susceptor concept for metal mesh susceptors, designed to achieve uniform in-plane temperatures during induction heating, is documented. The process involves redirecting eddy current flow patterns in the resistive mesh susceptor by specifically designed cut patterns in the mesh. A theoretical model was developed to predict heat generation in metal mesh susceptors with any prescribed network pattern. Initial results for meshes with cut patterns show significant changes in heating compared to an uncut mesh. Cut patterns can be optimized to reduce temperature gradients in the susceptor to within the processing window of the composite. Experimental results are presented for qualitative comparisons.

A Comparison of Oven-cured and Induction-cured Adhesively Bonded Composite Joints

The advantages of using adhesives for joining composite structures are now well accepted. Adhesive joints may offer, over bolted joints, advantages such as a lower assembly weight, a superior stress transfer and an improved fatigue resistance. However, in some applications the above advantages may be offset by the processingconditions required to cure the adhesive. Indeed, in the conventional oven curing process the thermal energy must diffuse through the composite layers to heat the joint interfaces, resultingin longand expensive processingtime as well as wasted energy. A novel method of achieving adhesive bonds is addressed in the present study. The method of electromagnetic heating is well suited for rapid and efficient localized heatingof adhesive bond lines, provided suitable susceptors are used at the interfaces. This paper presents the results of a study on the use of induction heating for bondingcomposite adherends. Single-lap shear tests and double-cantilever beam fracture experiments were performed on oven-cured and induction-cured adhesively bonded joints made from woven-fabric composites. The feasibility of usingthe induction-heatingtechnique for the hardeningof adhesives in composite bonded joints was demonstrated. It was found that the strength of single-lap shear joints was not significantly affected by the choice of the hardening method of the adhesive. Furthermore, the critical fracture energy of the oven-cured and induction-cured double-cantilever beams was found to be only dependent on the adhesive type. Induction-cured specimens were found to be as tough as corresponding oven-cured specimens.

A Time-domain Reflectometry Method for Automated Measurement of Crack Propagation in Composites during Mode I DCB Testing

This article describes a new technique for automated measurement of crack initiation, growth, and propagation in composite materials during mode I double cantilever beam (DCB) testing. The proposed method uses time-domain reflectometry (TDR) to detect changes in geometry and electromagnetic properties (dielectric or magnetic) along a transmission line that can be embedded in or bonded to the surface of the specimen. Two types of transmission line TDR sensors are evaluated (IM7 carbon fiber and ARACON) during DCB tests. A P-SPICE transmission-line simulation model is used to verify the baseline signal response for the DCB sensor and the sensitivity for crack detection, with good agreement. Comparison with standard visual methods in DCB testing showed excellent correlation in crack location, crack propagation (LC), and the interlaminar fracture toughness (GIC) values. The TDR sensor design and model-based parametric studies are carried out to determine optimal sensor geometry and configuration. The results demonstrate that the TDR-based method can measure crack propagation parameters at high resolution and accuracy, in an automated manner using low-cost sensors.

Accelerated Curing of Adhesives in Bonded Joints by Induction Heating

This paper presents the results of a study on the use of induction heating for rapid curing of a commercially available room-temperature curing paste adhesive, with a view to application in the repair of composite structures. The repair of damaged composite structures using adhesively bonded patches is often a very time-consuming process. This is partly due to the long times required for complete and satisfactory cure of the adhesive systems used. While the curing process can be accelerated by application of heat through devices such as heating blankets and lamps, these methods are inefficient since considerable heat is lost to the surrounding material and environment. The method of electromagnetic heating, however, is well suited for rapid and localized heating of the adhesive bondline provided suitable susceptors are used. In this paper, it is shown that induction heating can be successfully used to cure a room-temperature curing paste adhesive. Furthermore, results of single lap shear and double notched shear specimen tests show no substantial reduction in the strength of bonded joints with the use of embedded susceptors.

Broadband Electromagnetic Modeling of Woven Fabric Composites

We demonstrate a new method for predicting the broadband electromagnetic (EM) wave propagation characteristics of woven fabric composites. The method combines a rigorous EM model with effective media theory to predict the EM properties of structural composites from dc to 50 GHz. Experimental results are provided that demonstrate the validity of the method. We also describe the presence of large narrow band electromagnetic resonances that occur above 30 GHz. These resonances, which are shown to be guided mode resonances, can be predicted by solving a simple dispersion relation.

Effect of the manufacturing process on the interfacial properties and structural performance of multi-functional composite structures

A composite integral armor (CIA) structure consists of various layers such as ceramics, rubber and polymer composites assembled in a precise sequence to provide superior ballistic and structural performance at low areal density. CIA structures were originally manufactured in a labor-intensive multi-step process. In recent years, vacuum-assisted resin transfer molding (VARTM) has emerged as an affordable manufacturing method for CIA structures. In this paper, the relationship between the manufacturing processes (i.e. VARTM and multi-step) and the mechanical performance of CIA beams is investigated by four-point bend tests. The behavior of the CIA is found to be highly dependent on the mechanism of stress transfer between the layers and the structures are found to fail progressively and provide significant ductility and capacity. The VARTM process is found to produce structures with superior mechanical performance. Moreover, the level of interface adhesion achieved during processing is shown to control the structural behavior of the CIA. Consequently, the Mode I fracture testing of VARTM and multi-step manufactured double-cantilever beams, representative of one interface of the CIA, is characterized. The resistance to crack growth of the specimens is also related to the manufacturing process, with the VARTM specimens achieving the highest fracture toughness.

A study on the induction heating of carbon fiber reinforced thermoplastic composites

Recent work in the literature has identified a new heating mechanism during induction processing of carbon thermoplastic prepreg stacks: contact resistance between fibers of adjacent plies. An experimental methodology has been developed to estimate the contact resistance through heating tests based on the properties of the composite and geometry of the specimen. Measured values indicate comparable resistance values at the contact region, compared to resistance in the fiber direction, for AS-4/PEI prepreg stacks under vacuum pressure. The measured values can serve as inputs for induction heating models and process models of carbon thermoplastic prepreg stacks.

Normal Incidence Free Space Optical Data Porting to Embedded Communication Links

In this paper, various techniques for normal incidence, free-space optical data porting to embedded data busses are presented. In the first approach, externally powered optical transmitter and receiver devices are coupled to an embedded electrical bus. This embedded link proves to be reliable through environmental and mechanical testing, and demonstrates data transmission up to 10 kHz. A self-powered transceiver is then fabricated and coupled to an optical fiber, showing data transmission up to 1 MHz. Finally, a self-powered bus using a dye-impregnated optical fiber is demonstrated to operate at speeds greater than 15 MHz. All methods explored overcome many of the problems associated with traditional physical connectorization, and are suitable for normal incidence remote querying of embedded passive elements, active devices, sensors, or networks.

Development of a numerical model to predict in-plane heat generation patterns during induction processing of carbon fiber-reinforced prepreg stacks

A numerical model is proposed to describe in-plane heat generation spatial response during induction processing of carbon fiber-reinforced thermoplastics. The model is based on a unified approach that considers three possible heating mechanisms: fiber heating (Joule losses in fiber), noncontact junction heating (dielectric hysteresis), and contact junction heating (Joule losses at junctions). A lumped meshing scheme is used to construct a numerical representation for cross-ply and angle-ply orientations of 2-ply prepreg stacks. Heat generation patterns are calculated based on voltage and current conservation laws and verified with induction heating of AS4 carbon fiber-reinforced polyetherimide (AS4/PEI) prepreg stacks. Excellent agreement is found except at very low angle-ply orientations where the predicted heating patterns show significant deviations from the experiment results. A sensitivity analysis is also performed to assess the relationship between heating patterns and material and process parameters. The results show that the stack angle between plies and intrinsic prepreg microstructure can significantly affect the heating patterns.