CompositesWorld

DEC 2018

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NEWS CompositesWorld.com 19 data vs. a legacy time/temperature recipe. Cure cycles also can be optimized, enabling the use of microwave and induction heating to deliver highly targeted and near-instaneous temperature as needed to achieve fast cure rates without "overcooking" the entire composite part. Macro vs. microwire sensors •ermocouples are the most common temperature sensor used in composites processing today. Formed by two wires of di•erent metals joined at one end, they generate a current with change in temperature. •ermocouples are inexpensive and can provide accurate temperature readings, but they must be plugged into a voltmeter. Even though the individual wires may have a very small diameter, the completed, data-generating assembly cannot be embedded into a part or bondline without reducing struc- tural properties and also posing vacuum bag challenges (i.e., source of potential leak paths) which may lead to poor quality composite parts. In contrast, the microwire sensors in AvPro's •ermoPulse system are †.‡ˆ mm in diameter and ‰‡ mm long, and have successfully measured temperature while embedded beneath a carbon Šber-reinforced polymer (CFRP) laminate more than ‡ˆ mm thick. In lap shear test results, coupons with and without embedded sensors in the adhesive bondline are indistin- guishable. •e microwire sensors are made from amorphous metal alloys, mainly cobalt and iron. •eir magnetic properties are unique. First, they polarize in only two possible states — along the wire length in one direction, or the opposite direction. Furthermore, the polarity changes nearly instantaneously — referred to as a Barkhausen Jump. When an alternating electromagnetic Šeld is applied to a sensor, these Barkhausen Jumps cause sharp voltage pulses that can be detected remotely with an antenna. •e integral of each pulse is temperature-dependent. One more key component to this measurement mechanism is that the microwire metallurgy can be tailored to a speciŠc Curie Temperature, which is the temperature above which the voltage pulse will no longer occur. Note, this is a certiŠable physical property of the manufactured microwire. Precise temperature is able to be extracted from the microwire's voltage pulse because the magnitude of the integral decreases nonlinearly as the microwire's temperature approaches its Curie Temperature (Fig. ‡, p. ‡†). •us, the •ermoPulse antenna sends out a low-frequency electromagnetic Šeld to interrogate the embedded sensor and then receives the resulting voltage pulse, which is then converted by the reader box into a temperature measurement at that sensor location. •ermoPulse sensors are autocalibrating and are actually made up of three microwires encapsulated in a rigid tube. (It is the tube that is †.‡ˆ mm in diameter; each of the wires is †.†‰ mm Measuring inside bondlines FIG. 1 Embedded sensors, antenna and temperature control ThermoPulse microwire sensors are much smaller than thermocouples (center and right, respectively, left photo) currently used to measure temperature. These microsensors remain embedded in bondlines and parts without causing defects and are interrogated using a magnetic antenna (top photo) which plugs into a hot bonder (bottom photo) for data logging and analysis, temperature readout and control of heat blanket, autoclave, oven or other temperature/curing device. Source | AvPro Inc.

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