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Research Papers

Bondline Temperature Control for Joining Composites With an Embedded Heater

[+] Author and Article Information
Brandon P. Smith, Mahdi Ashrafi, Mark E. Tuttle, Santosh Devasia

Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received February 5, 2015; final manuscript received July 2, 2015; published online September 9, 2015. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 138(2), 021011 (Sep 09, 2015) (9 pages) Paper No: MANU-15-1073; doi: 10.1115/1.4031069 History: Received February 05, 2015

This paper applies estimation and control techniques to achieve the desired bondline temperature for out-of-autoclave, adhesive bonding of carbon-fiber composite components. Rather than heating the entire system to cure the adhesive, e.g., in an autoclave, this research uses controlled heating targeted at the bondline with an embedded heater. The main contribution of this work is control using estimates of the bondline temperature without embedded sensors and wiring (of materials not intrinsic to the system) that can lead to reduced bond strength. Sensors placed outside the bond region are used to accurately estimate the bondline temperature to within 2% of the temperature range over which the bondline temperature is changed. Experimental results show that the estimated temperature can be used with feedback to achieve the desired, bondline, time–temperature profile that is needed to cure the adhesive. Additionally, shear lap joint tensile tests show that the resulting joint strength is comparable to specimens bonded using an external heat blanket.

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Figures

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Fig. 1

Schematic of experimental system during joining. An embedded heater is used to cure the structural-film adhesive, which bonds two carbon-fiber composite adherends. The entire system is insulated and is under vacuum pressure.

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Fig. 2

The desired temperature profile Ti,d at the bondline, as recommended by the adhesive manufacturer. Note that the initial value Ti,d(0) at time t = 0 is room temperature, which depends on the experimental conditions.

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Fig. 3

Schematic of experimental system (without bonding) for controller and estimator design. The structural-film adhesive in Fig. 1 is replaced with Teflon insulating sheets to prevent adhesive bonding during experimental estimation of model parameters. The entire system is still under vacuum pressure as in the joining case.

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Fig. 4

Open-loop step response Ti of the thermal system: (dotted line) response of fitted model GT,i in Eq. (2) and (solid line) experimentally measured

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Fig. 5

Block diagram of control scheme when a sensor is available to measure the inner bondline temperature Ti

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Fig. 6

Schematic of the discrete-time implementation of the controller (C(z) in Eq. (18)) with a saturation block

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Fig. 7

Schematic of temperature model through adherend

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Fig. 8

Results of step response as in Fig. 4 for (a) the measured temperatures (inner bondline temperature Ti and outer temperatures To and T1) and the estimated temperature Ti,e calculated from Eq. (22), and (b) the error eE in the fit from Eq. (28)

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Fig. 9

RMS value of the error eE in the estimated temperature Ti,e due to variations in the parameters p1 = k1, p2=ka, and p3=ρLcp

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Fig. 10

Block diagram of control system during joining. The inner bondline temperature Ti  is controlled using the estimated inner bondline temperature Ti,e.

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Fig. 11

Experimental results with cure profile without adhesive with (a) measured inner bondline temperature Ti, estimated inner temperature Ti,e, and desired inner temperature Ti,d with the control as in Fig. 10, (b) estimator error eE  as in Eq. (28), and (c) tracking error ed as in Eq. (38)

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Fig. 12

Photo of the experimental setup during bonding with the ERH

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Fig. 13

Experimental results with cure profile during bonding with adhesive with (a) measured inner bondline temperature Ti, estimated inner temperature Ti,e, and desired inner temperature Ti,d, (b) estimator error eE as in Eq. (28), and (c) tracking error ed as in Eq. (38)

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Fig. 14

Temperature profiles for bonding with (a) an embedded heater, (b) an external heat blanket with fabric in the bondline, and (c) an external heat blanket without fabric in the bondline

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Fig. 15

Schematic of bonding with Heatcon HB with external heater

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Fig. 16

Schematic of specimens cut from single lap joint cut for tensile testing with top view (left), and side view after adding tabs (right)

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Fig. 17

Mean tensile test load at failure for single lap joints made with an ERH, HB without embedded fabric, and HB with embedded fabric, all with one standard deviation error bars

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