Modeling of Ultrasonic Processing Margaret Roylance John Player Walter Zukas David Roylance Foster Miller Inc Waltham Massachusetts Received 11 November 2003 accepted 21 December 2003 DOI 10 1002 app 20595 Published online in Wiley InterScience www interscience wiley com ABSTRACT Curing of ber reinforced thermoset polymer composites requires an elevated temperature to accelerate the crosslinking reaction and also hydrostatic pressure to consolidate the part and suppress the formation of voids These processing conditions can be provided by autoclaves of appropriate size but these are expensive and sometimes dif cult to schedule Ultrasonic debulking followed by oven cure is an attractive alternative to autoclave cure In this technique a movable horn driven at ultrasonic frequency is applied to the surface of the uncured part This generates pressure and at the same time produces heating by viscoelastic dissipation The part can be debulked to net shape INTRODUCTION Ultrasonic tape lamination UTL is a technique that uses high frequency loading to achieve heating and consolidation of ber reinforced composite materials via viscoelastic energy dissipation 1 In conjunction with E Beam curing or a variety of thermal techniques such as solid state cure UTL provides an avenue for out of autoclave curing of high quality large scale ber placed composite structures This approach offers the possibility of out of autoclave processing without the development and quali cation of new resin formulations Although the use of ultrasonic welding of metals and unreinforced polymeric materials is an important industrial process process innovations have only recently made the use of ultrasound for the consolidation of polymer matrix composites containing more than 35 to 40 by volume of reinforcing ber possible Among other process parameters experiments have demonstrated that the angle at which insoni cation occurs is a critical process variable Figure 1 shows the UTL head mounted on a lament winder To induce consolidation without damaging a ber reinforced Correspondence to D Roylance roylance mit edu Permanent address W Zukas U S Army Soldier and Biological Chemical Command Natick MA Permanent address Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge MA 02139 Journal of Applied Polymer Science Vol 93 1609 1615 2004 2004 Wiley Periodicals Inc and staged through the action of the ultrasound There are a large enough number of experimental parameters in ultrasonic debulking and staging to make purely empirical process optimization dif cult and this paper outlines numerical simulation methods useful in understanding and developing the process 2004 Wiley Periodicals Inc J Appl Polym Sci 93 1609 1615 2004 Key words composites computer modeling curing of polymers differential scanning calorimetry DSC viscoelastic properties composite a horn angle of less than 90o is required Changing the horn angle changes the stress state in the material during insoni cation and thus changes the relative amount of energy dissipated by viscoelastic heating of the matrix compared to ber disruption Experiments have shown UTL to be effective both for consolidation of thermoplastic matrix composite materials and debulking of B staged thermoset prepreg 2 In both processes the ultrasonic loading must generate suf cient heat to induce ow and provide suf cient support for consolidation but one added complication of UTL of thermosets is the potential for thermally induced curing An ability to model and control this chemical reaction is essential to the use of UTL as part of an out of autoclave processing scheme This paper will describe some of the development required for such a model and some examples of the results that can be obtained from it To clarify the relative importance of frictional and viscoelastic heating the effects of pressure and amplitude on UTL induced heating have been measured 3 The data showed that increasing the static pressure has no observable effect on the heat generation in the material during UTL suggesting that the viscoelastic heating is dominating the UTL heat generation This conclusion is in agreement with the results of Tolunay et al 4 which showed for soft polymers the interface did not have a signi cant effect on the amount of heat dissipated during ultrasonic welding of unreinforced materials They observed that in this case the heating occurs over the whole volume 1610 ROYLANCE ET AL Figure 1 Ultrasonic tape lamination during lament winding Further they observed that intensive heating of the material began only after a certain temperature was reached This temperature most probably corresponds to the glass transition temperature since as Tg is approached the level of viscoelastic energy dissipation as measured by loss modulus increases markedly As the material continues to heat above Tg the loss modulus drops again and Tolunay et al 4 observed that the heating rate also generally drops until the temperature remains constant Based on these results the UTL process model described here focuses on viscoelastic energy dissipation and reaction exotherm as volumetric heat sources rather than frictional heating at the interfaces VISCOELASTIC DISSIPATION OF UNCURED RESIN A rst step in developing a process model for ultrasonic consolidation is estimating the amount of thermal energy dissipated during a loading cycle This can be written in terms of the material s loss modulus E in the theory of linear viscoelasticity 5 The material s complex modulus E E iE is a function of both temperature and frequency and a convenient model for this dependency is the Wiechert viscoelastic expression N E k 0 j 1 k j j 2 1 j 2 1 k where the 0j are preexponential constants and E is an activation energy for viscoelasticity Each j is given the same activation energy which renders the model thermorheologically simple and amenable to time temperature shifting methodology Numerical parameters for this model have been obtained by means of dynamic mechanical analyses DMA of uncured prepreg in which an oscillatory load was applied to the surface of the prepreg lay up The prepreg utilized in this study was a unidirectional IM7 ber tow impregnated with Hexcel Corp s 8552 thermoplastic toughened high performance epoxy resin The prepreg was stored in a freezer at 30 C until used Plies were layered in a 0 90 con guration for DMA using a Perkin Elmer DMA 7e set up for dynamic compression experiments