XXIV ICTAM, 21-26 August 2016, Montreal, Canada
AN EXPERIMENTAL AND COMPUTATIONAL STUDY ON A HIGH TEMPERATURE NiTi/POLYIMIDE MATRIX COMPOSITE INTERFACE Hieu Truong1a), Ozden Ochoa1, Dimitris Lagoudas2, John Connell3 & Frank Palmieri3 Mechanical Engineering Department, Texas A&M University, College Station, Texas, USA 2 Aerospace Engineering Department, Texas A&M University, College Station, Texas, USA 3 Advanced Materials & Processing Branch, NASA Langley Research Center, Hampton, Virginia, USA 1
Abstract For the first time, shape memory alloys (SMA) were successfully co-cured in a polyimide matrix composite to create high temperature, smart, hybrid composite laminates. In the work presented herein, the nickel-rich nickel titanium (NiTi) foil was used as the SMA constituent. A combination of laser ablation and amide acid sol-gel surface treatment performed on the NiTi foil created robust adhesion between the foil and the polyimide matrix. The mode-I fracture toughness of the hybrid SMA-composite interface was investigated via the double cantilever beam (DCB) test with in-situ digital image correlation (DIC) measurements. The dominant mode of failure revealed after the DCB tests was cohesive. Phase transformation in the NiTi foil was observed during the isothermal DCB tests at 25oC. Finite element analyses of the hybrid DCB specimen indicated that the NiTi foil underwent stress-induced phase transformation in the vicinity of the crack tip prior to and during crack propagation.
INTRODUCTION Hybrid high temperature composite materials are excellent candidates as lightweight structural materials for supersonic and hypersonic vehicles. Recently, the integration of SMA actuators into polymer matrix composite (PMC) laminates has attracted significant interest as a means to create smart and morphing aerospace components. The active attribute of hybrid SMA-PMC composites is inherent in the thermo-mechanically induced phase transformation of the SMA constituent. The effectiveness of load transfer between the SMA actuators and the composites dictates functionality of the hybrid structures. This load transfer effectiveness depends on the ability of the interface between the SMA and the PMC to propagate energy. It has been reported that hybrid SMA-PMC structures have been created by adhesively joining the components [1]. However, in these adhesive joints, there are two distinct interfaces (SMA-adhesive and adhesive-PMC) and therefore require a unique adhesive that can form robust interfaces with the two distinct materials to assure the optimal structural integrity of the hybrid composite. In this study, the SMA was surface-treated by laser ablation followed by treatment with a custom synthesized sol-gel formulation, and the hybrid composites were co-cured. Thus, the SMA-PMC hybrid interface was created without using an adhesive. That is, adhesion was achieved directly between the treated SMA surface and the polymer matrix of the PMC upon curing. Fabrication and investigations on hybrid, co-cured interfaces between Al, Ti, NiTi and epoxy-based matrices were previously performed by Truong et al. [2-4]. This paper details our approach to develop high temperature, smart material systems using a custom designed robust interface to fabricate NiTi foil/polyimide matrix hybrid composites. The results of this experimental and computational investigation are presented herein. APPROACHES Experimental Approach The hybrid composite laminate was fabricated with a single layer of Ti-50.8at%Ni foil (127 µm thick) sandwiched between four layers of composite pre-preg on each side. The pre-preg used in this study was 8-harness satin weave T650 carbon fabric/AFR-PE-4 matrix (cured Tg = 392 oC). Two layers of 50-µm thick Kapton film were used to create the precrack for fracture toughness tests. Prior to placing the NiTi foil in the panel layup, the foil was heat treated at 500 oC for 1 hour followed by aging at 400 oC for 45 min, and water quenching to activate the shape memory effects. In addition, a combination of laser ablation [5] and amide acid sol-gel surface treatment technique [6] was performed on both sides of the NiTi foil surfaces to create strong adhesion through covalent bond formation with the polyimide matrix. No adhesive was used at the NiTi-PMC interface. The composite laminate was cured in a Wabash hot-press under a 12-hour curing cycle at temperature and pressure up to 371oC and 1.4 MPa, respectively. The quasi-static double cantilever beam (DCB) tests with in-situ DIC measurements were performed at room and elevated temperatures. Each DCB specimen was tested by displacement-controlled loading to an opening displacement of 25 mm then unloaded to zero displacement. Other characterizations of the hybrid laminates and NiTi foil included differential scanning calorimetry (DSC), thermalmechanical analysis (TMA) and dynamic mechanical analysis (DMA) as well as optical microscopy (OM) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) on the cross-sections and fracture surfaces of the tested DCB specimens. Computational Approach Two-dimensional finite element analyses (FEA) of the DCB specimen were carried out using the commercial FEA software ABAQUS to study the mode-I delamination behavior of the hybrid interface. The virtual crack closure technique (VCCT) with B-K mixed mode fracture criterion was used to model crack propagation at the NiTi-PMC interface. Linear incompatible plane strain elements (CPE4I) were used together with the built-in nonlinear geometric option in ABAQUS activated. Transversely isotropic linear elastic material properties were used for the PMC while the NiTi foil’s nonlinear isotropic material properties were modelled using a user material subroutine (UMAT) defining the SMA behaviors based on the constitutive relations developed by Lagoudas et al. [7]. a)
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PRELIMINARY RESULTS AND DISCUSSION The micro-roughness pattern created by laser ablation on the NiTi foil surface is shown in Figure 1(a). Figure 1(b) shows the fracture surfaces at the hybrid interface after a DCB test at 25 oC. It was observed that the dominant mode of failure was cohesive. The crack first started adjacent to the NiTi surface (adhesive failure) then migrated into the PMC region (cohesive failure) and remained there until the test completion. This suggests a strong hybrid NiTi-PMC interface was achieved in this work. The critical mode-I fracture toughness of this interface is 685±7 J/m2. DSC measurements showed that after undergoing laser ablation treatment and curing cycle, the NiTi foil still exhibited SMA behavior, and at 25 o C, the material was fully in the austenitic phase. The DCB test results indicated phase transformation occured in the NiTi foil during the test. This was evidenced by the residual opening displacement at no load after the test as illustrated in the load-displacement plot in Figure 1(c) as well as the actual specimen shown in Figure 2(b) where the gap between the two arms is ~1 mm. The FEA results illustrated in Figure 2(a) revealed that the residual displacement was 1.016 mm.
Figure 1. (a) Laser ablated NiTi surface (b) Fracture surfaces (c) Load-displacement curves from DCB tests at 25 oC and FEA
It can be seen from Figure 2(b) that stress-induced phase transformation occured in the NiTi foil in the vicinity of the crack tip. Note that before loading, the foil was completely in the austenitic phase (0% martensitic vol. fraction). Before crack growth from the pre-crack, the martensitic volume fraction in the foil reached 15.8% at the crack tip. As the crack propagated, the region containing martensitic phase in the NiTi foil grew accordingly with a martensitic vol. fraction of approximately 8%.
Figure 2. (a) Residual displacement in the DCB after unloading (b) Martensitic volume fraction in the NiTi foil near the crack tip
CONCLUSIONS NiTi foil was successfully joined to a polyimide matrix composite laminate by co-curing. Robust adhesion between the NiTi foil and composite was achieved by performing a combination of laser ablation and amide acid sol-gel treatment on the NiTi surfaces. The mode-I fracture toughness of the hybrid interface was investigated by the DCB tests as well as FE analyses. It was observed both experimentally and computationally that the NiTi foil underwent a phase transformation during the DCB tests at 25 oC. Current analyses using DIC measurements and FEA are carried out to investigate the effects of SMA phase transformation as well as interfacial architecture (topography created by laser ablation on the NiTi foil and woven fabric reinforcements) on the strain energy release rates upon crack propagation. ACKNOWLEDGEMENT The authors acknowledge the financial support of the NASA Space Technology Research Fellowship Grant No. NNX14AM41H. The fabrication, characterization, and fracture testing of the composites were performed at NASA Langley Research Center, Hampton, VA.
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XXIV ICTAM, 21-26 August 2016, Montreal, Canada
[3] Truong, Hieu TX, Marcias J. Martinez, Ozden O. Ochoa, and Dimitris C. Lagoudas. "An Investigation on Hybrid Interface using On-Line Monitoring Experiment and Finite Element Analyses." In ICCM 20: 20th International Conference on Composite Materials, Copenhagen, Denmark, 19-24 July 2015. ICCM, 2015. [4] Truong, Hieu, Marcias Martinez, Ozden Ochoa, and Dimitris Lagoudas. "Experimental and Computational Investigations of Hybrid Interfaces in Hybrid Composite Laminates." In American Society of Composites-30th Technical Conference. 2015. [5] Palmieri, Frank L., Kent A. Watson, Guillermo Morales, Thomas Williams, Robert Hicks, Christopher J. Wohl, John W. Hopkins, and John W. Connell. "Laser Ablative Surface Treatment For Enhanced Bonding of Ti-6Al-4V Alloy." ACS Applied Materials & Interfaces 5, no. 4 (2013): 12541261. [6] Park, C., S. E. Lowther, J. G. Smith, J. W. Connell, P. M. Hergenrother, and T. L. St Clair. "Polyimide–Silica Hybrids Containing Novel Phenylethynyl Imide Silanes as Coupling Agents for Surface-Treated Titanium Alloy." International Journal of Adhesion and Adhesives 20, no. 6 (2000): 457-465. [7] Lagoudas, Dimitris, Darren Hartl, Yves Chemisky, Luciano Machado, and Peter Popov. "Constitutive Model for the Numerical Analysis of Phase Transformation in Polycrystalline Shape Memory Alloys." International Journal of Plasticity 32 (2012): 155-183.
a)
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