ABSTRACT
A load applied to a composite is transferred from the matrix to the dispersed phase through their shared interface. The performance of the composite depends strongly on the interfacial adhesion between the constituents. Shear strength, resistance to delamination, and effective load transfer between the matrix and the inclusion are improved when the interfacial bond is optimized. The interfacial adhesion strength between the metallic inclusions, titanium (Ti) and a nickel titanium shape memory alloy (NiTi), and different polymeric matrices, namely polycarbonate (PC), polypropylene (PP), and high-density polyethylene (HDPE), was measured using pullout testing. The adhesion strengths were determined by dividing the force needed to produce debonding of the interface of the polymer/metal strip over the embedded strip area. Results show that PC exhibits the highest adhesion with no difference for either NiTi or Ti. Comparison of the adhesion strength values for PP/Ti and PP/NiTi show a higher and very distinct value for the PP and Ti. In the case of HDPE, the interfacial bonding between the polymer and the NiTi is stronger than with the Ti but the difference is not as big as the one found for the PP composites.
INTRODUCTION
The shear lag or load transfer theory, developed by Cox [1], predicts how an uniaxial tensile load applied to a continuous cylindrical fiber embedded in a weak matrix is redistributed when there is a strong adhesion between the two phases. Assuming no-slip between the interfaces of both constituents, and using elasticity theory, equations for the shear stress distribution and the tensile stress distribution in the fiber were developed. This theory was modified by Kelly [2] and used extensively to predict composites mechanical behavior [3, 4]. Several modifications to the load transfer theory have been introduced, which include: the effect of the load transferred to the end of the fiber to consider the effect of short fibers in the magnitude and distribution of stresses [5]; the potential existence of stress concentration in the matrix due to neighboring fibers [6]; and the incorporation of different inclusion geometries to take into account the fiber shape, such as conical [7] or rectangular layers [8]. In all cases a strong interfacial bonding between the surfaces of the inclusion and the matrix of the composite is assumed.
The mechanisms that are believed to play a role in the adhesion between two surfaces are molecular bonding, mechanical coupling, and thermodynamic adhesion [9]. Molecular bonding is due to intermolecular forces such as chemical bondings (metallic, covalent, and ionic), dipole-dipole interactions, and van der Waals forces. The mechanical coupling, known also as interlocking, is due to surface roughness, which increasing the surface area allows a greater molecular bonding interaction. The thermodynamics mechanism of adhesion is based on an optimization of the interfacial tension at the interface, which could occur when the surface free energy of the polymer is minimized. All these mechanisms are heavily dependent on the surface condition of the materials involved.
A variety of methods have been used to characterize the interfacial adhesion strength in composite materials. Peel test, lap shear, fiber pullout, fragmentation, and microindentation are some of them [9-14]. These tests each provide a mean for comparison of the interfacial strength but do not give a fundamental property measure.
In the peel test [9] adhesive tape is pressed against the surface of the sample with a rubber cylinder and quickly pulled off. The delamination toughness is calculated using the average peel force and the width of the sample. The lap shear test [14] uses an overlap configuration where two samples are bonded over a region and separated by using a tensile force. The fiber pullout test [10-13] has been used extensively to measure the force needed to pull a fiber of a known length out from a matrix. This test requires a tensile stress to pullout the fiber lower than its ultimate tensile stress. The embedded fiber length should be less than the critical length to ensure that the tensile stress inside the fiber does not reach the tensile strength producing fracture of the fiber. In the fragmentation test [10-13], a fiber embedded in a matrix is loaded with a tensile force producing multiple fractures in the fiber. The test ends when the fracture segments reach the critical length for load transfer. The microindentation test [11, 12] consists in applying a compressive stress to embedded single fibers perpendicular to a cut surface. The test aims to produce debonding and/or slippage of the fibers.
The pullout method has been used extensively to study the behavior of interfacial adhesion strength between the shape memory alloy NiTi and polymeric matrices. Paine et al. [15] used pullout tests to measured the interface adhesion between NiTi and a graphite/epoxy thermoset matrix, and a thermoplastic matrix after treating the surface of the NiTi wires using acid etching, hand sanding, polymer coating and sandblasting. They reported the maximum force on the pullout curve as the force needed for adhesion failure. Paine et al. found that the adhesion strength depends upon the matrix used and the preparation and roughness of the metallic surface. The strongest adhesion was reported for the sandblasted wires. They also reported that the embedded length does not have an effect on the interfacial shear strength.
Jonalagadda et al. [16] correlated the NiTi/polymer adhesion with the local displacements induced by stresses produced by NiTi shape memory wires embedded in an epoxy matrix. Pullout tests were carried out on samples with different surface treatments: acid etched, hand sanded and sandblasted. The results for the roughest surface, corresponding to the sandblasted wires, exhibited the lower wire displacements; the highest debond stress and the highest shear stresses in the matrix. The debond stresses were calculated using the maximum pullout force and the interfacial work of fracture following the work by Penn and Lee [17]. For the shear stresses, a shear-lag solution derived by Tyson and Davies [18] was used. Smith et al. [19] used pullout tests to compare the adhesion strength of untreated and treated NiTi surfaces with silane coupling agents. The adhesion strength was determined measuring the maximum force required to pullout the NiTi wire from the PMMA matrix divided by the length of the wire embedded in the polymer.
Al-Sheyyab et al. [20] used a different testing geometry to investigate the influence of surface coating pre-treatment on the bonding between plastic and metal (steel, and aluminum). Samples with double overlapping between polymer and metal were manufactured to measure the adhesion strength using tensile-shear test. The ratio between the metal thickness and the overlap length with the plastic was 1:5. The coated metal sheets were placed in a mold, where the polymer was fed by injection molding. The average shear stresses were calculated by dividing the total applied shear force over the contact area. Using a simulation software based on the finite elements method, they obtained the stress distribution over the contact area and identified the location with maximum local stresses. They reported that maximum stresses occurred at the sides of the overlapping area.
The study presented in this paper is part of a broader investigation to establish the key factors in the fracture of an inclusion within a polymer/metal-alloy composite during calendering [21]. Two metal inclusion materials were chosen, titanium and the shape memory alloy NiTi, and were tested in combination with three polymers, polycarbonate (PC), polypropylene (PP), and high-density polyethylene (HDPE). Our prior work has shown that the deformation and fracture behavior of the inclusion depends on the processing parameters and factors such as interfacial adhesion, the dimensions and the mechanical properties of the constituents, the deformation behavior of the polymeric matrices, and finally on how the load is transferred from the rollers to the samples. In a prior study the interfacial adhesion strength between NiTi, and three different polymers, polycarbonate, polypropylene, and high-density polyethylene [22] used in the cold rolling tests was studied by using pullout tests of a wire embedded in a polymeric matrix. The NiTi wires (0.032 in diameter) embedded in the polymers were tested at a crosshead speed of 5 mm/min. Results showed that the adhesion strength between the NiTi and the HDPE had the highest value, followed by the PC. The NiTi/PP exhibited the lowest interfacial adhesion strength. Tests performed with Ti wires of the same diameter were unsuccessful due to yielding of the wire during the pullout test. An alternative geometry was sought in order to characterize the adhesion strength between the material combinations of interest. The configuration chosen for the pullout sample was similar to the cold rolling samples to capture not only the effect of the bonding between the two surfaces in contact and avoid yielding of the embedded material, but also the effect of the shape and size of the inclusion.
EXPERIMENTAL METHODS
Tensile pullout tests were performed to measure the adhesion strength between polymers and NiTi, and Ti. The polymers used were polycarbonate, polypropylene, and high-density polyethylene, (McMaster, #8742K131, #8574K24, #8619K421, size 12" x 12" x 1/16"). Pseudoelastic NiTi, (Nitinol Devices & Components, NiTi foil, Alloy type SE 508, pseudoelastic at room temperature, 55.6 at% Ni, 0.0127 mm, and 0.6 mm thick), and Ti (Alfa Aesar, Ti foil, 99%, 0.0127 mm, and 0.5 mm thick), were used as the metal inclusions.
The strip pullout samples were made following the same procedure and process parameters used for the sandwich cold rolling samples described by Calcagno [22, 23]. Rectangular grooves 5 mm wide were cut into the stainless steel molds used to prepare cold rolling samples. Grooves with a depth of 1.02 mm were made on either side of the open 50 mm square near the center. These grooves housed the strips while the plastic was melted. The etched NiTi or Ti, approximately 50 mm long and 5 mm wide, were placed between two polymer rectangles in the molds such that roughly 20 mm of the metal was exposed in one of the grooves on the sides of the molds. This procedure was followed to make the pullout samples with thin and thick strips.
For the pullout test of the strip samples, a sample holder used to measure the adhesion between a wire and polymer interface minimizing the compressive stress to the polymer was used [24]. Such device consists of a 1" NPS stainless steel pipe nipple, and a pipe cap with a centered hole as shown in Figure 1.
For the tests of the strip pullout sample, the hole was enlarged to give the strip some degree of freedom during sample positioning to avoid torsion during the test. The sample was inserted inside the pipe with the strip going through the hole in the center of the pipe cap as seen in Figure 1(b). Two half wood cylinders were cut and placed inside the pipe nipple to keep the sample from curling, avoiding lateral compressive stress on the samples during test.
The pullout tests were performed in an Instron 5566 with a constant crosshead speed of 5 mm/min. The adhesion strength between the metal or alloy and the polymer interface was calculated by dividing the maximum force needed to produce debonding during a pullout tensile test over the embedded strip surface area.
RESULTS AND DISCUSSION
Calcagno [22] was successful in obtaining adhesion strength results for the pullout of NiTi wires (0.031 in diameter) embedded in PP, PC, and HDPE. However during the pullout tests of Ti wires (0.0313 in diameter) in PP, the wire yielded, and it fractured with PC or HDPE as matrices. In subsequent work Ti wires with a thicker diameter (0.0625 in) were used, but plastic deformation was also observed. At this point a different configuration for the pullout samples was sought. Two main factors that could affect the bonding were taken into consideration: similar surface roughness of the inclusion to the one used in the cold rolling samples, and a stress distribution during the pullout similar to the cold rolling. The double overlapping configuration used by Al-Sheyyab et al. [20] was chosen with the modification of having a strip of 5 mm wide embedded in the polymer as shown in Figure 1(a). The PP samples with thin NiTi strip were tested successfully, however during the tests of the PP/Ti samples, most of the thin Ti strips yielded, and torsion was observed during the pullout tests. This problem was overcome using thicker strips for both, Ti and NiTi. The surface of all strips was subjected to the same etching treatment applied to the cold rolling strips.
Figure 1: (a) Sketch of a pullout sample, lateral and frontal view, (b) Sketch of the sample holder for the pullout test, exploded and compressed view with applied load.
Typical stress/extension data obtained during pullout tests of PP/Ti, PC/Ti, and HDPE/Ti strip samples are shown in Figure 2. Upon continued loading, the force needed to induce failure of the polymer/strip interface is attained. That force is represented by the maximum in the curve, which in the case of PP/Ti corresponds to 2.34 N/mm2 for a strip with an embedded length of 12.11 mm. The PC/Ti sample debonded at the highest value of the three cases with adhesion strength of 4.57 N/mm2 and embedded strip length of 12.1 mm. For the HDPE/Ti the maximum strength is 2.00 N/mm2 for a strip with an embedded length of 10.3 mm. After this point, only friction opposes the sliding of the strip [3, 25, 26]. The load required to overcome the interfacial adhesion between both materials is much lower than the load needed to induce deformation in the strips of Ti, thus the adhesion strength was calculated by dividing this force over the surface area of the embedded strip.
Figure 2: Adhesion Stress vs. Displacement for PP/Ti, PC/Ti, and HDPE/Ti strips pullout samples
Similarly, Figure 3 portrays typical stress/extension data for pullout tests of PP/NiTi, PC/NiTi, and HDPE/NiTi strip samples. In the case of PP/NiTi the stress needed to induce debonding between the polymer and the strip, was 1.52 N/mm2 for a strip with an embedded length of approximately 10.88 mm. The PC/NiTi sample debonded at the highest value of the tree cases with adhesion strength of 4.15 N/mm2 and embedded strip length of 10.48 mm. The adhesion strength obtained for the HDPE/Ti was 1.8 N/mm2 for a strip with an embedded length of 8.63 mm.
Figure 3: Adhesion Stress vs. Displacement for PP/NiTi, PC/NiTi, and HDPE/NiTi strips pullout samples.
Sample preparation and pullout tests for the PP/Ti, and PP/NiTi were performed as planned. In the case of the HDPE/Ti and HDPE/NiTi, the results of few samples were discarded due to bending of the strip during the test caused by incorrect placing of the sample in the grips. Some of the PC/Ti and PC/NiTi samples contained bubbles in the polymer, which occurred during the manufacture of the samples. Results for these samples were also excluded from the adhesion strength computations. Further information for all the NiTi and Ti samples tested is included in reference [21]. The results of all pullout tests are summarized in Table 1.
A comparison of the adhesion strength values obtained for the six materials combinations tested is provided in Figure 4. The interfacial bonding strength for each polymeric matrix and metallic inclusion has very low standard error for each condition. The PC exhibits the highest adhesion with no difference between NiTi and Ti. Comparison of the adhesion strength values for PP/Ti and PP/NiTi show a higher and very distinctly different value for the PP and Ti. In the case of HDPE, the interfacial bonding between the polymer and the NiTi is stronger than with the Ti but the difference is not as big as the one founded for the PP composites.
Table 1: Summary for the average adhesion strength, and standard deviations for all strip pullout samples.
|
Constituent |
NiTi |
|
"i |
|
|
Adhesion Strength (N/mm2) |
Standard deviation (N/mm2) |
Adhesion Strength (N/mm2) |
Standard deviation (N/mm2) |
|
|
PC |
4.42 |
0.17 |
4.48 |
0.27 |
|
PP |
1.44 |
0.11 |
2.32 |
0.12 |
|
HDPE |
2.26 |
0.31 |
1.75 |
0.15 |
Figure 4: Comparison of Adhesion Strength for all NiTi and Ti composites.
CONCLUSION
The modified overlapping configuration sample for the pullout tests was successfully used to characterize the interfacial bonding strength for all the combinations of matrix/inclusion considered. The adhesion strength was reported as the maximum force attained to debond the two surfaces divided by the contact surface area. Comparison between the adhesion strength between the NiTi and Ti with each of the polymers used was performed. Results show that the PC exhibits the highest adhesion with no difference between NiTi and Ti. Comparison of the adhesion strength values for PP/Ti and PP/NiTi depict a higher and very different value for the PP with Ti inclusions.
