- Email: [email protected]
Simulating the microbond technique with macrodroplets
Composites
ELSEVIER
Science and Technology 54 (1995) 423-430 0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0266-3538/95/$09.50
0266-3538(95)00080-l
SIMULATING THE MICROBOND TECHNIQUE WITH MACRODROPLETS
Lawrence Department of Chemical Engineering,
P. Hann
& Douglas
E. Hirt*
Clemson University, Clemson, South Carolina 29634-0909,
USA
(Received 29 August 1994; revised version received 17 January 1995; accepted 1 June 1995) Abstract
one end of the fiber is connected to a load cell, and a microvise is then brought into contact with the drop (Fig. 1). The microvise is typically a pair of parallel plates that exerts a point load on each side of the drop. The microvise is continually moved downward until the fiber/matrix interface can no longer withstand the load and the drop debonds from the fiber. The debond strength is then calculated from the equation:
In this study, the microbond technique has been simulated by the use of macrodroplets. These larger scale specimens consisted of cone-shaped epoxy droplets of length 6 mm, on 1 mm diameter steel wire. The specimens were formed reproducibly in a mold to provide 30” and 45” contact angles between jiber and resin. The model geometry was used to examine the effects of contact angle, loading position, and loading type (point versus axisymmetric loading) on the debonding strength. The results indicated that a higher debonding strength was achieved at the higher contact angle and at loading positions farther from the tip of the cone-shaped drop. When the load was applied near the tip of the drop, which is typical in microbond experiments, there was no statistical difference in debonding strength for point loading and axisymmetric loading. Finally, with the macrodroplets, it was possible to visually observe a three-stage debonding process. Keywords: microbond
technique,
r=F
interface, composite,
1 INTRODUCTION There are several single-filament tests that may be used to probe the interfacial strength between a fiber and a polymer matrix. l-l6 These tests are often used to provide a measure of the effect of, for example, a new coupling agent on the interfacial strength in a shorter period of time and at a lower cost than would be required for a full-scale composite test (e.g. short-beam shear or transverse tensile tests). This research has focused on one single-fiber test known as the microbond technique.‘@16 In the microbond technique, a drop of resin is deposited on a single fiber. Once the drop is cured, should
(I)
where z is the debond strength, F the debonding force, d the fiber diameter, and L the embedded length. One of the disadvantages of the microbond test is that it is difficult to assess where the vise contacts the droplet. Traditionally, the two shearing plates are brought together until they touch the fiber and a frictional force is measured. The plates are then opened slightly so that the fiber can move between them. The problem with this method is that the gap distance between the plates is variable and the point of vise/microdroplet contact varies from drop to drop and for different operators. This problem has been addressed by using a gap of specified width,13-16 but the contact point is still a variable if drops of differing size are used. Another difficulty with the microbond test is that each specimen could exhibit a different contact angle. It has been shown17 that the contact angle is related to the shape parameters of the fiber and the drop. The important shape parameters are the length of the drop, the radius of the fiber, and the maximum radius of the drop. Figure 2 illustrates computer generated drop profiles for relatively large and small contact angles. On examination of these drop profiles, it may seem that the drop shape would not have a significant effect on the debonding strength, but it will be shown that the contact angle does make a significant difference. As noted above, the microbond test is a simple single-filament test, but it has a number of limitations. Among them are the variability of the vise contact
epoxy, contact angle
*To whom correspondence
XdL
be addressed. 423
L. P. Ham,
424
(measured
F by load cell)
ACTUAL GEOMETRY (Drop -200 km long)
Fig. 1. Schematic diagram of the microbond
technique.
point, the variability of the size and shape of the drop, the difficulty in observing the failure process, and the variability of the mechanical properties of the drops. In order to understand better the microbond technique, it is necessary to be able to replicate the
40.3”
Fig. 2. Computer-generated
drop profiles for relatively large and small contact angles.
D. E. Hirt test conditions. Chou et a1.16 reported a study in which many microdroplet specimens were formed and subsequent droplet dimensions were measured. Only specimens of a certain size were selected so that the drop size was relatively reproducible. Drops were debonded by using various distances between microvise plates to determine the effect of microvise gap width on the debonding strength. In their study, Chou et al. demonstrated that the debonding strength increased and then reached a plateau as the distance between the microvise plates increased. In this complementary study, a method has been devised to fabricate the droplets in a mold. The droplets are conical in shape to simulate the compressive stresses that are likely to be present in actual microdroplet experiments. Additionally, the cone-shaped drops are significantly larger than the typical microdroplets to improve ease of handling. Since the drops can be formed reproducibly, the debonding can be performed in a systematic manner to determine the effects of drop shape, loading position, and loading type (point and axisymmetric loading) on debonding strength. Moreover, the larger specimens allowed for visual inspection of crack propagation during debonding. 2 EXPERIMENTAL The material that was chosen for the conical drops was Epon 828 (supplied by Shell Chemical Company) and the curing agent used was Jeffamine D-230 (polyoxypropylene diamine) (supplied by Texaco Chemical Company). The ‘fiber’ in the enlarged specimens was 1 mm (O-039 in) diameter wire of SAE 1090 steel (supplied by Small Parts Incorporated). The drops were formed reproducibly in a mold to minimize the number of variables that are inherent in typical microbond specimens. The macrodrops had a conical shape (Fig. 3) where the maximum radius was controlled by the length of the drop and the contact angle between the drop and the fiber. One mold was used to produce drops that had a 30” contact angle, while another mold was used to fabricate drops that had a 45” contact angle. For both contact angles, the cavities were machined to produce drops nominally 6 mm long. The molds, which were made from Teflon, were designed so that a fiber was aligned in the center of a drop. A schematic illustration of a mold and its supports is shown in Fig. 4. A fiber length of 15 cm was used for all of the experiments. Prior to use, the fibers were cleaned by rubbing them with a cloth saturated with trichloroethylene to remove any oil that might have been present. After the fibers were cleaned and aligned in the mold, the resin was prepared by combining 100 parts by weight Epon 828 and 35 parts by weight Jeffamine D-230. The mixture was then injected into
Simulating the microbond F (measured by load ceil)
MODEL GEOMETRY (Drop -6 mm long) Fig. 3. Schematic diagram of the model geometry used for the macrobond tests. Cone-shaped epoxy drops were formed reproducibly on 1 mm diameter steel wire.
the conical cavities with a 5 cm3 syringe. In order to control the amount of resin injected into the cavities, the same number of drops were ejected from the syringe for each droplet at a given contact angle. The 45” samples required 25 drops and the 30” samples required 9 drops. The resin was cured in the mold at room temperature for 24 h. The mold was then placed in an oven (American Scientific Products, model Tempcon Oven S/P) and the resin cured at 80°C for 2 h and then at 120°C for 2 h. After the mold cooled, the drops were removed by carefully flexing
0000000 0000000
/
0000000
425
technique
the upper plate. Once the specimens were labeled, the exact embedment lengths were measured with calipers. The mechanical testing was performed on an Instron-type tensile testing machine made by ATS Incorporated. In order to simulate the microbond test, a special vise was designed to correspond to the physical dimensions of the ATS machine. A diagram of the vise, fabricated from stainless steel, is shown in Fig. 5. As shown schematically in Fig. 6, two types of loading conditions were studied (point loading and axisymmetric loading). The point loading was achieved by using two plates with a known separation distance of 1*6,2.4,4*0, or 4.8 mm between the plates. These plates were designed to fit the vise in such a way that the separation distance could be easily changed by sliding the plates back and forth and then bolting them into place. The axisymmetric loading was achieved using various plates with a 1.6, 2.4, 4.0, or 4.8 mm diameter hole drilled through the center. These plates were also designed to fit the vise so that they could be easily installed or removed. The four loading positions on 30” and 45” drops are shown schematically in Fig. 7. The test specimens were loaded in the testing machine by suspending the fiber from a suspension plate and the vise was brought into contact with a macrodrop. The force on a drop was increased by moving the crosshead at a rate of 1.3 cm/min until the
A Movable
Fig. 4. Illustration of the mold used to form the specimens. Conical cavities were machined in the top plate and the holes in the bottom plate were used to align the wires through the center of the cone-shaped drops.
plates
Fig. 5. Diagram of the macrovise. The movable plates were slid into position to set the proper gap distance for point loading. For axisymmetric loading, the movable plates were removed and replaced with a plate that had a hole of specified diameter drilled through it.
L. P. Ham,
426
D. E. Hirt on a 1 mm diameter steel wire. Although it will be extremely difficult, if not impossible, to scale this model geometry down to the actual size of a microdrop, it can be used to characterize the important variables in the actual microbond test. By studying these variables it might be possible to understand better the experimental results obtained in the actual test.
AXISYMMETRIC LOADING
POINT LOADING
Fig. 6. Schematic diagram illustrating the differences in vise design and vise/droplet contact for point and axisymmetric loading.
drop debonded from the fiber. After debonding, the peak force was recorded and the interfacial strength was calculated using eqn (1).
3 RESULTS
AND
DISCUSSION
The purpose of this research was to study the effects of varying the contact angle, contact point, and type of loading on the measured debonding strength in a simulated microbond test. The model drop that was fabricated for this study had a different geometry and was much bigger than an actual microdrop. Typically, a microdrop deposited on an 8 pm diameter carbon fiber may be 100-200 pm long, whereas the model geometry comprised a 6 mm long cone-shaped drop
3.1 Effect of contact angle for point loading The difference between drops fabricated with 30” and 45” contact angles under the influence of point loading is shown in Fig. 8 (each data point represents the mean of at least 30 debonded drops and the error bars represent a 99% confidence interval). As the vise contacts further down the length of the drop (moving from left to right in Fig. S), there is an increase in the measured interfacial strength. This also has been demonstrated mathematically by several investigators’8m2” and experimentally by Chou et al.” and reinforces the recommendation that the test be conducted using a constant gap distance. More importantly, the data in Fig. 8 demonstrate the significance of the contact angle on the measured interfacial strength. As can be seen, an increase in the contact angle results in an increase in the interfacial strength. This phenomenon is due to a higher interfacial stress concentration that is developed for the smaller contact angle and promotes debonding. In the light of the results shown in Fig. 8, there seems to be an indication that some of the scatter in the microbond test results may be a consequence of testing drops with different contact angles.
POINT
LOADING
401
1
z
1
I
8 $
25.
0
30”
contact angle
.
45”
contact angle
P 0 30” CONTACT
ANGLE
45” CONTACT
1
2
3
4
ANGLE
Fig. 7. Illustration of the four loading positions on the and 4.5” drops. Due to the relative shape of the 30” drop, vise contacts the resin further down the drop as macrovise gap distance for point loading, or hole size axisymmetric loading, becomes larger.
30” the the for
Axial Distance From Drop Tip To Loading Position (mm) Fig. 8. Plot of debond strength versus axial loading position for drops fabricated with 30” and 45” contact angles subjected to point loading.
427
Simulating the microbond technique 3.2 Point versus axisymmetric
loading for 45”
30” CONTACT
ANGLE
contact angle
A comparison between point and axisymmetric loading for drops made with a 45” contact angle is shown in Fig. 9. It can be seen that when the intermediate gap distances are used there is a statistically significant difference between point and axisymmetric loading. However, the most important characteristic of this plot is the lack of statistical difference between point and axisymmetric loading at the extreme gap distances. It has been proposed18 that applying an axisymmetric load to a microdrop would increase the contact area on the droplets and improve the accuracy of the test. However, the results in Fig. 9 indicate that when the force is exerted near the top of the drop (typically the case in the microbond test), the debonding strengths for point and axisymmetric loading are statistically identical, and the recommendation to use axisymmetric loading is not necessary. 3.3 Point versus axisymmetric contact angle
A comparison between point and axisymmetric loading for drops made with a 30” contact angle is shown in Fig. 10. As with Fig. 9, it can be seen that at the smallest gap distance (smallest axial loading position) there is no statistical difference between point and axisymmetric loading. However, there is a significant statistical difference in strengths for the intermediate gap distances. It can also be seen in Fig. 10 that there is no data point for point loading at the largest gap distance. At that gap distance the drops were not debonded from the fiber, but rather the drop
I
I
P
ANGLE
I Point Loading
q
m Axisymmetric 0
1
2
Loading
3
g
35-
3 s F
30-
2 v) z I:
25-
$
I
0
Point Loading
=
Axisymmetric
Loading
I 204 0
? 1
2
3
4
Axial Distance From Drop Tip To Loading Position (mm)
Fig. 10. Plot of debond strength versus axial loading position for drops fabricated with a 30” contact angle subjected to point and axisymmetric loading.
loading for 30”
45” CONTACT
4ol------
4
Axial Distance From Drop Tip To Loading Position (mm)
Fig. 9. Plot of debond strength versus axial loading position for drops fabricated with a 45” contact angle subjected to point and axisymmetric loading.
edges plates
were sheared off in cohesive failure. The vise contacted far enough down the 30” drops that
the majority of the load was being supported by the resin rather than being transferred to the interface. Therefore, the fracture strength of the epoxy was reached before that of the interface causing cohesive failure of the drop. This phenomenon has been addressed previously,19~20 where a finite-element analysis of a microdroplet specimen under loading was used to identify a locus of maximum tensile stress that could lead to cohesive failure in the region of the droplet meniscus. 3.4 Effect of contact angle for axisymmetric loading The difference between drops fabricated with 30” and 45” contact angles under the influence of an axisymmetric load is shown in Fig. 11. Unlike the data shown in Fig. 10, there is a decrease in the measured interfacial strength at the largest axial loading distance, and there is an overlap in confidence intervals between the 30” and the 45” data points at that position. This would seem to indicate that when axisymmetric loading is being applied to a drop at certain positions, there is no statistical difference in the interfacial strength between drops with different contact angles. The exception to this trend is exhibited when the load is applied near the top of the drop. As shown in Fig. 11, when the load is applied at the lowest axial loading position, there is a statistical difference in debonding strength between the 30” and 45” drops. Once again, this is an important observation because in the actual microbond technique, the load is typically applied near the tip of the
L. P. Hann, D. E. Hirt
428 AXISYMMETRIC
LOADING
P
0
30"
contact angle
1 45" contact angle
P 1
2
3
4
Axial Distance From Drop Tip To Loading Position (mm) Fig. 11. Plot of debond strength versus axial loading position for drops fabricated with 30” and 4.5”contact angles subjected to axisymmetric loading.
drop. Therefore, as with the results presented in Fig. 8, the contact angle has a significant impact on the debonding strength when the load (axisymmetric or point) is applied near the tip of the drop. As can be seen in Fig. 11, it was possible to obtain a data point for the 30” drops undergoing an axisymmetric load at the largest axial loading distance. As mentioned previously, the collection of this fourth data point was not possible when a point load was applied to the 30” drops. When the axisymmetric load was applied, it was uniformly distributed around the drop. Therefore, each point around the circumference carried a fraction of the load. This enabled the debonding strength to be reached before cohesive failure of the epoxy matrix. 3.5 Specimen failure The tests conducted with the vise fixtures that produced point loading allowed for visual inspection of the macrodrops debonding from the fiber. It appeared that the debonding process occurred in three stages. The first stage, which was termed the initiation and propagation stage, began with the initiation of a crack. For all of the gap distances, the crack initiated at the top of the drop. As shown previously,20 this would be expected because the stress concentration is highest near the tip of the drop and significant thermal residual stresses can cause premature interfacial failure at the tip. As the load increased, the crack propagated steadily down the interface until it was approximately two-thirds of the way down the drop. At that point the second stage, which was termed the pause stage, was reached. In this stage, the crack
remained stationary with increasing load until the interfacial failure strength was reached and, in the third stage, the specimen failed catastrophically. This failure process was filmed for a number of 45” samples under a point load with a gap distance of 4 mm (vise position 3) using a video camera and recorder. A series of pictures was then taken from the TV screen of the crack as it propagated down the interface. From these pictures it was possible to determine the length of the crack and the time it took, from crack initiation, for the crack to reach that length. These data were then plotted as shown in Fig. 12. As can be seen, the three stages of the debonding process can clearly be seen from this graph. The first stage is the linear region, the second stage is the plateau region, and the third stage is the instantaneous increase in the crack length, where the final crack length is the length of the macrodrop. 3.6 Comparison with results of Chou et a1.16 The data in Fig. 8 for both 30” and 45” drops subjected to point loading do not reach a plateau at higher axial loading positions, which contradicts the results presented by Chou et aZ.16This contradiction may be due to the variation in specimen geometry between microdroplets and the cone-shaped drops used in this research. This difference creates a somewhat different stress distribution in the two types of samples. Moreover, geometry- and loading-dependent frictional forces may affect debonding. As an example, note that for axisymmetric loading in Fig. 10 the debond strength increases to a maximum and then decreases. At low levels of the load, a crack initiates at the tip of the cone. With increasing load, this axisymmetric crack grows along the fiber/matrix interface resulting in two ‘free’ surfaces. At first the surfaces separate in
6-
z
5-
!z.. .c El, s -I
4-
3-
Y &% 6
2 1-
0,
0
9
10
11
12
13
14
Time (s) Fig. 12. Plot of crack length versus time for a 45” contact angle drop subjected to point loading at a gap distance of 4 mm.
Simulating
the microbond
typical fashion as the crack grows. However, for some critical crack length, the component of the force normal to the interface caused by the vise on the droplet makes the two surfaces come back into contact. This is a rather complicated process that depends on the vise position. This contact induces a frictional force that contributes towards a resistance to catastrophic debonding. From the experimental results for axisymmetric loading, it appears that there is an optimum vise position that maximizes this frictional resistance. When the vise position is either larger or smaller than this value, the frictional resistance, and therefore the load to failure, decreases. To further investigate these ideas related to debonding, modified model geometries will be studied in the future. The belief is that macroscale geometries that more closely approximate the shape of a microbond specimen may provide results for the debonding strength that reach a plateau. At the same time, a new geometry may affect the three-stage debonding process described above. This will also be studied in future work. 3.7 Scatter in the data Figure 13 shows a plot of the debond strength versus the embedment length for all of the data points generated from the point-loading experiments. As can be seen in this plot, there is significant scatter in the debond strength, which is the same trend that can be observed in the results from the actual microbond technique. Interestingly, the scatter may be even more pronounced in the microbond test due to fiber/surface heterogeneitieslO and drop-to-drop variation in cure.l These effects are minimized in the present work due to the relatively large size of the macrodroplets. Regardless, when the important variables are studied
technique
429
under controlled conditions (i.e. reproducible drops and loading), the scatter in the data can be reduced significantly (compare the y axes in Figs 8-11 to the y axis in Fig. 13). 4 CONCLUSIONS 1. The point where the vise contacts the drop had a direct effect on the measured debond strength. For point loading, the debond strength increased as the vise contact point moved axially down the drop. For axisymmetric loading, the debond strength increased to a point and then began to decrease as the vise contact point moved axially down the drop. 2. The contact angle had a direct effect on the measured debond strength. For point loading and under certain axisymmetric loadings, an increase in the contact angle resulted in an increase in the measured debond strength. 3. When the vise made contact near the tip of the drop, there was no statistical difference in debond strength between point loading and axisymmetric loading. 4. The drop appeared to debond from the fiber in a three-stage process. Stage one was the initiation and propagation stage where the drop began to debond from the fiber at the tip of the drop. The crack then propagated down the fiber/matrix interface until it reached stage two, which has been termed the pause stage. In this stage the crack seemed to stop propagating with increased stress until catastrophic failure occurred in stage three. ACKNOWLEDGEMENTS The authors would like to thank Dr Paul Joseph and Mr Gadi Kesavaram for the interpretation of the failure mechanism. The authors also gratefully acknowledge the financial support of this research through the National Science Foundation’s EPSCoR program. REFERENCES
5.5
Embedment
6.0
6.5
7.0
Length (mm)
Fig. 13. Plot of the debond strength versus the embedment length for all of the data points generated from point loading experiments.
Drzal, L. T. & Herrera-France, P. J., Composite fiber-matrix bond tests. In Engineered Materials Handbook Volume 3: Adhesives and Sealants. ASM International, Materials Park, OH, 1991, pp. 391-405. Broutman, L. J., Measurement of the fiber-polymer matrix interfacial strength. In Interfaces in Composites, ASTM STP 452. American Society for Testing and Materials, Philadelphia, PA, 1969, pp. 27-41. Jiang, K. R. & Penn, L. S., Improved analysis and experimental evaluation of the single filament pull-out test. Comp. Sci. Technol., 45 (1992) 89-103.
430
L. P. Ham.
4. Zhou,
5.
6.
7. 8.
9.
10.
11.
12.
L. M., Kim, J. K. & Mai, Y. W., On the single fibre pull-out problem: Effect of loading method. Comp. Sci. Technol., 45 (1992) 153-60. Whitney, J. M. & Drzal, L. T., Axisymmetric stress distribution around an isolated fiber fragment. In Toughened Composites, ASTM STP 937, ed. N. J. Johnston. American Society for Testing and Materials, Philadelphia, PA, 1987, pp. 179-96. Drzal, L. T., Rich, M. J. & Lloyd, P. F., Adhesion of graphite fibers to epoxy matrices: I. The role of fiber surface treatment. J. Adhesion, 16 (1982) l-30. Bascom, W. D. & Jensen, R. M., Stress transfer in single fiber/resin tensile tests. J. Adhesion, 19 (1986) 219-39. Netravali, A. N., Stone, D., Ruoff, S. & Topoleski, L. T. T., Continuous micro-indenter push-through technique for measuring interfacial shear strength of fiber composites. Camp. Sci. Technol., 34 (1989) 289-303. Grande, D. H., Mandell, J. F. & Hong, K. C. C., Fibre-matrix bond strength studies of glass, ceramic, and metal matrix composites. J. Muter. Sci., 23 (1988) 311-28. Miller, B., Muri, P. & Rebenfeld, L., A microbond method for determination of the shear strength of a fiber/resin interface. Comp. Sci. Technol., 28 (1987) 17-32. Gaur, U. & Miller, B., Microbond method for determination of the shear strength of a fiber/resin interface: Evaluation of experimental parameters. Comp. Sci. Technol., 34 (1989) 35-51. Miller, B., Gaur, U. & Hirt, D. E., Measurement and mechanical aspects of the microbond pull-out technique for obtaining fiber/resin interfacial shear strength.
D. E. Hirt Comp. Sci. Technol., 42 (1991) 207-19. 13. Latour Jr, R. A., Black, J. & Miller, B., Fracture mechanisms of the fiber/matrix interfacial bond in fiber-reinforced polymer composites. Surf. and Znterf Analysis, 17 (1991) 477-84. 14. McAlea, K. P. & Besio, G. J., Adhesion between polybutylene terephthalate and E-glass measured with a microdebond technique. Polym. Comp., 9 (1988) 285-90. 15. Rao, V., Herrera-France, P. J., Ozzello, A. D. & Drzal, L. T., A direct comparison of the fragmentation test and the microbond pull-out test for determining the interfacial shear strength. J. Adhesion, 34 (1991) 65-77. 16. Chou, C. T., Gaur, U. & Miller, B., The effect of microvise gap width on microbond pull-out test results, Comp. Sci. Technof., 51 (1994) 111-16. 17. Carroll, B. J., The accurate measurement of contact angle, phase contact areas, drop volume, and Laplace excess pressure in drop-on-fiber systems. J. Coil. Znterf. Sci., 57 (1976) 488-95. 18. Wu, H. F. & Claypool, C. M., An analytical approach of the microbond test method used in characterizing the fibre-matrix interface, J. Muter. Sci. Lett., 10 (1991) 260-2. 19. Herrera-France, P. J., Chiang, M. Y. M. & Drzal, L. T., The microbond technique for measurement of fibermatrix interfacial shear strength: A theoretical analysis. Proc. SPE ANTEC, Detroit, 3-7 May 1992, pp. 234-8. 20. Chiang, M. Y. M. & Herrera-France, P. J., Analyses of the techniques for fiber-matrix bond strength measurement. Proc. 6th Int. SAMPE Electronics Conference, Baltimore, 22-25 June 1992, pp. 69-81.
ELSEVIER
Science and Technology 54 (1995) 423-430 0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0266-3538/95/$09.50
0266-3538(95)00080-l
SIMULATING THE MICROBOND TECHNIQUE WITH MACRODROPLETS
Lawrence Department of Chemical Engineering,
P. Hann
& Douglas
E. Hirt*
Clemson University, Clemson, South Carolina 29634-0909,
USA
(Received 29 August 1994; revised version received 17 January 1995; accepted 1 June 1995) Abstract
one end of the fiber is connected to a load cell, and a microvise is then brought into contact with the drop (Fig. 1). The microvise is typically a pair of parallel plates that exerts a point load on each side of the drop. The microvise is continually moved downward until the fiber/matrix interface can no longer withstand the load and the drop debonds from the fiber. The debond strength is then calculated from the equation:
In this study, the microbond technique has been simulated by the use of macrodroplets. These larger scale specimens consisted of cone-shaped epoxy droplets of length 6 mm, on 1 mm diameter steel wire. The specimens were formed reproducibly in a mold to provide 30” and 45” contact angles between jiber and resin. The model geometry was used to examine the effects of contact angle, loading position, and loading type (point versus axisymmetric loading) on the debonding strength. The results indicated that a higher debonding strength was achieved at the higher contact angle and at loading positions farther from the tip of the cone-shaped drop. When the load was applied near the tip of the drop, which is typical in microbond experiments, there was no statistical difference in debonding strength for point loading and axisymmetric loading. Finally, with the macrodroplets, it was possible to visually observe a three-stage debonding process. Keywords: microbond
technique,
r=F
interface, composite,
1 INTRODUCTION There are several single-filament tests that may be used to probe the interfacial strength between a fiber and a polymer matrix. l-l6 These tests are often used to provide a measure of the effect of, for example, a new coupling agent on the interfacial strength in a shorter period of time and at a lower cost than would be required for a full-scale composite test (e.g. short-beam shear or transverse tensile tests). This research has focused on one single-fiber test known as the microbond technique.‘@16 In the microbond technique, a drop of resin is deposited on a single fiber. Once the drop is cured, should
(I)
where z is the debond strength, F the debonding force, d the fiber diameter, and L the embedded length. One of the disadvantages of the microbond test is that it is difficult to assess where the vise contacts the droplet. Traditionally, the two shearing plates are brought together until they touch the fiber and a frictional force is measured. The plates are then opened slightly so that the fiber can move between them. The problem with this method is that the gap distance between the plates is variable and the point of vise/microdroplet contact varies from drop to drop and for different operators. This problem has been addressed by using a gap of specified width,13-16 but the contact point is still a variable if drops of differing size are used. Another difficulty with the microbond test is that each specimen could exhibit a different contact angle. It has been shown17 that the contact angle is related to the shape parameters of the fiber and the drop. The important shape parameters are the length of the drop, the radius of the fiber, and the maximum radius of the drop. Figure 2 illustrates computer generated drop profiles for relatively large and small contact angles. On examination of these drop profiles, it may seem that the drop shape would not have a significant effect on the debonding strength, but it will be shown that the contact angle does make a significant difference. As noted above, the microbond test is a simple single-filament test, but it has a number of limitations. Among them are the variability of the vise contact
epoxy, contact angle
*To whom correspondence
XdL
be addressed. 423
L. P. Ham,
424
(measured
F by load cell)
ACTUAL GEOMETRY (Drop -200 km long)
Fig. 1. Schematic diagram of the microbond
technique.
point, the variability of the size and shape of the drop, the difficulty in observing the failure process, and the variability of the mechanical properties of the drops. In order to understand better the microbond technique, it is necessary to be able to replicate the
40.3”
Fig. 2. Computer-generated
drop profiles for relatively large and small contact angles.
D. E. Hirt test conditions. Chou et a1.16 reported a study in which many microdroplet specimens were formed and subsequent droplet dimensions were measured. Only specimens of a certain size were selected so that the drop size was relatively reproducible. Drops were debonded by using various distances between microvise plates to determine the effect of microvise gap width on the debonding strength. In their study, Chou et al. demonstrated that the debonding strength increased and then reached a plateau as the distance between the microvise plates increased. In this complementary study, a method has been devised to fabricate the droplets in a mold. The droplets are conical in shape to simulate the compressive stresses that are likely to be present in actual microdroplet experiments. Additionally, the cone-shaped drops are significantly larger than the typical microdroplets to improve ease of handling. Since the drops can be formed reproducibly, the debonding can be performed in a systematic manner to determine the effects of drop shape, loading position, and loading type (point and axisymmetric loading) on debonding strength. Moreover, the larger specimens allowed for visual inspection of crack propagation during debonding. 2 EXPERIMENTAL The material that was chosen for the conical drops was Epon 828 (supplied by Shell Chemical Company) and the curing agent used was Jeffamine D-230 (polyoxypropylene diamine) (supplied by Texaco Chemical Company). The ‘fiber’ in the enlarged specimens was 1 mm (O-039 in) diameter wire of SAE 1090 steel (supplied by Small Parts Incorporated). The drops were formed reproducibly in a mold to minimize the number of variables that are inherent in typical microbond specimens. The macrodrops had a conical shape (Fig. 3) where the maximum radius was controlled by the length of the drop and the contact angle between the drop and the fiber. One mold was used to produce drops that had a 30” contact angle, while another mold was used to fabricate drops that had a 45” contact angle. For both contact angles, the cavities were machined to produce drops nominally 6 mm long. The molds, which were made from Teflon, were designed so that a fiber was aligned in the center of a drop. A schematic illustration of a mold and its supports is shown in Fig. 4. A fiber length of 15 cm was used for all of the experiments. Prior to use, the fibers were cleaned by rubbing them with a cloth saturated with trichloroethylene to remove any oil that might have been present. After the fibers were cleaned and aligned in the mold, the resin was prepared by combining 100 parts by weight Epon 828 and 35 parts by weight Jeffamine D-230. The mixture was then injected into
Simulating the microbond F (measured by load ceil)
MODEL GEOMETRY (Drop -6 mm long) Fig. 3. Schematic diagram of the model geometry used for the macrobond tests. Cone-shaped epoxy drops were formed reproducibly on 1 mm diameter steel wire.
the conical cavities with a 5 cm3 syringe. In order to control the amount of resin injected into the cavities, the same number of drops were ejected from the syringe for each droplet at a given contact angle. The 45” samples required 25 drops and the 30” samples required 9 drops. The resin was cured in the mold at room temperature for 24 h. The mold was then placed in an oven (American Scientific Products, model Tempcon Oven S/P) and the resin cured at 80°C for 2 h and then at 120°C for 2 h. After the mold cooled, the drops were removed by carefully flexing
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the upper plate. Once the specimens were labeled, the exact embedment lengths were measured with calipers. The mechanical testing was performed on an Instron-type tensile testing machine made by ATS Incorporated. In order to simulate the microbond test, a special vise was designed to correspond to the physical dimensions of the ATS machine. A diagram of the vise, fabricated from stainless steel, is shown in Fig. 5. As shown schematically in Fig. 6, two types of loading conditions were studied (point loading and axisymmetric loading). The point loading was achieved by using two plates with a known separation distance of 1*6,2.4,4*0, or 4.8 mm between the plates. These plates were designed to fit the vise in such a way that the separation distance could be easily changed by sliding the plates back and forth and then bolting them into place. The axisymmetric loading was achieved using various plates with a 1.6, 2.4, 4.0, or 4.8 mm diameter hole drilled through the center. These plates were also designed to fit the vise so that they could be easily installed or removed. The four loading positions on 30” and 45” drops are shown schematically in Fig. 7. The test specimens were loaded in the testing machine by suspending the fiber from a suspension plate and the vise was brought into contact with a macrodrop. The force on a drop was increased by moving the crosshead at a rate of 1.3 cm/min until the
A Movable
Fig. 4. Illustration of the mold used to form the specimens. Conical cavities were machined in the top plate and the holes in the bottom plate were used to align the wires through the center of the cone-shaped drops.
plates
Fig. 5. Diagram of the macrovise. The movable plates were slid into position to set the proper gap distance for point loading. For axisymmetric loading, the movable plates were removed and replaced with a plate that had a hole of specified diameter drilled through it.
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D. E. Hirt on a 1 mm diameter steel wire. Although it will be extremely difficult, if not impossible, to scale this model geometry down to the actual size of a microdrop, it can be used to characterize the important variables in the actual microbond test. By studying these variables it might be possible to understand better the experimental results obtained in the actual test.
AXISYMMETRIC LOADING
POINT LOADING
Fig. 6. Schematic diagram illustrating the differences in vise design and vise/droplet contact for point and axisymmetric loading.
drop debonded from the fiber. After debonding, the peak force was recorded and the interfacial strength was calculated using eqn (1).
3 RESULTS
AND
DISCUSSION
The purpose of this research was to study the effects of varying the contact angle, contact point, and type of loading on the measured debonding strength in a simulated microbond test. The model drop that was fabricated for this study had a different geometry and was much bigger than an actual microdrop. Typically, a microdrop deposited on an 8 pm diameter carbon fiber may be 100-200 pm long, whereas the model geometry comprised a 6 mm long cone-shaped drop
3.1 Effect of contact angle for point loading The difference between drops fabricated with 30” and 45” contact angles under the influence of point loading is shown in Fig. 8 (each data point represents the mean of at least 30 debonded drops and the error bars represent a 99% confidence interval). As the vise contacts further down the length of the drop (moving from left to right in Fig. S), there is an increase in the measured interfacial strength. This also has been demonstrated mathematically by several investigators’8m2” and experimentally by Chou et al.” and reinforces the recommendation that the test be conducted using a constant gap distance. More importantly, the data in Fig. 8 demonstrate the significance of the contact angle on the measured interfacial strength. As can be seen, an increase in the contact angle results in an increase in the interfacial strength. This phenomenon is due to a higher interfacial stress concentration that is developed for the smaller contact angle and promotes debonding. In the light of the results shown in Fig. 8, there seems to be an indication that some of the scatter in the microbond test results may be a consequence of testing drops with different contact angles.
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30” the the for
Axial Distance From Drop Tip To Loading Position (mm) Fig. 8. Plot of debond strength versus axial loading position for drops fabricated with 30” and 45” contact angles subjected to point loading.
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Simulating the microbond technique 3.2 Point versus axisymmetric
loading for 45”
30” CONTACT
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contact angle
A comparison between point and axisymmetric loading for drops made with a 45” contact angle is shown in Fig. 9. It can be seen that when the intermediate gap distances are used there is a statistically significant difference between point and axisymmetric loading. However, the most important characteristic of this plot is the lack of statistical difference between point and axisymmetric loading at the extreme gap distances. It has been proposed18 that applying an axisymmetric load to a microdrop would increase the contact area on the droplets and improve the accuracy of the test. However, the results in Fig. 9 indicate that when the force is exerted near the top of the drop (typically the case in the microbond test), the debonding strengths for point and axisymmetric loading are statistically identical, and the recommendation to use axisymmetric loading is not necessary. 3.3 Point versus axisymmetric contact angle
A comparison between point and axisymmetric loading for drops made with a 30” contact angle is shown in Fig. 10. As with Fig. 9, it can be seen that at the smallest gap distance (smallest axial loading position) there is no statistical difference between point and axisymmetric loading. However, there is a significant statistical difference in strengths for the intermediate gap distances. It can also be seen in Fig. 10 that there is no data point for point loading at the largest gap distance. At that gap distance the drops were not debonded from the fiber, but rather the drop
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loading for 30”
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Axial Distance From Drop Tip To Loading Position (mm)
Fig. 9. Plot of debond strength versus axial loading position for drops fabricated with a 45” contact angle subjected to point and axisymmetric loading.
edges plates
were sheared off in cohesive failure. The vise contacted far enough down the 30” drops that
the majority of the load was being supported by the resin rather than being transferred to the interface. Therefore, the fracture strength of the epoxy was reached before that of the interface causing cohesive failure of the drop. This phenomenon has been addressed previously,19~20 where a finite-element analysis of a microdroplet specimen under loading was used to identify a locus of maximum tensile stress that could lead to cohesive failure in the region of the droplet meniscus. 3.4 Effect of contact angle for axisymmetric loading The difference between drops fabricated with 30” and 45” contact angles under the influence of an axisymmetric load is shown in Fig. 11. Unlike the data shown in Fig. 10, there is a decrease in the measured interfacial strength at the largest axial loading distance, and there is an overlap in confidence intervals between the 30” and the 45” data points at that position. This would seem to indicate that when axisymmetric loading is being applied to a drop at certain positions, there is no statistical difference in the interfacial strength between drops with different contact angles. The exception to this trend is exhibited when the load is applied near the top of the drop. As shown in Fig. 11, when the load is applied at the lowest axial loading position, there is a statistical difference in debonding strength between the 30” and 45” drops. Once again, this is an important observation because in the actual microbond technique, the load is typically applied near the tip of the
L. P. Hann, D. E. Hirt
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2
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Axial Distance From Drop Tip To Loading Position (mm) Fig. 11. Plot of debond strength versus axial loading position for drops fabricated with 30” and 4.5”contact angles subjected to axisymmetric loading.
drop. Therefore, as with the results presented in Fig. 8, the contact angle has a significant impact on the debonding strength when the load (axisymmetric or point) is applied near the tip of the drop. As can be seen in Fig. 11, it was possible to obtain a data point for the 30” drops undergoing an axisymmetric load at the largest axial loading distance. As mentioned previously, the collection of this fourth data point was not possible when a point load was applied to the 30” drops. When the axisymmetric load was applied, it was uniformly distributed around the drop. Therefore, each point around the circumference carried a fraction of the load. This enabled the debonding strength to be reached before cohesive failure of the epoxy matrix. 3.5 Specimen failure The tests conducted with the vise fixtures that produced point loading allowed for visual inspection of the macrodrops debonding from the fiber. It appeared that the debonding process occurred in three stages. The first stage, which was termed the initiation and propagation stage, began with the initiation of a crack. For all of the gap distances, the crack initiated at the top of the drop. As shown previously,20 this would be expected because the stress concentration is highest near the tip of the drop and significant thermal residual stresses can cause premature interfacial failure at the tip. As the load increased, the crack propagated steadily down the interface until it was approximately two-thirds of the way down the drop. At that point the second stage, which was termed the pause stage, was reached. In this stage, the crack
remained stationary with increasing load until the interfacial failure strength was reached and, in the third stage, the specimen failed catastrophically. This failure process was filmed for a number of 45” samples under a point load with a gap distance of 4 mm (vise position 3) using a video camera and recorder. A series of pictures was then taken from the TV screen of the crack as it propagated down the interface. From these pictures it was possible to determine the length of the crack and the time it took, from crack initiation, for the crack to reach that length. These data were then plotted as shown in Fig. 12. As can be seen, the three stages of the debonding process can clearly be seen from this graph. The first stage is the linear region, the second stage is the plateau region, and the third stage is the instantaneous increase in the crack length, where the final crack length is the length of the macrodrop. 3.6 Comparison with results of Chou et a1.16 The data in Fig. 8 for both 30” and 45” drops subjected to point loading do not reach a plateau at higher axial loading positions, which contradicts the results presented by Chou et aZ.16This contradiction may be due to the variation in specimen geometry between microdroplets and the cone-shaped drops used in this research. This difference creates a somewhat different stress distribution in the two types of samples. Moreover, geometry- and loading-dependent frictional forces may affect debonding. As an example, note that for axisymmetric loading in Fig. 10 the debond strength increases to a maximum and then decreases. At low levels of the load, a crack initiates at the tip of the cone. With increasing load, this axisymmetric crack grows along the fiber/matrix interface resulting in two ‘free’ surfaces. At first the surfaces separate in
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Simulating
the microbond
typical fashion as the crack grows. However, for some critical crack length, the component of the force normal to the interface caused by the vise on the droplet makes the two surfaces come back into contact. This is a rather complicated process that depends on the vise position. This contact induces a frictional force that contributes towards a resistance to catastrophic debonding. From the experimental results for axisymmetric loading, it appears that there is an optimum vise position that maximizes this frictional resistance. When the vise position is either larger or smaller than this value, the frictional resistance, and therefore the load to failure, decreases. To further investigate these ideas related to debonding, modified model geometries will be studied in the future. The belief is that macroscale geometries that more closely approximate the shape of a microbond specimen may provide results for the debonding strength that reach a plateau. At the same time, a new geometry may affect the three-stage debonding process described above. This will also be studied in future work. 3.7 Scatter in the data Figure 13 shows a plot of the debond strength versus the embedment length for all of the data points generated from the point-loading experiments. As can be seen in this plot, there is significant scatter in the debond strength, which is the same trend that can be observed in the results from the actual microbond technique. Interestingly, the scatter may be even more pronounced in the microbond test due to fiber/surface heterogeneitieslO and drop-to-drop variation in cure.l These effects are minimized in the present work due to the relatively large size of the macrodroplets. Regardless, when the important variables are studied
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under controlled conditions (i.e. reproducible drops and loading), the scatter in the data can be reduced significantly (compare the y axes in Figs 8-11 to the y axis in Fig. 13). 4 CONCLUSIONS 1. The point where the vise contacts the drop had a direct effect on the measured debond strength. For point loading, the debond strength increased as the vise contact point moved axially down the drop. For axisymmetric loading, the debond strength increased to a point and then began to decrease as the vise contact point moved axially down the drop. 2. The contact angle had a direct effect on the measured debond strength. For point loading and under certain axisymmetric loadings, an increase in the contact angle resulted in an increase in the measured debond strength. 3. When the vise made contact near the tip of the drop, there was no statistical difference in debond strength between point loading and axisymmetric loading. 4. The drop appeared to debond from the fiber in a three-stage process. Stage one was the initiation and propagation stage where the drop began to debond from the fiber at the tip of the drop. The crack then propagated down the fiber/matrix interface until it reached stage two, which has been termed the pause stage. In this stage the crack seemed to stop propagating with increased stress until catastrophic failure occurred in stage three. ACKNOWLEDGEMENTS The authors would like to thank Dr Paul Joseph and Mr Gadi Kesavaram for the interpretation of the failure mechanism. The authors also gratefully acknowledge the financial support of this research through the National Science Foundation’s EPSCoR program. REFERENCES
5.5
Embedment
6.0
6.5
7.0
Length (mm)
Fig. 13. Plot of the debond strength versus the embedment length for all of the data points generated from point loading experiments.
Drzal, L. T. & Herrera-France, P. J., Composite fiber-matrix bond tests. In Engineered Materials Handbook Volume 3: Adhesives and Sealants. ASM International, Materials Park, OH, 1991, pp. 391-405. Broutman, L. J., Measurement of the fiber-polymer matrix interfacial strength. In Interfaces in Composites, ASTM STP 452. American Society for Testing and Materials, Philadelphia, PA, 1969, pp. 27-41. Jiang, K. R. & Penn, L. S., Improved analysis and experimental evaluation of the single filament pull-out test. Comp. Sci. Technol., 45 (1992) 89-103.
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L. M., Kim, J. K. & Mai, Y. W., On the single fibre pull-out problem: Effect of loading method. Comp. Sci. Technol., 45 (1992) 153-60. Whitney, J. M. & Drzal, L. T., Axisymmetric stress distribution around an isolated fiber fragment. In Toughened Composites, ASTM STP 937, ed. N. J. Johnston. American Society for Testing and Materials, Philadelphia, PA, 1987, pp. 179-96. Drzal, L. T., Rich, M. J. & Lloyd, P. F., Adhesion of graphite fibers to epoxy matrices: I. The role of fiber surface treatment. J. Adhesion, 16 (1982) l-30. Bascom, W. D. & Jensen, R. M., Stress transfer in single fiber/resin tensile tests. J. Adhesion, 19 (1986) 219-39. Netravali, A. N., Stone, D., Ruoff, S. & Topoleski, L. T. T., Continuous micro-indenter push-through technique for measuring interfacial shear strength of fiber composites. Camp. Sci. Technol., 34 (1989) 289-303. Grande, D. H., Mandell, J. F. & Hong, K. C. C., Fibre-matrix bond strength studies of glass, ceramic, and metal matrix composites. J. Muter. Sci., 23 (1988) 311-28. Miller, B., Muri, P. & Rebenfeld, L., A microbond method for determination of the shear strength of a fiber/resin interface. Comp. Sci. Technol., 28 (1987) 17-32. Gaur, U. & Miller, B., Microbond method for determination of the shear strength of a fiber/resin interface: Evaluation of experimental parameters. Comp. Sci. Technol., 34 (1989) 35-51. Miller, B., Gaur, U. & Hirt, D. E., Measurement and mechanical aspects of the microbond pull-out technique for obtaining fiber/resin interfacial shear strength.
D. E. Hirt Comp. Sci. Technol., 42 (1991) 207-19. 13. Latour Jr, R. A., Black, J. & Miller, B., Fracture mechanisms of the fiber/matrix interfacial bond in fiber-reinforced polymer composites. Surf. and Znterf Analysis, 17 (1991) 477-84. 14. McAlea, K. P. & Besio, G. J., Adhesion between polybutylene terephthalate and E-glass measured with a microdebond technique. Polym. Comp., 9 (1988) 285-90. 15. Rao, V., Herrera-France, P. J., Ozzello, A. D. & Drzal, L. T., A direct comparison of the fragmentation test and the microbond pull-out test for determining the interfacial shear strength. J. Adhesion, 34 (1991) 65-77. 16. Chou, C. T., Gaur, U. & Miller, B., The effect of microvise gap width on microbond pull-out test results, Comp. Sci. Technof., 51 (1994) 111-16. 17. Carroll, B. J., The accurate measurement of contact angle, phase contact areas, drop volume, and Laplace excess pressure in drop-on-fiber systems. J. Coil. Znterf. Sci., 57 (1976) 488-95. 18. Wu, H. F. & Claypool, C. M., An analytical approach of the microbond test method used in characterizing the fibre-matrix interface, J. Muter. Sci. Lett., 10 (1991) 260-2. 19. Herrera-France, P. J., Chiang, M. Y. M. & Drzal, L. T., The microbond technique for measurement of fibermatrix interfacial shear strength: A theoretical analysis. Proc. SPE ANTEC, Detroit, 3-7 May 1992, pp. 234-8. 20. Chiang, M. Y. M. & Herrera-France, P. J., Analyses of the techniques for fiber-matrix bond strength measurement. Proc. 6th Int. SAMPE Electronics Conference, Baltimore, 22-25 June 1992, pp. 69-81.