The topographical features of fractured tensile, flexural, K 1C, and impact specimens of monolithic epoxy have been studied and correlated with mechanical properties and surface features of samples before fracture. The topographical features studied include waviness (W a), roughness average (R a), root mean square value (R q), and maximum roughness height (R max or R z). As surface notches generate triaxial state of stress, therefore, the crack propagation is precipitated resulting in catastrophic failure. Although surfaces can be examined before fracture for any deleterious topographical elements, however, fractured surfaces can reveal finer details about the topography. It is because, as discussed in this article, surfaces with specific topography produce fracture patterns of peculiar aesthetics, and if delved deeper, they can further be used to estimate about the topography of surfaces before fracture. In addition, treating the samples with surfaces of specific topography can help improve the mechanical properties of monolithic epoxy.
The tribological conservation of technical polymers, such as thermosetting epoxy, is getting increasing interest to use them in various engineering applications [
The influence of topographical features is momentous both at micro and macro level [
In current work, monolithic epoxy was treated with abrasive papers to change their topography to correlate with mechanical properties and fracture patterns. The topographical features were studied using an Infinite Focus (G4) Alicona optical microscope. The working principle of Alicona optical microscope is focus-follow technique which is a non-contact method. Non-contact methods are getting growingly famous to determine topography, usually for surfaces that may undergo damage by contact methods [
Bisphenol A-epichlorohydrin based epoxy (density ~1.3 g/cm3) and dimethylbenzylamineisophoronediamine based low viscosity fast curing hardener with (density ~1.1 g/cm3) were employed and purchased from Polyfibre, UK. The mixing ratio of hardener: epoxy was 1:2.
The hardener and epoxy were degassed for 1 h in separate beakers. The hardener and epoxy were mixed by tip sonicator of 750 W power and 250 kHz frequency with 5 s break, 10 s vibration, and 70% power (Vibra-cell model VC 750, USA). Then, vacuum degassing was repeated for 15 min. The resin mixture was cast into silicone molds and cured for 6 h at room temperature and then post-cured at 150˚C overnight. The bottom and top surfaces of each specimen were processed with abrasive papers on rotating wheels at 150 rpm for 1 min.
TAn Infinite focus Alicona G4 optical microscope was employed for microscopy and to measure topography. The working principle of the microscope is focus-follow method which is a non-contact method. ASTM Standard D792 (Equations (1) and (2)) was used to measure densification. The densities of water, hardener, and epoxy were 0.9975, 1.1, and 1.3 g/cm3, respectively. Vickers microhardness was measured using Buehler Micromet II hardness tester (200 g, 10 s). The schematics of mechanical test specimens are shown in
depth and 0.25 mm tip of radius) using Equation (6). The weight of impactor head was 400 g and length of impactor arm was 0.4 m. SEM FEI Quanta 200 was used to study the fractured surfaces of tensile samples to investigate the fracture patterns in the specimens. The fractured surfaces were trimmed from the samples and gold layer was employed using SC500A Emscope sputter coater.
The densification values and mechanical properties are summarized in
Sr. | Properties | As-cast | VC | 1200 P | 320 P | 60 P |
---|---|---|---|---|---|---|
1 | Densification (%) | 99.3 ± 0.37 | 99.1 ± 0.31 | 99.2 ± 0.34 | 99.4 ± 0.29 | 99.3 ± 0.36 |
2 | Microhardness (HV) | 278.2 ± 15.2 | 391.2 ± 10.2 | 437.9 ± 11.8 | 335 ± 18.6 | 298 ± 21.7 |
3 | Young's modulus (MPa) | 748.6 ± 24.7 | 776.1 ± 21.3 | 781.6 ± 29.4 | 741.9 ± 33.4 | 726.9 ± 36.4 |
4 | UTS (MPa) | 50.2 ± 2.7 | 51.5 ± 2.7 | 54.7 ± 2.3 | 49.6 ± 2.9 | 47.3 ± 3.1 |
5 | Tensile strain (%) | 7.2 ± 1.0 | 7.4 ± 1.1 | 8.5 ± 1.3 | 9.1 ± 1.7 | 12.3 ± 2.3 |
6 | Flex. Modulus (MPa) | 729 ± 38.3 | 787.1 ± 25.3 | 797.3 ± 30.5 | 762.9 ± 33.5 | 652.9 ± 42.6 |
7 | Flex. Strength (MPa) | 68.7 ± 6.9 | 71.5 ± 3.8 | 79.9 ± 2.9 | 65.6 ± 4.6 | 63.6 ± 8.3 |
8 | Flex. Strain (%) | 5.8 ± 0.06 | 5.8 ± 0.29 | 5.9 ± 0.31 | 6.2 ± 0.49 | 6.9 ± 0.4 |
9 | K1C (MPa.m1/2) | 1.02 ± 0.1 | 1.04 ± 0.15 | 1.04 ± 0.05 | 1.03 ± 0.1 | 1.02 ± 0.1 |
10 | G1C (J/m2) | 341.5 ± 51.5 | 546.6 ± 42.3 | 620.5 ± 47.9 | 684.7 ± 62.8 | 759.6 ± 69.8 |
11 | Charpy (kJ/m2) | 1.11 ± 0.15 | 1.15 ± 0.1 | 1.17 ± 0.09 | 1.12 ± 0.12 | 1.01 ± 0.2 |
performance of epoxy samples. The microhardness of specimens (as-cast) is 277 HV. When processed with velvet cloth (VC), the hardness enhanced to 392 HV (40% increase). When processed with 1200 P, hardness enhanced to 439 HV (57% increase). This improvement in hardness values can be related to the straightening of surfaces. High roughness values were recorded in samples (as-cast). When specimens have fluted surface, the edge of indenter may not hit completely on edge of the sample. When the indenter comes in contact with the flat surface, hindrance is presented by the surface beneath and high hardness was recorded. On the other hand, when indenter comes in contact with the corners, decreased hindrance is presented by the corners, and hence, the hardness values decreased. In specimens processed with VC and 1200 P, smoothness enhanced or sharp sections were truncated. Hence, an increment in hardness was recorded. The hardness degraded in specimens processed with 320 P and 60 P which indicates that indenter bears low hindrance due to fluted topography.
The stiffness improved from 749 MPa to 776 MPa (4.1% increase) when samples were processed with VC. The stiffness of epoxy processed with 1200 P also enhanced to 782 MPa (4.9% increase). Nevertheless, the stiffness of samples processed with 320 P degraded to 742 MPa (1% decrease). The maximum degradation in stiffness was recorded when the specimens were processed with 60 P and degraded to 727 MPa (2.9% decrease). The results indicate that stiffness can be improved by treating the epoxy with VC and 1200 P and degraded by treating the samples with 320 P and 60 P. The UTS of samples treated with VC changed from 49.8 MPa to 51.9 MPa (2.9% increase). The maximum enhancement in UTS was recorded when samples were processed with 1200 P and UTS became 55 MPa (3.9% increase). The UTS of samples processed with 60 P degraded to 48.9 MPa (1.9% decrease). When processed with 60 P, the UTS degraded to 46.8 MPa (4.9% decrease). The improvement in stiffness and UTS with VC and 1200 P can be because of the straightening of surfaces as samples (as-cast) had roughness of ±14 µm. In contrary, the roughness of samples processed with VC changed between ±4 µm while that of 1200 P fluctuated between ±3 µm. Hence, modulus and strength can be enhanced by treating the samples with VC and 1200 P. On the other hand, the roughness of samples processed with 320 P abrasive paper fluctuated between ±20 µm while roughness of 60 P fluctuated between ±30 µm. Therefore, roughness above ±20 µm shows deleterious impact on strength and stiffness of samples. The tensile strain increased with coarser topography which can be because of decreased strength and stiffness values. The tensile strain showed no marked variation with VC and marginally enhanced in 1200 P. Hence, improved tensile features can be obtained when specimens were processed with VC and 1200 P. Similar results were recorded for flexural properties.
It was observed that K1C values were unchanged. One reason can be the direction of roughness. It was observed that roughness perpendicular to loading direction does not significantly change the mechanical performance. The results indicate that standard deviation is disparate for different specimens. It may be attributed to the notch tip that was manually honed that does not replicate length and curvature of notch tip. In addition,
the distribution, size, and volume fraction of porosity can be an additional aspect that can affect the mechanical performance. The trend indicates that G1C improves with the coarsening of roughness. Nevertheless, as topography did not record any marked impact on K1C, we believe that this increase in G1C is not arising from the topography. In calculating G1C, K_1C^2 is divided by stiffness. As stiffness degraded with coarse topographical features, hence the increment in G1C is probably coming from degraded stiffness. No major difference was recorded in fracture toughness results. Nevertheless, processing of specimens with abrasive papers recorded a marked influence on Charpy impact results. The change in flexural stress-strain is presented in
The fractography surfaces of specimens are presented in
with 60 P and 320 P. The fracture patterns of K1C specimens differ from those of 3PBT specimens in a way that fracture was originated from the notch tip as the tip generated high levels of stress concentration. Although there were no diversions in crack path in case of as-cast epoxy, however, a bit coarser topography was recorded in samples processed with the abrasive papers. As the displacement rate is relatively low in K1C testing, the surface notches showed a significant impact on the topography of fracture surfaces. However, the influence of surface notches and topographical features on fracture patterns was marginalized in case of Charpy impact testing where the samples were suddenly impacted at the back of the notch by a heavy and pointed hammer. Sheer and straight fracture patterns were observed in Charpy impact specimens and fracture occured right from the notch tip.
The surface waviness (
The topographical features are summarized in
in the thermoset. The Rz values decreased when the specimens were processed with VC and 1200 P and increased when treated with 60 P and 320 P abrasive papers. As ravines were partially removed with 1200 P and VC, therefore, a reduction in Rz indicates that severe surface notches present in the as-cast samples were removed by the treatment with VC and 1200 P abrasive paper. In addition, an increase in mechanical properties when processed with VC and 1200 P further validates the removal of deep notches. On the contrary, in samples processed with 320 P and 60 P abrasive papers, the Rz values increased and were even higher than those in as-cast monolithic epoxy samples. Therefore, increase in Rz values and the presence of craters and trenches indicate that both abrasive papers 320 P and 60 P produced severe surface notches that caused the fracture. A decrease in mechanical properties when treated with 320 P and 60 P abrasive papers further corroborates the presence of severe surface notches which act as stress concentration sites and causes fracture. Therefore, Rz of fractured surfaces can be an indicator of the topographical features of the samples. Apart from Rz, Ra is another important parameter to consider. The decrease in Ra with increasing Rz may seem contradicting however can be explained on the basis of observed fractured patterns and surface roughness charts shown in
The mechanical performance is a function of topography and also depends on the orientation of topography. The topography of fractured patterns of monolithic epoxy can be used to approximate the topography of samples before fracture. It was recorded that epoxy (as-cast) had roughness that was decreased when processed with VC and 1200 P and enhanced by 60 P and 320 P. The highest enhancement in mechanical performance was recorded when specimens were processed with 1200 P. A high value of Ra (with low Rz value) can be on indicator of smoother samples surfaces. On the other hand, a low value of Ra (with high Rz value) indicates the presence of deep surface notches. A similar trend was observed in Rq values as in Ra values. However, no specific trend was observed in surface waviness and may not be indicative of topographical features. The fracture patterns suggest that when the surface notch goes beyond certain severity, brittle fracture occurs. In addition, treating the samples with surfaces of specific topography can help improve the mechanical properties of monolithic epoxy.
The authors would like to thank the Department of Mechanical and Construction Engineering, Northumbria University, UK for the provision of research facilities, without which the analysis of relevant data was not possible.
Atif, R. and Inam, F. (2016) Fractography Analysis of Monolithic Epoxy with Tailored Topography. World Journal of Engineering and Technology, 4, 517-527. http://dx.doi.org/10.4236/wjet.2016.44051