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Structural performance of the glass fiber–vinyl ester composites with interlaminar copper inserts
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Composites: Part A 39 (2008) 195–203 www.elsevier.com/locate/compositesa
Structural performance of the glass fiber–vinyl ester composites with interlaminar copper inserts A. Abu Obaid, S. Yarlagadda * Center for Composite Materials, University of Delaware, 202 Composites Center, Newark, DE 19716, United States Received 9 July 2007; received in revised form 29 October 2007; accepted 10 November 2007
Abstract Mechanical performance in tension, compression and flexure of glass fiber reinforced vinyl ester composites with interlaminar copper inserts was evaluated experimentally. Key variables in performance evaluation were relative size of the copper insert and surface treatment of copper. Test coupons were manufactured using VARTM method and tested following ASTM standards for tension, compression and flexure (four-point bend). Process-induced residual strains were measured during fabrication and proved to have minimal impact for the specific thermal cycles involved. Mechanical test results indicate that the change in insert size or surface treatment have minimal impact on tensile properties though the presence of copper inserts increases failure strain. Both insert size and surface treatment have a strong impact on compression and flexural properties associated with compression mode. Decrease in mechanical properties under compression and flexure are mainly due to poor adhesion between the copper surface and the composite and the resulting delaminations. For composites with large inserts, compatible surface treatments restored both compression and flexure properties to the equivalent baseline (no copper insert) performance. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Vinyl ester; A. Copper inserts; E. Surface treatments; B. Mechanical properties
1. Introduction The continuous increase in functional requirements of composite materials poses new challenges in materials and design. Of particular interest is the need for conductive pathways in composite materials, such as glass fiber reinforced systems. This may be due to smart structure designs, integrated health monitoring systems, conformal load-bearing antennas, etc. For these types of systems, conductive elements need to be integrated into a nonconductive composite material in a non-parasitic manner. Conductive element geometries can range from wire forms for transfer of electrical energy to and from sensor/ device, to large planar forms as required for antenna systems. For structurally integrated antennas, the mechanical properties of host structure can be strongly influenced *
Corresponding author. Tel.: +1 3028314941. E-mail address: [email protected] (S. Yarlagadda).
1359-835X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.11.006
by the presence of the embedded elements and the element/composite interface properties. In the literature, extensive theoretical and experimental studies were performed on conformal antennas at Northrop Grumann [1– 4]. Researches focused on developing conformal antennas with optimal characteristics with minimum structural losses while maintaining electromagnetic performance. Several issues were discussed in these studies such as; structural and electrical performance, material selection, manufacturing complexity, airframe location, electromagnetic interference/lightning strike, weight assessments, repair and cost [1–4]. The conformal antenna system also showed improved the radio frequency communication link range compared to conventional blade antennas [1]. You et al. [5–7] and Jeon et al. [8] evaluated a multi-layer sandwich structure concept for load-bearing antenna structures. A microstrip patch antenna was developed and integrated into the core of a conventional sandwich structure, with the through-thickness location tailored based on the frequency of operation.
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In all these studies, the common approach has been to use standard antenna substrate materials (various PCBs) and build structural configurations with appropriate joint interfaces for load transfer into and out of the antenna system. An alternative approach could be to evaluate integration of antenna elements directly into structural composite materials, specifically focusing on copper inserts in the interlaminar regions of glass fiber composites. Copper inserts can be of different configurations such as metal mesh grids, copper coated Kapton [1], patterned thin foil geometries such as spirals [2] and small copper batches printed with a grid configuration on a dielectric substrate [9]. The proposed approach has advantages from an integration perspective, as the antenna elements are part of the composite structural lay-up and manufacturing process. Several issues need to be addressed – the impact of impermeable inserts on the manufacturing process, the impact on mechanical performance, and the effect of using a structural substrate on electromagnetic performance. The current effort focus on glass reinforced vinyl ester composites as the host structure and copper foil elements as inserts. The effect of embedded large-scale planar inclusions has received some attention in the literature. Javidinejad et al. [10] investigated the mechanical degradation of graphite– epoxy laminated composites with embedded thermoplastic and copper strips. Static tensile properties were affected to a small extent (3–4.5% reduction in stiffness), however significant reductions were noted in fatigue – >60% reduction in fatigue life at >75% ultimate fatigue loads. Kim et al. [11] evaluated static and dynamic tensile properties of glass/epoxy composites with embedded copper foils. Dogbone tensile coupons with electrolytic tough pitch (ETP) copper strips embedded at the midplane along the entire length of the coupon were tested. While static properties did not show significant changes, large drops in fatigue properties (failure within 1–2000 cycles) were noted even as low as 35–50% of the ultimate load level. Use of a compatible film adhesive to increase interfacial bonding between copper and the resin significantly enhanced fatigue life (90% increase). There are several unresolved issues that need to be addressed. The effect of the composite cure cycle on the residual strain in the copper insert needs to be understood, due to the low tensile yield strain in copper (0.2%). The impact of interlaminar copper inserts on the compression and flexure performance has not yet been evaluated. It is well known that compression and fracture toughness properties of composite materials are very sensitive to the presence of interlaminar inclusions. There are also the issues of the relative size of the copper insert to the size of the host composite material, adhesion between the copper and composite and the effect of thickness location of the copper inserts. 2. Materials The reinforcement used was Saint-Gobain Vetrotex E-glass plain weave fabric 324-2407 with an areal density
Table 1 Atomic concentrations of Copper-AS foils, measured using X-ray photoelectron spectroscopy (XPS) [12] Cu (%)
Zn (%)
Cr (%)
O (%)
2
12
11
44
of 24 oz/yd2, and a proprietary sizing that is compatible with polyesters, vinyl esters and epoxies. The vinyl ester used was Derakane 411-C-50, supplied by Dow Chemicals. The vinyl ester system was a mixture of Derakane 411-C-50, Trigonox 239A (cumyl hydroperoxide) as the active ingredient and 6%-cobalt naphthalene solution (CoNap) as accelerator, where the mix ratios based on the weight of the vinyl ester resin, were 2 wt% of Trigonox 239A and 0.2 wt% of CoNap. Three types of electrolytic tough pitch (ETP) copper foils, made from 110 alloy and thickness of 0.07 mm, were evaluated: Bare copper foils, Copper-AS (roughened surface, chemically incompatible to vinyl ester) and CopperT (roughened surface, chemically compatible to vinyl ester). Bare and Copper-AS foils were supplied by Somers Thin Strip/Brass Group. The mechanical properties of the foils as given by the manufacturer are: tensile strength of 220 MPa, yield strength of 82.7 MPa, yield strain of 0.2%, and elongation at break of 29%. Copper-AS foils are supplied with a special surface modification that enhances the adhesion between the copper surface and epoxy resins and are used for PCB applications. Surface morphology and chemical composition of the surface were investigated previously [12] using SEM and XPS techniques and found that copper surface has a mechanical roughness in the range of 1 lm and is rich in oxygen containing functional groups (Table 1). Our previous study [12] showed that a surface treatment of Copper-AS with Epon 828 epoxy interlayer resulted in an improvement of 100% in the adhesion between the copper and the glass fiber/vinyl ester composite. For this treatment a mixture of diglycidyl ether of bisphenol-A (EponÒ 828) epoxy resin from Shell Chemical and 28% by weight of PACM 20 curing agent was used. The specific treatment methodology is outlined in [12]. This type of copper foil is denoted as Copper-T and provides both mechanical roughness and chemical compatibility with vinyl ester. 3. Process induced residual stress evaluation To evaluate the effect of the manufacturing process on the residual strain state in the copper foil, an eight-layer E-glass/vinyl ester composite panel was fabricated with a copper foil on the top surface. The glass fabric stack with copper foil on the top was infused with vinyl ester using vacuum assisted resin transfer molding (VARTM) method. For the VARTM process, a distribution media (DM) is usually placed between the top layer of fabric and the vacuum bag to speed up infusion process. It has been shown in
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our previous work [13], that the arrangement of the DM on the top of the panel plays a key role in the successful manufacturing of composites containing impermeable inserts (no voids underneath insets are obtained). The effects of the impermeable insert size, arrangement of the DM on the resin flow patterns were studied and methodologies outlined to prevent void formation below the insert. These methods were used in the VARTM process for panel fabrication. The panel was allowed to cure at 25 °C, followed by post-curing at 110 °C for 2 h. The post-curing cycle included a heating stage from 25 to 110 °C at rate of 5 °C/min, isothermal stage at 110 °C for 2 h and cooling stage to room temperature at rate of 5 °C/min. A strain gauge and a thermocouple were bonded to the copper foil surface prior to infusion and strain and temperature recorded during the infusion, curing and post-curing process. Fig. 1 shows the measured strain and temperature in the copper during the manufacturing process. There is a small increase in strain initially during vinyl ester cure, which can be attributed to the exothermic reaction. The maximum tensile strain in the copper reaches 0.15% at the 110 °C, which is just below the yield strain of copper (0.2%). The post-cure temperature of 110 °C is quite a bit lower than the typical 177 °C cure cycle for many high performance epoxies. With linear thermal expansion, a 177 °C cure cycle will induce 0.25% strain in the copper insert causing the copper to yield by 0.05%, which could have significant implications on its fatigue performance. The copper relaxes during the cool down phase of the post-cure cycle and at room temperature shows a small residual compression strain, though it is not clear whether this is real due to cure shrinkage of the vinyl ester or within experimental error. In summary, we may assume that for the process conditions in this study, there is minimal process-induced residual strain within the copper foil.
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4. Mechanical performance evaluation 4.1. Proposed evaluation methods The present work focuses on the static mechanical properties measured under tension, compression, and flexural loading for glass fiber–vinyl ester composites with copper inserts. The types of tests and copper inserts evaluated are listed in Table 2. 5. Fabrication and experimental procedure This paper addresses the tension, compression and flexure performance of glass fiber/vinyl ester composites with embedded copper inserts. In tension and compression measurements, the copper foil is positioned at the composite mid-plane, whereas in flexure, the copper foil is positioned below the top layer and tested in both tension (copper above bottom layer) and compression (copper below top layer) mode tests. For all mechanical tests, five different size copper foil inserts were evaluated. 5.1. Tension Tensile test specimens were prepared as in accordance with ASTM D3039. Copper insert dimensions were selected to provide five different area ratios: 0% (baseline), 3.6%, 7%, 14.3% and 28.6%. Area ratio of the insert (r) is defined as r¼
insert area 100 gauge length width
ð1Þ
Actual insert dimensions are 12.7 mm 12.7 mm, 12.7 mm 25.4 mm, 12.7 mm 50.82 mm and 12.7 mm 101.6 mm respectively and were cut from Copper-AS foils. The copper inserts were placed on the four layer of an eight-layer E-glass fabric lay-up (mid-plane). Then the assembly (glass fabric with copper inserts) was infused with vinyl ester resin using VARTM, followed by the curing and post-curing process described previously. Copper foil with vinyl ester compatible surface treatment was evaluated at the largest area ratio (28.6%). The panel was cut into tensile test specimens with final dimensions of 25.4 mm width, 6 mm thick and 524 mm length. At least five tensile specimens were prepared for each area ratio. Tension tests were conducted at a Table 2 Test matrix for structural performance evaluation
Fig. 1. Measured thermal strain in copper during composite post-cure cycle.
Test
Copper type
Insert area ratio, r (%)
Tension Same Compression Same Four-point bending Same Same
Copper-AS Copper-T Copper-AS Copper-T Bare copper Copper-AS Copper-T
0. 3.6, 7, 14.3 and 28.6 28.6 0, 0.9, 3.6, 14.3 and 57 57 0, 2.8, 11 and 44.4 44.4 Same
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cross-head speed of 0.127 cm/min (0.05 in/min) using a mechanical test frame with 133.5 kN (30,000 lb) load cell. For all measurements, the gauge length was 17.78 cm. 5.2. Compression Test specimens were prepared according to ASTM D3410 using the same materials and procedure of panel fabrication as in the tensile specimen case. It was technically very difficult to achieve the same insert area ratios (as in the tension case) with a specimen of 25.4 mm gauge length and 12.7 mm width according to the ASTM standard requirements. Therefore, the width of specimens was increased to 38.1 mm. Copper insert dimensions were 2.95 mm 2.95 mm, 5.9 mm 5.9 mm, 11.75 mm 11.75 mm and 23.5 mm 23.5 mm, were cut from Copper-AS foils. The final copper insert area ratios are 0% (baseline specimens), 0.9%, 3.6%, 14.3% and 57%. The largest area ratio was also evaluated for Copper-T foil. The difference in insert geometry between the tension and compression specimens is due to different gauge lengths. The panel was cut into test coupons with gauge length of 25.4 mm and dimensions of 4.6 mm thickness, 38.1 mm width and 152.4 mm length. All measurements were conducted at a cross-head speed of 0.127 cm/min (0.05 in/ min) using a mechanical test frame with 133.5 kN (30,000 lb) load cell. Uniaxial strain gauges were placed on both faces of the specimens at the centroid of the gauge section. At least five specimens were tested. 5.3. Flexural (4-pt. bending) Four-Point Bending tests were conducted on specimens, according to ASTM D 6272. Selection of the type and location of inserts were dictated by the assumption that there will be minimal impact in tension mode and maximum in compression mode. All three types of copper foils mentioned previously were evaluated. For the bare Copper case, the dimensions of the inserts were 25.4 mm 38.1 mm, 25.4 mm 50.8 mm, 50.8 mm 101.6 mm and 101.6 mm 203.2 mm. The inserts were placed right under the top layer in the eight-layer lay-up, which allows us to study the effect of the copper insert on the compression mode.
Copper-AS and Copper-T were also evaluated for the largest area ratio. The fabricated panels were cut into flexural specimens with final dimensions of 5 mm 152.4 mm 304.8 mm and with copper area ratios, r = {0%, 0.7%, 2.8%, 11% and 44.4%}. The measurements were performed using a mechanical test frame with 26.70 kN (6000 lb) load cell. For each test, the specimen was placed in the span fixture such that the copper side of the specimen was under compression (top surface), where the span length was 203.2 mm. Uniaxial strain gauges were placed on both top and bottom surfaces of the specimens and the load as a function of strain values for compression mode (eC) and for tension mode (eT) were recorded. 6. Results and discussion 6.1. Tension results Stress–strain curves were obtained for the specimens with different area ratios of inserts and show typical bi-linear behavior of woven fabric composites, with the ‘‘knee” phenomenon [14]. From these curves, the failure properties (strength r and strain at failure ef) and the elastic modulus Ee in a strain range of 0.1–0.3% and knee strain (ee) were calculated. The knee strain was defined as the strain transition point at which the mechanical response becomes bilinear. The tensile test results are documented in Table 3. T-Tests at 95% confidence were performed on the results to identify whether the copper insert size or surface treatment affect the mechanical properties of the composites. P-Values of T-test were obtained for the following comparative cases: baseline (no Copper insert, r = 0%) versus Copper-AS with different insert size {r = 3.6%, 7%, 14.3% and 28.6%} and Copper-AS with r = 28.6% versus Copper-T (r = 28.6%). A comparison between baseline samples and samples with Copper-AS inserts shows small P-values for strength and strain behavior, indicating statistically significant effects due to the presence of the copper insert. Samples with Copper-AS inserts on the average show higher values of failure strain, slightly lower values for elastic modulus and knee strain ee compared to the baseline samples. The size of the insert does not appear to have a significant impact. The presence of the copper insert introduces
Table 3 Measured tensile properties as a function of copper area ratio (r) and copper surface treatment Copper type
Copper-AS Copper-AS Copper-AS Copper-AS Copper-AS Copper-T
r (%)
0 3.6 7 14.3 28.6 28.6
Failure properties
Elastic properties
r (Mpa) (P-value)
ef 106 (P-value)
ee 106 (P-value)
Ee (GPa) (P-value)
326 ± 18 349 ± 21 345 ± 24 345 ± 27 325 ± 30 350 ± 30
14,961 ± 1992 20,190 ± 2239 (0.007) 18,743 ± 3438 (0.08) 19,076 ± 994 (0.02) 20,294 ± 6755 (0.20) 20,205 ± 4428 (0.98)
6853 ± 605 4915 ± 266 (0.004) 4609 ± 980 (0.004) 4827 ± 551 (0.004) 5329 ± 725 (0.002) 5334 ± 1054 (0.99)
25.1 ± 3.5 23.9 ± 2.0 23.8 ± 3.1 22.7 ± 0.3 22.5 ± 2.8 23.8 ± 1.5
(0.03) (0.08) (0.11) (0.11) (0.21)
(0.60) (0.60) (0.26) (0.26) (0.21)
P-Value of T-test is given for each comparative case: baseline with r = 0% versus Copper-AS with different insert size (r = 3.6%, 7%, 14.3% and 28.6%) and Copper-T versus Copper-AS (same r).
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two possible effects: tensile yield and subsequent plastic behavior of copper and adhesion of copper to the matrix. Tensile yield of copper occurs at 0.2% strain in the composite, after which the insert carries no load and functions like an interlaminar crack. A comparison between Copper-AS and Copper-T yields large P-values for failure and knee strain elastic results and relatively small P-values for modulus and strength results. From these results, improved adhesion between copper and vinyl ester slightly increased the strength and the modulus but did not significantly affect the failure and knee strain values. Delamination initiation and propagation at the copper/matrix interface depends on the level of adhesion, however, it has minimal impact on tension performance. Failure modes were fiber and matrix fracture for all the samples and significant delaminations were also observed in specimens with copper inserts, which is consistent with observations in the literature for static loading [11].
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6.2. Compression test results For each test, strain data from the gauges on both sides of the coupons were collected and averaged to create stress–strain curves and mechanical properties such as strength rcom, strain at failure ecom and elastic modulus Ecom were determined and properties are listed in Table 4. The P-values of T-tests are shown in Table 4 for the following comparisons: baseline samples (r = 0%) versus samples having Copper-AS with different insert size {r = 0.9%, 3.6%, 14.3%, and 57%} and samples having Copper-T inserts versus samples having Copper-AS inserts (both r = 57%). T-Tests on strength and strain data resulted in small P-values for baseline samples versus samples with Copper-AS inserts, and for samples with Copper-T versus samples with Copper-AS (r = 57%). T-Tests on modulus data yield relatively large P-values, indicating insignificant differences between samples being compared. Thus, the
Table 4 Measured compressive properties of specimens with copper inserts P-value of T-test is given for each comparative case: Copper-AS with r = 0% versus Copper-AS with different insert sizes (r = 0.9%, 3.6%, 14.3%, and 57%) Copper type
r (%)
rcom (MPa) (P-value)
ecom 106 (P-value)
Ecom (GPa) (P-value)
Copper-AS Copper-AS Copper-AS Copper-AS Copper-AS Copper-T
0 0.9 3.6 14.3 57 57
367 ± 25 349 ± 20 274 ± 50 288 ± 34 306 ± 20 366 ± 25
13,410 ± 2156 13,165 ± 1024 10,072 ± 2331 10,805 ± 1377 11,714 ± 2324 13,395 ± 1410
28.0 ± 1.5 28.6 ± 1.3 28.1 ± 1.3 27.6 ± 0.9 28.9 ± 4.2 28.8 ± 0.8
a
(0.21) (0.01) (0.005) (0.01) (0.005)a
(0.81) (0.04) (0.04) (0.29) (0.26)a
(0.50) (0.86) (0.60) (0.70) (0.97)a
P-Value for comparison between Copper-AS (r = 57%) and Copper-T (r = 57%).
Fig. 2. Fracture surfaces of specimens tested under compression: (A) baseline, (B) Copper-AS with 57.1% area ratio and (C) Copper-T with 57.1% area ratio.
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presence, size and surface type of the copper insert significantly affects strength and strain but not the elastic modulus. Results shown in Table 4 indicate that the strength and average strain at failure exhibit a general decreasing trend as the insert size becomes large. These results are expected since the adhesion between Copper-AS and the composite substrate is relatively weak, causing the formation and propagation of delaminations at the copper–vinyl ester interface. At the largest area ratio, the ultimate strength and strain decrease by 20% and 6–8% respectively. No significant change is seen in the elastic modulus due to the presence and/or size of the copper insert. For the specimens with treated copper inserts (CopperT), the compatible surface treatment results in an improvement of 20% in the compressive strength and 14% in the failure strain, in comparison to the specimens with CopperAS and is comparable to the baseline (no insert) values. This is due to the strong copper surface–substrate adhesion and reinforces our argument that poor adhesion causes debonding between copper and vinyl ester resulting in premature failure. The effect of thickness location of the insert (away from the mid-plane) is currently being investigated and will be documented in future work. The presence of copper inserts also caused an increase in the local buckling effect during the compression test. The buckling level STR was determined for each specimen tested applying the following equation: STR ¼
je1 e2 j je1 þ e2 j
or chemical treatment), Copper-AS (roughness with incompatible chemical treatment) and Copper-T (roughness and compatible). In all cases, the insert is below the top layer on the compression side (top) of the specimen. The associated flexural properties in the elastic region (elastic modulus, stress and strain) and at failure (ultimate stress and ultimate strain values) were calculated. The elastic modulus in both modes was determined from the slope of linear fit curve of stress-strain data in the range of 0.1– 0.3%. For each compression and tension mode, the linear fit (which is the elastic curve as shown in Fig. 3) was extended over the entire strain range and elastic stress and strain denote where the linear elastic fit starts to deviate from the actual response (Fig. 3). For tension and compression modes, the obtained flexural properties for the bare copper case are listed in Tables 5 and 6. T-Tests were performed on flexural data associated with compression and tension mode and P-values are also given in Tables 5 and 6. For compression mode data, P-values were obtained for samples with different insert sizes versus Baseline (r = 0%). Comparison between
ð2Þ
where e1 and e2 are the measured strains on two faces of the coupon. The buckling level above 10% (STR > 0.1) within strain range of 0.1–0.3% constitutes a failure in the specimen, according to the ASTM standard. Low area ratio specimens are generally buckling-free, whereas the Copper-AS specimens with r = 57% failed the buckling criterion (STR 19%). The strain level ebuckling at which the buckling criteria starts to break down (i.e. STR > 10%) was estimated to be ebuckling < 1%. For Copper-T specimens, STR was 7% confirming the improved adhesion effect on compression performance. Improved copper–substrate adhesion also explains the clean fracture surface (which is in transverse direction to the applied load) observed by the specimens with epoxy treatment (Copper-T) as shown in Fig. 2. Specimens with as received copper surface (Copper-AS) exhibit delamination extended over the entire gauge area in the loading direction (Figs. 2A and 2B), due to poor adhesion. 6.3. Flexure test results Flexure tests were performed using the four-point bend methodology and stress–strain curves were generated in tension and compression modes, using back-to-back strain gauges. Results from compression tests have previously shown the importance of copper-matrix adhesion, so three types of copper were evaluated: Bare Copper (no roughness
Fig. 3. Typical flexural stress–strain curves measured for compression mode (Fig. 3A) and tension mode (Fig. 3B), featuring the end of elastic regions, where the stress–strain curves deviate from their extended elastic linear fit.
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Table 5 Elastic and failure flexural properties measured in compression mode as a function of copper area ratio (r %) Copper type
Baseline Bare Bare Bare Bare
r (%)
0 0.7 2.8 11 44.4
Failure flexural properties
Elastic flexural properties 6
rfC (MPa)
efC 10
267 ± 32 218 ± 46 179 ± 42 156 ± 56 203 ± 37
15,820 ± 1710 10,447 ± 4401 8175 ± 2241 6231 ± 2281 6258 ± 3216
(0.00) (0.02) (0.01) (0.42)
(0.05) (0.0007) (0.0002) (0.005)
rfeC (MPa)
efeC 106
EC (GPa)
158 ± 15 109 ± 33 131 ± 30 78 ± 26 81 ± 19
6943 ± 436 6130 ± 604 (0.94) 5741 ± 1303 (0.78) 3823 ± 1669 (0.06) 3173 ± 699 (0.01)
21 ± 1 21 ± 2 23 ± 1 22 ± 3 25 ± 1
(0.35) (0.98) (0.04) (0.04)
(0.80) (0.07) (0.69) (0.004)
P-value of T-test is given for each comparative case: Bare copper with different insert size (r%) versus Baseline. rfC and efC are flexural strength and strain at failure measured in compression, respectively; rfeC, efeC and EC are elastic flexural stress, strain and modulus measured in compression, respectively.
Table 6 Elastic flexural properties measured in tension mode as a function of copper area ratio (r %) Copper type
Baseline Bare Bare Bare Bare
r (%)
0 0.7 2.8 11 44.4
Elastic flexural properties rfeT (MPa)
efeT 106
ET (GPa)
140 ± 2 138 ± 41 (0.46) 129 ± 38 (0.08) 104 ± 33 (0.39) 85 ± 6 (0.06)
7512 ± 590 6650 ± 2013 (0.43) 5846 ± 1438 (0.00) 4966 ± 1313 (0.02) 3411 ± 192 (0.07)
19 ± 1 21 ± 3 23 ± 1 21 ± 2 25 ± 1
(0.02) (0.00) (0.09) (0.00)
P-Value of T-test is given for each comparative case: Bare with different insert size (r %) versus Baseline. rfeT, efeT and ET are elastic flexural stress, strain and modulus measured in tension, respectively.
baseline samples and samples with Bare Copper inserts yields mostly small P-values, indicating significant differences in flexural properties. All the specimens tested failed in the compression mode (first-ply failure) due to the presence of the insert on the compression side. Failure properties in compression mode are significantly affected by the size of the insert and show a decreasing trend as the copper insert size is increased. During flexural loading it was observed that interfacial delamination starts and propagates from the edges of the copper inserts and top ply buckling failure was more pronounced in specimens with larger inserts sizes (Fig. 4). No buckling was observed in baseline specimens and specimens with insert of r = 0.7%. Under loading, the edges of the copper inserts are under high stresses (stress concentration effects), leading to the initiation of the interfacial fracture [15].
A similar decreasing trend with increased insert size is also seen for averaged elastic stress and strain values measured in compression (Table 5) and tension (Table 6) modes. For tension and compression modes, the elastic strain and stress exhibit a drop of 55% and 45%, respectively, from the baseline case (r = 0%) to the largest copper insert case (r = 44.4%). The decrease is mainly due to the delamination occurring at the weak composite/copper interface. In contrast, the modulus values tend to increase as the copper size increased due to the stiffness contribution from the copper insert, which on average has a modulus of 122.5 GPa, whereas the modulus of the composite is around 22 GPa. It is clear from the compression test data (Table 4) that the interfacial adhesion between the copper surface and the composite should significantly impact flexural performance
Fig. 4. Representative failure patterns a function of copper insert size: (A) r = 0%, (B) r = 0.7%, (C) r = 11.1% and (D) r = 44.4%.
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Table 7 Elastic and failure flexural properties measured in compression mode as a function of copper surface treatment Copper type
Baseline Copper-AS Copper-T
r (%)
0 44.4 44.4
Failure flexural properties
Elastic flexural properties 6
rfC (MPa)
efC 10
rfeC (MPa)
efeC 106
EC (GPa)
394 ± 5 414 ± 16 (0.05) 334 ± 12 (0.001)
15,950 ± 1490 15,232 ± 698 (0.50) 15,015 ± 935 (0.41)
113 ± 20 104 ± 9 (0.51) 165 ± 50 (0.13)
4373 ± 752 3861 ± 469 (0.37) 7696 ± 2318 (0.06)
25 ± 1 26 ± 1 (0.01) 26 ± 1 (0.25)
P-Value of T-test is given for each comparative case: Baseline versus Copper-AS and Copper-T.
Fig. 5. Comparative failure patterns for surface treated copper with r = 44%: (A) Bare copper, (B) Copper-AS and (C) Copper-T.
in the compression mode. Therefore, a second set of flexure specimens were fabricated with insert size r = 44.4%, using two types of copper foils, Copper-AS and Copper-T. Baseline measurements (r = 0%) were also performed for this case. Statistical analysis (T-tests) was performed on measured flexural data associated with compression mode and the results of P-value are listed in Table 7. For the Copper-AS case, failure strength and elastic modulus show small increases, where as failure strain and elastic properties are similar to the baseline. The Copper-T case shows similar results, however there is a significant increase in elastic knee strain and stress compared to both the baseline and CopperAS case. As noted previously, Copper-AS has surface roughness and a chemically incompatible surface treatment, while Copper-T has both roughness and compatible surface treatment. The improvement in extending the elastic region of Copper-T specimens is due the strong interface formed between the copper surface and the substrate. As expected, interfacial adhesion between the copper surface and the composite also affected the fracture patterns on the compression side of the specimens, as seen on Fig. 5. The Copper-AS and Copper-T specimens did not show the interfacial delamination and buckling failure modes that were observed by the specimens with Bare copper inserts, but exhibited fiber and matrix fracture typical of the baseline specimens. Fiber/matrix fracture was relatively more intensive and localized in Copper-T specimens compared to Copper-AS (Fig. 5). 7. Summary and conclusions In this effort, the process-induced strains and static mechanical properties of glass fiber/vinyl ester composites
containing interlaminar copper inserts were investigated in tension, compression and flexure as a function of the insert size and copper surface treatment. In both tension and compression samples, the insert was at the mid-plane and in flexure the insert was below the top ply (compression side). The process cycle for vinyl esters involved a 110 °C post-cure cycle that did not result in any significant residual strain effects. However, higher temperature cure cycles (180 °C or greater) may potentially cause thermal yield of copper during the cure cycle. In order to quantify the significance of the effects of copper inserts type and/or size on mechanical properties of composites, T-tests at 95% confidence were performed on the test results. P-Value was provided for each comparative case and the trends of the mechanical properties as a function of copper insert size and type were determined. In tension, the presence of inserts causes a small reduction in modulus and elastic strain, but an increase in failure strength and strain. The change in the size of the insert and surface treatment did not impact the tensile properties. Compression performance was strongly impacted by the size of the insert and its adhesion to the polymer in the composite. Compression properties (strength and strain at failure) showed a significant decrease in their values (up to 20% and 6–8%) as the insert size increased. The decrease in properties can be attributed to the poor adhesion between the copper surface and the composite leading to debonding, delamination and subsequent buckling failure observed in the samples. Improvement in adhesion through surface treatment restored the original baseline performance for the largest insert size tested. Similarly, specimens tested in flexure show a significant degradation in elastic stress/strain values and failure properties associated with compression mode as the size of the insert
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increased. Improved copper/composite adhesion through surface roughness and compatible surface treatments resulted in flexure performance that was similar to the baseline (no copper) but with an extended elastic region (increased knee stress and strain).
[5]
[6]
Acknowledgement The financial support, provided by the Office of Naval Research (ONR) under Grant N00014-03-1-0891 for the Advanced Materials Intelligent Processing Center at the University of Delaware, made this work possible. References [1] Alt KH, Lockyer AJ, Martin CA, Kudva JN. Application for smart skin technologies development of a conformal antenna insulation in vertical tail of a military aircraft. In: McGowan AR, editor. Smart structures and materials: smart electronics. Proceedings of SPIE, vol. 2448. 1995. p. 42–52. [2] Alt KH, Lockyer AJ, Kudva JN, Coughlin DP, Kudva JN. Overview of DoD’s RF multifunctional structural aperture (MUSTRAP) program. In: McGowan AR, editor. Smart structures and materials: smart electronics and MEMS. Proceedings of SPIE, vol. 2448. 1995. p. 137–46. [3] Lockyer J, Kudva JN, Coughlin DP, Alt KH, Martin CA, Durham MD. Prototype testing and evaluation of a structurally integrated conformal antenna insulation in the vertical tail of a military aircraft. In: McGowan AR, editor. Smart electronics and MEMS. Proceedings of SPIE, vol. 3046. 1997. p. 173–81. [4] Lockyer J, Alt KH, Kudva JN, Tuss J. Air vehicle integration issues and considerations for CLAS successful implementation. In: McGowan AR, editor. Industrial and commercial applications of smart
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structures technologies. Proceedings of SPIE, vol. 4332, 2001. p. 48– 59. You CS, Hwang W, Park HC, Lee RM, Park WS. Microstrip antenna for SAR application with composite sandwich construction: surface antenna structure demonstration. J Compos Mater 2003;37:351–64. You CS, Hwang W. Design of load-bearing antenna structures by embedding technology of microstrip antenna in composite sandwich structure. Compos Struct 2005;71:378–82. You CS. Design and fabrication of composite smart structures for communication. Chapter 2: effect of a dielectric cover on microstrip antennas by T/R methods. PhD. Dissertation, Pohang University of Science and Technology; 2004. Jeon JH et al. Design of microstrip antennas with composite laminates considering their structural rigidity. Mech Compos Mater 2002;38:447–60. Yeo J, Mittra R. Design of conformal multiband antennas based on fractal concepts. Microwave Opt Technol Lett 2003;36(5):333–8. Javidinejad, Joshi Shiv P. Design and structural testing of smart composite structures with embedded conductive thermoplastic film. Smart Mater Struct 1999;8:585–90. Kim H et al. Fatigue fracture of embedded copper conductors in multifunctional composite structures. Compos Sci Technol 2006;66:1010–21. Obaid A, Gocke A, Yarlagadda S, Advani S. Enhancement of the adhesion between copper surface and vinyl ester in glass fiber–vinyl ester composites. Compos Interf 2007;14(2):99–116. Lawrence JM, Pierre F, Abu Obaid A, Yarlagadda S, Advani SG. Simulation and validation of resin flow during manufacturing of composite panels containing embedded impermeable inserts with the VARTM process. Polym Composite 2007;28(4):442–50. Tsai SW. Strength characteristics of anisotropic and laminated composite materials. NASA CR-224; April 1965. Weidong X, Sitaraman SK. Investigation of interfacial delamination of a copper–epoxy interface under monotonic and cyclic loading: modeling and evaluation. IEEE Trans Adv Pack 2003;26(4):441–6.
Composites: Part A 39 (2008) 195–203 www.elsevier.com/locate/compositesa
Structural performance of the glass fiber–vinyl ester composites with interlaminar copper inserts A. Abu Obaid, S. Yarlagadda * Center for Composite Materials, University of Delaware, 202 Composites Center, Newark, DE 19716, United States Received 9 July 2007; received in revised form 29 October 2007; accepted 10 November 2007
Abstract Mechanical performance in tension, compression and flexure of glass fiber reinforced vinyl ester composites with interlaminar copper inserts was evaluated experimentally. Key variables in performance evaluation were relative size of the copper insert and surface treatment of copper. Test coupons were manufactured using VARTM method and tested following ASTM standards for tension, compression and flexure (four-point bend). Process-induced residual strains were measured during fabrication and proved to have minimal impact for the specific thermal cycles involved. Mechanical test results indicate that the change in insert size or surface treatment have minimal impact on tensile properties though the presence of copper inserts increases failure strain. Both insert size and surface treatment have a strong impact on compression and flexural properties associated with compression mode. Decrease in mechanical properties under compression and flexure are mainly due to poor adhesion between the copper surface and the composite and the resulting delaminations. For composites with large inserts, compatible surface treatments restored both compression and flexure properties to the equivalent baseline (no copper insert) performance. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Vinyl ester; A. Copper inserts; E. Surface treatments; B. Mechanical properties
1. Introduction The continuous increase in functional requirements of composite materials poses new challenges in materials and design. Of particular interest is the need for conductive pathways in composite materials, such as glass fiber reinforced systems. This may be due to smart structure designs, integrated health monitoring systems, conformal load-bearing antennas, etc. For these types of systems, conductive elements need to be integrated into a nonconductive composite material in a non-parasitic manner. Conductive element geometries can range from wire forms for transfer of electrical energy to and from sensor/ device, to large planar forms as required for antenna systems. For structurally integrated antennas, the mechanical properties of host structure can be strongly influenced *
Corresponding author. Tel.: +1 3028314941. E-mail address: [email protected] (S. Yarlagadda).
1359-835X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.11.006
by the presence of the embedded elements and the element/composite interface properties. In the literature, extensive theoretical and experimental studies were performed on conformal antennas at Northrop Grumann [1– 4]. Researches focused on developing conformal antennas with optimal characteristics with minimum structural losses while maintaining electromagnetic performance. Several issues were discussed in these studies such as; structural and electrical performance, material selection, manufacturing complexity, airframe location, electromagnetic interference/lightning strike, weight assessments, repair and cost [1–4]. The conformal antenna system also showed improved the radio frequency communication link range compared to conventional blade antennas [1]. You et al. [5–7] and Jeon et al. [8] evaluated a multi-layer sandwich structure concept for load-bearing antenna structures. A microstrip patch antenna was developed and integrated into the core of a conventional sandwich structure, with the through-thickness location tailored based on the frequency of operation.
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In all these studies, the common approach has been to use standard antenna substrate materials (various PCBs) and build structural configurations with appropriate joint interfaces for load transfer into and out of the antenna system. An alternative approach could be to evaluate integration of antenna elements directly into structural composite materials, specifically focusing on copper inserts in the interlaminar regions of glass fiber composites. Copper inserts can be of different configurations such as metal mesh grids, copper coated Kapton [1], patterned thin foil geometries such as spirals [2] and small copper batches printed with a grid configuration on a dielectric substrate [9]. The proposed approach has advantages from an integration perspective, as the antenna elements are part of the composite structural lay-up and manufacturing process. Several issues need to be addressed – the impact of impermeable inserts on the manufacturing process, the impact on mechanical performance, and the effect of using a structural substrate on electromagnetic performance. The current effort focus on glass reinforced vinyl ester composites as the host structure and copper foil elements as inserts. The effect of embedded large-scale planar inclusions has received some attention in the literature. Javidinejad et al. [10] investigated the mechanical degradation of graphite– epoxy laminated composites with embedded thermoplastic and copper strips. Static tensile properties were affected to a small extent (3–4.5% reduction in stiffness), however significant reductions were noted in fatigue – >60% reduction in fatigue life at >75% ultimate fatigue loads. Kim et al. [11] evaluated static and dynamic tensile properties of glass/epoxy composites with embedded copper foils. Dogbone tensile coupons with electrolytic tough pitch (ETP) copper strips embedded at the midplane along the entire length of the coupon were tested. While static properties did not show significant changes, large drops in fatigue properties (failure within 1–2000 cycles) were noted even as low as 35–50% of the ultimate load level. Use of a compatible film adhesive to increase interfacial bonding between copper and the resin significantly enhanced fatigue life (90% increase). There are several unresolved issues that need to be addressed. The effect of the composite cure cycle on the residual strain in the copper insert needs to be understood, due to the low tensile yield strain in copper (0.2%). The impact of interlaminar copper inserts on the compression and flexure performance has not yet been evaluated. It is well known that compression and fracture toughness properties of composite materials are very sensitive to the presence of interlaminar inclusions. There are also the issues of the relative size of the copper insert to the size of the host composite material, adhesion between the copper and composite and the effect of thickness location of the copper inserts. 2. Materials The reinforcement used was Saint-Gobain Vetrotex E-glass plain weave fabric 324-2407 with an areal density
Table 1 Atomic concentrations of Copper-AS foils, measured using X-ray photoelectron spectroscopy (XPS) [12] Cu (%)
Zn (%)
Cr (%)
O (%)
2
12
11
44
of 24 oz/yd2, and a proprietary sizing that is compatible with polyesters, vinyl esters and epoxies. The vinyl ester used was Derakane 411-C-50, supplied by Dow Chemicals. The vinyl ester system was a mixture of Derakane 411-C-50, Trigonox 239A (cumyl hydroperoxide) as the active ingredient and 6%-cobalt naphthalene solution (CoNap) as accelerator, where the mix ratios based on the weight of the vinyl ester resin, were 2 wt% of Trigonox 239A and 0.2 wt% of CoNap. Three types of electrolytic tough pitch (ETP) copper foils, made from 110 alloy and thickness of 0.07 mm, were evaluated: Bare copper foils, Copper-AS (roughened surface, chemically incompatible to vinyl ester) and CopperT (roughened surface, chemically compatible to vinyl ester). Bare and Copper-AS foils were supplied by Somers Thin Strip/Brass Group. The mechanical properties of the foils as given by the manufacturer are: tensile strength of 220 MPa, yield strength of 82.7 MPa, yield strain of 0.2%, and elongation at break of 29%. Copper-AS foils are supplied with a special surface modification that enhances the adhesion between the copper surface and epoxy resins and are used for PCB applications. Surface morphology and chemical composition of the surface were investigated previously [12] using SEM and XPS techniques and found that copper surface has a mechanical roughness in the range of 1 lm and is rich in oxygen containing functional groups (Table 1). Our previous study [12] showed that a surface treatment of Copper-AS with Epon 828 epoxy interlayer resulted in an improvement of 100% in the adhesion between the copper and the glass fiber/vinyl ester composite. For this treatment a mixture of diglycidyl ether of bisphenol-A (EponÒ 828) epoxy resin from Shell Chemical and 28% by weight of PACM 20 curing agent was used. The specific treatment methodology is outlined in [12]. This type of copper foil is denoted as Copper-T and provides both mechanical roughness and chemical compatibility with vinyl ester. 3. Process induced residual stress evaluation To evaluate the effect of the manufacturing process on the residual strain state in the copper foil, an eight-layer E-glass/vinyl ester composite panel was fabricated with a copper foil on the top surface. The glass fabric stack with copper foil on the top was infused with vinyl ester using vacuum assisted resin transfer molding (VARTM) method. For the VARTM process, a distribution media (DM) is usually placed between the top layer of fabric and the vacuum bag to speed up infusion process. It has been shown in
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our previous work [13], that the arrangement of the DM on the top of the panel plays a key role in the successful manufacturing of composites containing impermeable inserts (no voids underneath insets are obtained). The effects of the impermeable insert size, arrangement of the DM on the resin flow patterns were studied and methodologies outlined to prevent void formation below the insert. These methods were used in the VARTM process for panel fabrication. The panel was allowed to cure at 25 °C, followed by post-curing at 110 °C for 2 h. The post-curing cycle included a heating stage from 25 to 110 °C at rate of 5 °C/min, isothermal stage at 110 °C for 2 h and cooling stage to room temperature at rate of 5 °C/min. A strain gauge and a thermocouple were bonded to the copper foil surface prior to infusion and strain and temperature recorded during the infusion, curing and post-curing process. Fig. 1 shows the measured strain and temperature in the copper during the manufacturing process. There is a small increase in strain initially during vinyl ester cure, which can be attributed to the exothermic reaction. The maximum tensile strain in the copper reaches 0.15% at the 110 °C, which is just below the yield strain of copper (0.2%). The post-cure temperature of 110 °C is quite a bit lower than the typical 177 °C cure cycle for many high performance epoxies. With linear thermal expansion, a 177 °C cure cycle will induce 0.25% strain in the copper insert causing the copper to yield by 0.05%, which could have significant implications on its fatigue performance. The copper relaxes during the cool down phase of the post-cure cycle and at room temperature shows a small residual compression strain, though it is not clear whether this is real due to cure shrinkage of the vinyl ester or within experimental error. In summary, we may assume that for the process conditions in this study, there is minimal process-induced residual strain within the copper foil.
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4. Mechanical performance evaluation 4.1. Proposed evaluation methods The present work focuses on the static mechanical properties measured under tension, compression, and flexural loading for glass fiber–vinyl ester composites with copper inserts. The types of tests and copper inserts evaluated are listed in Table 2. 5. Fabrication and experimental procedure This paper addresses the tension, compression and flexure performance of glass fiber/vinyl ester composites with embedded copper inserts. In tension and compression measurements, the copper foil is positioned at the composite mid-plane, whereas in flexure, the copper foil is positioned below the top layer and tested in both tension (copper above bottom layer) and compression (copper below top layer) mode tests. For all mechanical tests, five different size copper foil inserts were evaluated. 5.1. Tension Tensile test specimens were prepared as in accordance with ASTM D3039. Copper insert dimensions were selected to provide five different area ratios: 0% (baseline), 3.6%, 7%, 14.3% and 28.6%. Area ratio of the insert (r) is defined as r¼
insert area 100 gauge length width
ð1Þ
Actual insert dimensions are 12.7 mm 12.7 mm, 12.7 mm 25.4 mm, 12.7 mm 50.82 mm and 12.7 mm 101.6 mm respectively and were cut from Copper-AS foils. The copper inserts were placed on the four layer of an eight-layer E-glass fabric lay-up (mid-plane). Then the assembly (glass fabric with copper inserts) was infused with vinyl ester resin using VARTM, followed by the curing and post-curing process described previously. Copper foil with vinyl ester compatible surface treatment was evaluated at the largest area ratio (28.6%). The panel was cut into tensile test specimens with final dimensions of 25.4 mm width, 6 mm thick and 524 mm length. At least five tensile specimens were prepared for each area ratio. Tension tests were conducted at a Table 2 Test matrix for structural performance evaluation
Fig. 1. Measured thermal strain in copper during composite post-cure cycle.
Test
Copper type
Insert area ratio, r (%)
Tension Same Compression Same Four-point bending Same Same
Copper-AS Copper-T Copper-AS Copper-T Bare copper Copper-AS Copper-T
0. 3.6, 7, 14.3 and 28.6 28.6 0, 0.9, 3.6, 14.3 and 57 57 0, 2.8, 11 and 44.4 44.4 Same
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cross-head speed of 0.127 cm/min (0.05 in/min) using a mechanical test frame with 133.5 kN (30,000 lb) load cell. For all measurements, the gauge length was 17.78 cm. 5.2. Compression Test specimens were prepared according to ASTM D3410 using the same materials and procedure of panel fabrication as in the tensile specimen case. It was technically very difficult to achieve the same insert area ratios (as in the tension case) with a specimen of 25.4 mm gauge length and 12.7 mm width according to the ASTM standard requirements. Therefore, the width of specimens was increased to 38.1 mm. Copper insert dimensions were 2.95 mm 2.95 mm, 5.9 mm 5.9 mm, 11.75 mm 11.75 mm and 23.5 mm 23.5 mm, were cut from Copper-AS foils. The final copper insert area ratios are 0% (baseline specimens), 0.9%, 3.6%, 14.3% and 57%. The largest area ratio was also evaluated for Copper-T foil. The difference in insert geometry between the tension and compression specimens is due to different gauge lengths. The panel was cut into test coupons with gauge length of 25.4 mm and dimensions of 4.6 mm thickness, 38.1 mm width and 152.4 mm length. All measurements were conducted at a cross-head speed of 0.127 cm/min (0.05 in/ min) using a mechanical test frame with 133.5 kN (30,000 lb) load cell. Uniaxial strain gauges were placed on both faces of the specimens at the centroid of the gauge section. At least five specimens were tested. 5.3. Flexural (4-pt. bending) Four-Point Bending tests were conducted on specimens, according to ASTM D 6272. Selection of the type and location of inserts were dictated by the assumption that there will be minimal impact in tension mode and maximum in compression mode. All three types of copper foils mentioned previously were evaluated. For the bare Copper case, the dimensions of the inserts were 25.4 mm 38.1 mm, 25.4 mm 50.8 mm, 50.8 mm 101.6 mm and 101.6 mm 203.2 mm. The inserts were placed right under the top layer in the eight-layer lay-up, which allows us to study the effect of the copper insert on the compression mode.
Copper-AS and Copper-T were also evaluated for the largest area ratio. The fabricated panels were cut into flexural specimens with final dimensions of 5 mm 152.4 mm 304.8 mm and with copper area ratios, r = {0%, 0.7%, 2.8%, 11% and 44.4%}. The measurements were performed using a mechanical test frame with 26.70 kN (6000 lb) load cell. For each test, the specimen was placed in the span fixture such that the copper side of the specimen was under compression (top surface), where the span length was 203.2 mm. Uniaxial strain gauges were placed on both top and bottom surfaces of the specimens and the load as a function of strain values for compression mode (eC) and for tension mode (eT) were recorded. 6. Results and discussion 6.1. Tension results Stress–strain curves were obtained for the specimens with different area ratios of inserts and show typical bi-linear behavior of woven fabric composites, with the ‘‘knee” phenomenon [14]. From these curves, the failure properties (strength r and strain at failure ef) and the elastic modulus Ee in a strain range of 0.1–0.3% and knee strain (ee) were calculated. The knee strain was defined as the strain transition point at which the mechanical response becomes bilinear. The tensile test results are documented in Table 3. T-Tests at 95% confidence were performed on the results to identify whether the copper insert size or surface treatment affect the mechanical properties of the composites. P-Values of T-test were obtained for the following comparative cases: baseline (no Copper insert, r = 0%) versus Copper-AS with different insert size {r = 3.6%, 7%, 14.3% and 28.6%} and Copper-AS with r = 28.6% versus Copper-T (r = 28.6%). A comparison between baseline samples and samples with Copper-AS inserts shows small P-values for strength and strain behavior, indicating statistically significant effects due to the presence of the copper insert. Samples with Copper-AS inserts on the average show higher values of failure strain, slightly lower values for elastic modulus and knee strain ee compared to the baseline samples. The size of the insert does not appear to have a significant impact. The presence of the copper insert introduces
Table 3 Measured tensile properties as a function of copper area ratio (r) and copper surface treatment Copper type
Copper-AS Copper-AS Copper-AS Copper-AS Copper-AS Copper-T
r (%)
0 3.6 7 14.3 28.6 28.6
Failure properties
Elastic properties
r (Mpa) (P-value)
ef 106 (P-value)
ee 106 (P-value)
Ee (GPa) (P-value)
326 ± 18 349 ± 21 345 ± 24 345 ± 27 325 ± 30 350 ± 30
14,961 ± 1992 20,190 ± 2239 (0.007) 18,743 ± 3438 (0.08) 19,076 ± 994 (0.02) 20,294 ± 6755 (0.20) 20,205 ± 4428 (0.98)
6853 ± 605 4915 ± 266 (0.004) 4609 ± 980 (0.004) 4827 ± 551 (0.004) 5329 ± 725 (0.002) 5334 ± 1054 (0.99)
25.1 ± 3.5 23.9 ± 2.0 23.8 ± 3.1 22.7 ± 0.3 22.5 ± 2.8 23.8 ± 1.5
(0.03) (0.08) (0.11) (0.11) (0.21)
(0.60) (0.60) (0.26) (0.26) (0.21)
P-Value of T-test is given for each comparative case: baseline with r = 0% versus Copper-AS with different insert size (r = 3.6%, 7%, 14.3% and 28.6%) and Copper-T versus Copper-AS (same r).
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two possible effects: tensile yield and subsequent plastic behavior of copper and adhesion of copper to the matrix. Tensile yield of copper occurs at 0.2% strain in the composite, after which the insert carries no load and functions like an interlaminar crack. A comparison between Copper-AS and Copper-T yields large P-values for failure and knee strain elastic results and relatively small P-values for modulus and strength results. From these results, improved adhesion between copper and vinyl ester slightly increased the strength and the modulus but did not significantly affect the failure and knee strain values. Delamination initiation and propagation at the copper/matrix interface depends on the level of adhesion, however, it has minimal impact on tension performance. Failure modes were fiber and matrix fracture for all the samples and significant delaminations were also observed in specimens with copper inserts, which is consistent with observations in the literature for static loading [11].
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6.2. Compression test results For each test, strain data from the gauges on both sides of the coupons were collected and averaged to create stress–strain curves and mechanical properties such as strength rcom, strain at failure ecom and elastic modulus Ecom were determined and properties are listed in Table 4. The P-values of T-tests are shown in Table 4 for the following comparisons: baseline samples (r = 0%) versus samples having Copper-AS with different insert size {r = 0.9%, 3.6%, 14.3%, and 57%} and samples having Copper-T inserts versus samples having Copper-AS inserts (both r = 57%). T-Tests on strength and strain data resulted in small P-values for baseline samples versus samples with Copper-AS inserts, and for samples with Copper-T versus samples with Copper-AS (r = 57%). T-Tests on modulus data yield relatively large P-values, indicating insignificant differences between samples being compared. Thus, the
Table 4 Measured compressive properties of specimens with copper inserts P-value of T-test is given for each comparative case: Copper-AS with r = 0% versus Copper-AS with different insert sizes (r = 0.9%, 3.6%, 14.3%, and 57%) Copper type
r (%)
rcom (MPa) (P-value)
ecom 106 (P-value)
Ecom (GPa) (P-value)
Copper-AS Copper-AS Copper-AS Copper-AS Copper-AS Copper-T
0 0.9 3.6 14.3 57 57
367 ± 25 349 ± 20 274 ± 50 288 ± 34 306 ± 20 366 ± 25
13,410 ± 2156 13,165 ± 1024 10,072 ± 2331 10,805 ± 1377 11,714 ± 2324 13,395 ± 1410
28.0 ± 1.5 28.6 ± 1.3 28.1 ± 1.3 27.6 ± 0.9 28.9 ± 4.2 28.8 ± 0.8
a
(0.21) (0.01) (0.005) (0.01) (0.005)a
(0.81) (0.04) (0.04) (0.29) (0.26)a
(0.50) (0.86) (0.60) (0.70) (0.97)a
P-Value for comparison between Copper-AS (r = 57%) and Copper-T (r = 57%).
Fig. 2. Fracture surfaces of specimens tested under compression: (A) baseline, (B) Copper-AS with 57.1% area ratio and (C) Copper-T with 57.1% area ratio.
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presence, size and surface type of the copper insert significantly affects strength and strain but not the elastic modulus. Results shown in Table 4 indicate that the strength and average strain at failure exhibit a general decreasing trend as the insert size becomes large. These results are expected since the adhesion between Copper-AS and the composite substrate is relatively weak, causing the formation and propagation of delaminations at the copper–vinyl ester interface. At the largest area ratio, the ultimate strength and strain decrease by 20% and 6–8% respectively. No significant change is seen in the elastic modulus due to the presence and/or size of the copper insert. For the specimens with treated copper inserts (CopperT), the compatible surface treatment results in an improvement of 20% in the compressive strength and 14% in the failure strain, in comparison to the specimens with CopperAS and is comparable to the baseline (no insert) values. This is due to the strong copper surface–substrate adhesion and reinforces our argument that poor adhesion causes debonding between copper and vinyl ester resulting in premature failure. The effect of thickness location of the insert (away from the mid-plane) is currently being investigated and will be documented in future work. The presence of copper inserts also caused an increase in the local buckling effect during the compression test. The buckling level STR was determined for each specimen tested applying the following equation: STR ¼
je1 e2 j je1 þ e2 j
or chemical treatment), Copper-AS (roughness with incompatible chemical treatment) and Copper-T (roughness and compatible). In all cases, the insert is below the top layer on the compression side (top) of the specimen. The associated flexural properties in the elastic region (elastic modulus, stress and strain) and at failure (ultimate stress and ultimate strain values) were calculated. The elastic modulus in both modes was determined from the slope of linear fit curve of stress-strain data in the range of 0.1– 0.3%. For each compression and tension mode, the linear fit (which is the elastic curve as shown in Fig. 3) was extended over the entire strain range and elastic stress and strain denote where the linear elastic fit starts to deviate from the actual response (Fig. 3). For tension and compression modes, the obtained flexural properties for the bare copper case are listed in Tables 5 and 6. T-Tests were performed on flexural data associated with compression and tension mode and P-values are also given in Tables 5 and 6. For compression mode data, P-values were obtained for samples with different insert sizes versus Baseline (r = 0%). Comparison between
ð2Þ
where e1 and e2 are the measured strains on two faces of the coupon. The buckling level above 10% (STR > 0.1) within strain range of 0.1–0.3% constitutes a failure in the specimen, according to the ASTM standard. Low area ratio specimens are generally buckling-free, whereas the Copper-AS specimens with r = 57% failed the buckling criterion (STR 19%). The strain level ebuckling at which the buckling criteria starts to break down (i.e. STR > 10%) was estimated to be ebuckling < 1%. For Copper-T specimens, STR was 7% confirming the improved adhesion effect on compression performance. Improved copper–substrate adhesion also explains the clean fracture surface (which is in transverse direction to the applied load) observed by the specimens with epoxy treatment (Copper-T) as shown in Fig. 2. Specimens with as received copper surface (Copper-AS) exhibit delamination extended over the entire gauge area in the loading direction (Figs. 2A and 2B), due to poor adhesion. 6.3. Flexure test results Flexure tests were performed using the four-point bend methodology and stress–strain curves were generated in tension and compression modes, using back-to-back strain gauges. Results from compression tests have previously shown the importance of copper-matrix adhesion, so three types of copper were evaluated: Bare Copper (no roughness
Fig. 3. Typical flexural stress–strain curves measured for compression mode (Fig. 3A) and tension mode (Fig. 3B), featuring the end of elastic regions, where the stress–strain curves deviate from their extended elastic linear fit.
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Table 5 Elastic and failure flexural properties measured in compression mode as a function of copper area ratio (r %) Copper type
Baseline Bare Bare Bare Bare
r (%)
0 0.7 2.8 11 44.4
Failure flexural properties
Elastic flexural properties 6
rfC (MPa)
efC 10
267 ± 32 218 ± 46 179 ± 42 156 ± 56 203 ± 37
15,820 ± 1710 10,447 ± 4401 8175 ± 2241 6231 ± 2281 6258 ± 3216
(0.00) (0.02) (0.01) (0.42)
(0.05) (0.0007) (0.0002) (0.005)
rfeC (MPa)
efeC 106
EC (GPa)
158 ± 15 109 ± 33 131 ± 30 78 ± 26 81 ± 19
6943 ± 436 6130 ± 604 (0.94) 5741 ± 1303 (0.78) 3823 ± 1669 (0.06) 3173 ± 699 (0.01)
21 ± 1 21 ± 2 23 ± 1 22 ± 3 25 ± 1
(0.35) (0.98) (0.04) (0.04)
(0.80) (0.07) (0.69) (0.004)
P-value of T-test is given for each comparative case: Bare copper with different insert size (r%) versus Baseline. rfC and efC are flexural strength and strain at failure measured in compression, respectively; rfeC, efeC and EC are elastic flexural stress, strain and modulus measured in compression, respectively.
Table 6 Elastic flexural properties measured in tension mode as a function of copper area ratio (r %) Copper type
Baseline Bare Bare Bare Bare
r (%)
0 0.7 2.8 11 44.4
Elastic flexural properties rfeT (MPa)
efeT 106
ET (GPa)
140 ± 2 138 ± 41 (0.46) 129 ± 38 (0.08) 104 ± 33 (0.39) 85 ± 6 (0.06)
7512 ± 590 6650 ± 2013 (0.43) 5846 ± 1438 (0.00) 4966 ± 1313 (0.02) 3411 ± 192 (0.07)
19 ± 1 21 ± 3 23 ± 1 21 ± 2 25 ± 1
(0.02) (0.00) (0.09) (0.00)
P-Value of T-test is given for each comparative case: Bare with different insert size (r %) versus Baseline. rfeT, efeT and ET are elastic flexural stress, strain and modulus measured in tension, respectively.
baseline samples and samples with Bare Copper inserts yields mostly small P-values, indicating significant differences in flexural properties. All the specimens tested failed in the compression mode (first-ply failure) due to the presence of the insert on the compression side. Failure properties in compression mode are significantly affected by the size of the insert and show a decreasing trend as the copper insert size is increased. During flexural loading it was observed that interfacial delamination starts and propagates from the edges of the copper inserts and top ply buckling failure was more pronounced in specimens with larger inserts sizes (Fig. 4). No buckling was observed in baseline specimens and specimens with insert of r = 0.7%. Under loading, the edges of the copper inserts are under high stresses (stress concentration effects), leading to the initiation of the interfacial fracture [15].
A similar decreasing trend with increased insert size is also seen for averaged elastic stress and strain values measured in compression (Table 5) and tension (Table 6) modes. For tension and compression modes, the elastic strain and stress exhibit a drop of 55% and 45%, respectively, from the baseline case (r = 0%) to the largest copper insert case (r = 44.4%). The decrease is mainly due to the delamination occurring at the weak composite/copper interface. In contrast, the modulus values tend to increase as the copper size increased due to the stiffness contribution from the copper insert, which on average has a modulus of 122.5 GPa, whereas the modulus of the composite is around 22 GPa. It is clear from the compression test data (Table 4) that the interfacial adhesion between the copper surface and the composite should significantly impact flexural performance
Fig. 4. Representative failure patterns a function of copper insert size: (A) r = 0%, (B) r = 0.7%, (C) r = 11.1% and (D) r = 44.4%.
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Table 7 Elastic and failure flexural properties measured in compression mode as a function of copper surface treatment Copper type
Baseline Copper-AS Copper-T
r (%)
0 44.4 44.4
Failure flexural properties
Elastic flexural properties 6
rfC (MPa)
efC 10
rfeC (MPa)
efeC 106
EC (GPa)
394 ± 5 414 ± 16 (0.05) 334 ± 12 (0.001)
15,950 ± 1490 15,232 ± 698 (0.50) 15,015 ± 935 (0.41)
113 ± 20 104 ± 9 (0.51) 165 ± 50 (0.13)
4373 ± 752 3861 ± 469 (0.37) 7696 ± 2318 (0.06)
25 ± 1 26 ± 1 (0.01) 26 ± 1 (0.25)
P-Value of T-test is given for each comparative case: Baseline versus Copper-AS and Copper-T.
Fig. 5. Comparative failure patterns for surface treated copper with r = 44%: (A) Bare copper, (B) Copper-AS and (C) Copper-T.
in the compression mode. Therefore, a second set of flexure specimens were fabricated with insert size r = 44.4%, using two types of copper foils, Copper-AS and Copper-T. Baseline measurements (r = 0%) were also performed for this case. Statistical analysis (T-tests) was performed on measured flexural data associated with compression mode and the results of P-value are listed in Table 7. For the Copper-AS case, failure strength and elastic modulus show small increases, where as failure strain and elastic properties are similar to the baseline. The Copper-T case shows similar results, however there is a significant increase in elastic knee strain and stress compared to both the baseline and CopperAS case. As noted previously, Copper-AS has surface roughness and a chemically incompatible surface treatment, while Copper-T has both roughness and compatible surface treatment. The improvement in extending the elastic region of Copper-T specimens is due the strong interface formed between the copper surface and the substrate. As expected, interfacial adhesion between the copper surface and the composite also affected the fracture patterns on the compression side of the specimens, as seen on Fig. 5. The Copper-AS and Copper-T specimens did not show the interfacial delamination and buckling failure modes that were observed by the specimens with Bare copper inserts, but exhibited fiber and matrix fracture typical of the baseline specimens. Fiber/matrix fracture was relatively more intensive and localized in Copper-T specimens compared to Copper-AS (Fig. 5). 7. Summary and conclusions In this effort, the process-induced strains and static mechanical properties of glass fiber/vinyl ester composites
containing interlaminar copper inserts were investigated in tension, compression and flexure as a function of the insert size and copper surface treatment. In both tension and compression samples, the insert was at the mid-plane and in flexure the insert was below the top ply (compression side). The process cycle for vinyl esters involved a 110 °C post-cure cycle that did not result in any significant residual strain effects. However, higher temperature cure cycles (180 °C or greater) may potentially cause thermal yield of copper during the cure cycle. In order to quantify the significance of the effects of copper inserts type and/or size on mechanical properties of composites, T-tests at 95% confidence were performed on the test results. P-Value was provided for each comparative case and the trends of the mechanical properties as a function of copper insert size and type were determined. In tension, the presence of inserts causes a small reduction in modulus and elastic strain, but an increase in failure strength and strain. The change in the size of the insert and surface treatment did not impact the tensile properties. Compression performance was strongly impacted by the size of the insert and its adhesion to the polymer in the composite. Compression properties (strength and strain at failure) showed a significant decrease in their values (up to 20% and 6–8%) as the insert size increased. The decrease in properties can be attributed to the poor adhesion between the copper surface and the composite leading to debonding, delamination and subsequent buckling failure observed in the samples. Improvement in adhesion through surface treatment restored the original baseline performance for the largest insert size tested. Similarly, specimens tested in flexure show a significant degradation in elastic stress/strain values and failure properties associated with compression mode as the size of the insert
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increased. Improved copper/composite adhesion through surface roughness and compatible surface treatments resulted in flexure performance that was similar to the baseline (no copper) but with an extended elastic region (increased knee stress and strain).
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Acknowledgement The financial support, provided by the Office of Naval Research (ONR) under Grant N00014-03-1-0891 for the Advanced Materials Intelligent Processing Center at the University of Delaware, made this work possible. References [1] Alt KH, Lockyer AJ, Martin CA, Kudva JN. Application for smart skin technologies development of a conformal antenna insulation in vertical tail of a military aircraft. In: McGowan AR, editor. Smart structures and materials: smart electronics. Proceedings of SPIE, vol. 2448. 1995. p. 42–52. [2] Alt KH, Lockyer AJ, Kudva JN, Coughlin DP, Kudva JN. Overview of DoD’s RF multifunctional structural aperture (MUSTRAP) program. In: McGowan AR, editor. Smart structures and materials: smart electronics and MEMS. Proceedings of SPIE, vol. 2448. 1995. p. 137–46. [3] Lockyer J, Kudva JN, Coughlin DP, Alt KH, Martin CA, Durham MD. Prototype testing and evaluation of a structurally integrated conformal antenna insulation in the vertical tail of a military aircraft. In: McGowan AR, editor. Smart electronics and MEMS. Proceedings of SPIE, vol. 3046. 1997. p. 173–81. [4] Lockyer J, Alt KH, Kudva JN, Tuss J. Air vehicle integration issues and considerations for CLAS successful implementation. In: McGowan AR, editor. Industrial and commercial applications of smart
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