enhancing the through-thickness modulus of carbon …
TRANSCRIPT
Copyright 2020 by Boston Materials, Inc.
WWW.BOMATERIALS.COM
ENHANCING THE THROUGH-THICKNESS MODULUS OF CARBON FIBER COMPOSITES USING Z-AXIS ORIENTED MILLED CARBON FIBER
Dr. Rasam Soheilian, Clara Stewart, Robert Mone, Anvesh Gurijala
December 11, 2020
ABSTRACT
Boston Materials presents a thin (0.12mm thick) Z-axis carbon fiber reinforcement that is integrated with a standard unidirectional carbon fiber prepreg. This material (SUPERCOMP® 1015 UD) is laminated using common press consolidation. The resulting panel was subjected to a stacked compression test in which strain gauges detect the through-thickness strain of the panel under a compressive load. A panel comprised completely of a standard unidirectional (UD) prepreg was also produced and tested as a comparative baseline. The SUPERCOMP 1015 UD panel (46% Z-axis milled fiber + 54% UD fiber) has a near two-fold increase in through-thickness modulus compared to the panel made completely with a standard UD prepreg. Keywords: Carbon fiber composite; through-thickness; Z-axis; milled fiber; enhanced composite
Copyright 2020 by Boston Materials, Inc.
WWW.BOMATERIALS.COM
INTRODUCTION
The transfer of through-thickness loads is a deficiency in virtually all forms of fiber-reinforced composite materials. This deficiency stems from the anisotropy (direction-dependent behavior) of fiber reinforcements, which are predominantly available as planar materials. While in many cases the design of structures using fiber-reinforced composite revolves around in-plane loads, through-thickness loads manifest at bonding interfaces, mechanical fastening points, and sealing features. Additionally, composite structures that undergo impact and rapid dynamic loads are subject to bulk through-thickness loads. From a first principles perspective, incorporating fiber reinforcement in the through-thickness (i.e., Z-axis) direction can mitigate through-thickness strain and failure.
Methods such as 3D-weaving, 3D-braiding, Z-pinning, and Z-stitching have been employed in the past to incorporate through-thickness fiber reinforcement in composite laminates. In all of these cases, the final through-thickness fiber content is typically less than 5% of the overall fiber reinforcement, and the process cannot be cost-effectively used to make thin sheets or films that can be used in a conventional ply layup process. Accordingly, the methods mentioned above are greatly limited in improving bulk through-thickness mechanical properties, producing thin structures, and integrating into ply lamination processes (such as automated tape laying). Historically, there has been no through-thickness fiber reinforcement that has the same ply thickness as a conventional carbon fiber unidirectional tape.
With the target of creating the first through-thickness aligned fiber reinforcement that is available as thin (less than 0.2mm thickness) prepregs and dry reinforcements, Boston Materials has developed ZRT™ technology. The ZRT technology leverages common milled carbon fibers (nominal fiber length ranging from 0.05 to 0.2mm) that are aligned in the Z-axis using a proprietary process. Boston Materials has integrated this alignment technique into a 60”-wide roll-to-roll process that produces films of Z-axis milled carbon fiber that can range from 0.05 to 0.2mm thickness, or 50 to 200gsm fiber areal weight, depending on the product. The SUPERCOMP product line comprises of Z-axis fiber layers that are laminated to in-plane continuous fiber plies, and the ZRT product line comprises of standalone Z-axis milled fiber layers for thermal, electrical, and local mechanical reinforcement.
Figure 1: 60”-wide production line installed at Boston Materials’ facility in Billerica, MA
Copyright 2020 by Boston Materials, Inc.
WWW.BOMATERIALS.COM
EXPERIMENTATION
Materials
Boston Materials manufactures the SUPERCOMP 1015 UD prepreg (made with NPT301 epoxy). This prepreg features a layer of standard modulus UD continuous fiber (140gsm) that is laminated to a layer of standard modulus Z-axis oriented milled carbon fiber (120gsm); the resin content is 37% by weight. This material was toll-processed into a prepreg by Mitsubishi Chemical Carbon Fiber Composites (MCCFC). A baseline UD prepreg (120gsm fiber areal weight, 37% resin content by weight) made with the same continuous fiber and NPT301 epoxy used in the SUPERCOMP 1015 UD prepreg was also sourced from MCCFC.
Panel Fabrication
Two panels (10” x 10” x ~0.25” thickness) were produced using a press consolidation process. The first panel comprised of thirty-seven layers of SUPERCOMP 1015 UD prepreg arranged in a quasi-isotropic (QI) layup. The second panel comprised of seventy-four layers of the baseline UD prepreg arranged in a QI layup. Both panels were subject to the following cure cycle: 100 psi; 3°F/min ramp to 275°F; hold for 60 minutes, cool to room temperature. A 150mbar vacuum was applied to the press chamber to reduce the risk of porosity and surface defects. Polished steel plates treated with Loctite FREKOTE 700-NC release were used as the press platens.
Stacked Compression Test
Each panel was cut into three nominally 1.5” strips. The strips were then stacked and glued with Loctite EA9460 adhesive, cured at 100°C for 1 hour along with 48-hour cure at room temperature to achieve full cure. Nominally 1.5” x 1.0” specimens were then cut from the strips. Strain gages were placed on the 1.5” sides on the middle coupon that was approximately 0.25” thick. The middle coupon is the active test material. Testing was performed on an MTS universal test frame with a load cell capacity of 66,500 pounds. Test rate was 0.05 inches/minute, data acquisition rate of 29Hz. Test loads were run to a sufficient load to establish the initial linear portion of the stress vs. strain plot, from which modulus was determined. No catastrophic failure was achieved for this test regime. This testing was performed by Highland Point, Inc. (A2LA accredited, certificate #5742.01).
Figure 2: Stacked Compression Test Setup
Copyright 2020 by Boston Materials, Inc.
WWW.BOMATERIALS.COM
RESULTS
The SUPERCOMP 1015 UD panel (46% Z-axis milled fiber + 54% UD fiber) has a 95% increase in through-thickness modulus compared to the baseline UD panel (100% UD fiber). Additionally, the baseline panel has a high coefficient of variation (CV). Considering that the two panels were produced with identical processing parameters, the high CV suggests that standard composite materials have limited repeatability of through-thickness properties.
Table 1: Stacked compression test results
Specimen ID Width
(in) Thickness
(in) Peak Load
(lb) Modulus (Msi)
Strain Gauge 1 Strain Gauge 2
SUPERCOMP 1015 UD (46% Z-axis + 54% UD)
SC-1 1.5420 1.0050 33594 2.4 2.3
SC-2 1.5215 1.0035 39487 2.3 2.4
SC-3 1.5420 1.0070 35746 1.9 1.9
SC-4 1.5345 1.0050 35010 2.2 2.0
SC-5 1.5435 1.0040 36730 2.1 2.1
Average 1.5367 1.0049 36113 2.2 2.1
SD 0.010 0.001 2206 0.2 0.2
CV(%) 1 0 6 9 10
BASELINE UD (100% UD)
B-1 1.5375 1.0130 30805 1.5 1.4
B-2 1.5145 1.0205 25564 0.7 0.7
B-3 1.5295 1.0100 25729 0.9 0.9
B-4 1.5390 1.0140 31459 1.6 1.5
B-5 1.5310 1.0220 26190 1.0 1.0
Average 1.5303 1.0159 27949 1.1 1.1
SD 0.010 0.010 2924 0.4 0.3
CV(%) 1 1 10 34 31
Copyright 2020 by Boston Materials, Inc.
WWW.BOMATERIALS.COM
CONCLUSIONS
A stacked compression test was used to determine the through-thickness modulus of carbon fiber composite panels. The SUPERCOMP 1015 UD panel has a near two-fold increase in through-thickness modulus compared to the panel made completely with a standard UD prepreg. Furthermore, this study demonstrates that shielding a dense layer (120gsm fiber areal weight) of Z-axis aligned carbon fiber between adjacent UD plies that have relatively low through-thickness modulus can still enhance the through-thickness load transfer capability of the bulk composite material. In standard composite materials, the through-thickness load modulus is dominated by interlaminar matrix polymer. Regions where the continuous fiber plies can nest and percolate with the adjacent plies can locally enhance the through-thickness load transfer efficiency, however this interaction is randomized and weak. The inclusion of Z-axis aligned milled carbon fiber between each continuous fiber ply creates consistent and bulk inter-ply percolation while leveraging the high stiffness of carbon fiber in the through-thickness direction.
Examples of applications that would immediately benefit from materials that have enhanced through-thickness mechanical properties include:
• gaskets in aircraft engines and automobile transmissions that operate in high temperature and high differential pressure environments
• composite airframes that have bolted and riveted fastening points, and adhesive-bonds
• micro-sandwich laminates used in handheld devices and speaker cones that are subject to bending
• implantable medical devices that require fixation to bone tissue
• composite plates and cylinders used in munitions that undergo rapid pressurization or impact
• additively manufactured brackets, fixtures, and jigs that require near-isotropic properties
Copyright 2020 by Boston Materials, Inc.
WWW.BOMATERIALS.COM
All data given is based on representative samples of the materials in question. Since the methods and circumstances under which these materials are processed and tested are key to their performance, and Boston Materials, Inc. (DBA Boston Materials) has no assurance of how the material will be processed by third parties, Boston Materials, Inc. cannot guarantee these characteristics or properties. SUPERCOMP® and ZRT™, and all other related characters, logos, and trade names are claims and/or registered trademarks of Boston Materials, Inc. Use of trademarks, trade names, and other intellectual property rights of Boston Materials, Inc. without prior written approval by such is strictly prohibited.