Ready-to-Ship Composites

Share

Fig. 1. CFRP J-frames for installation into lower skin section of Lear Fan fuselage assembly. Photo Credit: Lear Fan LTD and Abaris

From time to time, clients will contact me to discuss a problem they are having with a composite part that is not meeting the dimensional requirements called out on the drawing. When troubleshooting the problem, I will first ask to see two things: A photo or drawing of the part configuration and a copy of the ply table related to the layup of the part. Once it is determined that it is not a laminate springback issue (or other tooling concern), we turn the focus to the ply orientation scheme and the fiber form(s) employed in the layup. Manifestations of this issue have plagued the composites industry since my early career and continue to this day.

The first experience I had with major dimensional problems in a carbon fiber-reinforced epoxy part was back in the early 1980s. Our tooling group was tasked to build a set of composite molds for multiple “J”-frame sections for the Lear Fan 2100 all-carbon fiber fuselage (Fig. 1). We completed the tools and sent them to production, where they were used to make a set of pre-production parts, built with 8 plies of 3K, 8-harness-satin (HS) fabric and autoclave-cured at 350°F (177°C) and 90 psi. However, as the vacuum bag was removed from the first part, it demolded itself, looking more like a carbon fiber curly fry than the fuselage frame section it was intended to be. Of course, the production department pointed to the tooling as the culprit, but after further investigation, and much discussion with the M&P group, it became apparent that the root cause was something other than the tooling. But what was it?

After much deliberation, the theory put forth was that the coefficient of thermal expansion (CTE) of the carbon fiber in the cross section had a positive value (expands), while in the linear direction it had a negative value (shrinks) at the 350°F (177°C) cure temperature, thus “locking in” the fibers in this state at vitrification. The laminate then experiences a differential temperature of -280°F (-138°C) during cooldown to room temperature, ~70°F (21°C). At this point, the fibers want to return to their previous size while the resin slightly shrinks, causing increased stresses within the laminate and warpage, as depicted in the examples in Fig. 2. How do you remedy this?

Fig. 2. Four-ply CFRP unidirectional (UD) tape panels with symmetric and asymmetric layup schemes as shown. Note that the symmetric panel is flat as molded, and the asymmetric panels are warped about the dominate axis of orientation. Photo Credit: Abaris

It was determined that the asymmetric1 orientation scheme itself, [0↓/+45↓/90↓/-45↓/0↓/+45↓/90↓/-45↓], with all plies warp face down (indicated by arrow ↓), was problematic in that it did not compensate for thermally induced forces. It was clear that opposing forces were needed within the laminate design to offset the effect and maintain a balanced2 orientation with quasi-isotropic3 properties. A new orientation scheme was introduced, [0↓/+45↓/90↓/-45↓/-45↓/90↓/+45↓/0↓], resulting in a cured part that still had a slight twist in it from end to end. Why?

A closer look at the 8HS carbon fabric itself would shed more light on the cause. One side of the fabric has yarns running in the warp direction and the other side has them running perpendicular — 90° opposite of each other in the fill (or weft) direction. This is true for all HS weave fabrics (Fig. 3). This means that the orientation (up or down) of the warp face of each ply must also be controlled along with the fiber axial direction in the layup to produce a truly symmetric laminate: [0↓/+45↓/90↓/-45↓/-45↑/90↑/+45↑/0↑]. This scheme produces a dimensionally accurate part.

Warp vs. fill face.

Fig. 3. This 6K 5-HS fabric shows warp face and fill face characteristics. The warp yarn direction is parallel to the length of the roll (parallel to yellow Kevlar selvedge), and the fill yarn direction is perpendicular. Warp face orientation needs to be considered when cutting plies prior to layup. Photo Credit: Abaris

This effect is most dramatic with carbon fiber unidirectional (UD) tape and HS weave fabrics. It does not apply to plain and twill weave fabrics that have an equal amount of fiber (yarns) running in both the warp and fill directions on both sides of the fabric. Therefore, warp face control with these fabrics are not required.

What about glass fiber, which has a positive CTE value when heated and expands in both linear and cross-sectional directions? Glass fiber has an average CTE that is approximately four to five times greater than carbon fiber. Therefore, each ply of HS weave glass fabric is also considered asymmetric and still can contribute to part warpage (Fig. 4). Since glass fiber has a lower modulus, thin laminates made with glass fiber reinforcements can often be forced to conform to shape with minimal load, while laminates made with higher modulus carbon fibers cannot.

#1581 style glass.

Fig. 4. A single layer of #1581 style glass (8HS), used as a bleeder layer in a circular composite repair scheme, shows the effect of asymmetry after an elevated temperature cure at 250°F (121°C), subsequently cooled to room temperature. Photo Credit: Abaris

There are other considerations when using HS weave fabrics, one of which includes having fibers running on the same axis at each possible interface (nesting). This applies to layers of opposite axes that are adjacent to each other in the ply layup scheme. The idea is that nesting improves the (axial) load transfer between layers as depicted in Fig. 5, making for a more efficient laminate. Another requirement is that all laminates made with HS fabrics technically need to be designed with an even number of plies, otherwise, the laminate will be asymmetric. In addition, designers need to recognize that HS fabrics are not symmetric, even though they have equal yarn size and count in both the warp and fill directions. The outermost faces of the laminate will have greater stiffness in the dominant fiber axis compared to that of a plain (or twill) weave fabric that provides uniform stiffness.

Fig. 5. This illustration depicts an optimized layup scheme that takes advantage of nesting at five of the seven interfaces in this theoretical 8-ply laminate. Photo Credit: Abaris

While the composite industry generally has a fundamental understanding of basic fiber orientation and laminate symmetry, many designers overlook the intrinsic asymmetry of certain fiber forms such as HS (and stitched multiaxial) fabrics that can lead to dimensionally inaccurate parts. Paying attention to the type, orientation and interaction of fiber reinforcements early in the design stage can go a long way toward avoiding thermal stresses that can lead to warpage in the finished structure.

BARRDAY PREPREG
Composites One
world leader in braiding technology
Gurit Advanced Composite Materials & Solutions
Custom Quantity Composite Repair Materials
Keyland Polymer Webinar Coatings on Composite & AM
Eliminate Quality Escapes  With LASERVISION AI
Toray Advanced Composites hi-temperature materials

Related Content

Feature

A new era for ceramic matrix composites

CMC is expanding, with new fiber production in Europe, faster processes and higher temperature materials enabling applications for industry, hypersonics and New Space.

Read More
Automation

Large-format 3D printing enables toolless, rapid production for AUVs

Dive Technologies started by 3D printing prototypes of its composite autonomous underwater vehicles, but AM became the solution for customizable, toolless production.

Read More
ATL/AFP

PEEK vs. PEKK vs. PAEK and continuous compression molding

Suppliers of thermoplastics and carbon fiber chime in regarding PEEK vs. PEKK, and now PAEK, as well as in-situ consolidation — the supply chain for thermoplastic tape composites continues to evolve.

Read More
Aerospace

Combining multifunctional thermoplastic composites, additive manufacturing for next-gen airframe structures

The DOMMINIO project combines AFP with 3D printed gyroid cores, embedded SHM sensors and smart materials for induction-driven disassembly of parts at end of life.

Read More

Read Next

Design/Simulation

Modeling and characterization of crushable composite structures

How the predictive tool “CZone” is applied to simulate the axial crushing response of composites, providing valuable insights into their use for motorsport applications.

Read More
CAMX

VIDEO: High-rate composites production for aerospace

Westlake Epoxy’s process on display at CAMX 2024 reduces cycle time from hours to just 15 minutes.

Read More
Carbon Fibers

CFRP planing head: 50% less mass, 1.5 times faster rotation

Novel, modular design minimizes weight for high-precision cutting tools with faster production speeds.  

Read More
Composites One