CompositesWorld
Issue link: https://cw.epubxp.com/i/450689
FEBRUARY 2015 10 CompositesWorld DESIGN & TESTING » Composite parts never have the same dimensions as the tool on which they are processed. Mechanisms such as tool dimensional change during heat up and residual stress buildup within the part during cure/solidifcation and cool-down are the cause. Many are familiar with results such as "springback," which is the closing of angles due to strain anisotropy 1 . Tis is true for all composite materials and processes — only the mechanisms difer between them and then, only slightly. Dimensional change is a problem if its magnitude is greater than the part's dimensional tolerance requirements. Aerospace structure tolerances can be as tight as ±0.25 mm, and meeting tolerances this narrow can be difcult without a good dimensional-management strategy. Anyone who has baked a cake knows that how it turns out depends not only on the ingredients in the batter but also the type of pan and oven, the pan's location in the oven, the baking temperature and time, and the cool down and removal from the pan. Te same is true for composites processing. Dimensional management is a systems level problem, and many parameters afect cured-part dimensions. Te systems parameters that afect dimensional change (and any other outcome in a composites process) can be divided into three broad groups related to the part, the tooling and the process 2 : • Part: Geometry, material behavior and layup • Tooling: Geometry, material behavior • Process: Temperature, pressure, time and heat transfer Tese factors all interact to determine the outcome. If a tool is machined to the composite part's nominal engineering dimen- sions, dimensional measurements on multiple parts pulled from that tool will generally show a mean deviation from nominal and some vari- ability around that mean. If the total deviation from nominal is less than the dimensional tolerances, dimensional conformance is achieved and no further action is required. If not, the part, tooling and/or process have to be modifed to achieve dimensional conformance. Tis can be a costly and time-consuming iterative process, because it is difcult to anticipate the efect of system parameter changes if there is no predictive model. It is increasingly unacceptable to depend on trial and error to achieve dimensional conformance, especially after the tool is made and full-scale part production has begun. Alternatives include depending on experience, consulting expert opinion and performing tests. Te frst two often fall short if the part and process is complex or deviates from the previous experience base. Test data are often of limited use because cured dimensions depend on part, tooling and process, which can make scaling of results from small test coupons to the full-sized part/process misleading and, thus, risky. A more efective option, particularly for large, complex structures, is computer simula- tion, where a physics-based model is generated and links systems parameters related to the part, tooling and process to the process outcomes — in this case, dimensional change. To accurately predict that change, the model must include a high-fdelity description of the part, including its geometry, the layup and details of the composite material's behavior as its properties evolve during the cure/consolidation cycle. Also necessary is a good description of the tooling, including its geometry and its thermo-physical properties. Finally, the model must capture the process: temperature and pressure application over time, and heat transfer to the part and tool 3 . Tis type of multi-physics process model is, internally, fairly complex but can be relatively easy to set up and run if the right software-based solution package is selected. Tere are several software tools on the market that can be used for design and simulation. For the purposes of our example, we used Dassault Systèmes' CATIA and ABAQUS design and simulation software with Convergent Manufacturing Technologies' COMPRO process simulation software (Fig. 1, p. 10). Fig. 2 shows the predicted temperature gradient Getting part dimensions right in composites molding Simulating change Fig. 1: Finite element mesh of part and tool. Fig. 2: Calculated tempera- ture profle during heat-up and cure Fig. 3: Calculated dimen- sional change. Source | Convergent Manufacturing Technologies 248 244 241 235 232 228 225 221 217 214 210 207 201 Temperature (F) FIG. 2 FIG. 1 FIG. 3 4.08E-03 3.74E-03 3.40E-03 3.06E-03 2.72E-03 2.38E-03 2.04E-03 1.70E-03 1.36E-03 1.02E-03 1.70E-03 679.77E-06 339.89E-06 0.00E+00 U, Magnitude Deformation Scale: 5x T e m p e F I G . 2 D e f o r m a t i o n S c a l e : 5 x