Our previous article highlighted the increased complexity of placing prepregs on curved “3D” surfaces. We will now show you how these complexities are reflected in requirements and how they are measured and verified. Depending on the part that is manufactured, certain “design requirements” need to be met. Design requirements define, among other things, how strict manufacturing tolerances are. For CFRP parts made from unidirectional prepreg tapes, these tolerances come in the form of the following questions:
We will now look at each of these questions respectively and see why they are important and how we can measure the actual values. Please note, that the words “tape” and “stripe” are used interchangeably with a preference to “stripe”.
Usually, there are different restrictions on the size of the gap between adjacent stripes and the width of the gap between adjacent courses. These restrictions also vary depending on the complexity of the part. Flat parts and parts of a “simple curvature” have tighter tolerances than parts that are complexly curved.
Figure 1: Schema of design requirements of a CFRP component. When dealing with ¼in (6.35mm) prepregs, gaps between tapes are rarely allowed to exceed 1mm. Gaps between courses are rarely allowed to exceed 3mm. There are often additional constraints put on the total gap width within specified distances. Please note that these tolerances are not generally applicable to all components and are subject to change with respect to the application and surrounding conditions of the component.
How each ply must be staggered is usually defined in the laminate structure definition, where ply angles and boundaries are defined. We already covered staggering in a previous article. To summarize it very briefly, staggering is used to ensure the stability of laminates by shifting individual layers of the same orientation sideways to their material direction.
Figure 2: Schematic for the position tolerance around the nominal boundary. This value is usually below half the material width. For non-perpendicular angles (anything other than 0 or 90 in this figure), the boundary overlapping strategy is also defined. See our previous article about this subject.
Due to the curvature of the layup surface, keeping a prepreg stripe straight is rarely possible on complex surfaces. Depending on the condition of the underlying surface, stripes twist or shift sideways. In the most complex cases, the result is that a stripe steers toward the direction of the center of the curvature. There are different strategies to alleviate the problem where the angle of the stripe deviates from the planned, nominal angle. In the next article, we will showcase where this problem comes from and which strategies may be applied.
The prepreg orientation tolerance is usually specified in the CFRP component’s design requirements by an angle of ±5°, for instance. The steering tolerance is usually specified by the limits of the prepreg material (its matrix material and composition and its width) and either comes from the material specification sheet or manual experimentation.
This article about design requirements went deep into specifications and tolerances. In next week’s article about orientations in 3D, we will show how a stripe can deviate from its nominal angle and why surfaces that are curved along multiple directions will lead to gaps in the finished part.
Until then, stay safe and stay tuned.