Spring-in and warpage of fiber reinforced laminates are the two most prominent process-induced deformation modes in CFRP part manufacturing and led to a large number of investigations in the literature, ranging from analytical expres-sions to fully 3D coupled thermo-chemo-viscoelastic finite element simulations (among others: [17, 32, 118, 124–141]). These deformations are a result of residual stress introduced during the laminate manufacturing process by the following sources [17, 32, 127, 142]:
• Thermal expansion of the part
• Resin cure shrinkage
• Gradients in temperature, degree of cure, and fiber volume content
• Thermal tool expansion and tool-part interaction
The influence of the thermal expansion of the part on residual stress build-up was subject of early studies that neglect the impact of temperature gradients, which is applicable for thin laminates only [143, 144]. Given that the resin gels at an elevated temperature, the part is subjected to a temperature discrepancy between its stress-free net-shape state and its actual operating state. Material anisotropy leads to CTE anisotropy, which drives a directional negative thermal expansion during cool-down, resulting in a part distortion.
Volumetric resin cure shrinkage results in a reduction of the part’s dimensions.
Due to directional laminate stiffness, the resulting ply strains in fiber direction are small compared to the perpendicular fiber direction of a ply. The effect of shrinkage is thus similar to the effect of decreasing part temperature [17].
In thick laminates temperature gradients and accompanying cure gradients can occur, which contribute to the development of stress and deformation due to the spatially varying material response [39]. If non-uniform cure is present, three different scenarios may occur, shown in Figure 2-2: (a)outside−to−insidecure, (b)inside−to−outsidecure and (c)One−sidedcure [5]. Scenario (a) can lead to entrapped voids or volatile by-products of the curing resin and delaminations because the cured exterior region constrains the interior region [36, 44]. inside− to−outsidecure occurs at lower set-point temperatures thanoutside−to−inside cure. If the part interior gels earlier than the part surface, it leads to compression stresses in the core, which is favorable over the transverse tension stresses arising in scenario (a). However, the final degree of cure can be lower in some resin systems if inside−to−outside cure is chosen [5]. Finally, one−sided cure (c) is
induced if different mold temperatures are chosen on the top and bottom side of the laminate. Whereas this cure strategy leads to lower curing times without increasing the exothermic peak temperature, the residual stresses in thickness direction are not balanced, resulting in part distortion [5]. All of these cure strategies lead to an increase in residual stress compared to a uniform curing that causes a reduction in the laminate load carrying capability in general and may result in part distortion, delamination, microcracks, and void formation in particular [43, 145, 146].
Figure 2-2Possible non-uniform cure scenarios [5].
Fiber volume gradients in thickness direction, occurring in for example man-ufacturing processes using vacuum bags, can result in fiber volume content deviations by several percent as shown by Radford and others [108, 124, 126].
Hubert demonstrated that pressure gradients may induce an uneven resin flow.
In an autoclave set-up this can result in local laminate thickness variations as well as fiber volume gradients, especially in the presence of resin bleed [147]. Since the laminate stiffness is defined by the fiber volume content, these gradients lead to unsymmetrical laminate layup and warpage after the cool-down. In order to improve the mechanical properties, Naji and Hoa showed that the cure cycle has an effect on the fiber volume content and developed a modified curing process to gain uniform part thickness and fiber volume content [148, 149].
If CFRP laminates are produced on a metal tool, the tool features a significantly higher thermal expansion coefficient compared to the part in most cases, de-pending on the tool material. Thus, fibers close to the tool part interface exhibit stress since they are pressed onto the tool surface by the autoclave pressure and both tool and part are subjected to thermal expansion (see Figure 2-3).
Figure 2-3Sketch of warpage mechanism due to tool-part interaction [6].
This stress state decreases in thickness direction with increasing distance from the tool surface as the stress transfer in-between the fibers is poor [150]. Only the first layers follow the tool deformation. These stresses are "cured-in" and will be released upon demolding and, thus, result in part warpage as shown in Figure 2-3 (c) [6, 151]. To demonstrate this mechanism, Albert and Fernlund investigated the utilization of a fluorinated ethylene propylene (FEP) release sheet in-between tool and part with the result of significantly decreased spring-in of L- and C-shaped specimen [128]. If the tool material shows a similar thermal expansion as the part, this tool-part effect vanishes since the induced strain in the part due to tool-part interaction becomes very small. This is a major reason for the utilization of tools with low CTE, such as CFRP and Invar [152].
Moisture causes swelling leading to a geometrical volume change, which results in similar effects as cool-down or shrinkage. It has not been included in the de-scription above due to its reversibility and absence in most production processes using epoxy resin systems [153]. However, if the chemical resin reaction mech-anism is polycondensation, the dependency of the resin properties on moisture has to be considered, given that water is a byproduct of the resin reaction.
Overall, the residual stresses in the laminate during cure, cool-down, and af-ter demolding are a result of the transient stress and strain build-up during the whole production process. Compensation of different mechanisms can oc-cur. Viscoelastic resin behavior results in a reduction of the residual stress in general, but the impact depends on the resin itself as well as the applied cure cy-cle [117, 154]. In order to reduce residual stresses and improve laminate quality,
the investigations conducted found a homogeneous temperature and degree of cure evolution to be highly beneficial. Thus, the optimization of manufacturing processes to improve part laminate quality targets the reduction of temperature and degree of cure gradients in most cases [38, 41, 146, 155, 156].