Evaluation of Laminate composite Distortion by an Integrated Numerical-Experimental approach

Numerical Model - Phase 1


Preliminary work

Selection of a thermo-chemical model which defines the heat transfer between the part and the tool/the environment.



Definition of the thermochemical model


Selection of a cure kinetic model which predicts the evolution of the cure rate.


Definition of the cure kinetics model


Selection of a cure shrinkage model which defines the volume shrinkage during cure.


Definition of the cure shrinkage strain model


Selection of a non-linear model which simulates the resin modulus variation versus temperature


Definition of non-linear elastic model


Utilization of the homogenization technique in order to achieve the composite properties at various moments of time in the curing process


Composite properties at various temperatures obtained through homogenization technique



Numerical simulation - First results achieved


Distortion on Z axis in the skin coupon


Displacements in the coupon in the initial simulation amplified


Displacements in the C-shaped spar coupons


Displacements in the C-shaped spar coupons


Observation: The uncalibrated numerical tool has provided inaccurate distortion results. A coupling reaction can be observed for the skin coupon as for the C-shaped coupon, a warpage effect can be observed combined with spring-out of the flanges.


Calibration of the numerical tool

For the calibration of the numerical tool the following steps have been completed:
      - Insertion of the resin gel point as parameter in the simulation process according to the AIMEN experimental results
      - Insertion of a variable friction coefficient. Friction between the part and the tool starting with the resin gel point. No friction below gel point.
      - Insertion of a variable through-thickness fiber volume fraction which takes into account the uneven distribution of resin in the LRI (Liquid Resin Infusion) process in the C-shaped spar coupon and the maintaining of the constant fiber volume fraction for the skin coupon.



Displacements distribution in the skin coupon after calibration


Displacements distribution in the C-shaped spar coupon after calibration


FE plot of C-shaped coupon spring-in. The distorted part on top of the tooling


Experimental Approach - Phase 1


COUPON TESTING: Monitoring Manufacturing and Shape Distortion Analysis

Liquid resin infusion

Small scale coupons were manufactured by liquid resin infusion of RTM6 resin and dry carbon fiber preforms manufactured by AFP.

Steps followed in the experimental approach:


Monitoring manufacturing results

The manufacturing process was monitored with Direct Current (DC) sensors and Fiber Bragg Gratings (FBG). DC sensors indirectly measure resin cure behaviour through resistivity which changes during the curing reaction with the formation of polymeric chains and crosslinking. DC sensors measure temperature and electrical resistance of the resin. These parameters can be related to the resin behaviour during manufacturing. DC sensors were able to detect resin arrival, point of minimum viscosity, gel point and end of cure during the manufacturing cycle.


FBG sensors measure temperature and strain by means of a shift in a designed wavelength reflected. FBG temperature sensors were placed at both surfaces of the coupon detected the temperature distribution across the surface of the coupon and the through the thickness of the laminate.


FBG strain sensors detected a significant variation of strains (contraction) due to resin cure shrinkage at the gel point and more pronounced decreased of strain during the cooling stages. At the end of the curing cycle an uneven distribution of strains was observed across the coupon.


Post-manufacturing analysis of shape distortions

3DCMM measurements were performed after demoulding during several days monitoring changes on the final part after manufacturing. The same FBG sensors used during manufacturing were reconnected and continue monitoring strain evolution after demoulding.

Skin coupons after 14 days of manufacturing showed an increased in the curvature along the X axis. C-spar coupons showed warpage of the web and spring-in of the flanges. The strain sensors effectively monitored release of strains after the day of demoulding.


Prepreg

The same methodology was used for monitoring prepreg cure evolution, temperature profile and strain variations during the manufacturing cycle. After fabrication the geometry and strain evolution were also analysed in the same way as for LRI coupons. In this case, the sensors were also embedded in between layers to get a better understanding of the through-thickness variation of temperature and strain.


Embedment of sensors on prepreg skin coupon and C-spar coupon.


Numerical Model - Phase 2

A sub-section of a full wing-box assembly has been modelled. The part consists of three spars and skin. The tooling consists of the skin tool, the spar tools, fixtures and auxiliaries.

Downscale demonstrator Tooling Assembly


The FE model of the entire assembly needs a certain degree of simplification.


Simplified geometry of the downscale demonstrator


FE model of the tooling and part assembly


The meshed parts assembly manufactured in the 1.2m downscale demonstrator


Close-up view of the FE for the C-spar-Skin junction


Close-up view of the FE for the I-spar-Skin junction


The meshed parts assembly positioned on the Skin Tool (red)


In order to obtain a robust model that does not require high computing resources, the Skin Tool substructure and Spar Tools have not been modelled using 3D elements. Instead, constraints that bear the same role have been applied.


The fixture and Skin Tool substructure were replaced by constraints on the Z direction


Constraints applied to simulate the effect of the Spar Tools


Results


Degree of cure at the end of the curing process


Observation: At the end of the oven curing cycle, the degree of cure is not homogeneous but has a certain variation throughout the part, as described in the picture. The complete curing is obtain after a post-curing phase.


Z direction displacements [mm]


Distorted shape of the part amplified by 30 times


Conclusions: The first iteration of results is showing geometrical deformations of the spars that are similar to the spar coupons which have been simulated in the first numerical phase. The spars show spring-in of the flanges in the range of 0.3mm up to 2.18mm [Z displacement]. The skin shows different deformations compared with the skin coupons. A calibration phase is required in order to achieve a both accuracy and precision for the simulation.