In Liquid Composite Molding (LCM) process- and tool-design, the permeability of the fibrous reinforcement is a very important parameter to consider. The permeability of porous fibrous media is a geometric property and measures its hydraulic conductivity to fluid flow. Since a standard test method for evaluating permeability currently does not exist, a number of different methods have been developed. Experimental and simulation based approaches are the most promising with respect to accuracy. Both approaches have been pursued at the LCC since its foundation. The overlapping topic “Permeability measurement” deals with the application and further development of these available methods across the established groups of the institute.
At the LCC test benches for the two most widespread experimental methods; unidirectional (1D) and radial (2D) flow methods, are available. These two methods complement each other. By means of the unidirectional method, the saturated and unsaturated in-plane permeability tensor of porous textiles can be determined as well as the saturated off-plane permeability. The radial flow method enables the determination of the complete in-plane permeability tensor as well as the through-thickness compaction response of the material. However, the experimental methods are limited to the determination of permeability values of flat samples, whereas real parts usually contain curved sections and consist of different material lay-ups with various fiber volume fractions and reinforcement architectures. Furthermore, experiments are time and material consuming. These issues are addressed by the simulation approach, which allows fast calculation of permeability values even on complex material models. The material modeling process is based on digital image processing of micrographs, computer tomography (CT) scans or images. Nevertheless, experimental validation and verification of the simulation approach is mandatory. In the following, the different test benches available at the institute and the simulation approach are introduced more detailed:
Unidirectional in-plane flow experiments
The saturated and unsaturated permeability in direction of the applied pressure difference can be measured using this test bench according to the Darcy approach. The flow front and mass flow are tracked via pressure sensors and force transducers, respectively. The experimental data is gathered by a data acquisition system and is automatically analyzed after the test. Up to four experiments can be run in parallel for time saving reasons. Next to the 4-cavity setup shown in Figure 1, a single cavity setup has been established where the flow front is detected optically through a transparent mold half.
Radial in-plane flow experiments and measurement of compaction response
In this test facility the complete unsaturated in-plane permeability tensor is measured together with the through-thickness compaction response of the material within a single experiment. The setup is installed in an universal testing machine and the flow front is detected optically. The experimental data is gathered by a data acquisition system and is automatically analyzed after the test. The fiber volume fraction of the sample can continuously be adjusted and the compaction response can be investigated as a function of the mold closure velocity.
Unidirectional out-of-plane flow experiments
The setup allows the determination of the saturated permeability in the out-of-plane (through-thickness) direction by measuring the mass flow through the compacted fibrous textile. Flow and compaction in the thickness direction is enabled by the application of perforated plates. The fluid pressure at the in- and outlet is monitored by integrated pressure transducers.
By means of all these test facilities, the 3D permeability tensor according to Darcy’s law can completely be determined by assuming that the in-plane permeability tensor coincides with the textile plane. The latter is wide spread and justified especially for thin parts.
Simulation approach for permeability prediction
The principle of this approach is to set up a computer model of the fiber material and perform a computational fluid dynamics (CFD) simulation. The permeability is determined from the resulting fluid velocity field according to Darcy’s law. One important step is to gather all relevant input data for modeling the textile’s architecture. Thereto, digital images of the fabric are taken and analysed using digital image processing. Based on these geometrical parameters the material model is established.