Advanced Composites Processing
Exercise no. 5
Spring Semester 2012
Issue date
19.04.12
Deadline:
03.05.12
Exercise no. 5: Permeability
Introduction/Objectives
The scope of this laboratory is to let the students familiarize themselves with
permeability characterization. Permeability is a key parameter for impregnation
processes: it is the ability of a porous medium to transport fluids, in other words it
represents physically how favorable a preform of fabrics can be impregnated by, for
example, a resin system. It generally varies depending on the flow direction (e.g. outof-plane, in-plane and fiber direction or transverse permeability) and can assume
very different values depending on the textile used and the fiber volume content FVC
(as this affects the interconnect porous space between fibers in which fluid can flow).
In-plane permeability ranges typically from 10-9 to 10-12 m2. As permeability is indeed
an internal property of the fiber preform, it can change locally in the preform itself
(because of local variations in the free space between fibers) and in addition is
affected by preform handling, draping and compaction during a process.
Correct measurement of permeability is an on-going problem in the scientific
community. Permeability characterization research has been carried out for years
and yet there is still no common standard. Usually, in-plane permeability is measured
by a 1D flow test, with a fixed thickness cavity (so fixed fiber volume content),
injecting a testing fluid at set pressure. The flow velocity is recorded and it is
correlated to the permeability, through the following analytical formula:
2𝐾
𝑥𝑓 (𝑡) = √𝜙𝜇 ∙ 𝛥𝑝 · 𝑡
(1)
Where xf is the flow-front position at time t; μ the viscosity; ϕ the porosity, Δp the
difference of pressure and K is the value of permeability to calculate (in the direction
of the flow).
In this lab the students will perform a 1D test, and also a 2D permeability
measurement in the test rig in Figure 1, whose working principle is described in [1].
The 2D test is able to provide both in-plane values of permeability (fiber- and
transverse direction) from a single fiber sample in one single test. In this laboratory,
some tests will be run and permeability results analyzed. Students are asked to
answer the questions in the following section.
[1]
Louis, B.M., Di Fratta, C., Danzi, M., Zogg, M., Ermanni, P., Improving time effective
and robust techniques for measuring in-plane permeability of fiber preforms for LCM
processing. Proceeding at SEICO 11 Paris – 32nd International Conference, March 28th-29th
2011: p. 204-211.
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis
Page 1
Advanced Composites Processing
Exercise no. 5
Spring Semester 2012
Tasks Description
Please make clear the following
questions:
1. Describe the in-plane 2D
permeability
measurement
and summarize the factors
influencing the permeability.
Assess the final values of
permeability by the data
obtained from measurement;
2. Using the data obtained from
the visual 1D permeability
experiment performed in the
laboratory,
calculate
the
permeability of the glass fiber
preform tested. The FVC of
glass fiber was 47% and Δp
of 1 bar. Assuming that the
fluid viscosity is 0.038 Pa·s.
Figure 1 – In-plane permeability test rig
Impregnation time t [s] Impregnation length xf [cm]
Hint: use equation (1).
3. Is there a difference between permeability values measured in 1D and 2D tests? If
yes, could you provide an explanation? Also, what are the advantages/disadvantages
of both tests?
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis
Page 2