MATRIX DAMAGE HEALING IN FIBRE REINFORCED COMPOSITE
MATERIALS CONTAINING WOVEN-IN SHAPE MEMORY ALLOY
WIRES
T.C. Bor1, L.L. Warnet 1 and R. Akkerman 1
1 Chair of Production Technology, University of Twente, Drienerlolaan 5, 7522 NB Enschede,
The Netherlands – e-mail: t.c.bor@utwente.nl; l.warnet@utwente.nl; r.akkerman@utwente.nl Keywords: self-healing composite, shape memory alloy
ABSTRACT
Continuous fiber reinforced composite materials are susceptible to delamination upon impact. Shape Memory Alloy (SMA) wires can be used to assist the healing of damage employing various known or new healing methodologies. Upon a local heating stimulus the contracting action of pre-strained wires, oriented parallel to the out-of-plane direction of the composite material, can be utilized to compress a delaminated region.
In this work the healing behavior of a continuous fibre reinforced thermoplastic model material is studied with woven-in SMA wires. The thermoplastic character of the matrix material allows for welding/healing of delaminated regions under the generated compressive stress when the material is in a soft state at elevated temperatures.
The requirements for optimum matrix healing at the delaminated interface are studied as a function of temperature and local stress state for different SMA wire distributions and volume fractions. The progress of interface healing is based on a reptation model for the interface mobility of polymer chains of the thermoplastic matrix.
The healing process is critically dependent on the thermal and mechanical properties of both the thermoplastic matrix material and the shape memory alloy wires. The developed compressive stresses in the out-of-plane direction depend on the weave pattern of the SMA wires employed. Additional stresses develop in the in-plane directions upon heating due to the undulating character of the weave patterns of the SMA wires. Requirements for optimum healing conditions with assistance of 3D woven SMA wires are provided.
1. INTRODUCTION
Advanced continuous fibre reinforced composite materials are increasingly being used for structural applications where long term reliability of the mechanical and other properties is of prime importance. However, during the lifetime of a composite structure, relatively minor events like low energy impact can cause local damage, such as small matrix cracks and delaminations. In-situ (self) healing of the damage would definitely enlarge the lifetime, reliability and robustness of structural composite materials as well as prevent costly repairs.
In this work Shape Memory Alloy (SMA) wires are employed to compress delaminated regions in continuous fibre reinforced composite materials at elevated temperatures. In this way an intimate contact zone at the faces of the delaminated regions can be formed stimulating the thermal reptation process, where polymer chains of one side of the delamination interdiffuse with those across the interface providing new strength. The application of SMA wires to assist damage healing has been studied before both theoretically and experimentally employing straight wires oriented in the out-of-plane direction. Such a configuration requires special attention to the bonding between the wires and the matrix of the composite material to maintain proper force transfer. A possible alternative is the use of woven-in SMA wires. The undulating and continuous character of the wire distribution ensures good force transfer from the wires to the composite material without the risk of debonding/slippage at the wire-matrix interface. An example of a model system with a symmetric distribution of SMA wires is shown in Fig. 1 for a [0 90]s lay-up of plies with uni-directional fibers.
Figure 1: Schematic distribution of SMA wires in a continuous fibre reinforced thermoplastic composite material with a [0 90]s lay-up of UD glass fibers. The area in
the centre of the composite material denotes a delaminated region. 2. MODEL
The response of the continuous fiber reinforced model composite material with woven-in SMA wires is determined in a similar fashion as reported recently for straight SMA wires oriented in the out-of-plane direction [1]. The composite material contains an amorphous matrix characterized by glass transition temperature Tg. The
constitutive behaviour of the SMA wires has been modeled according to the work of Popov and Lagoudas [2], incorporating the shape memory effect and pseudo-elasticity. The mechanical response of the composite material to the compressive action of the wires can be determined from the effective in-plane and out-of-plane properties of the composite material along with proper boundary conditions. Here, it is assumed that only a small portion of the composite material is heated up for a healing treatment and since the rest of the composite material remains at ambient temperature, the in-plane strain of the heated region is zero.
3. RESULTS
The effective stiffness and thermal expansion coefficient of the composite material in a direction parallel to the SMA wires can be determined as a function of the angle
between the wires and the plane of the composite material. It is assumed that the wire density is sufficiently large to impose homogeneous deformation in the in-plane
SMA wires Glass fiber Delamination
and out-of-plane directions. The angle equals 0° for wires oriented in the plane of the composite material and 90° for wires oriented in the out-of-plane direction. Results are shown in Fig. 2 for temperatures sufficiently below and above Tg of the
amorphous thermoplastic matrix material [1]. The fiber volume fraction of the glass fibers equals 50%.
Figure 2 : Effective compliance and linear thermal expansion coefficient of the [0 90]s continuous glass fibre reinforced composite material with a thermoplastic matrix material in a direction parallel to the SMA wires below Tg (blue) and above Tg (red)
versus the angle of the SMA wires.
The compliance of the composite material in a direction parallel to the SMA wires is strongly dependent on the SMA wire orientation. For = 0°, the wires are oriented in the plane of the composite material and, as in-plane deformation is suppressed by the adjacent composite material (see above), the effective composite material compliance equals zero. The more the wires are oriented in the out-of-plane direction, the more the compliance increases and reaches the maximum value at
= 90°. A similar behavior is observed for the linear thermal expansion coefficient. Subsequently, the behavior of the continuous fibre reinforced composite model system has been studied during a healing treatment as a function of Tg, the fraction
of SMA wires, fSMA, the wire orientation and the level of prestrain of the SMA wires.
Here, wires are either not prestrained (fully detwinned) or prestrained to a value of 5% (fully twinned) which also indicates the maximum extent of the shape memory effect [2]. The other thermo-mechanical properties of the SMA wires employed can be found in [1].The healing treatment comprises heating to the healing temperature
Tg + 25 K, holding the composite material for a sufficiently long time to allow
interdiffusion of the polymer chains and cooling down to room temperature.
In previous work [1] it was shown that under certain conditions a mild compressive stress of -1 MPa can be exerted by the SMA wires in the out-of-plane direction at the healing temperature and a stress free state can be obtained after healing. In this way thermal healing can be accomplished without the occurrence of residual stresses after cooling to room temperature. Here, the same approach is followed to determine the optimal fraction of SMA wires fSMA as a function of the SMA wire angle for a range
of glass transition temperatures. The results are shown in Fig. 3 for non-prestrained SMA wires (a) and fully prestrained SMA wires (b).
0 30 60 90 0 0.5 1 1.5 2x 10 -10 T < T g T > T g 0 30 60 90 0 1 2 3 4x 10 -4 T < T g T > T g Se ff [ 1/Pa ] αeff [ 1/ K] a) b) ICSHM2013_________________________________________________________________________________ 704
Figure 3 : Required volume fraction of SMA wires at different orientation angles
for optimum healing conditions without residual stresses after cooling to room temperature for (a) non-prestrained SMA wires and (b) fully prestrained SMA wires.
At = 90° optimal healing conditions occur for fully prestrained SMA wires only at relatively low glass transition temperatures, whereas for non-prestrained SMA wires a wide temperature range is possible [1]. Low fractions of SMA wires are sufficient for high angles . However, the more the wire orientation deviates from 90°, the less effective the exerted force can be used to compress the composite material in the out-of-plane direction, requiring higher fractions of SMA wires to obtain the -1 MPa compressive stress at the healing temperature. The relatively stiff response of the composite material at small angles (see Fig. 2a) enhances this effect. Especially below 45°, fSMA increases so strongly, that the application of SMA wires renders
impractical. Similar results have been obtained if the SMA wires were replaced for wires made of carbon, steel, glass or aramide as will be presented in a future publication.
4. CONCLUSIONS
Woven-in shape memory alloy wires can be employed to exert sufficient compressive stresses to assist thermal healing of delamination damage in continuous fibre reinforced composite materials upon a thermal healing treatment. If the angle of the woven-in SMA wires with the plane of the composite material is too small, the required amount of SMA wires becomes impractically large.
REFERENCES
[1] T.C. Bor, L. Warnet, R. Akkerman, A. de Boer, Modeling of stress development during thermal damage healing in fiber-reinforced composite materials containing embedded shape memory alloy wires, J. Comp. Mat. 44, 22 (2010) 2547-2572.
[2] P. Popov, D.C. Lagoudas, A 3-D constitutive model for shape memory alloys incorporating pseudoelasticity and detwinning of self-accomodated martensite, Int. J. of Plasticity, 23 (2007) 1679-1720. 0 30 60 90 0 0.025 0.05 0.075 0.1 T g = 343K T g = 393K T g = 443K T g = 493K 0 30 60 90 0 0.025 0.05 0.075 0.1 T g = 293K T g = 343K a) b)
Non-prestrained Fully prestrained
fSM A fSM A ICSHM2013_________________________________________________________________________________ 705
