Debonding

Debonding (also called dewetting) is one mechanism of the failure of filler reinforced composites which are subjected to either continuous stress or fluctuating stresses. Debonding may also be used as a method of production for some of the materials discussed in Section 7.3. 

15 gives a simple description of the stress acting on an isolated particle. In reality, more particles are involved in the dissipation of local stresses in filled 

The rate of debonding decreases as the number of debonded particles increases and as the stress increases. The debonding constants characterize the interaction and the influence of neighboring particles. Their values depend on the filler concentration and on the adhesion of the filler to the matrix. The volume increase due to debonding is given by the equation  

The volume increase depends on the filler fraction and on the applied strain. This is confiimed in practice. Debonding correlates with loss of stiffness. The first part of the stress-strain curve (elastic stage) is related to the strains beyond which debonding occurs. In glass bead filled polypropylene, this strain was 0.7%.  

In mixtures of particles, the stress of debonding is not uniform. Higher stress is needed to debond from smaller particles. Adhesion is inversely proportional to the cube root of the diameter of the particles. Experiments confirmed that large particle sized filler decreased the tensile strength of composites. The filler concentration effect is not linear. Up to a certain concentration, filler did increase the tensile properties but beyond certain level there is a reverse effect. This may relate to the interactions described in previous sections where the quality of bonding (weak or strong) depended on filler concentration. 

---

Figure I represents a two-dimensional damage distribution of an impact in a 0/90° CFRP laminate of 3 mm thickness. Unlike in ultrasonic testing, which is usually the standard method for this problem, there is no shadowing effect on the successive layers by delamination echos. With the method of X-ray refraction the exact concentration of debonded fibers can be calculated for each position averaged over the wall thickness. Additionally the refraction allows the selection of the fiber orientation. The presented X-ray refraction topograph detects selectively debonded fibers of the 90° direction.

Fig. 1 High re.solution X-ray refraction topography of low energy impact (5J) at CFRP epoxy laminate. Image area 2 mm X 4 mm. Horizontal resolution 0.2 mm. The image represents selectively an area of debonded fibers of vertical fiber orientation.

Fig. 2 X-ray refraction topographs of a series of /OyPOj/s samples of different impact energies. The total damage of the laminates is characterized by addition of all debonded layers of0° and 90° fiber direction.

Fig. 3 Refraction values of both (0°+ 90°) fiber directions with respect to impact energy per layer. The fiber/matrix debonding of CFRP laminates correlates significantly to the impact energy per volume (energy density).

Another consideration is the difference in thermal expansion between the matrix and the reinforcement. Composites are usually manufactured at high temperatures. On cooling any mismatch in the thermal expansion between the reinforcement and the matrix results in residual mismatch stresses in the composite. These stresses can be either beneficial or detrimental if they are tensile, they can aid debonding of the interface if they are compressive, they can retard debonding, which can then lead to bridge failure (25). 

Carbon is a commonly used and successful weak interfacial coating. For high temperature appHcations, however, carbon is not the best solution, because it oxidizes, leaving a physical gap between the reinforcement and the matrix or allowing interfacial reactions that result in a strong interface bond. Much research has been conducted to develop alternative high temperature debond coatings, with tittle success to date. 

Artefacts Damaged piece of GFRP to show opacity caused by debonding. 

Fig. 1. (a) Adhesive vs. cohesive failure, (b) Close-up view of adhesive failure in the pre.sence of an interphase. The locus of failure may be adjacent to or within the interphase (as shown), and particles of material may be ejected during the debonding process. 

Fig. 16. Extensive deformation of foam core during debonding.

The deformation of the core significantly increases the peel force during debonding (see Fig. 16). 

The foam typically becomes the locus for failure during debonding. 

Upon peeling, each particle dissipates energy by stretching to several times its diameter before debonding from the surface. 

---