Particle/matrix debonding

If structural failure of the fabricated composite occurs by cracks in the matrix, fracture of the particle-matrix interface, or case debonding under any applied load, the extra and exposed surface will cause an enormous rise in the motor pressure during the combustion event with disasterous consequences. 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

If the adhesion is low, debonding at the rubber particle matrix interface can occur. In both cases voids are formed and this reduces the degree of stress triaxiality in the surrounding matrix and favors the further growth of shear bands. 

A threshold level of interfacial adhesion is also necessary to produce a triaxial tensile state around rubber particles as the result of the cure process. When the two-phase material is cooled from the cure temperature to room temperature, internal stresses around particles are generated due to the difference of thermal expansion coefficients of both phases. If particles cannot debond from the matrix, this stress field magnifies the effect produced upon mechanical loading. 

Decohesion and void growth are controlled by the debonding stress at the particle/matrix interface it only occurs when the local debonding stress is lower than the fracture stress (crazing stress) of the matrix itself G is correlated with the local toughness at the interface it is dependent on the particle/matrix adhesion energy. 

It was noted in the previous section that the carboxyl end groups on the CTBN elastomer affected the final performance of the material as a toughener since these groups would co-react with the epoxy resin and facilitate stress transfer from the brittle matrix to the phase-separated elastomer. Without this adhesion the particles could debond prematurely, which would lead to poor dissipation of the energy of the growing crack. It has also been noted that excessive adhesion between an epoxy resin and a thermoplastic could be deleterious to the performance (Williams et al, 1997). The process of toughening of a thermoset... 

In composite propellants, the cavitation (or debonding) process, which has been shown to take place near (or at) the particle-matrix interface, is dependent on pressure, deformation, and additional viscoelastic and dissipative considerations (10). 

Compared with BD13, at low strain of 0.2, the matrix appearance is similar, but less particles are debonded from the matrix. At 0.4 true strain, the matrix deformation and microvoids formation are less important than BD13 as well. Large matrix deformation and large quantity of microvoids appear when the tme strain reaches 0.6. The microfilaments across the interface can still be seen for some small size particles. Even though the microvoid formation and growth are developed greatly, the horizontal... 

It is sometimes suggested that rubber particles lose completely their ability to sustain a stress once they have cavitated. This is actually not true except only for a very few cases. First exception is when voids are formed along particle-matrix interfaces due to debonding (poor adhesion). Transfer of stress between the matrix... 

Matrix-rubber particle adhesion is an important parameter for rubber toughening. For effective rubber toughening, rubber particles must be well bonded to the thermoset matrix. The poor intrinsic adhesion across the particle-matrix interface causes premature debonding of particles, leading to catastrophic failure of the materials. Nearly all the studies [9, 193, 2-10] have been concerned with reactive rubbers as toughening agents, and showed that dispersed particles have interfacial chemical bonds as a consequence of chemical reactivity. 

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