Crack pinning

Fig. 10. Crack pinning by a SiC fiber in a glass matrix, photographed using an optical microscope and Nomarski contrast. Fiber ties perpendicular to plane of micrograph lines represent crack position at fixed intervals of time, crack mnning left to right.

The crack-pinning mechanism is not very efficient with low-modulus particles such as rubbers. But with stiff thermoplastics (Sec. 13.4), or with high-modulus particles such as inorganic fillers, this mechanism may have an important contribution. 

Figure 13.4 Scheme of the crack-pinning mechanism dp, interparticle distance. 

The effect of TP particle size is controversial. While crack pinning is favored by small particles, large particles can promote particle bridging. 

Schematic and TEM image of crack pinning by a nanoparticle and transgranular fracture induced by intragranular particle. 

Silica particles do not induce any modification of the stress state in the material, and so no extended plasticity in the matrix. Naiiosized silica particles can be considered as a modifier of polymer chain displacement, and not as a reinforcement filler. There is adsorption of PP on silica surface and consequent reduction of molecular mobility with a large increase of elastic modulus. We do not observe any process zone, and mechanisms as particle/matrix decohesion as well as crack pinning or blunting are not effective. 

Figure 2. Micrograph of Adhesive B showing good adhesion between the pulverized MEAc toughener and the matrix resin, with crack-pinning features discernible. ME Ac toughener was added at a 15-phr level and has a particle-size range of 5 to 45 pm (average 18 pm).

Figure 4. SEM micrographs of epoxy/Al20 composite fracture surface (a) D230 AA18 50% and (b) crack pinning tails in DGEBA/D230 with 10% AA10 AI2O3.

The design of the fuse pin has been used since 1982 and in a 7-year period there have been 15 incidents of cracked pins. It was discovered that the pin failures resulted from the lack of primer, cadmium plating, and a corrosion inhibition compound. Since... 

Crack bowing Bowing in the crack plane between second-phase crack-pinning points... 

Particulate thermoplastics toughen by crack bridging, crack pinning and by stress concentration which produces shear banding. Amine curing agents are most often used to cure thermoplastic modified epoxies. 

Crack pinning is possible only when the particle size is larger than the crack opening displacement, which is 1.7 pm for neat epoxy matrix. Hence, it is not possible in a nanocomposite system, where the particle size is much smaller than the limit [100]. 

Fig. 11.13 Schematic representation of the crack pinning mechanism (Drawn after Kinloch and Young (1983))...

Figure 17.6 The formation of microcracks and crack pinning in metal oxide nanoparticle reinforced...

Some of the models used to rationalize the toughening effect associated with plastic deformation of the matrix and second-phase particle at the crack tip are represented in Fig. 1. This deformation is achieved through crack pinning and broadening, particle bridging and cavitation, crack path deflection, shear band formation and crazing or microcracking for stress relaxation at a crack tip. 

---