Crack propagation through adhesive interface in glass driven by dynamic loading
Dynamic crack behaviors at glass interfaces were investigated to understand dynamic failure mechanisms of glass. To produce highly intensive and rapidly increasing loading, glass specimens jointed with epoxy adhesive in edge-to-edge configurations were impacted on their notched edges with plastic projectiles. Cracks developed from the notch and propagated into the interfaces between glass plates at the maximum speed. The patterns of crack propagation through the interfaces were observed to depend on the interface’s conditions. Crack propagation stops at the interface where no adhesive was applied. The crack penetrates through the interface where two glass plates were bonded directly without any space. If the interface has finite thickness of an adhesive layer, a crack passing through the interface branches into multiple cracks immediately when it extends to the second glass plate. Both of the slow crack speed in the epoxy adhesive and resistance for crack initiation in the second glass account for the delay in crack propagation at the interface. The surface conditions of glass at the interface affect the resistance for crack initiation. Mirror-like polished surfaces have better resistance than rough surfaces trimmed by a water jet. If the polished surface is etched with hydrofluoric acid to remove surface flaws, the glass surfaces have higher strength and resistance for damage. This etched glass even ceases crack propagation completely with a sufficiently thick adhesive layer. Crack branching has been an open topic. Exact explanation has not been given yet. As the consistent shape of crack branching are created with the proposed method, diagnostics experiments were conducted to reveal the nature of crack branching. To investigate interaction between stress propagation and crack branching, stress histories synchronizing with high speed images were measured. Two types of specimen were used to vary stress distribution during crack propagation. The apex angle of spreading branched cracks increases in specimens having smaller width in dimension. The reflected waves from boundaries reaches cracks earlier because of the short traveling distance in the direction transverse to the cracks. These reflected waves interact with the crack and cause change of the branching shape. The fluctuation of stress intensity factors were observed with methods of caustics. A dark circular shadow at the ends of crack tips represents the stress intensity. The primary crack propagating early and carrying main load from the projectile can be switched if it stops at interface. Then, other cracks begin to receive the intensive load and are eventually extended to the second glass through the adhesive layer while other cracks still stay at the interface. The crack initiation, propagation and its interaction with interfaces were simulated with peridynamics. Peridynamics is a mathematical reformulation of continuum model by integrating pairwise penitential functions between two particles. These bond-based mechanics can represent discontinuity in peridynamics while traditional continuum mechanics cannot handle the discontinuity. The results from peridynamics show good agreements with experimental results in terms of the crack speeds and the branching shapes. Although the dimension of adhesive layers was not modeled exactly because of limitation of grid spacing, the resistance from interface to prevent crack propagation was shown in analogue with experimental results. The size of horizon where particle deformation and failure are computed, affect the interaction of cracks with interfaces.
Chen, Purdue University.
Aerospace engineering|Mechanical engineering|Materials science
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