The dislocations and grain bounday.

When dislocation meet the grain boundary. The grain boundary present obstacles to dislocation motion. A usual point is that the dislocation will pile up against the grain boundary(But from the expriments , it seldomly see this phenomenon.). Macroscopic yielding occours when the adjacent grain can deform plastically Which maybe effected by the emission of dislocation from the grain boundary, or in another way , the pileup dislocation can produce a stress concentration which can active the dislocation source in the adjacent grain. As we knowe ,in polycrystalline, the material will express a Hall-Petch relationship between the yielding stress and the grain dimension. There are many models to explain the H-P relationship,such as pile-up dislocations, average fress distance....

Another argument such as dislocation can transmite the grain boundary. So how can we consider the slip resistance associated with slip penetration events across grain boundaries?
Also some scholars has claim that the grain boundary can absorb dilocations and aslo emite dislocations.

So the which mechanism is true in grain boundary, whether dislocation can pile up across boundary, when will a dislocation can penetrate to the adjacent grain, when can grain boundary absorb a dislocation , which and when cause the grain bounday to emite a dislocation.

I appreciate your points and discussion. Will you please express your points or attach the paper about this topic?