Research On The Mechanical Charcteristics Of Crack Tips In Crystalline Aluminum

The failure of materials is often caused by surface and internal cracks.The research on crack propagation has always been an important research hotspot.It is very important for the safe use of structures.In different materials,the mode of crack propagation is also different.The microstructure and material properties of materials lead to the complexity of crack propagation mode.Due to the complexity of crack propagation,it is necessary to clarify the propagation mechanism of cracks under different working conditions,to guide the preparation of new materials and prevent material failure.Based on this,this paper has carried out the following work:(1)For single crystal aluminum,the mechanical behavior of crack tip in a semi-infinite plane under uniaxial tensile stress is analyzed.The effects of crack length,crack depth and the angle between crack and free surface on the stress intensity factor at the crack tip are analyzed.Under the action of a single tensile load,the longer the crack length,the greater the dimensionless stress intensity factor KI*.When the crack is located in the horizontal position,the stress concentration far from the free surface is more obvious;At this time,it is easy to propagate towards the interior of the material,and this phenomenon will be aggravated with the propagation of the crack;With the increase of the angle,the length of the crack and the stress concentration at both ends of the crack will weaken,and the angle between the crack and the free surface will have a great influence on the crack propagation mode.When the crack is at about 45° to the free surface,the crack mainly propagates in the mixed mode of mode Ⅰ and mode Ⅱ,and the change of crack length has a great influence on the end of the crack away from the free surface.(2)For nano-polycrystalline aluminum,under the action of uniaxial tensile stress and without considering the dislocation free zone at the crack tip,when multiple slip surfaces are emitted from the crack tip and the crack length is small,the microcrack will first appear at the grain boundary.At this time,the main crack will merge with the microcrack to form a new deflection crack,and when the main crack is long,the main crack will expand directly through the crystal.With the increase of grain size,the number of dislocations that the grain can accommodate is more,and more dislocations play a greater role in inhibiting the crack propagation.In coarse-grained materials,crack propagation is more difficult than in fine-grained materials.With the increase of crack length,the number of dislocations emitted at the crack tip decreases gradually,and the crack propagation rate in nano-crystalline aluminum will increase.With the increase of crack length,the grain size has little effect on crack propagation.In the process of crack direct transgranular propagation,the number of emitted dislocations is small.When the dislocation free zone at the crack tip is not considered and the grain boundary is located in the direction conducive to crack generation,cracks are more likely to appear at the grain boundary.Different crystal directions will have an important impact on crack propagation.If the crack initiation along the grain boundary is considered,the deflection crack formed after the confluence of the main crack and the microcrack at the grain boundary will be longer.When considering the dislocation free zone at the crack tip,the energy at the dislocation free zone will meet the cracking requirements before the grain boundary and the crack tip.The micro-pores nucleate and develop in the dislocation free zone in front of the crack tip.The cracks are emitted and linked with each other through dislocations,which will effectively accelerate the expansion of the crack tip.(3)For nano-polycrystalline aluminum,when grain boundary slip occurs in nano-polycrystalline aluminum,dislocations are easy to accumulate at the intersection of grain boundaries.When the grain boundary is close to the crack tip,the dislocation density at the intersection of grain boundaries is high,because the shear stress on the grain boundary is relatively large,and microcracks are easy to form at the intersection of grain boundaries.Considering different grain boundary sizes,the shorter the grain boundary size,the more obvious the inhibition effect on crack propagation,the more serious the dislocation accumulation at the grain boundary,and the smaller the crystal size,the easier it is to produce cracks at the grain boundary.For the main crack under tensile load,the nano-crack is more likely to nucleate on the upper and lower sides of the crack plane than in the front of the crack tip.The generation of lattice dislocation in nano-crystalline metals is inhibited.In this case,the contribution of grain boundary dislocation movement to plastic deformation is greater.