Microstructural Origins of Dynamic Fracture in Ductile Metals

Becker, R ; Belak, J ; Campbell, G

Lawrence Livermore National Laboratory

关键词: Compression;    Fractures;    Space Shuttles;    Nucleation;    36 Materials Science;   

DOI  :  10.2172/15004869
RP-ID  :  UCRL-ID-150286
RP-ID  :  W-7405-ENG-48
RP-ID  :  15004869

美国|英语

来源: UNT Digital Library

【 摘 要 】

From the formation of microscopic cracks in the fuel pipe liner of the space shuttle to the safety of roadway bridges, the fracture of materials has enormous implications throughout our society. The ability to assess and design safe engineering structures requires a detailed knowledge of this failure process. The fracture process depends on both the loading history and the detailed microscopic structure (microstructure) of the material. Weak points, such as inclusions and grain boundary junctions, are the locations from which microscopic fractures (voids and cracks) originate. Once nucleated, these fractures quickly link together to form a macroscopic crack. Despite this qualitative understanding, little is known about voids nucleation, plastic deformation in the surrounding material, and the mechanisms of linking. Central to Stockpile Stewardship is an understanding of shock loading of materials. During the passage of a shock wave, the material is compressed at a very high rate. This compression produces a high density of dislocation defects and other changes to the microstructure that are poorly understood. When the shock wave reflects from a free surface, the compression is rapidly released and extreme tension is produced inside the material. If this tension exceeds the internal rupture strength, microscopic fractures form and link up to create a spallation scab--a thin scab that separates from the bulk of the material. In this project, we use the LLNL gas gun facility to produce a planar stress pulse with controlled duration and amplitude. The sample is carefully captured in soft foam while measuring the free surface velocity profile. The amount of change in the surface velocity during release is related to the spallation strength. We study light metals (Al, V, Ti, Cu) with known initial microstructure: single crystal, polycrystalline, and single crystal with engineered inclusions. Light metals enable direct measurement of the three dimensional distribution of damage using X-Ray tomography. After the tomography experiment is complete, the samples are sliced and analyzed using 2D metallography.