In tensile testing for a polycrystalline ductile material, why does the shear take place at 45 degree angle in 'cup and cone' fracture? will the shear angle not be different for different grains? Also, is the failure intergranular or transgranular? Does the failure take place finally due to breakage of atomic bonds or completely due to slip? Will the failure be transgranular or intergranular in brittle polycrystalline material? Will the failure be because of breaking of atomic bonds in this case?
Why does the shear take place at a 45-degree angle? According to Schmid's law (check out the video) shear stress is maximized on planes oriented at 45 degrees to the axis of the applied tensile stress. This is because, under uniaxial tension, the maximum shear stress occurs on planes that are inclined at 45 degrees to the direction of the applied load. In a ductile material, plastic deformation is driven by shear stress, leading to significant deformation along these planes. In ductile materials, deformation typically occurs through the process of slip, where dislocations move along slip planes. These slip planes are oriented in such a way as to accommodate the shear stress. The alignment of these planes at approximately 45 degrees to the tensile axis facilitates this slip process, leading to the characteristic 'cup and cone' fracture. Will the shear angle not be different for different grains? Indeed, individual grains in a polycrystalline material may have different orientations, leading to variability in the local shear planes. However, the macroscopic fracture surface results from the collective behavior of numerous grains. While the shear angle can vary at the microscopic level within individual grains, the overall shear plane tends to align with the 45-degree angle where the average shear stress is maximized. Is the failure intergranular or transgranular in ductile materials? Transgranular Fracture: In ductile materials, the fracture is predominantly transgranular, meaning it propagates through the grains rather than along the grain boundaries. This occurs because plastic deformation, which precedes fracture, tends to be more uniform within grains, allowing cracks to traverse through them. Does the failure take place finally due to breakage of atomic bonds or completely due to slip? Combination of Mechanisms: The failure in ductile materials involves a combination of mechanisms. Initially, plastic deformation occurs predominantly through slip, where dislocations move and atomic bonds are temporarily displaced but not permanently broken. As deformation progresses and the material reaches its ultimate tensile strength, voids and micro-cracks form and grow. Final fracture occurs when these voids coalesce and the atomic bonds are irreversibly broken. Thus, the final failure involves both slip and the breakage of atomic bonds. Failure in Brittle Polycrystalline Materials In brittle polycrystalline materials, the fracture can be either transgranular or intergranular. The mode of fracture depends on the material's properties and the presence of impurities or weaknesses at the grain boundaries. In many cases, brittle materials tend to fracture along grain boundaries (intergranular fracture) due to the higher brittleness of these regions. However, in the absence of significant grain boundary weaknesses, the fracture may be transgranular. In brittle materials, failure typically occurs due to the direct breaking of atomic bonds without significant plastic deformation. The stress concentrates at flaws or cracks, leading to rapid propagation of the crack and cleavage along crystallographic planes. This cleavage involves breaking atomic bonds along specific planes, leading to sudden and catastrophic failure. Long story short: In ductile polycrystalline materials, shear takes place at a 45-degree angle due to maximum shear stress, and fracture is predominantly transgranular with a combination of slip and atomic bond breakage. In brittle polycrystalline materials, failure can be intergranular or transgranular, primarily due to the breaking of atomic bonds along specific planes without significant plastic deformation.
In general, ductile materials under tension form voids which then coalesce and lead to fracture. However, in highly ductile materials like gold, this behavior is modified due to the unique mechanisms of vacancy and nanocrack dynamics (see the paper www.jstage.jst.go.jp/article/matertrans/47/2/47_2_298/_pdf/-char/en) 1. Vacancy Dynamics: In highly ductile materials, vacancies play a significant role in deformation. Gold, for example, generates and transports vacancies extensively during deformation. 2. Nanocrack Fragmentation: Nanocracks in gold contribute to void nucleation and growth, but due to the high mobility and continuous migration of these nanocrack fragments to the specimen surface, the voids do not coalesce centrally but rather along the length of the specimen. 3. High Internal Stress: The high internal tensile stress in materials like gold during low-speed deformation prevents the stabilization of voids at the center, promoting necking to a point instead. These factors combined suggest that the unique vacancy and nanocrack behaviors in highly ductile materials like gold lead to a deformation mechanism where the material necks down to a point without forming central voids, differing from the traditional ductile fracture process which typically involves central void coalescence. In essence, the high mobility of vacancies and nanocrack fragments in gold, coupled with the material's ability to sustain high internal stresses, results in the necking phenomenon observed in highly ductile materials.
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In tensile testing for a polycrystalline ductile material, why does the shear take place at 45 degree angle in 'cup and cone' fracture? will the shear angle not be different for different grains? Also, is the failure intergranular or transgranular? Does the failure take place finally due to breakage of atomic bonds or completely due to slip?
Will the failure be transgranular or intergranular in brittle polycrystalline material? Will the failure be because of breaking of atomic bonds in this case?
Why does the shear take place at a 45-degree angle?
According to Schmid's law (check out the video) shear stress is maximized on planes oriented at 45 degrees to the axis of the applied tensile stress. This is because, under uniaxial tension, the maximum shear stress occurs on planes that are inclined at 45 degrees to the direction of the applied load. In a ductile material, plastic deformation is driven by shear stress, leading to significant deformation along these planes.
In ductile materials, deformation typically occurs through the process of slip, where dislocations move along slip planes. These slip planes are oriented in such a way as to accommodate the shear stress. The alignment of these planes at approximately 45 degrees to the tensile axis facilitates this slip process, leading to the characteristic 'cup and cone' fracture.
Will the shear angle not be different for different grains?
Indeed, individual grains in a polycrystalline material may have different orientations, leading to variability in the local shear planes. However, the macroscopic fracture surface results from the collective behavior of numerous grains. While the shear angle can vary at the microscopic level within individual grains, the overall shear plane tends to align with the 45-degree angle where the average shear stress is maximized.
Is the failure intergranular or transgranular in ductile materials?
Transgranular Fracture: In ductile materials, the fracture is predominantly transgranular, meaning it propagates through the grains rather than along the grain boundaries. This occurs because plastic deformation, which precedes fracture, tends to be more uniform within grains, allowing cracks to traverse through them.
Does the failure take place finally due to breakage of atomic bonds or completely due to slip?
Combination of Mechanisms: The failure in ductile materials involves a combination of mechanisms. Initially, plastic deformation occurs predominantly through slip, where dislocations move and atomic bonds are temporarily displaced but not permanently broken. As deformation progresses and the material reaches its ultimate tensile strength, voids and micro-cracks form and grow. Final fracture occurs when these voids coalesce and the atomic bonds are irreversibly broken. Thus, the final failure involves both slip and the breakage of atomic bonds.
Failure in Brittle Polycrystalline Materials
In brittle polycrystalline materials, the fracture can be either transgranular or intergranular. The mode of fracture depends on the material's properties and the presence of impurities or weaknesses at the grain boundaries. In many cases, brittle materials tend to fracture along grain boundaries (intergranular fracture) due to the higher brittleness of these regions. However, in the absence of significant grain boundary weaknesses, the fracture may be transgranular.
In brittle materials, failure typically occurs due to the direct breaking of atomic bonds without significant plastic deformation. The stress concentrates at flaws or cracks, leading to rapid propagation of the crack and cleavage along crystallographic planes. This cleavage involves breaking atomic bonds along specific planes, leading to sudden and catastrophic failure.
Long story short:
In ductile polycrystalline materials, shear takes place at a 45-degree angle due to maximum shear stress, and fracture is predominantly transgranular with a combination of slip and atomic bond breakage.
In brittle polycrystalline materials, failure can be intergranular or transgranular, primarily due to the breaking of atomic bonds along specific planes without significant plastic deformation.
@@tec-science Thanks a lot...this was very helpful
Why does a highly ductile material ((eg. Gold) neck completely to a point, and not undergo void formation at center like in ductile material failure?
In general, ductile materials under tension form voids which then coalesce and lead to fracture. However, in highly ductile materials like gold, this behavior is modified due to the unique mechanisms of vacancy and nanocrack dynamics (see the paper www.jstage.jst.go.jp/article/matertrans/47/2/47_2_298/_pdf/-char/en)
1. Vacancy Dynamics: In highly ductile materials, vacancies play a significant role in deformation. Gold, for example, generates and transports vacancies extensively during deformation.
2. Nanocrack Fragmentation: Nanocracks in gold contribute to void nucleation and growth, but due to the high mobility and continuous migration of these nanocrack fragments to the specimen surface, the voids do not coalesce centrally but rather along the length of the specimen.
3. High Internal Stress: The high internal tensile stress in materials like gold during low-speed deformation prevents the stabilization of voids at the center, promoting necking to a point instead.
These factors combined suggest that the unique vacancy and nanocrack behaviors in highly ductile materials like gold lead to a deformation mechanism where the material necks down to a point without forming central voids, differing from the traditional ductile fracture process which typically involves central void coalescence.
In essence, the high mobility of vacancies and nanocrack fragments in gold, coupled with the material's ability to sustain high internal stresses, results in the necking phenomenon observed in highly ductile materials.