Why doesn’t the load passes through max shear stress direction for ductile materials?

- December 28, 2020 at 12:15 pm
  
  Rameez_ul_Haq
  Subscriber
  
  For a ductile material, we all know it fails in the direction of maximum shear stress. Doesn't that mean the load has to pass through that direction, but we say that the load path is parallel to the vector principal stresses. Why is that?n
- December 28, 2020 at 3:01 pm
  
  peteroznewman
  Subscriber
  
  nIn the image below, the brittle material has fractured along the plane of maximum shear stress while the ductile material shows necking (narrowing) prior to tensile fracture on a plane normal to the tensile stress. n
- December 29, 2020 at 9:17 am
  
  Rameez_ul_Haq
  Subscriber
  
  ,ohhh. So you mean to say that the ductile materials are not always supposed to fracture in the direction of maximum shear stress? Thats quite of a surprising statement for me. Because until now, what I have heard is that the brittle materials always fracture in the direction of the maximum principal stress while ductile does so in the direction of max shear. nMaybe the picture you shared is also doing the same, but the loading condition applied makes it appear as if they are doing the opposite (JK)nSo how do we judge, that will the ductile material fail in the direction of maximum shear or maximum tensile stress [or in other words, maximum principal stress]?.
- January 3, 2021 at 6:01 pm
  
  Rameez_ul_Haq
  Subscriber
  
  Array, expecting a reply from you on this one pelase? n
- January 3, 2021 at 7:59 pm
  
  peteroznewman
  Subscriber
  
  nn
- January 3, 2021 at 8:48 pm
  
  Rameez_ul_Haq
  Subscriber
  
  ,actually I have already seen this video by 'The efficient Engineer' and I was referring to his words where he says the ductile materials always fail in max shear direction while the brittle does so in the direction of max principal stresses. If you recall my comment where I mentioned that the picture you upload maybe is showing so because of the loading condition applied, I was referring to this video where he shows the failure of the ductile and brittle material under torsion.nPlease see what this guy says at and after 8:45.nn
- January 3, 2021 at 11:32 pm
  
  peteroznewman
  Subscriber
  
  ArraynYour original question mentions load path. The load path in a tensile test specimen is from one end to the other in a straight line. Failure begins with necking, which is when material starts sliding at 45 degrees to the load path, which is the direction of maximum shear stress. However, the final fracture is not at 45 degrees, but more straight across.nWe can simulate necking in ANSYS using a Plasticity material model to simulate the behavior of ductile materials.nThe image I used in December labeled brittle fracture may have been from a torsion test, it was not specific.n
- January 4, 2021 at 6:01 am
  
  Rameez_ul_Haq
  Subscriber
  
  ,so you mean to say is that the load still passes in the direction of maximum principal stress but the plastic failure for a ductile material starts to occur at 45 degrees to this direction i.e. 45 degrees to the load path. But my original question is why is the load supposed to pass through the direction of maximum principal stresses? In ANSYS Help, it is written that the locus of the directions of vector principal stresses suggests the paths of maximum load transfer throughout a body. My question is why?n