What is the more precise procedure in determining the yield point, is it from the bending of the curve of stress strain or from the 0.2% or 0.1% procedure?
Just for additional information. To get 0.2% offset there should be a clearly discernible linear elastic part in the stress-strain curve so that a parallel to this line that follows Hooke's law, can be drawn at 0.2% strain to intersect the stress-strain curve and the corresponding stress read off from the chart. However in materials such as gray cast iron, the stress-strain curve is parabolic throughout and it is conventional in such cases to mark off 0.5% strain, draw a vertical line from this point to intersect the stress-strain curve to get the so-called yield stress.
Dear Sadeem,
The answer to your question is depicted in the following paragraphs:
Yield point
The point in the stress-strain curve at which the curve levels off and plastic deformation begins to occur.
Offset yield point (proof stress)
When a yield point is not easily defined based on the shape of the stress-strain curve an offset yield point is arbitrarily defined. The value for this is commonly set at 0.1 or 0.2% plastic strain.The offset value is given as a subscript, e.g., Rp0.2=310 MPa.High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials.
Therefore the precise method to determine the yield point of aluminium is from the 0.1 or 0.2% plastic strain.
References:
Ross 1999, pp 56-59.
https://www.mtu.edu/materials/k12/experiments/tensile/
https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm
Hoping this will be helpful,
Rafik
Dear Sadeem,
The answer to your question is depicted in the following paragraphs:
Yield point
The point in the stress-strain curve at which the curve levels off and plastic deformation begins to occur.
Offset yield point (proof stress)
When a yield point is not easily defined based on the shape of the stress-strain curve an offset yield point is arbitrarily defined. The value for this is commonly set at 0.1 or 0.2% plastic strain.The offset value is given as a subscript, e.g., Rp0.2=310 MPa.High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials.
Therefore the precise method to determine the yield point of aluminium is from the 0.1 or 0.2% plastic strain.
References:
Ross 1999, pp 56-59.
https://www.mtu.edu/materials/k12/experiments/tensile/
https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm
Hoping this will be helpful,
Rafik
for Aluminum and other Ductile materials you must use from the 0.2% method. mechanics [Ferdinand P. Beer, E. Russell Johnston Jr., John T. DeWolf,] book is also suitable for further consideration.
for Aluminum and other ductile materials you must use from the 0.2%strain method. mechanics of materials [Ferdinand P. Beer, E. Russell Johnston Jr., John T. DeWolf,] book is also suitable for further consideration.
It is better to use 0.2% strain method for ductile material like AL.
If you are simply interested in characterizing the material and compare different types of materials, the 0.2% offset method is probably the recommended one, as it is the one used in industry and is reproducible. Preferably you would then also use standardized tensile sample geometries to avoid geometrical variation.
However, if you are going to use this yield stress in e.g. a material model in an FEM or plasticity code, you might want to do backwards extrapolation of the yield stress based on the plastic deformation properties of the material. This would reduce the error of the simulations or the calculations afterwards.
It is important to remember that the 'yield point' as we tend to used it, does not exist: it is just a convenient way to quickly (and roughly!) characterize and describe the material properties. Depending upon what you want to know about your material (or its application), a different definition of the yield point might be more appropriate.
for aluminum and its alloys there is no yield we use proof stress by offset o.2% or o.o2 %1 in the strain axis to intersect with the sress strain curve and so can get the proof stress
Just for additional information. To get 0.2% offset there should be a clearly discernible linear elastic part in the stress-strain curve so that a parallel to this line that follows Hooke's law, can be drawn at 0.2% strain to intersect the stress-strain curve and the corresponding stress read off from the chart. However in materials such as gray cast iron, the stress-strain curve is parabolic throughout and it is conventional in such cases to mark off 0.5% strain, draw a vertical line from this point to intersect the stress-strain curve to get the so-called yield stress.
Dear all
How about composite materials (lets say particulate type) where aluminum is a matrix in it, still the same rule applied?
http://www.wiley-vch.de/books/sample/3527313605_c01.pdf
If you refer to Figure 1.14 (page 15) in this document, you will notice that the schematic stress strain curves for the matrix, fiber and the composite are significantly different, both when the matrix is brittle (top figure) and ductile (bottom figure). 0.2% offset method is suitable for ductile materials and may not be acceptable for MMC when the fiber has little or no ductility. An approximate method of estimating the yield strength of the composite is based on the weighted average of the values of the fiber and the matrix. Hope this helps. To my knowledge MMC in practice is designed for elastic behavior and therefore determination of Young's modulus assumes greater importance.
Dear all
Thank you for your worth answers,
How about the porous materials, specially aluminum, Does the above procedure still applicable?
- Badges
- Science topic
- Similar topics
- Cell Biology
- Methods