yield strength compare to max stress

Question

how does the max stress experienced compare to the yield strength of the material and what does this mean in practical terms ? is the part also fit for purpose ?

the part is a carabiner

max stress is 8.735e+0.9

yield strength is 6.204e+0.8

I don't really understand what this means is the part fit for purpose I would say yes because the part did not break during the simulation but it did change shape

Answer 1

Yield strength

When subjected to stress, a material undergoes recoverable deformation. The yield strength of a material represents the stress beyond which its deformation is plastic. Any deformation that occurs as a result of stress higher than the yield strength is permanent. Because of the linearity of elastic deformation, yield strength is also defined as the greatest stress achievable without any deviation from the proportionality of stress and strain. Beyond this point, large deformations can be observed with little or no increase in the applied load. Yield strength is measured in N/m² or pascals.

The yield strength of a material is determined using a tensile test. The results of the test are plotted on a stress-strain curve. The stress at the point where the stress-strain curve deviates from proportionality is the yield strength of the material. Some plastics’ deformation is linearly elastic and once the maximum strength is attained, the material fractures. It is difficult to define an exact yield point for certain materials from the stress-strain curve. This is because these materials do not display an abrupt curve; rather the onset of yield occurs over a range. It is therefore practical to use proof stress as a representation of the yield strength.

Proof stress

Proof stress is measured by drawing a line at 0.2% of the plastic strain, parallel to the straight-line elastic region of the stress-strain curve. The stress at the point where this line intercepts the curve is the proof stress. The yield strength of a material can be increased by certain material processes.

Tensile strength

Often referred to as ultimate tensile strength (UTS), tensile strength is the maximum tensile load a material can withstand before fracture. It is a measure of a material's resistance to failure under tensile loading.

The tensile strength of a material is determined using a tensile test. It is the highest point on the stress-strain curve, which is plotted after the test. The tensile strength can also be determined using this formula:

σ_f = P_f/A_o

Where P_f is the load at fracture, A_o is the original cross-sectional area, and σ_f is the tensile strength, measured in N/m² or pascals. It is important to note that the tensile strength of a material is a specific value under controlled standard test conditions. However, in practical applications, tensile strength varies with temperature. At 100°C, the tensile strength of copper falls from 220Mpa at room temperature, to 209Mpa. These variations are compensated for by using a factor of safety, which is usually a fraction of the original tensile strength in design considerations.

Comparative analysis of yield strength and tensile strength

The following are some of the major differences between yield strength and tensile strength:

1. Yield strength is measured at the point of plastic deformation.
2. Tensile strength is measured at the point of fracture. Tensile strength is rarely used in the design consideration of structures made from ductile materials. This is because these materials undergo substantial deformation before their tensile strength is reached. Rather, yield strength is considered for ductile materials, while tensile strength is used for brittle materials.
3. During design considerations, tensile strength is analyzed only in uni-axial loading. Multi-axial stress states are estimated in yield strength analysis.
4. Deformation of materials occurs after yield strength has been surpassed, while tensile strength is reached after deformation has taken place. In brittle materials, tensile strength is reached with minimal or no yield.
5. Tensile strength is usually of a higher numerical value than the yield strength of a particular material.
6. The tensile strength of a material can be ascertained with 100% accuracy. However, yield strength has to be estimated for most materials.

Answer 2

If the max stress is greater than the yield strength then that means your part has yielded (or entered plastic deformation). There are some instances where a part yielding can fit a design purpose but in this case for a carabiner, I would say the part does not fit its purpose. Yielding would probably lead to a loss of functionality because a carabiner is used to quickly and reversibly connect components. If the parts no longer fit together (because part of it is longer than designed to be) and can't be released or put back together then it is no longer useful. Even though it didn't break in simulation its functionality may be lost.