Researchers and materials practitioners often come across the stress-strain diagram, which is a popular tool used to select materials based on their mechanical properties.
While the diagram is a useful resource for many applications, it is an issue that it is 𝐧𝐨𝐭 𝐜𝐨𝐫𝐫𝐞𝐜𝐭 to use the stress – strain diagram for fracture analysis - a fatal beginner's mistake. The reason behind can be understood in our today’s illustration below!
Let me know the comments below.
It is mainly due to the fact that normal steel has a steep drop in toughness.
The stress-strain plot shown in the figure is typically obtained by testing a standard tensile specimen made of steel under quasi-static conditions.
And the primary objectives of the tensile test is to find :
1. The maximum stress at which the tested material behaves elastically (Yield strength)
2. The maximum load the material can withstand without developing any flaws or necking (ultimate strength).
And for the design applications which are concerned only about the yield strength and ultimate strength, the stress-strain diagram can be used safely with some reasonable factor of safety. But beyond that, the data can not be used with high confidence.
The material's behavior starting from the formation of necking (which finally leads to the fracture of specimen) can not be accurately characterized by stress-strain plot obtained by the tensile test because of following reasons:
1. The material no longer remains homogeneous after necking.
2. The cross section area is not defined properly after necking due to presence of voids
3. The definition of stress, i.e., load/area, can not be used after necking since the actual cross-section area can not be calculated accurately.
4. The voids and other flaws raises the actual local stresses tremendously which are not captured by the stress-strain plot.
5. Also the state of stress becomes three-dimensional after necking.
Therefore, for fracture analysis the data obtained from tensile test can not be used for reliable design.
For the application to the fracture mechanics, a cracked specimen like compact tension specimen should be used to obtain the fracture toughness (KIC ) of the material. And the concepts of fracture mechanics like stress-intensity factor or J-integral should be incorporated in design. The fracture is governed by the cracks or crack like flaws which can be analyzed using concepts of fracture mechanics. And special experimental testing procedures are required to study the fracture phenomenon like ASTM E647.
Shiv Sahaya Shukla, thanks for your reply! Fully agree - apart from the fractur part. For this, we would highly recommend CMOD (crack mouth opening displacement) and GF (specific fracture energy) in order to generate authentic results.
K is an old measure with questionable meanings since no energy is considered. For this, you are invited to dive into our other topics here on Researchgate.
Hi @Martin Brandtner-Hafner. By "GF (specific fracture energy)", do you mean energy release rate?
Hi Mir Aamir Abbas : Thanks for your comment!
As a fact, GF (specific fracture energy) and GIc (strain energy release rate) are something totally different.
The first needs no crack length measurement, whereas the latter does.
The first is non-linear elastic whereas the latter is, and so on.
I only can tell you that GF is a game-changer in fracture analysis.
Here is the link: Article The Theoretical Basis of a Method to Determine the Fracture ...
There is some information available about fracture in stress strain diagram. In the case of ductile materials, UTS denotes the state at which void nucleation at inclusion sites takes place. Void coalescence and growth happens from UTS to Break point.
In the case of brittle materials, UTS denotes the state at which the microcracks in the material suddenly grow fast and join each other to cause the fracture of the component.
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