When I do a tensile test on 17-4 PH stainless steel, the stress-strain curve is very strange, two sections of Young’s modulus appeared. What is going on? The sample is made by SLM.
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According to our materials experts, you should always be careful interpreting the start of the force-displacement diagram. It is possible that the test needs to "set" itself, important issues are:
- the clamping of the test articles in the test bench vices
- alignment of the test article in the test bench
- size of the test articles (the load suggests small test articles/small diameter)
- how is displacement measured (displacement/elongation sensors "slip" easily depending on the expertise of the operator)
- artifacts in the test article?
Instead of proceeding the test till rupture (prior to the in-elastic/plastic region), it would have been beneficial if the test had been stopped at about 8 kN and relaxed to 0 kN to commence a new test.
From my own experience with creep test articles we see similar issues and apply a displacement offset so that the "second section" (using your words) is extrapolated to zero load. Usually this is just a few hundredths or tenths of a mm. In this case you could shift the whole diagram 0.3 mm to the right by applying a similar offset of about -0.3 mm.
