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.
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.