What kind of fibre are you testing, and how are you measuring load and strain? Do you have a mechanically homogeneous fibre? If not, how do you determine the area of cross-section?

If this is a polymer optical fibre, I would guess that you have elastic deformation up to a stress of 42 MPa at a strain just less than 1%.

Between 42 and 140 MPa, you appear to have plastic deformation of the fibre. Alternatively some element of the composite structure has failed, and the load is now carried by elements which either have lower Young's modulus, or have been loaded beyond their yield stress.

Between 18% and 72% strain there could be some kind of plastic flow. Alternatively, if you are testing a jacketed fibre, there could be slippage and stiction between inner and outer layers of the composite structure.

How are you gripping the fibre? A common problem with this type of testing is slippage of the cable at the grips. If you are measuring crosshead movement, rather than strain in the fibre, this can give misleading results.

If you are testing a silica-based glass fibre, the modulus of the glass core and claddng should be approximately 73 GPa. However, if the fibre is protected by a coating or polymer jacket, some of the load will be carried by the outer layers, and there are multiple Young's moduli to consider.

Thanks for your detail answer. I am using PMMA fiber by rubber clip to grip fiber.

Reported values for the Young's modulus of PMMA are typically in the range 2.4 GPa to 5 GPa, with tensile strength 50-76 MPa.

You should probably measure the slope over the initial rise in stress for strains below 1%. This is too compressed in your graph to judge whether the response is linear here.

https://web.archive.org/web/20190422003653/http://www.kayelaby.npl.co.uk/general_physics/2_2/2_2_2.html

https://designerdata.nl/materials/plastics/thermo-plastics/poly%28methyl-methacrylate%29

https://www.makeitfrom.com/material-properties/Polymethylmethacrylate-PMMA-Acrylic

rArticle Evaluation of polymethyl methacrylate resin mechanical prope...

Slope of the initial linear portion should be used to be absolutely sure that you are truly in the elastic region. Perhaps you need more data points at low stress/strain regions

Thanks Alan Robinson and Srini Vasan