## General Mechanical

#### Explicit Dynamics to find Elongation at break

• Kurrnin
Subscriber

Using ISO 527, I need to calculate the elongation at break of PLA that has been injection moulded at three separate mold temperatures.

One end is kept fixed, while the other is pulled at 50mm/min. When I try to simulate that every model just deforms to 50 mm without breaking, and when I try to make the simulation run for longer than 104 seconds the simulation fails.

Could anyone give me any tips for how to do this? For the PLA materials, I know Young's Modulus, Poissons Ration, Tensile Yield and Ultimate strength and density.

• peteroznewman
Subscriber

Do you mean that you know the Yield Strength and Ultimate Strength for each of the three mold temperatures?

• Kurrnin
Subscriber
Yes sorry that’s what I mean
• peteroznewman
Subscriber

So how much does the yield strength change with mold temperature, please show the numbers.

Also, can you get the full Stress-Strain curve for the three mold temperature?

• Kurrnin
Subscriber
At 30 deg C it is 23.4 MPa
At 60 deg C it is 25.4 MPa
At 90 deg C it is 25.8 MPa

I don’t have a Stress-Strain curve as I am using data from a report and it doesn’t have that included.
• peteroznewman
Subscriber

The report should have included the Elongation at Break.

The simplest material model of plasticity is Bilinear Hardening either Isotropic or Kinematic.

You have the first point on the curve, the yield strength, and you know the strain for that because you can divide that value by Young's Modulus.

The problem is you only have the ultimate tensile strength, which gives you the value on the stress axis, but you don't have the strain at which the sample failed so you can't calculate the Tangent Modulus.

I don't think you can simulate your way to missing material data.

• Kurrnin
Subscriber

Using an online program, and the values I have I was able to calculate the stress-strain graphs. Using them I found the Strain for the ultimate tensile strength as 0.11873 at 30 deg C. At 60 deg C it was 0.118438 and at 90 deg C it was 0.11888.

Will that help?

• peteroznewman
Subscriber

Only if you trust the online program. What equations did it use to calculate those values?  They should have come from the experimental material testing.

If you wish to proceed, then calculate the strain at yield, then calculate the tangent modulus from the two points (yield and ultimate) on the stress-strain curve.