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The J-R curve is determined in arc-shaped samples directly obtained from medium density polyethylene gas pipes by the modified normalization method. The elastic and plastic factors (η el and η pl ) used in the J-integral expression are geometry sensitive and their values for side-grooved arc-shaped samples in a three-point bending configuration are experimentally determined. The values of the elastic compliance and crack depth-to-width ratios are found to be related by the expression recommended in the ASTM E813-87 standard for the standard bend specimens. The η pl factor is determined from the exponent of the power law that best fits the experimental data of separation parameter versus ligament area and is also close to the theoretical value for three-point bending geometry. The J-R curve is also predicted by applying the procedure proposed in the EPRI/GE handbook and compared with the experimental multiple-specimen results.