The ASTM dicates a plain rectangular shape. I understand that the material necks pretty quickly but I frequently get breaks in the grips, particularly with films of thickness less than 0.002."
Dog bone samples seem as though they would help, so why not use those?
ASTM D882 is for plastic films. For thins films of many other materials, dog-bone specimens are more commonly used.
The deformation of dog-bone specimens is not homogeneous through the length of the full specimen. That means that the cross-head displacement of the testing machine is not necessarily equal to the displacement of the gage region. As a result, you will need some sort of extensometer to measure the change in length of the specimen which makes the test a bit more complicated.
Of course, it can be argued that the deformation is not homogeneous in a rectangular specimen either - which is why you run into breaks at the grips.
