I tested the strength of my samples of tyvek with Dr Mark Pridham in engineering. I used the Instron tensile testing machine that you can see in the top diagram. I put small samples of the tyvek into the machine. This gave me the maximum load, the tensile strength at maximum load, extension at machine break and tensile extension at maximum load. The samples of tyvek did not break in the machine but were stretched. I took samples of tyvek cut horizontally, vertically and diagonally to see if the direction made any difference to the strength. All of the results were similar, as you can see in the graph. I also tested a sample that was cut to see how easy it was to pull two pieces apart. This had less strength than the other samples. Each sample began to fail at around the same load and extension, therefore we can conclude that the structure of the tyvek is random. The strength really depends on the structure at that certain point, the thickness of the material varies in random patterns as well but the strength is similar throughout.
More information on tyvek can be found on the DuPont website: http://www2.dupont.com/Tyvek/en_US/tech_info/test_results.html
A tyvek handbook is also available which tell of the properties of tyvek and additional information: http://www2.dupont.com/Tyvek/en_US/assets/downloads/tyvek_handbook.pdf