What is shear strength of T6061?

Depends on what alloy you mean. If you are talking about pure Al (AISI1xxx), it is generally accepted to yield about 7-11Mpa.

Another thing to consider is that Al does not have as pronounced yield as say Fe systems. The 0.2% offset stress is used in most calculations regarding Al yield stress and is calculated as follows:

L = length of sample

->0.2%*L=0.2% offset strain.

Plot this point on stress/strain graph and draw line from this point on the strain axis parallel to the elastic slope of the material until it reaches the data points. This intersection is the 0.2% offset stress point.