If your edge margin is below 1.5 then the joint is critical for tear-out of the material or fatigue cracking.
Generally 2D for repair and 2D+ for new design (2 times the hole diameter). McDonnell Douglas nominally uses 2D + 1/16 inch for metal structures and 3D + 1/16 inch for graphite structures. Consult the maintenance manual as edge distance will vary with manufacturer and with the particular aircraft. Edge distance can be reduced depending on the load - it's all about the load.
History of Edge Distance:
In the 1950's Edge Distance was typically 1.5D or 1.5 times the hole diameter when measured from the center of the hole to the edge of the material. After the Comet disasters, it was changed in Great Britain to a minimum of 2D. Many manufacturers adopted this standard. Some old Boeing military aircraft used an edge distance of 1.7 +0.030 - 0.06 inch.
To add confusion, some manufacturers used the term "Edge Margin". Typically, Douglas often used the term "Edge Margin" while others used the term "Edge Distance". After Boeing purchased Douglas, the integration caused some confusion. To further the confusion, some sources used Edge Distance to mean the distance from the edge of the hole instead of the usual center of the hole distance.
Currently, most references use the term Edge Distance and measure from the center of the hole. Many engineering standards call for 2D +0.05 for metals. The 0.05 allows for maintaning the 2D distance after repair using oversize fasteners, and allowance for manufacturing and repair tolerances.
FAA AC65-15A page 165 states an Edge Distance of at least 2D minimum, with a recommended of 2 1/2 times rivet diameter.
Interesting strength data is contained MIL-HDBK-5 that shows significant strength reductions below 2D. Below 2D is sometimes used depending upon stress engineering data.
For composite structures 2.5D + 0.05 inch is used by several manufacturers. Composits are more sensitive to edge distances and hole spacings than metal joints. The brittle nature of composits and the absence of local yielding creates higher peak stresses.
Edge distance applies not only to sheet edges but to bend.
Nutplate minimum edge distance in primary structures is typically 4D.
Rivet tearing (bearing critical failure).