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My understanding of modern wood glue is that, if the proper glue is used and wood surfaces are properly prepared, it will form a bond stronger than the wood itself.
Is this true? If not what are the exceptions to that statement? If so does that mean that complicated and/or elaborate mechanical joints — e.g., dovetail joints — are now more decorative than practical?
Indeed, when done properly, the glue is stronger than the wood itself when both of the glued faces are edge-grain or face-grain.
A few circumstances come to mind when glue alone is not stronger than the wood:
When glue is applied to end-grain, the grain acts as little straws and draws a lot of the glue up the wood, away from the joint (similar to how the grain draws water up the tree when the tree is alive). In these cases, the majority of the glue is pulled away from the joint and (without more elaborate joinery techniques) a strong joint cannot be achieved, no matter how rough you sand or how tight your clamps are.
Joining with an end-grain face is unavoidable for 90-degree joints, such as when making boxes or drawers. This is why you'll often see dovetail or finger joints used in these cases. These joints not only provide physical limitations to how the wood can move, but also allow the glue to be applied to a greater surface area and along edge-grain faces, creating a much stronger joint.
Joining end-grain faces is also unavoidable for 180-degree joints, and a good round-up of techniques to strengthen this joint can be found at the question, How can I join two boards at the ends?
There are a lot of factors that determine how a piece of wood reacts to changes in humidity or contact with water. These include the type of wood, its density and shape, the direction of the grain, and its age. Therefore, the two pieces of wood that make up the joint will often react differently to the changing environment, putting unique stresses and tension on the joint. Furthermore, the direction of the tension is often different than what is expected under typical use of the joint.
When significant changes in moisture are anticipated, glued joints can fail as the different pieces of wood react differently to the change in moisture.
I've always been told that when you glue two boards together that the join will be stronger than the surrounding wood. This is primarily for joining boards to make a wider (or thicker) board. Long edge to long edge, and if you properly join them it seems to be true. When bending a board to break I've never seen it break along the seam. I have seen the glue fail over time and the board will split along the seam. Though this is mostly from years of seasonal movement and likely the two boards didn't move the same together.
I also tend to use my biscuit joiner to help reinforce these joints as well.
Now comparing that to dovetail joints is a very different subject. Dovetail joints help strengthen an angle. Glue doesn't do nearly as well with angles or pieces that will get a lot of stress from different angles. A corner is end-grain being attached to face grain, which already is a weaker glue joint. On top of that it will suffer more opposing stresses.
A dovetail adds physical strength from the wood itself, also give more surface area to glue. Other forms of joinery do the same thing, they are needed to hand different cross stresses and provide more strength.
In the tests performed by Matthias Wandel. He found that most glues performed equally as well. And when joints failed it was with wood coming out.
However the strongest part of wood is along the grain and grain is rarely straight. So the joint area would still be weaker than if it was a solid piece of wood.
Glue is not a joint. For ultimate longevity of a wooden structure go with physics not chemistry.
I should have been more concise in my response to this question. I was referring to structural joinery specifically...the joinery traditionally practiced by joiners in the construction of frameworks that have to carry loads with little deflection/deformation and no chance of failure. Covered bridges come to mind, but you would find similar joints in utilitarian furniture as well. Rather than depend on the adherence ability of glue (especially over time, and especially in variable environmental conditions) joiner's configured connections between the wooden members to take advantage of woods inherent resistance to compression.
Glue is stronger than lignin in wood, but weaker than wood fibers. If you glue along the grain, the wood will fail before the glue due to the weakness of the lignin in wood, but gluing end grain, the glue will fail before the wood since the wood fibers are massively stronger than glue.
To increase strength even more, taper the glued ends so that there's always overlapping fibers, the longer the taper, the stronger the resulting piece.
It looks like there's a few factors in determining if a glued joint will be stronger than some other joinery: is the wood stressed along or against the grain? Are there multiple points of stress? Is stress on the glue directly, or on a wood-glue-wood sandwich?
