Well I happen to find out the hard way with this.  I wanted to give 2x4s a try out to see if it would be some what safe for me to turn.  Not sure if I didn't use enough glue or let the glue cure long enough.  Started turning it using the roughing gouge I get half of it round.  By the time I started on the tail stock end of the piece I had it up to about 1100 RPM roughly notch 9 on the lowest setting of the belts on the delta.  As I put pressure on the gouge into the wood the next thing I know the piece basically snapped.  Not in half but length wize with the glue joint.  So the only thing I could think of is A I didn't use enough glue or B not enough cure time or option D both A and B.  Either way lesson learned.  I was using the live centers for this.  Any thoughts on this one?

 

 

Well, it was the glue joint that failed - no wood was harmed in the failure...

- Were the pieces flat so that so there was good contact or was the glue used as a filler?

- Did you use enough glue?  Almost looks starved?

- Was the wood dry enough, and what type of glue were you using?  What is the discoloration on the left 2X?

- How long did you leave them clamped up, and what type of clamps were you using?  And how many clamps did you use?

- How old is your glue?  Has it been frozen and then thawed?

- When did you clamp it up?  Was it left in freezing conditions after glue up during the cure time?

It looks like you are using the wrong glue.  Also 2x4's probably need to be run through the planer to be flat enough for the glue to work like it is supposed to do. But is does look like you got away from the super glue.

What kind of glue was it ? I’ve turned Douglas fir and it can have some pretty grain structure but it needs to be flat to produce a good glue joint. 
Paul

just regular wood glue from home depot.

I have the same thoughts as @Cal

Seeing the glue bunched up like it is in the picture says that the boards were not prepared correctly for gluing.  If you want to glue boards like that together at least sand them and then use a gap filling glue.  Glue does not stick well to mold either.  

In addition to the other very valid comments. When gluing a large surface the inside which is the part you are turning toward take a lot longer for glue to cure so that also affects the joint. When you look at dimensional lumber it looks flat but it is not so that is why jointing the lumber is very important.

More than likely after reading all this the glue wasn't left long enough for curing well I'll have to give it another go after the cold snap finishes off...

In addition to the above -- clean, flat, enough glue, enough pressure, no gaps --  wood glues have what they call a "chalk temperature."   This is a temperature at which the glue turns white (chalky) and has little strength.   For most PVA woodworking glues, this is 45-55 degrees.   This is the minimum temperature for both the glue AND the wood.  It does not help just to warm up the glue because it will cool when it hits a piece of wood at 40 degrees.

 

From the TiteBond site:
Chalk Temperature
When glue dries, the loss of water pulls the adhesive particles together with
enough force to form a continuous film. If the drying temperature is below a
critical point, water evaporation is not sufficient to pull the particles together,
leaving them in the joint. The dried film in the joint will appear whiter than
normal. This is known as "chalking" and the critical temperature is the "chalk
temperature." When chalking occurs, the glued joint loses strength and could
result in a failed bond.

TiteBond Aliphatic Resin ("yellow glue") chalks at 50 degrees Fahrenheit as does Thixotropic PVA ("trim & molding glue"). TiteBond III Ultimate ("Proprietary Polymer") goes 47 degrees F, while Crosslinking PVA (TiteBond II Ultimate) chalks at 55F.

Which is why I'm saying I'm going to wait until this cold snap is over supposed to be in the 20s here this week.

When I was wanting to learn to use a Skew I used 2X4s ripped in half, also bought a "whitewood" 4X4 fence post at Menards. It wasn't real expensive, was great to learn on, and I didn't have to glue it up.

1 hour ago, AndrewB said:

More than likely after reading all this the glue wasn't left long enough for curing well I'll have to give it another go after the cold snap finishes off...

From the other comments and what I see it is more than a temperature and not left long enough problem. The wood needs to be run across a jointer or at least sanded good. Also a lot more glue.