IMG_2485.JPG shows pretty well what happened. External fatigue cracking from the toe of the weld, at the "bottom" (referencing the circumference) of the weld. This failure didn't happen due to restraint in the lateral direction, rather in the rotating direction of the bushing. Excessive stiffness in the bushing could have contributed, though due to the location of the failure, I'd be willing to bet that the heat affected zone of the weld is excessively hard. If this is the case, they have some serious manufacturing issues.
If they try to feed you some bs about the arm failing due to hitting a large bump, don't buy it since that arm clearly failed due to fatigue.
As far as lab work is concerned, I've already given you your mode of failure. If you want, I could do a cross-section through the failure in my spare time (e.g. extremely cheap) and see if there are weld quality/hardness issues at work. PM me for more info if you want (and my credentials if you need them...).