No it's not normal for a bearing to fail after 18000 miles, which is why I think the noise is caused by something else. A worn bearing would in any case make a very different noise. You can check the bearing by jacking the wheel off the ground, then rotating it. A low pitched rumble would indicate wear; a scraping or rubbing sound is something else. Grab the wheel top and bottom and try to rock it. There should be no movement if the bearing is sound. (Doing the same thing holding the wheel at left and right can be deceptive because of movement in the steering joints.)
While the wheel is in the air look for damaged tyre tread, rubbing brake pad, a warped disc.
While at 40 mph is there any vibration at the steering wheel? Does the car steer true or does it try to wander at that speed?
Let us know the outcome of these tests and we can advise further.