The headline of this thread is underselling your problem, rather than “2 bikes broken in 18 months”, it could be " Brand new frame broken in 6 months"
Your photos show a 58cm frame with a fair ammount of seatpost stick out so guessing you must be 6’ plus, how much do you weigh, are you close to the 125kg rider limit?
Edit: misread the OP, 2nd frame did last 18months.
I never thought in a broken frame, but the idea of a frame being stressed on a turbo never looked healthy for me.
Also, I don’t think a towel avoid sweat to penetrate/reach the bike. The towel gets sweat and transfers it to the frame, handlebar, etc anyways. I’ve heard about wrapping with stretch film. Not that would be the cause, just a thought.
Question for @4ibanez , what model year are your 2 broken frames? Mine was a 2017.
As the other guy with the same problem I’m going to back up the OP on this. While my stock seatpost could have been accused of being short (thanks specialized) I had an aftermarket easton EC90 in mine that was much longer. I also never had a rocker plate with that bike and used a wheel-on trainer for most of its life until around the last year of it. As for saddle fore/aft I had a motion capture pro bike fit done at one point and my saddle position is pretty close to center of the marks on the rails of an offset seatpost. I dare say the seatpost was the only thing holding it together. Upon giving the area a tug without the seatpost in the frame the area completely gave way.
My thought is that this is just a weak point in the welding/frame design. Notice only one of the cracks goes along a weld while the rest find their own path on the tubes. Could be inadequate heat treating post welding combined with the all the stresses put on the seatpost mast concentrated due to being locked in a trainer with some nice corrosive salt there to weaken it from the inside. It’s a wonder if an o-ring and some grease where the seatpost collar meets the seatpost might provide something of a barrier to salt water penetration
I ordered the new Allez in April 2022, so it’s done about 18 months.
I’m 189cm / 6’ 2" and currently about 78kg. If it’s relevant 365w FTP and putting in 12-15 hours p/w (90% of that is on the trainer at this time of year)
It definitely can get gross indoors, but I have 3 fans and I lay a folded towel horizontally across the handlebar, to try and minimise sweat getting on the bike, but I realise some can transfer. Looking at the bike though, it’s basically mint condition apart from the crack.
I’m wondering if a carbon frame would be better, since I believe it doesn’t fatigue in the same way that metals do. But then there’s actually widely known issues with clamping on some carbon frames so…
I think the old bike was from around 2010-12, the new one was a 2022 - this is the exact one: Specialized Allez E5 2022 Road Bike from Wheelies
Sorry, misread the date in your original post. Its a major concern its cracked so soon, on what appears to be a normal setup well within Specialized guidance in the user manual. Hope you get a good response from the dealer/Specialized.
There was likely some contamination in the weld(s). Aluminum is a pretty brittle metal, so all it needs is a starting point and the crack is inevitable with repeated stress on that area. Being locked into a trainer does put stress on the frame (in different spots vs. riding outside), but nothing the frame shouldn’t be able to handle. I would expect Specialized will warranty it without any drama. Yeah, a carbon frame is less prone to cracking from fatigue cycles, so maybe something to consider if Specialized is willing to swap for carbon and you pay the difference.
Yeah, my first reaction to those pics was that it was a poor weld. I seriously doubt having the bike on the trainer caused so much undo stress that the frame cracked.
Im sure its said somewhere, maybe not - that is from the manufacturing process… not the trainer stess IMO, although the trainer probably does apply more force and bind that joint more.
If I was to take a stab at a failure analysis on that its from contaminated aluminum and poor joint design (the joint welded to a seat tube). I imagine this is a highly restrained joint and because manufacturing cost is pennies on these frames they are not using a stress-relief treatment after welding.
Specialized should not argue in the least with this.
I’m thinking that it’s big guy + lots of power = too much for light aluminum frame to handle long term. Maybe being clamped in a trainer exacerbates it as well.
Carbon might well be better for this application. Maybe even a beefy carbon mountain bike since it never leaves the trainer?
I don’t think you are doing anything wrong so I think warranty should be straightforward.
However, although your setup is within limits, you have got an awful lot of seatpost showing and your saddle is pretty far back, you’ve got a pretty good lever to maximize the effect your weight has on that joint.
I can’t convince myself either way on if I think the rocker plate would make it worse or not 
I think my gut instinct is it should actually make less stressful on that joint of the bike is able to naturally move with your weight rather than fighting against it 
The higher the power output the less force through the saddle-seatpost-seat tube assembly. My butt can attest to this.
I have a £250 road bike permanently setup on my trainer, stock bike weight was 12kg with 23mm tyres. The frame is plain gauge 6061 and its lasted 10,000km (virtual).
If I was going for ultimate reliability I’d choose a steel frame which ought to have a near infinite fatigue life compared to an Aluminium alloy one.
You aren’t always putting out a lot of power. Most of the time one is probably doing more sitting than high power output. The key point is probably him being a larger person with a good amount of seat post showing (lever) and putting the weight on the joint.
The frame shouldn’t break. Maybe third time’s the charm?
Personally, I’ve broken every steel frame I ever owned. 
Agree that steel frames are typically less prone to cracking than aluminum (and titanium), but carbon is less prone to fatigue cracks compared to all the common metal frames. Not trying to turn this into a carbon vs. x thread, but the fatigue resistance of carbon is one of it’s big benefits. I also personally wouldn’t choose steel for a dedicated trainer frame where sweat is often an issue. All of these frame materials should be fine on a trainer as long as they are designed and manufactured properly.
If I can get my money back I may put an old Super Six Evo on the trainer (feels a shame to do that with what was such a nice bike), but no doubt they’ll want to give me a like for like replacement.
Specialized will not give you money…zero chance of that happening.
Your only chance of getting “money” is if your LBS agrees to take the replacement frame, resell it and allow you to choose different frame.
As depressing as it is, that’s the extreme cheap end of “name brand” road bikes these days. Likely an intentional business decision from specialized on frame quality. Easy enough to do the math - what percentage of the folks buying a £1k bike are going to be pushing it as hard as you are? Most are probably putting less than 50 hours a year on them and are recreational riders pushing low power. It’s cheaper for specialized to just warranty the tiny percentage of situations where the bike can’t handle it vs. “overbuilding” the frame for the masses who will never stress it.
I know several very strong specialized sponsored riders who ride the “sprint allez” aluminum frame and it’s widely regarded as one of the best frames for the $ these days. Maybe worth considering an upgrade if you don’t want to keep messing with warranty claims.
Do you honestly think Specialized sat around and said “yeah, we don’t need a frame that will last more than a couple of hundred hours for this price point”?
Are concessions made in terms of weight, etc. to get a lower price point? 100%…but the idea that quality is also sacrificed is just not accurate. The companies I worked for (two of the largest in the industry) has QC frame standards that were universal…they either passed or failed. Didn’t matter of they were high end Ti frames or entry level steel frames.
