Hi, I'm carrying 2x 10lbs dumbells plus some water and stuff in my Trail 25 backpack. I've been using it for a month and it seems to be holding up well. I'd like to increase the weight and eventually carry 50lbs in my Trail 25. Should I expect the backpack to continue to hold together with the extra weight?
The trail 25 has no supportive belt and a very light frame so probably is designed to carry up to about 10-15lbs of stuff...generally the max amount that is comfortable directly on the shoulders and compressing your back. ymmv. You can carry more for the purpose of exercise as it seems you do but you will probably find it uncomfortable for any extended period and personally I wouldn't. Weight shirts are probably a better approach to this kind of thing because they distribute the weight evenly around your body...but not my thing so just speculation on my part.
50lb is a lot of weight to carry in a backpack and generally only backpacks of capacities of >50L are designed to carry that sort of weight. They generally have a stay or some kind of fairly substantial inner frame and a supportive padded hip belt that transfers most of the weight to your hips. Even with that capacity many of them run out of comfort at around the 35lb mark. Of course that is for hiking all day backpacking rather than for weight carrying exercise.
To add to what @OldGuyot said, carrying more than 10-15 pounds in a day pack is not only uncomfortable but it also puts a lot of strain on the back. Back packs are designed to carry more using a solid back stay or similar plus a hefty waistbelt to transfer the extra weight to the hips. But that adds strain to the knees. Younger hikers don't appreciate all this until it's too late. (Ask me how I know ;))
If OP wants to carry more weight in order to build muscle, lose weight, get more cardio exercise, etc. then I'd recommend hiking on an incline with a moderate amount of weight. There are no mountains near me so I use a treadmill with enough incline to get my heart rate into aerobic territory. It's more effective and much easier on the back, knees and hips.
Many thanks to you @OldGuyot and @Wanderer . I am kinda old (51) so trying to be aware of my physical limitations, while also surprised at how easy and fun rucking is. I will abandon the idea of adding more weight to the Trail 25, and look to transition to some other thing for my workouts like a weight vest. It's such a nice bag IMO, I'd like to make it last as long as possible. Thanks again!