My story has generally humanoid, human-sized creatures with insect-like exoskeletons. Assuming their joints and bone structure work as closely as possible to their internal-skeleton counterparts (and that lower gravity allows the exoskeleton not to collapse), would hand-to-hand fighting/fist-fighting have any major differences? Are there weak points/strong points in an exoskeleton that could be exposed with peak-human-level strength? I thought for example that a fighter may grab and tear the other’s shell away, or that a heavy blow could fracture chitin and suffocate an opponent. Could this generally work, or would hand-to-hand fighting be generally fruitless with an exoskeleton?
If there are no tools, it would seem that attacking joints to pierce, sever or tear limbs would be very effective. Depending on the biology, lost limbs could be a nuisance or they could be fatal.
From a sheer strength standpoint, unless the creatures have special snapping limbs to provide incredible impulse or momentum, I doubt a single hit could crack the "chest" or "back". For two reasons:
First, look at humans. No matter how hard a powerlifter or boxer punches another person in the chest, it is unlikely to cause complete destruction of the ribcage. Most humans simply cannot destroy others in a single hit. It takes repeated hits in one place to break bones for most attackers. In part, I think this has something to do with the idea that you (generally) cannot exert forces that would break your own body. It would just kill too many creatures by accident. If you could punch with enough force to shatter a skull or ribcage, you would be subject your own skeleton to some non-trivial amount of the hit as a counterforce.
The second reason is thematic. If they have exoskeletons, these creatures are probably hard to kill. It would seem silly to just have them able to crack each other open, when nothing else on the planet has evolved along the same lines.
The "smasher" varieties of mantis shrimp accelerate their club-like claws at over 10,000g to deliver shell crushing (150Kg force) blows to their victims.
Similarly for our humanoids, the "fists" of the exoskeleton are heavily armoured and used like a mace that can break bones. Highly elastic tendons of the triceps, like those found in a kangaroo's hamstrings, can store tension while the contracting muscles (biceps) hold the fist back until ready to strike, which when released happens in the blink of an eye.
Boxers knock people out not by destroying their skulls but by shaking their brains. A human head is the closest we have to an exoskeleton part of the body. People can be knocked out or killed even wearing steel helmets.
The major difference would be to target punches to areas overlying vulnerable organs. The strikes can transmit force through the carapace.
But just like humans almost any melee weapon is preferable in a fight to the death than your fists. If only as a force multiplier or puncture weapon.
would hand-to-hand fighting/fist-fighting have any major differences?
The Martial Arts that we have created on Earth are a specialized series of attacks and movements meant to deal with humans. Through generations of study, we created attacks methods to attack the organs and joints and deliver a series of blows that are meant to strategically corner your human opponent.
All of that goes out the window when you're fighting something that isn't a human. Martial Arts designed to deal with non-human targets would need to be redesigned to attack their new weak-points and defend against their new range of motion.
For example, many martial arts have grapples and holds that are designed to restrain a human efficiently, and put the attacker in an advantageous position. Those holds only work with a human's joints and skeletal structure. A Hold that would restrain a human-sized insect would need to adjust to the new joints, ranges of motion, and attack the weakest angles of their musculature.
Entire styles of fighting will be obsolete due to the difference in strength, number of arms, body height, body weight, etc.
Martial Arts in your world would be very different that anything we have come up with on Earth.