Spread the force...
When materials are subjected to forces, what happens to them is determined by their properties - their strength (sheer, tensile, compressive, etc.), their brittleness, and other factors. If the force is below the threshold they can handle, then they can transmit that force externally, onto other things; if the force is too great, it causes changes internally, and the material fails.
In other words, if you push something gently, then it can pass that on to what's behind it; push too hard, and it breaks.
Spread across space
Weapons are mounted on hardpoints, which have two main properties:
- They're made of something strong.
- They're constructed in such a way as to spread any force applied to them across as much of your craft's structure as possible.
This reduces the amount of force that any one part of your craft takes, helping to keep it below that critical threshold. The important part, however, isn't spreading the force over more space - it's time that's key.
Spread across time
With a weapon like a cannon or railgun, you can accelerate the projectile gradually over a long track or barrel, applying force the whole time by the expansion of gas, a magnetic field, or some other means. This means you can apply a (relatively) gentle force, which adds up over time to give the projectile a very large velocity. This reduces the force that the structure has to support at any given moment while that projectile is being accelerated.
On the other hand, when that projectile strikes a target, the material of that target will be attempting to bring the projectile to rest in a much shorter time. Force is defined as the rate of change of momentum, and the time you spent giving the projectile its momentum is much longer than the amount of time it spends punching through an armour plate - the peak forces are much higher, and therefore more likely to exceed the threshold at which the material fails. This explains how your ship can be made of the same stuff as theirs, but theirs breaks and yours doesn't. By keeping peak forces low, your ship can spread the force out and experience the weapon fire as a slight push applied to the whole ship, while theirs has the force concentrated and experiences it as a hole being punched somewhere.
Spread across mass
Conservation of momentum says that the total "nudge" your ship experienced has to add up to the same amount of momentum as the shove that their armour felt, but because your ship spread it over more mass, it's less noticeable. A weapon whose recoil changes the velocity of a multi-tonne ship by a few metres per second might send chunks of debris flying out at hundreds of metres per second because the chunks have so much less mass. Of course, their craft may have similar mass to yours - or even more - but because they couldn't spread the force, that didn't help them:
- Because the projectile came in at great speed, they weren't able to spread the force over more time. This meant that they exceeded the peak forces of their materials.
- Because their materials failed, they were unable to transmit the forces further through the ship and spread them over space.
- Because they couldn't spread the forces across more space, they couldn't affect a large enough of the ship to add up to much mass.
- Because they couldn't spread the force over more mass, the pieces that were affected ended up with greater velocity, which means that those pieces will then probably hit other bits of the ship at great speed...
And so the whole thing happens again until the pieces are small and/or slow enough that the materials they're hitting can stop them. This is particularly important in space, where you're likely to be in vacuum:
- There's no air to slow the pieces down, so this knock-on process is particularly effective
- There's no air to carry a shockwave, so this process is your most effective means of spreading damage through the enemy ship.
This process of generating as many fast-moving chunks as possible so as to spread the damage is called fragmentation. Sometimes your projectile might be designed to shatter on impact, or even explode beforehand; other times it's just about striking in such a way that you maximise the fragments thrown off (perhaps by having a round that will tumble erratically as it passes through the target rather than punching through pointy end first).