Crash sound effects can be made in a variety of ways depending on the type of crash sounds one is looking for. For metallic crash sound effects, sound designers and foley artists visit junk yards and pile up old pieces of metal, discarded tool and car parts, metal fences and the like and then throw a heavy object into the pile to make a clanging, clattering crash sound effect. Other crash sound effects might be made with rubble from a construction site. Glass, too, provides great fodder for crash sound clips and crash sounds. Different pieces of glass such as windows, bottles, dishware, etc, can be set up tightly together and then smashed to bits with a cinder block or bowling ball.
One particularly sought after type of crash sound fx is a car crash sound effect. This is because car chases and car action are a favorite of film producers. Gotta have that chase scene and the resulting car crash sound effect to add action to the film! In order to make authentic car crash sounds, professional sound designers and firms will actually buy cheap junker cars and smash them into each other, then add other crash sound clips elements on top of the recording, such as the aforementioned glass crash and meal crash sounds. Sometimes, too, strong bass elements will be added to crash sound fx to increase the subsonic output of the crash, which is great of course for 5.1 systems that use a subwoofer and theaters. So, there you have it, crash sound effects often include high end shattering sounds from glass, midrange clanking sounds from metal, rubble for richness and bass elements for impact.