Is it a crime if someone is aware a device looks like a bomb and brings that device to a public place?
Yes: Many states have laws against "hoax bombs" or "false bombs." E.g., in Texas:
(a) A person commits an offense if the person knowingly manufactures, sells, purchases, transports, or possesses a hoax bomb with intent to use the hoax bomb to:
- make another believe that the hoax bomb is an explosive or incendiary device; or
- cause alarm or reaction of any type by an official of a public safety agency or volunteer agency organized to deal with emergencies.
A person is guilty of placing a false bomb or hazardous substance in the second degree when he or she places, or causes to be placed, any device or object that by its design, construction, content or characteristics appears to be or to contain, a bomb, destructive device, explosive or hazardous substance, but is, in fact, an inoperative facsimile or imitation of such a bomb, destructive device, explosive or hazardous substance and which he or she knows, intends or reasonably believes will appear to be a bomb, destructive device, explosive or hazardous substance under circumstances in which it is likely to cause public alarm or inconvenience.
Even without the mens rea requirements of these crimes, the common-law standard for other crimes like Disturbing the Peace is the "reasonable person" doctrine. So even if you honestly didn't think your device resembled a bomb, if a court concludes that a "reasonable person" would think it did then you can be convicted of such lesser crimes.
This is my opinion based on my own travel experiences:
Assuming it is definitly not a bomb, but rather (for example) some mechanical engineers project (like a computer/sensor with wires going everywhere), that is fine. If you purposfully create panic, or present it in a questionable way, that is not ok and can be illegal. If it is supposed to resemble a bomb (but isn't one), thats not fine too, and you should have a valid reason to bring it and talk to the airport in advance that you will be bringing it (in a way that doesn't make them suspicious of your intentions). Either way, you'll likely expect to have security have a greater interest in you.
Many terrorism and public order laws have specific prohibitions of replica bombs and such, depending by country. To provide a comprehensive list would be difficult, especially as many anti-terrorism powers are quite generally and generously worded in grounds for arrest and confiscation.
Specifically regarding "if someone is aware", many offences in the UK have the mental component of subjective recklessness: seeing the risk and taking it anyway. For example, taking something bomblike and happening to scare people with it could constitute Assault (causing victim to apprehend immediate unlawful force).
Additionally, officials and security in public places such as at airports can confiscate more or less anything, especially anything which looks dangerous, at their discretion, without being particularly accountable to a higher power.