3 sentences

1. 既然新协议已经签署，旧约就此失效。

   jì​rán xīn xié​yì yǐ​jīng qiān​shǔ, jiù​yuē jiù​cǐ shī​xiào.

   Since the new agreement has been signed, the old treaty is now void.

   Source: Dong Chinese Dictionary

2. 由于过了保质期，这种药已经失效了。

   yóu​yú guò le bǎo​zhì​qī, zhè zhǒng yào yǐ​jīng shī​xiào le.

   Because it is past its expiration date, this medicine is no longer effective.

   Source: Dong Chinese Dictionary

3. 飞机的液压系统突然失效，导致紧急迫降。

   fēi​jī de yè​yā xì​tǒng tū​rán shī​xiào, dǎo​zhì jǐn​jí pò​jiàng.

   The aircraft's hydraulic system suddenly failed, resulting in an emergency landing.

   Source: Dong Chinese Dictionary