The heat exchange tubes of the liquid ammonia cooler have cracked

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For a project undertaken by my company, one of the heat exchangers was used to cool a medium with liquid ammonia. Due to inadequate consideration at the time, this liquid ammonia cooler was designed as a conventional fixed-tube-sheet shell-and-tube heat exchanger, with a design pressure of 0.8 Mpa and a design temperature range of -40 to 60°C for the tube side; the shell side had a design pressure of 1.2 Mpa and the same temperature range. After operating for some time, it was suddenly discovered that the heat exchanger was leaking, and the leakage occurred in the two rows of heat exchange tubes located at the top of the heat exchanger (these tubes were made of titanium). Upon removing those tubes, cracks were found to be longitudinal in nature (resembling cracks caused by expansion). I’m not sure of the exact cause; I would appreciate it if experts and industry professionals could help analyze this issue. Thank you