Thermomechanical　cycle　failure　was　analyzed　in　808-nm　high-power　AlGaAs/GaAs　laser　diode　bars　(LDBs)　in　detail.　Two　thermal　stress　cycle　experiments　were　carried　out　in　these　devices　with　peak　thermal　stresses　of　19.4　and　33　MPa.　The　threshold　current　increase　demonstrated　a　square-root　dependence　on　the　cycle　number,　and　a　tail　and　split　in　lasing　spectrum　were　observed.　This　result　was　attributed　to　nonradiative　recombination　increasing　with　cycle　number　due　to　the　diffusion　of　defects　that　accompanies　band-structure　renormalization.　Furthermore,　from　X-ray　diffraction　measurements　we　found　that　the　epitaxial　layer　remains　monocrystalline　during　thermal　stress　cycling.　We　calculated　the　out-of-plane　strain　and　in-plane　stress　induced　by　plastic　deformation　in　the　epitaxial　layer　based　on　the　interplanar　spacing　in　the　crystal.　Our　results　suggest　that　thermomechanical　strain　and　stress　were　induced　by　pulsed　operation,　which　led　to　degradation　of　the　high-power　LDBs.