The　triaxial　stress　constraint　and　the　effective　yield　stress　distribution　in　the　plastic　zone　for　strain　hardening　materials　are　studied,　and　then　a　modified　strip-yield　model　is　proposed　to　investigate　the　thickness　effect　of　CCT　specimens.　Consequently,　a　plastic　constraint　factor　alpha　is　defined　and　analysed　in　detail.　The　results　show　that　the　factor　alpha　can　comprehensively　account　for　the　influence　of　thickness,　crack　length,　loading　level　and　hardening　exponent.　A　simple　expression　for　the　plastic　zone　length　and　a　fitting　expression　involving　alpha　are　obtained.　Application　of　the　modified　strip　model　to　Newman＇s　crack　closure　model,　and　comparison　with　FEM　results,　show　that　the　model　can　account　for　the　influence　of　thickness　on　crack　closure.