when sheet will thicken and for a strain-hardening material, the tension will never reach a maximum

The statement you provided seems to be a part of a larger context, possibly discussing the properties of strain-hardening materials. However, without a specific question, it's difficult to provide a step-by-step answer.

Strain-hardening, also known as work-hardening, is the process of increasing a material's hardness and strength through plastic deformation. This usually happens when metal is bent, stretched, or otherwise deformed.

The sheet will thicken when it is compressed in a direction perpendicular to the direction of stretching. This is known as the Bauschinger effect.

As for the tension never reaching a maximum, it could mean that for a strain-hardening material, as you continue to deform it, it continues to harden, and the tension (or stress) required to continue deforming it increases. There isn't a maximum point because the material continues to harden as long as you're still deforming it.

Please provide more context or a specific question for a more detailed answer.