Product review (1 stars): They fail like clockwork
This is my 3rd sets of turn knobs on my $1000+ Jenn-Air cooktop. It seems that Jenn-Air/Whirlpool has no intention to improve the material of these knobs. They are made of light weight plastic coated with nice looking brushed aluminum surface. But the plastic sockets crack within a year for the most frequently used burners.  Once they cracked, it is not possible to hold them together with any type of tape. But I'd consider to wrap the perimeter of the sockets with duct tapes while they are still intact to reinforce the plastic wall.