How to capture TCP fragmentation in linux

Question

I want to capture the tcp fragmentation packets using wireshark. I wrote a tcp client and a tcp server. I have set MTU to 500, MSS to 1500. Also I have set IP_MTU_Discouver to IP_PMTUDISC_DONT. The send/receive buffer size is 1448 and the input file to the client is 15kb. But I cannot capture any fragmentation. What's the problem?

Thanks!