what does Atticus attempt to establish through heck Tate's testimony

Atticus wanted to call into question the concern, or rather lack of concern, Bob had for his daughter when he found her. More importantly, Atticus tries to illustrate that Mayella could not have sustained the injuries that Heck Tate witnessed. Tom was not physically capable of causing those injuries to Mayella. The subtext is that it was Bob Ewell who actually assaulted his own daughter.