leometwu的部落格

上: repost. Similar fracture surface as that shown below.
 
 
 
 

主裂面,  primary crack

 A

  A:側光oblique light:  beach marks are more obvious 貝殼紋更明顯. B:頂光top light. 箭頭所指為終裂區(final rupture) 面積粉小, < 10%, 代表 牙桿所受外力只有抗拉強度的 10%.
  B

調質後硬度(強度)太低(HRC 19~23), 牙輪輥牙時, 易被粘著撕去材料(adhesive wear; 2, 牙底; 3, 牙面), 形成應力集中位置 --> 引發疲勞裂紋 往內前進 --> 終至斷裂. 註:疲勞壽命與表面抗拉強度呈正比 Too low a surface hardness (HRC 19~23) caused adhesive wear on the roots of threads  during thread rolling. Notches thus produced at bottom of thread served as crack initiation sites, see red arrow 2 in the photo below.

平牙底, flat root, No Good.  圓牙底 Rounded root, Better.     Secondary cracks.