HADR is not a cluster manager. It does not fail-over the database to standby automatically, you must issue the HADR takeover command on the standby to do this.
DB2 comes with a cluster manager called TSA that can do the automatic fail-over. TSA is usually installed automatically when you install DB2, but you probably want the latest fixpack. TSA is somewhat complex, so you will need to invest some time to learn it.
Or you can use another cluster manager to do the fail-over (you will usually have to write the fail-over scripts that get executed when the cluster manager detects a problem with the primary server).
Keep in mind that if the primary server is down (or not in Peer State), you must issue the HADR takeover command with force option. Also, if you have a cluster manager, I would recommend that you use a single database virtual IP address that moves with the servers in addition to using ACR.
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390