Is ofxBox2d setDamping not working?

I tried to set damping froce for ofxBox2dCircle object.
I found I could use ofxBox2dCircle::setDumping();
But It seemed that it didn’t work.
Any help? thanks!

 ofPtr<ball> c = ofPtr<ball>(new ball);
        c.get()->setPhysics(1, 0.4, 0.4);
        c.get()->setup(box2d.getWorld(), touch.x, touch.y, radius);
        c.get()->touchID = touch.id;
        c.get()->setDamping(10000);
        myBalls.push_back(c);

I got it worked.
First, the value for Damping must between (1.0-0.0);
Second,we should use setDamping in update(). We also don’t need to set it up.

void testApp::update( ){

     for (int i=0; i<myBalls.size(); i++) {
            myBalls[i].get()->setDamping(0.99);
    }
}