// Makes the joint break when a force larger than 10 is applied
hingeJoint.breakForce = 10;
// Makes the joint unbreakable
hingeJoint.breakForce = Mathf.Infinity;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        hingeJoint.breakForce = 10;
        hingeJoint.breakForce = Mathf.Infinity;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        hingeJoint.breakForce = 10
        hingeJoint.breakForce = Mathf.Infinity