Abdominal workouts are a great way to flatten your stomach, but if you really want to strengthen your core and have visible abs then I recommend adding a weighted ab workout to your weekly routine.
Complete 10-12 reps of exercise A. Without resting move straight on to exercise B and perform as many reps as you can until failure. Without resting move on to exercise C and perform as many reps as you can until failure. Rest 2 minutes before repeating two more times.
A. Landmine 180 10-12 Reps (as heavy as you can go with good form)
B. Cable Crunch to failure
C. Hanging Leg Raise to failure
Rest 2 minutes and repeat two times before moving on to the next circuit.
D. Cable or Dumbbell Wood Chop 10-12 reps
E. Weighted Leg Lift to failure(stability ball between feet or ankle weights)
F. Plank Hold to failure (advanced exercisers can add a weight plate onto their back)
Remember to always warm up and cool down before and after exercise to help prevent injury. You want to lift as heavy as you can while still maintaining proper form. You only want to be able to complete the prescribed number of reps. If you are performing 10 reps and you reach 10 and feel like you could have done 5 more, then you need to increase your weight. If you want to know how to perform a certain exercise, you can find it here.