The fundamental principle of ultrasonic flaw detection

The fundamental principle of ultrasonic flaw detection

There are many kinds of ultrasonic flaw detectors,but in actual process of flaw detection,reflection of waves ultrasonic flaw detector is widely applied。Normally in even material ,existing flaw will generate,material discontinuity,this discontinuity will generate inconsistency of acoustic impedance ,from reflected theorem we can know,ultrasonic wave will happen to reflect when in the interface of two different acoustic impedance,the strength of reflected energy is related to the difference acoustic impedance、orientation and size of interface。Pulse reflection type ultrasonic flaw detector is designed according to this principle。

Current portable pulse reflection type ultrasonic flaw detector mostly are A scanning way,  so called A scanning displayed ways is abscissa of screen is travel time or travel distance of ultrasonic in the detected material。Vertical coordinates is amplitude of ultrasonic reflected wave。Such as there is a flaw in a steel workpiece , due to exist flaw all the time , form an interface in different medium between flaw and steel material ,the acoustic impedance is different between the interface,when ultrasonic meets this interface ,then it will happen to reflect, the energy that reflected and then received by probe,on the screen ,the certain position of abscissa will display reflected wave,the position of abscissa is the depth that flaw in the detected material,the height and shape of reflected wave will be different due to different flaw ,reflect the property of flaw。

Main advantages :

(1)  penetrating ability is powerful,penetrating depth can up to several meters

(2) High level of sensitivity, it can detect reflector that has similar reflection ability with air gap what has tenths  of a few millimeters。

(3) It is more accurate on making sure the orientation 、size、shape and property of internal reflector 

(4) It needs to approach to detected object from another side

(5) Provide flaw detection result instantly

(6) Operational safety,light equipment

Main disadvantages

(1) need to be rigidly operated by experienced person

(2) It is hard to detect if they are rough 、 irregular shape、small、thin or uneven material;

(3) It is hard to make very accurate qualitative、 quantitative characterization to discoverable flaw