The Ultrasonic Measurement for the Height of Crack which is Perpendicular Direction

by Jizhen Xia,Yongfeng Ong* and Buyao Zhang*

NDT teaching research lab. in NANCHANG Institute of Aeronautical Technology
The article is first published on "13th WCNDT(World Conference on Non-Destructive Testing)" (Sao Paulo,Brazil,18-23 October,1992)
* The Yongfeng Ong and Buyao Zhang were students of NDT speciality in NANCHANG Institute of Aeronautical Technology,graduated in 1991.
Abstract
The article described a simple method of ultrasonic measurement for the height of crack which is perpendicular direction on the bottom,for example,the spilly place in weld seam at root.This method is using common ultrasonic flaw detector and angle beam probe.It is suitable for initial assessment for crack's height during UT on site.The results shown that the accuracy is approx กภ0.3mm for 1.57~8mm crack height.
Introduction
At present,the measurement about crack height in the weld seam has concerned with the problem about fracture mechanics and safety assessment,and damage allowance design.This is a important problem how accurately measuring crack height as full as possible by means of simple method and common instrument on site.
In this article,it is attempt to use some angle beam probe with frequency 2.5MHz and different refraction angle,their piezocrystical size including 10x12mm,13x13mm,and 8x8mm.The crack height's measuring principle is according to the difference of propagating time between the root-echo and tip-diffracting wave of the crack.When the crack height is larger than the beam dia. so that can not cover the full height of crack,then need to measure depth difference between root-echo and tip-diffracting wave to evaluate the crack's height.
Testing process
It is using the plane block made of steel,on its back to groove perpendicularly with different depth(simulating crack height),it can be sawing or spark machining.
It is measuring the propagating time's difference between root-echo and tip-echo of the groove,and according to transverse wave's velocity to calculate the crack's height by angle beam probe with refraction angle 43กใ,50กใ,55กใ and 64กใ.
The method's basic principle is shown in fig.1.
For used common ultrasonic flaw detector,it should can to calibrate time base using larger proportion in order to observe


fig.1
and read-out the time difference on screen from the scale,and the instrument should possess of small linearity error (horizontal and vertical).In addition,it should is narrow pulse as full as possible for used probe in order to assure obtaining good resolution.
The testing results shown in Tab.1
Tab.1
real crack height(mm) real refraction angle of beam axis
in the sample		 piezocrystal size(mm) measuring height value(mm)
1.57 43กใ
50กใ
55กใ		 10x12
10x12
13x13		 1.53
1.54
1.31
2.0 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 2.05
2.19
2.10
2.98
2.18 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 2.12
1.73
1.80
2.41
2.7 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 2.73
2.31
2.46
2.29
3.1 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 2.70
2.58
2.76
2.39
3.31 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 2.87
3.0
3.06
2.4
3.43 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 3.42
3.12
3.55
4.60
4.0 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 3.46
3.47
3.7
4.78
4.63 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 4.34
4.24
4.24
4.78
5.0 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 4.67
4.60
4.33
-
5.9(inclined 5กใ) 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 6.17
5.78
5.62
8.05
6.8(inclined 5กใ) 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 6.84
6.85
7.79
5.4
8.0(inclined 5กใ) 43กใ
50กใ
55กใ
64กใ		 10x12
10x12
13x13
8x8		 7.52
7.7
7.79
6.0
During testing,first to find the largest echo-amplitude of the crack where the refraction beam's axis should is arrived the root of crack,then increase instrument gain in order to observe the tip-echo in front of the root-echo.Because of the tip diffracted echo-amplitude is smaller than root echo-amplitude, consumedly, so much as 20dB.Therefore,the tip-diffracted wave must be identified carefully.Then read-out the distance or time difference between two echo and calculate the crack height.
Conclusion
fig.2 fig.3
  fig.4



   fig.5
From above results,it can obtain following idea:
1.It has well measurement accuracy for refraction angle 43กใ. As the refraction angle to increase,so the measuring error to increase,it can be explained that the time difference ก๗t is to decrease as the refraction angle to increase,so that ก๗t is difficult to identify on screen,as shown in fig.2.
2.If piezocrystal size is smaller so that refracting beam can not cover the full crack height,then the tip-diffracted wave is difficult to observe,so the measurement is difficult to carry,as shown in fig.3.
3.If  crack height approach or more than beam diameter,it can use the method, i.e measure the depth difference between root-echo(sample's thickness) and tip-diffracting wave to evaluate the crack's heightis,as shown in fig.4.
4.If the crack don't perpendicular but is inclined a certain angle,the measurement's result will possess of different error according to different inclined direction,as shown in fig.5.