Whilst the base line is perfectly level the crystal is quiescent. Any peaks to the right
of the initial pulse indicate that the crystal has received an incoming pulse reflected
from one or more interfaces in the material. Since the spot moves at a very even
speed across the tube face, and the pulse of ultrasonic waves moves at a very even
velocity through the material, it is possible to calibrate the horizontal line on the
screen in terms of absolute measurement. The use of a calibration block, which
produces a reflection from the back wall a known distance away from the crystal
together with variable controls on the flaw detector, allows the screen to be calibrated
in units of distance, and therefore determination of origins of returned pulses
obtained from a test piece.
It is therefore possible not only to discover a defect between the surface and the back
wall, but also to measure its distance below the surface. It is important that the
equipment is properly calibrated and, since it is in itself not able to discriminate
between intended boundaries of the object under test and unintended discontinuities,
the operator must be able to identify the origin of each peak. Further as the pulses
form a beam it is also possible to determine the plan position of a flaw.
The height of the peak (echo) is roughly proportional to the area of the reflector,
though there is on all instruments a control, which can reduce or increase the size of
an indication - variable sensitivity in fact. Not only is party of the beam reflected at a
material/air interface but also at any junction where there is a velocity change, for
example steel/slag interface in a weld.
Probing all faces of a test piece not only discovers the three-dimensional defect and
measures its depth, but can also determine its size. Two-dimensional (planar)
defects can also be found but, unlike radiography, it is best that the incident beam
impinges on the defect as near to right angles to the plane as possible. To achieve
this some probes introduce the beam at an angle to the surface. In this manner
longitudinal defects in tubes (inner or outer surface) are detected.
Interpretation of the indications on the screen requires a certain amount of skill,
particularly when testing with hand held probes. The technique is, however,
admirably suited to automatic testing of regular shapes by means of a monitor - an
electronic device that fits into the main equipment to provide an electrical signal when
an echo occurs in a particular position on the trace. The trigger level of this signal is
variable and it can be made to operate a variety of mechanical gates and flaw
warnings. Furthermore, improvements in computer technology allow test data and
results to be displayed and out-putted in a wide variety of formats.
Modern ultrasonic flaw detectors are fully solid state and can be battery powered,
and are robustly built to withstand site conditions.
Since the velocity of sound in any material is characteristic of that material, it follows
that some materials can be identified by the determination of the velocity. This can
be applied, for example in S.G. cast irons to determine the percentage of graphite
nodularity.
ในขณะที่บรรทัดฐานคือระดับสมบูรณ์แบบ ผลึกจะไม่มีการทำ มียอดเขาทางด้านขวาของชีพจรเริ่มต้นบ่งชี้ว่า คริสตัลที่ได้รับการหมุนเข้ามาประจำจากอินเตอร์เฟส น้อยหนึ่งในวัสดุ ตั้งแต่ย้ายจุดที่แม้มากความเร็วหน้าท่อ และชีพจรย้ายคลื่นอัลตราโซนิกที่แม้มากความเร็วผ่านวัสดุ จำเป็นต้องปรับเทียบเส้นแนวนอนในการหน้าจอในวัดแน่นอน ใช้ของเทียบบล็อก ซึ่งทำให้เกิดการสะท้อนจากผนังด้านหลังห่างคริสตัลที่รู้จักพร้อมควบคุมตัวแปรในการตรวจจับข้อบกพร่อง ช่วยให้หน้าจอเพื่อใช้การปรับเทียบในหน่วยของระยะทาง และกำหนดจุดเริ่มต้นของพัลส์ที่ส่งคืนได้รับจากชิ้นทดสอบจึงสามารถจะค้นพบข้อบกพร่องระหว่างพื้นผิวและด้านหลังผนัง แต่ยังวัดระยะห่างของใต้ผิวหนัง เป็นสิ่งสำคัญที่จะอย่างถูกต้องมีการปรับเทียบอุปกรณ์ และ ตั้งแต่มันอยู่ในตัวเองไม่สามารถเหยียดระหว่างขอบเขตวัตถุประสงค์ของวัตถุภายใต้การทดสอบและ discontinuities ตั้งใจตัวดำเนินการต้องสามารถระบุจุดเริ่มต้นของแต่ละช่วง เพิ่มเติมเป็นการกะพริบแบบแสงก็ยังสามารถกำหนดตำแหน่งแผนของข้อบกพร่องความสูงของพีค (echo) เป็นสัดส่วนประมาณในพื้นที่หรือว่ามีอยู่ในมือทั้งหมด ตัวควบคุมซึ่งสามารถลด หรือเพิ่มขนาดของan indication - variable sensitivity in fact. Not only is party of the beam reflected at amaterial/air interface but also at any junction where there is a velocity change, forexample steel/slag interface in a weld.Probing all faces of a test piece not only discovers the three-dimensional defect andmeasures its depth, but can also determine its size. Two-dimensional (planar)defects can also be found but, unlike radiography, it is best that the incident beamimpinges on the defect as near to right angles to the plane as possible. To achievethis some probes introduce the beam at an angle to the surface. In this mannerlongitudinal defects in tubes (inner or outer surface) are detected.Interpretation of the indications on the screen requires a certain amount of skill,particularly when testing with hand held probes. The technique is, however,admirably suited to automatic testing of regular shapes by means of a monitor - anelectronic device that fits into the main equipment to provide an electrical signal whenan echo occurs in a particular position on the trace. The trigger level of this signal isvariable and it can be made to operate a variety of mechanical gates and flawwarnings. Furthermore, improvements in computer technology allow test data andresults to be displayed and out-putted in a wide variety of formats.Modern ultrasonic flaw detectors are fully solid state and can be battery powered,and are robustly built to withstand site conditions.
Since the velocity of sound in any material is characteristic of that material, it follows
that some materials can be identified by the determination of the velocity. This can
be applied, for example in S.G. cast irons to determine the percentage of graphite
nodularity.
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