Langstrecken-Ultraschalluntersuchung mit geführten Wellen

2011-01-25T13:40:47Z

Robyvecchio: unref tag

{{unreferenced}}

[[Image:UT vs GWT.jpg|thumb|right|400px|This illustrates the
difference in concept between conventional UT and guided wave
testing (GWT).]]

'''Guided Wave testing (GWT)''' is one of latest methods in the field of
[[Nondestructive_testing|non-destructive evaluation]]. The method
employs mechanical stress waves that propagate along an elongated
structure while guided by its boundaries. This allows the
waves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen many
engineering structures, particularly for the inspection
of metallic [[Pipeline_transport|pipelines]] around the world. In
some cases, hundreds of meters can be inspected from a single
location. There are also some applications for inspecting
[[Rail_track|rail tracks]], rods and metal plate structures.

Although Guided wave testing is also commonly known as Guided Wave
Ultrasonic Testing (GWUT) or Long Range Ultrasonic Testing (LRUT),
it is fundamentally very different to
[[Ultrasonic_testing|conventional ultrasonic testing]]. Guided wave
testing uses very low [[Ultrasound|ultrasonic]] frequencies compared
to those used in conventional UT, typically between 10~100kHz.
Higher frequencies can be used in some cases, but detection range is significantly reduced. In addition, the underlying physics of guided waves is more
complex than bulk waves. Much of the theoretical background has
been addressed in a separate [[Waveguide|article]]. In this
article, the practical aspect of GWT will be discussed.

== History ==

The study of guided waves propagating in a structure can be
traced back to as early as the 1920s, mainly inspired by the field
of seismology. Since then, there has been an increased effort on
the analytical study of guided wave propagation in cylindrical
structures. It was only in the early 1990s that guided wave testing was
considered as a practical method for the
[[Nondestructive_testing|non-destructive testing]] of engineering
structures. Today, GWT is applied as an integrated health
monitoring program in the oil, gas and chemical industries.

== How it works (Pipeline Inspections) ==

[[Image:Guided wave testing GWT.jpg|thumb|right|300px|An example of
pipeline inspection using guided wave testing (GWT). Mechanical stress wave is generated via transducer array mounted around the pipe surface. The electrical signal is driven by the portable electronic unit. After the collection, the result is displayed on the computer for further analysis.]]

[[Image:Typical GWT result.jpg|thumb|right|300px|A typical example of the GWT data showing both the A-scan type (bottom) and the C-scan type (top) results. The green band indicates the position of the transducer array.]]

Unlike conventional ultrasonics, there are an infinite number of
guided wave modes that exist for a pipe geometry, and they can be
generally grouped into three families, namely the torsional,
longitudinal and flexural modes. The acoustic properties of these
wave modes are a function of the pipe geometry, the material and the
frequency. Predicting these
properties of the wave modes often relies on heavy mathematical modeling which are typically
presented in graphical plots called [[Dispersion_relation|Dispersion]]
curves.

In Guided Wave Testing of pipelines, an array of low frequency
transducers is attached around the circumference of the pipe to generate
an axially symmetric wave that propagate along the pipe in both the forward and backward
directions of the transducer array. The Torsional wave mode is most commonly
used, although there is limited use of the longitudinal mode. The equipment operates in a pulse-echo
configuration where the array of transducers is used for both the
excitation and detection of the signals.

At location where there is a change of cross-section or a change in
local stiffness of the pipe, an echo is generated. Based on the
arrival time of the echoes, and the predicted speed of the wave mode at a
particular frequency, the distance of a feature in relation to the
position of the transducer array can be accurately calculated. GWT
uses a system of distance amplitude curves (DAC) to correct for
attenuation and amplitude drops when estimating the cross-section
change (CSC) from a reflection at a certain distance. The DACs are
usually calibrated against a series of echoes with known signal
amplitude such as weld echoes.

Once the DAC levels are set, the signal amplitude correlates well to the CSC of a defect. GWT does not measure the
remaining wall thickness directly, but it is possible to group the
defect severity in several categories. One method of doing this is
to exploit the mode conversion phenomenon of the excitation signal
where some energy of the axially symmetric wave mode is converted to
the flexural modes at a pipe feature. The
amount of mode conversion provides an accurate estimate of the
circumferential extent of the defect, and together with the CSC,
operators could establish the severity category.

A typical result of GWT is displayed in an A-scan style with the
reflection amplitude against the distance from the transducer array position.
In the past few years, some advanced systems have started to offer
C-scan type results where the orientation of each feature can
be easily interpreted. This has shown to be extremely useful when
inspecting large size pipelines.

==Features==
===Advantages===
#100% Volumetric coverage - full coverage within the inspection range.
#Long range inspection - potential to achieve hundreds of meters of inspection range.
#Limited access - insulated line with minimal insulation removal, corrosion under supports without need for lifting, inspection at elevated locations with minimal need for scaffolding, and inspection of road crossings and buried pipes.
#Data is fully recorded.
#Fully automated data collection protocols.

===Disadvantages===
#Interpretation of data is highly operator dependent.
#Difficult to find small pitting defects.
#Not very effective at inspecting areas close to accessories.
#Needs good procedure

[[Category:Nondestructive testing]]
[[Category:Guided Wave testing]]

Operation Clean-up

2010-02-10T11:22:00Z

Robyvecchio: Removed category Military operations; Quick-adding category Military operations involving Pakistan (using HotCat)

{{nofootnotes|date=April 2009}}
'''Operation Clean-up''' (also known as '''Operation Blue Fox''') was a [[military operation]] in [[Karachi]], [[Pakistan]] from 1992-1994 that sought to 'cleanse' the city of 'anti-social' elements. The operation initiated on June 19, 1992. The main target of the operation was the [[Mohajir Qaumi Movement]] (now renamed [[Muttahida Qaumi Movement]]).<ref name="abdul">{{cite news|url=http://www.jang.com.pk/thenews/spedition/sp_news15/p32.htm|title= Chronology of the Events (1991-2006) |last=Qadir|first=Abdul|publisher=Jang|accessdate=2009-08-08}}</ref> The operation took place during [[Nawaz Sharif]]'s first term as the [[Prime Minister of Pakistan]].<ref name="AMN">{{cite web|url=http://web.amnesty.org/library/Index/ENGASA330011996?open&of=ENG-PAK|title=Pakistan: Human rights crisis in Karachi|date=1996-02-01|publisher=Amnesty International|accessdate=2009-07-26}}</ref> The period is regarded as the bloodiest period in Karachi's history, with thousands killed or gone missing in the fighting.

On May 19 1992, Prime Minister [[Nawaz Sharif]] met Sindh CM [[Muzaffar Hussain Shah]], Home Minister [[Chaudhry Shujaat Hussain]], COAS General [[Asif Nawaz]], Sindh Corps Commander General Naseer Akhtar and other officials at GHQ Rawalpindi to decide on the modalities of Operation Clean-up in Sindh; operation to be carried out by the Rangers and Mehran Force with full backing by the Army.<ref>http://www.pakistanihistory.com/92.htm</ref>

Among the reasons given for the launching of the operation were the [[Jinnahpur]] affair and the [[Major Kaleem Case]]<ref>[http://thenews.com.pk/top_story_detail.asp?Id=24255 MQM shifts blame for 1992 operation from military to Nawaz], [[The News (Pakistan)]], 2 September 2009</ref>.

==References==
{{reflist}}

==See also==
*[[Jinnahpur]]

==External links==
* [http://www.scribd.com/doc/16399946/Rise-of-the-MQM-in-Pakistan-Politics-of-Ethnic-Mobilization Rise of the MQM in Pakistan: Politics of Ethnic Mobilization], Farhat Haq, [[Asian Survey]], Vol. 35, No. 11 (Nov., 1995), pp. 990-1004, [[University of California Press]]
* [http://www.scribd.com/doc/16400153/Ethnicity-and-State-Power-in-Pakistan-The-Karachi-Crisis Ethnicity and State Power in Pakistan: The Karachi Crisis], Moonis Ahmar, [[Asian Survey]], Vol. 36, No. 10 (Oct., 1996), pp. 1031-1048, Published by: [[University of California Press]]
* [http://books.google.com/books?id=Q9sI_Y2CKAcC&pg=PA35 A History of Pakistan and its Origins], pg 35, Christophe Jaffrelot
* [http://www.bitsonline.net/eqbal/articles_by_eqbal_view.asp?id=35&cid=7 Beyond this Battle of Karachi], [[Eqbal Ahmed]], [[Dawn (newspaper)|Dawn]], 17 August 1995

{{pakistan-stub}}

[[Category:History of Pakistan]]
[[Category:History of Sindh]]
[[Category:Muhajir people]]
[[Category:Political repressions in Pakistan]]
[[Category:Karachi District]]
[[Category:Military operations involving Pakistan]]

Wikipedia - Benutzerbeiträge [de]

Langstrecken-Ultraschalluntersuchung mit geführten Wellen

Operation Clean-up