The limitation and reliability of Ultrasonic flaw detection technique

Due to acoustician electronics engineer and flaw detector staff have worked for fifty years,make ultrasonic flaw detection technique based on principle of solid radar developed as a main method of nondestructive inspection。the published nondestructive books、information by the whole world ,at home and abroad nondestructive detection academic annual meeting ,ultrasonic detection takes up the first position，this represents ultrasonic detection has achieved brilliant results on engineering application, and keep improving,but it has little reports on limitation and reliability,so it generates a fake  appearance,looked like ultrasonic detection is already perfect，once exists products quality accidents,it will be thought that operators technique level is low,and then destroy the first production line workers’ enthusiasm,but summary and prevention from similar accidents has little improvement on promoting  ultrasonic technique 

1.Reliable “accident ”in Ultrasonic detection practice

1.1  the failure of “accident ” (1) ultrasonic flaw detector displays obvious flaw wave,dissect physical objectivity will find nothing,such as ,in 1964 the writer has ultrasonic detection on aluminum alloy  special shaped profile ,found there was a obvious flaw wave,and instantly detected and corrosion at the same time,each time planed 0.5mm,as a result ,nothing flaw had been found。(2) ultrasonic flaw detection according to standard to judge small flaw with flat bottom hole is   

Enormous,such as :in1979 detection,found there were some flaws,and exposed a length of 2mm tiny crack,used 2mm width bamboo split piece vertically insert this hole can up to 12mm,inclined insert can up to 23mm,used ultrasonic straight and inclined probe to  retest  and only get flaw wave with 2mm,however it is a enormous hole that ultrasonic will never detect successfully。

1.2 the “accident ”of science adventure successfully aim at exposed amount of flaw in ultrasonic detection qualified electric generator 

Observation and  statistics analysis and experiment,and found some group of clutter,when the quantify is lower than departmental standard(2mm flaw bottom hole) ,but according to accumulated experience ,it must find vast reflectivity of stripe,so refuse it strongly,and return the 12 large forgings to manufacturer in the end of 1979 。And asked them to put middle hole ,and had a deal that if no strips,and returned instantly,or  return the goods,as a result ,even one had been taken back。

2.  Ultrasonic flaw detection technique its limitation 。Ultrasonic flaw detection is take advantage of solid echo--elastic wave reflection to judge flaw and position in workpiece，

According to echo energy to estimate the size of flaw ,and use some skills of detecting technology  and then according to orderliness and experience of production technology to judge the properties of flaw,like above said,apparent,due To universal ultrasonic detector only detect three physical quantity;

1.  Elastic pulse travel time

2. Echo energy :

Decreasing degree of Mechanical oscillation passing some distance:relative value。However,we know nothing about flaw in workpiece:size,shape,depth，inclined angle,surface roughness，internal filler,whether they are big flaw or many small compacted flaws,unknown quantity has exceeded three。So from math to see,ultrasonic flaw detection is facing a unknown problem。Secondly,currently,acoustic theory basic research of ultrasonic flaw detection is stopping at the problem  that few regular flaw in solid reflect ultrasonic 、crystal in solid has scattering to ultrasonic，still solving what kinds of flaw can scatter whet kinds of ultrasonic，but the most important is solve ultrasonic flaw detection；；according to scatter condition can infer what kind of flaw is existing in workpiece，this is another problem that can not get a unique answer，we can know from experiment ,different shapes size inclined degree surface roughness can generate same height echo,so just based on echo height can not distinguish these flaw,and ,the questions needed to be solved by     ultrasonic flaw detection is more complex harder than parent technique--antiaircraft radar，because latter is only pursuing one goal that is spot shape flaw,but the former has spot shape,line shape,plane shape,volume shape and dispersive  flaw,to sum up,present ultrasonic flaw detection technique is still exist limitation,namely can not unique to make sure the detecting results,such as :: Poland knows from drendury tube,choose freely 32 full automatic welding joint and 47 semi-automatic welding joint,compare to ultrasonic and ray detecting result from the welding joint with same length,照Speaking to the number of flaw,discovered flaw in welding joint is ,discovered flaw by ultrasonic in semiautomatic is 71%,discovered by ray photographic method is 29%,in semiautomatic welding joint ,discovered by ultrasonic is 74%,  discovered by ray photographic is 26%,the flaw detected by ultrasonic mostly distributed at 90 and 270 polar coordinates,these flaw mostly are crack and incomplete fusion ,but detected by ray are mostly at the position between 150  and  300,most of flaws are ball shape volume flaw,this result expresses that use only one method to get a result is not complete,but for internal flaws in big shape machine,can only use ultrasonic method to do nondestructive flaw detection。

3. The reliability of Ultrasonic flaw detection technique is due to   ultrasonic itself exists limitation,so its reliability of course can not reach up to ideal ,actually any of practical detecting result is different from ideal requirements,no matter manual operation or automatic detection are not exceptional,the attached image appears this difference，from the image ,practical detection is always has leakage detection or false detection,leakage detection and false detection is depended on controlling level on affecting factors, but anyway ,the leakage detection and false detection are not avoidable,recently,there are 600 electronics companies in America subordinate ten thousand mainly use ultrasonic to detect controlling machine,  So that make sure normal operation of power plant, and ultrasonic inspection can up to 85%,for this technique that originally thought as unreliable and not visual ,can up to this level of detecting reliability,and scholars are very satisfied。 Flaw detecting staffs should comply with ,of not convenient to stipulate percentage,at least point that it can not up to 。 Due to historical reasons, production units at grass-roots level in our country 。Always think ultrasonic flaw detection that just control quality on important part  as high and new technique，commonly thought its reliability should be ,so no matter what errors are not allowed。For example, a factory in 18 years that just has two years that did not up to ,one year is 99.96%,  another year is 99.93%,even so,flaw detection staffs still been punished、 accused 。But actually ultrasonic flaw detection technique is the best to detect flaw position，for shape and tropism ,it just has medium ability of udgement,and is bad at detecting on properties and size,so it is necessary to explain to the public ,The real reliability and surface appearance reliability of ultrasonic flaw  detection。The formal can not up to ,the latter is possible to reach and up to this index under certain conditions。

4.  These advice is aim at reducing limitation of ultrasonic flaw         detection， improving its detecting reliability,maybe do the follows jobs  :（1）

Strength basic theory research,looking for new breakthrough。Summary 、    research formation mechanism of regular flaw and occasional flaw from     observing 、accumulating and sorting, Mechanics  、acoustics  features、properties and reactions to every ultrasonic beam，especially strengthen research on mechanics and acoustics, try best to find new mechanics and acoustics law and theory as physical base of brand new ultrasonic flaw detector，and some  electronics methods that realize it,(2)  research and develop new ultrasonic flaw detector,according to new discovery new rules of mechanics acoustics to develop new ultrasonic flaw detector 、energy converter and  accessory auxiliary device, reference from eddy current flaw detector, extend current ultrasonic into double frequency 、multiple frequency....,and take advantage of phase position frequency spectrum to improve ultrasonic detecting physical quantity,so as to get more evidence to judge flaw, improve current ultrasonic flaw detector design，make it equipped with some experience and techniques of present flaw detection technology (3) promote exchange and technique discussion of extended flaw detection staffs.  On the basis of extended gather successful experience and failure lessons,build the whole country ultrasonic flaw detection  practical database,and make into a  variety of software so as to be used by the ultrasonic flaw detecting staffs in forefront of production .(4) keep updating and revising standard and specification of every ultrasonic flaw detection ,make is more correspond to current state and development of practical production,especially add some key and reliable content in it 。(5)in every kind of training  material and magazines ,firstly introduce according to facts the limitation and reliability of current ultrasonic flaw detection principle,and admit the quantify and qualitative of current ultrasonic flaw detection technique to flaw are still quite incompetent，welcome and encourage aspiring young people keep discovering and finding new breakthrough,make the limitation of ultrasonic flaw detection technique gradually reduce ,and improving reliability。