1、Journal of Nondestructive Evaluation,Vol.20,No.3,September 2001(q 2001)Observation of Changes in Magnetic Images Due to a Strainor Fatigue Progress in Austenite Stainless Steels using aScanning Hall-Sensor MicroscopeA.Oota,1,*K.Miyake,1D.Sugiyama,2and H.Aoki2Received September 14,1999;Revised Septem
2、ber 15,2001Using a scanning Hall-sensor microscope with an active Hall element of area 50mm 3 50mm,wemeasured two-dimensional magnetic images of spontaneous magnetization on a surface of a numberof 304 stainless plates in a paramagnetic austenite phase.Stainless plates with a yield point of 31kg/mm2
3、were subjected to a strain or pulling-fatigue process at room temperature.In a strain state,most plates show the same progressive change in magnetic images resulting from a strain-inducedmartensite transformation,but others produce additional and/or different change although the originremains unsolv
4、ed.A pulling-fatigue process at a stress amplitude of 28 kg/mm2also produces somedifferences in the images from the strain process,at the early stage of fatigue progress.The studyshould provide a new horizon to elucidate the complex destructive-progress in strain or fatigueprocess of304 stainlessste
5、els that areused asstructural materials inelectric powerstations,chemicalplants and medical equipment.KEYWORDS:ScanningHall-sensormicroscope;magneticimages;spontaneousmagnetization;non-destructiveevaluation;stainless steel;austenite phase;strain-induced martensite transformation;fatigue process.tum
6、interference devices(SQUID)(2,3)and micro-HallI.INTRODUCTIONsensors(5,6)provides new and promising techniques forNDE of magnetic materials,because of many advantagesInvoking of social securities and environmentalover conventional techniques such as eddy current,ultra-problems demands higher levels o
7、f reliability and stabil-sound and x-ray imaging,etc.Magnetic imaging tech-ity for structural materials such as iron and stainlessnique using a SQUID has enabled a non-destructivesteels supporting infra-structuring in industries.Muchdetection of Lu ders bands in mild steels subjected toeffort has be
8、en made to develop non-destructive evalua-a strain process.(3)A scanning Hall-sensor microscopetion(NDE)techniquesto investigatethe destructivepro-(SHM)with an active Hall element of area 50mm 3gress under fatigue and/or strain processes in struc-50mm has also enabled a magnetic detection of smalltu
9、ral materials.(14)cracks(,10 mm long and,0.1 mm wide)in mild steelsRecent progress in magnetometers utilizing highly-caused by a fatigue process,(6)so that SHM can be usedsensitivemagneticsensorssuchassuperconductingquan-as a simple,economic and conventional tool for NDE ofmagnetic materials.We pres
10、ent here the magnetic imagesof spontaneous magnetization on a surface of 304 stain-*Corresponding author.E-mail:ootaeee.tut.ac.jpless steels subjected to strain or fatigue process at room1Toyohashi University of Technology,Tempaku-cho,Toyohashi,Aichitemperature observed using this microscope,and the
11、441-8580,Japan.influence of the destructive progress on the images of2Technical Research and Development Laboratories,Topy IndustriesLtd.,Akemi-cho,Toyohashi 441-8510,Japan.304 stainless steels.870195-9298/01/0900-0087/0 q 2001 Plenum Publishing Corporation88Oota,Miyake,Sugiyama,and Aoki2.EXPERIMENT
12、ALprogressive way.Measurements were repeated afterobserving the samples to higher levels and reducing theload to zero.In addition,a number of the plates A were2.1 Stainless Platessubjected to a pulling-fatigue test using a conventionalhydraulic-servo fatigue testing machine at a stress ampli-Commerc
13、ial 304 stainless steels with a yield pointof 31 kg/mm2and a tensile strength of 68 kg/mm2weretude of 28 kg/mm2and a frequency of 29.2 Hz,to eluci-date a destructive progress under a fatigue process.machinedintoa3.8mmthickplate.Theyhavethefollow-ing chemical composition except for iron:Cr,18.2 wt%;M
14、agnetic measurements were made as a parameter ofstress cycles in a progressive manner by stopping theNi,8.3 wt%;C,0.05 wt%;Si,0.43 wt%;Mn,0.89 wt%;P,0.03 wt%;S,0.05 wt%.As can be seen from Fig.machine and taking away the plate,until micro-cracksappeared in the sample.1,two types of stainless plates
15、A and B with differentgeometries were used for the study.There are notches onboth sides in the former,but not in the latter.2.2 MeasurementsToinduceaplasticdeformationatroomtemperature,Magnetic images of spontaneous magnetizationanumberofstainlessplatesoftypesAandBwereuniaxi-were measured on a surfa
16、ce of stainless plates under zeroally strained using a conventional Amsler-type testingexternal fields using SHM with an active area 50mm 3machine at a strain rate of approximately 0.001 s21along50 mm,of which the details have been published else-the length of the sample.Special attention was paid towhere.(6)The SHM was equipped with a micro-Hall sen-the environment around the machine so as not to magne-sor on a movable x-y stage using a stepping motor andtize the sample during the test.After re
