2 IntroductionHigh-frequency sound waves are sent out at a material to find material changesA pulser produces an electrical pulse that causes a piezoelectric transducer to send out a sound waveReflected waves are transformed back into electrical signals by the transducer and analyzedIts main applications are in thickness gauging and flaw detection
3 BackgroundOriginated from sonar technology, which was developed just before World War IISonar uses technique of bouncing echoes off of submerged objects to detect themUltrasonics, a form of nondestructive testing, was applied to “safe life” design, which ensures that structures don’t develop macroscopic defects during its life, and any detection led to its removal
4 Background (cont’d)In the early 1970’s, ultrasonic testing had made large strides and could detect extremely small defects or discontinuities in metalsAn unforeseen downside was that many manufacturers were now discarding more metal parts that were deemed satisfactory earlierThis in turn led to fracture mechanics, which allowed the determination of whether “a crack of a given size would fail under a certain load if the fracture toughness were known (ndt-ed.org).”Other laws were developed to predict the rate of growth of cracks under cyclic loading
5 Background (cont’d)Fracture mechanics allowed for the concept of fail safe design, which stated that structures could have defects as long as they would not grow to cause failure.Over the past few years, ultrasonic testing has become more popular due to advances in both computer and information technologyFor example,
6 Theory and TechniqueHigh frequency sound energy is used to find such things as material flaws and dimensional measurementItems needed to run an ultrasonic test:Pulser/receiver unitPiezoelectric transducerDisplay devices
7 The Role of Each DeviceA pulser/receiver creates a high voltage electrical pulse, which is sent throught the material in the form of propagating sound waves by the transducerAny discontinuity or flaw in the material will cause some energy to reflect backThe reflected waves are converted into an electrical signal by the transducer and amplified by the receiver for signal processing
8 The Role of Each Device (cont’d) The amplified electrical signal is then displayed on a screenThe reflected signal strength is often displayed vs. time b/w signal generation and echo receptionD= vt/2 for normal beam inspection of discontinuities, different for angle beamsThis can lead to info on the flaw’s size, location, and orientation among other things
9 Picture Gallery Transducer Test Specimen Pulser/Receiver Display Device
10 ProsAdvantagesSensitive to both surface and subsurface discontinuitiesPenetration depth is better than other NDT methodsWith pulse-echo, access to only one side is neededHighly accurate in regards to reflector size, shape, and locationMinimal part preparation
11 Cons Disadvantages Surface must be accessible to transmit ultrasound More training required relative to other methodsCoupling medium is normally required to promote transfer of soundHas difficulty inspecting rough, small, or irregularly shaped objectsLinear defects parallel to sound beam may go undetected
12 Piezoelectric Transducers PT’s contain polarized materialWhen electric charge is applied, dipoles are induced and dimensions changeIf a force is placed on the material, it will change dimensions and create an electric field
13 The Piezoelectric Effect Crystal material at rest: No forces applied,so net current flow is 0CrystalCurrent Meter= 0Charges canceleach other, sono current flow
14 The Piezoelectric Effect CrystalCrystal material with forces appliedin direction of arrows………..ForceCurrent Meterdeflects in +directionDue to properties of symmetry,charges are net + on one side &net - on the opposite side: crystal getsthinner and longer
15 The Piezoelectric Effect Changing the direction of theapplied force………..CrystalForceCurrent Meterdeflects in -direction…. Changes the direction ofcurrent flow, and the crystal getsshorter and fatter.
16 The electromechanical effect When the switch is closed, and you apply the exact amountof power to get the same current that resulted when you squeezedthe crystal, the crystal should deform by the same amount!!Crystal+ sidepower source(battery)- side…. and, the crystal should get shorter and fatter.
17 Transducer types Contact Immersion Direct contact w/ specimen Rugged casing and platesUses coupling material to remove air gapsImmersionNon-contactOperates in liquid