Ultrasonic energy distribution in aluminium single crystals

INTRODUCTIONInvestigation of acoustic fields and elastic energy distribution is one of the most effectivemethods for defect detection and visualisation. This task of field investigation is now routinein ultrasonic testing but has special features if the materials under study are anisotropic.The elastic anisotropy of single crystals leads to a number of effects complicating theinterpretation of the results. For example, if the propagation direction deflects away from thecrystallographic axis, the elastic waves spread as two or even three eigenmodes wherethe directions of the phase and ray velocities do not coincide with one another. The influenceof elastic anisotropy manifests itself even if the beam’s axis coincides with a high-symmetrydirection in the crystal. The lateral rays of the beam in this case are inclined in the relation tothe crystallographic axis. The polarisation of these rays differs from the polarisation of thecentralpartofthebeam.Theelasticenergydistributionhastoreflectthemedium’sanisotropy.The specific phenomena occur if the waves propagate near the acoustic axis and if the normalplanetothewavevectorisnottheplaneofmirrorsymmetry[1].Theinnerdefectvisualisationunder crystallographic elastic symmetry conditions has a number of peculiarities.Thestudyofthedisplacementdistributionofelasticwavespropagatinginaluminiumsinglecrystalsisthepurposeofthispaper.Aluminiumischosenastheobjectofinvestigationbecauseits elastic properties are well known, and in addition high-purity, high quality single crystalsare available. The effect of small beam deflection from the crystallographic axis is studied bythe laser interferometric method [2,3] and the experiments have been carried out when thewavespropagatealongboththe[001]and[111]axesaswellalongdirectionsclosetothe[111]axis.Ofparticularinterestistheshearwaveenergydistributioninconnection withtheinternalconical refraction (ICR) effect [4] if the wave propagates along [111] axis in a cubic crystal.Beyond that point the distortion of the elastic energy distribution caused by the inner defectexposed at the crystal surface has been studied.Theemploymentofthelaserdetectionmethod issuitableforthepurposesmentioned abovesinceitallowstheimagingoftheenergydistributionthroughthescanningprocess.Theresultsof the scanning process are strikingly illustrated. Special attention should be paid to thecircumstance where the output signal of the laser interferometer is mostly sensitive to out of