Non-destructive Testing Of Mechanical Strength Of Aerospace Alloy By Laser Induced Ultrasound And Thermal Infrared Radiometry

In the field of aerospace and astronautic industry,the safety of aircraft is regarded as an important issue worldwide.Strictly high standard of aircraft-building material in terms of its integrity and mechanical performance requires the implementation of NonDestructive Testing(NDT)techniques of various kinds.In most cases,NDT focuses on surface/internal fatigue or crack resulting from overloading,multiple cyclic loading or strong impact.However,at the very early stage of defect formation,miniscule but detectable changes have occured in the material which should be taken as potentially hazardous.This research focuses on the mechanical strength evaluation of light-weight aerospace aluminum alloy in terms of the stress-strain relation which quantitatively reflects its mechanical performance throughout the entire mechanical history from stress free to fracture.Two photoacoustic and photothermal based methodologies,i.e.Frequency Domain Laser Ultrasound(FDLU)and Laser Infrared Radiometry(LIR)were implemented to analyze the mechanical properties of the aluminum alloy.The FDLU method is an upgraded version of conventional ultrasonic testing.It uses harmonic-modulated laser ultrasound excitation and lock-in detection and thus enjoys the advantage of effectiveness and nonrigid-contact by photoacoustic coupling.Basically the elastic wave velocity of alloy may vary with different stress-strain state,which reslts in the change of FDLU amplitude and phase signal.In the first place,a laboratory-based tensile rig was designed and fabricated to fix the sample and provide with uniaxial tensile loading large enough to ultimately break the aluminum sample.The rig accommodates a water tank for ultrasound coupling purpose.An adhesive foil strain gauge was collaborated with the rig providing independent strain readings of the sample.Finite Element Method(FEM)was used to calculate the strain elastic limit of the tested sample by referring to the American Society for Testing Material(ASTM)standard.The strain elastic limit value was an important reference for the subsequent experiments.A series of FDLU tests were conducted when the sample undertook cyclic loading and unloading within the elastic regime.Being consistent with the linear stress-strain relation,FDLU amplitude and phase signal varied with strain in a reproducibile and reversibile pattern.In the next stage,when the sample undertook plastic deformation and finally fractured,the phase versus strain plot within the entire mechanical history showed a inflection point which was coincide with the elastic limit value from FEM calculation.The strain dependence of FDLU signal was attributed to two factors,the surface displacement from Poisson's effect and the change of elastic wave velocity.A thoeretical analysis based on General Hooke's Law was then performed to interpret the contribution from the two factors.The results show that the material exhibits elastic anisotropy when subjected to initial loading,FDLU signal thus changes accordingly.However,the vertical Poisson displacement contributes an additional signal change which makes quantification difficult by FDLU.In order to overcome the unfavorable sensitivity of surface displacement of FDLU signal,another photothermal detecting methodology is proposed.Based on the fact that thermal diffusion property changes with external stress,LIR is able to capture this subtle variation of thermal property and therefore evaluate the stress strain state undertaken.Expanded beam and single-point infrared detector was adopted to sufficiently suppress the influence of deformation.Within the elastic regime,two testing modalities,frequency scan under different loading condition and strain scan at constant frequency,were conducted in experiment.The data were fitted with a single-solid-layer thermal wave theoretical model and thermal effusivity and diffusivity were extracted.The result shows thermal conductivity is soly responsible for the change of both LIR amplitude and phase,which conforms well to the theoretical prediction.Then single frequency strain scan test was conducted throughout the entire mechanical history of the sample.The fitted dimensional-thermal parameter(thickness over square root of thermal diffusivity)exhibits excellent analogy with conventional stress-strain relation.LIR was proved to be able to work properply as a stress/strain gauge which provides both qualitative and quantitative evaluation of thermal-mechanical performance with non-contact feature.LIR was also validated for being able to assess the mechanical strength of NiCo nanocoated composite samples.A three-solid-layer thermal diffusion model was developed to perform respective thermal property extraction of both coating layer and substrate.The experimental data fitted by multiple-layer model indicates strain dependence thermal diffusivities of different layers.Such thermal-mechanical parameter can serve as a quantitative indicator of the extra strength provided by NiCo coating.In the last research,mid-infrared camera was introduced to work as the detector thus forms the Laser Infrared Lockin Thermography(LILT)technique.Focused laser beam was harmonically modulated in this configuration to improve the signal-to-noise ratio and the output signal was obtained as dynamic lock-in thermal wave amplitude and phase images.During the test,the sample was snapshotted by LILT at various stress-strain state throughout its entire mechanical history.A corresponding three-dimensional laterally infinite thin plate with thermal anisotropy was considered to analyze the images.Both theoretical and experimental phase images were processed with elliptical contour fitting and characeterized by the change of the ratios of axis length.The thermal anisotropy indicator was introduced to depict the stress-strain behavior of elastic and plastic deformed material throughout its entire mechanical history.In conclusion,this research explored the application of FDLU and LIR technqiues in mechanical strength evaluation of aerospacial aluminum alloys.FDLU method shows the advantage of high sensitivity and effectiveness which is more potential in detecting obvious defects such as crack,porosity and hole.For the case of stress-strain analysis,FDLU has the inherent deficiency of coupling requirement and spurious sensitivity to displacement that is not an optimal approach.On the contrary,LIR/LILT materializes mechanical strength evaluation by quantitatively determine the stress/strain dependence thermal physical property of the sample in a completely noncontact way,is very promising in early residual stress and fatigue inspection prior to the appearance of conspicuous defects.The muturally complementary features of the two methodologies make both of them potentially applicable in aerospace industry.