Analysis Of Ballistic Impacts On Composite Materials By .
The e-Journal of Nondestructive Testing - ISSN 1435-4934 - www.ndt.netMore info about this article: http://www.ndt.net/?id 25674APPLICATION NOTEAnalysis of Ballistic Impacts on Composite Materials by InfraredActive Thermography.In this study, we investigated the assessment of the damaged area on composites ballistic plates subjected to high velocityimpact. The active pulsed thermography technique was used for performing post-mortem analysis of the impactedspecimens. Quantitative analysis of the damaged areas shows consistent results with the size of the projectile suggestinghigh precision of the quantification done in this work. This quantitative defect analysis combined with knowledge ofprojectile velocity allows for characterization of absorbed energy and differentiation of generated defect types. This allowfor the evaluation of material efficiency in spreading absorbed energy over large areas. Our observations indicate thathigh velocity shots tend to induce smaller impact damage areas characterized primarily by fiber breakage, while lowvelocity shots tend to induce larger impact damage areas featuring predominantly delamination and matrix crackingdamage mechanisms.IntroductionSince the beginning of the last century, ongoing advancesin materials engineering have led to an unrestraineddevelopment of new technologies. Composite materialsare among those materials attracting the most attentiondue to significant advantages over their homogeneouscounterparts. These advantages include high specificstiffness and high specific strength combined with asignificant reduction in weight, making compositematerials attractive for many industrial applications. Oneof the most important fields of application is the defenseindustry where composite material properties such aslow weight, rigidity, strength and durability are of keyimportance. Composite materials made from artificiallyobtained high-strength fibers are particularly interestingfor high-performance defense applications includingballistic protection applications.Analysis of the extent of damage caused to a ballisticplate post-impact is of paramount importance for theresearch, development, and evaluation of compositeprotection equipment. Infrared thermography testmethods are particularly effective for post-impactcharacterization of composite materials as they allow fornon-destructive evaluation of damage to the internalstructure of the composite material.This work discusses the assessment of damaged areas oncomposite ballistic plates subjected to high velocityimpact using Infrared active thermography techniques.Quantitative defect size and depth analysis werecorrelated with projectile velocity to characterizematerial efficiency in spreading absorbed energy overlarge areas.TelopsActiveSolutions.ThermographyTelops has recently developed an Infrared ActiveThermography Solution called TESTD. The TESTD seriesoffer non-destructive testing (NDT) solutions for theevaluation of components or assemblies for subsurfacedefect detection without material damage. TESTDcombines high-capability thermal infrared cameras withdifferent external excitation sources and a user-friendlypost-processing software. Optical excitation sourcessuch as flash and halogen lamps or lasers are available,along with electromagnetic and mechanical sourcesincluding inductive coils and ultrasound generators.Telops designs a wide range of solutions from basicsystems to compact integrated systems with a variablelevel of automation depending on project needs andconstraints.High-Speed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops1
APPLICATION NOTEFigure 1 Telops TESTD basic (top) and compact (bottom)systems.Figure 2 Composite Ballistic sample used for NDTmeasurements (top) and shots description and projectilespeed (bottom).Time Dependent Results.Composite Ballistic Sample.The test sample used for experiments was designed forballistic protection applications. The sample consists of acarbon fiber-nylon composite panel with an areal densityof 5.45 kg/m2 and a matrix content around 35%. Thespecimen features a 0-90 weave and has a crosssectional thickness of approximately 5 mm. The ballisticimpacts on the specimen shown in Figure 2 are classifiedinto 3 categories: Perforation generated at highprojectile speed (green), partial perforation (red), andbouncing generated at low projectile speed (yellow).Theshots marked with white squares in Figure 2 top panelwere not analyzed in this work because the projectilespeed was not measured during the ballistic test.The TESTD pulsed thermography (PT) system was usedfor the following analysis in reflection mode. The samplewas submitted to the thermal pulse generated by theflash lamp which generated non-stationary heat flowwithin the sample. Figure 3 despites the thermal infraredradiation emitted by the specimen during the coolingprocess. The emitted radiation was measured by a TelopsFast IR camera as a function of time.High-Speed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops2
APPLICATION NOTEdensity. For the investigated composite sample, thedefective area is a mix of lower fiber density regions withopen air gaps. The thermal effusivity is lower in thoseregions compared to non-defective areas. Therefore, thenon-defective area is expected to dissipate heat moreefficiently than the defective area, which is consistentwith the experimental observation.For analysis, data are transformed from the time domainto the frequency spectra using the dimensional discreteFourier transform (DFT). This analysis technique appliedto pulsed thermography is referred to as Pulse-PhaseThermography (PPT).Data AnalysisTelops TESTD line of active thermography products isdelivered with a full-featured data analysis packagecalled Reveal Lab. This software provides an optimizedworkflow to utilize a wide range of powerful functions toanalyze and post-process images and image sequences.Discrete Fourier transform image processing is used tocompute the amplitude and the phase images (Figure 4).Figure 3: Thermal map of the specimen recorded after flashexcitation (upper panel); and transient temperature curve ofa defective (red) and non defective (blue) region of interest(lower panel).In this work, the damage induced by the ballistic shots onthe composite plate are categorized as defective area.The temperature profile for a defective and nondefective area show continuous non-periodical signaldecays as shown in the lower panel of Figure 3. Fewseconds after the flash excitation when the heatgenerated by the flash is still uniform at the surface ofthe specimen both areas behave similarly. Severalseconds later, higher temperatures were measured inthe defective zones compared to the non-defective area.This behavior can be explained by consideringdifferences in thermal effusivity between defective andnon-damaged areas. Thermal effusivity is a materialproperty describing its ability to exchange heat with itssurroundings. Thermal effusivity increases with materialFigure 4: Workflow of the pulsed-phase-thermographytechnique used by Reveal Lab software and example ofamplitude and the phase images computed by the software.Phase images are particularly interesting for NDT dataanalysis because they are less affected by environmentalreflections, emissivity variations, non-uniform heating,surface geometry and orientation. This is very importantfor quantitative defects characterization of materials.High-Speed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops3
APPLICATION NOTEDefect Type.Figure 5 depicts a phase image of the impacted ballisticcomposite sample. The defects are identified by thecontrast in the image resulting from their relativelocation in the material volume.expression z C1. C1. ( / f)0.5 where C1 is an empiricalconstant. It has been observed that, C1 1 when usingamplitude data [1], while C1 1.82 is typically adoptedwhen working with phase data [2,3]. Figure 6 shows thedepth range for the investigated composite sample as afunction of the optical excitation frequency. Lowerfrequencies allow inspection of deeper defects into thematerial.Figure 5: Phase image with different type of defectsDuring ballistic impact the main energy absorptionmechanisms include projectile kinetic energy impartedonto the sample, and energy absorption as a result ofshear plugging, tensile fiber failure, fiber rdelamination, and frictional energy absorbed duringinteraction of the bullet and laminate. The damagemodes observed for the investigated composite sampleare detailed in Figure 5.Investigated Depth RangeFor proper NDT material inspection, it is necessary tohave knowledge of the material thermal diffusivity ( ).Thermal diffusivity is an inherent material property thatdescribes the rate of heat transfer across a temperaturegradient for a given material. The investigated depthrange of a specimen is related to the thermal diffusionlength ( / f)0.5 which represents the depth at whichthe amplitude of the thermal wave is reduced to e-1 of itsvalue at the surface, with f being the modulationfrequency of the excitation source. The thermaldiffusivity of the investigated ballistic composite sample cm s was measured using the ParkerMethod. The depth range can be estimated using theFigure 6: Depth Range for the investigated composite sampleand 3 phase images at different depths within the sample.Quantitative Analysis.To estimate the defect size, the Reveal Lab coordinatesystem was calibrated on samples of known size prior toperforming NDT experiments. Defect sizes werecomputed for different depths within the sample.Central Impact Hole Size.Figure 7 shows the central impact damage area of allshots as a function of depth range. The projectiles usedfor the ballistic shots were 5.5 mm in dimeter and about7 mm in height, corresponding to a base area of about 24mm2. The mean damage area of the central impact for allshots appear to be quite consistent with the size of theprojectile, demonstrating good precision of thequantification done in this work.High-Speed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops4
APPLICATION NOTEFigure 8: Damage area for three shots (bouncing) as afunction of depth range.Total damage size of partial perforation shot.Figure 7: Central impact damage area of all the shots as afunction of the depth range.Total damage size of bouncing shots.Figure 8 shows the total damage area for three shots(bouncing) as a function of the depth range. The totaldamage includes the impact hole and regions displayingthe fiber delamination and compression/shear induceddamage type. Higher velocity shots appear to inducegreater damage. Bouncing type shots inducedelamination very close to the sample surface butsample damage size quickly decreases with increasingdepth. For example, shot 15 does not show any damageat 2.1 mm from the surface of the sample. This allows foran estimation of the depth of the impact damage for Shot15 to be approximately 1 mm.Figure 9 shows the total damage area for six shots (partialperforation) as a function of the depth range. Relativelyhigher delamination damage was observed for lowerspeed shots (shots 9 and 10) in the case of partialperforation shots. However, for some shots, theprojectile remained embedded in the sample after theballistic impact test, which may have had anundetermined impact on the NDT results. Additionalconsiderations include the effect of sample weakeningdue to repeated impacts. The sample was subjected to15 consecutive shots, later shots impacted a samplealready weakened by earlier shots. This can be seenwhen comparing the damage caused by shots 9 and 10(lower velocity) with shots damage caused by shots 1, 46 (higher velocity). Lower velocity shots seem to inducebigger damage. Considerations must therefore be madefor impacts recorded in the latter phases of this test toaccount for potential degradation of overall plateintegrity due to previous impacts. Additionally, in thisanalysis, shots 9-12 generated increased and overlappingareas of delamination damage compared to earlierimpacts, potentially due to the plate degradation byearlier shots. These large areas of spread delaminationdamage complicate efforts to precisely quantify thedamage area induced by each shot.High-Speed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops5
APPLICATION NOTEFigure 9: Damage area for six shots (partial perforation) as afunction of depth range.Total damage size of perforation shotsFigure 10 shows the total damage area for four shots(perforation) as a function of depth range as well asphase images measured from both sides of thespecimen. The damage size areas shown on figure 10 topleft panel were extracted from measurements done onthe front side, but same analysis could be done from therear side as well. At the sample surface (zero depthrange), the size of the damage shown in figure 10 top leftpanel corresponds to the projected penetration hole. Atdeeper depth range the sample experiencedelamination, increasing the total damage size. Around2.1 mm the total damage size decreases due to dampingof the delamination propagation. The NDT experimentalconditions allow for investigation up to 2.1 mm deepinside the sample from each side of the specimen.Because the composite sample is approximately 5 mmthick, our analysis provides insight into a large portion ofthe sample cross-section. For the perforation shots, thedefect sizes do not vary much with the projectile speedas expected in high velocity ballistic impact conditions.Compared to the partial perforation and bouncing shots,the perforation shots show less delamination damage.When the projectile completely perforates the materialmore energy is absorbed via the fiber breakagemechanism. Relatively limited delamination damage istherefore observed with corresponding smaller defectsize as compared to the partial perforation and thebouncing type.Figure 10: Damage area for four shots (perforation) as afunction of the depth range.ConclusionTelops offers a complete non-contact infrared activethermography solution which includes a high-capabilityTelops FAST-IR thermal camera, powerful software, andflexible illumination options including optical,mechanical, and electromagnetic sources. Our NDTsolutions ensure high quality industrial and researchmaterial analysis. Thanks to the high frame rates ofTelops cameras, the TESTD systems can detect damagefrom corrosion, delamination, decay, and otherirregularities even from very thin or from highlyconductive or diffusive materials which are notoriouslyhard to investigate. We conducted quantitative analysisof the damaged areas on composites ballistic platessubjected to high velocity impact. This quantitativedefect analysis combined with knowledge of projectilevelocity allows for characterization of absorbed energyand differentiation of generated defect types. Ourobservations indicate that high velocity shots tend toinduce smaller impact damage areas characterizedprimarily by fiber breakage, while low velocity shots tendto induce larger impact damage areas featuringHigh-Speed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops6
APPLICATION NOTEpredominantly delaminationdamage mechanisms.andmatrix crackingReferences[1] Busse G. and Rosencwaig A. “Subsurface Imaging withPhotoacoustics,” Appl. Phys. Lett., 36(10):815-816, 1980.[2] Thomas R. L., Pouch J. J., Wong Y. H., Favro L. D. andKuo P. K. “Subsurface Flaw Detection in Metals byPhotoacustic Microscopy”, J. Appl. Phys., 51(2): 1152–1156, 1980.[3] Meola C et al. J. Mater. Process. Technol., 155156:1132–1137, 2004.Telops Inc.100-2600 St-Jean Baptiste AveQuébec, QC, Canada, G2E 6J5 peed Infrared Imaging for Crack and Hot-Spot Formation in a Graphite-Fiber and Epoxy – 2020 Telops7
Analysis of Ballistic Impacts on Composite Materials by Infrared Active Thermography. In this study, we investigated the assessment of the damaged area on composites ballistic plates subjected to high velocity impact. The active pulsed thermography technique was used for performing post-mortem analysis of the impacted specimens.
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