Non-destructive detection of fatigue damage in thick composites by pulse-echo ultrasonics

Non-destructive detection of fatigue damage in thickcomposites by pulse-echo ultrasonicsA.P.Mouritz*,C.Townsend,M.Z.Shah KhanDefence Science and Technology Organisation,Aeronautical and Maritime Research Laboratory,PO Box 4331,Melbourne,Victoria,Australia 3001Received 14December 1998;received in revised form 29June 1999;accepted 22July 1999AbstractThe non-destructive evaluation (NDE)of fatigue damage in thick polymer composites used in large ships is examined by the use of low-frequency pulse-echo ultrasonics.The initiation and growth of fatigue-induced damage was monitored with ultrasonics during cyclic ¯exural loading of glass-®bre-reinforced polymer (GRP)composites between 7mm and 38mm thick.Fatigue loading the GRP to high stresses (above 50%of the static failure stress)induced extensive damage that consisted of debonding of the surface glass ®bre tows together with a complex array of cracks and delaminations within the composite,and this damage was easily detected by means of ultrasonics.Cyclic loading the composite to relatively low fatigue stresses (below 50%of the failure stress)caused much less damage.Short cracks developed within the resin-rich regions between the glass ®bre plies and within the ®bres tows,and this damage was just detectable by using ultrasonics.The cracks induced by low-stress fatigue were di cult to detect because they mostly grew parallel to the transmission path of the ultrasound waves.The ability to detect damage induced by either low or high fatigue stresses improved with increasing thickness of the GRP.The implications from this study for the NDE of fatigue-induced damage in thick composite ships by the use of pulse-echo ultrasonics is discussed.#2000Elsevier Science Ltd.All rights reserved.Keywords:A.Polymer-matrix composites;B.Fatigue;D.Non-destructive testing;D.Ultrasonics1.IntroductionGlass-®bre-reinforced polymer (GRP)composites are often used in maritime vessels such as yachts,lifeboats,®shing trawlers,patrol boats and naval minehunting ships [1].The appeal of GRP composites over more traditional maritime construction materials (such as steel and aluminium alloy)is their light-weight,good fatigue performance,excellent corrosion resistance in sea-water,and low through-life maintenance costs.However,GRP composites generally have low sti ness which means that large composite ships must have hulls much thicker than those in metallic ships.The hull thickness is typically a few millimeters for small sail-boats,between 5and 10mm for ®shing trawlers and patrol boats,and above 20mm for naval ships.One example of an extremely thick composite ship is the Gatea minehunter,which is operated by the Italian,UnitedStates,Malaysian,Nigerian and Australian navies (Fig.1).The superstructure of the minehunter varies in thick-ness between 6and 20mm while the thickness of the hull varies from about 50to over 200mm.Ships made out of composite material are expected to operate for 20±50years without any major structural re®ts.As a result,their endurance and safety is depen-dent on the long-term fatigue performance of the GRP.The fatigue resistance of GRP composites used in ships is generally good,although fatigue damage can develop in highly stressed regions by the repeated action of waves slamming against the hull and by the hogging-sagging bending motion along the vessel in heavy seas.This motion can pose a serious safety problem because most fatigue failures on composite ships do not usually display early signs of visible damage.Therefore,it is important for ship operators to undertake regular non-destructive inspections for fatigue damage.A range of non-destructive evaluation (NDE)techni-ques can be used on GRP composites,including tap testing,mechanical impedance,thermography,laser shearogaphy,and X-ray radiography [2±5].However,0266-3538/00/$-see front matter #2000Elsevier Science Ltd.All rights reserved.P I I:S 0266-3538(99)00094-9Composites Science and Technology 60(2000)23±32*Corresponding author.Tel.:+61-396268276;fax:+61-396268999.E-mail address:adrian.mouritz@.au (A.P.Mouritz).very few techniques are suitable for inspecting ships at sea or in a dockyard,and arguably the most popular is pulse-echo ultrasonics.The popularity of pulse-echo ultrasonics is due to a number of practical features of the technique,such as its safety and reliability,access to only one side of a ship structure (e.g.bulkhead,deck)is needed to make an inspection,and the ultrasonic instrumentation is portable.Despite these advantages,the ultrasonics method is normally only applied to detect fatigue damage in car-bon-®bre-reinforced epoxy (CFRP)composites used in aircraft structures that are less than several millimetres thick.Because aircraft composites are much thinner than the GRP used in ships they are usually easier to inspect using ultrasonics.Williams and Doll [6],Shoup et al.[7],and Schuster and Friedrich [8]were unable to detect fatigue damage in thin CFRP composites with through-transmission ultrasonics.Under certain fatigue conditions,however,Williams et al.[9],Bader and Boniface [10],Kellas et al.[11],Nayeb-Hashemi et al.[12],and Scarponi and Briotti [13]all successfully detected the initiation and spread of fatigue cracks in CFRP composites with through-transmission ultra-sonics.This technique was able to detect small delami-nation cracks induced by fatigue loading,but only when the cracks grew in the direction transverse to the trans-mission path of the ultrasound waves.Other types of fatigue damage were more di cult to detect,such as®bre splitting [11]or cracks parallel to the transmission path of the ultrasound waves [10].It is worth noting that Kasap et al.[14]and Forsyth et al.[15]were able to detect by ultrasonics the debonding of ®bres from the resin matrix as well as resin cracks in composites fati-gued by thermal cycling rather than stress cycling.While certain types of fatigue damage (e.g.delamina-tions,®bre debonding)can be detected in aerospace composites that are less than several millimetres thick by the use of through-transmission ultrasonics,little is known about the detection of fatigue damage in much thicker GRP composites with pulse-echo ultrasonics.Tsushima and Ono [16]recently reported that with the use of pulse-echo ultrasonics it was possible to monitor the initiation and growth of damage in marine-grade GRP composites generated by zero-to-tension fatigue loading.The composites were between 4.5and 5.8mm thick,which is about the hull thickness in racing yachts and small boats.Tsushima and Ono [16]monitored a steady increase in the attenuation of the ultrasonic signal with the number of fatigue cycles due to an increasing number of fatigue cracks being initiated within the resin matrix.Apart from this work,there have been no pub-lished studies of the NDE of fatigue induced damage in thick composites by the use of pulse-echo ultrasonics.The aim of this paper is to determine the use of pulse-echo ultrasonics for detecting the initiation and growth of fatigue induced damage in thick GRP composite.The composite was similar to that used in many large compo-site ships,and was the same as that used in the hull of the Gatea minehunter.The GRP composite,which varied in thickness from 7to 38mm,was fatigue tested over a range of ¯exural stress levels under four-point-bend cyclic load-ing.The ¯exural fatigue loading was used to induce damage similar to the fatigue damage that can occur in composite ships by the repeated action of waves slamming against the hull or by the hogging-sagging bending motion along the vessel.The research ®ndings will be used to determine whether ultrasonics is a useful NDE technique for the detection of fatigue induced damage in the Gatea minehunter,which in this paper is taken as a typical example of an extremely thick composite ship.2.Materials and experimental techniques 2.1.MaterialsThe GRP composite was made from E-glass woven fabric and an isophthalic polyester resin.The fabric (known commercially as DF1400Woven Roving)was woven by Colan Industries with plain roving tows in the warp direction and spun roving tows in the weft direc-tion.The fractions of glass in the warp and weft direc-tions was 0.36and 0.64,respectively,and the areal density of the fabric was 1.35kg/m 2.The compositewasFig.1.HMAS Huon operated by the Royal Australian Navy.This ship is similar to the Gatea minehunter,and is made from thick GRP composite.24 A.P.Mouritz et al./Composites Science and Technology 60(2000)23±32laminated by wet hand lay-up,which is the process most often used in ship construction,and the resin matrix was cold-cured under ambient conditions ($20 C,55%relative humidity).The resin and void contents of the cured composite were 43.4 1.2%and $3%,respec-tively,which are typical values for a marine-grade posite specimens were laminated into long slender beams with average thicknesses of 7,16,27or 38mm for fatigue testing,and with the warp ®bres aligned along the beams.It is important to note that the glass fabric had an unusually coarse weave that caused the composite surface to be rough.The peak-to-valley height of the surface contours was typically about 1.5mm.A second type of composite specimen was used to determine the minimum delamination size that can be detected in thick GRP composite using ultrasonics.These specimens were made containing a series of steps,where the thickness of the steps increased in increments of about 25mm from 25mm up to maximum of 150mm,as shown in Fig.2.During lamination of these specimens,a single rectangular strip of 0.1mm thick Te¯on ®lm was inserted between two plies about 10mm from the bottom to arti®cially replicate a thin delami-nation buried at increasing depths beneath the surface of the steps.Specimens were made containing either a 2,5,10,25,50or 150mm wide strip of the ®lm that ran the length of the block.2.2.Fatigue testingFig.3illustrates the layout of the four-point ¯exural test used to fatigue the composite beams.The specimen dimensions together with the load span (S )and support span (L )lengths are given in Table 1.These values were used in accordance to ASTM D790M [17]for the static ¯exural testing of polymer composites under quarter-point loading at a support span-to-thickness ratio of 32-to-1.The only deviation from the ASTM standard was that the specimens were 40mm wide,rather than being the recommended width of 25mm.Wider specimens were needed to allow adequate area for the ultrasonic probe (which has a diameter of 25mm).The specimens were fatigued in displacement control with a sine wave load pro®le at a frequency of 0.1Hz.This low frequency was used to ensure the fatigue machine had adequate time to reach the maximum displacement (which in some tests exceeded 30mm).As a consequence it was not practical to fatigue the specimens to failure,which in some tests would exceed 1,000,000cycles,and therefore the tests were terminated at 20,000cycles.Fig.2.(a)Dimensions and (b)ultrasonic inspection of a GRP step-shaped specimen containing an arti®cial delamination created by Te¯on ®lm.The ®lm extended along the entire length of thespecimen.Fig.3.Diagrams of the four-point ¯exural fatigue test when the speci-men was ultrasonically inspected in the (a)unloaded and (b)loaded states.Table 1Specimen dimensions,load spans and support spans used in the ¯ex-ural fatigue testing Specimen thickness (mm)Specimen length (mm)Load span (S )(mm)Support span (L )(mm)8307128256167102605202710404238463814405501100A.P.Mouritz et al./Composites Science and Technology 60(2000)23±32252.3.Ultrasonic testingThe step-shaped and fatigue specimens were inspected using a Krautkra mer-Branson USD15(Version 4.00.02)ultrasonic dialog ¯aw detector and a Panametrics 0.5MHz transducer probe with a 25mm diameter (which are shown in Fig.2b).A 10mm thick rubber block was attached to the probe surface to provide better impedance matching with the composite and to provide a time delay to ensure the pulse-echo signal was not masked by the initial pulse signal.The ultrasonic instrumentation was operated at a capacitance of 2000pF and damping of 500 in the time-of-¯ight mode.The average speed of sound for longitudinal waves through the GRP was measured at nearly 3000ms À1,but this speed varied by $5%pre-sumably due to variations in the glass and void contents.The A-scan records were transferred from the ultrasonic unit to a computer using Krautkra mer ULTRADOC software to keep a permanent record of the measurements.The fatigue tests were interrupted at regular intervals to monitor the development and growth of damage.Fig.3shows the location where the ultrasonic probe was placed on the specimen during fatigue testing.Two types of ultrasonic inspections were performed:(1)all of the specimens were inspected in the unloaded condition (Fig.3a)and (2)the 16,27and 38mm thick specimens were also inspected while held at the peak fatigue stress (Fig.3b).Inspections were performed while the specimens were loaded because it is expected that any fatigue cracks will open,and thereby make them more detectable.3.Results and discussion3.1.Detection of low-stress fatigue damageThe growth of fatigue damage in the GRP composite was monitored during the fatigue test by an increase in gain of the ultrasound signal using A-scan records.Examples of A-scans for a 16mm thick specimen taken at the start,near the middle,and at the end of a fatigue test are shown in Fig.4.The A-scans are characterised by a cluster of three large peaks at a depth of about 15to 18mm,and these signify the re¯ection of ultrasound waves from the back-surface of the specimen 1.Fig.4shows that at the start of this fatigue test the amount of gain needed to bring the pulse-echo signal to 80%full-screen height was 63dB (Fig.4a),and this rose to 65dB after 11,600cycles (Fig.4b),and reached 67.5dB after20,000cycles (Fig.4c).The A-scan records were usually consistent when taken at di erent locations along the specimen between the two load points,with the scatter in the gain values rarely exceeding 1dB.The change in gain with number of load cycles for the 16mm thick GRP tested at a relatively lownormalisedFig.4.A-scans of a 16mm thick specimen tested at a low normalised fatigue stress ('f a'0 0X 36)for (a)0,(b)11,600and (c)20,000cycles.The gain level to bring the peak to 80%of full-scale height is shown in the top right corner of the A-scans.1The re¯ection of ultrasound waves in a polymer composite is usually observed in an A-scan as a single peak rather than a cluster of peaks.However,the rough back surface of the GFRP,with a average peak-to-valley height of 1.5mm,caused di erences in the time-of-¯ight of the ultrasound waves when re¯ected from the peaks and valleys at the back-surface of the specimen resulting in multiple pulse-echo signals in the A-scans.26 A.P.Mouritz et al./Composites Science and Technology 60(2000)23±32fatigue stress ('f a'0=0.36)is shown in Fig.5.In this study the normalised fatigue stress is de®ned as the maximum ¯exural fatigue stress in the outer layers of the laminate ('f )divided by the maximum ¯exural strength of the outer layers of the composite ('0),which is 230MPa.In this ®gure the gain was measured when the specimen was unloaded and when held at the peak fatigue stress.The gain increased slightly with the ®rst 1000±5000cycles and then remained reasonably con-stant for the unloaded condition and increased gradu-ally for the loaded condition over the remainder of the fatigue test.These trends were observed for GRP speci-mens with the four di erent thicknesses when tested at any normalised fatigue stress below 0.5.It was found,however,that the increase in gain caused by fatigue was dependent on the thickness of the GRP composite.For example,Fig.6shows that after testing at 'f a'0=0.36for 20,000cycles there is a general increase in gain with thickness.This indicates that fatigue damage can be more easily detected in thicker composites,and when the composites are under load.After being tested for 20,000cycles at normalised fatigue stresses ('f a'0)below 0.5,the GRP composite was examined using scanning electron microscopy (SEM)to identify the damage responsible for the small increase in gain.The only fatigue damage observed was a low density of short cracks that grew in the through-thickness direction (ie.normal to the surface).These cracks grew in the same direction as the transmission path of the ultrasound waves,as shown in Fig.7.These cracks only occurred towards the back surface oftheFig.5.E ect of number of cycles on the increase in gain for a 16mm thick specimen tested at a low normalised fatigue stress ('f a'0 0X36).Fig.6.E ect of thickness on the change in gain after the composite was tested at a low normalised fatigue stress ('f a'0 0X 36)for 20,000cycles.The gain values shown are an average of the values measured over the course of the fatigue tests.The error bars represent one stan-dard deviation.A.P.Mouritz et al./Composites Science and Technology 60(2000)23±3227specimen,which was subject to a tensile stress under ¯exural loading.Other studies [10]have shown it is not possible to detect fatigue cracks that run parallel to the transmission direction of ultrasound waves.Therefore these cracks in the GRP would not be expected to increase the gain.However,some cracks grew from the resin-rich regions between the plies into ®breglass tows where they were forced to weave a tortuous path around the ®bres.As the cracks weaved around individual ®bres they were de¯ected from the through-thickness direction for short distances (typically less than 50m m).These are the only regions along the crack length that intersect the ultrasound wave path,and could be the cause for the slight increase in gain.3.2.Detection of high-stress fatigue damageRaising the normalised fatigue stress ('f a'0)above 0.5resulted in more substantial changes to the gain level than reported earlier.For example,Fig.8shows A-scans for a 16mm thick composite fatigued at 'f a'0=0.72for 0,9000and 20,000cycles.The gain increased from 65dB (Fig.8a)to 81dB (Fig.8b)after the ®rst 9000cycles,but further testing to 20,000cycles caused the gain to decline to 75.5dB (Fig.8c).This decline in gain was accompanied by a shift in the depth at which the ultrasonic signal is re¯ected from $16to $15mm.The e ect of the number of fatigue cycles on the gain level for this specimen is shown more clearly in Fig.9.The gain initially increased rapidly until $9000cycles,but then further fatigue loading caused a reduction in gain from the peak value.This trend was observed whenever the com-posite was tested at a normalised fatigue stress ('f a'0)above 0.5.Examination of the composite using SEM revealed after testing at normalised fatigue stresses ('f a'0)above 0.5the fatigue damage was much more extensive and complex than the damage that occurred when 'f a'0`0X 5.Soon after the start of the fatigue test thewarp tows along the top surface partially debonded from the underlying composite because of the high compressive stress generated in the outer layers by ¯ex-ural loading.The tows debonded in shortsegmentsFig.7.Low-stress fatigue cracking in the GRP tested at a low normal-ised fatigue stress ('f a'0 0X 36)for 20,000cycles.The arrow shows the transmission direction of the ultrasoundwaves.Fig.8.A-scans for a 16mm thick specimen tested at a high normalised fatigue stress ('f a'0 0X 72)for (a)0,(b)9000and (c)20,000cycles.28 A.P.Mouritz et al./Composites Science and Technology 60(2000)23±32between the warp/weft tow cross-over points,and the surface density of these failures increased steadily over the duration of the fatigue test.This damage is expected to have contributed to the steady rise in the gain during the ®rst 9000cycles of the test because the ultrasound waves would be scattered at the debonds.In addition to this damage,the composite contained a high density of cracks that extended in the through-thickness direction,and these were similar to the crack shown in Fig.7.A high number of through-thickness cracks were forced to weave between the ®bres within the tows,and this would have contributed to the increase in gain.Cracks also grew in directions other than the through-thickness direction,as shown in Fig.10a,and these would cause a greater attenuation of the ultrasound waves than the cracks that grew in the through-thickness direction.Where many of the resin cracks intersected a warp ®bre tow,they grew along the tow/matrix interface to create a small delamination that resulted in yet further increa-ses in the gain level of the ultrasound signal (Fig.10b).Delaminations were observed to grow in length during the fatigue test,and after 9100cycles caused the back surface ply to detach from the tensile-side of specimen.This ply detachment is observed in the A-scans (Fig.8)Fig.10.High stress fatigue damage in the GRP tested at a high normalised fatigue stress ('f a'0 0X 72).The arrow shows the transmission direction of the ultrasoundwaves.Fig.9.E ect of number of cycles on the increase in gain for a 16mm thick specimen tested at a high normalised fatigue stress ('f a'0 0X 72).A.P.Mouritz et al./Composites Science and Technology 60(2000)23±3229by a reduction in the apparent thickness of1±2mm after 9000cycles.The gain level falls from the peak value at 9000cycles because the ply detachment has reduced the beam-path length that reduces the attenuation.Fig.11shows the e ect of normalised fatigue stress on the gain after20,000cycles.In this®gure a compar-ison is made between the gain values for unloaded and loaded specimens.The gain values were on average1±2 dB higher when the fatigued composite was inspected under load.The curves show two fatigue regimes:.At low normalised stresses('f a'0`0X5)the gain rises slowly due to a gradual increase in the num-ber of fatigue cracks that are parallel to the trans-mission path of the ultrasound waves.It is only when the cracks are forced to weave between the ®bres that some scattering of the ultrasound waves is likely to occur(Fig.7)..At high normalised stresses('f a'0b0X5)the gain rises more rapidly because of the increasing num-ber and length of debonded surface tows,cracks, and short delaminations(Fig.10).However,the delaminations cause a slight reduction in the peak gain level when they grow large enough to cause ply debonding.3.3.Detection of fatigue damage in composite ships The detection of fatigue damage in the thick GRP specimens using pulse-echo ultrasonics indicates that this method may be useful in the inspection of compo-site ships for fatigue cracks.However,large ships such as the Gatea minehunters are between$160and200 mm thick along the keel,which is much thicker than the fatigue specimens studied here.While it was found that the detection of fatigue damage using pulse-echo ultra-sonics improved with the thickness of the GRP up to38 mm(Fig.6),this does not prove that fatigue damage can be detected in the thickest regions in the mine-hunter.This can only be proven by ultrasonically inspecting$200mm thick composite specimens con-taining fatigue damage.It was not possible to fatigue 200mm thick specimens using the four-point test equipment because of the need for an extremely long load span(3.2m)and support span(6.4m).One technique to determine whether high stress fati-gue damage in extremely thick composites can be detected using pulse-echo ultrasonics was to inspect for delaminations buried deep within the step-shaped spe-cimens.These delaminations,which were arti®cially created by Te¯on®lm,were similar to those formedbyFig.11.E ect of normalised fatigue stress on the increase in gain after20,000cycles.The gain was measured when the specimens were(a)unloaded and(b)loaded at the peak fatigue stress.30 A.P.Mouritz et al./Composites Science and Technology60(2000)23±32high stress fatigue.(It was not possible,however,to fabricate these specimens containing other types of fati-gue damage,namely the low stress and high stress resin cracks shown in Figs.7&10a,respectively.)The ability to detect a delamination buried at increasing depths below the surface using the ultrasonics method is shown in Fig.12.In this ®gure the open points represent where a delamination can not be detected whereas the closed points signify where the delamination is detectable.The minimum delamination size that can be detected increased gradually the deeper it was buried beneath the surface.This ®gure indicates that delaminations caused by high stress fatigue which grow larger than about 10mm may possibly be detectable in the minehunter hull.It is important to note,however,that the minimum detect-able size will be determined by a number of factors that were not investigated in this study,such as the delami-nation shape and the presence of other types of fatigue induced damage (e.g.resin cracks,tow debonding).A problem commonly encountered during the ultra-sonic inspection of thick composite structures,such as minehunter ships,is that their ultrasound properties vary due to variations in resin and void contents.Smith [1]reports that the amount of woven ®breglass used in marine vessels usually varies from 45to 55%while the porosity varies between 2and 5%.The variation in ultrasound attenuation that can occur in thick GRP composites was demonstrated by Mouritz and Town-send [18]during the NDE of three new Gatea -type minehunters built for the Royal Australian Navy.The minehunters were built from the same GRP composite as the fatigue and step-block specimens used in this study.Fig.13shows gain values measured by Mouritz and Townsend [18]at di erent locations through the minehunters where the thickness ranged from 6to 220mm.The amount of gain increased rapidly with the thickness of the composite.It is important to note,however,that for any thickness above $40mm the scatter in the gain values is between 5and 12dB.This suggests it would not be possible to detect damage in the minehunter induced by low stress fatigue (i.e.'f a'0`0X 5)because any increase in gain caused by fatigue (<3dB)would be masked by the large scatter in the values for the GRP.It would also be di cult to detect damage induced by high fatigue stress in the minehunter until the normal-ised fatigue stress exceeded $0.7,at which point the gain has risen by more than 12dB.Nevertheless,delaminations produced by fatigue loading at 'f a'0values between 0.5and 0.7may possibly be detected provided they are longer than $10mm.These observations demonstrate the di -culty that can be expected during the NDE of a thick composite ship when using pulse-echo ultrasonics.The detection of fatigue damage may be improved by using A-scan ultrasonics in combination with other NDE techni-ques such as Lamb waves,angle-probe ultrasonics,and lasershearography.Fig.12.Map of the e ect of depth on the minimum detectable delamination size.The ®lled points represent the detection of the delamination while the open points signify when the delamination could not be detected.The solid line shows the boundary between detectable and undetectable dela-minations.This map is for square-shape arti®cial delaminations in the GRP that does not contain fatigue damage.A.P.Mouritz et al./Composites Science and Technology 60(2000)23±3231。