EFFECTS OF DEFECTS ON INTERLAMINAR PERFORMANCE OF COMPOSITES

Andrew Makeev*, Yuri Nikishkov, Guillaume Seon, and Erian Armanios Department of Mechanical and Aerospace Engineering

University of Texas at Arlington, Arlington, Texas, U.S.A.

Abstract

This work presents a methodology for accurate assessment of the effects of manufacturing defects on the interlaminar tensile (ILT) strength and fatigue performance of composites used in rotorcraft fatigue-critical structural designs. A primary failure mechanism for such structures is delamination. Although the ILT strength and fatigue material properties are essential for the development of analytical predictive methods for initiation of delamination at sites that give rise to interlaminar stresses, there are no reliable methods to determine such properties. A major technical challenge is the extreme sensitivity of the ILT to manufacturing defects including porosity that could lead to unacceptable scatter in test results. In this work, X-Ray Computed Tomography (CT) measurements of porosity defects present in curved-beam test articles, are integrated into finite element based stress analysis to capture the effects of defects on the ILT strength and fatigue behavior. Once the effects of manufacturing defects are captured through transfer of the CT measurements into a three-dimensional finite element model, their contribution to the scatter in the test data could be quantified, and reliable interlaminar material properties determined. The ability to extract accurate ILT material properties from curved-beam tests is verified through an alternative short beam test configuration and method. The proposed test provides a basis for assessing the effects of defects.

1. INTRODUCTION1

Currently, composite structural designs are based upon the same factor of safety as that of metals, 1.5, to determine the ultimate design load from limit load. This is in spite of the susceptibility of composite parts to variations in manufacturing processes compared to metal parts. In addition to material variation in the resin content, bulk factor, and fiber alignment, part fabrication process variations such as operator skill, tooling setup, humidity fluctuation and equipment control, are common causes that contribute to variation in part quality. Consequently, the increased sensitivity of composite part quality to material and process variations lowers production yields. Production yields of greater than 90% remain a “hit-and-miss” target [1].

The effects of manufacturing process parameters on structural strength and durability are not well understood. In particular, the effects of inadequate design method and manufacturing process used to produce carbon/epoxy and glass/epoxy composite aircraft fatigue-critical, flight-critical components, result in defects such as porosity and voids, impacting

1 Presented at the 39th European Rotorcraft Forum, Moscow, Russia, September 3-6, 2013.

* Corresponding author, e-mail: makeev@uta.edu.

the residual capability and the useful life of these components.

Manufacturing defects can severely deteriorate matrix-dominated properties, resulting in degraded strength and fatigue structural performance. While it is not feasible to eliminate all defects in a composite part, it is possible to improve upon desing conservatism and address part durability and damage tolerance requirements once the defects and their effects are captured. Advanced structural methods that account for manufacturing defects in composite parts are needed to enable accurate assessment of their capability and useful life and enhance current design and maintenance practices.

One of the major barriers to accurately predict failure for polymer-matrix composites has been the lack of reliable matrix-dominated material properties which could be used as a basis for the development of accurate failure criteria [2, 3]. In particular, interlaminar tensile (ILT) strength and fatigue properties have been challenging to characterize. Accurate measurements of ILT strength and fatigue performance characteristics are needed to capture delamination failure which is one of the primary failure modes for rotorcraft composites. Glass-fiber and carbon-fiber reinforced epoxy-matrix prepreg composites, susceptible to delamination, are most commonly used in the design of rotorcraft composite structures.

Helicopter rotor systems are subject to extreme cyclic loads. The fatigue capability is a limiting factor for high-cycle components in dynamic systems [4]. Typical examples of the fatigue-critical primary composite structures include main rotor blade spars and yokes in commercial and military helicopters [5]. The primary failure mechanism for such structures is delamination. Accurate ILT and interlaminar shear (ILS) fatigue material properties integrated in reliable failure criteria are essential for the development of predictive methods able to capture fatigue delamination. In particular, Reference [3] successfully used ILT and ILS S-N fatigue curves in the formulation of stress-based and fracture-based fatigue failure criteria to predict the onset and propagation of matrix-dominated failure modes including delaminations. However, the development of failure prediction methods is currently limited by the lack of reliable ILT material properties.

A major technical challenge to characterizing ILT behavior is extreme sensitivity to manufacturing defects including porosity that could lead to significant scatter in test results. Composite helicopter rotor components are typically thick and oftentimes have areas with tight radius of curvature [4], which make them especially prone to porosity defects. Test articles representative of curved structural details in composite parts, adopted by the rotorcraft industry, are prime candidate to better understand the effects of porosity. However, there is no consensus on the accept/reject criteria for assessing the effects of porosity on strength and fatigue performance. It has been determined that porosity volume content is not a reliable basis for capturing the effects of defects.

A relatively simple test specimen that could become a standard across the industry for characterizing porosity in composite structures, along with the analysis methodology able to determine the key elements for capturing of the effects of defects on structural performance, would be extremely beneficial.

To the best of the authors’ knowledge, there are no standard test methods for the assessment of ILT fatigue performance in composites. However, there are two American Society for Testing and Materials (ASTM) standard methods for measuring ILT strength.

Currently, ASTM D 6415 curved-beam (CB) test is a standard practice for measuring the ILT strength [6]. Figure 1 shows a typical CB test setup for a 6.6 mm (0.26-inch) thick unidirectional carbon/epoxy tape system. The failure mode is tensile delamination that starts in the radius area, typically at about two third of the thickness corresponding to the maximum interlaminar tensile stress location, and quickly propagates through the flanges. ASTM D 6415 provides a closed-form approximation of the ILT strength [6–8].

Figure 1. ASTM D 6415 test setup and tensile delamination failure of a unidirectional IM7/8552 carbon/epoxy CB specimen. Dashed lines indicate the area subject to CT scanning.

The authors measured ASTM D 6415 CB strength for multiple unidirectional carbon-fiber and glass-fiber reinforced epoxy-matrix prepreg tape composites. Based on their experience, the curved-beam strength data typically exhibit large scatter. For example, for 6.6 mm (0.26-inch) thick CB coupons manufactured from Hexcel IM7/8552 unidirectional tape [9] and cured per manufacturer’s specifications, the average ASTM D 6415 interlaminar tensile strength value varies between 68.9 MPa (10 ksi) and 82.7 MPa (12 ksi) with a coefficient of variation (COV) usually higher than 20%. This raises doubts concerning the adequacy of the ASTM D 6415 standard as a coupon-independent test for measuring ILT material strength. Large scatter in material characterization test results may reflect manufacturing quality as much as the material properties.

Another method for assessing the ILT strength is ASTM D 7291 which applies a tensile force normal to the plane of a composite laminate using adhesively bonded thick metal end-tabs [10]. As noted in Ref [10], through-the-thickness strength results generated using this method will not, in general, be comparable to ASTM D 6415 since ASTM D 7291 subjects a relatively large volume of material to a quasi-uniform stress field while ASTM D 6415 subjects a small volume of material to a non-uniform stress field. Moreover, characterizing ILT strength using ASTM D 7291 is perhaps more challenging than ASTM D 6415 as failure could occur in the bond lines between the composite and the metal end-tabs.

Carbon/epoxy CB specimen configurations were studied in Ref [11] to establish a method for assessing ILT strength. It was noted that “Strength data obtained from curved beam test specimens can be very relevant to the design process. Specimens with curved geometries present unique manufacturing problems that do not occur in flat panels. Consequently, failure modes and strengths obtained

20 mm

from curved beam specimens with manufacturing flaws will closely resemble those in the actual structure with similar flaws. However, if a specimen does not contain any significant flaws, a true material property may be measured.” Reference [11] recorded a significant CB strength reduction (up to a factor of four) in low-quality CB specimens due to large voids detected based on a cross-section cut using a diamond saw. However, large scatter in the strength data observed in high-quality CB specimens could not be explained due to the lack of available three-dimensional nondestructive inspection techniques with micro-focus ability at that time. This work shows that microscopic defects present in high-quality CB specimens (with extremely low porosity content) cause large scatter in curved-beam strength and fatigue behavior.

The objective of this work is to develop efficient and accurate methods to capture the effects of porosity on the ILT strength and fatigue material behavior. Once the effects of manufacturing defects are understood, their contribution to the scatter in test data could be quantified, and reliable material properties extracted.

ADTM D 6415 CB specimens are well suited for assessing the effects of porosity on ILT strength and fatigue performance. Porosity can be easily introduced in the CB specimens; the CB radius area susceptible to delamination onset is far away from the attachments of the test fixture; and the nondestructive inspections can be limited to that area. Therefore, such specimens could arguably be the most suitable candidates to assess the effects of porosity on ILT strength and fatigue behavior.

The ability to extract accurate ILT material properties from CB tests must first be verified. To this end, the recently developed short-beam (SB) method [12] is expanded to enable measurement of the ILT strength for pristine material structure; and establish a rigorous basis for assessing the effects of manufacturing defects in the CB specimens. It is worth noting that failure of a SB specimen also occurs due stressing small volume of material. As the manufacturing process to produce flat SB specimens is simpler compared to CB specimens, the SB specimens are relatively free of defects and can potentially be used as a baseline for determining ILT material strength.

2. SHORT-BEAM METHOD

Short-beam (SB) specimens are prismatic coupons with uniform rectangular cross-sections and subject to three-point bending. Reference [12] provides the details of the methodology for assessing multiple stress-strain constitutive relations, based on the SB tests and the digital image correlation (DIC) technique-based deformation measurement. The dimensions of the SB coupons and the test fixture attachments were modified compared to ASTM D 2344 [13] in order to

generate accurate stress-strain curves as well as consistent failure modes [12, 14]. SB test coupons have been machined in the zero-degree and 90-degree directions from unidirectional tape panels and loaded in the 1-2 (in-ply), 1-3 (interlaminar) and 2-3 principal material planes to characterize the shear stress-strain relations; Young’s moduli in the zero-degree and 90-degree directions; and Poisson’s ratios. The fiber direction is denoted as 1 (zero-degree); the in-ply transverse direction as 2 (90-degree); and the laminate thickness direction as 3 (interlaminar direction.)

To characterize ILT material properties in this work, six SB specimens were machined in the thickness direction from a 106-ply thick IM7/8552 carbon/epoxy unidirectional tape panel cured at 177⁰C (350⁰F) per prepreg manufacturer’s specifications [9]. The cured ply-thickness was approximately 0.183 mm (0.0072 in) resulting in 19.4-mm (0.76-in) panel thickness. The SB specimen length was 19.4-mm (0.76-in). The coupons were 3.8-mm (0.15-in) thick, and 2.8-mm (0.11-in) wide. The coupon thickness corresponds to the in-ply 90-degree principal material direction; and the 2-3 principal material plane is the plane of loading. Figure 2 shows a schematic of a SB coupon and the loading conditions.

Figure 2. A SB coupon machined from a thick plate in the interlaminar (thickness) direction; loading conditions; and the interlaminar tensile failure.

To assess the ILT strength, the SB specimens were placed in an ASTM D 2344 test fixture with a 12.7-mm (0.5-inch) diameter loading nose and 3.18-mm (0.125-inch) diameter supports; and loaded in an electromechanical load frame at a constant 1.27 mm/min (0.05 in/min) crosshead displacement rate till failure. It is worth noting that the ASTM standard loading nose diameter is 6.35 mm (0.25 inches). The support length was 14.99 mm (0.59 inches). All SB specimens failed in tension in the middle of the specimen as schematically shown in Figure 2. The average failure load value was 209 N (47 lbs), which was too low to introduce any compressive damage

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The average 72.5 MPa (10.5 ksi) ASTM D 6415 CB strength value is significantly lower compared to the 116.5 MPa (16.9 ksi) value generated based on the SB tests. Also, the 26.5% COV in the CB strength is more than eight times higher compared to the 3% COV in the ILT strength reported from the SB tests.

4. REFINEMENT OF THE CURVED-BEAM METHOD TO ACCOUNT FOR POROSITY

ASTM Standard D 6415 underscores the sensitivity of measured ILT strength to void content. Consequently, the test results may reflect manufacturing quality as much as material properties [6]. An industrial computed tomography (CT) system with a 225 KV Microfocus X-ray tube and Varian 4030E series flat panel detector, built by North Star Imaging, Inc. was utilized in this work to detect porosity in the CB test specimens. The micro-CT scans were accomplished at 40 kV tube voltage and 400 μA target current at 0.5 frames per second and 10.5x magnification resulting in 0.5Χ10-3 in (12 μm) scan resolution. The dashed lines in Figure 1 outline the area of the CT scan. Specimen cross-sectional images were obtained from the 3D volumetric model reconstructed in the efX-CT software by North Star Imaging. About 1,000 through-the-width section images, approximately 1200×1600 pixels in size, were used for void detection in each specimen. Void identification, classification and measurement were performed by the Numerical Technology Company software. The software applied binary threshold transformation based on the pixel intensity found at local minima between air and material peaks on the pixel histogram (representing the number of pixels at a given intensity) for the full scan volume; and then used object labeling and elliptical shape fitting for void measurements and calculation of the porosity volume. The CT based measurements were conducted on all ten 12.7-mm (0.5-inch) wide specimens.

Figure 4. Void geometry in CB specimen 6.

Figure 4 shows examples of voids found in a representative CB specimen. Voids typically have the largest aspect ratios in the fiber direction resulting in thin curved shapes, and often degenerating into noodle-like shapes. Void thickness was found to be the smallest void dimension in all specimens. This geometry can be attributed to the expansion of air pockets that exhibit the largest resistance, in the thickness direction, due to curing pressure. It is worth noting that the porosity content in the CB specimens was low, e.g. the porosity content in the laminate thickness-based 6.6 × 6.6 × 6.6 mm3 (0.26 × 0.26 × 0.26 in3) cube volumes throughout the CT scans in the CB radius area was less than 0.12%. However, it is not the volume but the presence of individual voids at critical locations that reduces failure loads and causes large scatter.

The porosity volume alone is not sufficient for describing the effects or severity of voids. Indeed, a single large void located close to the mid-plane in the CB radius area is worse than a few dispersed smaller voids that add up to the same total volume. Reference [17] indicates that the porosity volume often does not correlate well with the degradation in structural performance.

Post-failure CT scans of the CB specimens confirmed that delamination progresses through longer voids located around the mid-section of the specimens. Figure 5 shows typical cracks in a representative failed specimen. For all specimens, critical voids were identified in the CT scans before specimen failure and confirmed as the origin of delamination in the corresponding post-failure CT scans. Critical voids are located in the vicinity of the maximum interlaminar tensile stress (about two third of the thickness in the middle of the radius area). Table 1 lists the geometry and reference locations of the critical voids.

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distance frominimum pofor the pointdistance coconsidered that distancinterlaminar distance of point-stress average-stre

Figure 10interlaminar stress (top)

The averageksi) for the ksi) for the acurved-beamand comparD 6415 [6]. values founagree with ttests. Noteaverage-streonly 2.1% aASTM D 64that the repristine (free

om the voidoint was ident-stress and arresponding as the optimce was use

tensile stre19 μm (7.3×1method and

ess method.

0. Charactetensile stre

and the aver

e ILT strengtpoint-stress average-stresm specimensred to the str The averagd using the the 116.5 MPe that COV ess based Iand 3.2% com415 approximefined ILT se of defects)

d, a uniquentified as shoaverage-stresto the minimal characteris

ed to determngth value. 10-4 in) was dd 89 μm (3.5

eristic distaength (ILTS) rage-stress (b

th values are method and ss method. Rs are summaresses calculae interlaminarefined CB t

Pa (16.9 ksi)in the poin

LT strength mpared to 26

mation. Low strength is amaterial prop

e and distinown in Figurss methods.

mum COV castic distancemine the re A characte

determined fo5×10-3 in) fo

ance and using the p

bottom) meth

124.3 MPa 110.4 MPa

Results for tharized in Tabated using Aar tensile stretest methodo) ILT from thent-stress and

calculations6.5% COV inscatter sugg

appropriate aperty, and th

nctive re 10 The

an be ; and

efined eristic or the or the

the point-hod.

(18.0 (16.0 e ten ble 2

ASTM ength ology e SB d the s are n the gests as a e CB

test ILT s

Tabinterwith

Spec

1

2

3

4

5

6

7

8

9

10

Avg

COV

5.

Thison faand curvASTservcell ratioFiguCBdelacurvthroulocaamocoinin delaobse

is useful forstrength.

le 2. Failurerlaminar tensporosity defe

cFailure Load, kN (lbs)

2.24 (503)

1.34 (302)

1.89 (425)

2.61 (586)

1.27 (286)

0.93 (210)

1.85 (416)

2.47 (555)

2.45 (550)

2.06 (463)

1.91 (430)

V 28.2%

EFFECTS OPERFORMA

s Section preatigue testing

12.7-mm (0ved-beam speTM D 6415 tvo-hydraulic lcapacity, and

o and 5 Hz ure 11 shows

specimensaminations inved-beam spugh the flantions of the

ong the specide with thethe poros

aminations aserved in som

r assessing t

e load, ASTsile strength (ects.

ASTM D 6415 ILTS, MPa (ksi)

Repome(ks

83.4 (12.1) 1

52.3 (7.6) 1

70.3 (10.2) 1

92.8 (13.5) 1

50.0 (7.3) 1

37.7 (5.5) 1

68.8 (10.0) 1

94.6 (13.7) 1

93.9 (13.6) 1

81.5 (11.8) 1

72.5 (10.5) 1

26.5%

OF DEFECTSANCE

esents S-N cg of CB coup0.5-in) wide ecimens wertest fixture woad frame wd subjected t

frequency, s examples o

after fatignitiated in tpecimens; anges. It ise delaminatecimens; ane location of sity-free sps well as delae of the curve

the effects of

TM D 6415 (ILTS) for CB

efined ILTS, oint-stress ethod, MPa si)

Reavme(ks

23.1 (17.9)

25.1 (18.1)

25.8 (18.2)

23.8 (18.0)

19.0 (17.3)

25.2 (18.2)

29.3 (18.7)

25.8 (18.2)

23.9 (18.0)

21.5 (17.6)

24.3 (18.0)

2.1%

S ON THE ILT

curves determpons. Twenty

IM7/8552 ure tested in fwas placed iwith 25 kN (5to cyclic load

to delaminaof fatigue delague testing.the radius aand quickly s worth notitions were nd did not the maximum

pecimen. amination braed-beam cou

f porosity on

and refinedB specimens

efined ILTS, verage-stress ethod, MPa si)

115.1 (16.7)

109.3 (15.8)

105.7 (15.3)

109.1 (15.8)

103.3 (15.0)

112.9 (16.4)

112.0 (16.2)

113.0 (16.4)

109.3 (15.9)

113.8 (16.5)

110.4 (16.0)

3.2%

T FATIGUE

mined basedy 36-ply thickunidirectionalfatigue. Thein a uniaxial5.5 kip) loads at 0.1 loadation failure.aminations in. Fatiguearea of the

propagateding that thenot uniformnecessarily

m ILT stressSecondary

anching wereupons.

n

d s

d k l

e l

d d . n e e d e

m y s y e

Figure 11. curved-beam

The ILT peacalculated fapproximatiodeformed shcalculation [loading up fatigue testinplotting the and the intethe number ksi) static ILASTM D 64

The interlamsquares fit 12. A largenoted. Sucreliable assebased uponapproximatiohomogeneoCB test spec

Post-failure typically pasthe radius apresence ofspecimens being locateradial locati(about 2/3 ovoids on theThe porositylow, e.g. thickness-ba0.26 in3) cubcurved-beamTherefore,

Examples m fatigue test

ak stresses dfrom a closeon listed inhape of each[6] was obtaito the peak

ng. The ILTratio of the in

erlaminar tensof cycles to

T strength w15 CB tests.

minar tensilefor the powee scatter witch a poor coessment of In the ASTMon is

ous (defect-frecimens [6 – 8

CT scans sses throughrea of the CBf larger indivgives reaso

ed relatively on of maximof the thicknee delaminatioy content in tthe porosityased 6.6 x 6be volumes thm radius ait is not th

of delaminatt specimens.

during fatigueed-form geom ASTM D

h specimen, uned from thek fatigue loa S-N curve w

nterlaminar tesile strength failure. The

as generated

e S-N curveer-law are ph a low R2 vorrelation doLT fatigue m

M D 6415. this standee) anisotrop8].

showed thh the longer B specimens.vidual criticaln for the defar away fro

mal interlaminess). The pon path is shohe CB fatiguy content 6.6 x 6.6 mmhroughout threa was lehe porosity

tions in IM7/

e testing weremetric ILT s6415 [6].

used in the se initial monoad value priowas generateensile peak sagainst the lo

e 72.5 MPa d based on th

e and the lrovided in Fvalue of 0.33es not provi

material propeThe ILT s

dard assupic material in

hat delaminvoids locate

. In particula voids in seelamination com the theorenar tensile sresence of laown in Figuree specimensin the lam

m3 (0.26 x 0.e CT scans i

ess than 0.volume but

/8552

e first stress

The stress otonic or to ed by stress og of (10.5 he 10

east-igure 37 is ide a erties stress umes n the

nation ed in r, the

everal crack etical

stress arger e 11.

s was minate

.26 x n the 12%. t the

prescauspresthe Ichar

Figuthe A

The to reof thporocalcPoindistatestsstresASTthe voidmeathe mmodlocamodpeakdefecalcchardetestrestheILT normplottfailustrenaver

The in Fthanexpois mfatigappr

sence of indises the larsented in FiguILT stresses racterization.

ure 12. ILT SASTM D 641

methodologyefine the ILThe CB spec

osity in the ulation that

nt-stress andances determs were usedsses at critic

TM D 6415 IL20 curved-bs were det

asurement mmeasuremen

dels. Displal FE model

dels of the cuk fatigue loadect-free peaulating the ILracteristic disermined in Sss method, t89 μm (3.5×

stresses amalized by tted against tre to generngth values rage-stress m

ILT S-N curvigure 13. B

n 2% relative onent value

more than twicue curve basroximation.

ividual voids rge scatter ure 12. The must be inclu

S-N curve for5 stress appr

y presented S-N curve b

cimens, micrradius area,accounted f average-strmined from d to determincal voids andLT stress appbeam fatiguetected from

methods descnts were transacement bouls were genurved-beam sds. For the

ak stresses LT stresses astance from

Section 4. Sthe ILT stres×10-3 in) charat the peahe correspothe log of t

rate the ILT based on t

methods are l

ves from bothoth S-N curvdifference in determined fce the exponsed on the AThe 0.92 R2

at critical loin the fat

effects of crituded for a re

r IM7/8552 taroximation.

in Section 4 based on the ro-CT meas and the refor the pororess based c

the static cne the peak

d replace theproximation. coupons te CT scanscribed in Sesferred into lo

undary condnerated fromspecimens spoint-stress

were deteat the 19 μm

the void’s Similarly, in tsses were avracteristic disak fatigue onding ILT she number

S-N curvesthe point-streisted in Secti

h methods aves are similthe exponen

from the refinent correspoASTM D 6412 value corre

ocations thatigue resultstical voids on

eliable fatigue

ape based on

was appliedfatigue testsurements of

efined stressosity defects.characteristiccurved-beamk fatigue ILT closed-form For each of

ested, criticals using thection 4, andocal FE sub-itions in the

m the globalubject to themethod, the

ermined by(7.3×10-4 in)surface, as

the average-veraged overstance. Theloads were

strength; andof cycles tos. The ILTess and theion 4.

re presentedlar, with lessnt value. Thened method,onding to the15 ILT stressesponding to

t s n e

n

d s f s . c

m T m f l

e d -e l

e e y ) s -r e e d o T e

d s e , e s o

the point-strstress methothe R2=0.34approximatioability of thecapture theperformance

Figure 13. stress methmethod (bot

6. CONCL

This work infor porosity assessmentto the need to an accuraenable botmaintenanceunderstand performancebecome a st

CT measurecomposite element moILT strengthdefects wermeasurememodel, the sreliable matwere determ

ress method od reflect a h4 from the on. The e refined ILTe effects oe.

ILT S-N curvhod (top cuttom curve.)

LUSIONS

ntroduced strin compositet of their capafor a fundam

ate assessmeth safe ane. Technolog

their effecte could enatandard prac

ements of potest articlesdels to asses

h and fatigue re captured

ents into a scatter in theterial ILT str

mined.

and R2=0.94higher correla

ASTM D reduced sc

T stress anaof porosity

ves corresponrve) and the

ructural methe parts and enability and us

mental shift frent of part cond economigies to measts on stren

able that shictice.

orosity defec were integss the effectsbehavior. Othrough trathree-dimen

e test data wrength and f

4 to the averation compar6415 ILT s

catter reflectsalysis method

on the fa

nding to the pe average-s

hods that accnable an accseful life. It prom fleet statondition in ordical usage sure defects

ngth and faift and ultim

cts present ingrated into s of defects o

Once the effecansfer of thensional strucwas reduced;fatigue prope

rage-red to stress s the ds to

atigue

point-stress

count urate

points tistics der to

and s and atigue mately

n CB finite n the cts of e CT ctural ; and erties

ThisStan6415the Cat a D 64the Cinto the IresuIM7/rotorthe S

The propfatigcorrethe I

The becoof pIndespecdelathe candstreninhecurvpracand not tvoidand The meaappl

The charassostructestsdestsmaeffecthe and usefachiconddisp

ACK

ThisResethe grate

s work presndard D 64155 significantlyCB coupon telow volume c

415 includesCB radius ara finite elemILT material

ults of the m/8552 carbonrcraft applicaSB based ve

methodologperties was ue behavior.esponding toIM7/8552 tap

methodologome a standorosity on th

eed, porositycimens. Theamination lim

radius area didates to stungth. Althou

erent attributeved-beam tesctical value toshould be cothe porosity cs at critical locauses scatrefined CB

ans for defectlicable to vari

potential bracterize theociated noctural failure s. This wtructive inspallest voids acts on structupart conditiosize, and the

ful life are eving a funddition-based osition will be

KNOWLEDG

s work is spearch under University oefully acknow

sents a mod5 CB methody underestimest data are content. The

s measuremerea and trans

ment stress astrength prop

modified CB n/epoxy tape

ations, are in rification test

gy to deterfurther exte

. As an exao a 0.1 load pe composite

gy presenteard practice

he ILT strengy can be re

susceptibilitymits the nond

and makesudy the effecugh scatter ie of the CB ssting and an

o understandionsidered. content but tocations thattter in the st

B methodolot-tolerant desious structura

benefits of t material stn-destructivemodels exte

work shows pection is nand paramoural performaon in terms oeir influence established

damental shifstructural

e realized.

GEMENTS

ponsored by a Grant Awa

of Texas Arlwledged. Th

dification to d. The curre

mates the ILTaffected by p

e refinement ent of the critsitions defec

analysis modeperties. The tests prese

e compositeexcellent agrs.

rmine the Iended to cample, the ILratio was desystem.

ed in this for assessin

gth and fatigeadily intrody of the CB ra

destructive ins such speccts of porosityn the ILT st

specimens, thnalysis has ing the effectThis work shhe presence t reduce the trength and fgy provides

sign applicatioal configuratio

the ability toructure and

e measuremend well beythat accura

needed to qunt in deter

ance. Once kof critical deon residual , the ultimaft from statist

substanti

the US Offiard N00014-ington. Thise specimens

the ASTMent ASTM D

T strength asporosity evento the ASTMtical voids in

ct informationel to captureILT strength

nted for thee, popular inreement with

ILT materialharacterizing

LT S-N curveetermined for

work couldg the effectsue behavior.uced in CBadius area to

nspections tocimens goody on materialtrength is anhe subject ofa significant

ts of porosity,hows that it is

of individualILT strengthfatigue data. a practicalons, and it isons.

o accuratelytransfer the

ments intoyond the CBacy of non-quantify thermining theirknowledge ofefect locationstrength andate goal oftics-based toiation and

ice of Naval11-1-0916 tos support is

s used in this

M D s n M n n e h e n h

l g e r

d s .

B o o d l

n f t , s l

h . l

s

y e o B -e r f n d f o d

l o s s

work were manufactured by Bell Helicopter Textron. The authors are grateful to Mr. Edward Lee, Principal Engineer, Manufacturing R&D in this regard, and wish to thank Mr. Brian Shonkwiler, Research Associate and Ms. Paige Carpentier, PhD student, University of Texas Arlington, for their assistance with running the tests.

COPYRIGHT STATEMENT

The authors confirm that they, and/or their company or organization, hold copyright on all of the original material included in this paper. The authors also confirm that they have obtained permission, from the copyright holder of any third party material included in this paper, to publish it as part of their paper. The authors confirm that they give permission, or have obtained permission from the copyright holder of this paper, for the publication and distribution of this paper as part of the ERF2013 proceedings or as individual offprints from the proceedings and for inclusion in a freely accessible web-based repository.

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