Jinghao Li, John F. Hunt*, Shaoqin Gong and Zhiyong Cai* Fatigue behavior of wood-fiber-based tri-axial engineered sandwich composite panels (ESCP)

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1 Holzforschung 215; op Jingho Li, John F. Hunt*, Shoqin Gong nd Zhiyong Ci* Ftigue ehvior of wood-fier-sed tri-xil engineered sndwich composite pnels (ESCP) DOI /hf Received April 11, 215; ccepted Octoer 2, 215; previously pulished online xx Astrct: The sttic nd ftigue ending ehvior of wood-fier-sed tri-xil engineered sndwich composite pnels (ESCP) hs een investigted y four-point ending tests. Ftigue pnels nd wekened pnels (wescp) with n initil interfce defect were mnufctured for the ftigue tests. Stress σ vs. numer of cycles curves (S-N) were recorded under the different stress levels. The primry filure mode in the ftigue tests ws oserved in the sher zone (epoxy deonding), which ws different from fce filure in the pure ending zone for the sttic ending test. For residul ending (RB) test, epoxy deonding filure occurred etween the pure ending zone nd sher zone. Mcro crcks long the core/fce interfce developed s the numer of cycles incresed during the ftigue life. The crck propgtion or dmge for the pnels sumitted to ftigue test cn e descried s three-stge dmge process of first non-liner portion, followed y liner dmge ccumultion, nd lstly nonliner ccelerted dmge. Bending stiffness degrdtion t the higher lod level hd fster degrdtion during ftigue life. The dissipted energy of the pnels ws smll due to the high stiffness of the mterils. Keywords: ending stiffness, ftigue ehvior, residul properties, sndwich pnel, S-N curve, sttic testing, wood-fier-composite *Corresponding uthors: John F. Hunt, Zhiyong Ci, USDA Forest Service, Forest Products Lortory, Mdison, WI 53726, USA, e-mil: jfhunt@fs.fed.us (J.F. Hunt), zci@fs.fed.us (Z. Ci) Jingho Li: Deprtment of Biomedicl Engineering, Wisconsin Institutes for Discovery, nd Deprtment of Mterils Science nd Engineering, University of Wisconsin-Mdison, Mdison, WI 53715, USA; nd USDA Forest Service, Forest Products Lortory, Mdison, WI 53726, USA Shoqin Gong: Deprtment of Biomedicl Engineering, Wisconsin Institutes for Discovery, nd Deprtment of Mterils Science nd Engineering, University of Wisconsin-Mdison, Mdison, WI 53715, USA Introduction Sndwich composite (SC) mterils hving high strength to weight rtio re widely used for vriety of pplictions such s shipping, erospce, uilding construction, nd trnsporttion (Dvlos et l. 21; Vsiliev et l. 21; Shrf nd Fm 211; Wei et l. 213). They re fricted sed on fom or honeycom core etween two stiff fces (Fn et l. 27; Shlfn et l. 213; Smrdzewski 213). For some higher performnce pplictions of SC pnels (SCP), luminum iso-grid cores hve een used. Iso-grid structures were more efficient thn either fom or honeycom structures (Gison nd Ashy 1997). The ris re ligned in different directions tht etter distriute the stress for multidirectionl lodings. Bi-directionl grids re the simplest structure in the iso-grid domin nd provide the simplest mnufcturing, wheres it offers little sher nd twisting stiffness compred with tri-grid or multigrid core ligned structure. The tri-grid ris hve three different orienttions to resist the sher stress (Hn nd Tsi 23). Therefore, this type of structures with improved performnce cpilities ttrcted lot of interest (Huyrecht et l. 22; Wodesenet et l. 23; Higgins et l. 24; Zhng et l. 25). Interlocked kgome grid SCPs were fricted sed on cron fier composites nd their mechnicl ehvior ws studied including out-of-pnel, in-plne compression, nd ending tests (Fn et l. 27). The energy sorption chrcteristics of grid-stiffened fierglss composites were lso investigted under trnsverse loding. Gn et l. (24) found tht iso-grid structures hve good dmge tolernce where most of the energy sorption occurs eyond the initil filure. Cicl et l. (212) investigted truss-core structure mde of hemp/epoxy iocomposite y tensile nd flexurl tests nd found tht this core exhiits etter specific sher modulus nd strength thn model mde of polymeric core. Zuhri et l. (214) focused on structurl mterils with either interlocked grid-core mde of co-mingled flx-fier reinforced polypropylene or polylctide polymers. The sttic compressive properties of these mterils were modeled with finite elements (FE) to estimte the compressive response nd energy-soring chrcteristics. However, the ftigue ehvior of iso-grid structures is not well investigted.

2 2 J. Li et l.: Ftigue ehvior of ESCP The Forest Products Lortory (FPL) develops iosed engineered sndwich mterils iming t etter performnce in vrious engineered pplictions including pllets or tcticl shelters (Li et l. 213, 214, 216,). There re mny mid-level performnce pplictions tht require higher stiffness nd some level of fire nd wter resistnce. Therefore, phenolic lminted pper ws selected s n initil wood-fier-sed composite mteril s prt of tri-xil iso-grid sndwich pnels. The serch for some niche pplictions t reduced costs elongs lso to the reserch gols. Compred to the other structurl mterils mde of nturl fier (Zuhri et l. 214), this trixil iso-grid structure with its lrge tringulr cellulr cores mde from wood-fier-sed lminted pper hs significntly higher stiffness nd my e dvntgeous in lrger pplictions. In sndwich type pnels, fce compression nd core deonding re oserved in cse of oth undmged nd initilly wekened pnels (wescp) (Belingrdi et l. 27). The ftigue strength is dependent of the kind nd mount of dhesive pplied ut the thickness of fce sheet is not influentil (Jen et l. 29,). The filure mechnisms of composites hve een investigted lso y in-plne sher lod-ftigue tests (Binchi et l. 212). The ftigue properties of engineered strnd lumer nd engineered strnd pnels hve een studied nd it ws demonstrted tht these mterils could esily survive 1 6 lod cycles if loded to < 4% ultimte strength levels (Eckelmn nd Winndy 1978). The ftigue ehvior of oriented strnd ord (OSB) ws tested y five-point lod ending (Ci et l. 1996). Bo et l. (1996) compred the ftigue properties medium density fierord, prticleord, plywood, nd oriented strnd ord. The non-liner ehvior of ftigue nd heting of wood ws investigted y Nkno (1997). However, no literture ws found concerning the ftigue ehvior of tri-xil engineered sndwich core pnels (shortly: ESCP). This is the reson why in the present study, the ftigue ehvior of ESCP (sed on lminted pper) will e investigted y four-point ending ftigue. The focus will e the ftigue ehvior of the pnels, where the pnels will e sujected to cyclic loding. The expecttion is tht the knowledge in terms of the mechnicl chrcteristics of this type of sndwich pnels will e etter understood. Mterils nd methods Mterils: Phenolic impregnted lminted pper (NP61) with the nominl thickness of 2.4 mm ws otined from Norplex-Micrt Inc. (Postville, IA, USA). In this context, the lminted pper s mchine direction nd cross-mchine direction re designted s MD (x-xis) nd CD (y-xis), respectively. Epoxy 635 resin is from US Composites (West Plm Bech, FL, USA), with rtio of epoxy to hrdener of 3:1. Pnel design: Tri-xil engineered sndwich core pnels (ESCP) were prepred in the USDA, Forest Products Lortory, WI, USA s illustrted in Figure 1. The core nominl height with the liner ris ws 33. mm. The slots were cut slightly oversized to ccommodte the 6 ngulr orienttion etween the ris when ssemled. The slot spcing ws mm, thus n equilterl tringle ws creted fter ssemling. Before pplying epoxy resin s dhesive, ll lminte pper fce surfces were first lightly snded on the glue side, nd then the resin ws spred on the fces. A totl of 18 pnels were fricted. The configurtion includes three centrlly locted liner ris (Figure 1) with the spcing of 11.6 mm. Mesures of the pnel: spn 914 mm, width 267 mm, nd nominl thickness 38 mm (see Figure 1). Three pnels were tested y sttic ending. Twelve pnels were tested y ftigue ending tests t four stress levels. Three pnels were fricted to simulte wekened pnel (wescp) nd to oserve crck propgtion without resin long 1 mm portion of =1 mm Interlocked structure Initil crck c Ivdt (mm) Figure 1: () The tri-xil sndwich configurtion with n initil crck, () ending test dimensions, (c) pnel under sttic lod during ending test.

3 J. Li et l.: Ftigue ehvior of ESCP 3 ri. This resinless section ws locted within the ending sher zone s shown in Figure 1. The verge mximum ending lod vlue served s the control lod to determine the ftigue levels of 5, 6, 7, nd 8%. For this study, the mximum numer of cycles ws 1 6. There were lso three wescps tht were ftigue tested, one t ech lod level of 5, 6, nd 7%. The lortory environment during the test ws round 65±3% RH nd 23±3 C. Sttic ending test: Three pnels were (45 kn lod cell on n Instron 5587 Test Mchine) were performed ccording to the four point lod configurtion, ASTM C393-6, with cross-hed speed of 5 mm-min -1. The mximum ending deflection t mid-spn ws mesured y liner vrile differentil trnsformer (LVDT); Figure 1c shows pnel during loding. The fce nd core sher ending stresses could e determined using equtions from ASTM C However, the sher stress eqution in the stndrd ssumes n equivlent solid core configurtion. This eqution ws indequte for determining equivlent sher stress for the liner ris of the core used in this study. Therefore, the eqution ws modified for n equivlent I-em structure sed on the equivlent sher stiffness. τ ri P = β ( d+ c) Where P is the pplied lod, d is the sndwich totl thickness, c is the core thickness, nd is the pnel width. β is the equivlent sher stiffness rtio, which cn e written s: A (1) S β= (2) A I Where A S is the cross-sectionl re of equivlent solid core sed on stndrd, nd A I is the cross-sectionl re of equivlent I-em of tri-xil structurl core (Li et l. 213). The ending stiffness cn e determined y Eq. 3: slope ( ) D= l (3) 48 Where the slope is the 2 4% rtio of the pplied lod P to midspn deflection, l nd represent the ending spn nd the distnce from the support to the lod point, respectively. Results nd discussions Sttic ending properties Figure 2 shows the plots of lod nd mid-spn deflection of three ESCPs nd their verge for sttic ending. The verge filure lod ws 11.6 kn with 31.4 mm mximum deflection t the mid-spn (Tle 1). The verge fce stress of 7.4 MP, ws determined s the ultimte mterils strength for compression filure on the fce (Figure 3). The core sher stress ws clculted s n equivlent I-em for the ri of core using Eq. 2 sed on equivlent sher stiffness (Li et l. 213). The primry filure for pnels in sttic ending occurred in the pure ending zone (Figure 3) either in compression on the top fce or in tension on the ottom fce. Totl filure stress of either fce compression of 195 MP or tension of 173 MP ws reched efore the core sher stress limit of 17.9 MP t the core/fce interfce (epoxy resin) ws reched in the sher zone of the pnel (Li et l. 216). The core ri mteril s sher stress limit ws 84.1 MP, lmost four times the sher stress cpcity of the resin. The sttic core/fce interfce interction ssocited with the epoxy interfce resisted sher filure forcing filure to occur in compression or tension. Ftigue properties The 12 pnels were ftigue tested t the four stress levels or three pnels t ech stress level. The dt re summrized in Tle 2. The ending lods for the ftigue tests t the 5, 6, 7, nd 8% lod levels were 5.6 kn, Ftigue test: The 12 pnels were ftigue tested sed on ASTM D t four lod levels with three replictes for ech level (MTS mchine with servo-hydrulic lod control ctutor with compression/tension cpcity of ±17.8 kn). The set-up detils were the sme s for the stticlly tested pnels. The cycle durtion ws 1 Hz due to the cpcity of the hydrulic servo controller. Cyclic loding levels were determined on 5, 6, 7, or 8% of the mximum sttic ending lod nd were pplied for ech group in the ftigue tests, respectively. The servo signl ws sinusoidl with constnt mplitude lod rtio R =.1. As the mximum lod incresed for ech of the four levels, the minimum lod lso incresed slightly s eing 1% of tht lod. The ftigue test ended fter 1 6 cycles. If pnel did not fil fter 1 6 cycles, they were re-tested stticlly to ending filure. These dt were clled residul ending (RB) properties. Ftigue dmge test: Three pnels with n initilly wekened ri (wescp) were tested y mens of the sme procedure s descried ove for 5, 6, nd 7% mximum sttic lod levels. Bending lod (kn) SP 3 SP 2 Averge SP 1 Averge SP 1 SP 2 SP Mid-spn deflection (mm) Figure 2: Sttic lod vs. deflection for three pnels.

4 4 J. Li et l.: Ftigue ehvior of ESCP Tle 1: Sttic ending chrcteristics of ESCP. Group A Bend. lod, F mx (kn) Mx defl. Δ mx (mm) Fce stress, σ fce (MP) Core stress, τ ri (MP) Bending stiffness, D (kn m 2 ) SP SP SP Avrg (12.1) (6.7) (9.9) (12.7) (7.4) SP Pnel sumitted to sttic test. % Vrition. c d Figure 3: () Sttic ending filure mode, () ftigue filure mode, (c) residul tested filure mode of the pnel fter 1 million cycles t 5% stress level, (d) typicl crcks t the core:fce interfce for the ftigue pnel prior to filure. Tle 2: Averge ftigue chrcteristics of test groups. Fce properties Ftigue lod properties Test method Fce stress (MP) Equiv. core stress (MP) Mx. lod (kn) Min. lod (kn) Cycles (N) Sttic 7.4 (9.9).62 (12.3) 5% % (51.6) 7% (93.7) 8% (78.6) 5% (wescp) (8) 6% (wescp) (15) 7% (wescp) (5) Coefficients of vrition, in percent. % Of ftigue cycles for wescp vs. ESCP. 6.7 kn, 7.8 kn, nd 8.9 kn, respectively. The minimum ftigue lod for ech group ws clculted y the mplitude lod rtio of R =.1. The results from the ftigue life, Figure 4, show tht t 5% mximum lod level, the pnels hve ftigue life ove 1 6 cycles. The pnels cycliclly loded to 6% stress level filed fter cycles. When the stress level ws incresed to 7%, the ftigue life drmticlly decresed to cycles. At

5 J. Li et l.: Ftigue ehvior of ESCP 5 35 Mid-spn deflection (mm) Solid lines: Deflection under mx. ftigue lod Dsh lines: Deflection under min. ftigue lod Sttic ending filure deflection 8% 7% Residul ending filure deflection 6% 5% 8% 7% 6% 5% Crck length (mm) c d Dmge crck length g f e.e+ 2.E+5 4.E+5 6.E+5 8.E+5 1.E+6 Numer of life cycles.e+ 1.E+4 2.E+4 3.E+4 4.E+4 Numer of life cycles c Stress level (%) y = ln (N) R 2 =.833 d Dissipted energy per volume (J/m 3 ) % stress level 6% stress level 7% stress level 8% stress level 8% 7% 6% 5% 4 1.E+ 2.E+5 4.E+5 6.E+5 8.E+5 1.E+6 Numer of life cycles 1.E+1 1.E+2 1.E+3 1.E+4 1.E+5 1.E+6 Numer of life cycles Figure 4: Ftigue ehviors s function of cycles. () typicl pnel mid-spn deflections t 5%, 6%, 7%, nd 8% stress. Sttic ending nd residul ending mid-spn pnel deflections re shown for comprison, () crck length for the wescp t 6% stress level, (c) stress level degrdtion regression, (d) typicl dissipted energy per volume under different stress levels. 8% stress level, the ftigue life ws further reduced to cycles. As expected, the deflections for the initil ftigue pnels exhiited similr deflections s otined t the sme lod level for the pnels under sttic ending (Figure 2). Figure 4 lso shows tht the pnels loded t the 5% stress level only incresed 4.9% reltive displcement s it pproched 1 6 cycles. However, for stress levels of 6, 7, nd 8%, the deflection rte incresed s lod levels incresed. Oviously, the higher stress levels hd n incresing effect on deflection nd ccumulted dmge. To reduce the dt file size for ech test, deflection dt ws only cptured for decresing numer of cycles with incresing cycle count. Deflection dt were collected cycle-wise from the 1 th to 1 th cycles; every 1 2 cycles from 1 2 to 1 3 cycles; every 1 3 cycles from 1 3 to 1 4 cycles, etc. Since filure did not occur exctly on one of these dt collection cycles, the finl deflection line ws drwn s stright line to the sttic residul ending filure deflection point t the lst known totl cycle numer, thus indicting filure. All ftigue pnels hd similr filure mode. Rndom smll crcks slowly developed t the core/fce interfce in the ending sher zone until mjor crcks egn to occur. As lod incresed, mjor crcks rpidly propgted long the core/fce interfce primrily long the longitudinl direction. The pnels did not fil immeditely even though the presence of crcks ws redily visile. At some point, filure occurred suddenly within the epoxy. There ws lso evidence of surfce deonding filure within the lminte etween the core/fce interfces (Figure 3d). Residul ending (RB) properties At the 5% lod level, the pnels hd not reched filure fter 1 6 cycles. These pnels were then tested in sttic ending to determine the residul ending (RB) properties compred to the properties otined t the initil sttic ending test. The dt concerning mximum filure

6 6 J. Li et l.: Ftigue ehvior of ESCP Tle 3: Residul ending (RB) properties for the non-filed pnels t 5% stress level fter 1 6 cycles. Residul ending Bending lod F mx (kn) Mx. defl. Δ mx (mm) Fce stress, σ fce (MP) Core stress, τ ri (MP) Bending stiffness EI (kn m 2 ) RB RB RB Avrg (9.5) (11.8) (9.6) (9.1) (1.8) Δ Coefficient of vrition, (%). Difference to sttic ending, Tle 1 (%). lod, deflection, fce stress, core stress, nd ending stiffness for these pnels re listed in Tle 3. It is importnt to notice tht the verge vlues indicted tht the properties of these RB pnels were only round 5% lower thn the vlues of the initil sttic ending tests. However, the verge mximum deflection of the RB pnels ws 2.8% lower thn those tht hd een only stticlly tested. This counter intuitive result could e explined sed on solute displcement. The RB pnels, fter 1 6 cycles, hd finl curvture or deflection due to cyclic dmge s visile in Figure 4. Therefore, the mximum strin-to-filure included the initil curvture plus the sttic deflection t filure. Becuse these RB pnels hd een removed from the test pprtus nd then re-tested, it would e difficult to reconstruct their potentil initil zero-lod deflection. The smll digonl crcks in the epoxy interfce oserved with the pnels in the ftigue tests my hve contriuted to the lower totl filure lods. It is lso possile tht the smll crcks provided etter stress redistriution so tht ll components shred the lod, s compred to more rigid originl pnel without micro-crcks. The typicl filure mode for the RB pnels ws interfce sher in the sher zone (Figure 3c), wheres the sttic ending pnels filed in the pure ending zone (Figure 3), s descried ove. The pnels in ftigue tests hd developed smll crcks in the epoxy t the core/fce interfce throughout the sher zone so tht filure occurred when sher lods reched levels tht could not e restrined y the crcked epoxy. Ftigue properties of wekened pnels (wescp) The pnels with n rtificilly wekened ri (wescp, otined y omission of the resin long 1 mm long section of the ri) served for comprison for the core/ fce interfce filure zone with those of the ESCP. The wescps were prepred to force crck propgtion for etter visiility of filure mechnism t known loction. The cyclic percentge rtio, wescp/escp t 5, 6, nd 7% stress levels filed t cycle percentge rtios of 8, 15, nd 5%, respectively (Tle 2). Figure 4 shows dmged crck length propgtion s function of cycles for the 6% stress level ftigue tested pnel. Initilly, the crck length propgted quickly cusing the dmged pnel to fil with only 15% of the totl numer of cycles for the ESCP. These results show tht the dmged section quickly propgted cusing premture filure if section of the core/fce interfce is not well onded. For the ESCP, the crcks propgted rndomly, wheres the crck propgtion filure mechnism ws oserved on the wescp t prescried loction. While cre ws tken during the ESCP friction process, it is possile tht some of the experimentl dt vrition ws due to the rndom defects from mnul friction techniques when dhering the fce to core. The wescps nd ESCPs hve similr filure modes, ut the crck propgtion in the former is initited y the sence of epoxy nd propgted long the interfce s cyclic loding occurred. The wekened portion within the epoxy minimized the smll-crck stge. The crck propgtion of wescp t the cyclic loding of 6% mximum stress level is shown in Figure 4. It ws oserved tht the initil 1 mm crck nonlinerly propgted with incresing numer of cycles nd reched length of 35 mm efore filure (corresponding 38% of the totl spn). The other two wescps t 7 nd 8% stress levels s the former one. Ftigue dmge nlyses Normlized dmge s function of reltive life cycle for pnels t 6, 7, nd 8% stress levels re presented in Figure 5. The normlized dmge scle ws clculted s rtio of the loss in mid-spn deflection:

7 J. Li et l.: Ftigue ehvior of ESCP Stge III Normlized dmge scle (D s ) Stge I.2 Stge II 6% stress level 7% stress level 8% stress level Averge Ftigue life rtio (r N ) Reltive ending stiffness Control level 5% stress level.9 6% stress level 7% stress level 8% stress level Ftigue life rtio (r N ) 5% 6% 7% 8% Figure 5: Ftigue ehviors s function of ftigue life rtio. () Normlized dmge for ftigue pnels. () Typicl reltive ending stiffness for pnels under different stress levels. d -d r r N D = (4) S d - d t r Where d is the mximum mid-spn deflection t cycle N; r N d is the initil mximum mid-spn deflection for cycle 1 r nd d t is the mximum mid-spn deflection t the filure cycle. The ftigue life rtio ws clculted y: N r = N (5) N t Where N is the ftigue cycle, nd N t is the totl ftigue cycle for ech pnel. Stge I ws defined s the non-liner portion where su-structures filure nd stress redistriution occurred. Trnsition to stge II occurred etween.1 nd.2 of ftigue life rtio. Stge II is defined s the liner ccumultion of dmge s function of cycles. This stge consists of pprox. 6% of the totl ftigue life. Stge III is defined s the non-liner ccelerted dmge s cycles incresed. At this point, mjor crcks were developing nd could e oserved in the epoxy until the core/fce interfce could not support ny further sher stresses nd filure occurred. Stge III occurred pprox. fter the dmge hd reched.6.7 totl ftigue life. The 5% stress level pnel did not rech stge III even fter 1 6 cycles. At lest for this pnel construction, without n initil dmged section, 1 6 cycles were not sufficiently long to egin stge III ehvior. Ftigue ending stiffness degrdtion The reltionships for reltive ending stiffness degrdtion for the pnels under different stress levels re presented in Figure 5. As visile, only slight stiffness degrdtion occurred during the ftigue life time. The mximum degrdtion ws round 5% less for the pnels t 8% stress level compred with the sttic lod. This decrese ws fter out cycles. For the pnel t 5% stress level, there ws pproximtely 1% decrese t the end of 1 6 cycles. A rpid increse in deformtion incresed for the lst severl cycles efore filure. At the higher lods, deonding (crcks) in the core/fce interfces were decoupled so tht stiffness quickly decresed due to the ending lod trnsfer degrded till filure. Ftigue regression model One ojective ws to determine n pproximte reltionship etween pplied stress nd ftigue life, S-N. The experimentl dt implies tht possile exponentil reltionship exists etween these prmeters. The following exponentil eqution might descrie the reltion: S= AIn( N) + B (6) Where S is the percentge of pplied stress to the predicted sttic stress nd N is the numer of cycles-to-filure. A nd B re constnts tht relte to the mteril properties. The regression nlyses helped determine constnts A nd B nd their correltion coefficient R 2 (Figure 4c) is.833. Presumly, correltion could hve een etter in cse of not hndmde pnels, which contin inevitly smll irregulrities. Ftigue energy dissiption The typiclly dissipted energy ws clculted y difference etween the initil form of loding phse nd finl

8 8 J. Li et l.: Ftigue ehvior of ESCP form of relesing phse of ech cycle. Figure 4d shows the fitted curves with this regrd under different lod mplitudes. These stiffened pnels disply n elstic ehvior under ending ftigue loding while the hysteresis loop ws very smll in the ftigue tests. Accordingly, dissipted energy per volume in ech stress level is lso very smll, prticulrly, under low lod of 5% stress level without filure. Expectedly, the initil dissipted energy incresed s ftigue ending lod incresed. For the pnel under 5% stress level without filure, the dissipted energy showed ner liner chrcteristic during the entire ftigue life. However, the pnel t incresing higher stress levels showed incresing dissipted energy per volume. The dissipted energy per volume vs. life cycles for oth pnels t 6 nd 7% stress levels showed similr trends for pnels under 8% stress level, ut the curve s yield point ws delyed to higher numer of cycles. Proly, the numer of crcks nd severity of the crcks incresed s function of the totl dissipted energy of ech pnel. Conclusions ESCP were fricted nd tested to investigte oth sttic nd ftigue ending properties y mens of third point loding ending. The pnels tested in the sttic ending tests filed in the fces in the pure ending zone while the pnels tested in the ftigue tests filed in the core:fce interfce in the sher zone. The ftigue results showed tht the pnels t 5% stress level did not fil even fter 1 6 cycles. As stress levels incresed to 6% nd ove, the pnels filed with decresing ftigue cycles. The primry filure mode in the ftigue tests t 6, 7, nd 8% stress levels ws micro crcks in the epoxy resin or deonding oserved in the sher zone, which ws different from the sttic ending test, where fce filure occurred in the pure ending zone. For the sttic RB tests, epoxy deonding filure occurred etween the pure ending zone nd sher zone. The normlized dmge dt shows crck propgtion or dmge, which cn e descried s three stge dmge process. The stndrd S-N digrms were fitted to evlute the ftigue performnce. The dissipted energy of the pnels ws smll due to the high stiffness of the mterils, while the pnels under higher stress level hd lrger dissipted energy growth rtio s life cycle incresed compred to the pnel under lower stress levels. To improve ftigue performnce of these pnels, n interfce reinforcement method should e studied. Acknowledgments: This work is supported y USDA, Forest Products Lortory nd the uthors grtefully cknowledge the support of Sr Fishwild, Jmes Bridwell, Mrshll Begel, Dve Simpson, nd Mrc Joyl of EMRSL group for the mechnicl testing. References ASTM stndrd (26) C393, Test method for core sher properties of sndwich constructions y em flexure. ASTM stndrd (212) D7774, Stndrd test method for flexurl ftigue properties of plstics. Bo, Z., Eckelmn, C., Gison, H. (1996) Ftigue strength nd llowle design stresses for some wood composites used in furniture. Holz. Roh. Werkst. 54: Belingrdi, P., Mrtell, P., Peroni, L. (27) Ftigue nlysis of honeycom-composite sndwich ems. Compos. A 38: Binchi, G., Aglietti, G.S., Richrdson, G. (212) Sttic nd ftigue ehvior of hexgonl honeycom cores under in-plne sher lods. Appl. Compos. Mter. 19: Ci, Z.Y., Brdtmueller, J.P., Hunt, M.O., Fridley, K.J., Rosowsky, D.V. (1996) Ftigue ehvior of OSB in sher. Forest Prod. J. 46: Cicl, G., Recc, G., Oliveri, L., Perikleous, Y., Scrp, F., Lir, C., Lorto, A., Grue, D.J., Ziegmnn, G. (212) Hexchirl trusscore with twisted hemp yrns: out-of-plne sher properties. Compos. Struct. 94: Dvlos, J.F., Qio, P.Z., Xu, X.F., Roinson, J., Brth, K.E. (21) Modeling nd chrcteriztion of fier-reinforced plstic honeycom sndwich pnels for highwy ridge pplictions. Compos. Struct. 52: Eckelmn, C.A., Winndy, J.E. FEHS , The Performnce Test Method for Upholstered Furniture. Federl Supply Service, Generl Services Administrtion. Wshington, DC, Fn, H.L., Meng, F.H., Yng, W. (27) Sndwich pnels with kgome lttice cores reinforced y cron fiers. Compos. Struct. 81: Gn, C., Gison, R.F., Newz, G.M. (24) Anlyticl/Experimentl investigtion of energy sorption in grid-stiffened composite structures under trnsverse loding. Exp. Mech. 44: Gison, L.J., Ashy, M.F. Cellulr Solids: Structure nd Properties. Cmridge University Press, Cmridge, Hn, D.Y., Tsi, S.W. (23) Interlocked composite grids design nd mnufcturing. J. Compos. Mter. 37: Higgins, J., Wegner, P., Viisorenu, A., Snford, G. (24) Design nd testing of the Minotur dvnced grid-stiffened firing. Compos. Struct. 66: Huyrecht, S.M., Meink, T.E., Wegner, P.M., Gnley, J.M. (22) Mnufcturing theory for dvnced grid stiffened structures. Compos. A 33: Jen, Y.M., Ko, C.W., Lin, H.B. (29) Effect of thickness of fce sheet on the ending ftigue strength of dhesively onded luminum honeycom sndwich ems. Eng. Fil. Anl. 16: Jen, Y.M., Ko, C.W., Lin, H.B. (29) Effect of the mount of dhesive on the ending ftigue strength of dhesively

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