J Wd Sci (1998) 44:348-353 The Japan Wd Research Sciety 1998 Yu-Gu Sun Takanri Arima Structural mechanics f wd cmpsite materials h Ultrasnic evaluatin f internal bnd strength during an accelerated aging test Received: Nvember 7, 1997 / Accepted: April 21, 1998 Abstract This research attempts t bserve indirectly the variatin f internal bnd characteristics fr wd cmpsite materials during accelerated aging test treatment using ultrasnic pulse-transmissin techniques. Particlebard (PB) and riented strandbard (SB) were the representative specimens. The transit time f the ultrasnic wave prpagating thrugh the samples alng the nminal length and thickness directins was recrded using an apparatus called PUNDIT (C.N.S. Electrnic, Lndn). The transit times were measured in the samples under an ven-dried cnditin after treating them with biling water at different treating time stages, and the velcity was then calculated based n the transit time. Examinatin f the internal bnd strength cnducted n the same samples was dne accrding t JIS A 598. A study f the relatins amng springback, internal bnd strength, and velcity indicated that a high crrelatin existed between ultrasnic velcity measured in the length r thickness directin and the internal bnd strength fr the PB specimen, but n significant crrelatin was bserved between the velcity measured in the length directin and the internal bnd strength fr the SB specimen. The results f this research suggested that ultrasund techniques can be applied t predict r evaluate the internal bnd state f sme wd-cmpsite materials made f relatively small particles, such as PB especially, during accelerated aging test treatment prcesses. Key wrds Wd cmpsite materials Accelerated aging test Springback - Internal bnd strength Ultrasnic velcity Y-G. Sun (U~). T. Arima Graduate Schl f Agricultural and Life Science, The University f Tky, Tky 113-8657, Japan Tel. +81-3-3812-2111 (ext. 5249); Fax +81-3-5684-299 e-mail: aa671@hng.ecc.u-tky.ac.jp Part f this paper was presented at the 47th annual meeting f the Japan Wd Research Sciety, Kchi, April 1997 Intrductin The accelerated aging test has been widely accepted as an alternative fr evaluating r predicting the severing life fr sme wd-cmpsite materials, because it is difficult t assess the variatin f mechanical prperties f the materials during practical use. The test entails biling the material in water and ven-drying it. This prcess cnstitutes an efficient methd generally utilized t evaluate the internal bnd strength and dimensinal stability f the materials as the treatment cycles are shrt. Hwever, fr sme wdcmpsite materials, such as particlebard (PB) bnded with urea-frmaldehyde resin, there is rapid lss f its internal bnd strength even during the first cycle f treatmentj Hence the bard is brken dwn, and the internal bnd strength cannt be evaluated accurately by this treatment methd. Therefre, an alternative is needed t reslve this prblem. The ultrasnic wave technique has successfully evaluated sme physical and mechanical prperties f wd prducts, such as varius static elastic and strength prperties. They crrelate with a dynamic elastic mdulus based n the velcity at which ultrasnic stress waves prpagate thrugh them. Gerhards 2 cited 15 investigatins in which meaningful crrelatins were fund between the stress wave mdulus and tensile, cmpressive, r flexural mdulus (ME) r strength (MR) f lumber r veneer. He als summarized the effects varius wd factrs have n stress wave velcity, which ranged frm 1 t 2ft/s parallel t the grain in defect-free wd at 9%-15% misture cntent. Wave velcity (1) decreases as the grain angle, wd temperature, r misture cntent increases; (2) is higher in latewd than in earlywd; and (3) is lwer in decayed wd. Rss and Pellerin 3 revealed that wave attenuatin, a measure f energy dissipatin prperties, is sensitive t bnding characteristics and is a valuable nndestructive technique that cntributes significantly t the predictin f tensile and flexural mechanical behavir f wdcmpsite materials. Using the velcity f ultrasnic waves prpagating thrugh wd-cmpsite materials t evaluate r predict
349 the internal bnd state has nt been studied, especially during an accelerated aging test. The bjective f this research was t identify an ultrasnic velcity sensitive enugh t assess the internal bnd state and ne that can predict the internal bnd strength, especially fr wdcmpsite materials bnded with sme wild adhesive. Experiments Befre the experiment we cnsidered varius ultrasnic frequencies that culd influence the velcities by which the waves prpagated thrugh the samples; fur cmmercial PB specimens with varying density were selected. The velcities f the waves passing thrugh them in the nminal three directins were investigated using 54- and 2-kHz prbes f the PUNDIT apparatus. The results indicated that different frequencies have n significant influence n the velcity measured in either the lngitudinal r the transverse directin. Mrever, the different frequencies seem t have had little influence n the velcity measured in the thickness directin. The velcity measured with 2-kHz prbes shwed relatively lwer values cmpared t thse measured with 54-kHz prbes, suggesting that the prpagatin f ultrasnic waves f a higher frequency is easily influenced by the internal structure f the specimen (cmpared t the waves f lwer frequency). In this case, because the tested PB samples were f a three-layer structure, and the surface layer was a mre precise structure than the cre layer during the ht-pressing prcesses, when the ultrasnic waves with higher frequencies pass thrugh the specimen in the thickness directin there are mre reflectins and mre lss f energy. This is due t the fact that waves must travel thrugh differential interfaces f the medium, 4 which als can be reflected in the velcities. Hwever, these phenmena may nt ccur with the waves prpagating in the length directin. These findings seems t indicate that waves f higher frequency are mre sensitive t the internal structure f medium. In this study, the 2-kHz prbes were selected fr measuring the transit time f the waves traveling thrugh the specimens. Materials and methds The cnstructin f the cnstituent elements in the wdcmpsite materials were bviusly different; cmmercial PB (bnded with urea-frmaldehyde resin) and SB (bnded with phenl-frmaldehyde resin ) were the representative specimens used in this experiment. Their air-dried densities were.63 and.62g/cm 3 and their nminal thickness 1.5 and 1.cm, respectively. All samples were cut frm each full bard t 5 5 cm, measured precisely in millimeters, within 1%. T minimize the variability, all samples were equilibrated t apprximately 12% misture cntent prir t testing. A ttal f 12 samples were used fr each specimen in this study; the samples were divided int 1 grups with 12 in each grup. ne grup was randmly chsen as a cntrl and was subjected nly t the vendrying treatment. The remaining nine grups were immersed in biling water fr the designated treatment time, increasing stepwise, and then dried t the ven-dried cnditin using an ven set at 15 C. The ven-dried cnditin was estimated by checking the samples until their weight reached apprximately cnstant values during the vendrying prcess. T avid the effects f misture cntent and temperature n the velcity f the ultrasnic wave prpagating thrugh the specimens, the treated samples were kept in a desiccatr with silica gel until their temperature decreased t rm temperature, at which time the velcities were measured. The transit time f ultrasnic waves prpagating thrugh the samples in bth nminal length (the dminant directin f particle arrangement inside the panels) and thickness directins, was recrded precisely t 1% in micrsecnds by the PUNDIT apparatus with 2-kHz prbes. The velcity f the ultrasnic wave was calculated accrding t the distance between the tw prbes divided by the transit time, as shwn in Fig. 1. The retentin f internal bnd strength f the sample after these measurements was examined accrding t the JIS A985 test methd in an effrt t analyze its relatin t the velcity. 5 Results and discussin Relatin between treatment time and springback In general, the swelling that ccurs when the bard is expsed t misture r immersin in water is the sum f tw cmpnents: (1) swelling due t absrptin f water by the hygrscpe particles f wd and (2) release f cmpressin stress imparted t the bard during pressing f the mat in the ht-press. The latter is nt recvered when the specimen is reverted t the dry cnditin. This unrecvered thickness swelling is called springback. 1 In this experiment, results shwed that the increase f springback crrespnds t the increase in treatment time fr bth PB and SB specimens. Fr the PB specimen, a higher rate f increase f springback was fund within 2min frm the start f the treatment. Subsequently, this increasing trend slwed with the increase in the treatment time (Fig. 2). Hwever, cmpared t the results with PB, the springback f SB increased rapidly nly within the first 1 minutes; after that n clear variatin was fund except that deviatins in the measurements became greates (Fig. 3). Such changes may be due t the fllwing factrs. First, the PB specimen used was a three-layer structure. The face layer, which cnsisted with smaller particles than the cre layer, was pressed mre precisely during the ht-pressing prcesses. Such structural characteristics may help t keep the biling water frm entering the internal space f the bard during treatment. The SB specimen, in cntrast, was f a nnlayered structure, and its cmpnent chips were gemetrically larger than the particles f the PB, which culd lead t biling water being absrbed mre easily
35 5 Prbes (2 khz) L 4 Specimen 3 ' 7" ' ~ 2 T. pulse R. pulse 1 (a) n Specimen t i I I I 1 2 3 4 5 6 Time &treatment (rain.) Fig. 2. Relatin between treatment time and springback fr particlebard (PB) specimen. Each vertical bar indicates standard deviatin frm 12 samples mll 3 (b) Fig. 1. Methds fr measuring transit time and calculating velcity in which ultrasnic waves prpagating thugh specimens, a, b Measuring in length and thickness directins, respectively. L, length f specimen; T, transit time; V, ultrasnic velcity; 7". pulse, transmitted pulse; R. pulse: receiving pulse during the treatment. Secnd, differences f the internal structures and dimensins f the cmpnent elements affected the regularity f the surface and the accuracy f dimensinal measurements after the treatment. ~- 2 Z 1 t t Relatin between springback and internal bnd strength Crrespnding t the increase in springback, the internal bnd strength decreased cnsiderably fr bth specimens. Fr the PB specimen (Fig. 4) the internai bnd values decreased t near zer when the springback reached abut 4%; after that n clear change was bserved. These findings agree with thse f thers) Figure 5 shws the relatin between springback and internal bnd strength fr the SB specimen. Cmpared t the PB results, the internal bnd strength decreased rapidly until the springback reached q I I I 1 2 3 4 5 6 Time f treatment (rain.) Fig. 3. Relatin between treatment time and springback fr riented strandbard (SB) specimen. Each vertical bar indicates standard deviatin frm 12 samples
351.8 3.5 3..6,z:: ~ 2.5 ~D "~.4 > 2. 1.5 1 2 3 4 5 e-.2 ~".9 ~-.7 1 2 3 4 5 Fig. 4. Effect f springback n internal bnd strength fr PB specimens d::..= 5.3 @.8 >.1 1 2 3 4 5 I ~.6 Fig. 6. Relatin f springback and ultrasnic velcity fr PB specimens ~.4 I,u,,.a.2 1 2 3 Fig. 5. Effect f springback n internal bnd strength fr SB specimens abut 2%. After that n marked variatin was fund, This finding means that the internal bnd state culd be damaged r weakened easily, as the biling water easily entered the internal space f the SB samples during the treatment. Relatin between springback and ultrasnic velcity Figure 6 shws the variatin in ultrasnic velcities in regard t the springback changes fr the PB specimen. The ultrasnic velcities measured in bth the length and thickness directins decreased linearly with an increasing springback. The crrelatin between the velcity and springback was clse, as expressed by the high crrelatin cefficients (R = -.88 and -.96 fr the velcity in length and thickness directins, respectively). These results suggest that the pathways f ultrasnic waves prpagating thrugh the samples change with the springback. In ther wrds, the pathways f the wave prpagatin became lnger than the apparent distances measured between the tw prbes. Figure 7 shws the relatin between the ultrasnic velcity and springback fr the SB specimen. The velcities measured in the thickness directin decreased linearly with an increase in springback (R -- -.78). These results shw a tendency similar t that seen with PB. Hwever, in the length directin, the velcity has n meaningful relatin with the variatin in springback. These results suggested that pathways f ultrasnic waves traveling thrugh the
352 -g, 6. ~. 3.5,a= 5.5 = 5. ~ 4.5 > 4. ~".8 % ',--J -7 '3 c~.6 e- "~.5 >.4 1 2 3 1 2 3 Fig. 7. Relatin f springback and ultrasnic velcity fr SB specimens % g~ "6 > 3. 2.5 2. 1.5 ~".7.5.l-a.3 +.a.1.2.4.6.8 a.2.4.6.8 Internal bnd strength (MPa) Fig. 8. Ultrasnic velcity dependence n internal bnd strength fr PB specimens SB samples in the length directin exhibited n remarkable change. These phenmena were due t the differences f dimensin and alignment f the cmpnent chips in the specimens. Relatin between ultrasnic velcity and internal bnd strength It is well knwn that internal bnd strength is determined by the quantity and quality f the bnding pints inside wd-cmpsite materials. The quantity f bnding pints is determined predminantly by dimensin, alignment directin, and the distributin state f the cmpnent elements in the materials if they were prduced under the same cnditins After the accelerated-aging test treatment, the internal bnd strength f the bard is decreased. In fact, crrespnding with the develpment f the treating prcesses, a cnsiderable mumber f bnding pints amng the cmpnent particles r chips wuld be damaged wing t the release f the cmpress-stress seen with ht-pressing during bard prductin. Figure 8 shws the relatin between internal bnd strength and ultrasnic velcities measured in the nminal length and thickness directins fr the PB specimen. With increased treating time, the internal bnd strength decreased, and the velcity f ultrasnic waves passing thrugh these samples decreased fr bth measuring directins. As described abve, vids r discntinuities ccur as a result the decreasing quantities f bnding pints inside the samples because f the change in springback during treatment. The ultrasnic pulses take much mre time t prpagate in the samples, as the pulses must circumvent the vids r discntinuities by the quickest rute. In ther wrds, the pathways f the ultrasnic waves prpagating thrugh the samples change depending n the degree f treatment. Fr the SB specimen (Fig. 9), the velcity in the thickness directin decreased with a tendency similar t that in the PB specimen. Hwever, in the length directin, n meaningful crrelatin was bserved between the velcity and internal bnd strength. In this experiment, as previusly mentined, cmpnent chips f the SB are much larger than particles f PB; sme f them are even larger than the size f the samples tested (5 cm). When the ultrasnic measurements are cnducted n such samples in the length directin, the waves passed directly alng the
Table 1. Sme crrelatin cefficients f linear regressin between experimental parameters Specimen Springback and Springback and Internal bnd and internal bnd velcity velcity 353 Length Thickness Length Thickness Particlebard -.94 -.88.96.81.96 riented strandbard.84 - -.78 -.79,-,, 6. Cnclusins 5.5 = 5. ~ 4.5.1 > 4. ~".8 "---'7 r~ =2.6.s >.4.......................... i....2.4.6.2.4.6 Internal bnd strength (MPa) The behavir f the velcity f ultrasnic waves prpagating thrugh PB and SB specimens after treatment with biling water and ven-drying was investigated. Ultrasnic velcities, measured in the thickness directin fr bth specimens, decreased crrespnding with the levels f treatment. Frm these results we deduced that as the treatment time increased the quantities f damaged bnd pints inside the samples increased as well; and as a result vids r discntinuities inside the samples increased. When the waves traveled in such samples, their pathways wuld be changed wing t the waves having t circumvent the vids r discntinuities. The velcities in the length directin were significantly different in PB and SB specimens as there was a dimensinal limitatin f SB samples in this experiment. High crrelatin cefficients between the experimental parameters were bserved by statistical analysis fr bth specimens The results indicated that the parameters bserved in the thickness directin have a clser crrelatin with the internal bnd strength than that in the length directin (Table 1). Therefre, it is strngly suggested that ultrasnic velcity is sensitive t the internal bnd status f wd-cmpsite materials bnded with sme wild adhesives, especially during accelerated aging treatment prcesses. ur next study will emply this technique t investigate the effects f the cmpnent elements n the internal bnd state. Fig. 9. Ultrasnic velcity dependence n internal bnd strength fr SB specimens straight fiber alignment in sme chips, and they uncvered vids r discntinuities in the samples. Such structural characteristics wuld leave the pathways f the waves unchanged even thugh sme bnding pints between the chips had been damaged. Hwever, acrss the thickness directin, because the cmpnent chips were aligned in the SB specimen in the same manner as particles in the PB specimen, except fr the larger dimensin the pathways f the waves wuld als be changed cnsiderably after treatment. References 1. Kajita H, Mukudai J, Yan H (1991) Durability evaluatin f particlebard by accelerated aging tests Wd Sci Technl 25:239-249 2. Gerhards CG (1982)Lngitudinal stress waves fr lumber stress grading: factrs affecting applicatins: state f art. Fr Prd J 32(2):2-25 3. Rss RJ, Pellerin RF (1988) NDE f wd-based cmpsites with lngitudinal stress waves. Fr Prd J 38(5):39-45 4. McGnnagle WJ (1961) Nndestructive testing. McGraw-Hill New Yrk, pp 214-215 5. Japanese Standards Assciatin (1994) JIS standard fr particlebard JIS A 598, Japan, Japanese Standards Assciatin