Investigation of wood fracture toughness using mode II fracture (shearing)

Transcription

1 JOURNAL OF MATERIALS SCIENCE 30 (1995) Investigatin f wd fracture tughness using mde II fracture (shearing) G. PROKOPSKI Civil Engineering Department, Technical University f Rzeszw, Pwstaricdw Warszawy 6, Rzeszdw, Pland The test results f fracture tughness fr three wd species, such as pine, alder and birch are presented. Examinatin f fracture tughness is carried ut using mde II fracture (shearing). Values f the stress intensity factr, K,c, are determined fr the three main anatmic directins f wd. Micrstructural tests f particular wd species, perfrmed n specimens alng the three main anatmic directins f wd, are discussed. Qualitative relatinships are fund t exist between the micrstructure f wd and the btained values f the stress intensity factr, K,c. 1. Intrductin A variety f building materials used fr engineering cnstructin have numerus drawbacks such as inaesthetic appearance, liberatin f varius substances detrimental t health during a prlnged utilizatin perid, r lack f resistance t the ersive influence f the envirnment, which causes degradatin f materials and f cmplete cnstructins. Wd and its cmpsites are relatively widely used fr such building cnstructin elements that have nt nly t satisfy strength cnditins, but t meet aesthetic, envirnmental and ther requirements as well. Such applicatins f wd, and particularly its cmpsites (such as plywds r glued wd), in high reliability cnstructins require precise determinatins f their strength parameters t be made. This has resulted in the implementatin f fracture mechanics methds int the examinatin f wd. The parameters defined in fracture mechanics, such as the stress intensity factr, K, and the fracture energy, G, characterize a state f stress at the tip f a defect at the mment f its nn-cntrllable grwth. Wd may underg failure during utilizatin due t fracture ccurring, in particular, alng its natural cleavage planes. In recent years sme research wrks have cnsidered the prblem f wd strength in terms f fracture mechanics. In the wrks f References [1-5] the results f testing different wd species and their cmpsites with the use f fracture mdes I and II are presented. The tests reprted in the abve wrks have prved the suitability f fracture mechanics fr evaluatin f wd fracture tughness and its cmpsites. The results btained shw a high "sensitivity" f the fracture mechanics quantities, e.g. the stress intensity factrs, Krc and KIIc, and the fracture energy, Gr~ and GIlt, depending n the particular wd species, its humidity, the mde in which it is laded, and als n the 002~ Chapman & Hall directin f sampling in the specimens during testing, i.e. the lcatin f primary cracks in relatin t the anatmic directins f wd. 2. Experimental prcedure Examinatin f the fracture tughness f wd was carried ut n specimens made f the fllwing three wd species: pine, alder and birch. The fllwing investigatins were carried ut 1. tests f the stress intensity factr, Knc (mde II, shearing; 2. micrscpic tests using a scanning electrn micrscpe. In additin t fracture tughness testing, tensile and cmpressin testings alng the fibres, and tests f bending strength were perfrmed (Table I). In the tests every ten specimens f each wd species were used. In the fracture tughness testing cube, specimens f 100 mm edge dimensin were used, with tw 50 mm lng primary ntches. The ntches were cut ut by milling. The specimens fr fracture tughness testing were taken frm a single balk alng the three main anatmic TABLE I The strength f wd Wd Cmpressive Tensile Flexural species strength strength strength (MPa) (MPa) (MPa) Pine 47.3 _ _ [6.0]" [t2.8] [27.1] Alder 40.5 _ [2.4] [18.1] [10.9] Birch 53.6 _ _+ 6.6 [6.1] [10.73 [22.5] a Values in square brackets dente cefficients f variatin (%). 4745

2 / p" ~%~\ r'" (/ ~...~ X-Ypltter Figure 1 Mde f taking the specimens fr the examinatins. Frce sensr Figure 2 Diagram f test stand. P Str;iggg2ge [ directins f the wd (Fig. 1). In each testing series seven specimens were tested. The graphs f lad-displacement relatinships were pltted using an x-y recrder. The specimens used fr the tests had the fllwing misture cntent and density values Misture Density cntent (wt%) (MG m- 3) Pine 10.0 _ Alder 11.5 _ Birch 12.0 _ Fracture tughness tests Fracture tughness tests were perfrmed accrding t mde II cracking (shearing) n the stand presented in Fig. 2. The lad was measured against the crack displacement with a strain gauge and registered n an x-y pltter. The stress intensity factr, Klle, was determined using the frmula derived by Dixn and Strannigan [6], in which the stress intensity factr, K.c, depends n a critical value f the frce, PQ 5.11PQ (na)l/2 Kn~- 2BW where PQ is the frce initiating cracking (grwth); B is the thickness under the crack, W is the height; and a is the length f the crack. Table II cntains values fr the stress intensity factr, KH, fr each batch f specimens. A lad-crack displacement curve was btained fr each specimen. Sme specimen curves are shwn in Fig. 3. In Fig. 4 the stress intensity factr, Knc, is pltted against wd species and type f sample. The btained stress intensity factr values, Knc, shw cnsiderable variatin in relatin t bth the wd species and specimen type (I, II and III). Fr type I specimens the btained values f KII e were decidedly the greatest. Type I specimens made f pine wd had Knr values five times greate r than thse f type II specimens, and abut tw times greater than the Knr values f type III specimens. In the tests f alder wd, type I specimens had KII e values seven times greater than the values f type II specimens, and abut tw times greater than thse f type III specimens. In the tests f birch wd, type I specimens had K. values five times greater than the values f type II specimens, and abut tw times greater than thse f type III specimens. The failure curves btained in the fracture tughness tests shw different behaviural tendencies f particular wd species and type f specimen (I, II and III). In the tests f type I specimens the existence f cnsiderable plasic defrmatins has been fund in all three wd species. The character f the failure prcess and the btained values f destructive frces were determined by mutual relatins between particular TABLE II Stress intensity factr, K.c, fr sample types I, II and III Pine Birch Alder I II III I II III I II III KI]e (MNm 3/2) " a a 1.130" 0.156" 0.429" 1.337" a 0.696" 0.049" " 0.079" 0.034" 0.076" a a 0.015" [6.6] b [16.9] [16.2] [7.0] [22.0] [17.7] [5.0] [9.0] [2.2] "Values are fr Kll _+ 6. b Values in square brackets dente cefficients f variatin. 4746

3 15.0 b z v, i (a) 5(mm) i (b) I 5(mm) Figure 3 Examples f lad-displacement fr alder (a), birch (b) and pine (p) curves btained frm the fracture tughness tests : (a) type I, (b) type II, and (c) type III. Z ! (c) 5(mm) cmpnents f the wd structure and the "rdering" level f the structure. The variatin f the values and prprtins f the stress intensity factr, K~c, f particular wd species was caused by differences in the structure Micrstructural examinatin Micrstructural examinatin was carried ut n samples taken alng the three main anatmic directins f particular wd species, with the use f a scanning electrn micrscpe. An area f abut 400 mm 2 was bserved fr each specimen. The specimens fr micrstructural examinatin were sprinkled with graphite pwder. The magnificatin used was frm 50 t 1000 times. The micrstructural examinatin shwed structural differentiatin amng the wd species tested and als an evident structural differentiatin alng the three main directins f the anatmic structure f wd. The best structural "rdering" in all directins was shwn by pine wd (Figs 5-7). On the fractures, distinctly frmed fibres were visible, with specific "tube-like" crss-sectins, situated alng the III directin, i.e. the directin f grwth f the tree, in the frm f a reticular structure (Fig. 7). In the pine wd structure, relatively less structural disrder was fund with increasing regularity f the structure. The mst chatic structure was fund in the case f alder wd (Figs 8-10), which had the clsest "packing" f particular elements f the structure. Numerus diversified structural elements were seen, which created disturbances in the structure. The character f the structure clse t that f alder wd, while having fewer structural disrders, is shwn by birch wd (Figs 11-13). In the micrgraphs, fibres can be seen which are develped in the directin f grwth (Fig. 13) and have defined "tubelike" crss-sectins (Fig. 12), with numerus structural disrders lateral in relatin t the directin f the fibres. 4747

4 1.4 E z (a) ibireh Alder Pi ne A E z Figure 5 M.icmstr, uet~re f t~me wd, fracture type I, shwing a visibly rge~ed ~al ~fr~ac~e ~31f ~e ~zlear,1.y s ",t~bular" fibres f pine,~:~d ~ ~e~veqy,large <~s~sectfi:~al dimensin I I I ( b ) Birch Alder Pine 0,7 #- E Z % t ~ ~ Figure 6 MicmStructure~f pj~re wd, fracture type II, s'hwing the visible fibrjas -~trtmtca~e i" ~pi~e 'wd, ~wit, h taxerally #tuated elements f str~uctu-re ~c~neefi~g ~e '~bres I L I c ) Birch Alder Pine Figure 4 Stress intensity factr, Kn~, fr specimens: (a)type I, (b) type II, and (c) type III. 3. Results and discussin The examinatin f fracture tughness carried ut using mde II fracture, shearing, and the micrstructural tests have shwn the ccurrence f qualitative relatinships between the btained values f the stress intensity factr, Knc, and the micrstructure f particular wd species. Significantly higher values f KII c have been fund fr tw species f hardwd (alder and birch) in relatin t thse f sftwd (pine). This is mainly true fr type I specimens, fr which Kn values fr alder (1.130 MN m-3/z) and birch (1.337 MN m-3/2) clearly exceed the value f Kn~ btained in the studies f pine (0.783 MN m- 3/z). Figure 7 Micrstructure f pine wd, fracture type III, shwing the visible irregular, reticulated surface ~f!ttae pine 'wd s which frms a natural cleavage plane. 4748

5 Figure 8 Micrstrucmre f' aide~ wd, fracture type I, shwing a visibly lateral fractnrezfttie chatic fibre structure f alder wd f small crss-sectirr, with' mutually intersecting structural elements. Figure 11 Micrstructure f birch wd, fracture type I, shwing a lateral fracture f the cmplicated structure f birch wd, with visible "tubular" crss-sectins f fibres f small dimensin and elements cnnecting the fibre layers. Figure 9 Micrstructure f alder wd, fracture type II, shwing the visible fibrus structure f alder wd, with numerus lateral micrfibres that cnnect the main fibres. Figure 12 Micrstructure f birch wd, fracture type II, shwing the visible irregular structural fibres f birch wd, with numerus lateral micrfibres that cnnect the main fibres. Figure 10 Mierstructu~e f alder wd, fracture type III, shwing the visible, fairly regular; fibrus structure f alder wd, with a few structural elements; situated in different directins. Figure 13 Micrstructure f birch wd, fracture type III, shwing the clearly visible fibrus structure f birch wd, with a large cntent f main fibres. 4749

6 Such behaviur f particular wd species under breaking lad is related t their density and t the bserved structure f specimen fractures. The cherent structure f pine wd is the best rdered f all, with clearly frmed cleavage planes alng directins II and Ill (Figs 6 and 7). The pine wd fracture acrss the fibres (directin I) is regular and tubular, withut any additinal structural elements. The fractures f alder wd, and particularly thse f birch wd, have a cmpact and very cmplex structure (Figs 8 and 11). The structure f these fractures is cmplicated and shws a wide variety f frms that cnnect the particular elements f the main fibres. This is directly related t the differences existing between the micrstructure f hardwd and sftwd. Hardwd is cmpsed f fur cell types, while sftwd is nly cmpsed f tw fairly lsely bund cell types in the structure. Figs 8 and 11 illustrate the mre cmplicated and rugged failure surfaces f alder and birch wd, while the fracture surface f pine wd (Fig. 5) is regular and significantly less cmplicated. The greatest stress intensity factr values in the examinatin f II and III type specimens, are assciated with birch wd, and are apprximately 50% greater than the values btained in the examinatin f pine and alder wd. Micrscpical analysis f the fractures f these specimens has shwn that birch wd has the mst cmplex micrstructure f all the three wd species. The mutually perpendicular elements f the birch wd structure (Figs 12 and 13) cause the energy necessary fr failure t be greater than that fr the ther wd species. In Fig. 7 a flat grid f mutually perpendicular fibres is seen, which indicates easier shearing f pine wd alng the natural cleavage plane, i.e. alng directin III. Such arrangement f the fibre plane prmtes crack prpagatin at a relatively small frce which in this case is, hwever, cmparable with the frce necessary fr the failure f alder wd. The relatively high (as cmpared t alder wd) fracture tughness f pine wd alng directins II and III can be explained by sme similarity f the micrstructure f these wd species (Figs 6, 9 and 7, 10) and similarity f density. The evidently mst cmplex micrstructure alng directins II and III (a great number f highly develped elements that are lateral t the fibres, Figs 12 and 13) is shwn by birch wd, fr which the btained values f Knc are the greatest, being, respectively MN m- 3/z fr directin II, and MN m-3/2 fr directin III. The fracture tughness results btained have shwn that the use f fracture mechanics-based research methds in cnjunctin with micrstructure studies in relatin t wd is justified. Significant variatin f the KII c values fr particular wd species indicates that wd is sensitive t this kind f examinatin. References 1. S.M. CRAMER and A. D. PUGEL, Int. J. Fracture 35 (1987) K. WRIGHT and M. FONSELIUS, in "Prceedings, First Internatinal RILEM Cngress", Vl. 2 (Chapman & Hall, Lndn, 1987) pp A. VAUTRIN and B. HARRIS, J. Mater. Sci. 22 (1987) G. PROKOPSKI, ibid2 28 (1993) P. TRIBOULOT, "Reprt de D.E.A.', Universite de Metz (1978/79). 6. J. R. DIXON and J.S. STRANNIGAN, J. Strain Analysis 7 (1972) 125. Received 15 March 1993 and accepted 15 March