Relationships between small-specimen and large panel bending tests on structural wood-based panels

Transcription

1 Relationships between small-specimen and large panel bending tests on structural wood-based panels J. Dobbin McNatt Robert W. Wellwood Lars Bach Abstract Four test methods were used to measure bending strength and stiffness of nine different structural woodbased panels. Four plywoods, four oriented strandboards, and one waferboard were evaluated by one small-specimen test and by three large-panel tests: pure moment (Post flexure), midspan-loaded machine stiffness rating, and third-point loading. The three large-panel tests yielded higher modulus of elasticity (MOE)values and lower modulus of rupture (MOR) values than the small-specimen test. Regression equations were developed that accurately predict MOR and MOE values between test methods.these equations were applicable to the nine structural woodbased panels in both the panel length and width directions. Large-panel tests have been the suggested method of evaluating construction plywood for about 30 years (9). If small specimens are used, strength-reducing characteristics such as knots and veneer gaps can make results extremely variable. Variability can also be a problem when testing small specimens of other structural-use panels such as waferboard and oriented strandboard (OSB)(15, 20). Post (20) concluded that the most reliable data for use in engineering design are obtained from test panel sizes comparable to those in actual use. Problems can develop at the large deflections required to cause failure when bending thin sheets over long spans. Some panels deflect enough to slip off the end supports when a simply supported midspan or two-point load bending test is used. Also, at large deflections, axial forces develop in addition to the bending moment, which can affect calculated property values (19,21,24). For these reasons, Post (18) developed the pure-moment bending test for large panels, and it is now a standard test method for structural-use panels (3). The pure-moment bending test features direct application of equal moments to the ends of the panels. The frames applying the moment are free to move toward each other to avoid directly imposed axial forces at large deflections. Other large-panel bending test methods have been proposed for structural-use panels. A joint committee of the International Union of Testing and Research Laboratories for Research and Materials/International Council for Building Research and Documentation (RILEM/CIB) developed recommended test methods for structural plywood (21). Essentially the same tests have been proposed for other structural wood-based panels. The stated intent of these recommendations is that researchers...will follow these recommendations to permit an easier exchange of test results between research and commercial testing organizations throughout the world (21). Nondestructive machine stress rating (MSR) equipment has also been developed. It is designed to do repetitive static midspan tests on full-size panels in production conditions (1,4,16). The purpose of this study was to establish relationships between bending properties determined by three large-panel tests and a small-specimen quality control test for different types and thicknesses of structural woodbased panels. The work was a cooperative venture of the USDA Forest Service, Forest Products Laboratory (FPL), Madison, Wis., and the Forest Products Program of the Alberta Research Council (ARC), Edmonton, Alb., Canada. The authors are, respectively, Research Forest Products Technologist, USDA Forest Serv., Forest Prod. Lab., One Gifford Pinchot Dr., Madison, WI ; Manager, Forest Prod. Testing Lab.; and Manager, Special Projects, Forest Prod. Program, Alberta Research Council, Edmonton, AB T6H 5X2 Canada. This paper was received for publication in January Forest Products Research Society Forest Prod. J. 40(9): SEPTEMBER 1990

2 Literature review Several comparisons have been made between various panel sizes using different test methods. Bier (6) investigated the influence of panel widths (50,150,300, and 600 mm) on the bending properties of two thicknesses (5-ply, 15 mm; 7-ply, 21 mm) of radiata pine plywood. The panels were loaded at the third points over a 1050-mm span, He also loaded 50-mm-wide specimens at the center of a span of 48 times the panel thickness. He found no significant width effect on modulus of rupture (MOR) or modulus of elasticity (MOE). When comparing the third-point-loaded and midspan-loaded panels (different spans), Bier found no effect on MOE, but MOR from third-point loading was only 75 to 85 percent of MOR from midspan loading. In another series of tests using third-point loading of 600-mm-wide panels, Bier (5) found the following relationship between MOE based on total panel deflection (E ap. parent) and deflection between load points (E true) (E is in GPa): Wilson and Parasin (26) compared two widths of plywood (300 and 900 mm) using the pure-moment bending test in ASTM D 3043 (3). They found the mean MOR and MOE values from the 300-mm-wide panels to be about 7 percent lower than the 900-mm-wide panels. Szabo (23) compared two widths of waferboard (610 and 1220 mm) using the pure-moment bending test and found no differences in the MOR or MOE values. Wilson (25) compared MOE and MOR of 1/2-inch, 5-ply plywood using pure-moment bending of 4- by 4-foot panels and midspan loading of 2- by 26-inch small clear specimens. The MOR values of the small clear specimens averaged 58 percent greater than the MOR values of the large panels. In contrast, he found the MOE values of the small clear specimens averaged 10 percent less than the MOE values of the large panels. Those relationships were similar to those reported by Doyle and others (9). Szabo (22) compared the bending properties of waferboard using the pure moment test on 42- by 48-inch panels and midspan loading of 3-inch-wide specimens with a span of 24 times the thickness. The average MOE value of large panels was about 20 percent greater than that of the small specimens, and the average MOR value of the small specimens was about 20 percent greater than that of the large panels. McNatt (15) tested waferboard, OSB, flakeboard, and veneered composite panels using quarter-point loading of four different panel sizes and midspan loading of 3-inchwide specimens. He found the MOE values of small specimens to be 10 to 20 percent less than the MOE values of the large panels, but the MOR values of the small specimens were 6 to 30 percent greater than the MOR values of the large panels. Lehmann and Schaffer (14) proof loaded 2- by 4-foot flakeboard panels using midspan loading prior to cutting out small bending specimens. The panel MOE value averaged 23 percent higher than the small-specimen MOE value. Gerhards and Floeter (10) determined MOE values of 4- by 8-foot laboratory-made flakeboards using measurements of panel deflection under their own weight. Average MOE values were 23 percent higher than MOE values determined from the midspan-loaded small specimens that were cut from the flakeboards. Hanley and others (11) compared 16-mm- and 19-mmthick commercial structural particleboards tested in midspan loading using large panels (1500 by 900 mm or 1800 by 900 mm) and small specimens (304 by 100 mm or 354 by 100 mm). On the average, the small specimens were 30 percent stronger in bending; however, they were about 10 percent less stiff. Bach (4) correlated panel stiffness of structural woodbased panels measured by the MSR equipment (EI MSR ) with panel stiffness (EI PF) and maximum moment (M MAX ) measured by the ASTM pure-moment method (3). He reported these relationships (EI = 10 6 N mm 2 /mm, M MAX = N mm/mm): and In summary, the literature generally indicates that large-panel bending tests yield larger stiffness values but lower strength values than those of small-specimen bending tests. The method of loading large panels or small specimens may have a very small effect. Panel width does not appear to affect test results. The effect of test span apparently has not been investigated separately. Materials Nine different structural wood-based panel products were included in this study. Five panel products were manufactured in Canada: 1/2-inch and 5/8-inch sheathinggrade spruce plywood, two 7/16-inch OSB products, and one 3/4-inch OSB product. The other four panel products were manufactured in the United States: 3/8-inch southern pine sheathing-grade plywood, 1/2-inch Douglas-fir sheathing-grade plywood, 5/8-inch OSB, and 3/8-inch waferboard. Forty 4- by 8-foot sheets of each product were purchased. The five Canadian products were purchased locally by the ARC. The four U.S. products were purchased locally by the FPL. Test methods Half the sheets, 20 of each product, were marked for testing in bending with the span parallel to the 8-foot direction, and half were marked for testing with the span parallel to the 4-foot direction. All 160 sheets of the U.S. products were then cut in two 4- by 4-foot sections, and one section was shipped to the ARC Laboratory in Edmonton. In the same way, two hundred 4- by 4-foot sheets of the Canadian products were shipped to the FPL. Figure 1 shows the cutting diagrams for the 4- by 8-foot sheets. After sheets were cut to size, they were conditioned to equilibrium moisture content (EMC) at 65 percent relative humidity (RH). Bending properties of the nine product types were evaluated by the following three large-panel tests and one small-specimen test: FOREST PRODUCTS JOURNAL Vol. 40, No. 9 11

3 Figure 1. - Method of cutting 4- by 8-foot sheets for bending tests. Figure 3. - Schematic of pure-moment bending test equipment specified in ASTM D 3043, Method C (3). In this study, we tested 4- by 4-foot panels. D = 44 inches, S = 16 inches, L = 14 inches. Figure 2. - Schematic of equipment for machine stiffness rating of panel products. 1. Nondestructive measurement of bending stiffness of 4- by 4-foot panels using the MSR machine developed at ARC (4,8). 2. Pure-moment bending of 4- by 4-foot panels to obtain bending strength and stiffness (3). 3. Third-point loading of 300- by 1000-mm panels simply supported over a 900-mm span to obtain bending strength and stiffness (RILEM) (21). 4. Midspan loading of 3-inch-wide specimens over a span of 24 times the thickness to obtain bending strength and stiffness (ASTM/CSA) (2,7). The MSR and pure-moment bending tests were conducted at the ARC Forest Products Testing Laboratory. The 300- by 1000-mm panels and the small specimens were tested at the FPL. A schematic of the MSR machine for determining panel stiffness is shown in Figure 2. The machine applied a small load across the center of a 44-inch span. A second increment of load was then applied. Both loads were selected to fall within the linear portion of the load deflection range of the particular panel. The two loads Figure 4. - Third-point loading of 300- by 1000-mm panel; span is 900 mm. and incremental deflections were recorded and used to compute panel stiffness. These same panels were then loaded to failure using the pure-moment bending test described in ASTM D 3043, Method C (3) and shown in Figure 3. Bending strength (maximum moment and MOR) and stiffness were determined. The 300- by 1000-mm panels were tested by thirdpoint loading over a 900-mm span (Fig. 4), following the RILEM test method for plywood (21). The 3-inch-wide specimens were loaded at the center of a span of 24 times the nominal panel thickness as described in ASTM Standard D 1037 (2) and Canadian Standard CAN (7). 12 SEPTEMBER 1990

4 Results and discussion Results of all the bending tests are summarized in Tables 1to 3. Tables 1and 2 also include EMC and density of the various panel types at 65 percent RH. Moisture content and density Of the nine products, waferboard was the heaviest at 43 lb./ft 3 and had the lowest average EMC, 8-1/2 percent. The OSB averaged 41 1b./ft 3 and 8-1/2 to 9-1/2 percent moisture content (MC). Of the three plywood products, the southern pine panels were heaviest at 36-1/2 lb./ft. 3, and the spruce panels were lightest at 27 to 28 lb./ft. 3 The Douglas-fir plywood averaged just under 35 lb./ft. 3 and EMC averaged about 9-1/2 percent. The EMC of the spruce and southern pine plywood averaged 10-1/2to 11percent. Bending strength and stiffness Tables 1to 3 indicate that bending strength and stiffness values for structural wood-based panels are influenced by test method. The small-specimen (ASTM/CSA) test yielded lower MOE values, but higher MOR values than the large-panel tests, which agrees with results of other studies (12-16,24,26). In this study, third-point loading of 300- by 1000-mm panels yielded the highest MOE values, averaging 25 percent greater than that of the smallspecimen test. Pure-moment and MSR bending of 4- by 4-foot panels yielded MOE values that averaged 20 percent and 10 percent greater, respectively, than those of the small-specimen test. The pure-moment and third-point bending tests gave generally the same MOR values, and MOR values from the small-specimen test averaged 35 percent higher than those of the pure-moment and RILEM tests. Ratios of plywood stiffness along and across the grain direction of the face plies reflect panel layups. Stiffness TABLE 1 -Modulus of elasticity of structural wood-based panels parallel to panel length from four test methods. a TABLE 2. - Modulus of elasticity of structural wood-based panels perpendicular to panel length from four test methods. a FOREST PRODUCTS JOURNAL Vol. 40, No. 9 13

5 TABLE 3. - Modulus of rupture of structural wood-based panels from three test methods." TABLE 4. - Linear regression relationships between bending strength and stiffness of commercial structural wood-based panels for four test methods." Figure 5. - Excessive deflection of 300- by 1000-mm panel of 3/8-inch, 3-ply plywood with face grain perpendicular to span. ratios decrease as the number of plies increase when averaged over the four test methods: 3/8-inch 3-ply southern pine 8:l 1/2-inch 4-ply spruce 5.5:l 1/2-inch 5-ply Douglas-fir 3.5:l 5/8-inch 5-ply spruce 3.5:l Ratios of strength along and across the grain direction of the face plies averaged a little greater than 2:l for the 1/2- and 5/8-inch plywoods. For the 3/8-inch southern pine plywood, the bending strength ratio averaged almost 3:1 from the small-specimen test but less than 1.5:l from the pure-moment panel test. The ratio for the RILEM test could not be calculated because the panels tested with face grain perpendicular to the span deflected off the supports before maximum load was reached (Fig. 5).This was to be expected with the stationary supports and a span:depth ratio of about 94:l (900 to 9.5 mm). Ratios of OSB stiffness and strength along and across the direction of face strand layer alignment orientation averaged 2.5:l and 1.5:1, respectively, for all four products. Linear regressions were used to determine how well a particular test method could predict the MOE or MOR values from one of the other test methods. For example, a mill could predict what bending strength and stiffness could be expected from the large-panel Post flexure (PF) (pure-moment) test using the results of a small-specimen (SS ) quality control test: Ifthe bending test proposed by the joint RILEM/CIB committee as an international standard (21) were to be adopted, then mills could predict the strength and stiffness expected from that test using the small-specimen test results: [1] [2] SEPTEMBER 1990

6 [3] Equations [1] to [4] and eight additional relationships between test methods are given in Table 4. The MOE values from an in-plant MSR machine can be used to predict MOR, but correlations are somewhat lower than for other relationships (r 2 values range from 0.74 to 0.82, Table 4). The correlation between small-specimen and PF MOR is lower than expected (r 2 = 0.87). The small-specimen MOR value for the 3/8-inch plywood, parallel direction (8,080 lb./in. 2 ) is more than 90 percent greater than the corresponding PF MOR value (4,200 lb./in. 2 ). For all the other material s parallel and perpendicular directions, the small-specimen MOR value is no more than 48 percent greater than the PF MOR value. Although no attempt was made to avoid knots when cutting small specimens from the 4- by 8-foot sheets (Fig. 1), a larger proportion of the 3/8-inch southern pine plywood specimens were knot-free compared to the Douglas-fir or spruce plywood specimens. The result was a somewhat inflated MOR average for the small specimens of 3/8-inch plywood. Hirashima and others (12,13) and Panacek (17) also reported that MOR value difference between small specimens and large panels were affected by the presence of knots in the large panels. The relationships given in Table 4 can be compared to similar relationships reported in the literature. Bach (4) used panel stiffness (EI) from the MSR machine to predict EI from the PF machine for four thicknesses of structural sheathing panels: [4] Equivalent MOE data for the nine products in this study were converted to EI for calculating the following relationship: [5] Bach (4) also used EI MSR to predict maximum bending moment (M MAX ) expected from the PF test: The equivalent equation for data from this study is: [6] Predicted values for M MAX are practically identical for the two equations within the range of data considered. McNatt (15) included quarter-point loading of 1- by 2-foot samples in his study on panel size effects. This is similar to the third-point loading of 300- by 1000-mm samples (RILEM) used in this study. Identical small-specimen (ASTM/CSA) tests were included in both studies. The MOE relationship between small-specimen and RILEM tests in this study and between small-specimen and 1- by 2-foot panel tests in the earlier study by McNatt are presented in Figure 6. A single regression line through both sets of data shows very good correlation: [7] Figure 7 shows the equivalent plot for MOR data from the same series of tests. Again, a single regression line represents the data well: [8] Conclusions Values for MOR and MOE varied depending upon which of the four bending test methods was used: 1) PF Figure 6. - Relationship between MOE from small-specimen bending test and two similar panel bending tests. Figure 7. - Relationship between MOR from small-specimen bending test and two similar panel bending tests. FOREST PRODUCTS JOURNAL Vol. 40, No. 9 15

7 (pure-moment bending, ASTM D 3043, Method C); 2) nondestructive machine stress rating (midspan loading of 4 by 4-ft. panels); 3) third-point loading of 300- by 1000-mm panels (RILEM/CIB recommendation); and 4) midspan loading of small specimens (ASTM D 1037 and CAN M85). The midspan loading of small specimens method yielded lower MOE values but higher MOR values than the other three methods, which agree with earlier studies. Regression equations were developed that can accurately predict MOR and MOE values between test methods. For example, small-specimen MOE SS is related to large-panel pure-moment MOE PF by Equation [1]. All 12 regression equations (Table 4) developed were applicable to all plywood, OSB, and waferboard products tested in both the length and width directions. Literature cited Printed on recycled paper 16 SEPTEMBER 1990