PanelMSR An on-line stiffness tester for engineered wood panels

Transcription

1 PanelMSR An on-line stiffness tester for engineered wood panels Lister, Peter F. 1, Lau, Kenneth K. 2 ABSTRACT CAE Machinery Ltd. has developed a non-destructive on-line tester for measuring panel bending properties, such as stiffness and modulus of elasticity. Named the PanelMSR, the machine can test panels from 5/16 to 1-1/4 thick and runs at line speeds of up to 600 feet/minute. This paper describes the new PanelMSR tester, how it works and selected results from tests of the prototype machine. The use of on-line panel measurement data for both quality assurance and process control is also discussed. INTRODUCTION Engineered wood panels like plywood, oriented strand board (OSB) and particleboard are used in a wide range of applications where structural and durability performance are extremely important. For example, in wood frame housing, plywood and OSB are used extensively for roof, wall and floor sheathing. In these applications, panels must provide adequate strength and stiffness over the wide range of environmental conditions encountered during the lifetime of the structure. To ensure that panel products perform adequately in these applications, codes and standards have been developed. In North America, CSA 0325 (Canada) and PS-2 (United States) are the most widely accepted standards for OSB panels. These standards specify the minimum requirements for internal bond, thickness swell, modulus of elasticity (MOE) and modulus of rupture (MOR) as well as other properties. Similar standards also exist in Europe and Japan. Manufacturers producing structural panels are required to have quality control programs to ensure that the minimum requirements are met. Typical mill quality control programs involve testing as few as two panels per shift. Small samples cut from the panels are tested using off-line laboratory equipment and the results compared to the mill standards. While the results from these tests provide valuable information, infrequent sampling and small sample size mean that undesirable process variability may not be detected. Many hours can elapse between tests, and if problems occur, considerable amounts of off-grade panels could be produced. Additionally, the small number of panels tested, representing only a very small fraction of the daily production, significantly reduces the ability to detect any changes in the mean panel properties from the normal process variations. The limitations of current quality control practices mean that timely feedback on panel properties is not available for process control purposes. As a result, considerable variation in panel properties may occur. In one study, coefficient of variation (COV) values for particleboard panels varied from 14% for thickness swell to 6% for average panel MOE (Xu and Suchsland, 1998). In another study of OSB panels, COV values for MOE ranged from 4.9% to 8.1% (ARC, 1994). Other studies have also been carried out to investigate property variations within individual panels. P. Lau (1988) found that for waferboard panels, COV values for within-panel MOE/MOR ranged from 8.7% to 13.3%, and COV values for Internal Bond (IB) were as high as 21.7%. Ganev (1998) compared between-panel and within-panel properties of OSB panels from seven mills and found that COV values for between-panel MOE ranged from 11% to 16% while within-panel variation was over twice as large. These results not only demonstrate the large variation that can exist in properties within panels, but also cast some doubt on the effectiveness of small-sample tests to accurately reflect average panel properties. In practice, full-panel tests would more accurately reflect in-use panel performance. Variation in panel properties means that manufacturers must produce products that on average would greatly exceed the minimum code requirements. In a study of OSB panels produced by three different manufacturers, the average MOE 1 Product Development Manager, CAE Machinery Ltd., Vancouver, B.C., Canada. 2 Principal Consultant, KKL and Associates Consulting., Vancouver, B.C., Canada. 1

2 values were 14%-29% above the minimum average required by CSA 0437 (Bach, 1998a). This extra stiffness was achieved with increased panel density, which would require more wood per panel. As wood cost accounts for more than 40% of the variable manufacturing cost for most North American mills, even a small increase in the panel density would impact the total cost considerably. On-line non-destructive measurement of panel properties can help mills reduce manufacturing costs by optimizing the manufacturing process. Because the properties of every panel are almost immediately known, the effects of small changes in the process or raw materials can be quickly identified. With this information, the manufacturing process can be more accurately controlled so panel variability is minimized. Large safety margins are not required because any panels not meeting code requirements can be automatically identified and rejected. As a result, mean panel properties can be adjusted closer to the minimum requirements, thereby reducing panel costs. On-line measurements can also provide a valuable marketing tool. Panels having higher than average strength properties can potentially be sorted out and sold to specialty markets at premium prices. Manufacturers can also promote their products as being 100% tested and thereby guarantee the performance levels. This can help differentiate the products of value-added manufacturers from commodity producers. Until recently, equipment for non-destructive on-line measurement of panel properties was limited mainly to weigh scales, moisture meters, thickness sensors and blow detectors. Now, CAE Machinery Ltd. in Vancouver, Canada has developed an on-line tester for measuring panel bending properties. Named the PanelMSR, this new tester measures both panel stiffness and MOE. This paper discusses how the tester works, describes its main features and presents selected results from tests of the prototype machine. METHODS FOR MEASURING PANEL BENDING PROPERTIES A number of different methods can be used to measure panel stiffness and MOE (Welling, 1997). These include natural frequency measurement, wave speed measurement and load-deflection methods. Of these, load-deflection techniques are most widely used because they provide the most direct measurement of stiffness and because they are simple and reliable, which is an advantage for mill use. The most commonly used method for measuring panel bending properties is the static bending test prescribed by the ASTM D1037 standard. The method is illustrated in Figure 1 and involves testing samples loaded at the centre and simply supported at the ends. The small samples are cut from full-size panels and placed in a laboratory test machine. The tester loads the samples at a specified rate and measures the exerted load. Stiffness and MOE can be determined from the slope of the load-deflection curve using the well known beam equation given in Figure 1. Modulus of rupture (MOR) can also be measured if the test is continued until the sample fails. The static bending test described above can also be successfully applied to full-size panels. The Alberta Research Council in Edmonton, Canada has promoted this concept and has developed a tester for full-size panels that can be used for off-line measurements (Bach, 1985). The method has been accepted by CSA Test Methods for Construction Sheathing, but has not been widely adopted at the mill level. A more popular technique for measuring bending properties of full size panels is the Post Flexure method illustrated in Figure 2. The method involves applying a steadily increasing moment to both ends of a sample and measuring the resulting panel curvature. Stiffness, MOE and MOR can all be calculated from the resulting load-deflection information and maximum moment upon rupture. The method has been widely adopted by testing agencies in North America and is considered the standard method for testing bending properties of full-sized panels. For on-line use, a tester must be capable of measuring the bending properties of full-sized panels non-destructively at production line speeds. The static bending test can be adapted to meet these requirements by passing panels through a series of rollers that deflect the panel a known amount. By measuring the corresponding loads, load-deflection data is generated. This information can be utilized to estimate the slope of the load-deflection curve and the equation in Figure 1 used to calculate stiffness (EI) and MOE. This method has a significant advantage over other techniques because variations in bending properties along the panel can be detected as the panel travels through the rollers. Methods based 2

3 on vibration or sound wave measurement normally provide only average panel properties and are difficult to implement on-line because the panels would need to be stopped and clamped to make the required measurements. Two different approaches have been proposed for adopting the static bending method for on-line use (K. Lau and Yelf, 1987; Bach and Cheng, 1998). These are shown schematically in Figure 3. Both methods use two deflection stages to estimate the slope of the load-deflection curve shown in Figure 1. In the S-shape approach, however, the panel is bent equally in both directions, while in the W-shape approach, the panel is bent in a single direction. To understand how these two different measurement methods affect stiffness measurements, Figure 3 shows the schematic load-deflection curves of the two methods. In the S-shape method, panel stiffness is estimated from the loaddeflection points obtained by bending the panel in two directions. The slope of the line joining these two points is used to calculate the panel stiffness, and is an average of the bending properties for each side of the panel. The S-shape method will tend to slightly underestimate the stiffness of OSB panels because the load-deflection curve is typically non-linear at low deflection levels. This non-linearity is primarily caused by deviations in panel flatness. As long as a panel is relatively flat, the S-shape method will provide a good estimate of stiffness. If panels are severely dished, the S-shape method may underestimate the panel MOE. To avoid this problem, the W-shape approach was developed. In this method, the panel is bent in one direction and the bending properties estimated from two load-deflection points that lie in the linear region in the load-deflection curve. This concept is illustrated in Figure 3. Panel stiffness can be accurately estimated with this method, but only for one side of the panel. This is not considered to be a major limitation, because the stiffness difference between the two sides of the panel is normally small and most panels used in common structural sheathing applications are loaded from one side; e.g. snow load for roof sheathing, wind load for wall sheathing and occupancy loads for floor sheathing. Thus, the stiffness and MOE of panels in the in-use direction is of primary importance and can be very accurately measured using the W- shape method. In designing the PanelMSR tester, the pros and cons of the S- and W-shape approaches were carefully studied and a series of mill tests were carried out to investigate differences between the two methods. As a result of this work, the W- shape concept was chosen because of its greater accuracy in measuring panel properties in the in-use direction. PanelMSR ON-LINE STIFFNESS TESTER The PanelMSR tester measures stiffness and MOE in the panel s longitudinal direction and is designed to be installed in a mill s production line directly downstream from the press and trim saws. Hot panels entering the tester are deflected in two measurement stages and stiffness values calculated. These hot panel values are then used to predict stiffness and MOE of the panels at in-use ambient service condition. The PanelMSR tester is designed to run at feed speeds of 100 to 600 ft/min and to accept panels from 5/16 to 1-1/4 thick, up to 51 in width and 72 or more in length. All machine control and data acquisition is performed simultaneously using an industrial PC and custom software. An MS-Windows based user interface allows operators to easily set the machine operating parameters and view the measurement results. Specifications and overall dimension for the machine are given in Figure 4. A schematic drawing of the PanelMSR is given in Figure 5 and illustrates the main components of the machine, which are the main frame, the by-pass conveyor and the two measurement stages. Each measurement stage consists of a deflection roll at the centre and roll groups at each end. The roll groups consist of isolation rolls, which prevent outside forces from affecting the section of a panel in the test span, and reaction rolls, which resist deflection forces. The panel deflection is achieved by moving the infeed and outfeed roll groups and the deflection rolls to one of eight preset positions. Each setting is used for a small range of panel thicknesses and is designed so that panel bending stresses are approximately 10% and 30% of MOR for the first and second stages respectively. These stress levels are high enough that data collected by the machine are in the linear region of the load-deflection curve, but low enough that panels are not damaged during testing. In operation, the nominal panel thickness is entered into the control computer and the machine automatically adjusts to the correct thickness group settings. Feed speed is also automatically adjusted to match the mill s production line speed. 3

4 When a hot panel enters the machine, the PanelMSR measures the loads on each deflection roll as the panel moves through the two measurement stages. For a standard North American 8-ft long panel, data are collected for the central 5- ft portion. The data are then used to calculate the within-panel stiffness and MOE values. MOE calculations are based on either nominal panel thickness or actual values from the optional thickness sensors. The optional thickness sensor provides the more accurate MOE values and also provides valuable information on the panel thickness profile. As the PanelMSR is designed to be located directly down stream from a mill s press, the panels are tested when they are still hot. The in-service ambient temperature (cold) panel properties can be estimated from the hot panel values using correlation equations and the average panel temperature measured by an infrared (IR) temperature sensor. The relationship between hot and cold panel bending properties has been investigated in several studies that have shown cold panel properties can be accurately predicted from hot panel values (K. Lau, 1987; Bach, 1998b; Ohlmeyer and Kruse, 1999). The coefficients of the correlation equation depend on the panel composition and mill process parameters, therefore, these coefficients have to be determined for each mill. The PanelMSR software displays average MOE and standard deviation for each panel, along with upper and lower control limits and summary statistics. These values are saved in a data file, which can be viewed at any time. Data files can also be easily imported into a spreadsheet for more detailed analysis. RESULTS OF PROTOTYPE TESTING The prototype PanelMSR tester was built at the CAE plant in Vancouver and completed in April Figure 6 shows the tester installed in the CAE testing facility, along with rudimentary infeed and outfeed modules. Once the initial calibration and alignment work was completed, tests were conducted to verify that mechanical components, control hardware and data processing software were all working correctly. Tests were also carried out to measure natural frequencies of the structural components and to assess the dynamic effects of panels passing through the roll groups. These tests confirmed that the machine met basic design criteria for stiffness, dynamic loads and signal quality. An extensive series of panel tests were also carried out to assess measurement repeatability and accuracy. The repeatability tests involved running panels through the PanelMSR tester 30 times for each panel and determining the variation of the readings. The machine repeatability was expressed as the percent difference of each reading against the grand mean of the 30 runs. The 95% confidence interval of the machine repeatability was then determined from the measurement repeatability errors. Repeatability tests were conducted on a number of different panels for each thickness group and a typical result is shown in Figure 7. The figure shows a histogram of the errors associated with each run for a 23/32 thick panel. Measurement repeatability, characterized by the 95% confidence interval, was ± 0.942% for this particular test. The results of extensive testing of a full range of panel thicknesses have indicated that machine repeatability is well within ± 2% for all panel thicknesses tested. Accuracy tests were also conducted to compare the average stiffness measured by the PanelMSR with static measurements made using both static bending and Post Flexure methods. In one test, (150) 7/16 thick and (150) 23/32 thick OSB panels from four different mills were purchased at a local building material supply store. The panels were run through the PanelMSR tester and the stiffness values recorded for each panel. The full size panels were then measured on a static bending tester and the results were compared. Figure 8 shows the results of this particular test. The figure shows stiffness values for each panel as measured by the PanelMSR and the static bending tester. While the data for the 7/16 panels was tightly grouped together, the 23/32 panel data showed more stiffness variation, and were distinctly clustered by panel manufacturer. The data in Figure 8 was analyzed statistically to estimate how well the PanelMSR stiffness values matched the values measured by the static bending tester. A linear correlation equation of the data is shown in the figure. The slope of this line, 1.005, shows that the PanelMSR stiffness values are within 0.5% of the static bending tester. The R 2 correlation 4

5 coefficient of indicates very little scatter in the data. This is also reflected in the narrow width of the 95% confidence interval, which is virtually indistinguishable from the regression line shown on the graph. To study the relationship between the PanelMSR and Post Flexure stiffness values, the 300 panels from Figure 8 were sent to the APA, an independent testing agency that provides certification for engineered wood products produced by mills in Canada and the United States. The Post Flexure test results are shown in Figure 9 along with the corresponding stiffness values measured by the PanelMSR. The linear correlation line is also shown in the figure, as are the equation coefficients and the R 2 value. The slope of the correlation equation shows that the PanelMSR stiffness values were within 5% of the static Post Flexure measurements. The R 2 value of this relationship was 0.997, only slightly lower than that in Figure 8. A comparison of Figures 8 and 9 shows slightly lower correlation between the PanelMSR and the Post Flexure test than for the static bending test. This is because the Post Flexure method uses a different loading condition than the PanelMSR and static bending tester. In the Post Flexure test method, a panel is loaded to a uniform bending moment across the test span, while in the PanelMSR and static bending test method, the moment is the greatest at the centre of the test span and decreases linearly to zero at the end supports. As a result of the difference between these test methods, the within panel stiffness variability of a test panel would contribute to slightly different average panel stiffness measurements. Not surprisingly, the correlation of the PanelMSR test results with those of the static bending tester is better because these two methods have the same loading condition NEXT STEPS Extensive testing at CAE Machinery has demonstrated that the PanelMSR tester provides accurate and repeatable bending stiffness measurements for a wide thickness range of panels at production line speeds. These measurements also agree very well with measurements made using both static panel bending and the Post flexure methods. The next step in testing the prototype tester will involve installing the machine on-line in an OSB mill. Hot to cold panel correlations will be developed and further tests will be completed to assess the reliability, repeatability and accuracy of the tester. The data from these tests will be used to investigate how panel stiffness data can be used for process control purposes and to assess how panel properties can be improved to reduce panel costs. CONCLUSIONS On-line testing of panel bending properties provides mills with valuable information for both quality assurance and process control. By testing every panel, process parameters can be controlled more accurately so more consistent panels are produced. By reducing panel variability, mean panel properties can be adjusted closer to minimum requirements. This can result in considerable cost savings if raw material requirements can be reduced. In addition, more consistent panel stiffness and MOE would allow engineers and architects to design structural systems with better performance characteristics. For on-line use, load-deflection methods provide the most reliable and most direct measurement of panel bending properties. Of the several techniques available, the so-called W-shape method used by the PanelMSR provides the most accurate measurement of panel stiffness and MOE. The PanelMSR stiffness tester being developed by CAE Machinery Ltd. provides non-destructive on-line measurement of panel stiffness and MOE. The prototype machine has been extensively tested and provides excellent measurement repeatability and accuracy. Field tests are now being planned to fully evaluate the performance of the tester and to investigate its value as a process control tool. ACKNOWLEDGEMENT The authors would like to acknowledge to assistance of Dr. Lars Bach, Alberta Research Council and Dr. Simon Liu, National Research Council who both provided valuable assistance and advice on the PanelMSR project. Funding for the project was received from the NRC-IRAP program and the Science Council of British Columbia. 5

6 REFERENCES Alberta Research Council (1994). Machine Stiffness Rated Panel Products. 1993/94 year-end report for the Structural Board Association. Alberta Research Council, Forest Products Program, Edmonton, Canada. Bach, L. (1985). Machine stress rating panel products. 5 th Washington State University, Pullman, USA. Symposium on Non-Destructive Testing of Wood. Bach, L. (1998a). MOE variability within and between panels. Report prepared for CAE Machinery Ltd. Alberta Research Council, Forest Products Program, Edmonton, Canada. Bach, L. (1998b). MOR/MOE of hot, cold and reheated 7/16 OSB. Report prepared for CAE Machinery Ltd. Alberta Research Council, Forest Products Program, Edmonton, Canada. Bach, L. and Cheng, J.-J. (1998). Method and apparatus for on-line testing of the stiffness or strength of panels and especially of wood panels. US Patent 5,804,738. Ganev, S. (1998). Variability in OSB. Report. Forintek Canada Corp., Sainte-Foy, Canada. Lau, K. K. and Yelf, J. T. (1987). Temperature compensating continuous panel tester. US Patent 4,708,020. Lau, P. W. C. (1988). Structural performance of waferboard (5/8 ) for floor systems. Report prepared for the Structural Board Association. Forintek Canada Corp., Sainte-Foy, Canada. Ohlmeyer, M. and Kruse, K. (1999). Property Changes of particleboard after pressing. Poster presented at the 33 rd Particleboard/Composite Materials Symposium. April 13-15, Washington State University, Pullman, USA. Welling, J. (1997). Directory of Non-Destructive Testing Methods for the Wood Based Panel Industry. European Commission, Directorate General XII, Science Research and Development, B-1049 Brussels. Xu, W. and Suchsland, O.; (1998). Variability of particleboard properties from single and mixed-species processes. Forest Products Journal 48(9): FIGURES Static Bending Test P y L w t Load, P Load-Deflection Curve Deflection, y Equations: 3 L P EI =, 48 y EI = stiffness where E = modulus of elasticity P/ y = slope of load- deflection curve w t I = moment of inertia = 12 3 Figure 1. Static bending test for measuring panel stiffness, MOE and MOR. 6

7 M Post Flexure Test M t R w Equations: EI = M R, EI = panel stiffness E = modulus of elasticity M = bending moment R = radius of curvature I = moment where of intertia 3 w t = 12 Figure 2. Post flexure method for measuring panel bending properties. Methods for Stiffness Measurement S-Shape Method Bends panel in both directions and gives average stiffness W-Shape Method Bends panel in the in use direction and gives effective stiffness P 1, y 1 P 1, y 1 P 2, y 2 P 2, y 2 Load-Deflection Curve Load-Deflection Curve actual curve Load, P estimated curve Deflection, y P 2, y 2 Load, P actual curve P 1, y 1 estimated curve P 2, y 2 P 1, y 1 Deflection, y Figure 3. S-shape and W-shape approaches for on-line panel measurement. 7

8 PanelMSR Specifications Machine configuration: Two stage load-deflection, panel loaded in in-use direction. Bypass conveyor. Overall machine dimensions: 144 L x 122 W x 80 H Estimated machine weight: 2500 lbs Panel dimensions: 5/16 to 1-1/4 thick 51 wide (maximum) 72 long (minimum) Feed speed: 100 ft/min to 600 ft/min. Matches speed of infeed conveyor. Maximum panel speed/thickness: 100 ft/min (5/16 ) to 600 ft/min (1-1/4 ) Minimum panel spacing: 12 Thickness group settings: 9 including zero position Control and data acquisition: Industrial PC and custom software Measured data: Stage 1 and 2 loads, average panel temperature, panel thickness (optional) Calculated data: Average panel stiffness and MOE, standard deviation, max. and min. values Data files: Panel summary statistics, within panel stiffness and MOE Trending display: Control limits and averages for stiffness, MOE and thickness (optional). Number of panels tested, number of high/low panels Figure 4. Specifications and overall dimensions of the PanelMSR on-line tester. Infeed Roll Group CAE PanelMSR Stage 1 Stage 2 Outfeed Roll Group Panel In Bypass Panel Out - deflection roll - nip roll - isolation roll Figure 5. Schematic layout of the PanelMSR. 8

9 Figure 6. PanelMSR prototype installed in the CAE test facility. Number of panels Mean Stiffness: 1.8x10 6 lb-in 2 95% Confidence Interval: (+/ %) PanelMSR Repeatability Test 23/32" OSB Panel 30 Replications -1.00% -0.75% -0.50% -0.25% 0.00% 0.25% 0.50% 0.75% 1.00% More Deviation from the Mean [%] Figure 7. Repeatability test results for a 23/32 thick OSB panel. 9

10 2,500,000 Correlation between PanelMSR and Static Bending Tester Static Bending Stiffness [lb-in 2 ] 2,000,000 1,500,000 1,000, ,000 Manufacturer A - 23/32" Manufacturer B - 23/32" Manufacturer A - 7/16" Manufacturer B - 7/16" Upper 95% CI Low er 95% CI Regression Line Correlation Equation: y=1.005x R 2 = ,000 1,000,000 1,500,000 2,000,000 2,500,000 PanelMSR Stiffness [lb-in 2 ] Figure 8. Comparison of panel stiffness measured by the PanelMSR and static bending tester. Correlation between PanelMSR and Post Flexure Tester 2,500,000 Manufacturer A - 23/32" Post-flexure Stiffness [lb-in 2 ] 2,000,000 1,500,000 1,000, ,000 Manufacturer B - 23/32" Manufacturer A - 7/16" Manufacturer B - 7/16" Upper 95% CI Low er 95% CI Regression Line Correlation Equation: y=1.05x R 2 = ,000 1,000,000 1,500,000 2,000,000 2,500,000 PanelMSR Stiffness [lb-in 2 ] Figure 9. Panel stiffness measured by the PanelMSR and Post flexure tester. 10