CE0L Student Lab Manual Wood Flexural Strength Introduction This section outlines the steps necessar to perform the Wood Flexural Strength lab experiment. The purpose of the lab is to determine the Modulus of Elasticit, E, and the Shear Modulus, G, of a given sample of wood using static bending. This lab test generall follows procedures in ASTM D 4 Standard Methods of Testing Small Clear Specimens of Timber. Using the Tinius Olson 5000 load frame, we will deflect two wood samples to specified rate while load and beam deflection measurements are recorded using a computer-based data acquisition sstem. With this data the modulus of elasticit and shear modulus can be found using deflection equations. Onl one sample of wood will be tested to failure. Relevant Theor Refer to Figure for the following discussion. Deflection in the beam for one point loading occurs at the mid-span of a beam, since this is the point at which the moment is greatest. L P Figure. Simpl-supported beam nomenclature with single point load. Tpical shear and moment diagrams are also shown. Note that the span distance, L in Figure, is the support-to-support distance. The actual beam member ma extend past the supports, but length of the beam between the supports is the span distance. Theoreticall speaking, this deflection (Δ) is caused from both bending and shear that act on the beam. However, in CE 242, Mechanics of Solids, it is learned that a pure bending assumption can be made for beam deflection. We can ignore the shear contribution if the beam length-toheight ratio is greater than 5. It is for this case that the percentage of shear contributing is ver small, which is wh we can neglect this. However; if the length-to-height ratio is less than 5, we Department of Civil Engineering, Southern Illinois Universit Edwardsville 6/2/2009 Page of 5
CE0L Student Lab Manual must include the shear contribution since its percentage of deflection contribution is large. For the case in which the length-to-height ratio is between 5 and 5, (5 < L/H < 5), the shear percentage of contribution must be calculated to determine if it ma be neglected. Deflection, Δ, can be determined using the following equations: b s Bending b F s Shear where P = force applied L = length between supports E = modulus of elasticit I = moment of Inertia F = shape factor G = shear modulus A = cross-sectional area of the beam Relevant Equations and Information The following information is given: The shape factor, F, for this lab experiment is.2 because it is a beam of rectangular cross-section. The following information will be collected: Length between supports, L (in.) Width, b, and Depth, h, of wood sample (in.) Tpe of sample used (i.e. white pine, ellow pine, etc.) Force applied, P (lbf) Deflection, Δ (in.) The following information will be calculated using the above collected data: Moment of Inertia, I (in 4 ) I * b* h 2 Beam cross-section area, A (in 2 ) A b* h Modulus of Elasticit, E (psi), and Shear Modulus, G (psi) Department of Civil Engineering, Southern Illinois Universit Edwardsville 6/2/2009 Page 2 of 5
CE0L Student Lab Manual F, or Materials and Equipment Verif the following equipment has been setup and materials are present: Wood specimen, approximatel 0 x ½ x ½ Linear scale Tinius Olson 5-kip load frame (preset for this experiment) Load cell (2-kip, minimum capacit) with signal conditioner/digitizer 2-inch Linear displacement transducer (LDT) with signal conditioner/digitizer Computer with GENTEST data acquisition software Digital camera Safet Considerations This experiment involves working with a powered load frame capable of producing a force of over 5,000 pounds. Alwas be sure to keep clear of the moving parts when the machine is operating. Keep the guards in place while machine is operating. When installing or removing the grips turn off the main power switch. Alwas wear safet glasses around this machine when it is operating. Procedure The initial set up of the machine and computer will be done prior to the lab. Each group will conduct the experiment once using two samples of wood, a long specimen, loaded to failure, and a short specimen not loaded to failure. It is the job of each group to conduct this experiment on their own with appropriate supervision. ) Carefull place either wood specimen on the supports. For the short specimen the distance between the supports, L should be approximatel 7 inches, and for the long specimen, the distance should be approximatel 28 inches. Make sure that the support holder is centered with the machine center (i.e. under the point load). There are lines marked on each to indicate these points. Once the specimen is placed on the supports, and it is checked that it is centered and smmetric about the point load, the distance between the supports (center to center) must be measured. 2) The machine will then be turned on and the point load will then be lowered to the specimen using a nominal force of up to 0 pounds. The machine force indicators must be zeroed and the LDT must be set up so that it touches (with no more than a tenth of an inch) the deflection plate. At this point be sure to check that the wood sample is still centered on the supports, beneath the point load, and also be sure to check that it appears level. ) Tare the force indicator connected to the computer if necessar and tare the deflection indicator. When starting the computer program, ou should be sure to make a thorough description of the lab. For instance, be sure to include the lab title, tpe of specimen, length, etc., so that when ou work on the lab report, ou will have that information available to ou. Enter a unique, descriptive file name. 4) Once the entire setup is complete, the force will be applied at a loading rate of 0. in/min. Department of Civil Engineering, Southern Illinois Universit Edwardsville 6/2/2009 Page of 5
CE0L Student Lab Manual 5) Maximum applied load will be done differentl for the short and long samples: a) The short specimen is tested up to a load of at least 500 pounds, but no more than the 2000-pound load cell capacit. b) The long specimen will be tested to failure. We are looking for a peak in the deflectionforce curve which will be displaed b the computer that collects the data. It should be noted that after the initial ielding of the specimen, it can still withstand some force. We will continue testing until the specimen has adequatel ielded and provided good results. Following the experiment for the long specimen, a sketch should be made of the failure. 6) Once the data for the first specimen has been collected and recorded to a file, the second specimen will be tested in a similar manner. Report requirements The following information must be included in the report: Brief description of the lab (memo) Summar of results including E and G All measured data All calculated data Load-Deflection Curves for both samples A sketch of the rupture pattern in the wood sample tested to failure Sample Calculations Note: Erroneous data ma be thrown out of the calculations as long as there is sufficient correct data to support our work. 28 Specimen,.5 x.5 cross-section: L/H: 28 /.5 = 8.7 > 5 pure bending is assumed Load: P = 9.6 lbf Corresponding Deflection: 0.528 in 0.528 9.6*28 48* E *( )*.5*.5 2 E = 70000 psi (7507.6 psi, use for later calculations, but give the answer to three significant figures) 7.5 Specimen,.5 x.5 cross-section: L/H: 7.5 /.5 = 5 (must include shear) Load: 90.8 lbf Corresponding Deflection: 0.069 in Department of Civil Engineering, Southern Illinois Universit Edwardsville 6/2/2009 Page 4 of 5
CE0L Student Lab Manual F G = 28000 psi 90.8* 7.5.2*90.8* 7.5 0.069 ( ) ( ) 2 48*7507.6*( )*.5*.5 4* G *.5 2 This value was obtained from the test on the long sample. Department of Civil Engineering, Southern Illinois Universit Edwardsville 6/2/2009 Page 5 of 5