GB/T17657-2013 Test Method for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels: Determination of Static Bending Strength and Elastic Modulus (Three Point Bending)
- Determination of static bending strength and elastic modulus (three-point bending)
7.1 Principle
The static bending strength and elastic modulus of three-point bending are measured by applying a load to the middle of the specimen supported by two points. Static bending strength is the ratio of the bending moment and flexural section modulus of a specimen under maximum load; Elastic modulus is the ratio of stress to strain generated by a load on a specimen within the elastic limit range of the material.
7.2 Instruments and equipment
7.2.1 Universal mechanical testing machine. Select an appropriate load range according to product requirements, with a measurement accuracy of 1% of the load value. The testing machine consists of the following parts:
a) Two parallel cylindrical support rollers (see Figure 8), the length of which should exceed the width of the specimen. When the basic thickness of the plate t ≤ 6mm, the diameter of the support roller is (10 ± 0 5） mm ; When the basic thickness of the plate t>6 mm, the diameter of the support roller is (15 ± 0 5） mm。 The distance between the support rollers should be adjustable.
b) When the basic thickness of the plate t ≤ 6mm, the diameter of the cylindrical loading roller (see Figure 8) is (10 ± 0 5）mm ; When the basic thickness of the plate t>6 mm, the diameter of the load rolling is (30 ± 0.5) mm. The loading roller is placed parallel to the support roller and has an equal distance between the two support rollers.
Unit: unit meter
blob.png
Figure 8 Schematic diagram of the device for measuring static bending strength and elastic modulus (three-point bending)
c) A deformation measuring instrument (such as a dial gauge or similar measuring tool) is placed in the middle of the support roller to measure the deformation of the specimen, with a graduation value of 0 01 mm。
d) A measurement system that can measure the load applied to the specimen with an accuracy of 1% of the measured value.
- 2. 2 vernier calipers with a graduation value of 0 1 mm. Select ranges from 0mm to 300mm, 0mm to 600mm, and 0mm to 1500mm based on the length of the test piece.
- 2. 3 micrometers. Graduation value 0 01 mm, select ranges ranging from 0mm to 25 mm, from 25 mm to 50 mm, and from 50 mm to 75 mm based on the thickness of the specimen.
- 2. 4 stopwatches.
7.3 Test pieces
7.3.1 Specimen size
Length l2 ≥ (20t+50) mm, t is the basic thickness of the specimen, and 150 mm ≤ l2 ≤ l050 mm; Width b=(50 ± 1) mm.
For hollow structural plates with tube holes parallel to the length of the test piece, such as hole shaped or honeycomb shaped plates, the width of the test piece should be at least twice the width of the cross-sectional unit of each tube hole (i.e. twice the tube diameter plus two wall plate thicknesses). The test piece has a symmetrical cross-section, as shown in Figure 9. If the tube hole of the test piece is perpendicular to the length of the test piece, the loading roller should be located directly above the wall panel.
Unit in millimeters
blob.png
Figure 9 Cross section of hollow slab
When measuring the static bending strength, if the deflection deformation of the specimen is large and the specimen is not damaged, the distance between the two supports should be reduced, but not less than 100 mm. The testing report should specify the distance between the supports at the time of specimen failure. If such a situation occurs, the test piece should be taken again for measurement.
Plywood specimens should have no significant characteristics that affect their strength.
7.3.2 Sample balance treatment
If necessary, place the test piece in an environment with a temperature of (20 ± 2) ℃ and a relative humidity of (65 ± 5)% until its mass remains constant. The difference between the results of two weighings spaced 24 hours apart shall not exceed 0.1% of the mass of the specimen, which is considered to be a constant mass.
7.4 Method
7.4.1 Measure the width and thickness of the test piece. The width is measured at the center of the long side of the specimen; The thickness is measured at the intersection of the diagonal lines of the specimen.
7.4.2 Adjust the span between the two supports to at least 20 times the basic thickness of the specimen. The minimum is 100 mm and the maximum is 1000 mm. Measure the center distance between supports to the nearest 0.5 mm.
7.4.3 Place the test piece flat on the support. The long axis of the test piece is perpendicular to the support roller. The center point of the test piece is below the loading roller (see Figure 8).
7.4.4 Apply constant speed loading throughout the entire experiment. Adjust the loading speed to reach the maximum load within (60 ± 30) seconds. Measure the deflection deformation of the specimen at the midpoint (directly below the loading roller) to the nearest 0.1 mm. Draw a load deflection curve based on the deformation and corresponding load values. The load is accurate to 1% of the measured value. If the deflection deformation is measured in increments, at least 6 pairs of load deflection values should be taken.
7.4.5 Record the maximum load to the nearest 1% of the measured value.
7.4.6 Take two sets of test pieces according to the longitudinal and transverse directions of the board for testing. Within each group of specimens, half of the specimens face upwards and half of the specimens face upwards during measurement.
7.5 Result representation
7.5.1 Static bending strength
7.5. 1. 1. Static bending strength of the specimen σ Calculate b according to equation (6), accurate to 0 1 MPa：
blob.png (6)
In the equation:
σ B – Static bending strength of the specimen, in megapascals (MPa);
Fmax – maximum load at failure of the specimen, in Newton (N):
L1- Distance between two supports, in millimeters (mm);
B – specimen width, old position in millimeters (m in);
T – thickness of the specimen, in millimeters (mm)
7.5.1.2 The static bending strength of each group of test pieces of a plate (see 7.4.6) is the arithmetic mean value of the static bending strength of all test pieces in the same group, accurate to 0.1 M Pa
7.5.2 Elastic modulus
7.5. 2. Calculate the elastic modulus of the test piece according to equation Eb (7), accurate to 10 MPa:
blob.png (7)
In the equation:
Eb – elastic modulus of the specimen, in megapascals (MPa);
L – Distance between two supports, in millimeters (mm);
B – specimen width, in millimeters (mm):
T – thickness of the specimen, in millimeters (mm);
F2-F1- The increase in load within the line segment in the load deflection curve (Figure 10-F1 value is about 10% of the maximum load; F2 value is about 40% of the maximum load). The unit is Newton (N);
A2-a1- The increase in deformation in the middle of the specimen, that is, the change in specimen deformation between force F2 and F1. The unit is millimeters (mm):
blob.png
Figure 10 Load deflection curve within the elastic deformation range
7.5. 2. 2 The elastic modulus of each group of test pieces (see 7.4.6) of a plate is the arithmetic mean value of the elastic modulus of all test pieces in the same group, accurate to 10 MPa.
