1 Scope
This document specifies the test principles, test equipment, specimens, test conditions and specimen state adjustment, test procedures, test results and processing and test reports for the bending performance of the sandwich structure.
This document applies to the determination of the apparent bending strength and apparent bending modulus of the laminated structure when bending, the bending stiffness and shear stiffness of the laminated structure, the stress and elastic modulus of the laminated structure panel, the shear stress and shear modulus of the core of the laminated structure, and also applies to the determination of the bending adhesive layer strength between the panel and the core of the laminated structure.
2 Normative references
The content of the following documents constitutes an essential provision of this document through normative references in the text. Among them, note the date of the reference document, only the date of the corresponding version applies to this document; do not note the date of the reference document, its latest version (including all the revision of the list) applies to this document.
GB/T 1446 General rules for test methods for the properties of fibre-reinforced plastics
GB/T 3356-2014 Test methods for flexural properties of oriented fibre-reinforced polymer matrix composites
GB/T 3961 Terminology for fibre-reinforced plastics
3 Terms and definitions
The terms defined in GB/T 3961 and the following terms and definitions apply to this document.
3.1
Tensile or compression strength of facing
The maximum stress to which a sandwich structure is subjected when the panel is damaged under bending load.
3.2
core shear strength
The maximum shear stress to which the sandwich structure is subjected in the event of core failure under bending load.
3.3
Flexure adhesive layer strength
Sandwich structure under bending load, the maximum shear stress it bears when the adhesive layer is damaged.
3.4
Overhanging beam three point bending
A long beam with an overhanging arm, with a concentrated load in the span, with three displacement transducers installed in the span, at the two overhanging points, to measure the deflection of the span, left and right overhanging points of the bending test.
3.5
Apparent bending strengthAppearance bending strength of sandwich structures.
3.6
appearance bending modulus
appearance bending modulus
The bending modulus of the sandwich structure.
3.7
Nominal thickness of facing
The thickness of a panel obtained from theoretical calculations (design values) or from measuring a blank panel.
4 Test principle
Determination of panel tensile strength by three-point bending of long beam specimens of sandwich structure, determination of core shear strength by three-point bending of short beam specimens of sandwich structure, determination of bending stiffness and shear stiffness by three-point bending of outstretched beam specimens of long beam specimens of sandwich structure, so as to determine the modulus of elasticity of the panel and the shear modulus of the core.
5 Test equipment
5.1 Testing machine should comply with the provisions of GB/T 1446.
5.2 loading indenter and support see Figure 1. loading indenter pad width of the flat part should meet 10 mm ≤ W ≤ 30 mm. support at the free rotation of the pad, the width of the flat part should meet 10 mm ≤ Ws ≤ 20 mm, the length of the flat part should be greater than the width of the specimen.
6 Specimen
6.1 The specimen is a rectangular shape with a rectangular cross-section.
6.2 The thickness of the specimen shall be the same as the thickness of the sandwich structure. When the thickness of the sandwich structure is not determined, in order to determine the tensile strength of the panel, the shear strength of the core or the shear modulus of the core, it is recommended that the thickness of the core be taken as 15 mm and the thickness of the panel be taken as 0.3 mm~1.0 mm.
6.3 The width of the specimen should be less than one-half of the span and not less than twice the total thickness. The recommended specimen width is 60 mm. For honeycomb, corrugated and other lattice cores, the specimen width should include at least four complete lattices.
6.4 The length of the specimen is the span plus 40 mm or one half of the thickness, whichever is greater, and the length should meet the span when an outstretched arm is required:
7 Test conditions and specimen condition adjustment
Test environment conditions and specimen condition adjustment according to the provisions of GB/T 1446.
8 Test procedure
8.1 Specimen appearance inspection in accordance with the provisions of GB/T 1446.
8.2 Qualified specimens will be numbered, measuring the width and thickness of any three places within the span of the specimen, to take the arithmetic mean. Panel thickness to take the nominal thickness of the panel or the average thickness of 10 panels of the same batch of specimens. The accuracy of the measurement is in accordance with GB/T 1446. For lattice type cores of sandwich structure specimens, record the number of lattice walls on the cross-section.
8.3 Adjust the support according to the selected span, and install the displacement transducer in the span, see Fig. 1. when measuring the bending stiffness or shear stiffness of the sandwich structure, install three displacement transducers according to the method of three-point bending of the outer extension beam, see Fig. 2.
8.4 The loading speed is suitable to make the specimen reach the failure condition within 3 min~6 min, in general, the loading speed is 2 mm/min~5 mm/min for the determination of panel tensile strength or core shear strength, and 0.5 mm/ min~1.0 mm/ min for the determination of the bending stiffness or shear stiffness of the sandwich structure.
8.5 Determination of panel tensile strength or core shear strength, the specimen placed on the support, plus the loading indenter, and a hard rubber gasket between the indenter and the specimen, adjust the zero point of the test machine, according to the selected loading speed, uniform continuous loading until the specimen damage, read the damage load, observe and record the form of damage in accordance with Table 1.
8.6 To determine the bending stiffness or shear stiffness of the sandwich structure, apply a preload (15%-20% of the damage load), eliminate the gap between the specimen and the support, and unload to 5% of the damage load. Adjust the displacement transducer zero point and load to 40%-50% of the damage load at the selected loading rate. If the whole load-deflection data is needed, it should be measured until the damage, when there is a drop in the load value or stagnation phenomenon, then the load value for the damage load. If the specimen is M-shaped (generally the shear stiffness of the core is small, the bending stiffness of the panel is large), can be in accordance with Appendix B, multi-span method to determine the bending stiffness or shear stiffness of the sandwich structure. Core shear strength and core shear modulus of the determination of the appropriate test according to GB / T 1455.
8.7 Determination of apparent bending strength, apparent bending modulus, test speed according to GB/T 3356-2014 of 7.3 for loading.
Note: Apparent bending strength, apparent bending modulus is only applicable to the same structure between the samples, when the core type is different or different panel pavement while referring to the use.
9 Test results and processing
9.1 Plot the load-deflection curve.
9.2 The shear stress of the core is calculated according to equation (3):
10 Test report
The test report generally includes the following:
a) This document number and the name of the test item;
b) the source and preparation of the specimen, material species and specifications
c) Specimen number and shape. dimensions, calculated thickness, external quality and quantity of the panels used (in the case of anisotropic materials, the longitudinal or transverse direction should be indicated) and calculated modulus of elasticity of the panels used (when the modulus is known);
d) test temperature, relative humidity and condition of the specimen;
e) the name and type of test equipment, etc;
f) test speed and test span;
g) test results: performance values, arithmetic mean, standard deviation, dispersion coefficient and failure mode of each specimen are given;
h) test personnel, test date and others.
Appendix A (Informative) Span Reference Tables
Appendix B (Informative) Multi-span method for the determination of bending stiffness and shear stiffness of sandwich structures
Foreword
1 Scope
2 Normative references
3 Terms and definitions
4 Test principle
5 Test equipment
6 Specimen
7 Test conditions and specimen condition adjustment
8 Test procedure
9 Test results and processing
10 Test report
