RU2540460C1 - Method to determine mechanical properties of brittle materials at tension - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: pads with dimensions and shape identical to the sample which are made from the material providing for total rigidity of both pads that is less or equal to the rigidity of the sample being tested, are glued to two opposite surfaces of the sample thus a laboratory assembly is produced and then set in collet clamps of a testing machine. Each clamp is located between the edge of the end face and the beginning of fillet arc of the assembly. An extensometer is installed on the assembly surface. Load is applied to the assembly and basing on the extensometer readings the curve "deformation - stress" of the laboratory assembly is drawn up and used to restore the diagram of the sample deformation. Stress in the sample σ_s is expressed via the stress of the laboratory assembly σ_la and the pad σ_p, provided with deformation equality, according to the formula σ_s=3·σ_la-2·σ_p.

EFFECT: possibility to implement Saint-Venant principle and provision for homogeneous stress in the working part of a sample made from brittle material, provision for uniaxial tension in the working part of the sample from tested material, prevention of bending, provision for more force measurement points on the equal deformation base.

2 cl, 4 dwg

Description

Technical field

The invention relates to testing equipment, namely to mechanical testing of samples of composite materials, the tensile behavior of which is close to brittle. Scope - aircraft manufacturing, shipbuilding, mechanical engineering, nuclear energy, etc.

State of the art

A known method of tensile testing of metallic and nonmetallic materials (GOST 1497-84, "Metals. Methods of tensile testing"), which consists in the fact that the standard test specimen is loaded to failure, while the load and deformation are measured, which determine the modulus of elasticity, tensile strength, yield strength and tensile strength.

The disadvantages of the method when applied to the testing of brittle materials are difficulties in ensuring reliable fastening in the grips of the testing machine, as well as the inability to track the non-linear section of the deformation preceding fracture, since brittle materials are more sensitive to off-center loading and, accordingly, bending to a uniaxial stress state .

A known method of tensile testing of reinforced plastics ("Methods of static testing of reinforced plastics", Ternopolsky Yu.M., Kantsis T.Ya., M., "Chemistry", 1981), which consists in loading a standard sample type double-sided blades with overlays glued to the end parts to failure, measure loads and deformations, which determine the mechanical characteristics of plastics.

The disadvantage of this method is the underestimation of the values of deformation ability and tensile strength of the composite material in contrast to the corresponding characteristics determined during testing of full-scale structures. Brittle materials have mechanical characteristics that are different in tension and compression, which is true for most composites. With this method of testing samples of composite materials, difficulties arise in fulfilling the Saint-Venant principle: in samples of materials with properties that depend on the type of stress-strain state, the zones of the edge effect increase significantly, which prevents the creation of a uniform stress state in the working part of the sample. "

The author’s certificate of the Russian Federation No. 1335844, dated 11/12/1985, G01N 3/08, “Method for the mechanical testing of samples of materials”, Authors: Efimov O.Yu., Sakhno A.I., Mamleev R.F. A sample of the type of double-sided blade with overlays is placed in the grips of the testing machine, loaded with a test load, the load and the relative movement of the grippers are recorded. Pads of brittle material with a strength 3-10 times lower than the strength of the sample material are in contact with grippers and placed on the surfaces of the transition of the working part of the sample to the gripping part. Before loading the test load, a preliminary force is applied to the sample until the linings break, which is then reduced. Next, the sample is loaded to failure. Signs that coincide with the essential features of the claimed invention are: a sample with pads is placed in the grips of the testing machine, loaded with a test load, the load is recorded, the relative movement of the grippers is recorded, the pads are placed on the surfaces of the sample and come in contact with the grippers, the sample is loaded to failure.

The disadvantage of the prototype can be considered that with the described method for testing samples of brittle materials, difficulties arise in fulfilling the Saint-Venant principle and, accordingly, in creating a uniform stress state in the working part of the sample. In addition, with an increase in the width of the free ends of the sample, the relative volume of the “useful” working part decreases, the material consumption and the cost of manufacturing samples increase. It is also impossible to apply this method to the study of explosive compositions, the tensile behavior of which is close to brittle, because of the danger of initiating an explosion in places of contact with captures through the lining destroyed in the preliminary loading.

Disclosure of invention

The problem to which the invention is directed is to create a method for determining the mechanical properties of brittle materials, including explosive compositions, in the transition region preceding fracture, when they are stretched; identification of reserves of deformation ability of the studied material; improving the accuracy and information content of the experimental results.

The technical result achieved in solving this problem is to fulfill the Saint-Venant principle and, accordingly, to create a uniform stress state in the working part of the sample from brittle material; creating uniaxial tension in the working part of the sample from the material being studied, eliminating bending; obtaining more points of measurement of effort on the same basis of deformation.

To obtain the specified technical result in the proposed method for determining the mechanical properties of brittle materials under tension, including attaching the pads to the sample, placing the sample with pads in the grips of the testing machine, loading the sample with pads, registering the deformation of the working part of the sample, according to the invention, pads are identical in size and the forms are made of material providing the total stiffness of both pads less than or equal to the stiffness of the test sample. The pads are glued on two opposite surfaces of the sample. The result is a laboratory assembly, which is placed in the collet grips of the testing machine. Moreover, each grip is installed between the edge of the end and the beginning of the arc fillet assembly. An extensometer is installed on the assembly surface. Apply load to assembly. According to the testimony of the extensometer, a “strain-stress” curve of the laboratory assembly is obtained from which the strain diagram of the sample is restored. The stress in the sample σ _{o is} expressed in terms of the stresses of the laboratory assembly σ _ls and lining σ _p , provided that the deformation is equal, according to the formula σ _o = 3 · σ _ls -2 · σ _p .

This allows you to more accurately implement the principle of Saint-Venant, to increase the accuracy of determining the deformation ability and tensile strength of a sample of brittle material. It becomes possible to use collet grips that can center along the axis of loading. By increasing the rigidity of the object of study on the same strain base, a greater number of force measurement points are recorded, which increases the accuracy and information content of the experimental results. This allows us to determine the diagram of deformation of brittle materials under tension, including in the transition region preceding fracture.

On the surface of the pads in the area of contact with the grippers, it is possible to apply grooves, marks, roughnesses. This allows you to apply this method to the study of the mechanical properties of explosive compositions without the danger of an explosion when slipping in the grips of a testing machine.

Brief Description of the Drawings

Figure 1 shows the laboratory assembly.

Figure 2 shows a diagram of the deformation of the laboratory assembly.

Figure 3 shows a diagram of the deformation of the sample.

Figure 4 shows a table of the results of experimental studies, where

ε _growth (%) - tensile strain,

σ _growth (MPa) - tensile strength,

E _rust (MPa) - modulus of tensile elasticity.

Embodiments of the invention

As shown in FIG. 1, the test sample 1 is made of a brittle material in the form of a double-sided blade. On two opposite surfaces of sample 1, pads 2 are glued. The pads 2 are made of the same size and shape as sample 1 from a material that provides the total stiffness of both pads 2 less than or equal to the stiffness of the test sample 1. Sample 1 with glued pads 2 forms a laboratory assembly 3. Assembly 3 are placed and fixed in collet grips of the testing machine. Grips are imposed on the surface of the four faces of the assembly 3 so that the edge of each grip is located between the edge of the end 4 and the beginning of the arc of the fillet 5 of the assembly 3. On the surface of the assembly 3 in the contact zone 6 with the grips, grooves or risks or roughnesses are applied.

An extensometer 7 is installed on the surface of the working part of assembly 3. A load is applied to assembly 3, the extensometer 7 is read and a graph of the dependence of the deformation of the working part of the laboratory assembly 3 on the applied force (in Fig. 2) is used to restore the deformation diagram of sample 1. The voltage is the specimen σ _{о is} expressed in terms of the stresses of the laboratory assembly σ _ls and overlays σ _p , provided that the deformation is equal, according to the formula σ _o = 3 · σ _ls -2 · σ _p . The deformation diagram of the laboratory assembly 3 is shown in figure 2. This diagram contains four sections: section 8 of elastic deformation of the laboratory assembly 3; section 9 of plastic deformation of sample 1 as part of a laboratory assembly 3 with elastically deforming pads 2; section 10 of the destruction of sample 1 (the passage of a transverse crack); the deformation section 11 of the assembly 3 with the sample 1 having a through transverse crack.

Figure 3 shows a diagram of the deformation of sample 1 without overlays 2. Comparison of the diagrams in figure 2 and figure 3 allows us to conclude that standard tests of samples 1, in contrast to laboratory assemblies 3, do not reveal the area-section 9 preceding failure ( figure 2) of nonlinear deformation of the material of sample 1, the behavior of which is close to brittle. The diagram in figure 3 is almost linear until the destruction of sample 1.

Figure 4 presents a table of the results of statistical processing of experimental data obtained in experiments on stretching pads 2 from plexiglass, samples 1 from explosive composition (BC) and laboratory assemblies 3. At least 18 experiments were conducted and processed for each type of test. The table also shows the parameters of the diagram of the deformation of the material of the aircraft of samples 1, obtained from the test results of laboratory assemblies 3. Analysis of the data presented in the table allows the following conclusions:

- the use of the proposed design of laboratory assembly 3 during tensile tests made it possible to identify significant (1.77 times) reserves of the deformation ability of the aircraft material of samples 1, which cannot be determined during standard tests of samples 1;

- a different degree of differences in deformation ability and tensile strength (1.77 and 1.58 times, respectively) obtained in tests of laboratory assemblies 3 and samples 1 without overlays 2, indicates the nonlinear behavior of the material of the aircraft of samples 1 in the region preceding the fracture - section 9 ( figure 2). Standard tests of samples 1, in contrast to laboratory assemblies 3, do not allow us to identify this region of material deformation — section 9 (in FIG. 2): “stress-strain” relationship (FIG. 3 is almost linear until fracture);

- the use of the proposed design of the laboratory assembly 3 during tensile tests allows obtaining more stable (with a smaller dispersion) results, unlike standard tests of samples 1. This, in turn, allows one to obtain more accurate estimates of the strength and reliability of structures containing parts from brittle materials;

- the modulus of elasticity of the material of sample 1, determined using laboratory assemblies 3, practically coincides (the difference does not exceed 2%) with the value of this characteristic obtained in standard tests of samples 1. Which confirms the correctness of the derivation of the formula for reconstructing the deformation diagram of the material of aircraft BC of sample 1 from the curve deformation-stress "of the laboratory assembly 3 and the validity of the methodological assumptions used in the design of the laboratory assembly 3, and the method of testing it.

It should be noted that the destruction of all samples tested in laboratory assemblies occurred in the working part. This result proves that the proposed method allows to fulfill the Saint-Venant principle, to create a uniform stress state in the working part of the sample from brittle materials.

Thus, the method expands the arsenal of technical means for determining the mechanical properties of brittle materials under tension in the transition region preceding fracture, reveals the reserves of the deformation ability of the material, which cannot be determined during standard tensile testing of samples, allows to increase the accuracy and information content of the experimental results.

Industrial applicability

The most effective invention can be used in aircraft, shipbuilding, mechanical engineering, nuclear energy. To assess the strength and stiffness of any structural material whose tensile behavior is close to brittle, it is subjected to mechanical tests. The reliability of information about the strength and stiffness of the material determines the efficiency of its use and the operational capabilities of structures containing parts from it. The practical implementation of the described method confirmed the receipt of a technical result. This shows its performance and confirms industrial applicability.

Claims (2)

1. The method of determining the mechanical properties of brittle materials under tension, including attaching the pads to the sample, placing the sample with pads in the grips of the testing machine, loading the sample with pads, registering deformation of the working part of the sample, characterized in that the pads are identical in size and shape to the sample, made from a material providing the total stiffness of both pads less than or equal to the stiffness of the test sample, stick on two opposite surfaces of the sample, resulting in a floor they start the laboratory assembly, which is placed in the collet grips of the testing machine, each gripper is installed between the end edge and the beginning of the fillet arc of the assembly, an extensometer is placed on the assembly surface, a load is applied to the assembly, the strain-stress curve of the laboratory assembly is obtained from the extensometer, from which reduced deformation diagram sample voltage at the sample σ _o is expressed as the voltage assembly laboratory _dissolved σ and σ _n laths provided equality deformation of the form e _o = σ · σ 3 _hp -2 · σ _n. 2. The method according to claim 1, characterized in that the grooves in contact with the grippers are applied to the surface of the plastic linings, risks, roughnesses.

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