Method for calculating fracture toughness of blending modified composite material based on molecular dynamics

Abstract

The invention provides a method for calculating the fracture toughness of a blending modified composite material based on molecular dynamics, which is characterized in that the fracture toughness of a polyamide 6 (PA6) material is improved by adding a certain proportion of high density polyethylene (HDPE); comprising the following steps: constructing PA6 / HDPE composite material models with different mass ratios, carrying out balance simulation, carrying out tensile simulation on a crack-containing model, and calculating the fracture toughness of a material in a simulation result by using a J integral method. In addition, the fracture toughness of part of materials is tested by using an EWF method, and the experimental result is well matched with the change trend of simulation analysis. The method for calculating the fracture toughness of the PA6 / HDPE composite material has feasibility, and theoretical basis and reference can be provided for researching the design of a high-toughness hydrogen storage cylinder liner material.

Description

Technical Field

[0001] The invention belongs to the technical field of toughening materials for hydrogen storage cylinder liners, and in particular relates to a fracture toughness calculation method for composite materials based on molecular dynamics blending and modification. Background Art

[0002] As a zero-carbon clean energy, hydrogen energy has broad application prospects. High-pressure gaseous hydrogen storage is currently the most common way to store hydrogen, and Type IV high-pressure hydrogen storage cylinders have become one of the important containers for high-pressure hydrogen storage in many countries due to their advantages such as light weight, high hydrogen storage density, and fast charging and discharging speed. The liner material of Type IV hydrogen storage cylinders is usually composed of high-density polyethylene (HDPE) and polyamide (PA). Due to its strong molecular polarity and hydrogen bond interaction, polyamide has lower hydrogen permeability than high-density polyethylene, especially polyamide 6 (PA6), which exhibits higher gas barrier properties. However, polyamide 6 has the disadvantages of high brittleness and poor dimensional stability. It is easy to crack when using polyamide 6 as a liner material, which limits its performance in practical applications. Therefore, PA6 is selected to blend with HDPE to meet the requirements of the liner material in terms of toughness and gas barrier properties. When the amount of PA6 in the PA6 / HDPE composite material reaches a certain amount, a multi-layer cross-overlapping continuous layered structure will appear in the composite material. This structure will make the penetration path of the solvent more complicated, thereby enhancing the gas barrier properties of the material. The addition of HDPE can make the composite structure more stable. Experimental analysis of the fracture toughness of materials is often costly and time-consuming. As an alternative, molecular dynamics (MD) simulation can not only significantly reduce research costs, but also provide molecular-level understanding. Therefore, the molecular dynamics method can be used to study the fracture behavior of composite materials from a microscopic perspective, and the fracture toughness of composite materials can also be calculated, thereby providing a theoretical basis and reference for the design of high-toughness hydrogen storage cylinder liner materials. Summary of the invention

[0003] Purpose of the invention: The technical problem to be solved by the present invention is to provide a method for calculating the fracture toughness of composite materials based on molecular dynamics blending and modification, in view of the deficiencies in the prior art, which can realize the calculation of the fracture toughness of composite materials.

[0004] The present invention comprises the following steps:

[0005] Step 1, establishing an amorphous unit cell;

[0006] Step 2, relaxing the constructed amorphous unit cell so that the unit cell model reaches a stable state before dynamic simulation;

[0007] Step 3, create a crack and perform a stretching simulation on the relaxed model under normal temperature and pressure conditions;

[0008] Step 4, plot the stress-strain results obtained in step 3 into a curve, and use the J-integral method to calculate the fracture toughness of the material.

[0009] In step 1, six amorphous unit cells with mass ratios of polyamide 6 (PA6) to high-density polyethylene (HDPE) of 10 / 0, 7 / 3, 6 / 4, 4 / 6, 3 / 7 and 0 / 10 were established.

[0010] In step 1, use Materials Studio software to construct the molecular chain of the polymer and perform geometric optimization. Then, use the Amorphous Cell module in Materials Studio software to construct amorphous unit cells of composite materials with different mass ratios. Then, perform geometric optimization on the unit cells and convert the unit cell files into data files that can be recognized by LAMMPS.

[0011] In step 2, during relaxation, the energy of the unit cell is first minimized, and the conjugate gradient method is used to exclude atoms that are too close by force calculation. Then, the NVT (canonical ensemble, abbreviated as NVT, which means that the system maintains a certain number of particles N, volume V, and temperature T) ensemble is relaxed for 10 ps at 298 K. Finally, a 100 ps NPT simulation is performed at 298 K and 0.1 MPa to stabilize the energy of the system. The damping coefficients of temperature and pressure are 50 fs and 500 fs, respectively. The ReaxFF force field is used to describe the interaction between atoms. During the simulation, the order of bonds is updated, allowing the creation and separation of bonds, which can better describe the fracture phenomenon. The Nose-Hoover method is used to adjust the temperature and pressure of the system during the relaxation process.

[0012] In step 3, a crack is created, and the unit cell model uses the NPT ensemble (isothermal and isobaric ensemble (constant-pressure, constant-temperature), abbreviated as NPT, which means that the system maintains a certain number of particles N, pressure P, and temperature T) for tensile simulation at a temperature of 298K. In order to capture the Poisson effect, the pressure component perpendicular to the loading direction of the unit cell model is set to 0.1MPa.

[0013] In step 4, the calculation formula of J integral is:

[0014]

[0015] Where P is the axial load, b is the unbroken length, Δ is the tensile displacement, B is the specimen thickness; Δ represents the displacement; d is the differential; Represents the partial differential symbol.

[0016] In step 4, the displacement Δ includes an elastic part Δ el and the plastic part Δ p , so the J-integral calculation formula is:

[0017]

[0018] In step 4, the calculation formula of J integral is simplified to:

[0019]

[0020] Among them A P It is the plastic area under the load-displacement curve during tension.

[0021] The method of the present invention also includes step 5: using the EWF method (Essential Work of Fracture, EWF, which can be used to evaluate high-toughness polymer materials. The linear elastic fracture mechanics (LEFM) method is usually used for brittle materials, but due to the importance of the plastic deformation zone, this method is not always suitable for ductile materials. The Essential Work of Fracture (EWF) method is based on post-yield fracture mechanics and is usually used to evaluate the toughness of tough polymers) to test the fracture toughness of some materials, compare the trend of the simulation results with the experiment, and verify the validity of the simulation results.

[0022] The present invention also provides an electronic device, comprising a processor and a memory, wherein the memory stores program code, and when the program code is executed by the processor, the processor executes the steps of the described method.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention uses molecular dynamics simulation to calculate the stress and strain of PA6 / HDPE composite materials with different mass ratios, plots them into curves, and uses the J-integral method to calculate the fracture toughness of the material. At the same time, the fracture toughness of the composite material is tested by the EWF method. The variation trend of the experimental and simulation results is highly consistent, which provides a theoretical basis and reference for the selection and design of high-toughness polymer linings for hydrogen storage cylinders. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more clear.

[0026] Figure 1 It is a general simulation calculation flow chart of an embodiment of the present invention.

[0027] Figure 2aSchematic diagram of a single chain model (HDPE) in step 1 of an embodiment of the present invention.

[0028] Figure 2b It is a schematic diagram of a single chain model (PA6) in step 1 of an embodiment of the present invention.

[0029] Figure 3 is a stress-strain curve diagram of step 4 of an embodiment of the present invention;

[0030] Figure 4 The uniaxially stretched plate with a single-side crack in step 4 of the embodiment of the present invention;

[0031] Figure 5a It is a snapshot of the composite material of step 4 of the present embodiment at a strain of 50% (PA6).

[0032] Figure 5b It is a snapshot of the composite material of step 4 of the present embodiment at a strain of 50% (4PA6 / 6HDPE). DETAILED DESCRIPTION

[0033] like Figure 1 As shown, this embodiment provides a method for calculating the fracture toughness of a composite material based on molecular dynamics blending and modification, which specifically includes the following steps:

[0034] Step 1: Use Materials Studio software to construct the molecular chains of HDPE and PA6 (such as Figure 2a , Figure 2b The Amorphous Cell module in the software is then used to construct amorphous unit cells of composite materials with different mass ratios of PA6 / HDPE (10 / 0, 7 / 3, 6 / 4, 4 / 6, 3 / 7 and 0 / 10). The unit cells are then geometrically optimized and the unit cell files are converted into data files that can be recognized by LAMMPS.

[0035] Step 2: Use LAMMPS software to minimize the energy of the unit cell, use the conjugate gradient method, and calculate the force to exclude atoms that are too close. Then use NVT (canonical ensemble, abbreviated as NVT, which means that the system maintains a certain number of particles N, volume V, and temperature T) ensemble relaxation for 10ps at 298K. Finally, perform 100ps NPT simulation at 298K and 0.1MPa to make the energy of the system reach a stable state. The damping coefficients of temperature and pressure are 50fs and 500fs, respectively. Use ReaxFF force field (ReaxFF force field is a force field based on bond order. During the simulation, the order of bonds is updated, allowing the creation and separation of bonds, which can better describe the fracture phenomenon) to describe the interaction between atoms. Use the Nose-Hoover method to adjust the system temperature and pressure during the relaxation process.

[0036] Step 3: Create a crack. The model uses the NPT ensemble (constant-pressure, constant-temperature, abbreviated as NPT, which means that the system has a certain number of particles N, pressure P, and temperature T) for tensile simulation at a temperature of 298K. In order to capture the Poisson effect, the pressure component perpendicular to the loading direction is set to 0.1MPa.

[0037] Step 4: Plot the stress-strain results obtained by simulation calculation in step 3 into a curve, such as Figure 3 As shown, the fracture toughness of the material is calculated using the J integral method. The calculation formula of the J integral is:

[0038]

[0039] Where P is the axial load, b is the length of the fracture, Δ is the tensile displacement, B is the specimen thickness, Δ represents the displacement; d is the differential; represents the partial differential symbol, the model is as follows Figure 4 .

[0040] The displacement Δ can be expressed as the elastic part Δ el and the plastic part Δ p , therefore, the J-integral calculation formula is:

[0041]

[0042] In the present invention, the material exhibits obvious nonlinear behavior, and the contribution of the plastic region to J is much higher than that of the elastic behavior. Therefore, the calculation formula of the J integral is simplified to:

[0043]

[0044] Among them AP It is the plastic area under the load-displacement curve during tension.

[0045] After simulation calculation, the J integrals of PA6, 7PA6 / 3HDPE, 6PA6 / 4HDPE, 4PA6 / 6HDPE, 3PA6 / 7HDPE and HDPE are 3.035N / m, 3.096N / m, 3.291N / m, 3.441N / m, 3.619N / m and 4.436N / m respectively. The improvement of 7PA6 / 3HDPE, 6PA6 / 4HDPE, 4PA6 / 6HDPE, 3PA6 / 7HDPE and HDPE relative to PA6 are 2%, 8%, 13%, 19% and 46% respectively. Figure 5a , Figure 5b The snapshots of PA6 and 6HDPE / 4PA6 at a strain of 50% are shown. The results in the figure show that the presence of HDPE effectively enhances the structural stability of the composite material, and many atoms remain in the crack area, which effectively inhibits the expansion of the crack.

[0046] Step 5: Use the EWF method (Essential Work of Fracture, EWF, which can be used to evaluate high-toughness polymer materials. The linear elastic fracture mechanics (LEFM) method is usually used for brittle materials, but due to the importance of the plastic deformation zone, this method is not always suitable for ductile materials. The essential work of fracture (EWF) method is based on post-yield fracture mechanics and is usually used to evaluate the toughness of tough polymers) to test the fracture toughness of some materials. The J integrals of PA6, 6PA6 / 4HDPE, 3PA6 / 7HDPE and HDPE were tested and were 78.12KJ / m 2 、86.47KJ / m 2 、89.61KJ / m 2 and 112.67KJ / m 2 The improvement of 6PA6 / 4HDPE, 3PA6 / 7HDPE and HDPE relative to PA6 is 11%, 15% and 44% respectively. The experimental results are in good agreement with the trend of simulation analysis.

[0047] The present invention provides a method for calculating the fracture toughness of a composite material based on molecular dynamics blending and modification. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A method for calculating the fracture toughness of composite materials based on molecular dynamics blending modification, characterized in that: The following steps are involved: Step 1, establishing an amorphous unit cell; Step 2, relaxing the constructed amorphous unit cell so that the unit cell model reaches a stable state before dynamic simulation; Step 3, create a crack and perform a stretching simulation on the relaxed model under normal temperature and pressure conditions; Step 4, plot the stress-strain results obtained in step 3 into a curve, and use the J-integral method to calculate the fracture toughness of the material.

2. The method according to claim 1, characterized in that In step 1, six amorphous unit cells with mass ratios of polyamide 6 to high-density polyethylene being 10 / 0, 7 / 3, 6 / 4, 4 / 6, 3 / 7 and 0 / 10 respectively are established.

3. The method according to claim 2, characterized in that In step 1, use Materials Studio software to construct the molecular chain of the polymer and perform geometric optimization. Then, use the Amorphous Cell module in Materials Studio software to construct amorphous unit cells of composite materials with different mass ratios. Then, perform geometric optimization on the unit cells and convert the unit cell files into data files that can be recognized by LAMMPS.

4. The method according to claim 3, characterized in that In step 2, during relaxation, the energy of the unit cell is first minimized, and the conjugate gradient method is used to exclude atoms that are too close by force calculation. Then, the NVT ensemble is relaxed for 10 ps at 298 K. Finally, an NPT simulation is performed for 100 ps at 298 K and 0.1 MPa to stabilize the energy of the system. The ReaxFF force field is used to describe the interaction between atoms, and the Nose-Hoover method is used to adjust the temperature and pressure of the system during the relaxation process.

5. The method according to claim 4, characterized in that In step 3, a crack is created, and the unit cell model uses the NPT ensemble for tensile simulation at a temperature of 298K. In order to capture the Poisson effect, the pressure component of the unit cell model perpendicular to the loading direction is set to 0.1MPa.

6. The method according to claim 5, characterized in that In step 4, the calculation formula of J integral is: Where P is the axial load, b is the unbroken length, Δ is the tensile displacement, B is the specimen thickness; Δ represents the displacement; d is the differential; Represents the partial differential symbol.

7. The method according to claim 6, characterized in that In step 4, the displacement Δ includes an elastic part Δ el and the plastic part Δ p , so the J-integral calculation formula is:

8. The method according to claim 7, characterized in that In step 4, the calculation formula of J integral is simplified to: Among them A P It is the plastic area under the load-displacement curve during tension.

9. The method according to claim 8, characterized in that It also includes step 5: testing the fracture toughness of some materials using the EWF method, comparing the trend of the simulation results with the experiment, and verifying the validity of the simulation results.

10. An electronic device, characterized in that: The method comprises a processor and a memory, wherein the memory stores program codes, and when the program codes are executed by the processor, the processor executes the steps of the method according to any one of claims 1 to 9.

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