Design method of wet-forming composite structures for civil aircraft considering the porosity effect

Abstract

The application discloses a design method of a wet forming composite material structure of a navigable airplane considering the influence of porosity, and comprises the following steps: preparing a plurality of wet forming composite material test plates; randomly selecting test samples from all the test plates, drying the test samples and testing the densities of the test samples; testing the mechanical properties of all the test samples with the measured densities and testing the porosities of the test samples, and establishing a strength reduction curve of the mechanical properties of the wet forming composite material influenced by the porosities according to the test structure; statistically analyzing the porosity data of all the test samples, and determining a high-probability interval of the porosities; determining a load coefficient by using the established strength reduction curve of the mechanical properties of the wet forming composite material influenced by the porosities and the determined high-probability interval of the porosities, and designing the wet forming composite material structure by using the determined load coefficient. The design method can guarantee the safety of the airplane structure, avoid overdesign of the airplane structure, and improve the economy of the airplane.

Description

Technical Field

[0001] This invention relates to the field of composite material structure design for general aviation aircraft, and specifically provides a design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity. Background Technology

[0002] In the field of general aviation aircraft manufacturing, new aircraft models are increasingly using composite material structures, and the application and popularization of composite materials are increasingly influencing the development of the general aviation industry. Aircraft composite material structures are typically manufactured using wet molding processes (including major load-bearing components such as wing spars and fuselage frames), and must pass CS23 or FAR23 airworthiness certification. Wet molding manufacturing technology is low-cost, mainly due to inexpensive raw materials, low equipment costs, and low manufacturing barriers. Furthermore, parts manufactured using this process are easier to maintain and repair. Adopting this process in the design and manufacture of general aviation aircraft can significantly improve economic efficiency, making the products more competitive.

[0003] While wet molding is a low-cost manufacturing technology with superior economic and maintainability advantages, it suffers from drawbacks compared to processes like autoclaving, including higher porosity, poorer stability, and difficulty in ensuring consistency. Porosity is one of the most common defects in composite parts, caused by inappropriate tooling, incorrect layup, and curing processes. High porosity reduces structural strength and significantly decreases properties dependent on the resin or fiber / resin interface, particularly the compressive and interlaminar properties. Parts manufactured using wet molding inherently tend to have higher porosity, reaching 5%–6%.

[0004] Typically, for aircraft composite structures, inherent defects must be considered during the design process to verify the ultimate strength capability when such defects are present. Based on this, the high porosity inherent characteristics of wet-molded composite structures must be taken into account during the design of general aviation aircraft composite structures, especially wet-molded composite structures, as part of the damage tolerance design. In response to the above problems, existing technologies usually adopt over-safety and over-margin design methods, which will lead to unnecessary weight increases and fail to fully utilize the weight reduction advantages of composite materials.

[0005] Therefore, proposing a design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity, and ensuring the safety of wet-molded composite material structures for general aviation aircraft under high porosity while giving full play to the weight reduction advantages of composite materials, has become an urgent problem to be solved. Summary of the Invention

[0006] In view of this, the present invention provides a design method for wet-molded composite material structures for general aviation aircraft that takes into account the influence of porosity, in order to solve the problem that the high porosity of composite material structures is not considered in the design of existing wet-molded composite material structures for general aviation aircraft, or that overly safe or excessive margin designs are adopted.

[0007] The technical solution provided by this invention is: a design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity, comprising the following steps:

[0008] Step 1: Prepare a large number of wet-molded composite material test plates, wherein the material of the wet-molded composite material test plates is the same as the material of the wet-molded composite material structure to be designed;

[0009] Step 2: Randomly select samples from all test plates and dry them to obtain a large number of dried samples. Then, perform density tests on all dried samples.

[0010] Step 3: Perform mechanical property tests and porosity tests on all samples with measured density to obtain mechanical property data and porosity data of the samples, and use the mechanical property data and porosity data to establish the strength reduction curve of the wet-molded composite material under the influence of porosity.

[0011] Step 4: Perform classification and statistical analysis on the porosity data of all samples to determine the high probability intervals of porosity occurrence;

[0012] Step 5: Using the strength reduction curve of the wet-molded composite material under the influence of porosity established in Step 3 and the high probability interval of porosity determined in Step 4, determine the load factor for the structural design of the wet-molded composite material, and use the determined load factor to design the wet-molded composite material structure. The method for determining the load factor is as follows:

[0013] The ratio of the mechanical properties corresponding to the lower limit of the high probability range of porosity to the mechanical properties corresponding to the upper limit of the high probability range of porosity is used as the load factor.

[0014] Preferably, in step 1, the test plate includes two main types of layup systems: unidirectional layup and multidirectional layup.

[0015] Further optimization involves selecting samples for a test plate in step 2 using a compact arrangement and random selection principle.

[0016] Further optimization, in step 2, the density test method is as follows: the density of the sample is tested using a density balance with the displacement method, and the sample density is calculated using the following formula:

[0017]

[0018] Where: ρ c —The density of the sample, in g / cm³ 3 ;

[0019] ρ w —The density of water, measured in g / cm³ 3 ;

[0020] M i —The weight of the sample in air, in grams;

[0021] M b —The weight of the sample in water, in grams.

[0022] Further preferred, in step 3, the method for obtaining the porosity of the sample is as follows:

[0023] After decomposing the sample using acid ablation, the porosity of the sample is measured and calculated using the following formula:

[0024]

[0025]

[0026] V v =100-V m -V f

[0027] Where: V m —Volume content of resin in the sample, in %;

[0028] V f —The volume content of fibers in the sample, in %;

[0029] V v —The volumetric content of pores in the sample, i.e., the porosity of the sample, in %;

[0030] ρ m —The density of the resin, in g / cm³ 3 ;

[0031] ρ f —Fiber density, in g / cm³ 3 ;

[0032] M f —The weight of the fibers in the sample, in grams.

[0033] Further optimization, in step 3, the method for establishing the strength reduction curve of the wet-molded composite material under the influence of porosity is as follows:

[0034] By utilizing the one-to-one correspondence between the mechanical property data and porosity data of all samples, a scatter plot was established and fitted to determine the influence of porosity on the mechanical properties of the wet-molded composite material, and a strength reduction curve of the mechanical properties of the wet-molded composite material affected by porosity was established.

[0035] Further optimization, in step 4, the method for determining the high-probability interval of porosity is as follows:

[0036] Step 41: Determine the significance level of all porosity data using a goodness-of-fit test of the normal distribution;

[0037] Step 42: Determine whether the significance level is greater than a preset value. If so, determine that the porosity data conforms to a normal distribution and use the normal distribution to calculate the high probability interval of porosity occurrence. Otherwise, proceed to step 43.

[0038] Step 43: Calculate the significance level of the Weibull or other distributions, and determine whether the significance level is greater than a preset value. If so, use the Weibull or other distribution methods to calculate the high probability interval of porosity. Otherwise, proceed to step 44.

[0039] Step 44: Use non-parametric methods to determine the high probability range of porosity.

[0040] Further optimization, in step 41, the formula for calculating the significance level OSL is as follows:

[0041]

[0042] In the formula:

[0043] n—Total porosity data;

[0044] AD – Anderson-Darling statistic.

[0045] This invention provides a design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity. By testing a large number of wet-molded composite material samples, mechanical property and porosity data can be obtained. Using this data, a strength reduction curve for the mechanical properties of the wet-molded composite material under the influence of porosity can be established. Categorical statistical analysis of the porosity data can determine the high-probability intervals where porosity occurs. Then, combining the established strength reduction curve with the determined high-probability intervals, a load factor for structural design can be determined. Finally, the load factor is considered to complete the design of the wet-molded composite material structure. The wet-molded composite material structure designed using this method has high safety and does not introduce unnecessary weight increases.

[0046] This invention provides a design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity. Through probabilistic statistical methods, it can determine the porosity distribution of low-cost composite material structures and the degree of influence of composite porosity on mechanical properties. It innovatively proposes introducing a load factor as a new input for the design of composite material structures for general aviation aircraft, thereby ensuring aircraft structural safety. This method can reduce testing costs, avoid excessive structural design, and improve aircraft economy. It can be widely applied to the design of wet-molded composite material structures for domestic general aviation aircraft under high porosity conditions, providing design and compliance verification references for addressing the safety issues of composite material structures caused by high porosity. Attached Figure Description

[0047] Figure 1 A flowchart of the design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity, provided by the present invention;

[0048] Figure 2 This is a wet-molded composite material wing root structure for a certain model used for design and verification.

[0049] Figure 3 Schematic diagram of the test plate specimen for mechanical properties and porosity;

[0050] Figure 4 This is a diagram showing the porosity and distribution of a one-way plate.

[0051] Figure 5 This is a diagram showing the porosity and distribution of a multi-directional plate.

[0052] Figure 6 This is a normal distribution probability density curve of porosity;

[0053] Figure 7 A cumulative probability curve for porosity below a certain threshold;

[0054] Figure 8 A cumulative probability curve for porosity above a certain threshold;

[0055] Figure 9 The graph shows the strength reduction curves illustrating the effect of porosity on compressibility.

[0056] Figure 10 This is a load-displacement curve of the residual strength test after fatigue damage tolerance of the wing root assembly. Detailed Implementation

[0057] The present invention will be further explained below with reference to specific implementation schemes, but this explanation does not limit the scope of the invention.

[0058] The present invention provides a design method for wet-molded composite material structures for general aviation aircraft that considers the influence of porosity, used for designing wet-molded composite material structures for general aviation aircraft (in this embodiment...). Figure 2 Taking the root structure of a certain type of wet-molded composite material airfoil as an example, the process includes the following steps:

[0059] Step 1: Prepare a large number of wet-molded composite material test plates, wherein the material of the wet-molded composite material test plates is the same as the material of the wet-molded composite material structure to be designed. The test plates contain two main types of layup systems: unidirectional layup and multidirectional layup (the actual layup used at the wing root).

[0060] Step 2: Randomly select samples from all test plates and dry them to obtain a large number of dried samples. Then, perform density tests on all dried samples.

[0061] For a single test plate, the selection of samples follows a compact and random arrangement principle. A typical test plate and sample arrangement diagram is shown below. Figure 3 (This refers to one of the test plates and the sample prepared using it).

[0062] The preferred drying conditions are: drying the sample at 50°C for 24 hours.

[0063] The density test method is as follows: The density of the sample is tested using a density balance via the water displacement method, and the sample density is calculated using the following formula:

[0064]

[0065] Where: ρ c —The density of the sample, in g / cm³ 3 ;

[0066] ρ w —The density of water, measured in g / cm³ 3 (The density of water varies at different temperatures);

[0067] M i —The weight of the sample in air, in grams;

[0068] M b —The weight of the sample in water, in grams;

[0069] Step 3: Perform mechanical property tests and porosity tests on all samples with measured density to obtain mechanical property data and porosity data of the samples, and use the mechanical property data and porosity data to establish the strength reduction curve of the wet-molded composite material under the influence of porosity.

[0070] The method for obtaining the porosity of the sample is as follows:

[0071] After decomposing the sample using acid ablation, the porosity of the sample is measured and calculated using the following formula:

[0072]

[0073]

[0074] V v =100-V m -V f

[0075] Where: V m —Volume content of resin in the sample, in %;

[0076] V f —The volume content of fibers in the sample, in %;

[0077] V v —The volumetric content of pores in the sample, i.e., the porosity of the sample, in %;

[0078] ρ m —The density of the resin, in g / cm³ 3 ;

[0079] ρ f —Fiber density, in g / cm³ 3 ;

[0080] M f —The weight of the fibers in the sample, in grams;

[0081] The porosity data of all samples were classified and statistically calculated to establish scatter plots and dispersion histograms of porosity for different test plates. The statistical results of unidirectional lay-up are as follows: Figure 4 As shown in the figure, the statistical results of the multidirectional layup are as follows. Based on the two figures, the porosity concentration can be determined, with most areas ranging from 2% to 5%.

[0082] The method for establishing the strength reduction curve of wet-molded composite materials under the influence of porosity is as follows:

[0083] Using the one-to-one correspondence between the mechanical property data and porosity data of all samples, a scatter plot was established and fitted to determine the influence of porosity on the mechanical properties of the wet-molded composite material, and a strength reduction curve of the mechanical properties of the wet-molded composite material affected by porosity was established.

[0084] Figure 9The curves established for this embodiment illustrate the effect of porosity on the mechanical properties of wet-molded composite materials. The curves show that porosity reduces notched compressive strength. When the porosity is less than 2%, the curve slope is steep, indicating severe degradation of notched compressive strength, with a degradation rate of 15%. When the porosity is greater than 2%, the curve slope decreases, and the degree of performance degradation slows down. At a porosity of 5%, the performance degrades to 79% of its original value.

[0085] Step 4: Perform classification and statistical analysis on the porosity data of all samples to determine the high probability intervals of porosity occurrence;

[0086] The method for determining the high-probability interval of porosity is as follows:

[0087] Step 41: Determine the significance level (OSL) of all porosity data using the goodness-of-fit test of the normal distribution;

[0088] The formula for calculating the significance level is as follows:

[0089]

[0090] In the formula:

[0091] n—Total porosity data;

[0092] AD – Anderson-Darling statistic;

[0093] Step 42: Determine whether the significance level is greater than a preset value (in this embodiment, the preset value is selected as 0.05). If so, determine that the porosity data conforms to a normal distribution and use the normal distribution to calculate the high probability interval of porosity occurrence. Otherwise, proceed to step 43.

[0094] Step 43: Calculate the Weibull or other distribution significance level (OSL) and determine whether the significance level is greater than a preset value (preferably 0.05). If so, use the Weibull or other distribution method to calculate the high probability interval of porosity. Otherwise, proceed to step 44.

[0095] Step 44: Use non-parametric methods to determine the high probability range of porosity.

[0096] In this embodiment, the method for determining the high-probability interval of porosity is as follows:

[0097] Step 41: First, perform classification and statistical analysis on the porosity data of all samples. Use the goodness-of-fit test of the normal distribution to determine the significance level of all porosity data. Perform the Anderson-Darling test on all porosity data to calculate its significance level (OSL). In this embodiment, there are 108 porosity data for the unidirectional plate and 66 porosity data for the multidirectional plate, totaling 174 porosity data. The OSL value of the normal distribution is 0.067, which is greater than 0.05, and the data conforms to the normal distribution.

[0098] Step 42: Determine that the significance level is greater than the preset value of 0.05. Therefore, use a normal distribution to calculate the high probability interval of porosity occurrence. The specific steps are as follows:

[0099] Step 421: Calculate the mean μ and variance σ of all porosity data. 2 By using a two-parameter normal probability distribution, the intervals in which the porosity observations are concentrated can be determined. The formula for the normal distribution probability density function is as follows:

[0100]

[0101] Figure 6 The porosity probability density curve of the sample can be used to confirm the judgment in step 3 that "porosity is concentrated between 2% and 5%".

[0102] Step 422: Integrate the probability density function to obtain the cumulative distribution function. Then, based on the cumulative distribution function, calculate the probability interval (a conservative value accepted during aircraft design) for the occurrence of 90% porosity at a preset confidence level (95% in this embodiment). Figure 7 The cumulative probability curve for a porosity value less than a specific value is attached. Figure 8 This is the cumulative probability curve for porosity values ​​greater than a specific value. Based on this, the high probability range of porosity can be determined to be 2.11% to 4.74%. Therefore, a porosity of 2.11% is selected as the baseline value, and a porosity of 4.74% is selected as the limit value.

[0103] Step 5: Using the strength reduction curve of the mechanical properties of wet-molded composite material affected by porosity established in Step 3 and the high probability interval of porosity determined in Step 4, determine the load factor for the structural design of wet-molded composite material, and use the determined load factor to design the structure of wet-molded composite material.

[0104] The method for determining the load factor is as follows:

[0105] The ratio of the mechanical properties corresponding to the lower limit of the high-probability porosity range to the mechanical properties corresponding to the upper limit of the high-probability porosity range is used as the load factor. According to... Figure 9 Regarding compressive performance (porosity has little impact on other properties or the structure will not experience failure modes corresponding to other properties, so only compressive performance is considered), the performance at 2.11% porosity is used as the benchmark. The strength drops to 94% at 4.74% porosity; therefore, the load factor is equal to 1.06. In this embodiment, to ensure structural safety, the load-bearing capacity of the complete structure should be 1.06 times the ultimate load, that is, the wing root structure is designed according to 1.06 times the ultimate load.

[0106] The design method provided by this invention will be verified through a full-size wing root assembly test.

[0107] The assembly was tested under its maximum load condition, including static and fatigue damage tolerance tests, to verify structural compliance. During loading, the static test selected the condition with the highest wing shear force and bending moment, a symmetrical positive overload condition representing the most severe load. A vertical upward tensile force of 26600 N was applied at the loading point. The fatigue damage tolerance test considered the Design Service Goal (DSG) and employed load amplification and life amplification methods, selecting a load amplification factor (LEF) of 1.15 and a life amplification factor of 1.5, resulting in a center-of-gravity overload spectrum for a total of 60578 loading cycles. The root assembly underwent a strength failure test after fatigue damage tolerance; the load-displacement curve at 143.6% load is shown in the appendix. Figure 10 The test piece was damaged, and the test was stopped due to the detachment of the lower skin. Its safety margin was greater than 1.43.

[0108] The design of the entire wing root assembly meets the requirements of an environmental load factor of 1.3 and a porosity-induced load factor of 1.06, with a certain margin, fully leveraging the advantages of composite materials.

[0109] The above description is merely an implementation of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity, characterized in that, Includes the following steps: Step 1: Prepare a large number of wet-molded composite material test plates, wherein the material of the wet-molded composite material test plates is the same as the material of the wet-molded composite material structure to be designed; Step 2: Randomly select samples from all test plates and dry them to obtain a large number of dried samples. Then, perform density tests on all dried samples. Step 3: Perform mechanical property tests and porosity tests on all samples with measured density to obtain mechanical property data and porosity data of the samples, and use the mechanical property data and porosity data to establish the strength reduction curve of the wet-molded composite material under the influence of porosity. Step 4: Perform classification and statistical analysis on the porosity data of all samples to determine the high-probability intervals of porosity occurrence. The method for determining the high-probability intervals of porosity occurrence is as follows: Step 41: Determine the significance level of all porosity data using a goodness-of-fit test of the normal distribution; Step 42: Determine whether the significance level is greater than a preset value. If so, determine that the porosity data conforms to a normal distribution and use the normal distribution to calculate the high probability interval of porosity occurrence. Otherwise, proceed to step 43. Step 43: Calculate the significance level of the Weibull or other distributions, and determine whether the significance level is greater than a preset value. If so, use the Weibull or other distribution methods to calculate the high probability interval of porosity. Otherwise, proceed to step 44. Step 44: Use non-parametric methods to determine the high-probability range of porosity; Step 5: Using the strength reduction curve of the wet-molded composite material under the influence of porosity established in Step 3 and the high probability interval of porosity determined in Step 4, determine the load factor for the structural design of the wet-molded composite material, and use the determined load factor to design the wet-molded composite material structure. The method for determining the load factor is as follows: The ratio of the mechanical properties corresponding to the lower limit of the high probability range of porosity to the mechanical properties corresponding to the upper limit of the high probability range of porosity is used as the load factor.

2. The design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity as described in claim 1, characterized in that: In step 1, the test plate includes two main types of layup systems: unidirectional layup and multidirectional layup.

3. The design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity as described in claim 1, characterized in that: In step 2, for a test plate, the selection of samples adopts the principle of compact arrangement and random selection.

4. The design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity as described in claim 1, characterized in that: In step 2, the density test method is as follows: The density of the sample is tested using a density balance with the displacement method, and the sample density is calculated using the following formula: ； in: —The density of the sample, in g / cm³ 3 ; —The density of water, measured in g / cm³ 3 ; —The weight of the sample in air, in grams; —The weight of the sample in water, in grams.

5. The design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity according to claim 4, characterized in that: In step 3, the method for obtaining the porosity of the sample is as follows: After decomposing the sample using acid ablation, the porosity of the sample is measured and calculated using the following formula: ； ； ； in: —The volume content of resin in the sample, in % % —The volume content of fibers in the sample, in % % —The volumetric porosity of the sample, expressed as % (%). —The density of the resin, in g / cm³ 3 ; —Fiber density, in g / cm³ 3 ; —The weight of the fibers in the sample, in grams.

6. The design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity according to claim 1, characterized in that: In step 3, the method for establishing the strength reduction curve of the wet-molded composite material under the influence of porosity is as follows: By utilizing the one-to-one correspondence between the mechanical property data and porosity data of all samples, a scatter plot was established and fitted to determine the influence of porosity on the mechanical properties of the wet-molded composite material, and a strength reduction curve of the mechanical properties of the wet-molded composite material affected by porosity was established.

7. The design method for wet-molded composite material structures for general aviation aircraft considering the influence of porosity according to claim 1, characterized in that: In step 41, the formula for calculating the significance level OSL is as follows: ； In the formula: ; —Total porosity data; —Anderson-Darling statistic.

Images