A method for predicting the fatigue life of needle-punched composite material considering fatigue notch factor

Abstract

The application discloses a kind of fatigue notch coefficient's consideration needle-punched composite material fatigue life prediction method, it is related to composite material fatigue strength theory field.The method of the application includes, the calculation of material stress concentration coefficient, the influence of establishing stress level to material, the relationship between stress concentration coefficient and fatigue notch coefficient is established, the influence of stress concentration coefficient and fatigue notch coefficient is introduced by establishing stress level, the fatigue life of notch is calculated by means of smooth test piece S-N curve (stress-life curve), equation simplification.The fatigue life prediction method of needle-punched composite material considering fatigue notch coefficient proposed in the application has been verified by corresponding test of needle-punched ceramic matrix composite material.The proposed method has clear physical meaning, and can be used for constant amplitude fatigue life prediction of needle-punched composite material containing notch.

Description

Technical Field

[0001] This invention belongs to the field of fatigue life prediction of composite materials, and particularly relates to fatigue life prediction of notched parts of needle-punched long fiber reinforced composite materials. Background Technology

[0002] Composite materials, due to their high specific strength and excellent corrosion resistance, are widely used in aircraft engines, high-end automobiles, and other fields. Needle-punched composite materials are mainly composed of fibers and a matrix, forming a structure consisting of long fiber layers, a mesh layer, and needle-punched fiber bundles. The introduction of needle-punched fiber bundles into the composite material structure gives it a certain strength in the thickness direction. Because needle-punched composite materials are subjected to various alternating loads over long periods of service, and are frequently affected by bolted connections in practical applications, the presence of notches leads to a localized stress field near the notch that is much higher than the applied stress. Therefore, it is necessary to propose a method for predicting the fatigue life of notched structures in needle-punched composite materials. Summary of the Invention

[0003] The purpose of this invention is to propose a method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient. This invention obtains the finite width coefficient of the material itself using the finite width coefficient method, taking into account the influence of the pore size on the material. Based on this, a relationship between the stress concentration factor and the fatigue notch coefficient is established. The influence of stress level is introduced to obtain a relationship between the finite width coefficient and the fatigue notch coefficient considering the stress level. Furthermore, the fatigue notch coefficient is incorporated into the SN curve (stress-life curve) to achieve fatigue life prediction of notched structures in needle-punched composite materials.

[0004] The technical solution adopted in this invention is a fatigue life prediction method for needle-punched composite materials considering the fatigue notch coefficient. The implementation steps of this method are as follows:

[0005] Step (1): Calculation of the stress concentration factor of the material. The mathematical expression for the stress concentration factor is:

[0006]

[0007] In the formula, K t The stress concentration factor is... d is the width coefficient of the infinite plate, w is the notch size of the notched part, and M is the width of the material. M is the relationship between the notch and the composite material.

[0008]

[0009] E1

[0010]

[0011] If the gap is 0 When this item is 0, then... In the formula, E1 is the elastic modulus parallel to the direction of the long fiber layer, E2 is the elastic modulus perpendicular to the direction of the long fiber layer, and G... 12 v is the shear modulus in the plane perpendicular to the long fiber layer. 12 is Poisson's ratio in the plane of the long fiber layer.

[0012] Step (2): Consider the effect of stress level on the material. Most composite materials are notch-insensitive. At higher stress levels, the fatigue life is mainly determined by interlaminar damage and notch effects. At lower stress levels, the effect of notches on fatigue life approaches that of notch-sensitive materials. Therefore, stress level has a certain influence on the material. This can be expressed as follows:

[0013]

[0014] In the formula, α represents the effect of stress level on the material, and σ nominal For the nominal stress of the notched component, σ t This represents the static tensile strength of the notched component.

[0015] Step (3): Establish the relationship between the stress concentration factor and the fatigue notch factor. The specific expression is:

[0016]

[0017] In the formula, K f σ is the fatigue notch coefficient. α and β are the fitted parameters, and σ is the fatigue notch coefficient. nominal With σ smooth These represent the nominal stress of the notched composite material structure and the gauge length stress of the smooth specimen, respectively, when the notched and smooth specimens have the same fatigue life.

[0018] Step (4): Introduce the influence of stress level on stress concentration factor and fatigue notch factor. The specific expression is:

[0019]

[0020] Step (5): Calculate the notched fatigue life using the SN curve (stress-life curve) of the smooth specimen. The SN curve of the smooth specimen is as follows:

[0021] σ smooth =cN m

[0022] In the formula, σ smoothdenoted as , where is the stress on the gauge length of the smooth specimen; N is the fatigue life; and c and m are material constants obtained through fitting. Substituting the fatigue notch coefficient considering the stress level into the SN curve of the smooth specimen, the fatigue life prediction expression for the needled composite material including the notch structure is obtained as follows:

[0023]

[0024] Step (6): Simplify the equations. When the load applied to the notched part is equal to the static tensile load, N is 1, and σ nominal =σ t The above equation becomes

[0025]

[0026] Simplify to obtain

[0027]

[0028] Therefore, the fatigue life prediction expression for needled composite materials containing notched structures is:

[0029]

[0030] This equation is the final fatigue life prediction equation for through-hole components made of ceramic matrix composites.

[0031] The advantages of this invention are: it proposes a fatigue life prediction method for needled composite materials that considers the fatigue notch coefficient. This method combines the actual size of the component, not only taking into account the influence of stress level on the material, but also considering the solution of fatigue notch coefficient by introducing stress concentration factor. The proposed life prediction model can be applied to constant amplitude fatigue of notched parts and has certain practical engineering significance. Attached Figure Description

[0032] Figure 1 A flowchart for predicting the fatigue life of composite structures containing notched needle-punched structures;

[0033] Figure 2 This refers to a needle-punched ceramic matrix composite specimen with a notched structure. D is the diameter of the central hole, W is the specimen width, and t is the specimen thickness.

[0034] Figure 3 A graph showing the stress concentration factor of needle-punched ceramic matrix composites;

[0035] Figure 4 The stress-life curve (SN curve) for a smooth part of a needle-punched ceramic matrix composite material;

[0036] Figure 5 Figure 1 shows the predicted lifespan and actual lifespan of needle-punched ceramic matrix composites. Detailed Implementation

[0037] The present invention will be described in conjunction with the accompanying drawings;

[0038] The composite material is a needle-punched ceramic matrix composite. Since the main stress during cyclic loading is borne by the long fiber layer, its engineering constant only needs to consider the long fiber layer. The specimens used in the experiment were centrally through-hole specimens, as detailed below. Figure 2 As shown. The fatigue test load used in the experiment was a symmetrical cyclic load.

[0039] A method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient, the specific calculation method is as follows:

[0040] Step (1): Calculation of the material stress concentration factor. Stress concentration factor (e.g., ...) Figure 3 The mathematical expression for (as shown) is:

[0041]

[0042] In the formula, K t The stress concentration factor is... d is the width coefficient of the infinite plate, w is the notch size of the notched part, and M is the width of the material. M is the relationship between the notch and the composite material.

[0043]

[0044]

[0045] If the gap is 0 When this item is 0, then... In the formula, E1 is the elastic modulus parallel to the direction of the long fiber layer, E2 is the elastic modulus perpendicular to the direction of the long fiber layer, and G... 12 v is the shear modulus in the plane perpendicular to the long fiber layer. 12 is Poisson's ratio in the plane of the long fiber layer.

[0046] Step (2): Consider the effect of stress level on the material. Most composite materials are notch-insensitive. At higher stress levels, the fatigue life is mainly determined by interlaminar damage and notch effects. At lower stress levels, the effect of notches on fatigue life approaches that of notch-sensitive materials. Therefore, stress level has a certain influence on the material. This can be expressed as follows:

[0047]

[0048] In the formula, α represents the effect of stress level on the material, and σ nominal For the nominal stress of the notched component, σ t This represents the static tensile strength of the notched component.

[0049] Step (3): Establish the relationship between the stress concentration factor and the fatigue notch factor. The specific expression is:

[0050]

[0051] In the formula, K f σ is the fatigue notch coefficient. α and β are the fitted parameters, and σ is the fatigue notch coefficient. nominal With σ smooth These represent the nominal stress of the notched composite material structure and the gauge length stress of the smooth specimen, respectively, when the notched and smooth specimens have the same fatigue life.

[0052] Step (4): Introduce the influence of stress level on stress concentration factor and fatigue notch factor. The specific expression is:

[0053]

[0054] Step (5): Using the SN curve (stress-life curve) of a smooth specimen (e.g.) Figure 4 (As shown) Calculate the notched fatigue life. The SN curve for the smooth part is:

[0055] σ smooth =cN m

[0056] In the formula, σ smooth denoted as , where is the stress on the gauge length of the smooth specimen; N is the fatigue life; and c and m are material constants obtained through fitting. Substituting the fatigue notch coefficient considering the stress level into the SN curve of the smooth specimen, the fatigue life prediction expression for the needled composite material including the notch structure is obtained as follows:

[0057]

[0058] Step (6): Simplify the equations. When the load applied to the notched part is equal to the static tensile load, N is 1, and σ nominal =σ t The above equation becomes

[0059]

[0060] Simplify to obtain

[0061]

[0062] Therefore, the fatigue life prediction expression for needled composite materials containing notched structures is:

[0063]

[0064] This equation is the final fatigue life prediction equation for notched ceramic matrix composite specimens. The predicted life of the specimen and the life obtained from experiments are as follows: Figure 5 As shown, all values ​​are within 10 times the band, which meets the engineering requirements of this material.

[0065] This invention provides a method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient, relating to the field of fatigue life prediction for needle-punched long fiber reinforced composite materials. The method comprises the following steps: Step (1) Calculation of the stress concentration factor of the material; Step (2) Establishment of the influence of stress level on the material; Step (3) Establishment of the relationship between stress concentration factor and fatigue notch coefficient; Step (4) Introduction of stress level to establish the influence of stress concentration factor and fatigue notch coefficient; Step (5) Calculation of notched fatigue life using the SN curve (stress-life curve) of a smooth specimen; Step (6) Simplification of the equation. The fatigue life prediction method for needle-punched composite materials considering the fatigue notch coefficient proposed in this invention has been verified through corresponding experiments on needle-punched ceramic matrix composite materials.

Claims

1. A method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient, characterized in that, The implementation steps of this fatigue life prediction method are as follows: Step (1): Calculation of the stress concentration factor of the material; the mathematical expression for the stress concentration factor is: In the formula, K t The stress concentration factor is... d is the width factor of the infinite plate, w is the notch size of the notched part, and M is the width of the material. If the gap is 0 When this item is 0, then... In the formula, E1 is the elastic modulus parallel to the direction of the long fiber layer, E2 is the elastic modulus perpendicular to the direction of the long fiber layer, and G... 12 v is the shear modulus perpendicular to the plane of the long fiber layer. 12 The Poisson's ratio in the plane of the long fiber layer; Step (2): Consider the effect of stress level on the material; at higher stress levels, the fatigue life of the material is mainly determined by interlaminar damage and notch effect, while at lower stress levels, the effect of notch on fatigue life is similar to that of notch-sensitive materials, as specifically expressed by: In the formula, α represents the effect of stress level on the material, and σ nominal For the nominal stress of the notched component, σ t The static tensile strength of the notched component; Step (3): Establish the relationship between stress concentration factor and fatigue notch factor; The specific expression is: In the formula, K f σ is the fatigue notch coefficient; α and β are the fitted parameters, and σ is the fatigue notch coefficient. nominal With σ smooth The nominal stress of the composite material structure with notch and the gauge length stress of the smooth specimen are respectively the fatigue life of the specimen with notch and the specimen with smooth. Step (4): Introduce the influence of stress level on stress concentration factor and fatigue notch factor; The specific expression is: Step (5): Calculate the notched fatigue life using the SN curve (stress-life curve) of the smooth specimen; the SN curve of the smooth specimen is: σ smooth =cN m In the formula, σ smooth Let be the stress on the gauge length of the smooth specimen; N be the fatigue life; c and m are material constants obtained through fitting; substituting the fatigue notch coefficient considering the stress level into the SN curve of the smooth specimen, the fatigue life prediction expression of the needled composite material including the notch structure is obtained as follows: Step (6): Simplify the equations; when the load applied to the notched part is equal to the static tensile load, N is 1, and σ nominal =σ t The above equation becomes Simplify to obtain The fatigue life prediction expression for needled composite materials with notched structures is as follows: The fatigue life prediction expression for needled composite materials is the final fatigue life prediction equation for notched ceramic matrix composite parts.

2. The method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient according to claim 1, characterized in that: The influence of stress level on the material in step (2) confirms that interlaminar damage and notch effect dominate the low stress level and high stress level of the material, respectively, and are related to the size of the material itself and the applied stress load.

3. The method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient according to claim 1, characterized in that: The relationship between the stress concentration factor and the fatigue notch factor in step (3) is adopted when the fatigue life is 10. 6 The fatigue notch coefficient is obtained by fitting the following, and it satisfies that when the part is smooth, the fatigue notch coefficient is 1; 4. The method for predicting the fatigue life of needle-punched composite materials considering the fatigue notch coefficient according to claim 1, characterized in that: The fatigue life prediction equations obtained in steps (5) and (6) are used to predict the constant amplitude fatigue life of notched needle composite material structures by obtaining the stress concentration factor of the material, the SN curve of the smooth part, and the fatigue notch factor of at least two sizes of notch structure.

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