Influence of Manufacturing Tolerance on Strength Dispersion of Composite Bolted Joint Structure

Volume 32, Issue 4, Manufacturing Tolerance, Influence on Strength Dispersion of Composite Bolted Connections Zhao Libin H, Shan Meijuan 2, Peng Lei 3, Ji Shaohua 3, Jia Xiwen 3, Xu Jifeng 3 (1. Beijing University of Aeronautics and Astronautics, School of Aerospace, Beijing 100191; 2. Beijing University of Aeronautics and Astronautics, Beijing 100191; 3. Beijing Commercial Aircraft Technology Research Center, China Commercial Aircraft Co., Ltd., Beijing 102211, China) Material bolts are connected differently, resulting in dispersive strength of bolted joints. The problem is to take the manufacturing tolerances of the inch high-lock bolt and the hole in the orifice as an example to study the distribution of the double-shear and four-nail bolt connection under different matching clearances, and further adopt the improved characteristic curve method, the improved strength envelope method and The progressive damage model predicts the strength of the bolted joint structure, and the dispersion intervals of the four-nail joint failure strength are respectively obtained. Journal of Composite Materials, 2015, 32(4): 1092-1098. The gap is an inevitable important factor. When predicting the strength of composite bolted joints, the characteristic curve method, the strength envelope method and the progressive damage method developed in the past ten years are mainly used. For the first two, the matching clearance caused by the manufacturing tolerance will directly affect the calculation result of the nail load distribution, which leads to the fixed dispersion of the predicted strength. For the progressive damage method, the matching gap directly affects the progressive damage expansion process of the connected structure. , so that the predicted intensity has a fixed dispersion.

In this paper, the four-row single-row double-shear bolt connection structure of composite materials is used to calculate the nail load distribution of the joint structure by considering the direct stiffness method with the clearance. The three-dimensional finite element model of the joint structure is established based on ABAQUS software, and the improved characteristic curve method is adopted. Improved strength envelope method and progressive damage model, calculate the strength of the bolted joint structure for different fitting clearances caused by manufacturing tolerances, and analyze the influence of different fit gaps on the joint structure strength, design and analysis of composite bolted joint structure provide.

1 bolt connection structure and matching clearance composite bolt connection structure adopts four rows of single row double shear connection form, and the test piece geometry is as shown. The test piece is made of 800 grade composite materials, the layup order is s, and the single layer thickness is 0.185mm. The basic mechanical properties of the TT800 grade composite material are shown in Table 1, where the value of the mark "" is assumed by the transverse isotropic assumption. And engineering experience is determined. The British high-lock bolt HST12-6-7 is used as the fastener. Without the gasket, the bolt diameter is 4.76=028mm, and the installation tightening torque is 5Nm. For the convenience of description, the bolts are numbered from left to right. For the composite laminate bolt hole, the bolt hole diameter is 476t:=mm when the bolt-hole interference fit is not considered. Therefore, the minimum fit clearance is 0, which is the ideal fit; the maximum fit clearance is 0.047mm. The bolt-hole matching clearance range manufactured by the relevant standard is 00.047mm. The four-row single-row double-shear bolt connection test piece diagram Table 1 The basic mechanical properties of the T800 grade composite material 2 The effect of the clearance gap on the nail load distribution For the four-row single-row double shear bolt connection The structure, using the direct stiffness method to calculate the ideal fit when the nail load distribution is 0.329:0.234:0.204:0.233, it can be seen that the nail load coefficient of nail 1 is the largest, corresponding to the key hole, nail and hole serial number as shown.

In engineering practice, the bolt-to-hole mating clearance due to manufacturing tolerances will affect the nail load distribution of the joint structure. In order to clarify the influence of the clearance on the position of the key hole, the nail hole 1 is first taken as the maximum gap, and the other nail holes are ideally matched, that is, the nail hole 1 is the safest state in all possible working conditions. At this time, when the bolt connection structure starts to load, the nail hole 1 does not bear the force due to the gap, and the other three nail holes jointly bear the external load, and the nail load coefficient gradually increases linearly from zero, as shown in (a).

When the load is increased to about 10kN, the nail hole 1 starts to bear the force due to the deformation of the bolt, the contact area of ​​the bolt hole, etc. At this time, the slope of the nail transmission load curve of the nail 1 is the largest, and the slope of the nail transmission load curve of the other nails Slightly decreased. As the load increases, the nail transfer load curve of the staple 1 intersects the nail transfer load curve of the other staples.

When the load is increased to about 50 kN (far less than the ultimate strength of the bolted joint structure under ideal fit), the nail hole 1 is subjected to the maximum nail load and is a critical hole. Therefore, it can be considered that the matching clearance of the four rows of single-row double-shear connections does not change the position of the key holes of the connection structure.

The effect of the matching clearance on the key hole of the bolt connection and the load factor of the key hole Fig.2 Effectoffitclearanceoncritical has different gaps for the key holes and other nail holes are ideal fit, (b) gives the change of the nail load coefficient of the nail hole 1 In the figure: C is the gap size of the nail hole 1. It can be seen that with the decrease of the gap, the nail loading coefficient of the nail hole 1 gradually approaches the ideal matching condition, and the key hole 1 has the largest nail loading coefficient under the ideal cooperation condition.

Through the above analysis, each bolt and hole can be in two extreme states: ideal fit and 0.047 mm clearance fit, then there are 15 possible gap distributions. The nail load factor of the key hole hole 1 was analyzed by the direct stiffness method considering the gap, and the results are as shown.

It can be seen that all the nail load coefficient curves do not intersect. On the upper side of the straight line of the load factor 1 of the working condition, with the increase of the load, the initial nail load factor of the initial load factor is also large; the ideal fit for the nail 1 and the maximum clearance fit for the other nails (conditions) 12), the nail load factor is the largest. Under the working condition 1 under the straight line of the nail loading coefficient, the initial nail loading coefficient of all working conditions is zero. With the increase of the load, the nail loading coefficient is gradually increased, but the nail 1 has the largest gap, and the other nails ideally fit. (Working condition 5) The corresponding nail loading coefficient is the smallest. Therefore, the influence range of the matching clearance on the nail loading coefficient is within the envelope range of the working condition 5 and the working condition 12.

Considering that the strength of the joint structure is proportional to the nail load factor of the key hole, in order to study the influence of the fit gap on the strength dispersion of the joint structure, the working conditions 1, the working condition 5 and the working condition 12 are respectively studied. The strength prediction result (ie, the standard value) of the connection structure in the case of the ideal fit, and the minimum and maximum values ​​of the predicted strength in the presence of the fit gap, as shown in the conditions A, B, and C in Table 2.

3 The influence of the fit clearance on the bolt joint strength is based on the above-mentioned fit clearance condition and nail load distribution result. The improved characteristic curve method, the improved strength envelope method and the progressive damage model are used to analyze the connection under the three conditions A, B and C. The failure strength of the structure, and the dispersion of the strength of the joint structure caused by the fit gap was studied.

3.1 Improved characteristic curve method The characteristic curve method is a commonly used method for predicting the mechanical joint strength of composite materials. Based on the traditional characteristic curve method, Zhang et al. introduced the shear feature size, proposed a three-parameter characteristic curve based on the tensile, compressive and shear feature sizes, and established a three-parameter characteristic curve for the composite double shear connection failure prediction. The method was also verified by a multi-ply double-shear joint test of 800 grade composite materials. Wherein, the expression of the three-parameter characteristic curve is a radius; Ri, Rc and Rs are respectively the tensile feature size, the compression feature size and the shear feature size 0 are clockwise or counterclockwise rotation from the extrusion load direction to the tensile failure plane. Angle.

The three-parameter characteristic curve method can more accurately predict the failure mode and failure load of composite multi-nail double-shear connection structure, and effectively improve the structural design efficiency. When the three-parameter characteristic curve method is used to predict the multi-nail connection strength of the composite material, the key hole with the most serious load and its load ratio are obtained by nail load distribution analysis. Then, the characteristic size test is designed according to the key hole information, and the hole plate is pulled. The tensile failure load, the loaded plate extrusion failure load and the loaded orifice shear failure load, and the tensile, compression and shear feature sizes obtained by stress analysis according to the feature size definition, and then substituted into the formula (1) can be obtained three Parameter characteristic curve. Therefore, the determination of feature size is the core link of the three-parameter curve method to predict the failure of multi-nail connection.

In the subsequent research, Zhang et al. proposed a method for determining the numerical feature size of a simple feature size test piece using a progressive damage model instead of a test to determine the feature size.

After determining the three-parameter characteristic curve, the finite element method is used to analyze the stress analysis of the single-nail connection structure based on the key hole parameters, and the intensity ratio cloud image can be obtained. For each single layer, the points of the same intensity are connected to an equal stress curve, wherein the single layer of the iso-stress curve furthest from the edge of the hole is defined as the key layer. As the external load increases, the equal stress curve of the key layer gradually expands outward, as shown in the above half, where the decay, coincidence is the equal stress curve corresponding to the increasing external load. When the stress curve of the critical layer is in contact with the three-parameter characteristic curve (as shown in the generation of the above half), it is considered that the single-nail connection is broken to determine the failure load and failure mode of the key hole. Finally, the failure load of the multi-nail connection structure is obtained according to the key hole load ratio, and the failure mode of the key hole is the failure mode of the multi-nail connection.

Based on the numerical feature size determination method and the three-parameter characteristic curve method, the failure prediction of composite four-row single-row double-shear connection under three working conditions is carried out. For the three working conditions A, B, and C shown in Table 2, the equal stress curves are first contacted with the tensile feature points, so the tensile failure occurs in the joint structure under the three working conditions, and the strength under working condition A For 58.72kN, the strength under working condition B is 59.99kN, which is 2.16% higher than that of working condition A, and the strength under working condition C is 56.45kN. Compared with working condition A, the strength of the improved strength of the envelope is reduced by 3.2. The covered wire method is a common method for predicting the mechanical joint strength of composite materials commonly used in engineering. Based on the traditional strength envelope method proposed by Hart-Smith, Lm et al. consider the influence of the tensile stress of the hole edge on the extrusion strength of the hole edge caused by the bypass load. It is considered that the extrusion strength of the bolt connection increases with the increase of the bypass load. Gradually improve, and then the strength envelope method for the bolt strength of the bolted bypass load under the tensile load is proposed, and the T800 grade composite multi-nail double shear connection test is used to verify. On this basis, Zhao et al. made progress on different stress concentration mitigation factors, considering the influence of stress concentration mitigation factors on bolting strength, and gave the best experimental determination scheme for stress concentration mitigation factors. When the strength of the multi-nail connection structure is predicted by the strength envelope method, after the strength envelope is obtained, the key hole transmission load-bypass load relationship fold line is drawn respectively, and the intersection point with the strength envelope line is the failure point, corresponding to The sum of the nail transmission load and the bypass load is the final failure strength of the joint structure.

In this paper, the failure strength of four rows of single-row double-shear joint structures under three working conditions is predicted by the strength envelope method modified by bypass load and stress concentration mitigation factor. The results are shown in the figure. It can be seen that the intersections of the three working conditions are located on the straight line MN, which indicates that the joint structure has tensile failure under the three working conditions. The strength of the joint structure under working condition A is 6278kN, and the strength under working condition B is 65.26kN. Compared with the working condition A, the increase is 3.95%, and the working condition C is 60.26kN, which is 4.01% lower than the working condition A. 3.3 Progressive damage method The progressive damage method is an analysis method based on damage mechanics. The stress distribution state of the composite structure is obtained by the stress analysis model, the damage or failure of the material is evaluated according to the appropriate failure criterion, and the mechanical behavior of the damaged or failed material is described by a certain degradation model. Through the progressive damage analysis, not only the initial damage site of the composite structure can be predicted, but also the damage propagation path of the structure can be traced, and the whole process from initial damage to ultimate damage can be simulated, and the ultimate strength and residual strength of the structure can be determined. The above advantages of the progressive damage method have attracted wide attention from scholars since its inception, and have evolved numerous concrete implementation forms.

The double-shear bolt is connected with the progressive damage model. Based on the ABAQUS software, the finite element model of the four-row single-row double-shear bolt connection structure is established by C3D8 unit. As shown, the improved failure criterion such as Tserpes is selected, and the corresponding material degradation model is adopted. The progressive damage model of the four-row single-row double-shear bolt connection structure was established, and the progressive damage analysis was carried out. The failure criterion and material degradation model adopted by the progressive damage model are shown in Table 3. The deterioration factor in the table indicates the proportional coefficient between the attribute of the material after degradation and the attribute before degradation; the subscript f represents fiber failure, and m represents matrix failure, 3 The failure map of the laminate in the progressive damage analysis of the connection structure under the working condition, in which the gray represents the failure area, it can be seen that the failure of the connection structure occurs in the nail hole 1 under the three working conditions, and the failure mode is tensile failure, other nails The pores were in a similar state and all had minor crush damage. The progressive damage method is used to predict the failure strength of four rows of single-row double-shear joints. It can be seen that the strength under working condition A is 65.52kN, and the strength under working condition B is 68.89kN. Compared with working condition A, the strength is increased by 5.14%. The lower strength is 63.45kN, which is 3.16% lower than that of working condition A. Table 4 gives the prediction results of the three methods and the comparison with the standard values. It can be seen that the trend of the prediction results of the three methods is the same, the failure strength under the condition B is the highest, and the failure strength under the condition C is the lowest. When the improved characteristic curve method, the improved strength envelope method and the progressive damage model are used to predict the failure strength of the joint structure, the intervals of the influence of the gap on the strength of the joint structure are respectively, and the deviations are not more than 6%. Table 3 T800 level composite Material Failure Criterion and Material Degradation Model 4 Conclusions (1) For the four-row single-row double-shear connection structure of T800 composite material, the direct stiffness method considering the clearance fit is used to calculate the nail load distribution of the joint structure. It is known that the matching gap does not change the key hole. Position; for the key hole, the larger the matching clearance, the smaller the nail loading coefficient; for the nail hole with the matching clearance, the nail hole coefficient curves of the key holes do not intersect. Corresponding to the ideal fit of the key nail hole 1 and the maximum clearance fit of the other nails, the key hole nail load coefficient is the largest; the corresponding nail 1 has the largest fit clearance, while the other nail ideal fit, the key hole nail load coefficient is the smallest; It is the upper and lower limits of the nail hole with various clearance fits.

(2) Using the improved characteristic curve method, the improved strength envelope method and the progressive damage method to predict the strength of the bolted joint structure with clearance fit, the dispersion interval of the joint structure failure strength is respectively, and the deviation is less than 6%. , indicating that if the fasteners and bolt holes meet the manufacturing standards, the impact on the strength does not exceed 6%.

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