Bio-inspired hierarchical design of composite T-joints with improved structural properties

The biological principle of hierarchical (multi-scale level) design was used at the structural and laminate levels to design a novel carbon/epoxy T-joint with improved structural properties for potential use in light-weight aircraft structures. The bio-inspired structural modification mimics tree branch–trunk joints by embedding the stiffener flange into skin plies. This design concept results in increased fracture toughness due to crack branching and deflection. Simultaneously, bio-inspired ply angle optimisation was used to mimic the tailored arrangement of cellulose micro-fibrils observed in the wood cells contained within tree branch joints. The optimisation procedure minimises the interlaminar stress concentration in the T-joint radius bend and increases strength while maintaining similar global laminate stiffness properties. The hierarchical joint resulted in a significantly improved tensile strength compared to a conventionally designed T-joint. The new design additionally exhibited higher absorbed strain energy to failure load for bending and tension loading. Additionally, the hierarchical T-joint had a significantly reduced critical joint cross-sectional area (weight) due to the embedded design.     

Bio-inspired design of aerospace composite joints for improved damage tolerance

This paper uses a bio-inspired design strategy based on tree branch joints to improve the damage tolerance of co-cured composite T-joints. The design of tree branch joints at different length scales from the microstructural to the macro-length scale was investigated. X-ray computed tomography of a pine tree revealed three main features of tree branch joints which provide high structural efficiency and damage tolerance: integrated design with the branch embedded into the centre of the trunk; three-dimensional fibril lay-up in the principal stress directions; and variable fibril density to achieve iso-strain conditions through the joint connection. Research presented in this paper adapts the embedded structural feature of tree joints into a carbon/epoxy T-joint. The flange plies were embedded to 25%, 50% and 75% of the depth of the skin of the composite T-joint to mimic the design of tree branch joints. Experimental testing revealed that the bio-inspired T-joint design with integrated adherends had increased normalised inelastic strain energy (defined as ductility), increased normalised absorbed strain energy to failure, and higher load-carrying capacity following damage initiation (damage tolerance) compared to a conventionally bonded T-joint. However, these improvements were achieved at the expense of earlier onset of damage initiation in the T-joints.     

弯曲载荷下机织复合材料T型接头渐进失效分析

根据树脂传递模塑(RTM)成型的缎纹机织复合材料T型接头的结构特征和纤维布局特点, 基于ANSYS有限元数值分析平台, 建立符合其真实结构的几何模型和有限元分析模型。基于渐进失效强度预测方法的基本思想, 使用有限元计算软件ANSYS的参数化设计语言(APDL)开发相应的程序, 实现改进形式的Hashin失效准则。采用合适的最终失效评价方法, 建立二维机织结构复合材料T型接头受弯曲载荷时的渐进失效预测方法, 能够有效地模拟从初始加载到最终失效过程中机织复合材料T型接头结构的力学响应及损伤的萌生与发展, 并预测结构的静强度。      

RTM成型非对称复合材料T型接头的拉伸失效机制

采用树脂传递模塑(RTM)工艺制备了结构对称和非对称两种复合材料T型接头试样,并对其进行了静态拉伸力学试验,对比分析了两种结构的拉伸破坏模式、结构刚度及破坏载荷。同时基于T接头内聚力模型(CZM),研究了两种不同结构T型接头的拉伸破坏过程及失效机制,并对比分析了不同偏转角下T接头的层间应力。结果表明：不同结构T型接头的拉伸破坏模式不同,偏转角的存在使结构非对称T型接头夹角大侧圆弧受力明显高于小侧圆弧,导致接头首先在大侧夹角圆弧与三角区界面定向萌生初始裂纹,随后裂纹主要沿大侧腹板翻边与蒙皮的界面扩展,进而导致接头最终破坏,最终失效载荷较对称T型接头提高了15.3%,且结构刚度更大。有限元结果表明T型接头三角区的初始失效主要由层间正应力及剪应力引起,有限元分析的失效模式与试验一致,结构对称及非对称T型接头最终失效载荷与试验值均吻合较好;且随着偏转角的增加,腹板圆弧处层间应力逐渐减小,初始失效载荷将随之增大;初始破坏位置将转移至大侧夹角圆弧末端。     

填充物、胶膜和Z-pin对复合材料T型接头强度影响对比

为提高复合材料T型接头结构的拉伸强度,对接头中胶膜属性、圆弧区填充物属性和Z-pin增强三种结构参数对T型接头强度的影响进行了研究。设计了两种不同胶膜属性、两种不同填充材料和有无Z-pin的同尺寸试验件,完成拉伸试验,测得极限位移和极限拉伸强度,并进行了对比分析,同时研究了不同T型接头的损伤演化过程。结果表明：J299胶膜复合材料T型接头的极限位移和极限载荷相比于J116B胶膜分别提高了57.8%和64.7%;ZXC195增强芯复合材料T型接头的极限位移和极限载荷相比于单向带材料分别提高了51.7%和30.3%;Z-pin钉对复合材料T型接头的极限位移和极限载荷分别提高了190.8%和31.9%。三种结构参数均只影响接头的极限载荷和极限位移的大小,接头的整体刚度没有改变。胶膜属性对接头极限载荷的提高影响最大,而Z-pin对接头的极限位移提高影响最大。     

3-D non-linear stress analysis on the adhesively bonded T-joints with embedded supports

In the present study, it is aimed to compare mechanical behaviors of T-joint types with embedded and non-embedded supports subjected to bending moment. For this purpose, after experimental studies on the two different T-joint types were conducted, stress analyses in the T-joints were performed with a three-dimensional finite element method by considering the geometrical non-linearity and the material non-linearities of the adhesive (DP460) and adherend (AA2024-T3). Finally, stress analyses and experimental results show that the variation of the geometry of the bonding zone, e.g., embedding the supports, would change the stress distributions and strength of the joint. Additionally, it is seen that T-joints with embedded supports carry 30% more load than T-joints with non-embedded supports although their bending stiffnesses decrease.     

Numerical analysis and experiment of composite sandwich T-joints subjected to pulling load

This paper presents an investigation into the failure mechanism and alternative design of composite sandwich T-joints subjected to pulling load. Based on a conventional design of sandwich T-joint as the baseline, numerical modeling and analysis using finite element (FE) method was performed to assess the strength against pulling load. The effect of a cutout in the web panel near the joint has been considered. To validate the models, sandwich T-joint samples were manufactured and tested. Detailed FE analysis and inspection of the experimental results indicated that the failure was mainly due to the excessive stress in the adhesive between the cleat flange and the T-joint base panel. The manufacture defects, which reduced the strength of the T-joint test samples had also been investigated. This has been further demonstrated by experimental results of repaired T-joint samples. A very good correlation between the test data and FE results were obtained. An unconventional design of T-joint for simpler manufacture process was proposed. Based on the design, T-joint samples were modeled, manufactured and tested to demonstrate the manufacture process and evaluate the improved strength.     

Particle swarm-based structural optimization of laminated composite hydrokinetic turbine blades

Composite blade manufacturing for hydrokinetic turbine application is quite complex and requires extensive optimization studies in terms of material selection, number of layers, stacking sequence, ply thickness and orientation. To avoid a repetitive trial-and-error method process, hydrokinetic turbine blade structural optimization using particle swarm optimization was proposed to perform detailed composite lay-up optimization. Layer numbers, ply thickness and ply orientations were optimized using standard particle swarm optimization to minimize the weight of the composite blade while satisfying failure evaluation. To address the discrete combinatorial optimization problem of blade stacking sequence, a novel permutation discrete particle swarm optimization model was also developed to maximize the out-of-plane load-carrying capability of the composite blade. A composite blade design with significant material saving and satisfactory performance was presented. The proposed methodology offers an alternative and efficient design solution to composite structural optimization which involves complex loading and multiple discrete and combinatorial design parameters.     

Fatigue Behaviour of Composite T-Joints in Wind Turbine Blade Applications

This paper presents a study of fatigue performance of composite T-joints used in wind-turbine blades. A T-joint with various fibre reinforcement architectures were selected to investigate its fatigue behaviour. The 3D angle interlock T-joint was found to have the best performance in both static and fatigue loading. Increasing the static properties increases fatigue performance while the increasing rate in life performance is changed with the number of fatigue cycles. A finite element (FE) model was developed that can determine the stress distribution and the initiation and propagation of a delamination crack. The location for through-thickness reinforcement is very important to improve fatigue performance of composite T-joints. Fatigue performance is significantly improved for the web with through-thickness reinforcement while fatigue performance is decreased if the through-thickness reinforcement is applied to the flange-skin regions. The interlaminar veil significantly increases the ultimate strength under static load but fatigue performance at high stress cycles is increased but not significantly.     

Fatigue crack growth in tubular welded connections

A series of tests have been conducted on tubular welded T-joints using out-of-plane bending. The complete test series is designed to measure the stress distribution and the fatigue strength, for this size of T-joint, under random loading. The work presented here includes the experimental strain analysis, together with Finite Element results, and fatigue crack growth measurements. These results show that it will be possible to estimate the fatigue life of T-joints using a fracture mechanics approach.     

Numerical analysis and experiment of composite sandwich T-joints subjected to pulling load

This paper presents an investigation into the failure mechanism and alternative design of composite sandwich T-joints subjected to pulling load. Based on a conventional design of sandwich T-joint as the baseline, numerical modeling and analysis using finite element (FE) method was performed to assess the strength against pulling load. The effect of a cutout in the web panel near the joint has been considered. To validate the models, sandwich T-joint samples were manufactured and tested. Detailed FE analysis and inspection of the experimental results indicated that the failure was mainly due to the excessive stress in the adhesive between the cleat flange and the T-joint base panel. The manufacture defects, which reduced the strength of the T-joint test samples had also been investigated. This has been further demonstrated by experimental results of repaired T-joint samples. A very good correlation between the test data and FE results were obtained. An unconventional design of T-joint for simpler manufacture process was proposed. Based on the design, T-joint samples were modeled, manufactured and tested to demonstrate the manufacture process and evaluate the improved strength.     

Stressed state of equal-diameter T-joints of pipelines under the action of torques and bending moments

We analyze the stressed state of pipeline T-joints in which the connecting pipe makes an angle of 90° or 45° with the main pipe under the action of torques and bending moments causing the out-of-plane torsion and bending of the T-joint. The influence of the angle of orientation of the connecting pipe on the behavior of the stress tensor is studied for various loading modes. For all investigated loading modes, we determine the stress concentration factors according to the Maxell-More theory of specific distortion strain energy. Translated from Problemy Prochnosti, No. 3, pp. 69–75, May–June, 2000     

Parametric analyses of stitched composite T-joints by the finite element method

Numerical methods were employed to perform a detailed parametric study on composite T-joints with transverse stitching using the finite element method. This analysis was accomplished to discern the effects of key joint parameters including fiber insertion tow modulus, fiber insertion filament count, fiber insertion depth, and resin-rich interface zone thickness on T-joint displacement and damage initiation load. T-joint load conditions included flexure, tension, and shear. Significant results of the parametric finite element analysis indicate that under flexural loading, increasing the fiber insertion tow modulus and tow filament count increases the T-joint damage initiation load; increasing the fiber insertion depth reduces T-joint deflection; and reducing the web-to-flange interface thickness reduces the T-joint deflection. Fiber insertion tow filament count and modulus have a negligible effect on T-joint deflection under tension and initial damage load under shear.     

Optimization of a composite scarf repair patch under tensile loading

Mechanics of the composite repair under tensile loading with and without overlay plies was examined for nontraditional patch ply orientations. Three-dimensional nonlinear analysis was performed for repair failure prediction and good baseline comparison for open hole scarfed panels and panels repaired by using standard ply-by-ply replacement patch composition was achieved. Multidimensional optimization was performed to calculate the repair patch ply orientations which minimize the von Mises stresses in the adhesive. These optimal stacking sequences achieved significant reduction of the stress levels and resulted in predicted up to 85% and 90% strength restoration for flush and single ply thickness over-ply repair. These results are intended to illustrate additional design variables available for efficient composite repair design, namely the composition of the repair patch.     

An experimental and finite element study of the transverse bending behaviour of CFRP composite T-joints in vehicle structures

The behaviour of a woven fabric carbon/epoxy composite T-joint (representing a simplified version the T-joint located at the connection between the B-pillar and the longitudinal rocker in a car body structure) is investigated using experimental and numerical methods. Details of the manufacturing process and experimental design factors are considered to understand their influence on the performance of the T-joint structure. The experimental results reveal the influence of manufacturing process and experimental setup on the load-carrying capacity and failure mode of the T-joint. Numerical simulation accurately predicts the stress distribution and load-carrying capacity of the T-joint obtained from experimental tests. The FEM model, which includes the adhesive interface layers at the edges, convincingly represents the experimentally found stiffness: the error is less than 3%. According to Hashin matrix tension criteria, the first ply failure occurs at 3.746 kN when the Hashin failure index (R) becomes equal to 1. Whereas, in the case of experimental tests, the first ply failure occurs around 3.4 kN, at which force the first load drop is observed.     

Analysis of the stressed state in equal-diameter T-joints of pipelines. Part 1. Action of internal pressure and bending moments in the plane of a T-joint

We analyze the stressed state of two types of T-joints in which the union makes angles of 45° and 90° with the main pipe. The character of the distribution of stresses in a T-joint illustrates the distinctive features of its operation under the action of internal pressure and bending moments applied in the plane of the T-joint. On the basis of the theory of strength for specific distortion strain energy, we determine the stress concentration factors for all considered loading modes. The numerical values of the stress concentration factors are compared with the experimental data obtained by other authors. Translated from Problemy Prochnosti, No. 6, pp. 116–123, November–December, 1998.     

Moment Distribution around Circular/Elliptical/Triangular Cutouts in Infinite Symmetric Laminated Plate

In this article, a general solution for moment distribution around a circular/elliptical/triangular shaped hole in a laminated composite plate subjected to a bending/twisting moment at infinity is presented. The numerical results are obtained for graphite/epoxy, glass/epoxy, boron/epoxy, etc. The effect of loading factor, stacking sequence, material parameters, and hole geometry on moment distribution is studied for cross ply and angle ply laminates. The results are compared with existing literature and found to be in good agreement.     

环口板加固T型方钢管节点在轴压作用下极限承载力研究

从理论分析、试验测试及有限元模拟三个方面对环口板加固T型方钢管节点的极限承载力进行了初步的研究工作。首先基于塑性铰线模型推导出了环口板加固T型方钢管节的极限承载力计算公式。然后对2 个环口板加固T型方钢管节点试件及2 个对应的未加固节点试件进行了在轴压作用下的承载力试验测试,结果表明环口板可以明显提高管节点的极限承载力。通过有限元法对试验试件进行了数值模拟,其结果与试验结果吻合较好,因而使用有限元法对9 个不同节点尺寸的加固模型及对应的9 个未加固节点模型进行了模拟,结果发现加固后节点的承载力均大于未加固节点的承载力。环口板加固T型方钢管节点的极限承载力计算公式在环口板有足够刚度,节点破坏模式为由局部屈曲导致形成塑性铰线破坏时可以获得较为精确的结果。     

面内线性变化载荷作用复合材料层合板的振动与屈曲优化

采用广义微分求积（GDQ）法开展了不同边界条件下承受面内线性变化载荷作用复合材料层合板振动与屈曲的分析与优化。针对GDQ法求解面内线性变化载荷工况复合材料层合板屈曲问题存在计算振荡、不收敛现象,提出载荷扰动策略实现了GDQ法对复合材料层合板屈曲问题的稳定高效求解。基于基础圆频率和临界屈曲载荷系数的归一化指标,分析了铺层角度对复合材料层合板综合性能的影响,并结合直接搜索模拟退火算法开展了复合材料层合板的铺层顺序优化。结果表明:铺层角度变化对屈曲性能的影响明显强于频率特性;面内线性变化载荷中,以弯曲载荷作用下复合材料层合板的优化综合性能受边界条件变化的影响最小,而优化铺层角度受边界条件变化的影响最大。研究结果为复杂载荷作用下复合材料层合板的设计提供了参考。      

Strengthening mechanics of thin and thick composite T-joints reinforced with z-pins

This paper presents an experimental and analytical study into the importance of the skin–flange thickness on the strengthening mechanics and fracture modes of z-pinned composite T-joints. The structural properties of unpinned and z-pinned carbon fibre–epoxy T-joints that had skin–flange thickness values between 2 mm (thin) and 8 mm (thick) were determined under tension (stiffener pull-off) loading. Experimental testing revealed that the capacity of z-pins to improve the structural properties was strongly dependent on the T-joint thickness. The joint properties increased at a quasi-linear rate with the skin–flange thickness, and z-pin pull-out tests showed that this was due to the increased crack bridging traction load and traction energy. The increase to the structural properties of the z-pinned T-joints with increasing thickness is explained using the bridging traction laws for z-pinned laminates.