Strength improvement to composite T-joints under bending through bio-inspired design

This paper shows that a bio-inspired design methodology is an effective method to strengthen composite T-joints under bending loading. The ply angles in the laminate of a carbon/epoxy T-joint were tailored using an optimisation program mimicking the evolutionary process of adaptive growth in which the wood microfibril orientation in and around the tree branch-trunk joint is tailored to the prevailing bending loading condition. A single objective optimisation program with four ply angle input variables was used to compute the optimal design of the ply stacking pattern which minimised the interlaminar tensile stress in composite T-joints where delamination damage is initiated. FEA and experimental testing were performed to compare the structural properties of the bio-inspired T-joint against a base-line T-joint with a quasi-isotropic ply stacking pattern. The bio-inspired T-joint exhibited a higher bending failure initiation load (improved by 40%) and elastic strain energy capacity (increased by 75%) than the base-line T-joint.     

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。     

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.     

Treatment of residual stress in failure assessment procedure

The effect of residual stress on component failure has been investigated using the distributions from current failure assessment procedures, and a residual stress profile simple to apply with less conservatism has been proposed for the weld geometries of T-plate and tubular T-joint. The stress intensity factors (SIFs) in the two weld geometries under various types of loads have been calculated using the Green’s function method. The Green’s functions were determined not only for the T-plate but also for the tubular T-joint with the built-in ends. The use of a linear (bending) stress profile, derived from an analysis of measured residual stress distributions in T-plate and tubular T-joints, has been examined. The profile was validated with experimentally measured residual stress distributions in two materials, a high strength and medium strength ferritic steel and two geometries, a T-plate joint and a tubular T-joint for crack lengths up to half the plate or pipe thickness. Whereas the recommended residual stress distributions are geometry and material specific, it is shown that a simplified linear bending profile provides a possible guideline, applicable to a range of materials and geometries, where detailed information on weld procedures or residual stress profiles are unavailable.     

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.     

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.     

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.     

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。     

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     

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.     

Z-pin/缝合对复合材料T型接头剪切承载能力的影响

利用三维有限元模型对Z-pin/缝合增强试验件进行有限元分析,采用内聚力模型模拟界面的破坏情况,通过在分层的上下界面加入非线性弹簧元来模拟Z-pin/缝合的增强机制,非线性弹簧元的力学性能（桥联律）由细观力学方法获得。通过与试验结果的对比发现,由于未进行界面增强的T型结构的剪切承载能力已较高,Z-pin/缝合增强较难提高T型接头的剪切承载能力。从少数几种可提高T型接头的剪切承载能力的增强方案中可看到,应选择拉伸强度较高而拉伸模量较低的缝线来进行T型接头剪切界面增强设计。     

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。     

Fatigue strength assessment of tubular welded joints by an alternative structural stress approach

The tubular joints, frequently employed in the offshore industry, are submitted to stresses resulting from elementary loadings: tension/compression, in-plane bending and out-of-plane bending. This work concerns the analysis of the recommendations commonly used for the fatigue design of welded joints submitted to combined loadings. Particularly, it deals with the fatigue behaviour of T-joints submitted to deviated-bending: first, a finite element analysis was developed and a post-processing based on the structural stress approach, as proposed by the International Institute of Welding (IIW). Then, fatigue tests were conducted on T-joints submitted to deviated-bending. Comparisons between experimental and numerical results showed that this kind of recommendations is not systematically conservative. Thus, an alternative approach based on structural stresses and taking into account the multi-axial stress state in the weld toe was developed in order to complete the recommendations for the fatigue design of tubular welded joints.     

3D reinforcement of stiffener-to-skin T-joints by Z-pinning and tufting

Carbon fibre/epoxy T-stiffener-to-skin joint was reinforced through the thickness, either by insertion of Z-Fiber^® before autoclave cure of prepreg or by tufting of dry preform with a glass thread before resin injection and cure. The joints pull-off resistance increased significantly for both types of T-joints under both quasi-static and fatigue loading conditions. In the case of the tufted joints, the delamination between the skin and the stiffener stopped completely and the samples failed in bending. It is shown that a finite element model is successful in reproducing qualitatively the cracking progression in the unreinforced and 3D reinforced T-joint provided that the action of the through-the-thickness reinforcement is modelled by discrete nodal forced placed so as to replicate the physical reality.     

Z-pin参数对复合材料T型接头拉脱承载能力的影响

利用二维平面应变模型对Z-pin增强T型接头试样进行失效分析,采用内聚力模型模拟界面的破坏情况,通过在分层的上下界面加入非线性弹簧元模拟Z-pin的增强作用,非线性弹簧元的力学性能(桥联律)由细观力学方法获得,数值结果与试验值吻合较好。在已验证有限元方法的基础上,研究了Z-pin直径、密度及植入角度等对T型接头拉脱承载能力的影响。结果表明:Z-pin增强可显著提高T型接头的拉脱承载能力,与未Z-pin增强的T型接头相比,Z-pin增强明显延缓了掉载;T型接头的拉脱承载能力随Z-pin直径和密度的增加而增大,随植入角度的增大而减小;在所研究的角度范围内,当植入角度为60°时,T型接头的拉脱承载能力最好;Z-pin直径和密度对拉脱承载能力的影响远比植入角度的影响显著。     

基于NCF技术的RTM成型复合材料T型接头高效制造与验证

选用多层非屈曲经编织物(NCF)作为增强体, 提高了预成型体的铺覆效率及准确性, 同时根据T型接头的结构特点将其组合成工字梁形式并依此设计成型模具, 采用树脂传递模塑(RTM)成型工艺实现了复合材料T型接头的高效制造, 并对其拉伸和压缩力学性能进行了试验研究。结果表明: T型接头拉伸破坏由孔边蒙皮纤维拉断和腹板分层拉脱造成; 而压缩破坏则由腹板翻边分层屈曲和接头中部蒙皮纤维压断造成; T接头拉伸破坏载荷高于其压缩的破坏载荷。     

Defects and tensile properties of 6013 aluminum alloy T-joints by friction stir welding

In this paper, 6013-T4 T-joints were successfully fabricated with different welding parameters by friction stir welding in two different combination modes of skins and stringers. The distribution features and formation mechanisms of defects in T-joints were observed and analyzed. The effect of defects and welding parameters on tensile properties of T-joints was investigated. The result shows that the T-joint without tunnel defect only can be obtained with the traverse speed of 100 mm/min in this experiment, and the welding parameters influence the features and sizes of kissing bond defects. The fracture of T-joints along the shin is attributed to the kissing bond defect and the tunnel defect is the main factor affecting the tensile properties along the stringer.     

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.     

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.