Full impact damage tolerance assessment requires the ability to properly mimic the repeated impact response and damage behaviour of composite materials using quasi-static approximations. To this aim, ...this paper reports an experimental investigation evaluating two quasi-static methods for mimicking repeated impact response and damage behaviour of Carbon Fibre Reinforced Polymer (CFRP) composite laminates. In this study, an 8.45-J single impact was repeated 225 times and mimicked with 225 times 6.51-J quasi-static (energy equivalent) indentations and with 225 quasi-static (force equivalent) indentations following the recorded impact peak force variation. Results show that the loading rate and the inertial effect are the two major factors affecting the responses of the composite laminates under out-of-plane concentrated loading. Both the energy- and force-equivalent quasi-static indentations failed to reproduce the impact responses greatly associated with high loading rate and inertial effect. The force-equivalent quasi-static indentations were performed in a semi-automatic way and induced damage states more similar to those of the repeated impacts than those of the energy-equivalent quasi-static indentations, whereas the latter can be better automated and has better reproducibility compared to that of the repeated impact responses, as it is less dependent on high loading rate and inertial effect.
To further investigate the effects of in-plane and out-of-plane stresses on the delamination initiation for composite laminates under out-of-plane loading, this paper reports a joint experimental and ...numerical study, in which the fully clamped rectangular CFRP composite laminates were subjected to out-of-plane quasi-static indentations. The results show that the combination of the out-of-plane shear and in-plane tensile stresses together determined the initiation of delamination, whereas the influences of the out-of-plane compressive stress on the delamination initiation can be neglected. For the purpose of understanding the effects of deformations on the out-of-plane shear and compressive stress distributions, a concise analytical model was developed, which was validated against the numerical and experimental results. As a key take-away, this study reveals that the common impact tests at the geometric centre of the panel may not resemble sufficient similitude with stiffened panels where panel flexure is suppressed by various geometrical stiffening concepts.
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•Increasing the rivet-hole clearances can enhance the joints’ fatigue life but also reduce joints’ strength.•A 3D FE model integrating riveting and fatigue loading processes, was ...employed to analyze stress amplitude and CFRP damage.•The fatigue life of the joint and the path of crack propagation are directly affected by the stress amplitude.•Using the method of counting the number of damaged elements to quantitatively analyze the degree of CFRP damage.
The CFRP/Al countersunk rivet joints are one of the most fatigued dangerous points of the aircraft fuselage. To deeply investigate the failure mechanisms of the riveted joint structures, this paper utilizes electromagnetic riveting technology to prepare single-lap joint specimens and investigates the quasi-static tensile and fatigue failure behaviours of the joints under different rivet-hole clearances and stress levels. In addition, to assist this investigation, a three-dimensional finite element model, combining riveting and fatigue loading, is developed. This model takes into account the strain rate effects during riveting process and progressive failure behavior of various typical damage modes in CFRP laminates. The results indicate that Aluminum alloy sheets fracture dominates the fatigue failure of the joints. Increasing the clearance can reduce the stress amplitude on the shear plane and further enhance the fatigue life of the joints. Furthermore, all additional CFRP damage during the loading process is the growth of the initial riveting damage, and the damage-induced fibre bearing capacity decrease is considered the reason for the increase in stress amplitude of the Al sheet. Last, excessive clearance enlargement does not appreciably diminish the initial riveting damage, instead, it could result in a joint strength reduction.
Abstract The electromagnetic force dynamic installation (DI) method of interference fit fasteners has significant advantages in improving the quality of the assembly interface of composite structure. ...This study aims to investigate how improving assembly interface quality may help in achieving high‐precision interference‐fit joints in composites. Single‐lap joint specimens were prepared using electromagnetic installation technique, the fatigue life and failure behaviors of the joints under different bolt‐hole clearances were measured and analyzed experimentally. In addition, a three‐dimensional finite element model was developed to predict the radial pre‐compression stresses around the hole and the progressive failure behavior of various typical damage modes in carbon fiber‐reinforced polymer induced by the interference. The results show that the critical interference range for the DI‐composite bolted structure is 1.0%–1.2%. The fatigue life of the best interference (1.0%) and the worst interference (0%, neat fit) have a difference of 36 times in amplitude. The residual stress with uniform distribution and high amplitude reduces the stress concentration of the hole wall and improves the failure evolution of the loaded hole. The contribution of this study provides vital guidance for improving the fatigue performance of CFRP bolted structures using a novel DI method. Highlights I = 1.0% is the critical damage‐enhancing interference size for dynamically installation interference fit bolts with electromagnetic forces. The residual stress amplitude introduced by the interference does not increase uniformly as the interference size increases. Excessive interference significantly reduces the uniformity and magnitude of residual stresses. Under subsequent cyclic loading, the fifth layer near the inlet and outlet of the extruded lower laminate appears to be the critical ply of extrusion deformation, and severe damage is concentrated in the critical ply of deformation. The decrease of stress amplitude and the variation of mean stress are the mechanisms of fatigue life enhancement in interference fit joints under external cyclic loading.
Cold Expansion and Interference Fit are two well-known techniques in hole strengthening, having been widely used in aviation industry. Double cold expansion is a new strengthening process which ...effectively combined with the two techniques. A modelling method based on ABAQUS finite element method is proposed in order to study the effect of double cold expansion, which will include cold expansion, reaming process, interference fit and springback after that, the influence of edge distance ratio (EDR) on the stress-strain distribution has been studied. Proto-LXRD device has been used to measure the residual stress distribution in the surface of the specimens. Simulation and experimental results show that the stress distribution of the simulation is in an higher level than that of experimental, but the trends of the stress distribution are in a good agreement.
冷挤压和干涉配合作为飞行器制造中2种主要的孔强化工艺，可有效提高结构的疲劳寿命。二次冷挤压强化作为以上2种强化工艺的结合，具有较单一强化更好的强化效果，但其强化机理尚不清晰，从而限制了这一先进工艺的有效应用。提出了一种用于研究二次冷挤压强化机理的数值建模方法，并根据TC4-DT钛合金耳片孔二次冷挤压强化的实际工艺流程，综合考虑了开缝衬套冷挤压、铰孔、压合衬套干涉配合及回弹等过程，同时研究了边距比对二次冷挤压强化耳片孔周应力分布的影响，并采用X射线残余应力测量设备测量试件表面的残余应力。通过测量结果与模拟结果的对比表明：采用该建模方法得到的残余应力变化趋势与实验测量结果具有良好的一致性，可用于二次冷挤压强化机理的深入研究。
During rice domestication and improvement, increasing grain yield to meet human needs was the primary objective. Rice grain yield is a quantitative trait determined by multiple genes, but the ...molecular basis for increased grain yield is still unclear. Here, we show that NUMBER OF GRAINS 1 (NOG1), which encodes an enoyl-CoA hydratase/isomerase, increases the grain yield of rice by enhancing grain number per panicle without a negative effect on the number of panicles per plant or grain weight. NOG1 can significantly increase the grain yield of commercial high-yield varieties: introduction of NOG1 increases the grain yield by 25.8% in the NOG1-deficient rice cultivar Zhonghua 17, and overexpression of NOG1 can further increase the grain yield by 19.5% in the NOG1-containing variety Teqing. Interestingly, NOG1 plays a prominent role in increasing grain number, but does not change heading date or seed-setting rate. Our findings suggest that NOG1 could be used to increase rice production.
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