•A benchmark of cylindrical shell for investigating the effect to KDF from pure geometric imperfections is provided.•The Fourier series method is compared with the scatter points method for ...computation efficiency and prediction accuracy.•Guidance in dimensional tolerance is given for maximizing the load-carry capacity of cylindrical shells in manufacturing.
Imperfections from manufacturing process can cause a scattered reduction of the load-carrying capacity or buckling load of axially compressed cylindrical shell structures. To isolate the influence of geometric imperfections from other imperfections such as welding, a sub-scaled, integrally manufactured cylindrical shell with small-amplitude geometric imperfection was manufactured, analyzed and tested in this study. A test facility and measurement system (including imperfection measurement and buckling test) were constructed. Finite element (FA) numerical procedure for predicting the buckling load was developed. Results indicate that the buckling load predicted by the FE analysis is very close to that from the test. Knockdown factor (KDF) is discussed with reference to the NASA design document. Furthermore, the influence of pure geometric imperfections including imperfection component and amplitude on the buckling behavior is discussed based on Fourier series method. Some guidance for the dimensional tolerance in manufacturing process relating to the load-carrying capacity of thin-walled structures is provided.
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•A novel dual-sub-fold Miura foldcore is proposed by introducing sub-fold into the cell walls of conventional Miura foldcore.•The improvement in initial peak force and average force ...are demonstrated in novel Miura compared to the conventional Miura.•The novel Miura are found to exhibit a predictable and stable deformation mode for a wide range of geometric parameters.
A modified Miura foldcore geometry was developed by introducing sub-folds into the cell walls of a conventional Miura foldcore. Similar to other sub-fold Miura foldcores, stable plastic hinge lines were generated at sub-fold sites under the guidance of the sub-folds and transformed into traveling hinge lines or stationary hinge lines in the subsequent crushing process. Therefore, in comparison to the conventional foldcore, the dual-sub-fold Miura foldcore exhibited a higher average crushing force with an improvement of 60.8 % in the optimum case. The dual-sub-fold Miura foldcore exhibited relatively lower stiffness at the sub-fold sites, effectively reducing the initial peak crushing force. This reduction in peak crushing force reached a maximum decrease of 70 %. Moreover, this dual-sub-fold foldcore was glued to two parallel rigid plates (top and bottom), making it more suitable for engineering applications. The parametric study indicated that the dual-sub-fold Miura foldcore exhibited predictable and stable deformation modes. It was found that the average crushing force could be effectively enhanced by reducing the core folding angle, elevating the sub-fold position, decreasing the sub-fold size, and elongating the foldcore. The theoretical model for predicting the energy absorption performance of the foldcore was also established.
The huge computational cost is a main barrier of structural reliability assessment for complex engineering. Surrogate models can release the CPU burden of reliability assessment, however, it is very ...challenging to guarantee the prediction accuracy of failure probability. In this study, we propose an Adversarial Learning-Based Kriging model (ALBK), where two models learn from and compete with each other to achieve an improved model accuracy. First, the initial models are established, and fitting accuracy is evaluated by each other with the proposed criterion. Then, the modeling parameters are optimized according to the evaluation results. The data-driven criteria and adversarial relationship promote the evolution of modeling parameters. Moreover, a triple-indicator method is provided to choose the final model and avoid oscillation. The ALBK adjusts modeling parameters with alternative evolution, while the predicted values are more accurate than those of Kriging. Finally, an adaptive ALBK method is provided with new samples added to improve the accuracy of reliability assessment. Through several numerical examples, it can be seen that the ALBK always provides the best results with the fewest assessment calls, and the robustness is also good.
For cylindrical shells under axial compression, the essence of initial geometric imperfections is the superposition of local out-of-plane deformations of various forms, which may facilitate the ...development of buckling deformations, thus leading to a significant knockdown of the load-carrying capacity. It is very challenging for existing methods to provide an accurate prediction of the lower bound on a load-carrying capacity before the structure is fabricated. Therefore, it is crucial to find a type of assumed imperfection that will allow us to approximate lower bounds for shells in the design stage. Five 1-m-diameter unstiffened shells, termed W1-W5, are designed, analysed and tested. The measured imperfection approach, single-perturbation load approach (SPLA), worst multiple-perturbation load approach (WMPLA), and a Combined Approach for measured imperfections and superimposed radial point load imperfections are compared with test results. The results show that the SPLA-based methods produce higher KDFs than the test results and are sensitive to the distribution of the measured imperfections. In contrast, the KDFs predicted by the WMPLA and the Combined Approach are similar to one another and very close to the test results. From the comparison results, it can preliminarily be concluded that the WMPLA is able to envelop the small- and large-amplitude measured imperfections, which has the potential to predict a rational lower bound on the buckling loads of unstiffened cylindrical shells. The WMPLA should be extended to the design of other types of thin-walled structures with caution, because the manufacturing signature may be distinctly changed for different processes, and the buckling tests of other types of structures would be carried out in future study.
Thin-walled tube as an energy absorption device is indispensable in vehicles. Recently, the crashworthiness performance of metal/carbon fiber reinforced polymer (metal/CFRP) hybrid tubes has been ...widely investigated. However, those tubes are mainly straight, which tend to collapse in natural modes with lower energy absorption. Additionally, research has shown that the well-designed origami patterns applied to ends of conventional metal tubes can trigger the diamond mode with excellent performance. Therefore, the similar origami patterns are also applied to the ends of internal and external metal tubes of hybrid tubes. The two metal tubes will deform in diamond mode under the guidance of origami pattern, and then forcing the CFRP tube to deform the same high-performance collapse mode with more materials destroyed. The experimental results show that the hybrid tube indeed collapses in diamond mode with an excellent crashworthiness performance. The SEA can be increased by about 135% and 85.7% compared to conventional metal tube and CFRP tube. The CFE is also higher than other types of hybrid tubes. Besides, this design ensures that the CFRP tube is still straight, which greatly facilitates the manufacturing process.
•A 45-degree chamfer is introduced into one end of the CFRP tubes with circular or slotted cutout.•The chamfer can dominate the failure mode and successfully trigger the progressive crushing mode.•In ...most cases, the chamfer can eliminate the negative effects of cutouts and improve the energy absorption capacity.
Carbon fiber reinforced polymer (CFRP) tubes have been widely used as energy absorption devices. Because it is inclined to deform in progressive crushing mode (PCM) with low initial peak force Fp and high specific energy absorption SEA (the energy absorption per unit mass). In many cases, cutouts are introduced into these tubes to meet engineering requirements. However, the cutouts can cause a collapse in the middle height (i.e., mid-height collapse mode), and thus the SEA of the CFRP tubes is even reduced by 56%. Therefore, in this research, a chamfer is introduced into the CFRP tubes with cutout to trigger the PCM, thereby increasing the energy absorption capacity. A series of validation experiments show that the chamfer can trigger the PCM with about 64% decrease of Fp and 52% increase of SEA. The parameter study is conducted to investigate the effect of size and position of the cutout on the energy absorption of CFRP tubes with a 45-degree chamfer. The result indicates that the chamfer can eliminate the negative effects of cutouts and improve the energy absorption capacity in most cases. Furthermore, the contrastive analysis between CFRP and metal tubes reveals the difference in energy absorption characteristics of these two types of tubes and further explains the advantages of energy absorption of the CFRP tubes with a 45-degree chamfer and a cutout.
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In order to improve the global optimizing ability of surrogate-based optimizations, an enhanced variable-fidelity surrogate model (VFSM) optimization framework is developed based on the Gaussian ...process regression algorithm and the fuzzy clustering algorithm, which makes full use of the information fusion of high-fidelity model (HFM) and low-fidelity model (LFM). Firstly, a screening criterion is proposed for determining competitive sampling points based on low-fidelity surrogate model (LFSM). Then, the competitive sampling strategy is combined with the fuzzy clustering algorithm, which can adaptively obtain the reduced design space with more possibility to find the optimal result. In the reduced design space, the VFSM is constructed by means of the Gaussian process regression algorithm. Based on the VFSM, a gradient-based optimization can start from the competitive sampling point to search out the optimal solution quickly. In order to verify the effectiveness of the proposed optimization method, three test functions and an engineering example of hierarchical stiffened shells are carried out. Optimization results indicate that, under the similar computational cost, the proposed optimization method achieves significantly higher global optimizing ability than the high-fidelity surrogate model (HFSM) optimization method, the LFSM optimization method and the traditional VFSM optimization method by direct sampling.
•A screening criterion is defined for determining competitive sampling points.•The design space is reduced by fuzzy clustering algorithm and competitive sampling.•An enhanced variable-fidelity surrogate-based optimization method is developed.•The high global optimizing ability of the proposed optimization method is verified.
•Loading deviations are quantified by displacement gauges data in real time.•Strain field based on digital twin (DT) is built by strain and displacement data.•DT can combine all the advantages of ...multi-type data of FEA and sensor data.•Considering real-time loading deviations is helpful to improve the accuracy of DT.
For the static loading test, digital twin (DT) constructed by multi-type data fusion can combine the advantages of these data, which has potential application in test monitoring. Affected by assembly and manufacturing defects of test system, the loading deviations are difficult to avoid and quantify, and it has an important effect on the strain state of structures. However, the current DT is built by data fusion of strain gauges data and strain field of finite element analysis (FEA) only, and how to build DT considering real-time loading deviations by fusing multi-type sensor data remains a challenging task. Therefore, a strain field reconstruction method based on DT considering the real-time loading deviations (DT-SFRM-LD) is proposed to improve the accuracy of DT. In the test, the loading deviations calculated by displacement sensors data are used as FEA database input to obtain FEA strain field considering real-time loading deviations. The FEA strain field is combined with strain gauges data to construct DT in real time, which combines all the multi-type data advantages of displacement gauges, FEA strain field and strain gauges. A cylindrical shell test is performed to validate the high accuracy of DT-SFRM-LD. Results indicate that the AvgErr (5.0%) and MaxErr (13.6%) of DT-SFRM-LD are 4.3% and 10.0% lower than those of conventional DT method, and the AvgErr of DT-SFRM-LD is less affected by the eccentricity distance. In general, under different test conditions, the accuracy of DT-SFRM-LD is always higher than that of conventional DT method, indicating that considering the real-time loading deviations by displacement gauges is helpful to provide accurate FEA strain field distribution and to improve the accuracy of DT-SFRM-LD.
Stiffened shells in launch vehicles are very sensitive to various forms of imperfections. In this study, the imperfection sensitivity of a 4.5mdiam isogrid stiffened shell under axial compression is ...investigated. The measured imperfection, NASA SP-8007 and several types of assumed imperfections, including eigenmode-shape imperfection and dimple-shape imperfections (produced by the single perturbation load approach (SPLA) and worst multiple perturbation load approach (WMPLA)), are introduced into FE model to predict the knockdown factors (KDFs), respectively. Then, the buckling test of this full-scale stiffened shell under axial compression is carried out to validate the above numerical approaches. It can be found that the KDF predicted by the WMPLA is very close to the test results, while the ones predicted by eigenmode-shape imperfection and NASA SP-8007 are extremely conservative. Besides, the measured imperfection and other assumed imperfections are proven to be risky, because these methods overestimate the actual load-carrying capacity. Finally, it can be concluded that the WMPLA is a potential and efficient approach to predict KDFs in the design stages for future launch vehicles.
•The buckling test of a full-scale 4.5mdiam stiffened shell under axial compression was carried out.•Various forms of imperfections were considered in the numerical analysis of stiffened shells.•The numerically and experimentally predicted knockdown factors were compared in detail.
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