A study on residual fatigue bending strength and damage behavior of CFRP composites subjected to impact loadings

This paper evaluates the static and fatigue bending strengths of CFRP (carbon-fiber reinforced plastic) laminates having impact damages, e.g., foreign object damages (FOD). Composite laminates used in this experiment are CF/EPOXY and CF/PEEK orthotropy laminated plates with two-interfaces [0°₄/90°₄]_s A steel ball launched by an air gun collides against the CFRP laminates to generate impact damages. The damage growth during a bending fatigue test is observed by a scanning acoustic microscope (SAM). When the impacted side is compressed, the residual fatigue bending strength of CF/PEEK specimen P is greater than that of CF/EPOXY specimen B. On the other hand, when the impacted side is in tension, the residual fatigue bending strength of CF/PEEK specimen P is smaller than that of CF/EPOXY specimen B. In the case of impacted-side compression, the fracture is propagated from the transverse crack generated near the impact point. In the case of impacted-side tension, however, the fracture develops toward the impact point from the edge of interface-B delamination.     

蜂窝芯结构力学特性的实验与仿真研究

负泊松比蜂窝芯结构具有独特的力学性能,有着良好的应用前景,蜂窝芯结构弹性模量的研究是夹层结构设计的基础。文章对蜂窝芯结构模型进行力学性能实验以及ANSYS仿真分析研究,结果表明蜂窝芯结构的弹性模量都是随着t／b的增大而增大,且与理论计算值为同一个数量级,为蜂窝芯结构的工程实用化研究奠定了基础。     

铝蜂窝夹芯板低速动态冲击响应研究

为研究不同结构参数对质量相同、强度不同的两种铝蜂窝夹芯板低速动态冲击响应的影响，建立了铝蜂窝夹芯板受半球型落锤低速冲击的数值模型，并将有限元计算结果与试验结果进行对比，检验了模型的可靠性。在此基础上，对比研究了不同上下铝板厚度和不同蜂窝芯壁厚对两种铝蜂窝夹芯板在低速冲击下吸能效果的影响。结果表明：在质量相同的情况下，强度小、高度大的夹芯板在低速冲击下力-位移曲线更易出现双峰模式，增加蜂窝芯壁厚或是上下铝板厚度都会使第一次的峰值力增加，第二次峰值力降低；强度小、高度大的夹芯板蜂窝芯在低速冲击中吸能占比更多，强度大、高度小的则是上层铝板吸收的能量更多，前者的质量、体积比吸能更高；铝蜂窝夹芯板质量比吸能和体积比吸能与壁厚边长比、板厚芯高比均呈幂次关系。     

Identification and prediction of cyclic fatigue behaviour in sandwich panels

The work describes the development of analytical model to identify and predict the cyclic fatigue behaviour of composite sandwich panels subjected to cyclic fatigue loading under 3-point bending conditions. Sandwich samples made from CFRP skin and Nomex core have been loaded with a mean displacement corresponding to 60% of the failure deformation, and subsequently subjected to cyclic loading under displacement control with different loading levels. The fatigue tests show that the stiffness degradation over the number of cycles is characterised by three different phases according to the loading level used. The evolution of the energy dissipated per unit volume versus the number of cycles has also been considered. The cyclic history of the stiffness degradation is developed here following an alternative approach to the one currently adopted. The approach involves the use of interpolation through empirical functions of the experimental data, with the coefficients of the interpolation based on the material properties of the sandwich structure and the type of loading. The alternative modelling approach presented in this work allows the prediction of the fatigue behaviour in sandwich structures without using a large number of test data currently needed in fatigue testing of sandwich panels.     

Impact damage and hygrothermal effects on fatigue bending strength of orthotropy composite laminates

This paper focuses on fracture mechanisms experimentally based on the scanning acoustic microscope (SAM) when subjected to impact damages, i. e., foreign object damages (FOD), and also on the influence of impact damages and hygrothermals on residual fatigue bending strength of CFRP laminates. Composite laminates used in the experiments are CF/Epoxy orthotropy laminated plates, [0₄/90₄]_s. A steel ball launched by an air gun impacts on CFRP laminates to generate impact damages. Bending fatigue tests are periodically interrupted for a nondestructive evaluation (NDE) measurement of the progressive damages to build the fracture mechanism based on impact damages, and three-point fatigue bending tests are carried out to investigate the influence of hygrothermals on the effect of the residual bending fatigue strength of CFRP laminates.     

Effect of Cycling Low Velocity Impact on Mechanical and Wear Properties of CFRP Laminate Composites

The mechanical and wear properties of CFRP laminate were investigated using a method of cycling low velocity impact, to study the trend and mechanism of impact resistance of the CFRP laminate under repeated impact during its service process. The interface responses of CFRP laminate under di erent impact kinetic energy during the cycling impact process were studied were studied experimentally, such as impact contact duration, deformation and energy absorption. The worn surface morphologies were observed through optical microscopy and a 3?D surface profiler and the cross?sectional morphologies were observed through SEM to investigate the mechanism of impact material dam?age. Based on a single?degree?of?freedom damping vibration model, the normal contact sti ness and contact damp?ing of the material in di erent wear stages were calculated. It shows the failure process of CFRP laminate damaged by accumulated absorption energy under the cycling impact of di erent initial kinetic energy. The results indicate that the sti ness and damping coe cients will change at di erent impact velocities or cycle numbers. The damage mechanism of CFRP laminates under cycling low kinetic energy is delamination. After repeated experiments, it was found that there was a threshold value for the accumulated absorption energy before the failure of the CFRP laminate.     

Impact damages and residual bending strength of CFRP composite laminates subjected to impact loading

The purpose of this study is to confirm the decreasing in residual bending strength, and the failure mechanisms experimentally when CFRP composite laminates are subjected to foreign object damage (FOD). Composite laminates used in this test are CFRP orthotropic laminated plates, which are stacked with two-interfaces [0^o ₆/90^o ₆] _sym and four-interfaces [0^o ₃/90^o ₆/0^o ₃] _sym . When the specimen was subjected to transverse impact by a steel ball, the delamination area generated by the impact damage was observed by using the SAM (scanning acoustic microscope). Also, the fracture surfaces obtained by three-point bending test were observed by using the SEM (scanning electron microscope). Further, failure mechanisms were investigated based on the observed delamination areas and fracture surfaces.     

Dynamic response of aluminum honeycomb sandwich panels subjected to hypervelocity impact by porous volcanic rock projectile

The dynamic response and damage behavior of aluminum honeycomb sandwich panels (HC/SPs) subjected to hypervelocity impact by volcanic rock projectiles were investigated by hypervelocity impact tests and hydrocode simulations. The experiments were conducted using a two stage light gas gun and the results showed that the failure modes in HC/SPs subjected to hypervelocity impact by volcanic rock projectiles mainly took forms of front-face denting and circular perforation, honeycomb core collapsing and rapture, rear-face petal-ling and perforation etc. A 3D discrete configuration of the porous volcanic rock projectiles was set up. The hypervelocity impact behavior of the HC/SPs was investigated through hydrocode modeling, within a Lagrange-SPH coupling method in LS-DYNA solver. It was found that the dynamic response and failure modes in the HC/SPs were significantly influenced by the impact location and the impact velocity of the volcanic rock projectile.

     

穿孔对平纹编织面板蜂窝夹芯结构侧向压缩性能的影响

建立平纹编织面板蜂窝夹芯结构的渐进损伤分析模型来分别研究无损伤面板和单侧面板含穿孔损伤的蜂窝夹芯结构的侧向压缩性能,并将该结果与试验结果进行对照,以验证所建立模型的正确性。考虑到模型具有高度的材料非线性,选用ABAQUS/Explicit求解器进行蜂窝夹芯结构准静态侧向压缩性能的模拟,通过编写VUMAT子程序,分别设置面板和芯子的失效准则及刚度退化模型,选用内聚力模型模拟胶层,完成侧向压缩下蜂窝夹芯结构的渐进损伤分析。研究结果表明：无损伤面板的蜂窝夹芯结构侧向压缩强度受面板的屈曲行为控制,含穿孔损伤的蜂窝夹芯结构侧向压缩强度受含穿孔侧面板基体的抗压缩能力控制,且穿孔损伤会严重降低蜂窝夹芯结构的侧向压缩强度。     

高强钢/碳纤维增强复合材料热冲压-连接一体化工艺可行性及试样弯曲性能研究

提出一种高强钢/碳纤维增强复合材料(CFRP)的多材料零件连接成形一体化加工工艺,利用热冲压工艺对高强钢和CFRP预浸料进行连接并成形高强钢/CFRP多材料零件。研究热冲压过程中不同的淬火温度下多材料复合板的制备工艺,并对试样中钢板进行微观观测,最后对复合件进行弯曲试验。微观检测结果表明,该工艺下钢板组织为全马氏体组织。三点弯曲试验结果表明,未铺设CFRP试样、铺设2层CFRP试样和铺设4层CFRP试样的弯曲角度分别为130°、110°和104°。对弯曲试验的力位移曲线进行积分,计算试样的能量吸收量,得到如下结论：未铺设CFRP试样、铺设2层CFRP试样和铺设4层CFRP试样的能量吸收量分别为9410 J、9692 J和10 050 J;铺设2层CFRP的试样和铺设4层CFRP的试样相比于未铺设CFRP的试样,能量吸收量分别增加2.9%和6.8%。     

Low velocity impact denting of HSSA lightweight sandwich panel

Slow speed impact by a small mass can cause residual denting without perforation of a fibrous core sandwich panel that has thin facesheets. Denting depends on the kinetic energy, compliance and nose shape of the colliding body as well as the compliance and mass density of the sandwich panel. Collision experiments were carried out with fibrous core sandwich panels of different sizes struck by colliding spheres at small velocities. Analytical models based on either quasi-static or dynamic deformation of plates were developed to calculate the impact force during low speed impact on circular sandwich panels. Finite element analysis using ABAQUS was performed to calculate impact damage on sandwich panels. Results of the analytical and numerical models and the experimental measurements were compared. The dependence of damage on both structural parameters and impact variables was investigated.     

应力波载荷作用下CFRP、GFRP层板动态拉伸性能的实验研究

介绍了Hopkinson杆冲击拉伸实验设备以及实验技术,推导了应力、应变的计算公式。利用Hopkinson杆加载装置对CFRP、GFRP层合板进行了冲击拉伸实验研究,得到不同应变率下CFRP、GFRP层板的应力、应变(бε)曲线,以及断裂强度、拉伸模量、断裂应变等力学参数,以期对复合材料层板在冲击拉伸情况下动态力学行为和变形、破坏机理有一个初步的认识。     

Comparison of drilled hole quality evaluation in CFRP/CFRP stacks using optical and ultrasonic non-destructive inspection

In aeronautical industry, stringent requirements relate to the quality of drilled holes in carbon fiber reinforced plastic (CFRP) composite laminates as low hole quality determines poor assembly tolerance, structural properties reduction, and risk for long-term part performance. Non-destructive quality control techniques were applied to drilled CFRP laminate stacks for aeronautical applications to characterize the material damage induced by drilling in order to assess the hole quality for product acceptability. Experimental metrology procedures, including optical measurements and ultrasonic non-destructive evaluation, were employed to appraise both external and internal induced material damage in holes machined under diverse drilling conditions. The optical inspection procedure, comparable to the visual inspection method regularly utilized in industry, provided delaminated area evaluations that are underestimated in the case of severe drilling conditions by up to 7% for hole exit and up to 5% for hole entry. In the case of less severe drilling conditions, the underestimation was limited to <2.5% for both hole exit and hole entry, which can be considered a practically negligible disparity.     

The response of honeycomb sandwich panels under low-velocity impact loading

This paper describes the results of an experimental investigation and a numerical simulation on the impact damage on a range of sandwich panels. The test panels are representative of the composite sandwich structure of the engine nacelle Fan Cowl Doors of a large commercial aircraft. The low-velocity impact response of the composites sandwich panels is studied at five energy levels, ranging from 5 to 20 J, with the intention of investigating damage initiation, damage propagation, and failure mechanisms. These impact energy levels are typically causing barely visible impact damage (BVID) in the impacted composite facesheet.A numerical simulation was performed using LS-DYNA3D transient dynamic finite element analysis code for calculating contact forces during impact along with a failure analysis for predicting the threshold of impact damage and initiation of delaminations. Good agreement was obtained between numerical and experimental results. In particular, the numerical simulation was able to predict the extent of impact damage and impact energy absorbed by the structure. The results of this study is proving that a correct numerical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design a more efficient impact-resistant aircraft structure.     

负泊松比可变弧角曲边内凹蜂窝结构的力学性能

在环形蜂窝结构的基础上,提出了一种具有负泊松比效应的可变弧角曲边内凹蜂窝结构。分析了刚架结构的弯曲内力,利用能量法给出了曲边内凹蜂窝结构的横/纵向等效弹性模量和等效泊松比的解析公式。当弧角为180°时,所提出的曲边内凹蜂窝结构可退化为已有的环形蜂窝结构。所得解析解、已有理论解和有限元结果三者有较好的吻合度,表明了所提出方法的有效性。基于所得解析解讨论了微结构几何参数对等效弹性模量和等效泊松比的影响,研究结果表明：等效弹性模量受弧角的影响显著;等效泊松比受弧角、竖直壁板长度和两竖直壁板间宽度的影响较为明显,而受连接部分长度的影响不明显。     

激光干涉在蜂窝夹心层板振动模态测量中的应用

采用激光电子散斑干涉测量技术(electronic speckle pattern interferometry,简称ESPI)对悬臂矩形损伤蜂窝夹心复合板稳态振动下的振型进行了实验研究。通过完整试样的振型测量结果与有限元模拟均匀悬臂金属板结构振型的类比,验证了有关蜂窝夹心复合材料具有横观各向同性假设的观点,获得了悬臂矩形蜂窝夹心复合层板前14阶离面振动模态的振型图。实验结果表明,结构的损伤导致其局部刚度改变,使得共振频率减少,损伤对其高阶振型的形状改变较为明显,有时甚至是模态跃迁。     

The suggestion of the standardized finite element model through the experimental verifications of various railway vehicle structures made of sandwich composites

In this study, a standardized finite element model for the carbody structures of various railway vehicles made of sandwich composites was suggested. The standardized finite element model for composite carbody structures was introduced and proposed by comparing the results of real structural tests under vertical, compressive, twisting load and natural frequency tests of various railway vehicles. The results showed that the quadratic shell element was suitable to model the reinforced metal frame used to improve the flexural stiffness of sandwich panels compared to beam elements, and layered shells and solid elements were recommended to model the skin and honeycomb core of sandwich panels compared to sandwich shell elements. Also, the proposed standard finite element model has the merit of being applied to crashworthiness analysis just by minor modifications, such as contact conditions and constraint equations.     

复合材料蜂窝夹层板低速冲击损伤模拟及实验研究

复合材料蜂窝夹层板因其良好的力学特性及质量轻等优点在工程中得到了广泛应用,但其抗冲击能力较差。本文研究了复合材料蜂窝夹层板受低速冲击后的变形和损伤情况,采用光滑粒子动力学结合有限元数值模拟方法分析了复合材料蜂窝夹层板受不同能量冲击后的响应,并通过试验和模拟计算结果对比分析,给出了不同冲击能量下复合材料蜂窝夹层板的位移和损伤。研究结果表明:给出的复合材料蜂窝夹层板冲击数值模型能够合理的模拟低速冲击行为,能为工程中复合材料蜂窝夹层板结构受冲击损伤的测定提供参考依据。     

蜂窝夹层结构复合材料的力学性能研究

设计了不同铺层形式的蜂窝夹层结构复合材料实验板 ,研究了胶膜、铺层方式以及界面缺陷对蜂窝夹层结构的力学性能影响 ,对比了蜂窝夹层结构中的面板与叠层板的性能。     

高温合金蜂窝芯高速铣削材料去除机理与损伤行为

高温合金蜂窝芯材料具有高比刚度、轻质和能量吸收特性好等优异性能,被视为下一代高超声速飞行器热防护结构极具潜力的材料。高速铣削是高温合金蜂窝芯零件成型过程中重要的减材制造工艺,在蜂窝芯材料高速铣削时,蜂窝芯材料面内刚度低且高温合金塑性好,较小的切削力就会使蜂窝壁产生较大的塑性变形,导致蜂窝芯加工精度较低、加工损伤难以控制,对后续焊接、装配等工序产生不利影响。基于有限元仿真对蜂窝壁切削材料去除机理进行了深入研究,探索了铣削参数、刀具类型和铣削方式对铣削过程中铣削力和加工损伤的影响。研究结果表明,蜂窝壁切入角是影响蜂窝芯材料切削加工过程中瞬时应力分布和成屑机理的关键性因素。得到了铣削参数、刀具类型和铣削方式对高温合金蜂窝芯加工过程中加工损伤的影响规律。对于铣削参数,过大的进给量会导致芯格变形等加工损伤,降低切削速度会提高微小毛刺等加工损伤发生的频率;本文采用的三种刀具的对比结果表明,立式铣刀加工质量最好。插铣方式会产生明显的轴向冲击,而侧铣方式可以有效避免轴向冲击。研究成果为高温合金蜂窝芯低损伤高性能加工提供了理论依据和工艺技术储备。