加载速率对复合材料粘接修理金属结构力学性能的影响

基于复合材料补片基体和胶层树脂黏弹性的特点,分别采用光固化和传统热固化方式制备了一系列不同树脂配方的试样,实验研究了树脂配方对修理结构应变率效应的影响,测定了复合材料粘接修理结构在不同加载速率下的力学性能.结果表明,树脂体系的交联度较低时,应变率效应更明显;加载速率的提高通常会使补片更好地发挥分担载荷的作用,但是也会导致变形协调能力的下降;这两个因素共同影响复合材料粘接结构的最终修理效率.加载速率对于复合材料粘接修理结构的破坏模式也有影响.     

加载速率对FRP粘接修理铝板结构力学性能的影响

通过FRP修复金属结构在不同加载速率下的拉伸试验,研究了树脂组成和纤维布种类对加载速率效应的影响,分析了修复结构的破坏模式、载荷-位移关系、抗拉强度、弹性模量和断裂延伸率随加载速率的变化关系。同时,从树脂和复合材料的角度对不同加载速率下修复结构力学性能的变化进行了初步探讨。结果表明:低加载速率下,修复结构的抗拉强度和断裂延伸率随加载速率增加而增大,修复结构更多地表现出韧性材料的特点;当加载速率较大时,修复结构的抗拉强度和断裂延伸率变化不大且有所下降,加载速率效用也逐渐减弱。同时,修复结构的非线性区域逐渐向高载荷、低应变区域移动,表现出较为刚性的特点,弹性模量则始终变化不大。     

一种飞机结构快速修理新材料的研究

光固化复合材料修理补片是一种用于飞机结构快速修理的新材料。运用光敏树脂浸渍纤维增强材料制成了预浸料修理补片,采用正交设计方法确定了制备补片的最佳工艺条件,进而对补片的相关特性进行了分析与实验;最终通过模拟某歼击机机翼下蒙皮应力水平最高处的试验;得出结论:使用该材料对飞机结构进行修理所需设备少、结构增重小、修补强度高、通用性强,可适用于多种材质、多种损伤模式下复杂结构的快速修理。     

加载速率对合成纤维力学性能影响的力热效应

用于深海大型结构系泊的合成纤维系缆在不同加载速率下的力学行为对结构系泊效能有着重要影响。本文认为系缆在不同加载速率下拉伸时自身温度变化是系缆呈现不同性能的主要原因。论文首先对合成纤维材料应力-应变关系进行了双线性化处理并建立了双线性参数与温度关系模型,其次,根据塑性变形功的热能转化和传热学理论推导出纤维材料在不同加载速率下温度场的分布规律,从而以温度为中间参数建立及预测合成纤维材料在不同加载速率下拉伸时力学性能变化规律的分析模型。最后以尼龙66材料为例对上述现象开展实验研究,先在准静态条件下测量20℃、40℃、60℃三种温度的拉伸应力-应变曲线,然后根据上述理论预测出在室温条件下以50mm/min加载速率拉伸时尼龙材料的应力-应变曲线,并与实验值进行对比分析,二者吻合较好。以上研究为大尺度系缆力学性能预测提供了可行方法。     

服役温度对铝合金-碳纤维增强树脂复合材料粘接接头准静态失效的影响

为了研究服役温度对铝合金-碳纤维增强树脂(CFRP)复合材料粘接接头准静态失效的影响，本文加工了铝合金-CFRP复合材料粘接接头。考虑车辆实际运行工况下的服役温度，选取低温(?40℃)、常温(20℃)和高温(80℃)三种环境温度，结合设计的Arcan夹具对铝合金-CFRP复合材料粘接接头分别进行1 mm/min和100 mm/min的准静态试验，得到不同温度下铝合金-CFRP复合材料对接接头(BJs)、45°嵌接接头(45°SJs)和剪切接头(TSJs)的准静态失效强度，并结合失效断面对接头失效形式进行分析，建立了失效准则方程和三维响应曲面。结果表明:不同加载速率下的铝合金-CFRP复合材料粘接接头失效强度在高温环境下均呈明显的下降趋势，在低温环境下均呈一定程度的上升趋势。高温下的失效模式为胶层的内聚失效，低温下的失效模式中纤维撕裂的比例上升。相对于1 mm/min加载速率下的准静态失效强度，各温度和应力状态下的铝合金-CFRP复合材料粘接接头在100 mm/min加载速率下的准静态失效强度明显提高。      

光固化复合材料快速修复金属损伤结构

讨论了复合材料粘接修复损伤金属结构相对传统机械修理方法的优点和不足。光固化不仅可以快速实现复合材料固化,而且可以不需要高温,可以避免残余热应力的产生,比通常的复合材料粘接修理更具优势。采用光固化的方法实现了损伤金属结构的修复,分析了铺设层数和表面处理对修复效果的影响。     

拉伸速率对盘条力学性能影响的探讨

探讨了拉伸速率在标准ＧＢ２２８－８７规定范围内以及超出范围的情况下，对日检盘条的力学性能值的影响，找出试样力学性能值最具有代替性的拉伸速度范围，从而提高检验质量。     

不同加载速率下高性能水泥基复合材料断裂性能研究

为研究加载速率对高性能水泥基复合材料(High-performance cement-based composites,HPCC)断裂性能的影响,本研究对带预制裂缝的HPCC矩形梁进行了三点弯曲测试.以裂缝嘴张开位移(Crack mouth opening displacement,CMOD)为加载控制参数,加载速率分别为0.001 mm/s、0.01 mm/s和0.1 mm/s,试件内钢纤维体积掺量分别为0％和2％(均为质量分数).基于荷载-裂缝嘴张开位移(P-CMOD)曲线分析了第一裂纹应力、弹性模量、弯曲强度、失稳韧度以及断裂能等一系列断裂特征参数随加载速率的变化规律.试验结果表明:(i)第一裂纹应力和弹性模量几乎不受钢纤维含量和加载速率的影响;(ii)HPCC弯曲强度与应变率比呈对数关系,且含钢纤维HPCC的弯曲强度率效应更明显;(iii)含钢纤维HPCC的失稳韧度和断裂能有很大程度的提升,但其对加载速率的敏感性较低.掺钢纤维能够有效提高HPCC材料抵抗冲击荷载的能力.     

拉伸速率对PP/GB复合材料力学性能的影响

应用Instron材料试验机考察了室温下拉伸速率对玻璃微珠填充聚丙烯复合材料的弹性模量和屈服强度的影响，结果表明，随着微珠含量的增加，试样的弹性模量（Ec）增大，且与拉伸应变速率呈近乎幂律函数关系，而屈服强度（бyc）则随着微珠体积分数（Φf）的增加呈线性函数形式下降，并随着拉伸应变速率的增加呈幂律函数形式提高，此外，讨论了Ec、Φf与бyc三者之间的关系。     

The stress intensity factor for a cracked stiffened sheet repaired with an adhesively bonded composite patch

The problem of a cracked, stiffened metallic sheet adhesively bonded by a composite patch is analyzed. The composite patch is assumed to be either an infinite orthotropic sheet or an infinite orthotropic strip normal to the crack. Due to the high stress concentration around the crack and on the interface, an elliptical disbond is assumed to exist around the crack. The crack is asymmetric with respect to the stiffener's locations as well as to the patch's center. The effect of thermal stresses in curing process is also considered. The fracture problem is solved by the displacement compatibility method, using the complex variable approach and the Fourier integral transform method.The problem is dealt with in two steps. First, starting with an uncracked, patched stiffened sheet, the stress at the prospective location of the crack is determined in a closed-form solution. The second step is to introduce a crack into the stiffened patched sheet. The multivalue of the analytical formulation is treated in detail to ensure proper implement in the computer. The results show that the effect of the stiffeners on the stress intensity factor is not significant for a crack fully covered by a patch.For the repairs by Boron/Epoxy patches, the difference in KI between the infinite sheet patch and the infinite strip model is only minor (less than 5 percent) in the absence of the curing thermal stresses and it becomes more pronounced when these stresses are taken into consideration. The stress intensity factor for a crack repaired by an infinite composite strip also can be estimated with a good or reasonable accuracy via a simplified analysis in which the patch is considered as an infinite strip in the first step and is treated as an infinite sheet in the second step of the solution procedure mentioned above.The latter simplified analysis is based on the approach originally proposed by Rose for a relatively simple repair configuration. For most cases, that approach seems to work well for the repair of a stiffened sheet by an infinite composite strip with the effects of thermal stresses and a disbond included. It should be emphasized that the present methodology can apply to the problem of a crack in a metallic stiffened sheet growing beyond the patch's boundary and also to the repairs by an infinite adhesively bonded composite strip parallel to the crack.     

Fatigue crack growth analysis of stiffened cracked panel repaired with bonded composite patch

Fatigue crack growth behavior in a stiffened thin 2024-T3 aluminum panel repaired with one-sided adhesively bonded composite patch was investigated through experiments and analyses. The patch had three plies of unidirectional boron/epoxy composite. 2024-T3 aluminum stiffeners were riveted as well as bonded on the panel. Stiffeners were oriented in the loading direction and were spaced at either 102 mm or 152 mm with a crack centered between them. Also, un-repaired cracked panel with and without stiffeners were studied. Experiment involved tension-tension fatigue at constant amplitude with maximum stress of 120 MPa and stress ratio of 0.05. Bonded composite patch repair increased fatigue life about five-fold in the case of stiffened panels while it increased about ten fold in the case of un-stiffened panels. Fatigue life also increased with decrease of the distance between the stiffeners for both repaired and un-repaired panels. A three-dimensional finite element method was used to analyze the experiments. Residual thermal stresses, developed during patch bonding, requires the knowledge of temperature at which adhesive becomes effective in creating a bond between the structure and patch in the analysis. A simple method to estimate the effective curing temperature range is suggested in this study. The computed stress intensity factor versus measured crack growth relationships for all panel configurations were consistent and in agreement with the counterpart from the test material. Thus, the present approach provides a means to analyze the fatigue crack growth behavior of stiffened structures repaired with adhesively bonded composite patch.     

Crack opening displacement in plate with bonded repair patch

Mathematical techniques are extended to compute crack opening displacements in a cracked plate with an adhesively bonded composite patch. The plate and the patch are considered as orthotropic materials. The problem is reduced to the solution of integral equations. A software program is written to compute shear stresses in adhesive, stress intensity factors in the plate and the crack openings at the centreline of the crack. The effects of adhesive thickness, adhesive modulus, patch thickness and plate thickness on crack openings are investigated. A test program is carried out to obtain crack opening displacements in plate with bonded patch. A good agreement with analytical predictions is obtained. The effects of patches bonded on one or both sides of a plate on stress intensity factors are evaluated.     

Fatigue crack growth behavior of cracked aluminum plate repaired with composite patch

In this study, we investigated the fatigue crack growth behavior of cracked aluminum plate repaired with bonded composite patch especially in thick plate. Adhesively bonded composite patch repair technique has been successfully applied to military aircraft repair and expanded its application to commercial aircraft industry recently. Also this technique has been expanded its application to the repair of load bearing primary structure from secondary structure repair. Therefore, a through understanding of crack growth behavior of thick panel repaired with bonded composite patch is needed. We investigated the fatigue crack growth behavior of thick panel repaired with bonded composite patch using the stress intensity factor range (ΔK) and fatigue crack growth rate (da/dN). The stress intensity factor of patched crack was determined from experimental result by comparing the crack growth behavior of specimens with and without repair. Also, by considering the three-dimensional (3D) stress state of patch crack, 3D finite element analyses were performed to obtain the stress intensity factor of crack repaired by bonded composite patch. Two types of crack front modeling, i.e. uniform crack front model and skew crack front model, were used. The stress intensity factor calculated using FEM was compared with the experimentally determined values.     

Effect of load amplitude change on the fatigue life of cracked Al plate repaired with composite patch

In this study, the fatigue behavior of aluminum alloy 2024T3 v-notched specimens repaired with composite patch under block loading was analyzed experimentally. Two loading blocks were applied: increasing and decreasing at two stress ratio: R = 0 and R = 0.1. Failed samples were examined under scanning electron microscope at different magnifications to analyze their fractured surfaces. The obtained results show that under increasing blocks, the crack growth is accelerated for both repaired and unrepaired specimens. This is attributed to the increase of the loading amplitude in the second block. A retardation effect was observed for decreasing blocks loading in unrepaired specimens. However, this retardation effect is attenuated by the presence of the patch which lead to lower fatigue life for repaired specimens.     

The effects of size and stacking sequence of composite laminated patch on bonded repair for cracked hole

The performance of a bonded repair for cracked holes has been studied using the three dimensional finite element method, linear elastic fracture mechanics and strain energy density theory. Increasing the composite patch size reduces the strain energy level at the crack tip; increasing the patch length normal to the crack is a better choice. The stacking sequences of the laminated patch have little influence on the strain energy distribution in the vicinity of the crack. To repair the cracked holes of aircraft components subjected to variable direction loading during flight, the orientations of the patch ply, 90° and ±45° with respect to the crack direction, are the optimum selection in bonded repairs.     

Analysis of the plastic zone size ahead of repaired cracks with bonded composite patch of metallic aircraft structures

In this study, the three-dimensional and non-linear finite element method is used to estimate the performance of the bonded composite repair of metallic aircraft structures by analyzing the plastic zone size ahead of repaired cracks. Several calculations have been realized to extract the plasticized elements around the crack tip of repaired crack. The obtained results show that the presence of the composite patch reduces considerably the size of the plastic zone ahead of the crack. The effects of the adhesive properties and the patch thickness on the plastic zone size ahead of repaired cracks were analyzed.     

Elastic analysis of hybrid bonded joints and bonded composite repairs

The authors extend the closed-form bonded joint linear elastic analysis method of Delale et al. [Delale F, Erdogan F, Aydinoglu MN. Stresses in adhesively bonded joints: a closed-form solution. J Compos Mater 1981;15:249–71] and Bigwood and Crocrombie [Bigwood DA, Crocombe AD. Elastic analysis and engineering design formulae for bonded joints. Int J Adhes Adhes 1989;9(4):229–42] to include the composite deformation mechanisms and the thermal residual strains that arise in hybrid metal-composite joints such as those presented by bonded composite repairs applied to metallic aircraft structures. The analytical predictions for the adhesive stresses and the compliance are compared to the results of a linear elastic finite element model that has itself been validated by comparison with experimental results. The results are applied to the problem of coupled linear extension and bending of a bonded composite repair applied to a cracked aluminum substrate. The resulting stress intensity factor and crack-opening displacement in the repaired plate are compared to the results of a three-dimensional finite element analysis, and also exhibit excellent results. Throughout the text, observations are made regarding the practical application of the results to failure prediction in hybrid joints, whereby the authors demonstrate the need for consistency in the analytical methods used to determine the fatigue and failure of composites from the coupon level to the analysis of the final structural details.     

Fracture toughness analysis of inclined crack in cylindrical shell repaired with bonded composite patch

Adhesively bonded composite patch repair has been widely used to restore or extend the service life of cracked structural components due to its efficiency and cost-effectiveness compared to mechanical repair technique. Current available knowledge on patch repair mainly focus on flat damaged structures and the corresponding analysis methods and empirical databases are computationally efficient. In contrast, only limited work has contributed to studying patch repair to curved damaged structures. Authors have developed an adhesive element in conjunction with a shell element to investigate the effect of curvature on the adhesive stresses and mode I fracture toughness of the cracked host shell in the curved repairs. In this paper, this technology is again employed to model an adhesively bonded composite patch repair to a cylindrical shell embedded with an inclined through-thickness crack. The total strain energy release rate (SERR), calculated by the modified virtual crack closure technique (VCCT), is used to evaluate the mix-mode fracture toughness of the damaged structure and further to estimate the efficiency of patch repair. An automatic mesh generation scheme is proposed to conduct a quick parametric analysis, which can also be used to structural optimization design of composite patch repair. The numerical results are presented to show the effect of curvature and inclined angle of the through-thickness crack on fracture toughness of the repaired structure subject to different loads.