复合材料衬套螺栓干涉连接安装过程损伤机制

复合材料干涉连接结构由于能够显著降低孔周应力集中,提高结构承载能力和疲劳寿命,而成为一种先进的连接形式。但是由于复合材料层间强度低的特点,不合理的干涉螺栓连接结构形式、尺寸以及安装方式很容易引起孔壁分层,降低结构承载能力。针对该问题,本论文提出了一种基于衬套螺栓的干涉连接结构,并对该结构开展了安装过程中承载机制的试验和有限元研究。试验中测量了插钉力变化和孔壁损伤情况,有限元模型中充分考虑了层内损伤和层间分层因素,建立了基于复合材料渐进损伤和内聚力单元的损伤预测模型。通过干涉量为2.2%的衬套螺栓安装过程中的插钉力和损伤的有限元模型与试验结果对比分析发现,二者拟合度良好,证明模型的准确性。通过不同干涉量下的分层情况对比分析以及临界干涉量求解,解释了衬套螺栓提高孔壁质量的原因并提出了可靠干涉量范围。     

应用FEM与圆环镦粗实验测定TC4钛合金高温变形时的摩擦因数

采用圆环镦粗与有限元模拟相结合的方法测定了TC4钛合金高温变形时摩擦因数.利用有限元模拟确定了摩擦因数的理论标定曲线,采用圆环镦粗实验确定圆环内径变化百分比与圆环高度压缩百分比的关系曲线,将圆环镦粗后的内径减小百分比与模拟得到的摩擦因数理论标定曲线进行对比,发现TC4钛合金在干摩擦和玻璃润滑剂的条件下摩擦因数分别为0....     

氟化石墨改性适配器PTFE层摩擦磨损性能研究

目的研究适配器PTFE层氟化石墨填充改性后的摩擦磨损性能,提高适配器的耐磨性。方法以质量分数为2%,5%,8%,11%的氟化石墨为填料制备PTFE基复合材料,分别在20,40,60,80 r/min的转速下测试试样摩擦因数。通过三维视频显微镜采集试样表面磨损数据,并计算其体积磨损率。扫描电子显微镜(SEM)观察磨痕微观形貌。结果同一转速下,试样的摩擦因数随着氟化石墨质量分数的增加而增大。填充氟化石墨能显著降低试样的体积磨损率,填充物质量分数超过8%后,试样体积磨损率趋于稳定,试样摩擦因数得到明显增大。结论氟化石墨填充PTFE层可显著提高适配器的耐磨性,但质量分数不能超过8%,否则,会造成适配器与运输筒间的摩擦因数增大,增加航天器装填及出筒阻力。     

铝/钢薄板无铆连接接头成形过程及形貌控制

目的 研究铝/钢薄板无铆连接过程中接头形貌的影响因素。方法 采用有限元模拟方法分析了铝/钢薄板在平底模具下接头形貌的成形过程。结合变形区域的金属流动情况,解释了互锁结构的形成机理,并分析了不同工艺参数对接头形貌的影响。结果 接头互锁结构主要是依靠铝板填充钢板的凹陷部位而形成的,抑制钢板与模具接触一侧的金属流动有助于接头底部和侧壁的钢板拉薄,进而形成内部凹陷,促进互锁结构的形成。摩擦因数对接头形貌参数影响较大,增大摩擦因数可显著提高接头的互锁量。结论 冲头半径、冲头圆角、凹模深度以及摩擦因数对颈厚值和互锁量均有显著影响。通过控制这些影响因素,可以得到良好的接头。此外,接头的失效形式以铝板颈厚较薄处的剪断失效为主,因此对于采用铝上钢下的无铆连接,保证颈厚值相对于互锁量更加重要。     

5052/6061异种铝合金薄板搅拌摩擦焊接工艺研究

目的 研究焊接速度、搅拌头旋转速度和下压量对异种铝合金搅拌摩擦焊接头力学性能的影响,找出最佳工艺参数,从而进一步提高接头的力学性能。方法 采用正交实验法对1.5 mm厚5052/6061异种铝合金搅拌摩擦焊接进行实验设计,焊接完成后,观察焊缝宏观形貌,然后将试件制成标准拉伸试样进行拉伸实验,拉伸实验完成后用扫描电镜观察焊接接头的断口形貌,最后运用极差分析法和方差分析法分别对实验结果进行分析。结果 在选取的工艺参数范围内,搅拌头旋转速度影响最大,其次是焊接速度,下压量影响最小。当焊接速度为120 mm/min、转速为1 400 r/min、下压量为1.5 mm时,接头抗拉强度达到了最大值194 MPa,伸长率也达到了最大值9.62%。结论 在一定工艺参数范围内,提高焊接速度或搅拌头的旋转速度能显著提高接头的力学性能,而下压量对接头力学性能影响不显著。     

配合精度对复合材料单钉双剪螺栓连接静强度的影响

为研究不同配合精度(间隙配合及干涉配合)对复合材料单钉双剪螺栓连接静强度的影响,基于ABAQUS软件建立了三维渐进损伤有限元分析模型,并将分析结果与实验结果进行比较验证模型的有效性.结果表明,间隙的增加会导致接头挤压强度及刚度的降低,但在0％ ～2％ 间隙范围内极限挤压强度变化不大.间隙配合下螺栓-孔接触面积的减小引发应力集中,致使孔边出现损伤区域集中.适当的干涉配合能增强接头刚度及挤压强度;适当的干涉量能降低孔边应力集中,但过大的干涉量会导致孔周出现严重的初始损伤.     

CNTs含量对CNTs/Al微观组织及力学性能的影响

为增加碳纳米管(CNTs)在铝基体中的分散性,利用机械球磨-真空热压烧结工艺制备碳纳米管/铝(CNTs/Al)复合材料,采用扫描电子显微镜(SEM)、电子万能试验机和万能摩擦磨损实验机,研究了CNTs质量分数对CNTs/Al复合材料微观组织、力学性能及摩擦磨损性能的影响.结果表明:CNTs经超声波预先分散后分散性增加;当CNTs质量分数为2.0%时,复合材料中CNTs与Al粉之间表现出较好的相容性;随着CNTs含量进一步增加,CNTs团聚现象严重;热压烧结温度600℃时,随着CNTs添加量的增加,铝基复合材料的屈服强度和抗拉强度呈现出明显的先增大后降低的趋势,同时,CNTs/Al复合材料的摩擦因数和磨损率随CNTs含量的增大先减小后增加;CNTs质量分数为2.0%时,复合材料的屈服强度最大值为116 MPa,抗拉强度最大值为245 MPa,与纯Al基体相比,分别提高了78%和1.9倍.2.0%CNTs/Al复合材料可获得较好的摩擦磨损性能,其摩擦系数和磨损率呈现平缓趋势,复合材料的磨痕最浅.     

硬脂酸处理钛酸钾晶须对坡缕石纤维树脂摩擦材料摩擦性能的研究

研究了硬脂酸对钛酸钾晶须的改性效果,用处理了的钛酸钾晶须增强坡缕石复合制备摩擦材料,研究复合两种矿物纤维的增强机制.发现15%(质量分数)的钛酸钾晶须添加量在复合材料中分散不易均匀,摩擦因数波动较大,不利于复合材料的摩擦性能;13%以下的钛酸钾添加量对复合材料的摩擦因数的影响不大;10%的钛酸钾晶须添加量制备的复合材料磨损率最低,比未添加的坡缕石复合材料,耐磨性提高了27%,对改善坡缕石的耐磨性有显著的作用,特别是250℃以上的高温阶段的耐磨性得到显著改善.     

TC4钛合金表面激光熔覆NiCrCoAlY-Cr3C2复合涂层的摩擦和高温抗氧化性能

为了提高TC4 钛合金表面摩擦磨损和高温抗氧化性能,以 NiCrCoAlY+20%(质量分数)Cr3 C2 混合粉末作为熔覆粉末,采用激光熔覆技术在TC4 钛合金表面制备NiCrCoAlY-Cr3 C2 复合涂层,利用OM,SEM,XRD,EDS等分析涂层的显微组织和物相组成;采用 HXD-1 000TB 显微硬度计测量涂层显微硬度;采用 MMG-500 三体磨损试验机与 WS-G1 50 智能马弗炉对涂层和基体进行摩擦磨损及高温抗氧化实验.结果表明:利用激光熔覆技术在 TC4 钛合金表面可以制备形貌良好、无裂纹和气孔等缺陷的复合涂层.熔覆区显微组织结构致密,多为针状晶和树枝晶;结合区的显微组织主要由平面晶、胞状晶和树枝晶组成,生成了多种可提高耐磨性和高温抗氧化性的碳化物、氧化物和金属间化合物.复合涂层的最高显微硬度为 1344HV,约为钛合金基体 350HV的 3.8 倍;复合涂层的摩擦因数为0.2～0.3,较钛合金基体的摩擦因数0.6～0.7 明显下降;相同条件下复合涂层的磨损失重为0.00060 g,是钛合金基体磨损失重 0.06508 g 的0.9%;恒温 850 ℃氧化 100 h后复合涂层氧化增重为 6.01 mg·cm－2 ,约为钛合金基体氧化增重 25.10 mg·cm－2的24%.激光熔覆技术有效改善了TC4 钛合金表面的摩擦磨损和高温抗氧化性能.     

热镀锌钢板表面润滑处理及其摩擦特性

模具与冲压件间的摩擦磨损性能极大地影响冲压件的质量。为了提高汽车用热镀锌钢板的冲压成形性能,采用含多种无机离子的润滑剂对热镀锌钢板表面进行润滑处理。通过摩擦因数测量和拉深试验方法分别研究了热镀锌钢板的表面摩擦特性与冲压成形性能。结果表明:经无机润滑处理的热镀锌钢板表面摩擦因数明显降低、具有良好的表面润滑性能与稳定的摩擦特性;拉深试验时其最大拉深力下降、最大拉深杯体高度增大,即钢板的冲压成形性能得到了提高。     

碳纤维增强环氧树脂复合材料层合板干涉连接插钉轴向力建模与分析

碳纤维增强环氧树脂复合材料（CFRP）构件干涉配合连接的插钉轴向力过大会引起层合板弯曲和分层,严重影响产品的安全性。针对CFRP层合板的高锁螺栓干涉连接过程,分析了其制孔、插钉及拧紧等装配连接工艺,将其干涉插钉过程划分为4个阶段,并对各个阶段进行了详细的力学行为分析;对螺栓杆处和倒角处的挤压力和摩擦力分别进行力学建模,并结合各作用力的边界条件与阶段划分,构建了干涉插钉全过程的轴向力模型;通过ABAQUS有限元模拟了CFRP层合板干涉插钉工艺过程,并开展了干涉螺栓安装实验,对比分析了层合板孔周径向挤压应力分布和插钉轴向力变化规律,解析结果与模拟和实验结果吻合较好,为后续CFRP层合板的插钉分层损伤和工艺优化研究奠定基础。      

Influence of the dimensions of spheroniser plate protuberances on the production of pellets by extrusion-spheronisation

The influence of the dimensions of square-patterned pyramidal protuberances on a spheronisation plate on pellet yield, size and shape distributions, surface tensile strength and surface morphology was investigated using a 45 wt% microcrystalline cellulose/water paste. Tests were conducted using four extrudate diameters (1.0, 1.5, 2.0 and 2.5?mm) generated by screen extrusion and seven plate geometries, including a flat plate as a control. Geometrical analyses of the protuberance shapes provide some insight into the observed differences. Sharper protuberances reduced yield (promoting breakage and attrition) but tended to give narrower size distributions and less even pellet surfaces. The pellet yield for 1?mm extrudates was also subject to losses caused by fragments passing through the 1.0?mm gap between the plate and the spheroniser wall. Pellet tensile strengths were noticeably greater for 1.0?mm diameter extrudates, which is attributed to the greater extensional strain imparted on the paste during the extrusion step. For some geometries there is an optimal ratio of extrudate to protuberance dimensions.     

孔挤压过程中孔边应力应变演化规律的试验与模拟研究

该文采用挤压棒直接挤压的方法对LY12CZ铝合金进行孔挤压试验,测试了在不同过盈量、不同润滑条件下挤压时挤压力和孔边应变变化曲线。建立三维有限元模型,对挤压过程开展了数值模拟。结合试验和模拟结果,揭示了挤压过程中孔边应力应变的演化和分布规律;挤压力的变化特征以及摩擦系数和过盈量对挤压力的影响;孔的深度方向切向应力在挤压过程中的分布及孔边残余压应力的来源与演变。模拟值与试验测量值吻合良好,验证了有限元模拟的可靠性。     

T300/QY8911 π/4层压板机械连接干涉疲劳

本文对T300/QY8911π/4层压板机械连接干涉配合进行了静拉伸挤压和拉-拉疲劳试验。与滑配合比较,干涉使静挤压强度略有提高,但能使疲劳寿命增加数倍。文中用有限元方法计算了两种配合的孔边应力分布。并对疲劳损伤用X射线作了检测,初步分析了干涉配合的增寿原因。      

Stress distribution modeling for interference-fit area of each individual layer around composite laminates joint

The discussion about nonuniform stress distribution around interference-fit joint is particular significance in the design of composite laminates structures. In order to investigate the stress distribution of interference-fit area around composite laminates joint, an analytical model is developed for stress distribution based on the Lekhnitskii's complex potential theory. The normal and tangential stresses of contact are achieved by the relationship of deformation between pin and hole. The effects of ply orientation and interference percentage on stress components distributions of each individual layer around symmetrical laminates joint are discussed. In order to verify the validity of the analytical model, extensive 3D finite element models are established to simulate the stress components of laminates interference-fit joint. The results show that the analytical model is valid, and the laminate property and ply orientation have a significant effect on stress distribution trend while interference percentage mainly affects stress magnitude.     

The effect of interference-fit fasteners on the fatigue life of central hole specimens

Fatigue tests were carried out on 2024‐T351, thickness 1.6 mm, central hole specimens containing pins installed with five different interference‐fit levels. Tests clearly demonstrated the beneficial effect of interference fit on fatigue resistance, up to the maximum value examined, 2.5%. A three‐dimensional (3D) finite‐element model was used in order to characterize the stress field around the hole. A large specimen, with a 40‐mm‐diameter hole filled with interference‐fit pin, was instrumented by strain gauges and statically tested in order to check FEM results. A very good correlation existed between measured and numerically evaluated strains. FEM results demonstrated the well‐known effect of interference‐fit fasteners on reducing stress ranges. By increasing the interference level, the stress range was practically unchanged, while the mean stress decreased. Interference‐fit produces a biaxial stress state, which must be taken into account for fatigue evaluation. In the present case, a simple criterion, based on hoop strain, predicted the fatigue results quite well with the exception of open hole fatigue test results, which were overestimated.     

Experimental and numerical investigations about the combined effect of interference fit and bolt clamping on the fatigue behavior of Al 2024-T3 double shear lap joints

In this paper the fatigue behavior of double shear lap joints treated by different combinations of interference fit and bolt clamping have been investigated both experimentally and numerically. To do so, specimens made from aerospace structural material of aluminum alloy 2024-T3 plates were interference fitted at the sizes of 1.5% and 4.7% and torque tightened with 2 and 4 N m to be prepared for fatigue tests. Consequently, the joints were subjected to cyclic load at different levels to obtain fatigue life. Finite element (FE) analysis was also performed to find the stress and strain distributions and the results were used to help explain the trends observed in the experimentally obtained S–N data. The experimental tests showed that during the interference fit process a protruded region is created at around the hole in the exit plane due to directional material plastic flow as a consequence of the oversized bolt force fitting. This protruded region has a bigger height for the bigger interference fit size. The finite element results showed that the protruded region generally localizes the compressive effect of bolt clamping and reduces its capability in fatigue life enhancement, by relaxing the clamping force. The fatigue test results showed that a better fatigue life improvement was achieved by employing the combination of a smaller interference fit size and bigger clamping force.     

Using finite element modeling to examine the flow processes in quasi-constrained high-pressure torsion

Finite element modeling was used to examine the flow processes in high-pressure torsion (HPT) when using quasi-constrained conditions where disks are contained within depressions on the inner surfaces of the upper and lower anvils. Separate simulations were performed using applied pressures from 0.5 to 2.0 GPa, rotations up to 1.5 turns and friction coefficients from 0 to 1.0 outside of the depressions. The simulations demonstrate the distribution of effective strain within the depressions is comparable to the prediction by ideal torsion, and the applied pressure and the friction coefficient outside the depressions play only a minor role in the distribution of effective strain. The mean stresses during processing vary linearly with the distance from the center of the disk such that there are higher compressive stresses in the disk centers and lower stresses at the edges. The torque required for rotation of the anvil is strongly dependent upon the friction coefficient between the sample and the anvil outside the depressions.     

The effect of interference-fit on fretting fatigue crack initiation and ΔK of a single pinned plate in 7075 Al-alloy

The effect of interference-fit on fretting fatigue crack initiation and ΔK was studied numerically for available experimental results in a single pinned plate in Al-alloy 7075-T6. The role of interference ratio was investigated alongside friction coefficient through finite element. Cyclic stress distributions in the plate ligament and fretting stresses on the contact interface were evaluated using 3-D elastic–plastic finite element models. Additionally a 3-D elastic finite element model was utilized to discuss ΔK of cracks emanating from interference fitted holes. Results demonstrate that fretting was the main reason for crack nucleation, and furthermore, the location was precisely predicted and fatigue life enhancement was explained.     

Fatigue crack growth and life prediction of a single interference fitted holed plate

To understand the different aspects of fatigue behaviour of complex structural joints it will be much helpful if the effects of different parameters are studied separately. In this article, to study the isolated effect of interference fit on fatigue life a pined hole specimen is investigated. This specimen is a single‐holed plate with an oversized pin which force fitted to the hole. The investigation was carried out both experimentally and numerically. In the experimental part, interference fitted specimens along with open hole specimens were fatigue tested to study the experimental effect of the interference fit. In the numerical part, three‐dimensional finite element (FE) simulations have been performed in order to obtain the created stresses due to interference fit and subsequent applied longitudinal load at the holed plate. The stress distribution obtained from FE simulation around the hole was used to predict crack initiation life using Smith–Watson–Topper method and fatigue crack growth life using the NASGRO equation with applying the AFGROW computer code. The predicted fatigue life obtained from the numerical methods show a good agreement with the experimental fatigue life.