制孔分层损伤对CF/PEEK复合材料拉伸性能和表面应变分布的影响

目的 研究钻削制孔表面分层损伤与拉伸载荷下开孔碳纤维增强聚醚醚酮（CF/PEEK）复合材料表面应变分布的相关性。方法 通过对CF/PEEK复合材料层合板进行钻削制孔实验,分析不同进给速度对钻削温度、钻削轴向力、制孔出口表面分层和孔壁表面损伤的影响。采用数字图像相关技术（DIC）和力学实验相结合的方法,研究分层损伤程度对开孔CF/PEEK复合材料层合板拉伸性能和表面应变分布的影响。使用扫描电镜观测开孔试件的断裂形貌,分析开孔试件受拉伸载荷时的破坏模式。结果 随着进给速度的增加,钻削温度降低,钻削轴向力提高,出口表面分层和孔壁损伤程度加剧。随着分层损伤程度的增加,层合板的拉伸强度呈现出降低的趋势,试件的拉伸强度从558.4 MPa降低到525.63 MPa,降低了5.87%。在中应力和高应力状态下,试件x方向的最大负应变随着分层损伤程度的增加而增加。在高应力状态下,试件y方向的最大正应变随着分层损伤程度的增加而增加。试件的断裂方式主要是基体开裂、分层和纤维撕裂,断口有纤维脱落和纤维拔出,垂直于载荷方向的纤维破坏模式为剥离破坏,与载荷方向一致的纤维破坏模式为拉伸破坏。结论 钻削制孔表面分层损...     

V型切口板材试件拉伸断裂过程中的声发射特征

制作了直V型切口和斜V型切口板材试件,设计了板材拉伸断裂声发射监测试验。结合金属断裂力学性能和红外热图,分别对两种切口板材主要声发射参数与断裂过程的相关性进行深入分析,研究试件损伤过程的声发射参数表征以及某些参数特征。结果表明,声发射能量计数、振铃计数、撞击、幅度和频谱峰值能较好地表征试件损伤过程。证明了声发射技术应用于钢制结构损伤实时监测和预警的可行性,并得出某些声发射参数的重要特征。     

2D-C/SiC陶瓷基复合材料拉伸试验的声发射特性

对2D-C/SiC陶瓷基复合材料试样在室温条件下单调拉伸试验和循环拉伸试验的损伤声发射信号进行研究,利用无监督层次聚类分析方法对单调和循环拉伸试验的声发射信号进行损伤模式识别,得出了两种拉伸试验下试样都有相同的损伤分类。对每次单调加/卸载试验分别进行应力和声发射信号分析,得到了在循环加载区间和卸载区间试样的损伤情况。对比分析两种拉伸试验的声发射信号,得到两次试验中首次加载相同应力时,两个试样有同一种类的声发射损伤信号,从而说明循环加载对试样的主要损伤影响较小。     

Evaluation of fatigue damage in reinforced concrete slab by acoustic emission

The applicability of acoustic emission (AE) technique for evaluation of fatigue damage in reinforced concrete (RC) slabs under cyclic loadings in both laboratory and as a structure in service is studied. The fundamental test performed in laboratory shows that the cracking process can be practically monitored by the measurement of AE signals. Analysis of the relationship between loading phase and AE activity indicates that the final stages of the fracture process can be evaluated by detecting AE signals generated near the minimum loading phase. Comparison of the results from the structure and that from the laboratory specimen demonstrates that AE energy can be an effective parameter for the evaluation of fatigue damage in RC slabs in service.     

不同拉伸条件下碳／环氧复合材料损伤的声发射特性

为了研究复合材料在动载荷和恒载情况下的损伤机理,在不同加载速度和恒载情况下,对T700／环氧复合材料进行了拉伸损伤声发射试验研究。从其损伤过程和声发射特性发现,在较低速率下,该复合材料断裂强度对拉伸速率变化不够敏感;但随着拉伸速率的增大,基体及界面损伤的高峰出现时间逐渐提前,断裂时损伤越来越不完全;恒载声发射效应反映了复合材料抗蠕变损伤的能力,而其收敛能力的大小反映了复合材料构件历史损伤的严重程度。     

Comparison of Acoustic Emission Characteristics for C/SiC Composite Component Under Combination of Heating and Mechanical Loading

The acoustic emission(AE) characteristics of C/SiC composite component under various conditions were compared, with the purpose of identifying the possible damage and failure mechanism. During the process of the single mechanical loading, the highest amplitude of the AE signal was less than 85 dB and the main damage forms of matrix cracking and interface debonding were involved. For the heating process, high-energy AE signals with an amplitude more than 85 dB were detected and fiber fracture mechanism was determined as well due to the thermal stress caused by the mismatch of the thermal expansion coefficient between the reinforced fiber and matrix. During the combination process of the heating and mechanical loading, it was concluded that the degree of damage was much severer than the simple superposition of damage produced by the individual mechanical loading and the individual heating process.     

Post-impact damage characterisation on natural fibre reinforced composites using acoustic emission

The present study aims to characterise damage due to low velocity impact on jute fibre reinforced polyester composites. To attain this goal, a number of post-impact mechanical tests have been carried out, including tensile tests, three-point bending and indentation, using either a staircase or a continuous loading programme. On all these tests acoustic emission activity (AE) was monitored. The results, compared with damage observed under an optical microscope, show that AE is able to perform a reliable measurement of the level of damage also, on a natural fibre reinforced laminate. The main limitations of this study are owing to the rather low ultimate stress of the material and to the need to apply a loading to evaluate the damage produced by the impact event.     

声发射方法测定SiCw／Al复合材料的界面强度

对ＳｉＣｗ／Ａｌ复合材料形变断裂过程中的声发射（ＡＥ）特征的研究表明，用ＡＥ技术能准确测定ＳｉＣ纤维的断裂次数，并能测定其断枝长度，从而测定了纤维和基体间的界面强度和ＳｉＣ纤维的断裂强度。     

Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis

The sequence of microscopic fracture mechanisms in locally loaded cross-ply carbon-fiber composites was studied by analyzing acoustic emission (AE) signals in combination with the modal analysis of Lamb waves, using microscopic and ultrasonic examination of the specimen after load interruption. The first 70 AE events were analyzed, which were detected during the initial loading segment when the first sudden load drop and gradual load recovery were observed. Characteristics of the detected waves were compared with the S₀- and A₀-mode Lamb waves produced by a spot- or line-focused YAG laser. The internal damage progression of the composite specimen was determined to be the fiber fracture in the front lamina, transverse cracks in the mid-lamina, delamination and splitting.     

剪切修正GTN模型在拉剪变路径加载问题中的适用性

为评估含剪切修正项GTN模型在非比例加载条件下的适用性,以一组缺口圆棒试样的拉剪变路径加载试验结果为依据,运用有限元软件ABAQUS/Explicit模块及用户子程序VUMAT进行平行仿真分析。研究结果表明:在大预变形下的拉剪变路径加载条件下,增长的预应变会加快后续拉伸过程中孔洞的增长率,导致材料提前断裂。同时,结合断口宏观与微观形貌分析,研究了非比例加载过程材料的损伤演化机理。AA2024铝合金在拉剪变路径加载条件下的断裂机制为拉-剪混合断裂,随着预扭作用的增强,剪切断裂逐渐取代拉伸断裂成为主导的断裂模式。     

热障涂层系统的热梯度机械疲劳应力分析

基于IN738高温合金基体上涂覆的热障涂层系统(Thermal barrier coating system,TBCs),分析热循环和热梯度机械疲劳加载条件下涂层的应力分布及演变。通过有限元分析研究了热生长氧化层(Thermally growth oxidation,TGO)的应力分布,以预测不同载荷作用下TBCs的失效行为。结果可知,在热循环的基础上施加应变载荷会造成TGO应力性质及大小的改变。只施加温度载荷,在加热过程中TGO/粘结层(Bond coat,BC)界面波峰位置会承受轴向较大的拉伸应力,裂纹多会在此处萌生,且以层间开裂的方式失效。而在温度与机械载荷的共同作用下,冷却过程中会承受较大的拉伸应力,显著增大的轴向应力与径向应力共同作用,使垂直于TGO/BC界面的裂纹沿着界面方向扩展,从而造成陶瓷层(Top coat,TC)剥落。进一步对比分析了同相和反相加载时的应力分布,结果表明反相加载时一次循环周期内会产生拉伸平均应力,更易发生TBCs的失效。     

Acoustic emission activities and damage evaluation of reinforced concrete beams strengthened with CFRP sheets

In this study, the applicability of acoustic emission (AE) techniques to monitor damage evolution in reinforced concrete (RC) beams strengthened in flexure with carbon fiber reinforced polymer (CFRP) sheets is investigated. The objective is to initiate the creation of a user-friendly health monitoring system for RC structures strengthened with CFRP sheets using AE techniques. Five beams, 200 mm×300 mm in cross-section, were tested under three-point bending over a span of 1700 mm. One of the beams was tested in its virgin condition to serve as reference; the remaining four beams were tested after being strengthened with CFRP sheets bonded on the tension face. The parameters investigated in this study include both the amount of CFRP sheets and construction imperfections (the CFRP sheets were intentionally bonded without adhesive in the centermost 10% and 20% bonding area). The AE signals were collected and analyzed for all specimens. The AE parameters were analyzed for four levels of damage based on initial crack, propagation, yielding of main bars, and fracture or rip-off of the CFRP sheets. The frequency-peak magnitude distribution of the AE parameters was used to determine the b-value, defined by the Gutenberg–Richter relationship, for evaluating the damage evolution and fracture process of RC beams strengthened in flexure with CFRP sheets. From the results of this study, the signal characteristics – event, amplitude versus frequency, and amplitude versus duration – show clear differences in the different loading stages, depending upon the active damage mechanism. The b-value is correlated to the fracture process of the RC beams bonded with CFRP sheets and the degree of localization of damage. The AE technique is a useful nondestructive technique for monitoring the behavior of RC beams that are externally reinforced in flexure with CFRP sheets.     

TiAl合金涡轮压缩损伤与拉伸断裂行为

为研究TiAl合金涡轮经压力作用后的组织损伤机制及性能弱化规律,设计了对TiAl合金涡轮先压缩再拉伸的实验方法。利用扫描电镜(SEM)对压缩后的涡轮轴颈表面及内部的滑移和微裂纹进行了分析,并观察了拉伸断口形貌。实验结果表明：随着前期涡轮所受的压力的增大,压缩后的TiAl涡轮剩余抗拉强度逐渐降低,当压力为610 MPa时,剩余抗拉强度仅为86 MPa,强度损失率高达70%。TiAl合金压缩过程中形成了以沿层裂纹为主、穿层裂纹为辅的变形损伤特征。与压缩轴成45°的最大剪应力方向上的沿层裂纹是TiAl合金压缩损伤的主要形式。压缩损伤后的TiAl合金涡轮拉伸断裂均发生在靠近涡轮浇铸冒口侧的细轴颈部位。受压变形后的片层组织中的微小裂纹在随后拉应力作用下继续扩展直至韧带桥被贯穿,小裂纹合并成大裂纹,在断口上表现出沿层和穿层的混合断裂形貌。     

胶合板拉伸过程中的声发射特征

针对传统的力学试验方法对胶合板破坏过程的表征不足,提出了用声发射检测技术全程监测胶合板拉伸破坏过程。试验表明,材料的损伤破坏具有阶段性,不同阶段的声发射信号特征有明显的区别。研究发现声发射特征参数能够表征材料受载过程中的损伤演化规律和损伤类型,能直观地反映材料的损伤特征。因此,声发射检测技术可作为材料测试、质量评定的有效手段,也是传统力学方法的有益补充。     

圆筒内嵌钨合金球爆炸加载条件下断裂机制研究

针对钨合金作为预制破片战斗部穿甲后的易碎性,采用圆筒内嵌钨合金球战斗部,研究了97.5W-Ni-Fe合金在圆筒内嵌式爆炸加载穿靶前后的微观组织以及断裂机制。结果表明,97.5W-Ni-Fe合金爆炸加载后在钨颗粒内部产生大量形变孪晶,穿透靶板后形变孪晶诱发大量微观裂纹,微裂纹在穿靶后的拉应力作用下扩展并与W-W界面断裂相互连接,使钨合金球断裂成有效破片,增强了钨合金的二次毁伤能力。     

Microstructure and tensile behavior of multiply needled C/SiC composite fabricated by chemical vapor infiltration

The microstructure and tensile behavior of a multiply needled C/SiC composite fabricated by chemical vapor infiltration were investigated. Results showed that the tensile stress–strain curves exhibited a typical nonlinear behavior and can be divided into three regions: a very small initial linear region followed by a large nonlinear region and finally a quasi-linear region. Needling process caused a crimp around needling fibers and reduced the bearing fibers in plane. Needling process induced damages were the main reasons for the failure of the composite. The fracture mainly occurred at the cross of needling fibers and unidirectional fibers, with the fibers showing multi-step fracture and extensive pullout. The multi-step fracture of clusters and nonlinear curves indicated a typical non-brittle failure behavior of the multiply needled C/SiC composite due to the various damage patterns.     

Modal analysis of acoustic emission signals from CFRP laminates

As a result of its continuous and in situ detection capabilities, the acoustic emission (AE) technique is the prime candidate for damage monitoring in loaded composite structures. None of the AE analysis techniques used in laboratory studies has, however, proven to be capable of consistently dealing with the difficulties encountered in larger structures: large amount of data, the elimination of noise sources and the influence of wave propagation effects (attenuation, dispersion). This work will use the modal acoustic emission (MAE) technique as a more intelligent and efficient way of analysing AE results. AE waveforms obtained during tensile and bending testing of CFRP laminates will be presented. It will be demonstrated how taking into account the modal nature of AE waves can in future lead to more quantitative and accurate results.     

基于声发射技术的CFRP钢管混凝土受弯破坏过程

研究了CFRP钢管混凝土在弯曲荷载作用下的破坏过程及声发射变化规律。结果显示,整个破坏过程可分为弹性段、弹塑性段、下降段和软化段四个阶段,而声发射参数变化及波形特征与试件破坏过程表现出较好的对应关系。证明了利用声发射技术能有效地监测CFRP钢管混凝土弯曲过程损伤程度和破坏历程。     

碳钢–橡胶硫化黏接拉伸力学行为与黏接失效机理

目的 研究碳钢-橡胶黏接结构件的拉伸力学行为和黏接失效机理,同时获得黏接工艺参数对碳钢-橡胶黏接界面质量的影响规律。方法 引入内聚力模型来描述黏接界面的黏接失效过程,通过黏接结构件界面力学性能实验拟合,获得内聚力参数,建立碳钢-橡胶黏接三维拉伸数值模型,从拉伸应力、剥离应力、损伤因子和刚度弱化的角度分析碳钢-橡胶在拉伸载荷作用下的黏接失效行为,研究刚度、初始损伤强度和断裂能对碳钢-橡胶黏接界面力学性能的影响规律。结果 在实验得到的内聚力参数下,碳钢-橡胶黏接界面中部最大拉伸应力比边缘增大了0.42 MPa,黏接界面边缘的最大剥离应力比最大拉伸应力增大了0.77 MPa。在拉伸位移为1.35 mm时,黏接界面中部最先进入损伤演化。当拉伸位移增至1.8 mm时,黏接界面边缘进入损伤演化,黏接界面中部完全失效,刚度的减小使得黏接界面的抗拉伸能力增强,刚度越小黏接界面的抗拉伸能力增强效果越明显。初始损伤强度小于3.42 MPa,黏接界面损伤演化速度明显较慢,在较大拉伸位移下才会发生黏接失效。不同断裂能的黏接界面进入损伤演化时的拉伸位移一致,断裂能越大的黏接界面损伤演化至完全失效的速度越慢。结论...     

加载速率对结构钢力学性能和断裂韧度的影响

在常温下进行了常用建筑结构钢(16Mn和Q235B钢)动态拉伸和动态断裂韧度试验。试验结果表明，材料在动载下无论是屈服强度还是抗拉强度均有一定的提高，塑性有了一定的降低；由于缺口尖端区域温度的升高，常温下动载对于16Mn钢母材和焊缝的断裂韧度是有利的；Q235B钢常温下，焊缝的断裂韧度值较低，且动载对母材的断裂韧度影响较大，和静载相比动载下断裂韧度值降低了4倍有余，可以看出，Q235B钢的抗震性能较差。通过研究发现，对于低韧度水平的材料，研究动载下结构的断裂行为时．材料的强度也应该作为一个考虑因素。