不同应力和频率下环状ABS/GF的疲劳性研究

通过对玻璃纤维(GF)改性ABS的环状试样进行疲劳试验,从而比较了在环状下填料在不同循环应力条件下对ABS的疲劳性能的影响;作出了ABS/GF环状试样在不同应力和频率下的疲劳曲线并获得其疲劳变化的趋势,分析了其原因.结果表明,随着GF用量的增加,ABS/GF疲劳性能大幅下降;同时,随着循环应力的增加,复合材料的疲劳断裂周次大幅度下降.在低频率(0.25 Hz)和中等频率(0.5 Hz)下,ABS/GF疲劳性随着GF用量的增加而提高,但在高频率下,疲劳性就随着GF用量的增加而降低.     

不同应力条件下环状ABS/SBS疲劳性比较及外推法求出疲劳极限

通过在不同的应力条件下,对环状ABS/SBS试样的疲劳性能进行研究,同时通过外推法获得不同应力下的疲劳极限,并得到疲劳性能变化的趋势.实验结果表明,低应力(1.08 kN)和中应力(1.46 kN)下,环状ABS/SBS复合材料的动态力学性能比较好,疲劳性能好,并且在SBS的质量分数为6%时出现波峰;在高应力(1.87 kN)下,环状ABS/SBS复合材料力学性能并未出现波峰,而是随着SBS用量的的增加不断上升,同时其相对于低中应力下的疲劳断裂周次明显下降.     

环状ABS/SBS试样疲劳破坏断口分析

采用(丙烯腈/丁二烯/苯乙烯)共聚物/(苯乙烯/丁二烯/苯乙烯)共聚物(ABS/SBS)材料成型了圆环状试样,使用自行设计的试验装置对环状试样进行疲劳试验,分析比较在环状下的断口形态学,测试出不同含量SBS对ABS环状试样在不同应力和频率下的裂纹扩展速度及断口形貌.从而获得了比标准试样更准确的填料特性对材料不同加工条件疲劳性能的影响情况.最后提出在不同条件下提高材料疲劳强度的方法.     

玻璃纤维增强聚丙烯复合材料的疲劳性能研究

利用自制的新型疲劳实验装置,对玻璃纤维(GF)增强聚丙烯(PP)复合材料进行疲劳性能研究,得到PP/GF复合材料在不同初始应力、不同振幅、不同频率下疲劳性能与GF含量的关系.结果表明,初始应力、振幅、频率越大,材料疲劳拉伸性能越差.初始应力较大时,初始应力的变化对PP/GF材料疲劳性能的影响更为明显,振幅在75 ～80...     

基于XFEM短纤维聚醚醚酮复合材料结构稳定性数值模拟分析

熔融沉积成型(FDM)制备短纤维增强聚合物复合材料成型技术日趋完善。采用FDM-3D挤压工艺制备了纤维含量为10%的短碳纤维(CF)和玻璃纤维(GF)增强的高性能聚醚醚酮复合材料,通过拉伸实验获取聚醚醚酮(PEEK)、碳纤维聚醚醚酮复合材料(CF/PEEK)以及玻璃纤维聚醚醚酮复合材料(GF/PEEK)试样应力-应变曲线。在此基础上,以单边缺口弯曲断裂试样为对象,基于扩展有限单元法(XFEM)建立三种材料的数值模拟模型,分别讨论了含初始裂纹缺陷和无预制裂纹状况下试样的结构稳定性。结果表明,在相同条件下CF/PEEK复合材料构件先于纯PEEK材料、GF/PEEK复合材料发生结构失效行为,且当构件中存在热裂纹的状况下,CF/PEEK复合材料构件更有可能发生断裂失效。     

SAM和SAG对玻璃纤维增强ABS复合材料性能的影响

通过熔融共混法研究了苯乙烯-丙烯腈-马来酸酐(SAM)和苯乙烯-丙烯腈-甲基丙烯酸缩水甘油酯(SAG)两种带有反应性官能团的三元共聚物对玻璃纤维(GF)增强丙烯腈-丁二烯-苯乙烯共聚物(ABS)复合材料性能的影响。结果表明,SAM和SAG对ABS/GF复合材料都具有优异相容化效果,都可用作ABS/GF复合材料的相容型偶联剂使用。同等添加量时,添加SAG的ABS/GF复合材料具有更优异的物理力学性能,并且随着SAG添加量的增加性能提高更加明显;添加SAG的ABS/GF复合材料颜色更浅,并具有更优异的长期高温色变性。     

B_4C/2009Al复合材料的高周疲劳性能研究

本研究采用体积分数为17%的碳化硼颗粒(B_4C)增强2009Al复合材料,然后进行热挤压加工(挤压比为90∶1)。对复合材料室温下高周疲劳性能进行了测试,同时通过分析疲劳断口、观察微观结构深入研究其失效机理。在拉-压循环载荷条件下测试了B_4C/2009 Al复合材料的高周疲劳性能。B_4C/2009 Al复合材料的疲劳破坏机制为:微裂纹主要萌生于试样表面划痕和B_4C与Al之间界面的脱粘;微裂纹萌生后首先在基体中扩展,当微裂纹遇到B_4C颗粒时,裂纹发生偏折或者停止;随着微裂纹继续扩展,裂纹尖端塑性区变大,B_4C颗粒断裂和B_4C与基体界面的脱粘增多;最后,微裂纹不断扩展聚集造成了B_4C/2009 Al复合材料最终断裂。     

酚醛树脂/玻璃纤维复合材料界面接枝ABS研究

采用溶液接枝聚合方法,在偶联剂处理后的玻璃纤维(GF)表面接枝(丙烯腈/丁二烯/苯乙烯)共聚物(ABS)而形成一层有极性的柔性"可变形层",以提高酚醛树脂/GF复合材料的界面粘结性能.采用傅立叶变换红外光谱仪、能量色散光谱仪和扫描电子显微镜研究了接枝前后GF表面官能团、元素含量和表面微观形貌变化.研究了ABS接枝率对酚醛树脂/GF复合材料力学性能的影响.结果表明,ABS成功地接枝到GF表面;酚醛树脂/GF复合材料的力学性能随着ABS接枝率的提高先升高后降低:当ABS的接枝率为2.8%、3.0%、4.3%、4.7%时,酚醛树脂/GF的层间剪切强度、拉伸强度、弯曲强度、冲击强度分别达到最高,为70 MPa、620 MPa、450 MPa、226 kJ/m2.     

单向碳纤维/环氧树脂复合材料疲劳裂纹扩展的研究

为了模拟单向碳纤维／环氧树脂复合材料在拉－拉疲劳实际工作条件下疲劳裂纹扩展的情况，利用单向碳纤维增强环氧树脂复合材料制成的拉伸试样（CTS），在应力强度因子比R为0.5的条件下，就加载频率、应力强度因子幅△κ对Ⅰ型裂纹的扩展速度及裂纹扩展机理的影响作了较系统的研究，为材料的安全可靠使用条件及作用周期提供参考依据。     

玻纤增强共聚聚丙烯高性能化的研究

利用韧性优良的共聚聚丙烯(PPR)作为增强基体,通过玻纤(GF)与PPR制备高性能PPR/GF复合材料,研究了流动改性剂、马来酸酐接枝聚丙烯(PP-g-MAH)和玻纤的含量以及挤出次数对PPR/GF复合材料结构与性能的影响.结果表明:自制的流动改性剂可大幅增加PPR/GF的熔体质量流动速率,流动性可适用于注塑工艺;PP-g-MAH增加了PPR基体与GF之间的界面相互作用,提高PP/GF复合材料的力学性能;随玻纤含量增加,PP/GF复合材料的拉伸强度和模量大幅增加,缺口冲击强度和断裂伸长率有所降低,但材料的韧性仍保持较高水平,所制备PPR/GF/PP-g-MAH共混材料的性能与ABS相当,可替代ABS工程塑料作为结构件使用;多次挤出加工会降低PPR/GF复合材料中玻纤的平均长度和材料的力学性能.     

Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm surface length was made on each specimen. The crack-healing behavior under cyclic stress of 5 Hz, and resultant cyclic fatigue strengths at healing temperatures of 1100° and 1200°C, were systematically investigated. The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have an excellent ability to heal a crack at 1100° and 1200°C. (2) This sample could heal a crack even under cyclic stress at a frequency of 5 Hz. (3) The crack-healed sample exhibited quite high cyclic fatigue strength at each crack-healing temperature, 1100° and 1200°C.     

Fatigue Crack Growth Rates in Adhesive Joints Tested at Different Frequencies

Mode I fatigue crack growth tests were conducted on joints bonded with a filled adhesive (A) at 20 Hz and 2 Hz and on joints bonded with a filled and toughened adhesive (B) at 20 Hz, 2 Hz, 0.2 Hz and 0.02 Hz. Strain energy release rate, G, and J-integral were evaluated based on elastic and elastoplastic finite element analyses (FEA) of the joints bonded with adhesive A and B, respectively. For the configurations considered, J was found to be path-independent and did not differ much from G. The fatigue crack growth rate (FCGR), da/dN, in the joints bonded with adhesive A was relatively independent of frequency while it increased with decreasing frequency at given δ for the joints bonded with adhesive B. The fatigue processes in both adhesives involved the cracking of the filler particles and subsequent linkage of the resultant microcracks. The process zone in adhesive B is larger than that in adhesive A and it increases with decreasing frequency. It is suggested that this variation in process zone size can account for the observed fatigue behaviour. The fatigue crack growth velocity, da/dt, was also calculated for the joints bonded with adhesive B and the variation of da/dt with test frequency at given δG is much smaller than the variation in da/dN, suggesting a creep effect in the fatigue crack growth.     

Fatigue Crack Growth Rates in Adhesive Joints Tested at Different Frequencies

Mode I fatigue crack growth tests were conducted on joints bonded with a filled adhesive (A) at 20 Hz and 2 Hz and on joints bonded with a filled and toughened adhesive (B) at 20 Hz, 2 Hz, 0.2 Hz and 0.02 Hz. Strain energy release rate, G, and J-integral were evaluated based on elastic and elastoplastic finite element analyses (FEA) of the joints bonded with adhesive A and B, respectively. For the configurations considered, J was found to be path-independent and did not differ much from G. The fatigue crack growth rate (FCGR), da/dN, in the joints bonded with adhesive A was relatively independent of frequency while it increased with decreasing frequency at given δ for the joints bonded with adhesive B. The fatigue processes in both adhesives involved the cracking of the filler particles and subsequent linkage of the resultant microcracks. The process zone in adhesive B is larger than that in adhesive A and it increases with decreasing frequency. It is suggested that this variation in process zone size can account for the observed fatigue behaviour. The fatigue crack growth velocity, da/dt, was also calculated for the joints bonded with adhesive B and the variation of da/dt with test frequency at given δG is much smaller than the variation in da/dN, suggesting a creep effect in the fatigue crack growth.     

The effects of frequency on fatigue threshold and crack propagation rate in modified and unmodified polyvinyl chloride

The cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC‐M) and without (PVC‐U) chlorinated polyethylene (CPE) impact modifier, was studied. The effect of impact modifier upon fatigue crack growth rate and threshold was evaluated at frequencies of 1, 7, and 20 Hz. It was shown that the addition of CPE lowered the threshold stress intensity factor amplitude for crack growth (ΔK_th) of PVC‐M compared to that of PVC‐U at lower frequencies, and that the effect became more pronounced at lower frequency. At lower stress intensity factor amplitudes (below ΔK = 1 MPa·m^1/2), there was a slight difference between the crack growth rates of U‐ and M‐PVC. The crack advance mechanism is investigated by microscopic observation of the crack tip process zone. Although the zone is relatively large in PVC‐M, associated with higher toughness, it did not improve the fatigue crack growth resistance significantly. Fracture surface observations reveal a higher density of fibrils on the fatigued surface of PVC‐M with the density, relative to that observed in PVC‐U, reducing with frequency. It is therefore hypothesized that accelerated fibril failure is a mechanism of fatigue. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Crack initiation in PVC for subsequent linear elastic fracture mechanics analysis

Reproducible starter-cracks for subsequent linear elastic fracture mechanics analysis have been grown in PVC by fatigue cycling at 80 Hz. The crack growth rate has been related to the fracture surface markings and to the opening mode stress intensity factor (K_I) of the fatigue cycle. Termination of the fatigue crack growth when crack growth rate is constant ensures a smooth mirror fracture surface and a sharp crack tip.     

Origin of the Static Fatigue Limit in Oxide Glasses

Oxide glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. The log of crack velocity decreases linearly with decreasing stress intensity factor in Region I. For some glasses, at a lower stress intensity, K_o, log v asymptotically diminishes where there is no measurable crack growth. The same glasses exhibit static fatigue, or a decreasing strength for increasing static loading times, as cracks grow and stress intensity eventually reaches the fracture toughness. In this case, some glasses exhibit a low stress below which no fatigue/failure is observed. The absence of slow crack growth under a low stress intensity factor is called the fatigue limit. Currently, no satisfactory explanation exists for the origin of the fatigue limit. We show that the surface stress relaxation mechanism, which is promoted by molecular water diffusion near the glass surface, may be the origin of the fatigue limit. First, we hypothesize that the slowing down of slow crack growth takes place due to surface stress relaxation during slow crack growth near the static fatigue limit. The applied stress intensity becomes diminished by a shielding stress intensity due to relaxation of crack tip stresses, thus resulting in a reduced crack velocity. This diminishing stress intensity factor should result in a crack growth rate near the static fatigue limit that decreases in time. By performing Double Cantilever Beam crack growth measurements of a soda‐lime silicate glass, a decreasing crack growth rate was measured. These experimental observations indicate that surface stress relaxation is causing crack velocities to asymptotically become immeasurably small at the static fatigue limit. Since the surface stress relaxation was shown to take place for various oxide glasses, the mechanism for fatigue limit explained here should be applicable to various oxide glasses.     

CRACK GROWTH IN ADHESIVELY BONDED JOINTS SUBJECTED TO VARIABLE FREQUENCY FATIGUE LOADING

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Crack Shielding in Ce-TZP/Al2O3 Composites: Comparison of Fatigue and Sustained Load Crack Growth Specimens

Crack shielding stress intensities in in situ loaded compact tension specimens of two types of ceria-partially-stabilized zirconia/alumina (Ce-TZP/Al₂O₃) composites with prior histories of subcritical crack growth in sustained and tension-tension fatigue loading were directly assessed using laser Raman spectroscopy. Crack-tip stress fields within the transformation zones were measured by measuring a stress-induced frequency shift of a peak corresponding to the tetragonal phase. The peak shift as a function of the applied stress was separately calibrated using a ball-on-ring flexure test. Total crack shielding stress intensity was estimated from the far-field applied stress intensity and the local crack-tip stress intensity assessed from the measured near-crack-tip stresses. The shielding stress intensities were consistently lower in the fatigue specimens than in the sustained load crack growth specimens. The reduced crack shielding developed in the fatigue specimens was independently confirmed by measurements of larger crack-opening displacement under far-field applied load as compared to the sustained load crack growth specimens. Thus, diminished crack shielding was a major factor contributing to the higher subcritical crack growth rates exhibited by the Ce-TZP/Al₂O₃ composites in tension–tension cyclic fatigue. Calculations of zone shielding considering only the dilatational strains in the transformation zones accounted for 81% and 86% of the measured values in the sustained load crack growth specimens, but significantly overestimated the shielding in the fatigue specimens. Possible reasons for the diminished crack shielding in the fatigue specimens are discussed.     

Experimental and numerical prediction of effect of frequency on bending fatigue performance of polyamide 66/hectorite nanocomposite

An increased use of thermoplastics in components and structures that are subjected to cyclic loads necessitates a specific attention to variables that affect the hysteretic heating. Hysteretic heating effect in polyamide 66/hectorite nanocomposite has been investigated under bending strain control mode using a custom-built bending fatigue test setup in a laboratory environment. Dynamic mechanical analysis (DMA) results revealed a considerable rise in loss modulus with a decrease in frequency from 1 to 0.1?Hz irrespective of the temperature of the specimen. Alternatively, a reduction in fatigue test frequency from 2 to 0.5?Hz resulted in a significant decrease in cyclic softening. Fatigue behaviour predicted from DMA results using coupled structural/thermal finite element analysis is fairly in agreement with the experimental one. An accelerated crack initiation at decreased specimen temperature and high cyclic steady state stress reduced the fatigue life at 0.5?Hz compared with 2?Hz.