连续疲劳干预对膝关节主被动生物力学特性的影响

目的 探究连续疲劳干预对膝关节主被动生物力学特性的影响。方法 招募27位健康大学生在等速力量测试系统上进行疲劳实验。疲劳实验包括3个疲劳周期,每个疲劳周期至少30次股四头肌连续等长收缩。对比不同疲劳周期的最大被动力矩、平均最大自主收缩(maximum voluntary contraction, MVC)力矩、股外侧肌和股二头肌肌电积分值以及协同收缩指数(co-contraction index, CI)。结果 疲劳周期3后,最大被动力矩显著下降(P<0.05)。疲劳周期1、2、3过程内平均MVC力矩、股外侧肌和股二头肌肌电积分值都显著下降(P<0.05),然而二者CI全程无显著变化(P>0.05)。结论 股四头肌等长收缩训练剂量为90次的连续疲劳干预会显著影响膝关节主被动生物力学特性。疲劳干预后,膝关节抵抗被动屈伸能力下降。并且随着疲劳干预进程,股四头肌主动收缩能力下降,股四头肌和腘绳肌肌肉激活也下降,但二者肌群协同收缩水平不变。这种温和的肌群协同收缩模式有益于保持良性膝关节力学加载环境。研究结果有助于理解疲劳后膝关节主被动生物力学特性。

45号钢对称应变“间断及连续疲劳性能”对比试验

本文想要说明对称应变“间断”疲劳寿命N^2f与“连续”疲劳寿命Nf有无显著差别。通过一组45号铜光滑园试样，在应变范围±0．4％下，测得Nf的均值＝4813．Nf的均值，N^-f＝5325。在显著水平α＝0．005下，N^2f与Nf的总体值μf和μf之间无显著差别的概率达99％，N^-f的99％，置信区间是（2470．7156）。N^-f的99％置信区间是（2381．8269），为减轻长时间的连续疲劳试验给人员带来的疲劳，使疲劳试验更符合某些实际，采用间断疲劳试验代之，本文可起抛砖引玉作用。

分级不连续循环载荷作用下混凝土的疲劳特性

为研究混凝土在不连续周期荷载作用下的疲劳特性,在常规疲劳加载路径中加入时间间隔,对比开展了混凝土的常规疲劳试验和不连续疲劳试验.研究结果表明:在不连续疲劳试验中,加入时间间隔后的应力循环产生的残余应变(塑性变形)明显大于加入时间间隔前的应力循环产生的残余应变;时间间隔效应从0.062的应力水平开始影响混凝土的疲劳性能,并逐步增强,至应力水平为0.25后趋于稳定;时间间隔后循环内混凝土的弹性模量大于时间间隔前循环内混凝土的弹性模量;不连续疲劳损伤累积速度明显大于常规疲劳损伤,混凝土的不连续疲劳寿命远小于常规疲劳寿命;混凝土不同组分间的残余应力在时间间隔内持续作用,致使裂隙持续扩展,造成时间间隔后循环内残余变形较快发展.

连续管疲劳试验装置研制和实物试验研究

为了能比较真实地模拟连续管在实际作业工况条件下,即在弯曲和内压联合作用下的疲劳寿命,研制出了一套专门用来进行连续管低周疲劳寿命的实物试验装备。该装置可以在0￣70MPa的内压条件下测试直径范围为12.7￣88.9mm的连续管的疲劳寿命。通过对直径和壁厚为38.1mm×3.2mm的CT-80连续管进行初步实物试验,获得了连续管疲劳寿命(循环次数)与内压的关系曲线、连续管循环次数与胀径关系的曲线、连续管循环次数与椭圆度的关系曲线以及连续管循环次数与壁厚的关系曲线。实物试验结果与国际先进的软件预测结果一致。

A model of continuum damage mechanics for high cycle fatigue of metallic materials

A non-linear continuum damage model was presented based on the irreversible thermodynamics framework developed by LEMAITRE and CHABOCHE. The proposed model was formulated by taking into account the influence of loading frequency on fatigue life. The parameters H and c are constants for frequency-independent materials, but functions of cyclic frequency for frequency-dependent materials. In addition, the expression of the model was discussed in detail at different stress ratios （R）. Fatigue test data of AlZnMgCu1.5 aluminium alloy and AMg6N alloy were used to verify the proposed model. The results showed that the model possesses a good ability of predicting fatigue life at different loading frequencies and stress ratios.

A fatigue damage model for asphalt mixtures under controlled-stress and controlled-strain modes

A fatigue damage model based on thermodynamics was deduced for asphalt mixtures under controlled-stress and controlled-strain modes. By employing modulus of resilience as the damage hardening variable, a damage variable related with dynamic modulus was extracted as the evaluation index. Then, the damage evolution law under two control modes was proposed, and it has a similar form to the Chaboche fatigue model with a nonnegative material parameter m related to its loading level. Experimental data of four loading levels were employed to calibrate the model and identify the parameter in both control modes. It is found that the parameter m shows an exponential relationship with its loading level. Besides, the difference of damage evolution under two control modes was explained by the law. The damage evolves from fast to slow under a controlled-strain mode. However, under a controlled-stress mode, the evolution rate is just the opposite. By using the damage equivalence principle to calculate the equivalent cycle numbers, the deduced model also interprets the difference of damage evolution under two control modes on the condition of multilevel loading. Under a controlled-strain mode, a loading sequence from a low level to a high level accelerates damage evolution. An inverse order under the controlled-stress mode can prolong fatigue life.

Damage assessment of a continuous beam bridge based on the strain mode

Based on the method of strain mode, damage identification of continuous beam bridges by comparing the variance of several curves of strain modes difference is studied. Three cases of numerical simulation demonstrate that the proposed method is applicable to detecting many a damage in a continuous beam bridge, which accurately identifies the damaged positions of the bridge, and detects the damage severity of an element by its according peak value of the curve of strain modes difference that is found to increase with the increasing damage severity.

High Cycle Fatigue Life Prediction and Reliability Analysis of Aeroengine Blades

Based on the nonlinear continuum damage model （CDM） developed by Chaboehe, a modified model for high cycle fatigue of TC4 alloy was proposed. Unsymmetrical cycle fatigue tests were conducted on rod specimens at room temperature. Then the material parameters needed in the CDM were obtained by the fatigue tests, and the stress distribution of the specimen was calculated by FE method. Compared with the linear damage model （LDM）, the dam- age results and the life prediction of the CDM show a better agreement with the test and they are more precise than the LDM. By applying the CDM developed in this study to the life prediction of aeroengine blades, it is concluded that the root is the most dangerous region of the whole blade and the shortest life is 58 211 cycles. Finally, the Cox propor- tional hazard model of survival analysis was applied to the analysis of the fatigue reliability. The Cox model takes the covariates into consideration, which include diameter, weight, mean stress and tensile strength. The result shows that the mean stress is the only factor that accelerates the fracture process.