针对传统静态损伤识别测量方法存在测量系统布置烦琐、测点有限以及基于应变参量的损伤识别受加载条件限制的问题,一种基于归一化应变梯度变化(Differential Gradient of Normalized Strain,DGNS)的损伤识别方法被提出.通过对只含有矩...针对传统静态损伤识别测量方法存在测量系统布置烦琐、测点有限以及基于应变参量的损伤识别受加载条件限制的问题,一种基于归一化应变梯度变化(Differential Gradient of Normalized Strain,DGNS)的损伤识别方法被提出.通过对只含有矩形槽的试件和同时含有方形通孔和裂纹的试件的数值计算,说明该方法能够有效识别塑性变形、裂纹等损伤.实验中,提出采用应变等值线密度变化(Differential of Strain Contour Density,DSCD)取代DGNS用以识别结构损伤,两者具有相同的物理意义,但DSCD更加有效地保证云图的平滑性和可视性,实验证明DSCD可以有效地识别塑性损伤.研究表明,DGNS(DSCD)可以很好地刻画出结构塑性变形、裂纹损伤特性,剔除由于结构自身不均匀造成的应变集中区域的影响,有效地孤立出损伤区域;DGNS(DSCD)是结构的本征参数,不依赖于外部载荷幅值;相比于传统的应变损伤识别,DGNS(DSCD)具有更加敏感、更早捕捉损伤的优势.展开更多
对一砂质泥岩风化土在300,400,500 k Pa及600 k Pa固结围压下,分别进行了原状土样与重塑土样的动三轴试验,得到了该砂质泥岩风化土的阻尼比曲线。试验结果表明,原状土与重塑土在各固结围压试验下,阻尼比随归一化剪应变γ取值的增大而增...对一砂质泥岩风化土在300,400,500 k Pa及600 k Pa固结围压下,分别进行了原状土样与重塑土样的动三轴试验,得到了该砂质泥岩风化土的阻尼比曲线。试验结果表明,原状土与重塑土在各固结围压试验下,阻尼比随归一化剪应变γ取值的增大而增大,且在γ取值相同时,原状土阻尼比值大于相应重塑土;原状土与重塑土试样的λd~γ关系曲线均随固结围压的升高而逐渐降低,且降低幅度随着γ值的增大而减小,并在γ值接近于1时各关系曲线基本交叉重合;在同等试验条件下,通过经验公式与绘图两种方式求得的最大阻尼比值相差很小,但随着固结围压的增大,两种方式求得的最大阻尼比值相差变大。展开更多
This paper studies how the sample rotation method is applied to the case where item nonresponse occurs in surveys. The two cases where the response to the first occasion is complete or incomplete are considered. Using...This paper studies how the sample rotation method is applied to the case where item nonresponse occurs in surveys. The two cases where the response to the first occasion is complete or incomplete are considered. Using ratio imputation method, the estimators of the current population mean are proposed, which are valid under uniform response regardless of the model and under the ratio model regardless of the response mechanism. Under uniform response, the variances of the proposed estimators are derived. Interestingly, although their expressions are similar, the estimator for the case of incomplete response on the first occasion can have smaller variance than the one for the case of complete response on the first occasion under uniform response. The linearized jackknife variance estimators are also given. These variance estimators prove to be approximately design-unbiased under uniform response. It should be noted that similar property on variance estimators has not been discussed in literature.展开更多
文摘针对传统静态损伤识别测量方法存在测量系统布置烦琐、测点有限以及基于应变参量的损伤识别受加载条件限制的问题,一种基于归一化应变梯度变化(Differential Gradient of Normalized Strain,DGNS)的损伤识别方法被提出.通过对只含有矩形槽的试件和同时含有方形通孔和裂纹的试件的数值计算,说明该方法能够有效识别塑性变形、裂纹等损伤.实验中,提出采用应变等值线密度变化(Differential of Strain Contour Density,DSCD)取代DGNS用以识别结构损伤,两者具有相同的物理意义,但DSCD更加有效地保证云图的平滑性和可视性,实验证明DSCD可以有效地识别塑性损伤.研究表明,DGNS(DSCD)可以很好地刻画出结构塑性变形、裂纹损伤特性,剔除由于结构自身不均匀造成的应变集中区域的影响,有效地孤立出损伤区域;DGNS(DSCD)是结构的本征参数,不依赖于外部载荷幅值;相比于传统的应变损伤识别,DGNS(DSCD)具有更加敏感、更早捕捉损伤的优势.
文摘对一砂质泥岩风化土在300,400,500 k Pa及600 k Pa固结围压下,分别进行了原状土样与重塑土样的动三轴试验,得到了该砂质泥岩风化土的阻尼比曲线。试验结果表明,原状土与重塑土在各固结围压试验下,阻尼比随归一化剪应变γ取值的增大而增大,且在γ取值相同时,原状土阻尼比值大于相应重塑土;原状土与重塑土试样的λd~γ关系曲线均随固结围压的升高而逐渐降低,且降低幅度随着γ值的增大而减小,并在γ值接近于1时各关系曲线基本交叉重合;在同等试验条件下,通过经验公式与绘图两种方式求得的最大阻尼比值相差很小,但随着固结围压的增大,两种方式求得的最大阻尼比值相差变大。
基金This work was supported by the National Natural Science Foundation of China (Grnat Nos. 10071091 and 19831010).
文摘This paper studies how the sample rotation method is applied to the case where item nonresponse occurs in surveys. The two cases where the response to the first occasion is complete or incomplete are considered. Using ratio imputation method, the estimators of the current population mean are proposed, which are valid under uniform response regardless of the model and under the ratio model regardless of the response mechanism. Under uniform response, the variances of the proposed estimators are derived. Interestingly, although their expressions are similar, the estimator for the case of incomplete response on the first occasion can have smaller variance than the one for the case of complete response on the first occasion under uniform response. The linearized jackknife variance estimators are also given. These variance estimators prove to be approximately design-unbiased under uniform response. It should be noted that similar property on variance estimators has not been discussed in literature.
