DETERMINATION OF CREEP PARAMETERS FROM INDENTATION CREEP EXPERIMENTS

The possibilities of determining creep parameters for a simple Norton law material are explored from indentation creep testing. Using creep finite element analysis the creep indentation test technique is analyzed in terms of indentation rates at constant loads. Emphasis is placed on the relationships between the steady creep behavior of indentation systems and the creep property of the indented materials. The role of indenter geometry, size effects and macroscopic constraints is explicitly considered on indentation creep experiments. The influence of macroscopic constraints from the material systems becomes important when the size of the indenter is of the same order of magnitude as the size of the testing material. Two methods have been presented to assess the creep property of the indented material from the indentation experimental results on the single-phase-material and two-phase-material systems. The results contribute to a better mechanical understanding and extending the application of indentation creep testing.

DETERMINATION OF CREEP PROPERTIES OF THERMAL BARRIER COATING(TBC)SYSTEMS FROM THE INDENTATION CREEP TESTING WITH ROUND FLAT INDENTERS

Indentation creep behavior with cylindrical flat indenters on the thermal barrier coating (TBC) was studied by finite element method (FEM). On ike constant applied indentation creep stress, there is a steady creep rate for each case studied for different creep properties of the TBC system. The steady creep depth rate depends on the applied indentation creep stress and size of the indenters as well as the creep properties of the bond coat of the TBC and the substrate. The possibilities to determine the creep properties of a thermal barrier system from indention creep testing were discussed. As an example, with two different size indenters, the creep properties of bond coat of the TBC system can be derived by an inverse FEM method. This study not only provides a numerical method to obtain the creep properties of the TBC system, but also extends the application of indentation creep method with cylindrical flat indenters.

犬离体心室肌粘弹性特性实验研究

目的:通过生物力学实验,探讨印压检测心肌粘弹性的可行性及心肌梗死后心肌粘弹性改变。方法:通过结扎前降支方法建立犬陈旧性心肌梗死模型(N,8只)。应用印压实验装置,对正常和陈旧性梗死离体心室肌试件进行蠕变试验。结果:正常心肌组粘弹性特征参数:E164±9kPa、E2565±148、E∞57±7kPa、η1.7±0.9×105kPa.s;陈旧性梗死组:E1130±30kPa、E21076±466kPa、E∞113±27kPa、η2.8±1.0×105kPa.s。两组比较存在显著差异(P<0.05)。结论:印压可检测心室肌粘弹性特性。心肌梗死后弹性模量和粘性阻力都增高,心肌顺应性下降。应用基于导管超声的印压方法可望在活体研究不同病理状态下舒张期心肌粘弹性特性。

Determination of multiaxial stress rupture criteria for creeping materials:A critical analysis of different approaches

Materials in engineering applications are rarely uniaxially-loaded.In reality,failures under multiaxial loading has been widely observed in engineering structures.The life prediction of a component under multiaxial stresses has long been a challenging issue,particularly for high temperature applications.To distinguish the mode of failure ranging from a maximum principal stress intergranular damage to von Mises effective stress rupture mode a multiaxial stress rupture criterion(MSRC)was originally proposed by Sdobyrev and then Hayhurst and Leckie(SHL MSRC).A multiaxial-factor,α,was developed as a result which was intended to be a material constant and differentiates the bias of the MSRC between maxi-mum principal stress and effective stress.The success of the SHL MSRC relies on accurately calibrating the value ofαto quantify the multiaxial response of the material/geometry combination.To find a more suitable approach for determining MSRC,the applicability of different methods are evaluated.Given that the resulting analysis of the various approaches can be affected by the creep failure mechanism,princi-ples in the determination of MSRC with and without using continuum damage mechanics approaches are recommended.The viability of uniaxial material parameters in correlating withαthrough the analysis of available data in literature is also presented.It is found that the increase of the uniaxial creep dam-age tolerance parameterλis accompanied bythe decreaseof theα-value,whichimplies thatthe creep ductility plays an important role in affecting the multiaxial rupture behavior of materials.

Prediction of tensile power law creep constants from compression and bend data for ZrB_(2)-20 vol% SiC composites at 1800℃

Here we consider our four-point flexure and compression creep results obtained under Ar protection at 1800℃ to predict the tensile creep behavior of a ZrB_(2)-20 vol%SiC ultra-high temperature ceramic.Assuming power law creep,and based on four-point bend data,we estimated the uniaxial creep parameters using an analytical method present in the literature.Both predicted and experimental compressive stress exponents were found to be in excellent agreement,1.85 and 1.76 respectively,while observation of the microstructure suggested a combination of diffusion and grain boundary sliding creep mechanisms in compression.Along with the microstructural evidence associated with the tensile regions of the flexure specimens,the predicted tensile stress exponent of 2.61 exceeds the measured flexural value of 2.2.We assert an increasing role of cavitation to the creep strain in pure tension.This cavitation component adds to the dominant grain boundary sliding mechanism as described below and elsewhere for flexural creep.