界面摩擦对热障涂层压痕响应的影响

以四层结构的热胀涂层薄膜／基体材料为基础，利用有限元建立模型分析了界面摩擦对压痕响应的影响。分析表明，考虑界面摩擦情况下，随着摩擦系数的增大，压痕响应越趋近于完全结合时的压痕响应；水平方向的最大位移值随着摩擦系数的增大也逐渐增大。界面光滑接触时接触界面力学响应可以忽略。     

C/SiC复合材料纳米压痕有限元仿真

利用ABAQUS软件对C/SiC复合材料的纳米压痕实验进行有限元分析,引入内聚力模型来组建材料的本构模型。在细观力学层面上利用Oliver-Pharr方法对不同位置压痕点的载荷位移曲线进行分析,研究复合材料各组分原位力学性能的影响因素,揭示界面强度、界面厚度对纳米压痕过程中载荷位移曲线、材料硬度、弹性模量的影响规律。该仿真为C/SiC复合材料的工程应用、加工去除机理的研究及纳米压痕实验的参数优化提供理论依据与高效方法。     

封严涂层性能评定的新方法

利用研制成功的新型电子冲击刮削试验发现，可测试航空发动机封严涂层的冲击载荷－位移曲线，由此得出的屈服载荷点、最大载荷点、冲击刮削韧性等参数可用于定量综合评估封严涂层的可磨耗性、抗冲蚀性以及涂层与基体结合强度。     

维氏压痕对常压固相烧结碳化硅陶瓷材料力学性能的影响

为了研究维氏压痕裂纹对常压固相烧结碳化硅陶瓷(SSiC)材料力学性能的影响, 通过扫描电镜观察了0.1~100 N的压痕载荷下产生的表面裂纹及裂纹剖面的状况, 并测试了相应载荷下的力学性质, 探讨了压痕法测量SSiC材料硬度、韧性等力学性质的适当压力载荷. 结果表明, SSiC材料压痕裂纹起始的临界压力载荷介于0.1~0.2 N; 当压痕载荷小于0.5 N时, 裂纹尺寸小于5 μm, SSiC材料的平均弯曲强度受影响程度较小. 此外, 当压痕载荷为10 N以上时, 压痕法测得的维氏硬度值趋近定值, 且所得到的裂纹是半圆形裂纹, 因此, 10 N为采用压痕法准确测量SSiC材料硬度及韧性的最低压痕载荷值.     

平头压痕试验确定薄膜弹塑性参数的研究

本文研究用平头压痕试验确定薄膜-基体材料中薄膜材料弹塑性参数的可行性,重点研究了薄膜材料的屈服强度和硬化模量的确定方法.利用有限元(FEM)进行了模拟计算,给出了平头压痕下典型的等应力分布,以及载荷-压入深度的曲线.通过对载荷-压入深度曲线的研究,给出了通过平头压痕试验确定薄膜屈服强度和薄膜硬化模量的方法.     

平头压头下基体对压痕规律的影响研究

本文通过对软薄膜/硬基体两相材料体系的平头压痕弹塑性模拟.重点研究了平压头压入过程中,不同屈服强度比(软薄膜屈服强度与硬基体屈服强度之比)以及不同压头尺寸下硬基体对压痕规律的影响.研究发现硬基体对压痕规律的影响与屈服强度比近似满足线性关系,且这种线性关系不随压头尺寸的改变而改变,相同压头半径下,屈服强度比越大,影响就越明显;相同屈服强度比下,压头半径越大,影响就越小.研究还发现压头压入过程中,材料的堆积对压入深度没有影响.     

磷酸二氢钾(KDP)晶体纳米压痕过程的有限元分析

为了求得KDP晶体的应力-应变曲线以及材料的屈服应力,基于圣维南定理和实验得到的材料性能参数建立了KDP晶体的压痕过程仿真模型,利用ABAQUS有限元分析软件对KDP晶体压痕过程进行了有限元仿真,得到了KDP晶体的载荷-位移曲线和加/卸载过程中的等效应力变化规律.仿真结果表明:加载过程中最大应力集中在压头尖角处,卸载后最大应力分布在压头棱边所留下的压痕处,KDP晶体材料的屈服应力为120MPa.     

化学强化铝硅酸盐玻璃表面微观力学行为的有限元模拟

采用有限元方法研究化学强化铝硅酸盐玻璃的表面硬度、模量等微观力学行为,并与纳米压痕实验结果进行对照.结果表明,通过有限元计算获得的纳米压痕载荷-位移曲线与实验结果吻合度良好,根据有限元模拟得到的载荷-位移曲线,采用Oliver和Pharr方法计算出的硬度和杨氏模量值与实验得到的硬度和杨氏模量值非常接近.模拟过程中采用了根据Larsson塑性公式计算出的铝硅酸盐玻璃屈服强度与塑性区域的应力应变关系,模拟结果与实验结果吻合度高说明根据Larsson塑性公式获得的铝硅酸盐玻璃塑性参数准确度较高.根据有限元模拟得到的应力分布情况,分析了铝硅酸盐玻璃在化学强化前后的弹塑性变形行为,结果表明化学强化产生的表面压应力层对铝硅酸盐玻璃的弹性区影响较大,对塑性区影响较小.     

100nm厚铜薄膜的拉伸性能

以聚酰亚胺为基体制备了厚度为100nm的金属铜薄膜,利用基体的高弹性变形测定了铜薄膜的屈服应力,并研究了铜薄膜的形变与断裂行为．结果表明：使用铜薄膜／聚酰亚胺复合体能够测量出铜薄膜的屈服强度,厚度100nm的铜薄膜的屈服强度明显高于厚铜膜的屈服强度,厚度100nm铜薄膜的断裂为Ⅰ型沿晶断裂．超薄铜薄膜较高的屈服强度归因于小尺度材料中纳米量级的薄膜厚度和晶粒对位错运动的约束作用．     

火焰喷涂尼龙/纳米TiO2复合涂层性能的研究

为了探讨纳米TiO2对火焰喷涂尼龙1010涂层力学及抗老化性能的影响,利用电子拉力机对火焰喷涂尼龙1010/纳米TiO2复合涂层的力学性能及耐老化性能进行了测试.结果表明,当复合涂层配比为m(PA1010):m(n-TiO2)=100.0:0.5时,复合涂层综合性能较佳,涂层自拉伸强度为43.10 MPa,涂层与基体结合强度为40.23 MPa;涂层经240 h紫外线老化后,强度保持率分别为97.0%和87.2%.纳米TiO2能够显著提高涂层力学性能和抗老化性能.     

On the effect of substrate properties on the indentation behaviour of coated systems

The indentation behaviour of an elastoplastic coating–substrate system is investigated using a combination of dimensional and finite element analyses. Scaling functions relating the indentation load–depth curves to coating and substrate mechanical properties are given. Based on these scaling functions, the indentation behaviour of various coated systems is examined. The critical indentation depth to coating thickness ratio below which the substrate material has a negligible effect on the indentation response of the coated system is identified for various generic coating–substrate systems. Such ratio is given in terms of the yield strength and Young’s modulus of the coating and substrate, i.e. σ_yc/σ_ys and E_c/E_s. The results of parametric studies revealed that the commonly used rule that the maximum indentation depth should be less than 10% of the coating thickness, is applicable only when σ_yc/σ_ys<10. However, indentation experiments should be carried out up to a maximum depth of 5% of the film thickness to avoid any influence from the substrate when σ_yc/σ_ys≥10 and E_c/E_s>0.1.     

Effects of TGO Roughness on Indentation Response of Thermal Barrier Coatings

In this paper, an axisymmetric indentation model is set up to calculate the effects of the roughness of the thermally grown oxide (TGO) layer, which was modeled as a sinusoidal wave, on the indentation response of the thermal barrier coatings. It is found that the amplitude, wavelength, and thickness of the thermally grown oxide layer have obvious influences on the indentation response, while the effect of the indenter position can be neglected. In the top coating layer, residual stress mainly occurs below the indenter and around the nearest two peaks of the thermally grown oxide layer to the indenter. Only when the indentation depth is less than 10% of the thickness of the top coating layer, the influence of TGO roughness on the force versus displacement curves of the indentation can be ignored. Correlating this work with the experimental data from indentation test may lead to improved characterization of the mechanical properties of TBC systems.     

A dual conical indentation technique based on FEA solutions for property evaluation

The sharp indenters such as Berkovich and conical indenters have a geometrical self-similarity so that we can obtain only one parameter from an indentation loading curve, which makes different materials have the same load-displacement relation. Most studies to evaluate elastic-plastic properties by using the geometrical self-similar indenter have therefore tried to use dual/plural indentation techniques, on the basis of the concept of representative strain/stress varying with the indenter angle. However, any suggested representative concept is not universally operative for real materials. In this work, we suggest a method of material property evaluation without using the concept of representative strain. We begin the work by studying the characteristics of load-depth curves of conical indenters via finite element (FE) method. From FE analyses of dual-conical indentation, we investigate the relationships between indentation parameters and load-depth curves. The projected contact diameter is expressed as a function of the indenter angle, tip-radius, and material properties, which allows us to simply predict the elastic modulus. Two mapping functions for two indenter angles (45° and 70.3°) are presented to find the two unknowns (yield strain and strain-hardening exponent) via dual indentation technique. The method provides elastic modulus, yield strength and strain-hardening exponent with an average error of less than 5%. The method is valid for any elastically deforming indenters. We also discuss the sensitivity of measured properties to the load-displacement curve variation, and the difference between conical and Berkovich indenters.     

3-D Modeling of nanoindentation experiment on a coating-substrate system

The validity of indentation tests for the characterization of the mechanical properties of coatings relies greatly on the indentation depth. Deep indentation concludes to unreliable results due to the substrate effect on the measured properties. At shallow depths the size effect can also be an important error factor. The purpose of the present study is the determination of the critical ratio of coating thickness to indentation depth, up to which the substrate properties have a negligible effect on the force versus indentation depth curve and thus on determined mechanical properties of the coating. The analysis required, was conducted using finite element method. A spherical (ball) indenter was used and a three dimensional model of the indenter/coating-substrate system was applied. The effect of the coating to substrate Yield strength ratio, on the critical coating thickness to indentation depth ratio, was investigated for three different coating to substrate Young's modulus ratios. The results of this work provide considerable insight for the determination of the confidence indentation depth during micro-indentation for layered systems with different properties.     

弹塑性比例叠加的锥压入半解析模型与试验方法

基于能量密度等效,考虑圆锥压入(锥压入)过程中线性律纯弹性和幂律纯塑性的应变能比例叠加,提出了弹塑性应变能比例叠加的锥压入载荷-位移模型(Load vs. Displacement Model based on Proportional Superposition of elastoplastic-energy under conical indenting,LDM-PS),进而提出了获取材料Ramberg-Osgood律(R-O律)应力-应变关系的锥压入试验方法。针对80种设定材料,通过LDM-PS预测的载荷-位移曲线(正向预测)与有限元分析结果密切吻合,并且以有限元分析(Finite Element Analysis,FEA)所得载荷-位移曲线作为试验模拟曲线,采用两种锥角圆锥压头分别对平滑材料表面进行两次单锥压入加载(双锥压入),可通过对双锥压入的两个载荷-位移曲线的加载阶段按抛物律(Kick律)回归可实现R-O律参数的求解。由LDM-PS预测的R-O律应力-应变关系曲线(反向预测)与FEA的条件关系曲线密切吻合;针对8种金属材料完成了双锥压入试验,通过锥压入试验新方法预测的应力...     

基于纳米压痕及区间优化的Al 2024-T3铝合金本构参数识别研究

纳米压痕实验由于试样准备简便、使用范围广等优势,在材料力学测试领域得到了广泛关注。本研究建立了一种考虑纳米压痕实验不确定性的Al 2024-T3铝合金材料塑性参数识别方法。首先,针对Al 2024-T3铝合金开展了纳米压痕实验,获取了载荷-位移曲线,由于材料存在不均匀性,所以实验曲线存在不确定性。基于超参数优化的人工神经网络,建立了材料性能参数与压痕响应加载曲线的关联。基于区间优化理论,引入压痕实验曲线的不确定性,以压痕实验曲线加载曲率为不确定性量,提出了基于双层嵌套遗传算法的材料参数反分析识别区间优化模型,并进行了参数识别反问题的求解。该方法的优势在于能够考虑到实验测量的不确定性,识别结果更可信。所建立方法的有效性在Al 2024-T3铝合金塑性参数识别中得到了验证,识别误差分别为：屈服应力-0.87%,硬化指数2.76%。该识别方法可用于小尺寸试样局部力学性能的检测领域。     

设置耗能壳板的高强钢圆管桥墩轴压试验研究

为探讨设置耗能壳板的新型高强钢圆管桥墩的受力机理,对4组共8个新型Q460高强钢圆管桥墩试件开展相应的轴压试验研究,获得各试件破坏的荷载-位移曲线、荷载-应变曲线.通过对比分析轴压作用下试件的破坏模式、承载能力、延性性能等力学特征,探讨设置耗能壳板后高强钢圆管桥墩承载能力和变形性能的变化规律.试验结果表明,新型高强钢圆...     

On the factors affecting the critical indenter penetration for measurement of coating hardness

The nanoindentation test is the only viable approach to assess the properties of very thin coatings (<1 μm) since it can operate at the required scale and provides a fingerprint of the indentation response of the coating/substrate system. To measure the hardness of the coating only it is traditionally assumed that, as a rule-of-thumb, when the relative indentation depth (RID, i.e. the penetration divided by the coating thickness) is less than 0.1, the substrate will not affect the measured hardness of the coating. However, it is found that this rule is too strict for some and too loose for other coated systems. In this paper we present a comprehensive investigation of the factors influencing the critical relative indentation depth (CRID) using finite element simulation. The CRID is very sensitive to tip radius for soft coatings on hard substrates. For most coating/substrate combinations at reasonable penetration depths the 0.1 rule-of-thumb is a safe estimate. It is shown that the elastic property mismatch between coating and substrate also has an important effect on the measured hardness and this means that the Oliver and Pharr method generally used to extract hardness from nanoindentation data may give inaccurate results in coating/substrate systems with significant elastic mismatch.     

Mean stress effects on crack propagation rate and crack closure in 7050-T76 aluminum alloy

High strength aluminum alloys that are cyclically loaded in tension are known to show increased fatigue crack propagation rates when the mean stress is increased. It has been suggested that this increase in growth rate may be due to a lower crack closure stress. A smaller closure stress results in a higher effective fatigue crack propagation stress. Measurements of the closure stress were made using an extensometer placed across the crack in order to study the effects of crack closure on the fatigue crack growth rate. The closure stress was determined from the change in slope of load-displacement curves. The growth rate data at various mean stresses were plotted as a function of effective stress intensity factor. In all tests the cyclic stress amplitude was 80 MPa. For specimens run at mean stress levels of 44 MPa and 70 MPa the crack faces were found to close, but on increasing the mean stress to above 120 MPa no closure was observed. The growth rate data from specimens run at mean stresses from 44 MPa to 120 MPa coincided when plotted vs effective stress intensity factor but that crack growth rates for a mean stress of 226 MPa were higher than those obtained at lower mean stresses. We concluded that effects other than crack closure also influence the crack growth rate. Much greater scatter was found in the fatigue crack growth rates of our thicker specimens (5.6mm thick) than in our thinner specimens (2.5 mm thick). Indirect measurements of the length over which the crack faces were closed showed that the length closed decreased sharply with increasing mean stress. At a mean stress of 44 MPa the length closed at minimum stress was several millimeters, while at 70 MPa it had been reduced to less than 1mm.