采用无缝试件确定混凝土岩石的断裂韧度

该文建立了由无缝试件确定混凝土和岩石断裂韧度的理论模型及其解析表达式。该模型考虑了混凝土骨料粒径与岩石颗粒尺寸的重要影响,仅需小尺寸无缝试件的峰值荷载,即可直接确定出无尺寸效应的混凝土与岩石的断裂韧度。进一步,进行了含不同裂缝长度的岩石试件的系列断裂试验,研究结果表明:基于该文模型由无缝试件确定的岩石断裂韧度,与采用含1 mm预制浅裂缝试件的确定结果相吻合,与基于回归分析方法由含不同长度预制裂缝试件确定的断裂韧度基本一致。同时,基于所提模型,对其他学者完成的岩石与混凝土无缝与含缝试件的试验成果进行了深入分析,验证了所提模型与方法的合理性及适用性。该文研究为由实验室小尺寸无缝试件确定混凝土与岩石无尺寸效应的断裂韧度提供了新思路。     

面外约束对韧性材料的断裂韧度的影响

与脆性断裂的断裂韧度随厚度的增加而逐渐趋近于一个常数的变化趋势不同,韧性较好材料的弹塑性断裂韧度的特征为:在一定范围内随着厚度的增加,弹塑性断裂韧度逐渐增加。主要研究在保持试件的面内约束(a/w)保持不变的条件下,面外约束(厚度的变化)对同种材料的试件的断裂韧度的影响。首先采用韧性较好的材料进行断裂韧性实验,通过实验得到结果:试件的弹塑性断裂韧度J积分及临界裂纹尖端张开位移CTOD随着试件厚度的增加而线性增加。然后从能量平衡的角度出发,考虑在裂纹扩展过程中的所有的宏观能量耗散机制,根据通过实验验证的假设,化简后,得到最终的结果:一定程度上表征能量耗散的断裂韧度随着厚度的增加而增加,与实验结果吻合得较好。     

Influence of ultraviolet radiation on the low velocity impact response of laminated beams

In this study, an E-glass fiber reinforced epoxy composite laminate is subjected to ultraviolet radiation (UV) for up to 48 h. The intensity of the UV radiation is 1.5 J/cm²/min. A total of 39 specimens are prepared and tested, in which three specimens are used as control and 36 specimens are conditioned. For the conditioned specimens, 18 specimens are exposed to air and 18 specimens are immersed in a water base while subjected to UV radiation. Low velocity impact tests are conducted on the conditioned specimens and control specimens using instrumented drop-weight impact machine. Compression after impact (CAI) tests are conducted using a MTS machine to determine the residual load carrying capacity of the impact damaged specimens. Microscopic observation is conducted to aid in the analysis of the damage mechanism. The test results show that UV radiation alone has a significant effect on reducing the residual load carrying capacity of impact damaged laminated beams. The presence of water enhances the damage effect of UV radiation.     

Q690D高强钢基于连续损伤模型的断裂破坏预测分析

高强钢材在实际钢结构中已经开始逐步得到应用,该文针对Q690D高强度结构钢材,进行了钢材圆棒试样在单调加载和超低周循环加载下的断裂破坏试验,研究了试样裂纹的起始位置,分析了加载制度对试样承载能力和变形能力的影响,并采用扫描电镜观察了试样断口的微观形貌,断口呈现韧窝形式的延性断裂特征。基于Q690D钢材缺口圆棒试样的单调拉伸试验结果,结合有限元分析,标定了钢材的连续损伤模型参数。最后,应用钢材的连续损伤模型,对圆棒试样和带初始间隙试样在不同加载制度下的断裂破坏进行预测分析,得到试样的裂纹起始位置、荷载-位移曲线、断裂位移和疲劳寿命均与试验结果吻合良好。     

Experimental Evaluation of Dynamic Test Methodologies for Assessing the Elastic Constants of Granitic Rocks

Two dynamic test methodologies, the resonance method and the ultrasonic method, are used for assessing the elastic behaviour of granitic rocks. The experimental results obtained using two types of specimens: (i) rectangular beams (parallelepiped specimens); and (ii) cylindrical specimens (rod specimens); show that the resonance method used in this work is more adequate when both Young's modulus and Poisson's ratio are to be determined in these types of rocks. Moreover, it is demonstrated that test results obtained by resonance techniques are less dependent on specimen's geometry, size, and even surface roughness.     

带约束拉杆壁式钢管混凝土柱拟静力试验研究

为避免在多高层钢结构建筑中柱凸出填充墙,提出了设置约束拉杆的截面高宽比为3.0的壁式钢管混凝土柱,通过2个带约束拉杆壁式钢管混凝土柱足尺试件的拟静力试验,研究其抗震性能。试验结果表明:约束拉杆可以有效抑制两侧柱壁局部屈曲;试件破坏模式为柱底四周钢板受压鼓曲、钢板间纵向焊缝撕裂;滞回曲线较为饱满,无明显的捏拢现象;屈服位移角在1/147 rad~1/121 rad,极限位移角在1/51 rad~1/41 rad,位移延性系数在2.61~3.11,承载力退化系数在0.86~1.04,破坏时的等效粘滞阻尼系数在0.3以上。在试验研究的基础上,建立了该类试件的精细化有限元模型,对比可知有限元结果与试验结果吻合较好,验证了模型的正确性,可为此类构件的数值模拟提供参考。研究成果丰富了矩形钢管混凝土组合构件的类型,促进装配式钢结构建筑的发展。     

Measurement and monitoring of microwave reflection and transmission properties of cement-based specimens

The results of measurement and monitoring of reflection and transmission properties of cement-based specimens (blocks of mortar, concrete) obtained by using a simple and an inexpensive measurement system at microwave frequencies (X-band) are presented. Dependencies of the reflection and transmission coefficients on water-to-cement (w/c) ratio, preparing and curing conditions of the specimens are demonstrated. It is shown that the amplitudes of reflection and transmission coefficients, together with thickness of the specimens, determine the complex dielectric permittivity of the hardened cement-based specimens. The expected applications of the results for the determination of physical properties of cement-based materials are discussed. The causes and effects of measurement errors and uncertainties are also discussed.     

Effects of crystal orientation on bendability of aluminum alloy sheet

We investigated the effects of crystal orientation on the bendability of aluminum alloy sheets by experiments using single crystal specimens and finite element analysis using a crystal plasticity model. In the experimental investigation, single crystal specimens having cube and Goss orientations were made from a coarse-grained Al-Mg-Si alloy sheet. The cube single crystal specimens have shown an excellent bendability regardless of the bending direction. Meanwhile, the bendability of the Goss single crystal specimens strongly depended on the bending direction. The finite element analysis results are remarkably consistent with the experimental results.     

High-speed deformation of aluminum by cold rolling

High-speed deformation of aluminum was carried out by use of a high-speed rolling machine, and the results were compared with those of impact compression. The rolled specimens were electro-polished and observed under an electron microscope. As compared with the microstructures observed in the impact compression specimens, the microstructures observed in the rolled specimens showed almost the same tendency as impact compression for vacancy loop formation, but a different tendency for dislocation cell formations. The difference in the results is explained by the variation in strain rate during deformation in rolling. The results indicate that in high-speed rolling, vacancies and dislocations are produced independently during different periods. Specifically, in the high-strain period, during which dislocations are not generated, deformation proceeds without dislocations.     

Cohesive crack modeling of influence of sudden changes in loading rate on concrete fracture

The results of an experimental study of a sudden change in loading rate on the fracture behavior of normal- and high-strength concrete specimens of three different sizes are reported. Geometrically similar three-point bend specimens were subjected to either a sudden 1000-fold increase or a 10-fold decrease of the loading rate. It was observed that for a large increase of the loading rate, the post-peak softening can be reversed to hardening followed by a second peak of the stress-strain diagram. A sudden decrease of the loading rate initially causes, a steeper softening slope of this diagram. The results are similar for normal and high strength concrete specimens. The viscoelastic cohesive crack model with the rate-dependent softening law is used to model the experimental results.     

Effect of shot peening on fatigue limit of surface flawed samples

The effects of shot peening on the fatigue limit of specimens having a semicircular notch of varied surface length, 2a , are investigated. In the case of un-peened specimens, the fatigue limit of specimens having a notch of a = 0.05 mm was equal to that of the un-notched specimens. However, the fatigue limit of a = 0.3 mm was 46% smaller than that of the un-notched specimens. On the contrary, in the case of peened specimens, the fatigue limit of a = 0.2 mm was equal to that of the un-notched specimens and furthermore, that of a = 0.3 mm was only 5% smaller than that of the un-notched specimens. Multiple non-propagating cracks were observed in peened specimens after fatigue testing. The stress intensity factor of the maximum non-propagating crack size corresponded to that of a = 0.2 mm notch. These results indicate that shot peening increases fatigue limit and decreases the likelihood that a surface flaw will result in failure.     

纤维增强复合材料声发射的Felicity效应

本文对正交铺层的层间剪切试样和含表面裂纹试样的玻璃纤维增强复合材料板进行了声发射试验研究,记录了在单向拉伸下声发射的总数,计数率和幅度分布曲线。结果表明,两种试样的声发射幅度分布几乎相似,只是在加载的最后阶段才显示出由纤维破坏而引起的差别。这说明含表面裂纹板的破坏方式主要是以层间剪切破坏为主。两种试样的声发射总数—载荷关系曲线表明:在低载荷水平时,试样显示Kaiser效应,而在高载荷水平时则显示Felicity效应。      

粘弹性阻尼材料力学参数测试实验用双边附加自由结构阻尼试件设计方法研究

悬臂梁弯曲共振法测量粘弹性阻尼材料力学参数的实验中常以双边附加自由结构阻尼试件为实验对象。实验中试件设计的合理程度直接影响实验的测试精度。为了指导试件设计,通过灵敏度分析提出了影响系数的概念,并将其应用于分析试件的厚度比、共振频率比、密度比、损耗因子等参数的测量误差对粘弹性材料力学参数测试精度的影响中,进而对试件的设计提出一些有重要意义的技术规范。最后,基于悬臂梁弯曲共振法获取粘弹性材料力学参数的实验,分析了双边自由阻尼结构试件的适用范围。     

Application of resonant acoustic spectroscopy to asphalt concrete beams for determination of the dynamic modulus

In this paper, a new application of resonant acoustic spectroscopy (RAS) is examined for constructing asphalt concrete mastercurves from seismic testing. The frequency-dependent material properties can be characterized from multiple modes of vibration through the use of RAS. Beam-shaped asphalt specimens are tested at multiple temperatures to determine the resonance frequencies of the specimens. The resonance frequencies are estimated by applying a small load impulse and measuring the resulting acceleration through the specimens. Using RAS, the material properties of the specimens are determined numerically using the measured resonance frequencies. The results presented show that the frequency-dependent dynamic modulus of the asphalt concrete specimens can be characterized using several modes of vibration at each testing temperature.     

Pre-strain effect on fatigue strength characteristics of SUH660 plain specimens

Precipitation particles in precipitation-strengthened materials are considered to be cut by pre-strain treatment, which affects the fatigue strength. In this study, fatigue tests were performed on precipitation-strengthened stainless steel SUH660 to investigate the effect of pre-strain on fatigue crack initiation and propagation characteristics. Fatigue test results showed that pre-strained specimens have a shorter fatigue crack propagation life compared to non-strained specimens; this is the opposite of results observed in carbon steel. The accelerated fatigue crack growth observed in pre-strained specimens was first assumed to be caused by precipitate cutting. A dislocation accumulation model for the fatigue crack tip in the precipitation-strengthened material was then suggested. Buff-polished specimens were also used for the fatigue tests, and the results showed that the work-hardened layer had a significant impact on the fatigue strength and fatigue life.     

Analysis of applicability of small-sized specimens to prediction of temperature dependence of fracture toughness

The paper addresses the use of small-sized specimens of various types, including those with deep (50%) side grooves, for the purpose of fracture toughness prediction. The experimental data for numerous (more than 500) small-sized specimens prepared from materials of various degrees of embrittlement are compared to the test results for full-sized specimens of C(T) type. The concepts of Master Curve and Unified Curve are applied for the processing of experimental data. To handle the test results for small-sized deep-grooved specimens a calculation procedure has been elaborated, which adjusts the calculation method specified in the ASTM Standard E 1921. We provide recommendations of how to use precracked Charpy type deep-grooved (50%) specimens for prediction of a representative temperature dependence of fracture toughness. Translated from Problemy Prochnosti, No. 2, pp. 5–26, March–April, 2009.     

超细纤维增强复合材料 Ⅰ 型层间断裂韧性分析

采用静电纺丝技术制备了厚度约0.1mm的超细纤维无纺布薄膜, 并入层合板中间界面, 固化成型后加工为双悬臂梁(DCB)试样。根据ASTM D5528标准测试了 Ⅰ 型层间断裂韧性。实验结果表明, 增强试样比空白试样的 Ⅰ 型临界应变能释放率(G_{Ⅰ C})提高了约35％。同时采用有限元分析方法研究了含无纺布薄膜试样和空白试样的裂纹扩展过程, 数值结果与实验结果吻合较好, 更好地解释了含无纺布薄膜层合板的层间断裂机理。      

管线钢管屈服强度的稳定性试验

针对目前API Spec 5L-2009《管线钢管规范》中采用冷压平方法测试管线钢管屈服强度的不稳定性问题,采用双肩圆试样拉伸、展平率法板试样拉伸、预拉伸至弹性极限Rp0.02板试样拉伸和预拉伸至弹性极限Rp0.01板试样拉伸四种方法对不同取样方向的管线钢管试样进行了拉伸试验,并对四种方法测得的屈服强度结果进行分析比较。结果表明:双肩圆试样拉伸法由于不是全厚度试样,因而不能真实地反映管线钢管的屈服强度,且试样加工比矩形平板状试样更为繁琐;展平率法板试样拉伸方法由于同一取样角度只计算一个试样的展平率,因而试验结果比较分散,且需要采用滞后环法对试验数据进行处理,工作量较大;预拉伸至弹性极限板试样拉伸方法可以方便、准确地计算出管线钢管的屈服强度,但预拉伸至弹性极限Rp0.02时的非比例伸长延伸率稍大,因此建议采用预拉伸至弹性极限Rp0.01板试样拉伸的方法测试管线钢管的屈服强度。     

Determination of Remnant Residual Stresses in Fracture Toughness Specimens Extracted From Large Components

Abstract: Material fracture toughness data are required to undertake fitness‐for‐service assessments of engineering components containing cracks. Calculations of crack driving force in the component are compared with material fracture toughness values to assess the likelihood of subsequent failure. Experimental measurements of fracture toughness are usually made on small specimens extracted from a larger ‘parent’ component following strict experimental guidelines, formulated to ensure measured toughness values in the fracture specimens are appropriate for use in the full‐size component. Implicit in this procedure is the assumption that the extracted fracture specimens contain no residual stresses, with any residual stresses in the full‐size component being accounted for in the crack driving force calculation. This paper considers a recent conjecture within the structural integrity community that the extracted fracture specimens may themselves contain a residual stress field which may influence measurements of fracture toughness. This could potentially lead to a degree of ‘double accounting’, i.e. the effect of residual stresses may be included in both the material toughness and the crack driving force. This, in turn, could lead to unnecessary conservatism in safety assessments. To explore this conjecture, the results of numerical modelling and neutron diffraction measurements of residual stresses in fracture specimens extracted from two different welded parent components are presented. One of the components is significantly larger than the extracted specimens, with the other being marginally larger than the extracted specimens. Results confirm the intuitive expectation that the residual stresses in specimens extracted from much larger components are negligible, whereas if the dimensions of the extracted specimens are comparable with the larger component then significant residual stresses may remain.     

Triaxial moisture-controlled creep tests of hardened cement paste at high temperature

Creep tests of sealed specimens as well as unsealed specimens exposed to either pressurized water or air have been conducted at various constant temperatures from 100 to 300°C and durations up to 24 hours. Apart from the need to study multiaxial deformations, the triaxial loading is required to prevent moisture escape from the sealed specimen. The specimens are sufficiently small to achieve uniform temperature and moisture content in less than 3 minutes. Overcoming various difficulties, a triaxial testing device was built. Specimens heated at the time of loading and before loading were tested. Some new and unexpected results are found: (a) unsealed specimens exposed to pressurized hot water creep much less than sealed specimens and the difference in creep rate becomes greater after a few hours; (b) deviatoric creep of sealed specimens is at high temperatures much larger than the volumetric creep; (c) at 100°C, unsealed specimens loaded at the time of heating creep much more than sealed specimens, but at 200°C the situation is reversed; (d) the shrinkage and the additional creep due to drying exhibit great delay after drying. Furthermore, with a several hour increase of the period of preheating before loading, creep is substantially reduced, and the axial creep appears to depend nearly linearly on the axial stress superimposed on a hydrostatic pressure. The results appear to have some new implications for the accident analysis of reactor vessels as well as fire resistance.