Failure properties of a PBAN propellant in multiaxial stress fields

Experimental data have been obtained which duplicate the stress and strain field in a solid-propellant rocket-motor grain under pressurization. Development of a hollow ellipsoidal specimen has made possible the acquisition of these data. This test also allows a more thorough evaluation of the failure mechanism in multiaxial stress and strain fields. Failure data were obtained in the tensile-tensile-compressive (++?) stress and biaxial tensile-compressive (+?) strain octants by applying pressure to the internal surface of an ellipsoidal specimen. Data were obtained in the triaxial compressive (???) stress and biaxial tensile-compressive (+?) strain octants by simultaneous application of pressure to both internal and external surfaces of the ellipsoidal specimen and a gradual reduction of external pressure until failure. A stress analysis of the specimen is presented and data-reduction techniques are discussed. Data obtained with a hollow spherical specimen and with the newly developed hollow ellipsoidal specimen are compared. These data are obtained in the same stress and strain octants; however, the relative magnitudes of the parameters and, therefore, the positions within the stress and strain octants are different. This results in a change in the magnitudes of the failure parameter. Data from uniaxial specimens of the same propellant also are presented.     

An Integrated Approach to the Separation of Principal Surface Stresses Using Combined Thermo-Photo-Elasticity

A new instrument capable of the full-field separation of principal stresses on the surface of a component is presented. The instrument combines the techniques of thermoelastic stress analysis and reflection photoelasticity in a single optical head, permitting the simultaneous capture of both data from the same point of view. A single strain witness coating is employed for the acquisition of both the thermoelastic and photoelastic data, which is both birefringent under applied stress conditions and opaque at the infrared wavelengths to which the thermoelastic analysis system is sensitive. This enables the combined technique to be performed continuously from the same surface during loading. The performance of the new instrument is validated in the analysis of a classical laboratory specimen of known geometry. Separated stress data from the experiment is compared to simulated data, demonstrating that the accuracy of the stress separation technique is comparable to that of the individual thermoelastic and photoelastic techniques, and it is concluded that combined thermo-photo-elasticity is a powerful tool for the experimental separation of principal surface stresses.     

Specimen Size and Strain Rate Effects on the Compressive Behavior of Concrete

Three high-performance concrete (HPC) materials with different specimen geometries were characterized using Kolsky compression bar techniques to study the strain rate and specimen size effects on their uniaxial compressive strength. A large-diameter Kolsky bar and recently established annular pulse shaping technique were used to achieve dynamic stress equilibrium and constant strain-rate deformation in the experiments. A complimentary effort was conducted using a 19-mm-diameter Kolsky compression bar to understand the strain rate and specimen size effects on failure strength and dynamic increase factor (DIF) for concrete. It was found that, for all three concrete materials investigated, the failure strength is highly dependent on the specimen geometry, however such a relationship is not apparent for the DIF. The DIF observed in this study shows significantly lower values compared to historical data, which may indicate the importance of well-controlled dynamic testing conditions on the accuracy and validity of experimental results for concrete materials.     

On-Chip Mechanical Characterization using an Electro-thermo-mechanical Actuator

A new micro-system for the on-chip mechanical characterization of thin polysilicon films was designed, fabricated and tested. The device contains a micro electro-thermo mechanical actuator which is able to load a specimen until rupture in purely tensile conditions. The elongation of the specimen is measured during the test through the capacitance variation of a set of parallel plate capacitors, while the force in the specimen can be computed starting from the applied voltage. Through a unique loading cycle in which the voltage is first increased until specimen rupture and then decreased, the new device allows for the determination of the specimen elastic stiffness and nominal tensile strength. The obtained experimental results were compared with values previously obtained by means of different on-chip test devices. Multi-physics FE simulations were performed for additional comparison with analytical formulae used in the data reduction procedure and with the obtained experimental results.     

接触应力对轮轨材料滚动摩擦磨损性能影响

利用MMS-2A型微机控制摩擦磨损试验机研究了接触应力对轮轨材料的滚动摩擦磨损性能影响.结果表明：随接触应力的增加,滚动摩擦系数呈增加趋势,车轮和钢轨试样磨损加剧;相同接触应力水平下,车轮试样磨损量大于钢轨试样,表面损伤严重;随接触应力的增加,车轮试样表面从犁沟且轻微剥落向严重剥落损伤转变,钢轨试样表面损伤主要表现为犁沟效应并伴随有剥落现象,但相比车轮试样的剥离损伤要轻微.     

Analysis of dynamic stress intensity factors of three-point bend specimen containing crack

A new formula is obtained to calculate dynamic stress intensity factors of the three-point bending specimen containing a single edge crack in this study. Firstly, the weight function for three-point bending specimen containing a single edge crack is derived from a general weight function form and two reference stress intensity factors, the coefficients of the weight function are given. Secondly, the history and distribution of dynamic stresses in uncracked three-point bending specimen are derived based on the vibration theory. Finally, the dynamic stress intensity factors equations for three-pointing specimen with a single edge crack subjected to impact loadings are obtained by the weight function method. The obtained formula is verified by the comparison with the numerical results of the finite element method (FEM). Good agreements have been achieved. The law of dynamic stress intensity factors of the three-point bending specimen under impact loadings varing with crack depths and loading rates is studied.     

Analysis of dynamic stress intensity factors of three-point bend specimen containing crack

A new formula is obtained to calculate dynamic stress intensity factors of the three-point bending specimen containing a single edge crack in this study. Firstly, the weight function for three-point bending specimen containing a single edge crack is derived from a general weight function form and two reference stress intensity factors, the coefficients of the weight function are given. Secondly, the history and distribution of dynamic stresses in uncracked three-point bending specimen are derived based on the vibration theory. Finally, the dynamic stress intensity factors equations for three-pointing specimen with a single edge crack subjected to impact loadings are obtained by the weight function method. The obtained formula is verified by the comparison with the numerical results of the finite element method (FEM). Good agreements have been achieved. The law of dynamic stress intensity factors of the three-point bending specimen under impact loadings varing with crack depths and loading rates is studied.     

一种绸布的冲击拉伸性能实验

采用改进的分离式Hopkinson拉杆测试技术对低强度材料进行动态拉伸。该装置采用铝杆作为透射杆,并且透射杆采用半导体应变片。针对一种芳纶绸布材料,进行了试样两端应力平衡、实验速度、半导体应变片与电阻应变片测量的验证,获得了不同速度下被测材料的本构曲线,并通过高速照相机清楚地观察了试样的整个破坏过程。实验结果表明,设计的实验方法行之有效。     

Specimen-Bar Impedance Mismatch Effects on Equilibrium and Rate Constancy for Kolsky Bar Experiments

During a Kolsky bar, also known as a split Hopkinson pressure bar (SHPB), experiment, stress equilibrium and strain rate constancy conditions directly contribute to the measurement quality for rate-sensitive materials. A Kolsky bar specimen is initially at rest, and then gradually accelerated to a desired rate. Stress equilibrium is incrementally achieved by multiple stress pulse reflections inside the specimen to reach the desired mean stress. The critical time to achieve constant strain rate and equilibrium stress depends on the impedance mismatch between the bars and the specimen. This paper examined this critical time based on using linear elastic specimens under uniaxial compression. In the first part, the critical time is experimentally measured for PMMA specimens loaded by aluminum, titanium, and steel bars using linear ramp incident pulses. The results show that increasing impedance mismatch increases the time to reach a constant rate, while the time to satisfy equilibrium remains nearly the same. In the second part, optimal bilinear-shaped incident loadings were evaluated and shown to achieve both conditions faster than linear loadings. The time to satisfy both conditions was mapped via simulation using various bilinear pulses over a wide range of impedance mismatches. The analysis shows bilinear loadings with initial rise time between 1.75 and 2.15 transits in the sample require minimum time to equilibrium. There exists an optimum region of bilinear loadings that can reduce the time to reach constant rate. Within such region, the bilinear slope ratio can be approximated to be a reciprocal function of initial rise time.     

Research for the strain energy release rate of complex cracks by using point-by-point closed extrapolation approach

IntroductionTheresearchforcalculatingstrainenergyreleaserateisaveryimportanttaskforsolvingfracturemechanicsproblems.Inthepreviouscalculationforenergyreleaserate,externalforceworkwassubtractedbystrainenergy ,whichisthemethodoffreeenergy[1],thenodalforcesandnodaldisplacementsbetweentopandbottomsurfacesofcrackwasalsousedtocalculateenergyreleaserate[2 - 5 ].Theyarebothapproximatecalculations.Theformeriscomplex ,althoughtherearesomeadvisablefeaturesindefinition ,andawholecalculationforstrainenergy …     

Yield behavior of photoplastic materials

A mixture of flexible and rigid polyester resins has been used previously by Morris and Riley¹⁶ and by Zachary and Riley⁹ to model plastic deformations. The last of these papers furnished mechanical and optical properties under uniaxial tension and compression for several different mixture ratios of the polyester resins and also presents some results under multiaxial-stress conditions from thin cylinders under internal pressure. In a recent paper, Burger, Gomide and Scott¹⁴ used the rigid polyester resin at elevated temperature to model plastic deformations in upset rings; the behavior of the rigid polyester was verified with diametrically compressed disks and uniaxial-compression specimens. A very important similitude requirement for model to prototype scaling in photoplasticity work is that the macroscopic yield behavior of model and prototype materials must be the same. Thus, not only uniaxial tension and compression properties must be examined, but also yield properties under multiaxial-stress states have to be determined. The purpose of this paper is to provide additional information on the yield behavior of polyester mixtures which appear suitable for model studies of manufacturing methods such as rolling and extruding. For these processes, mixture ratio, test temperature and strain rate can be used to control the shape of the stress-strain curve and the yield behavior. The experimental procedure used to determine the initial yield locus of the photoplastic materials employed a new specimen geometry proposed by Arcan, Hashin and Voloshin¹⁸ which produces uniform biaxial-stress fields of opposite sign in one section of the specimen. Both polycarbonate and polyester materials were evaluated using this procedure and results are compared with those available in the technical literature.     

Experimental study of thickness-tapered unidirectional composite compression specimens

Various untabbed, thickness-tapered compression specimen geometries were studied experimentally. Both shear-loaded and end-loaded compression test methods were used to measure the strength and stiffness of carbon/epoxy and glass/epoxy unidirectional composite material systems. The compressive strength was found to be strongly dependent on specimen geometry, loading conditions, and the cracks that initiated in the taper regions and propagated to various lengths prior to catastrophic failure. A specific thickness-tapered specimen geometry that produced compressive strengths significantly higher than conventional specimen configurations resulted from the experimental optimization process. A method of determining compressive strains, and thus the compressive modulus, using this same specimen geometry and strain gages was also demonstrated.     

A Kolsky Torsion Bar Technique for Characterization of Dynamic Shear Response of Soft Materials

A novel Kolsky torsion bar technique is developed and successfully utilized to characterize the high strain rate shear response of a rate-independent end-linked polydimethylsiloxane (PDMS) gel rubber with a shear modulus of about10 KPa. The results show that the specimen deforms uniformly under constant strain rate and the measured dynamic shear modulus follows reasonably well the trend determined by dynamic mechanical analysis (DMA) at lower strain rates. For comparison, Kolsky compression bar experiments are also performed on the same gel material with annular disk specimens. The dynamic moduli obtained from compression experiments, however, are an order of magnitude higher than those obtained by the torsional technique, due to the pressure caused by the radial inertia and end constraints.     

基于数字图像相关技术的岩样破坏实验的应用研究

为了解喀喇昆仑公路(中国段)沿线岩石裂纹演化及受力后的力学性能(位移和应变),采用数字图像相关技术,对三点弯曲加载条件下的灰白石英粉砂岩和片麻岩试件在万能试验机上进行了破坏实验,得到了外力作用下裂纹生成、扩展等演化过程及两种岩样在加载直至破坏过程中的位移场和平均应变曲线。实验结果表明,在相同条件下,灰白石英粉砂岩比片麻岩更易脆断,片麻岩在受力断裂破坏时出现了多条裂纹,导致试件加剧断裂,研究成果为了解这一地域的岩石强度及破坏规律提供了实验依据。     

In-situ tensile testing of nano-scale specimens in SEM and TEM

We present a new experimental method for the mechanical characterization of freestanding thin films with thickness on the order of nanometers to micrometers. The method allows, for the first time, in-situ SEM and TEM observation of materials response under uniaxial tension, with measurements of both stresses and strains under a wide variety of environmental conditions such as temperature and humidity. The materials that can be tested include metals, dielectrics, and multi-layer composites that can be deposited/grown on a silicon substrate. The method involves lithography and bulk micromachining techniques to pattern the specimen of desired geometry, release the specimen from the substrate, and co-fabricate a force sensor with the specimen. Co-fabrication provides perfect alignment and gripping. The tensile testing fits an existing TEM straining stage, and a SEM stage. We demonstrate the proposed methodology by fabricating a 200 nm thick, 23.5 μm wide, and 185 μm long freestanding sputter deposited aluminum specimen. The testing was done in-situ inside an environmental SEM chamber. The stress-strain diagram of the specimen shows a linear elastic regime up to the yield stress σ _y MPa, with an elastic modulusE=74.6 GPa.     

冲击荷载下内衬八边形钢管空心钢筋混凝土柱的动力响应

以一种内衬八边形钢管空心钢筋混凝土柱为研究对象,在自主研发的无导轨式大型落锤试验机上,完成了3种不同边界条件、2种不同冲击高度以及2种不同轴压比的侧向冲击实验。研究了不同冲击高度、边界条件和轴压比下试件的破坏形态、冲击力时程曲线和跨中位移时程曲线,并选取一根普通空心钢筋混凝土试件与同条件的内衬钢管空心钢筋混凝土试件进行了对比。结果表明:用内衬钢管替代内置钢筋笼使得构件的抗冲击性能明显优于相应的普通空心钢筋混凝土构件,且在轴压比不超过0.3的范围内,轴压比的改变对冲击结束后本内衬八边形钢管空心钢筋混凝土试件的残余挠度影响不大。     

Biaxial testing of composite tubes

The development of a tubular test specimen for the characterization of advanced composite-material lamina and laminate mechanical properties is presented. The specimen design and supporting analyses are given for a practical tubular specimen that minimizes end effects. Test results for simplified loading conditions are shown to be in good agreement with values obtained in other tests. Failure strains in laminates are higher than those obtained in comparable flat-coupon tests. The results obtained indicate that the specimen and apparatus are useful for obtaining response properties and failure values under general in-plane loading.     

岩石失稳破裂的变形突跳研究

针对在非刚性加载条件下岩石失稳破裂过程中变形突跳的特性, 根据时间的不可逆性, 研究了岩石试样在普通试验机下的变形规律及突跳特性, 得到了与尖点突变模式相似的突变机构。再利用突变理论中尖点突变模型研究由试验机和岩石试样组成的力学系统的失稳机制, 结果表明：两者得到的岩石变形突变量非常相似, 且具有同一变化趋势。