Spallation in cylinder-plate impact

A combined theoretical-experimental study has been made of spall-type fracture induced in steel plates impacted by blunt cylinders whose diameter is comparable to the plate thickness. Since attention is restricted to normal impact, the problem is two dimensional, involving rotational symmetry. Analysis of shock-wave patterns induced by impact is achieved by employing a two-dimensional computer program, HEMP, in which the equations of motion are combined with realistic constitutive relations appropriate for the dynamic high-pressure regimes encountered. Experimentally, a series of tests was conducted in which the impact velocity was increased incrementally until penetration occurred. Recovered targets were sectioned and examined for evidence of incipient internal fracture as well as extensive plastic deformation. Correlation with theoretical predictions is considered good.     

Laser-Driven Flyers and Nanosecond-Resolved Velocimetry for Spall Studies in Thin Metal Foils

Experimental Mechanics - We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time...     

激光陀螺随机噪声分析及其性能评价

本文讨论了环形激光陀螺（ＲＬＧ）的随机噪声及其产生机理，并介绍了国外常用的用于评估ＲＬＧ和其它精密测量仪器性能的Ａｌａｎ方差法，最后，应用Ａｌａｎ方差法来分析某型号环形激光陀螺的误差特性。     

Characterization of a Laser Extensometer for Split Hopkinson Pressure bar Experiments

This paper examines a laser extensometer to calculate the strain in a sample during split Hopkinson pressure bar (SHPB) experiments. This method offers a non-contact, direct method for measuring sample strain which does not rely on one-dimensional wave propagation assumptions. First a single bar experiment is used to compare the extensometer’s accuracy and frequency response against a laser vibrometer and an accelerometer. The extensometer showed a close match with the vibrometer up to a bandwidth of 10 kHz. With the performance validated, the extensometer is applied to a SHPB experiment with silicone samples. For low strains, the extensometer shows a good match to the strain determined by strain gauges on the bars. At higher strains, the radial expansion of the sample can interfere with the measurement beam from the extensometer and produce inaccurate results.     

海水环境中环氧值对环氧树脂涂层摩擦学性能的影响研究

环氧树脂是一类具有优异的粘接、耐腐蚀和电气绝缘等性能的高强度热固性高分子材料,在黏结剂、防腐涂料和复合材料方面展现出广阔的应用前景,其摩擦学行为与环氧值密切相关.本研究中采用线棒涂布器在铸铁表面分别制备了3种具有不同环氧值的环氧树脂涂层.借助傅里叶红外光谱仪(FT-IR)、X射线光电子能谱仪(XPS)、扫描电镜(SEM)、纳米压痕仪、热重/差热综合热分析仪(TG/DTA)、UMT-3摩擦磨损试验机和表面轮廓仪研究了涂层的结构、硬度、耐热性及摩擦学性能.研究结果表明:涂层在干燥条件和海水环境中的摩擦系数和磨损率均随环氧值的增大而升高;海水环境中涂层的摩擦系数和磨损率均较干燥条件下低.涂层的磨损机理在干燥条件下包括了疲劳磨损、黏着磨损和磨粒磨损,而在海水中仅为疲劳磨损.本研究成果为高性能防腐耐磨涂层的设计与应用提供理论指导.     

Characterization of Heterogeneous Response of Al Bicrystal Subject to Micro Scale Laser Shock Peening

This letter shows the ability to perform characterization of the strain field in an aluminium bicrystal subject to plane strain condition induced by micro scale laser shock peening. Intensity contrast method, previously used in topographic measurements of strain fields in thin films is employed here. Our results show that this method is applicable for measurements of the plastically deformed bulk materials. Moreover, heterogeneous response of the grain boundary is observed as periodic nature of deformation.     

Application of Laser Deposition to Mechanical Characterization of Advanced High Strength Steels Subject to Non-Proportional Loading

Background

Characterization of hardening and fracture limits of advanced high strength steels (AHSSs) undergoing strain path changes (SPCs) are particularly challenging for plane strain condition, which commonly occurs in sheet metal forming. There is a need for a simple, engineering-friendly method to characterize materials subjected to complex loading paths that mimic stress conditions in actual forming processes.

Objective

Experimental additive manufacturing techniques have been applied to reinforce AHSS specimens subjected to SPCs in order to broaden capabilities for characterizing hardening behavior and fracture limits.

Methods

Hardening curves subject to SPCs (e.g. uniaxial tension or equi-biaxial tension followed by plane strain) have been obtained with a programmable biaxial tensile testing system using cruciform-shaped specimens with load-bearing arms reinforced by laser deposition. A notched specimen selectively reinforced by laser deposition was newly designed to characterize fracture limits subjected to SPCs ending with plane strain condition.

Results

Complex loading histories were successfully enabled by applying laser deposition technology. Results show that both hardening behavior and fracture limits of a TRIP-assisted steel and a dual-phase steel are dependent on loading history.

Conclusions

It appears that the laser deposition technique can be used for material characterization under specific SPCs. Hardening behavior of AHSSs under SPCs ending with plane strain is quite different from traditional uniaxial tension-uniaxial compression tests. For materials sensitive to SPCs, multi-step forming can be a great option to reach the targeted forming shape.

     

Mixed-Mode Dynamic Crack Growth in Functionally Graded Glass-Filled Epoxy

Compositionally graded glass-filled epoxy sheets with edge cracks initially along the gradient are studied under dynamic loading conditions. Specimens with monotonically varying volume fraction of reinforcement are subjected to mixed-mode loading by eccentric impact relative to the crack plane. The optical method of Coherent Gradient Sensing and high-speed photography are used to map transient crack tip deformations before and after crack initiation. Two configurations, one with a crack on the stiffer side of a graded sheet and the second with a crack on the compliant side, are examined experimentally. To elucidate the differences in fracture behavior due to functional grading, a homogeneous sample is also tested. The differences in both pre- and post-crack initiation behaviors are observed interms of crack initiation time, crack path, crack speed and stress intensity factor histories. When a crack is situated on the compliant side of the sample, it kinks significantly less compared to when it is on the stiffer side. Crack tip mode mixity histories show small but positive values during crack growth from the stiffer side of the sample towards the compliant side whereas a small but negative mode mixity prevails for the opposite configuration.     

Representation of Normal Cone Inclusion Problems in Dynamics Via Non-linear Equations

Analyzing non-smooth mechanical systems requires often the solution of inclusion problems of normal cone type. These problems arise for example in the event-driven or time-stepping simulation approaches. Such inclusion problems can be written as non-linear equations, which can be solved iteratively. In this paper we discuss three different methods to derive the non-linear equations representing the inclusions arising in the event-driven simulation approach. First, we formulate inclusions describing the individual non-smooth constraints and solve them successively. Secondly, we interpret the non-linear equations as the conditions for the saddle point of the augmented Lagrangian function. As a third possibility we discuss the exact regularization of set-valued force laws. All three methods lead to the same numerical scheme, but give different insight into the problem. Especially the factor r occurring in the non-linear equations is discussed. Two iterative methods for solving the non-linear equations are presented together with some remarks on convergence.     

Characterization of Laser Generated Lamb Wave Modes after Interaction with a Thickness Reduction Discontinuity Using Ray Tracing Theory

The effect that a circular discontinuity (due to thickness reduction) in an Aluminum plate has on the direction of Lamb wave propagation was experimentally and theoretically studied. Broadband Lamb waves were generated by a pulsed Nd:YAG laser and optically detected with a photo-EMF detector to increase spatial resolution. The experimental results show that thickness reduction modifies the time of flight (TOF) for S0 and A0 vibration modes and generates a change in direction of the ultrasonic Lamb wave. The change in the TOF as a function of distance and thickness reduction was numerically determined using ray theory and then compared to experimental results. It is shown that the change in the direction of propagation depends on the vibrational mode and frequency of the Lamb waves and this can affect the detection and characterization of a hidden discontinuity.     

Laser interferometry in shock-wave research

The various laser-interferometer instrumentation techniques which have been applied to the study of plane-stress-wave propagation in solids are reviewed and discussed. The capabilities and limitations of present systems are described.     

Dynamic and Quasi-Static Propagation of Compaction Waves in a Low-Density Epoxy Foam

We conducted dynamic and quasi-static compression experiments with low-density (ρ = 120 kg/m³) epoxy foam specimens. The specimens had a 10.0-mm-square cross-section and a length of 19.3 mm. Dynamic experiments were conducted with a modified split Hopkinson pressure bar (SHPB), and the quasi-static experiments were conducted with a hydraulic load frame device (MTS-810). In both cases, the specimens were loaded from one end at a constant velocity. Equally spaced grid lines were marked on the specimens to monitor the deformation history. Digital images taken at equally spaced time intervals gave the positions of each grid line. These images showed that a constant end-face velocity V produced a compaction wave front that traveled at a constant velocity C in both dynamic and quasi-static experiments. We described these results with a shockwave analysis that used a locking solid material model.     

Cyclic Strain Heterogeneity and Damage Formation in Rolled Magnesium Via In Situ Microscopic Image Correlation

Experimental Mechanics - Inherently tied to the complexity of their deformation mechanisms, Magnesium alloys show a high propensity for strain localization at multiple length-scales. Understanding...     

碳/环氧复合材料加工引起的残余热变形的微观云纹干涉方法研究

本文提出了一种新的微观云纹干涉法来测量碳/环氧复合材料的热变形.文中对该方法的原理和测量技术进行了详细的讨论,用碳/环氧复合材料制作了一个16层[90°2,0°2,+45°2,-45°2]的试样板,测量了由于加工引起的层间残余变形,计算了层间应变分量εx,εy,γxy.实验结果表明这种微观云纹干涉法能分辨出几十到几百微米范围内的云纹条纹.比传统的云纹干涉法具有更高的分辨率.