电流变阻尼器的抗冲击性能研究

从理论和试验两个方面研究了所设计的电流变阻尼器在大冲击下的抗冲击性能,分析了电流变液性能与阻尼器结构参数对抗冲击性能的影响。认为采用高性能的电流变液体及改变结构参数,都可以使电流变阻尼器的高速缓冲性能提高。电流变液流速对电流变液的屈服应力影响显著,其值随流速的增加按指数规律减小。从定性、定量两个方面分析了电流变阻尼器作为阻尼器效果不明显的原因为：由电流变效应引起的阻尼力在整个液压阻力所占比例太小,不能通过改变电压来使液压阻力具有很大的调节可控范围。     

The deformation of B4C particle in the B4C/2024Al composites after high velocity impact

In the present work, B₄C/2024Al composites with volume fraction of 45% were prepared by a pressure infiltration method. The microstructure of the crater bottom of B₄C/2024Al composite after impact was characterized by transmission electron microscope (TEM), which indicated that recovery and dynamic recrystallization generated in Al matrix, and the grain size distribution was about from dozens of nanometer to 200 nm. Furthermore, the plastic deformation was observed in B₄C ceramic, which led to the transformation from monocrystal to polycrystal ceramic grains. The boundary observed in this work was high-angle grain boundary and the two grains at the boundary had an orientation difference of 30°.     

Effect of fiber orientation,stress state and notch radius on the impact properties of short glass fiber reinforced polypropylene

Short glass fiber reinforced polypropylene (sgf-PP) is increasingly used in the automotive industry with the impact properties as key parameter. Experimentally, the impact behavior strongly depends on the specimen design, test set-up as well as temperature, and thus the characterization method should always be attuned to the occurring impact conditions of the final part. However, in order to deduce some general design criteria for sgf-PP, in this study a wide range of experimental parameters were investigated, specially focusing also on the effect of the governing, local fiber orientation distribution (FOD). Therefore, the effects of stress state (tensile, puncture and bending test), amount of stress concentration (notch radius) and temperature are characterized and discussed. The results proved that, as expected, distinctly different levels of impact strength and different dependencies on notches and notch radii are obtained for the various test set-ups. However, similarities in the temperature dependence are observed for specimens with similar governing fiber orientation.     

Understanding the impact properties of polymeric sandwich structures used for motorcyclists' back protectors

Conventional back protectors are comprised of two main parts: elastomeric foams to absorb the impact energy; and thermoplastic polymers to distribute the impact force on a wider area before the absorption process. Thermal comfort is usually maintained by vent holes within the structure. In the present work, the impact behavior of a number of samples made of materials commonly used for manufacturing such protectors was studied. Nitrile butadiene rubber as the soft layer and polyethylene thermoplastic as the hard layer were considered. The variables for the analyses were the thickness of the layers, the sample temperature and the distribution of the vent holes in the sample. The key findings are: the force distribution capability of the hard part and the stability of the impact properties with respect to temperature variations are fairly dependent on the thickness of the soft part; and a reasonable distance between two consecutive vent holes is required for achieving optimal impact protection.     

Miniaturized Compression Test at Very High Strain Rates by Direct Impact

A modified miniaturized version of the Direct Impact Compression Test (DICT) technique is described in this paper. The method permits determination of the rate-sensitive plastic properties of materials up to strain rate ∼10⁵ s⁻¹. Miniaturization of the experimental setup with specimen dimensions: diameter d _S = 2.0 mm and thickness l _S = 1.0 mm, Hopkinson bar diameter 5.2 mm, with application of a novel optical arrangement in measurement of specimen strain, makes possible compression tests at strain rates from ∼10³ s⁻¹ to ∼10⁵ s⁻¹. In order to estimate the rate sensitivity of a low-alloy construction steel, quasi-static, Split Hopkinson Pressure Bar (SHPB) and DICT tests have been performed at room temperature within the rate spectrum ranging from 5*10⁻⁴ s⁻¹ to 5*10⁴ s⁻¹. Adiabatic heating and friction effects are analyzed and the final true stress versus true strain curves at different strain rates are corrected to a constant temperature and zero friction. The results have been analyzed in the form of true stress versus the logarithm of strain rate and they show two regions of a constant rate sensitivity : relatively low up to the strain rate threshold ∼50 s⁻¹, and relatively high above the threshold, up to strain rate ∼4.5*10⁴ s⁻¹.     

Non-linear strain rate dependent micro-mechanical composite material model for finite element impact and crashworthiness simulation

The present study aims at implementation of a strain rate dependent, non-linear, micro-mechanics material model for laminated, unidirectional polymer matrix composites into the explicit finite element code LSDYNA. The objective is to develop an accurate and simple micro-mechanical, rate dependent material model, which is computationally efficient. Within the model a representative volume cell is assumed. The stress-strain relation including rate dependent effects for the micro-model is derived for both shell elements and solid elements. Micro-failure criterion is presented for each material constituent and failure mode. The implemented model can deal with problems such as impact, crashworthiness, and failure analysis under quasi-static loads. The developed material model has a wide range of applications such as jet engine jackets, armor plates, and structural crashworthiness simulation. The deformation response of two representative composite materials with varying fiber orientation is presented using the described technique. The predicted results compare favorably to experimental values.     

Particle impact on metal substrates with application to foreign object damage to aircraft engines

Foreign object damage (FOD) occurs when hard, millimeter-sized objects such as gravel or sand are ingested into aircraft jet engines. Particles impacting turbine blades at velocities up to about 300 m/s produce small indentation craters which can become sites for fatigue crack initiation, severely limiting the lifetime of the blade. A framework for analyzing FOD and its effect on fatigue cracking is established in this paper. Finite element analysis is used to determine the residual stresses and geometric stress concentration resulting from FOD. The roles of material rate sensitivity and inertia are delineated. The most important non-dimensional parameters governing impact indents are identified, significantly reducing the set of independent parameters. The second step in the analysis focuses on the potency of cracks emerging from critical locations at the indents. The results have been used to address the question: When and to what extent do the residual stresses and stress concentration caused by FOD reduce the critical crack size associated with threshold fatigue crack growth? For deep indents, it is found that elastic stress concentration is the dominant factor in reducing critical crack threshold when the applied cyclic load ratio, R, is large, otherwise the residual stresses are also important. Comparisons with a set of experiments conducted in parallel with the theory show that the numerical approach can account for various phenomena observed in practice.     

Modelling of Ground Moling Dynamics by an Impact Oscillator with a Frictional Slider

This paper describes current research into the mathematical modelling of a vibro-impact ground moling system. Due to the structural complexity of such systems, in the first instance the dynamic response of an idealised impact oscillator is investigated. The model is comprised of an harmonically excited mass simulating the penetrating part of the mole and a visco-elastic slider, which represents the soil resistance. The model has been mathematically formulated and the equations of motion have been developed. A typical nonlinear dynamic analysis reveals a complex behaviour ranging from periodic to chaotic motion. It was found out that the maximum progression coincides with the end of the periodic regime.     

Design of an Impact Loading Machine Based on a Flywheel Device: Application to the Fatigue Resistance of the High Rate Pre-straining Sensitivity of Aluminium Alloys

This paper presents a device that has been designed for tensile loading at medium impact rates (up to 10³ s^–1) and for performing either interrupted or failure tests. This machine allows us to apply prescribed pre-straining to the specimen, and then apply subsequent loading histories such as impact fatigue. Two specimen loading systems are considered, which make it possible to carry out tests with various ranges of force and various durations of time. A multi-CCD camera system is triggered by a chosen threshold from the force signal. The system is dedicated to the displacement measurement and gives both qualitative and quantitative information about the stretching mechanism leading to fracture. To illustrate the performance of the device, experimental results concerning impact tensile tests at a strain rate of about 300 s^–1 are presented, as well as consecutive impact-fatigue tests on two aluminium alloys.