基于相对即时密度的泡沫铝材料力学性能研究

通过对圆柱形泡沫铝试件进行静态压缩和冲击实验，考察了泡沫铝的初始密度、孔径和尺寸等因素对材料应力应变关系的影响，研究了基于相对即时密度的泡沫铝材料的塑性行为。实验所用泡沫铝试件包含四种尺寸，三种孔径及多种初始密度。实验结果表明，初始密度对泡沫铝的应力应变关系有着显著的影响，而其他因素，如孔径、试件尺寸等的影响较小。基于实验结果，提出了一种新的泡沫铝材料力学性能的描述方法，即用材料的相对即时密度与应力的关系来描述泡沫铝材料的塑性行为。该关系适用于静态和动态加载情况，只是两种情况下的参数不同。基于该方法，发现泡沫铝的塑性行为可以用单一的应力一相对即时密度关系描述，这一关系甚至不依赖于材料的初始密度，这将使泡沫铝材料塑性行为的描述大大简化。     

泡沫铝的单向力学行为

本文对不同孔径的开孔泡沫铝材料的单向拉伸性能和单向压缩性能进行了研究,揭示了泡沫铝材料的变形机理,并且发现相对密度不是确定材料力学属性的唯一参数,孔径大小对材料的力学性能也有一定的影响。基于实验数据,我们讨论了材料的宏观力学性能和微观结构的联系,并利用Ramberg-Osgood模型描述了材料的单轴拉伸一维应力应变关系。     

不同孔隙率及孔径泡沫铝的力学与吸能特性研究

对三种孔径两种孔隙率共六种泡沫铝试件进行了静态压缩试验,发现对于中孔隙率材料,孔隙率对材料力学性能和吸能性能仍有明显影响:孔隙率越大,其吸能能力越强,但是屈服极限越小.孔径对材料吸能能力也有一定影响.同时,试验中还发现,对于孔隙率为55%的材料,当孔径≤1mm时,泡沫铝材料的孔径对其力学性能及吸能性能影响甚小.     

开孔泡沫铝填充圆管的准静态压缩行为

采用开孔结构泡沫铝填充到薄壁圆形铝管中,制备出开孔泡沫铝夹芯铝管,并进行压缩实验,研究了这种结构材料的压缩力学行为和变形特征以及材料的结构特征参数对压缩力学性能和能量吸收特性的影响。在压缩过程中,泡沫铝夹芯铝管的载荷-位移曲线呈现出弹性段、波动的屈服平台段和压实段3个阶段特征;铝管的径厚比及泡沫铝本身的参数和强度对填充管的屈服强度、平均压溃力和吸能特性均有着非常显著的影响。填充泡沫铝后铝管的压缩变形方式发生改变,管壁只发生向外翻折变形,产生的环状褶皱减少。     

泡沫铝合金动态力学性能实验研究

利用分离式霍布金森压杆(SHPB)实验技术和MTS材料实验机对两组不同孔径、不同密度的开孔泡沫铝合金进行了准静态和动态压缩实验研究。实验结果表明:泡沫铝合金的静态和动态变形过程均具有泡沫材料变形的三个阶段特征。开孔泡沫铝合金的变形是均匀变化过程，并不出现局部的变形带。与相对密度对力学性能的影响相比，孔径大小的影响可以忽略不计。在考察的应变率范围内，屈服应力对应变率并不很敏感。     

爆炸荷载下泡沫铝材料中冲击波衰减特性的实验和数值模拟研究

通过实验和数值模拟对泡沫铝中冲击波传播特性进行了研究,结果表明:冲击波在泡沫铝中传播时显示明显的衰减特性;与此同时波头升时逐渐增加。这种衰减耗散特性主要来源于泡沫铝本身的本构粘性效应,而追赶卸载效应又会进一步促进冲击波的衰减。这为泡沫铝作为新型抗冲击缓冲材料提供设计基础。     

泡沫铝泡孔动态变形特性研究

在SHPB实验中采用应变冻结法将试件冻结于设定的压缩应变,然后观察内部泡孔的变形情况并讨论其变形机理。分别对同一相对密度(40%)、不同基体材料的两种泡沫铝以及两种相对密度(19%和8%)、相同基体材料的泡沫铝进行实验,讨论了基体材料的本构关系、泡沫的相对密度对变形模式的影响。利用ANSYS/LS DYNA软件对泡沫铝在动态压缩时泡孔的变形进行数值模拟计算,进一步说明泡沫铝泡孔变形的模式与基体材料本构关系及相对密度的关系。     

铝/硅橡胶复合材料动态压缩行为的研究

通过向开孔泡沫铝中填充硅橡胶而制备了铝 /硅橡胶复合材料 ,在Hopkinson压杆实验装置上对这种材料进行了动态压缩实验 ,分析了其动态压缩应力 应变响应特征 ,并与开孔结构泡沫铝的压缩行为进行了比较。结果表明 :铝 /硅橡胶复合材料的压缩应力 应变响应具有两个阶段的特征 ,即弹性和塑性变形阶段 ;这种复合材料具有较强的应变率效应 ,随应变率的提高 ,其屈服强度和流动应力显著上升。     

聚氨酯泡沫铝动力学性能实验及本构模型研究

为了改进泡沫铝的动态吸能性能,将聚氨酯填充到开孔泡沫铝中制备成复合材料。通过霍普金森杆(SHPB)冲击实验,研究包含相对密度、应变、应变率和聚氨酯含量等影响因素的聚氨酯泡沫铝材料的动力学性能,并建立了动态本构模型。实验结果表明,聚氨酯泡沫铝的动态弹性模量与相对密度无关,屈服强度和流变应力与应变率和泡沫铝的相对密度成正比;聚氨酯泡沫铝的屈服强度与泡沫聚氨酯质量增加近似呈线性关系。所建立的动态本构模型在相对密度和应变率在一定的变化范围内与实验数据吻合较好。     

数字图像相关方法在闭孔泡沫铝压缩试验中的应用

为了了解相对密度与胞孔结构对闭孔泡沫铝力学性能的影响,本文采用放大成像及数字图像相关技术对两种不同密度的泡沫纯铝试样进行了实验研究.利用数字图像相关方法对泡沫纯铝变形前后的图像进行相关计算,获得了弹性范围内静态压缩情况下闭孔泡沫铝材料表面的全场变形及局部孔结构的变形,同时根据试验结果计算了试件的名义弹性模量.实验结果表明泡沫铝整体孔结构的变形与泡沫金属材料相对密度有关,而单个孔结构的变形主要与孔壁面光滑程度和皱褶有关.实验结果还表明图像相关方法能够有效地应用于闭孔泡沫金属的力学性测量和评估的研究.     

泡沫铝内衬对抗内部爆炸钢筒变形的影响

为提高承受内部爆炸载荷钢筒的抗爆性能，研究了泡沫铝内衬对钢筒变形的影响。首先通过对比实验，发现在本文的实验条件下，泡沫铝内衬导致钢筒变形增大，甚至发生了严重的破坏；进而建立有限元模型，研究了钢筒变形随爆炸当量、泡沫铝内衬厚度的变化机理和规律。结果表明，添加足够厚度的泡沫铝内衬能够减小钢筒变形，但泡沫铝厚度不足时，则可能起到相反的效果。对于固定尺寸的含泡沫铝内衬钢筒，随着爆炸当量增加，泡沫铝内衬对钢筒塑性变形的影响主要包含3种模式。模式1，泡沫铝可通过塑性变形吸收爆炸载荷，从而减小钢筒变形。模式2，泡沫铝内衬导致钢筒承受的载荷强度增大，钢筒塑性变形增大。模式3，泡沫铝对载荷强度的影响可忽略，泡沫铝通过增大结构质量减小钢筒塑性变形。     

Analytical modeling and finite element simulation of the plastic collapse of sandwich beams with pin-reinforced foam cores

Analytical predictions are presented for the plastic collapse strength of lightweight sandwich beams having pin-reinforced foam cores that are loaded in 3-point bending. Both polymer and aluminum foam cores are considered, whilst the facesheet and the pins are made of either composite or metal. Four different failure modes are account for: metal facesheet yield or composite facesheet microbuckling, facesheet wrinkling, plastic shear of the core, and facesheet indentation beneath the loading rollers. A micromechanics-based model is developed and combined with the homogenization approach to calculate the effective properties of pin-reinforced foam cores. To calculate the elastic buckling strength of pin reinforcements, the pin-reinforced foam core is treated as assemblies of simply supported columns resting upon an elastic foundation. Minimum mass design of the sandwich is then obtained as a function of the prescribed structural load index, subjected to the constraint that none of the above failure modes occurs. Collapse mechanism maps are constructed and compared with the failure maps of foam-cored sandwich beams without pin reinforcements. Finite element simulations are carried out to verify the analytical model and to study the performance and failure mechanisms of the sandwich subject to loading types other than 3-point bending. The results demonstrate that the weaker the foam is, the more optimal the pin-reinforced foam core becomes, and that sandwich beams with pin-reinforced polymer foam cores are structurally more efficient than foam- or truss-cored sandwich beams.     

空心玻璃微珠及玻璃纤维混合填充环氧树脂力学性能实验研究

制备了一种空心玻璃微珠质量比、四种玻璃纤维质量比的空心玻璃微珠及玻璃纤维混合填充环氧树脂复合泡沫材料。通过一系列准静态压缩、拉伸实验,研究了玻璃纤维填充量对复合材料的密度、强度、弹性模量等性能的影响,并分析了其影响原因。通过三点弯曲实验,研究了复合材料弯曲弹性模量、弯曲强度等力学性能与玻璃纤维填充量的关系。通过拉伸断口SEM照片分析了玻璃纤维增强复合材料力学性能的原因。研究表明:随着玻璃纤维填充量增加,复合材料的拉伸和弯曲强度、压缩屈服极限及弹性模量等力学性能都有较大的提高,且仍然保持了较好的弹性性能和塑性特征。     

PVDF应力测试技术及其在多孔材料爆炸冲击实验中的应用

通过精确测量Hopkinson杆子弹速度,实现了对PVDF压力传感器的动态标定,测试数据线性度良好,误差不超过1.9%,得到动态灵敏度系数K=32.83 pC/N,采用500 g TNT对“钢板-泡沫铝-钢板”复合结构进行爆炸冲击加载,测量结构间应力波的传播情况。研究结果表明:电压测试信号可以较为准确地反映弹性波与塑性波的加载时间和传播速度,PVDF对弹性应力波段高频信号的动态响应灵敏准确,与理论数据的相对误差为3.5%。测得泡沫铝材料中塑性波的传播速度为590 m/s,A₁-B₁界面塑性波透射系数达到了0.53,远高于弹性波透射系数。从机理上对应力时程曲线中出现的特殊现象进行了阐述,为相关爆炸测试提供参考。     

泡沫铝夹层结构抗冲击性能的近场动力学模拟分析

针对某光学舱所采用的泡沫铝夹层防护结构在破片冲击下的抗冲击性能问题，采用Monte-Carlo方法创建了泡沫铝结构的二维细观模型，在常规态型近场动力学理论中引入了Mises屈服准则和线性各向同性强化模型，建立了近场动力学塑性本构的数值计算框架。基于近场动力学计算程序模拟了低速冲击作用下泡沫铝夹层结构的塑性变形以及有机玻璃背板的裂纹扩展形态，分析了泡沫铝芯材孔隙率对该夹层结构抗冲击性能和损伤模式的影响规律。结果表明：泡沫铝夹层结构良好的塑性变形能力是其发挥缓冲与防护作用的主要因素，并且在一定范围内，泡沫铝芯材孔隙率越高，则夹层结构具有更好的抗冲击性能；当泡沫铝孔隙率从0.4提升到0.7时，泡沫铝对冲击物的动能吸收率从90%提高到99%；模拟结果与实验结果具有较好的一致性，验证了模拟结果的准确性和分析结论的有效性。通过数值模拟，预测了有机玻璃背板的裂纹扩展形态，发现提高泡沫铝的孔隙率能获得更好的防护效果。     

An indentation technique for estimating the energy density as fracture toughness with Berkovich indenter for ductile bulk materials

A technique is proposed to estimate the energy density as fracture toughness for ductile bulk materials with an indentation system equipped with a Berkovich indenter based on the theory of plastic deformation energy transforming into the indentation energy of fracture. With progressive increase of penetration loads, the material damage is exhibited on the effective elastic modulus. A quadratic polynomial relationship between the plastic penetration depth and penetration load, and an approximate linear relationship between logarithmic plastic penetration depth and logarithmic effective elastic modulus are exhibited by indentation investigation with Berkovich indenter. The parameter of damage variable is proposed to determine the critical effective elastic modulus at the fracture point. And the strain energy density factor is calculated according to the equations of penetration load, plastic penetration depth and effective elastic modulus. The fracture toughness of aluminum alloy and stainless steel are evaluated by both indentation tests and K_IC fracture toughness tests. The predicted S_cr values of indentation tests are in good agreement with experimental results of CT tests.     

The elastic–plastic behaviour of foam under shock loading

An experimental investigation of the elastic–plastic nature of shock wave propagation in foams was undertaken. The study involved experimental blast wave and shock tube loading of three foams, two polyurethane open-cell foams and a low-density polyethylene closed-cell foam. Evidence of precursor waves was observed in all three foam samples under various compressive wave loadings. Experiments with an impermeable membrane are used to determine if the precursor wave in an open-cell foam is a result of gas filtration or an elastic response of the foam. The differences between quasi-static and shock compression of foams is discussed in terms of their compressive strain histories and the implications for the energy absorption capacity of foam in both loading scenarios. Through a comparison of shock tube and blast wave loading techniques, suggestions are made concerning the accurate measurements of the principal shock Hugoniot in foams.     

泡沫铝防护钢筋混凝土板的抗爆性能

为研究多孔吸能材料泡沫铝板对工程结构的抗爆防护作用，开展室外爆炸破坏实验，分别对设置不同泡沫铝防护层的钢筋混凝土（reinforced concrete，RC）板在爆炸荷载下的动态响应及破坏模式进行了研究，并运用LS-DYNA软件建立了有限元模型。通过与实验对照，验证了模型的可行性，对比分析了有、无泡沫铝防护层钢筋混凝土板的损伤破坏规律，并讨论了泡沫铝密度梯度分布和纵筋配筋率的影响。结果表明:有限元模型能够有效分析含泡沫铝防护层RC板的动态响应及其破坏形态；泡沫铝防护层能够有效减小钢筋混凝土板的挠度变形，降低试件的破坏程度；泡沫铝密度由下到上递增情况对RC板的减爆效果最好；增大配筋率可以提升泡沫铝防护RC板整体抗爆性能。     

泡沫塑料中冲击波的传播

在现有 Ｓ Ｈ Ｐ Ｂ技术的基础上,将泡沫塑料作为子弹撞击铝杆,利用铝杆上的应变片测得的应力波形及泡沫塑料的应力应变曲线,合理地分析了冲击波在泡沫塑料中的传播特性,并提出了泡沫塑料中的破坏波（压实波）概念。     

Evolution of subsequent yield surfaces and elastic constants with finite plastic deformation. Part II: A very high work hardening aluminum alloy (annealed 1100 Al)

Results are presented on the evolution of subsequent yield surfaces with finite deformation in a very high work hardening annealed 1100 aluminum alloy. In Part I [Khan, A.S., Kazmi, R., Stoughton, T., Pandey, A., 2009a. Evolution of subsequent yield surfaces and elastic constants with finite plastic deformation. Part 1: a very low work hardening aluminum alloy (Al-6061–T6511) 25, 1611–1625.] of this paper, similar results are presented for a very low work hardening aluminum alloy. Those results were very different from the present ones, and all the results were for proportional loading paths. The subsequent yield surfaces are determined in tension, free end torsion and combined tension–torsion proportional and non-proportional loading paths, using 10 με deviation from linearity definition of yield. Yield surfaces are also determined after linear, bi-linear, and non-linear unloading paths after finite deformation under tension, free end torsion, and combined tension–torsion loading. The initial yield surface is closer to the von-Mises surface and the subsequent yield surfaces show distortion, expansion, positive cross-effect, and “nose” in the loading direction. Additionally, the subsequent yield surfaces after non-proportional loading paths show shrinkage and compounded distortion. The yield surfaces after unloading depict strong anisotropy, positive cross-effect and exhibits different proportion of distortion in each loading conditions. The Young’s and shear modulus decrease with plastic deformation and this decrease is much less than those reported in the published literature.