Polymeric foam behavior under dynamic compressive loading

Polymeric foams are commonly used in many impact-absorbing applications and thermal-acoustic insulated devices. To improve their mechanical performances, these structures have to be modeled. Constitutive equations (for their macroscopic behavior) have to be identified and then determined by appropriate tests.Tests were carried out on polypropylene foams under high strain rate compression. In this work, the material behaviour has been determined as a function of two parameters, density and strain rate. Foams (at several densities) were tested on a uniaxial compression for initial strain rates equal to 0.34 s⁻¹ and on a new device installed on a flywheel for higher strain rates. This apparatus was designed in order to do stopped dynamic compression tests on foam. With this testing equipment, the dynamic compressive behaviour of the polymeric foam has been identified in the strain rate range [6.7.10⁻⁴s⁻¹, 100s⁻¹].Furthermore, the sample compression was filmed with a high speed camera monitored by the fly wheel software. To complete this work, picture-analysis techniques were used to obtain displacement and strain fields of the sample during its compression. Comparisons between these results and stress-strain responses of polypropylene foam allow a better understanding of its behaviour. The multiscale damage mechanism, by buckling of the foam structure, was emphasised from the image analysis.     

Anisotropic damping behavior of closed-cell aluminum foam

Closed-cell aluminum foams were prepared by molten body transitional foaming process. The anisotropic damping property of closed-cell aluminum foam was measured in two directions using the forced vibration method. The measured results show that the loss factors of the TD specimens are higher than that of the LD specimens. The loss factors ratio β_L/β_T depends linearly on the shape-anisotropy ratio R. Anisotropic damping behavior is due to the variation of Young’s modulus resulting from anisotropic cell morphology.     

Effect of processing parameters on cell structure of an aluminum foam

A closed cell aluminum foam with the same composition but different cell sizes and structures was prepared by changing air injection rate and impeller speed during foaming process to study the influence of air injection rate and impeller speed on cell structure. The foams prepared under the foaming conditions are characterized as roughly equiaxed polyhedral cells with density range of 0.1–0.22 g/cm³ and cell diameter of 4–11 mm with different cell wall thickness and Plateau border size. Cell size of the aluminum foam is increased with increasing air injection rate, and higher impeller speed results in a much smaller cell size at given air injection rate. Cell wall thickness and Plateau border size of the aluminum foams are decreased with the increase in cell size. Moreover, the higher impeller speed produces smaller size of the foam cells with thicker cell wall and Plateau border size, resulted in higher density foam in contrast to the foam with the same cell size prepared at lower impeller speed.     

晶须增强铝基复合材料的热压缩变形行为研究

采用试验和数值模拟相结合的方法研究了纵向、横向和倾斜分布的晶须增强铝基复合材料热压缩变形行为.研究表明:SiCw/6061Al复合材料300℃压缩的应力-应变行为与晶须取向角密切相关;在热变形过程中,纵向分布晶须折断严重,导致复合材料表现为应变软化行为;而倾斜于压缩方向30°的晶须折断和转动明显,引起相应复合材料应变软化;横向分布晶须没有转动而折断程度很小,使复合材料呈现出加工硬化行为.     

Prediction of compressive properties of closed-cell aluminum foam using artificial neural network

The design of artificial neural network (ANN) is motivated by analogy of highly complex, non-linear and parallel computing power of the brain. Once a neural network is significantly trained it can predict the output results in the same knowledge domain. In the present work, ANN models are developed for the simulation of compressive properties of closed-cell aluminum foam: plateau stress, Young’s modulus and energy absorption capacity. The input variables for these models are relative density, average pore diameter and cell anisotropy ratio. Database of these properties are the results of the compression tests carried out on aluminum foams at a constant strain rate of 1 × 10⁻³ s⁻¹. The prediction accuracy of all the three models is found to be satisfactory. This work has shown the excellent capability of artificial neural network approach for the simulation of the compressive properties of closed-cell aluminum foam.     

颗粒增强铝合金基泡沫铝材料压缩性能的研究

利用熔体发泡法制备了颗粒增强铝合金基闭孔泡沫铝,进行了准静态压缩和动态压缩实验,并且与铝合金基泡沫铝的相关性能进行了比较.研究了不同相对密度的泡沫铝准静态压缩和动态压缩性能.添加颗粒能增强基体合金性能,改善泡沫压缩效果.结果表明,在动态压缩和准静态压缩中,随着密度增加,颗粒增强基泡沫铝的平台应力和弹性模量逐渐增大;动态情形下的能量吸收能力要高于准静态情形下的能量吸收能力.向熔体中添加适当比例的粉煤灰颗粒可产生显著的基体增强效果,有效提高泡沫铝的压缩性能.     

短纤维对泡沫铝压缩力学性能与吸能特性的影响研究

为探索闭孔泡沫铝加入短纤维后的力学性能和吸能特性变化规律.利用熔体发泡法在铝熔体中加入短碳纤维后制作得到纤维增强泡沫铝,通过万能材料试验机和高速液压伺服材料试验机在常温下分别对泡沫铝、纤维增强泡沫铝进行准静态和中应变率下(0.001～100s-1)的动态力学性能测试,分析了纤维长度、纤维含量对泡沫铝力学性能和吸能特性变...     

不同温度下泡沫铝压缩行为与变形机制探讨

利用Hopkinson杆与MTS实验装置分别研究泡沫铝在不同温度下的动态与静态力学性能,实验结果表明,泡沫铝有很强的温度软化效应,坍塌应力与平台应力和“应力降”的大小均随温度的升高而降低。动态高温下应力应变曲线与静态低温下应力应变曲线类似,反映材料应变率与温度之间的等效关系。低温下泡沫金属强度较高,脆性较强,泡沫结构易脆性坍塌,并伴有脆性裂纹,随着温度的升高,基体材料逐渐软化,泡沫金属强度降低,胞孔结构在压缩过程中从低温下脆性失稳逐渐变成以胞壁屈曲与塑性变形为主,且在不同温度段,应变率敏感度不同。     

Correlation between structure and compressive strength in a reticulated glass-reinforced hydroxyapatite foam

Glass-reinforced hydroxyapatite (HA) foams were produced using reticulated foam technology using a polyurethane template with two different pore size distributions. The mechanical properties were evaluated and the structure analyzed through density measurements, image analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). For the mechanical properties, the use of a glass significantly improved the ultimate compressive strength (UCS) as did the use of a second coating. All the samples tested showed the classic three regions characteristic of an elastic brittle foam. From the density measurements, after application of a correction to compensate for the closed porosity, the bulk and apparent density showed a 1 : 1 correlation. When relative bulk density was plotted against UCS, a non-linear relationship was found characteristic of an isotropic open celled material. It was found by image analysis that the pore size distribution did not change and there was no degradation of the macrostructure when replicating the ceramic from the initial polyurethane template during processing. However, the pore size distributions did shift to a lower size by about 0.5 mm due to the firing process. The ceramic foams were found to exhibit mechanical properties typical of isotropic open cellular foams.     

晶须转动对铝基复合材料热压缩变形行为的影响

采用平面应变有限元方法研究了晶须转动对晶须增强铝基复合材料热压缩变形行为的影响.数值结果表明:晶须转动不仅引起晶须承载能力的下降,而且也影响基体的塑性变形行为,同时复合材料在热压缩变形过程中表现出明显的应变软化行为.数值分析研究进一步证明:晶须转动是复合材料出现应变软化的重要原因.     

Hot compressive deformation behavior of the as-quenched A357 aluminum alloy

The objective of the present work was to establish an accurate thermal-stress mathematical model of the quenching operation for A357 (Al–7Si–0.6Mg) alloy and to investigate the deformation behavior of this alloy. Isothermal compression tests of as-quenched A357 alloy were performed in the temperature range of 350–500 °C and at the strain rate range of 0.001–1 s⁻¹. Experimental results show that the flow stress of as-quenched A357 alloy decreases with the increase of temperature and the decrease of strain rate. Based on the hyperbolic sine equation, a constitutive equation is a relation between 0.2 pct yield stress and deformation conditions (strain rate and deformation temperature) was established. The corresponding hot deformation activation energy (Q) for as-quenched A357 alloy is 252.095 kJ/mol. Under the different small strains (≤0.01), the constitutive equation parameters of as-quenched A357 alloy were calculated. Values of flow stress calculated by constitutive equation were in a very good agreement with experimental results. Therefore, it can be used as an accurate thermal-stress model to solve the problems of quench distortion of parts.     

液态扩散焊制备泡沫铝夹芯板及其疲劳行为

采用超声辅助液态扩散焊接的方法制备冶金复合泡沫铝夹芯板,利用光学显微镜（OM）和SEM观察冶金复合样品的界面组织和结构,发现连接界面发生了侵蚀作用,接头均匀连续;EDS线扫结果表明,连接界面处焊接合金（Zn-10Al）和铝基体间的元素扩散现象明显,表明在超声作用下,基体材料表面氧化膜被破坏,枝晶在界面附着生长,形成良好的冶金连接。将制备的冶金复合样品和胶黏泡沫铝夹芯板样品进行三点弯曲疲劳对比试验,结果显示,冶金复合样品和胶黏样品的疲劳极限分别达到3 058 N和2 829 N。在相同载荷下,冶金复合样品的疲劳寿命（S-N）远远长于胶黏样品。两种样品的疲劳破坏方式完全不同,胶黏样品表现为面板和芯层黏接面的脱黏剪切破坏,冶金复合样品的疲劳剪切破坏出现在泡沫铝芯层,没有出现面板脱离现象。     

Optimization and compressive behavior of composite 2-D lattice structure

Abstract

In an effort to optimize axially compressed members of a wooden structure, composite 2-D lattice structures were designed and manufactured with wood-plastic composites as the panel and glass fiber reinforced plastic as the core by using a simple insert-glue method. The compressive behavior of composite 2-D lattice structures made of cores with different diameters and inclination angles was investigated. Analytical models were employed to predict the theoretical load capacity, equivalent compressive strength, and failure types of lattice structures. Reinforcing hoops were then made at the two ends of cores according to its force characteristics and failure type.     

The effects of foam filling on compressive response of hexagonal cell aluminum honeycombs under axial loading-experimental study

In this paper the effects of foam filling of honeycomb panels on their plastic behavior and mechanical properties are studied experimentally. Five types of Al 5052-H39 honeycombs in bare and foam filled conditions are subjected to quasi-static axial compressive loading. The panels are selected so that the effects of parameters such as the cell size, the cell walls thickness and the panel thickness on the mean crushing strength, energy absorption capacity and the wavelength of the folds could be investigated. Tests show that foam filling of panels increases their mean crushing strength and energy absorption capacity up to 300% and the less the honeycomb density the greater the effect of foam filling. Furthermore, mean crushing strength of foam filled panels is larger than the sum of the mean crushing strengths of bare honeycomb and foam alone. The wavelength of folds and densification strain in foam filled panels are smaller than those of bare honeycombs. These tests also showed that unlike the theoretic formula the panel thickness influences the mean crushing strength of honeycomb.     

Effect of cell shape anisotropy on the compressive behavior of closed-cell aluminum foams

The closed-cell Al–Si foams have been prepared by molten body transitional foaming process using TiH₂ foaming agent. The cell shape anisotropy ratio of specimens with various relative densities was measured. The quasi-static compressive behavior of Al–Si foams in both longitudinal and transverse directions were investigated. The results show that Al–Si foam loaded in the transverse direction exhibits a lower stress drop ratio. The relationship between plastic collapse stress ratio and cell shape anisotropy is in accordance with Gibson and Ashby model. The plastic collapse stress and the energy absorption property of Al–Si foams increase following power law relationship with relative density. Moreover, Al–Si foams exhibit higher plastic collapse stress and the energy absorption property in the longitudinal direction than that in the transverse direction.     

泡沫铝夹心板芯层泡沫结构的研究

研究了采用轧制复合-粉末冶金发泡法制备的泡沫铝夹心板的生产工艺,探讨了主要工艺条件对芯层泡沫结构的影响,得到了优化的工艺参数.研究结果表明:轧制复合工艺可以使芯层粉末达到很高的致密度,为发泡过程创造了有利条件.轧制复合板适宜的发泡温度为615～620℃,温度过高会导致芯层出现大尺寸连通孔.发泡时间对熔融态泡沫体的凝固过程有显著影响,时间过长会使泡沫层塌陷,发泡温度为620℃时,经4～6min发泡芯层可形成良好的泡沫结构.     

Experimental and theoretical investigation on the compressive behavior of aluminum borate whisker reinforced 2024Al composites

The compressive behavior of Al₁₈B₄O₃₃w/2024Al composites fabricated by squeeze casting was investigated under low and elevated temperature. Microstructure shows that the compression exerts a significant effect on whisker fracture and rotation. The theory of synergistic effects caused by different strengthening mechanisms is used to predict the yield strength. Experiments show that compressive yield strength of composites improves by 47% compared with those of 2024Al at 623 K and agrees relatively well theoretical value. The compressive deformation depends on matrix mainly at lower temperature and the main failure mode is shear fracture. Additionally, fracture mechanisms are investigated further through fracture surface analysis. During hot compression, the predominated softening mechanisms also include dynamic recrystallization and strain softening except for dynamic recovery, which corresponds well with the shape of flow curves, microstructural observation and change of activation energy. Lastly, the optimum process parameters are determined to be about 0.1 s^− 1 and 723 K based on Dynamic Material Model and validated by microstructure evolution. Experiments show that the strain rate has a mixed effect on whisker fracture.     

Experimental studies on the quasi-static bending behavior of double square tubes filled with aluminum foam

Quasi-static experiments were performed on empty tubes and aluminum foam-filled single and double tubes to study the effects of different filler arrangements on their three-point bending behavior. The load-carrying capacity and energy absorption of different structures are compared. The results confirm the advantage of the foam-filled structures. In particular, the double tube structure with aluminum foam filler enhances the load-carrying capacity, crashworthiness, and total and specific energy absorptions of the structure, in comparison with the foam-filled single tube. It was also found that increasing the wall thickness of the inner tube improves the performance of the structure within the experimental range, and adhesion between foam and tube has a negative effect.     

Fracture and microstructure of open cell aluminum foam

SEM and EDS measurements were used to scrutinize the microstructure of Duocel open cell 6101 aluminum foam in relation to its fracture properties. In-situ SEM tensile tests on the open cell aluminum foam were performed to investigate the different fracture modes of struts and Aramis/Digital Image Correlation software was used to map the strain in individual struts. Observations during tensile tests showed that the microstructure of the struts has a great influence on the fracture behaviour of the foam. In particular AlFeSi-precipitates, which are due to the casting of the 6101 aluminum alloy, and the morphology of the foam alters the fracture mode of the struts in the foam from transgranular to intergranular. Less energy is needed for intergranular fracture of struts and the strain to failure of the foam is decreased due to weak individual struts.