Large deformation rate-dependent stress-strain behavior of polyurea and polyurethanes

The thermoplastic elastomer polyurethane and the elastomeric thermoset polyurea are finding new applications in increasing the survivability of structures under impact loading, including those encountered in blast and ballistic events. However, the mechanical behavior of polyurea and polyurethane materials under these high rate conditions is relatively unknown. Here, the rate-dependent stress-strain behavior of one polyurea and three representative polyurethane materials is studied by dynamic mechanical analysis, quasi-static compression testing and split Hopkinson pressure bar (SHPB) testing. The polyurethane chemistries were chosen to probe the influence of the hard segment content on the mechanical behavior, where the volume fraction and the amorphous vs. crystalline structure of the hard segment domains were varied. The large strain stress-strain behavior of polyurea and polyurethane shows strong hysteresis, cyclic softening, and strong rate-dependence. The polyurethane with a non-crystalline well-dispersed hard segment morphology did not exhibit cyclic softening. The materials are observed to transition from a rubbery-like behavior under low strain rate (∼10⁻³-10⁰ s⁻¹) loading conditions to either a leathery or glassy-like behavior under high strain rate (∼10⁻³ s⁻¹) loading conditions.     

Effects of temperature,strain rate,and cyclic loading on mechanical behavior of silane-modified polyurethane sealant

To evaluate the mechanical properties of modified polyurethane sealants in engineering applications, the influences of temperature, strain rate, and cyclic loading on the mechanical properties of silane-modified polyurethane sealant were experimentally investigated. The monotonic tensile experiments with various strain rates and temperatures were conducted, and strain rate and temperature dependent nonlinear stress–strain curves were obtained. The results showed that the silane-modified polyurethane sealant exhibited temperature dependence at constant strain rate and rate dependence at room temperature. However, it is shown no obvious rate dependence at temperature of 150°C. In addition, the multi-step cyclic loading experiments with mean strain decrease and increase at each step were carried out to analyze the influence of cyclic loading and cyclic loading history at different temperatures. The results demonstrated that the viscous behavior of the materials was evidently observed in the first step and disappeared in other steps for the four-step cyclic loading with mean strain decrease case. Moreover, the cyclic stress relaxation of the materials was not obvious due to the prior cyclic loading with higher mean strain history, while the cyclic stress relaxation of the material continued to occur for the prior cyclic loading with lower mean strain history, and the cyclic strength of the materials decreased with the increase of temperature.     

熔融沉积工艺参数对热塑性聚氨酯弹性体静动态力学性能的影响

为揭示通过熔融沉积成型（FDM）工艺制备的热塑性聚氨酯弹性体（TPU）的静动态力学性能及工艺参数对其力学性能的影响,采用万能材料试验机和分离式霍普金森压杆（SHPB）实验装置对使用3种打印速率（10、40、70 mm/s）和3种喷头温度（200、220、240 ℃）制备的TPU开展准静态（0.01 s^-1）和动态（1 000 s^-1）加载下的力学性能试验,并进行工艺参数优选,同时进一步获取了材料在较宽应变率范围（0.001~2 500 s^-1）的应力?应变样本空间数据。结果表明,准静态和动态加载下,喷头温度220 ℃、打印速率40 mm/s为最优工艺参数;试样在准静态和动态下均具有应变率效应;准静态下试样超弹性特征显著,动态下结合朱?王?唐（ZWT）方程构建的材料黏弹性本构模型拟合曲线与实验曲线吻合较好;采用最优工艺参数制备的试样出现明显“微相分离”现象。     

Fine colloidal silica as reinforcing filler in polyurethane polymers

Colloidal silica in the particle size range 1.4–10 nm was extracted in to tetrahydrofuran (THF). Alkali silicate solutions with SiO₂ : M₂O (M = Li, Na, K, Cs) ratios ranging from 3:1 to 20:1 were used as source of silica particles in the size range 1.4–4.4 nm whereas commercial silica sols were used for particles in the range 5–10 nm. Films of polyester- and polyether-based polyurethane-containing colloidal silica were prepared and their mechanical properties measured. The reinforcing effect increased with increasing silica content and showed a maximum between 1.5 and 2.5 nm for polyether-based polyurethane and between 4 and 6 nm for polyester-based polyurethane. The area under the hysteresis loops of the stress–strain curves also showed maximum in the same particle size ranges for the two types of polyurethane. Reinforcement mechanisms are discussed in terms of interactions between small particles and hard or soft segments of the polymer chains.     

Temperature‐ and strain rate‐dependent constitutive modeling of the large deformation behavior of a transparent polyurethane interlayer

The dynamic mechanical behavior of a polyurethane used as an interlayer in a laminated windshield construction is studied by dynamic mechanical analysis, compression testing at various strain rates (0.001/s to 7000/s), and various temperatures (?40°C to 25°C) by using the universal testing machine and split Hopkinson pressure bar (SHPB) equipped with temperature controllers. The obtained results show that the mechanical behavior of the polyurethane interlayer is sensitive to temperature and strain rate. Under dynamic loading condition, stress‐strain curves at ?40°C exhibit the transition from “rubbery” to “glassy”. On the basis of the constitutive theory and the experimental data, a one‐dimension thermal‐hyper‐viscoelastic constitutive equation is proposed to describe the large compressive deformation response of the polyurethane interlayer over wide ranges of strain rates and temperatures. The parameter of the function is significant to describe the trend of the stress–strain curve at larger strain. The stress–strain curves at low strain rate and low temperature can overlap the stress–strain curves at higher strain rate and higher temperature; there may be an equivalent relationship between temperature and strain rate. POLYM. ENG. SCI., 55:1864–1872, 2015. © 2014 Society of Plastics Engineers     

Dynamic mechanical properties of polyurethane elastomers using a nonmetallic Hopkinson bar

A modified split Hopkinson-bar apparatus, in which the striker and input/output bars are made of polycarbonate instead of metal, was used to study three typical examples of a high-density flexible polyurethane elastomer (PORON) in sheet form. This variation of the device reduces a mismatch in impedance between the input/output bars and the specimen, thus allowing the stress in the specimen to reach a uniform state before significant engineering strain is induced. Dynamic compressive stress-strain curves were obtained from the measured incident, transmitted, and reflected waves. This article presents the behavior of these foams as a function of strain rate; for PORON 4701-05-20125-1637 under strain rates of 2.67 × 10⁻³ s⁻¹ to 4500 s⁻¹, the stress-strain response can be described by a function comprising a rate-dependent modulus and a strain-dependent factor, while for PORON 4701-01-30125-1604 and 4701-12-30062-1604, only loading at high strain rates yields similar characteristics. Empirical equations were derived to characterize these mechanical properties; in addition, characteristics relating to energy-absorption capability as well as deformation under approximately constant stress were also studied. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 619–631, 1997     

A new test method to obtain biaxial tensile behaviors of solid propellant at high strain rates

A new test method was proposed and applied for studying the biaxial tensile behaviors of hydroxyl-terminated polybutadiene (HTPB) propellant at high strain rates. The biaxial tensile stress responses of the propellant at room temperature and at different strain rates (0.40–85.71 s^?1) were obtained through the use of biaxial tensile strip samples, a new designed aluminum apparatus and a uniaxial Instron testing machine. A high-speed camera and scanning electron microscop (SEM) were employed to observe the biaxial tensile deformation and the damage of HTPB propellant under the test conditions. The results indicated that strain rate could remarkably influence the biaxial tensile behaviors of HTPB propellant. The effect of strain rate on the characteristics of stress–strain curves, mechanical properties and fracture mechanisms was consistent with that in uniaxial tension. However, the biaxial weakening of HTPB propellant was obvious. The strain at biaxial maximum tensile stress was between 10 and 30 % lower than that at the corresponding uniaxial case. Finally, the correlations between the fracture mechanisms and the mechanical properties of HTPB propellant, stress state and the damage of HTPB propellant were discussed. The damage of the propellant under the biaxial tensile test was less serious than that under uniaxial tension at the same strain rate. In addition, continuously increasing strain rate could change the fracture mechanism of the propellant under the biaxial and uniaxial tensile tests. In this investigation, the dominating fracture mechanism of HTPB propellant changed from the dewetting and matrix tearing at lower strain rate to the particles fracture at higher strain rate.     

Fatigue and fracture characteristics of a fine-grained (Mg,Y)–PSZ zirconia ceramic

The fatigue and fracture characteristics of a partially-stabilized fine-grained zirconia with spinel additions, (Mg,Y)–PSZ, were studied. Fracture toughness, crack growth resistance curves and fatigue crack growth (FCG) behavior, under both sustained and cyclic loading, were evaluated. Mechanical fatigue effects were clearly evidenced by (1) remarkable crack growth rate differences under cyclic and static loading and (2) significant loading ratio effects. Comparing the cyclic and the static FCG behavior allows to deduce a higher cyclic fatigue sensitivity of the fine-grained (Mg,Y)–PSZ with respect to a commercial peak-aged Mg–PSZ used as a reference material. By in situ observation of crack extension under cyclic loading, the fatigue mechanisms could be resolved. Mechanical degradation of bridging ligaments, as already known for coarse-grained Mg–PSZ, is one source of cyclic fatigue. An additional source attributed to the particle dispersed microstructure of the (Mg,Y)–PSZ is the interaction between crack faces and hard spinel particles. The sensitivity of (Mg,Y)–PSZ and Mg–PSZ to cyclic fatigue is discussed in terms of the respective microstructures, prevalence and operativity of distinct mechanical fatigue mechanisms.     

Microstructure of geopolymer accounting for associated mechanical characteristics under various stress states

Geopolymer was prepared with various SiO₂/Na₂O mole ratios and mechanical tests and microstructural analyses are performed to investigate how the constituents affect its mechanical behavior in distinct stress states. Laboratory results reveal that the SiO₂/Na₂O ratio affects the polymerization by influencing the formation of silicon Q⁴(mAl) structures. The proportion of Q⁴(4Al) correlates positively with the mechanical characteristics of geopolymer, and the proportion of Q⁴(2Al) correlates negatively with the mechanical characteristics of geopolymer. The proportions of Q⁴(mAl) affect the stress–strain curve and the failure modes of geopolymer under various confining pressures. Three types of stress–strain curves with different peak strengths and plastic deformations are obtained. Incomplete polymerization generates a geopolymer with an imperfect microstructure, which determines the plastic deformation while unloading. Polymerization of a geopolymer affects its apparent cohesion and friction angle. However, the friction-induced strength declines drastically when the failure mode changes from the split mode to the shear mode.     

Tensile mechanical properties and constitutive model for HTPB propellant at low temperature and high strain rate

To investigate the mechanical properties and fracture mechanisms of hydroxyl‐terminated polybutadiene (HTPB) propellant at low temperature and high strain rate, uniaxial tensile tests were conducted over the range of temperatures 233 to 298 K and strain rates 0.4 to 14.14 s^?1 using an INSTRON testing machine, and scanning electron microscope (SEM) was employed to observe the tensile fracture surfaces. The experimental results indicate that the deformation properties of HTPB propellant are remarkably influenced by temperature and strain rate. The characteristics of stress–strain curves at low temperatures are different from that at room temperature, and the effects of temperature and strain rate on the mechanical properties are closely related to the changes of properties and the fracture mechanisms of HTPB propellant. The dominating fracture mechanism depends much on the temperature and changes from the dewetting and matrix tearing at room temperature to the particle brittle fracture at low temperature, and the effect of strain rate only alters the mechanism in a quantitative manner. Finally, a nonlinear viscoelastic constitutive model incorporating the damage evolution and the effects of temperature and strain rate was developed to describe the stress responses of this propellant under the test conditions. During this process, the Schapery‐type constitutive theories were applied and one damage variable was considered to establish the damage evolution function. The overlap between experimental results and predicted results are generally good, which confirms that the developed constitutive model is valid, however, further researches should be done due to some drawbacks in describing the deformation behaviors at very large strain. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42104.     

A bilinear constitutive response for polyureas as a function of temperature,strain rate and pressure

Polyurea is widely applied as a coating material to increase the survivability of structures under impact loading. In this study, a systematic experimental program about the stress–strain behaviors of two polyaspartic esters polyureas is conducted over the temperature range of 233 K–293 K and strain rate range of 0.001/s–15,000/s, including the high‐pressure effects on the response of the polyureas. Based on the experimental results, the effects of temperature, strain rate, and pressure on the stress–strain behaviors are analyzed, the mechanical properties of the two polyureas are compared. The temperature and strain rate dependences of the Young's modulus, yield stress, and strain hardening slope are modeled. Finally, a bilinear constitutive model is proposed to describe the temperature, strain rate, and pressure dependences of the stress–strain behaviors. The model predictions, which agree well with the experimental results, provide basis for the application of the two polyureas. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45256.     

Mechanical response of dibenzyl-based polyurethanes with diol chain extension

A systematic investigation was made of the effects of varying hard and soft segment chemistry, crosslinking and preparation procedures, on the mechanical response of melt-cast polyurethane elastomers. In particular, two hard segments were compared, based on the diisocyanates: 4,4′-methylene bis(phenyl isocyanate) (MDI) and 4,4′-dibenzyl diisocyanate (DBDI). Rotation around the central -CH₂-CH₂- bridge in DBDI allows alignment of aromatic rings and hence crystallization within the hard phase, which is not available with MDI in melt-cast polyurethanes. Thus, new polymers were achieved, with a controlled ordering of copolymer hard segment blocks on the macromolecular chain. Wide angle X-ray diffraction of the as-moulded polymers revealed the presence of crystallinity in some cases, in the DBDI-based PU materials. Mechanical tests included load-unload cycles at constant rate of extension, with measurement of hysteresis and strain recovery, and stress relaxation tests. The presence of DBDI hard segments instead of MDI led systematically to increases in: the input strain energy to a given elongation, hysteresis and residual strain under cyclic loading, and stress relaxation. The results were interpreted in terms of a physically-based constitutive model framework previously proposed. This revealed that the observed effects of varying hard segment could all be explained by the hard domains having a higher flow stress in the presence of DBDI relative to MDI, associated with increased hydrogen bonding in DBDI-based polymers, which is enhanced in some cases by hard segment crystallinity. Materials with mixed MDI and DBDI hard segments were found to give the optimum combination of high input strain energy, but minimum residual strain, compared to equivalent materials based on MDI or DBDI alone.     

Experimental Investigation on High Strain Rate Tensile Behaviors of HTPB Propellant at Low Temperatures

To study the high strain rate tensile behaviors of hydroxyl‐terminated polybutadiene (HTPB) propellant at low temperatures, uniaxial tensile tests were conducted at different strain rates (0.4–42.86 s⁻¹) and temperatures (233–298 K) using an INSTRON testing machine. Scanning electron microscopy (SEM) was employed to observe the tensile fracture surfaces. Experimental results indicate that strain rate, temperature and test environment remarkably influence the tensile behaviors of HTPB propellant. The stress‐strain curves exhibit three different shapes. The elastic modulus and maximum tensile stress increase with decreasing temperature and increasing strain rate. However, the strain at maximum tensile stress decreases with increasing strain rate at low temperatures and there is a maximal value at 298 K and 14.29 s⁻¹. The effects of strain rate, temperature and test environment on the tensile behaviors are closely related to the changes of properties and fracture mechanisms of HTPB propellant. The dominating fracture mechanism depends on not only temperature but also strain rate, and it changes from the dewetting and matrix tearing at room temperature and lower strain rate to the particle brittle fracture at low temperatures. Based on the time‐temperature superposition principle (TTSP), the master curves of mechanical parameters for HTPB propellant were obtained.     

Mechanical properties of nanosilica/polypropylene composites under dynamic compression loading

This article presents results on the dynamic mechanical properties of PP‐SiO₂ nanocomposites, with nanosilica contents of 1, 3, and 5% by weight, at various strain rates using a Split Hopkinson Pressure Bar (SHPB) apparatus. The specimens were prepared using a hot compression technique. The dynamic mechanical characteristics, of PP‐SiO₂ nanocomposites, are illustrated in terms of stress–strain curves, up to nearly 1100 s⁻¹ of strain rates. From the results, the yield stress, compression modulus, and compressive strength of the composites, were significantly influenced by the strain rates and nanosilica contents. The values of strain rate sensitivity, and dissipation energy of the composites at various strain rates, were also determined. It was found that the strain rate sensitivity, and the dissipation energy, increased with increasing strain rates. In addition, it was observed that the composites experienced more severe damage under a high strain rate loading, compared to a low strain rate loading. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Coarse-grained modeling of model poly(urethane urea)s: Microstructure and interface aspects

Poly(urethane urea) elastomers are versatile and can be tailored to exhibit a broad range of mechanical response under high strain rate deformation. In this work, we utilize coarse-grained molecular dynamics simulations to elucidate the molecular mechanisms, particularly the effects of hard segment content, intermolecular interaction, and rigidity of the interface between the hard and soft segments on local morphology and rate-dependent stress-strain behavior in the ballistic regime. Simulation results qualitatively agree with available experimental data, where analysis of hard segment orientation during tensile and compression deformation and dynamic strain rate sensitivity was also performed. Further study of the intermolecular interaction on the stress-strain behavior reveals that it has a strong effect on strain hardening, particularly for a rigid interface, once the hard segment content reaches the percolation threshold. Simulation results also show that interface intermolecular interaction could become more dominant over interface rigidity in the initial stress-strain response, particularly below percolation.     

BOPP纵拉过程的应力应变行为

采用高温下挤出厚片纵拉过程的模拟反映出来的应力应变行为，对厚片高温纵拉过程的形变特征进行分析研究，结果表明，EB在加工过程中具有更大的形态能力和更宽范围拉伸速度的适应性，在实验范围内，不同温度下的拉伸屈服应力与对数应变速率均呈线性关系，同时也受到相对分子质量的影响：高温下拉伸比的变化不会改变厚片形变的基本特征，但拉伸比提高后，应力应变曲线后端的硬化现象比较明显；深入理解BOPP拉伸温区结构变化现理，对于提高挤出厚片的形变能力，降低拉伸过程中薄膜的破损，提高生产效率将起到重要作用。     

Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG)

Uniaxial and plane strain compression experiments are conducted on amorphous poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG) over a wide range of temperatures (25-110 °C) and strain rates (.005-1.0 s⁻¹). The stress-strain behavior of each material is presented and the results for the two materials are found to be remarkably similar over the investigated range of rates, temperatures, and strain levels. Below the glass transition temperature (θ_g=80 °C), the materials exhibit a distinct yield stress, followed by strain softening then moderate strain hardening at moderate strain levels and dramatic strain hardening at large strains. Above the glass transition temperature, the stress-strain curves exhibit the classic trends of a rubbery material during loading, albeit with a strong temperature and time dependence. Instead of a distinct yield stress, the curve transitions gradually, or rolls over, to flow. As in the sub-θ_g range, this is followed by moderate strain hardening and stiffening, and subsequent dramatic hardening. The exhibition of dramatic hardening in PETG, a copolymer of PET which does not undergo strain-induced crystallization, indicates that crystallization may not be the source of the dramatic hardening and stiffening in PET and, instead molecular orientation is the primary hardening and stiffening mechanism in both PET and PETG. Indeed, it is only in cases of deformation which result in highly uniaxial network orientation that the stress-strain behavior of PET differs significantly from that of PETG, suggesting the influence of a meso-ordered structure or crystallization in these instances. During unloading, PETG exhibits extensive elastic recovery, whereas PET exhibits relatively little recovery, suggesting that crystallization occurs (or continues to develop) after active loading ceases and unloading has commenced, locking in much of the deformation in PET.     

The influence of the stress relaxation and creep recovery times on the viscoelastic properties of open cell foams

The aim of this work is to study the mechanical behavior of flexible polyurethane foams used in cushioning applications. In particular, the differences between slow recovery (SR) and fast recovery (FR) foams are highlighted. To characterize the flexible polyurethane foams, creep and hysteresis tests were performed at different strain rate, stress levels, and temperatures. Significant differences were observed between the SR and FR foams, particularly in terms of residual deformation after unloading, hysteresis area, and creep behavior at different stress levels. Creep compliance at different stress levels was compared with a Voigt‐Kelvin model. Stress–strain loading curves were compared with a phenomenological model originally modified to account for the strain rate dependence. In both cases, it is possible to show that the main differences observed in the behavior of the foams are due to the different relaxation and recovery times of the foams. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

超高韧性聚氨酯复合材料性能影响因素

通过研究异氰酸酯指数(R值)、外加剂掺量、以及养护龄期对超高韧性聚氨酯复合材料性能的影响,优化材料制备技术,得到超高韧性聚氨酯复合材料的最佳制备技术。结果表明:R值决定着聚氨酯材料的软硬段比例,对力学性能影响较大,外加剂的掺入降低了材料的孔隙率,提高了材料致密度,聚氨酯材料的后熟化过程在7 d左右;超高韧性聚氨酯复合材料的最佳配合比组成为:R值1.25,外加剂加入量7.5%,浇注后的养护龄期为7 d。制备的超高韧性聚氨酯复合材料作为桥梁加固材料具有轻质、硬化快、可实现免胶加固等特点。