Compressive mechanical properties of sawdust/high density polyethylene composites under various strain rate loadings

This article is concerned with the static and dynamic mechanical properties of high‐density polyethylene (HDPE) reinforced with sawdust (SD) at a strain rate of up to 10³ s^?1. In this study, the static and dynamic properties of HDPE/SD composites with different filler loadings of 5, 10, 15, 20, and 30 wt% SD were deliberated at different levels of strain rates (0.001, 0.01, 0.1, 650, 900, and 1100 s^?1) using a conventional universal testing machine and the split Hopkinson pressure bar apparatus. The results showed that the stress–strain curves, yield behavior, stiffness, and strength properties of the HDPE/SD composites were strongly affected by both the strain rate and the filler loadings. Furthermore, the rate sensitivityof the HDPE/SD composites showed a great dependency on the applied strain rate, increasing as the strain rate increased. However, the thermal activation values showed a contrary trend. Meanwhile, for the postdamage analysis, the results showed that the applied strain rates influenced the deformation behavior of the tested HDPE/SD composites. Moreover, for the fractographic analysis at dynamic loading, the composites showed that all the specimens underwent a severe catastrophic deformation. J. VINYL ADDIT. TECHNOL., 24:162–173, 2018. © 2016 Society of Plastics Engineers     

Crystallization of Nylon 11 under compressive high strain rates

The mechanical behavior of semicrystalline Nylon 11 was studied at strain rates between 10^?3 and 8800 s^?1. X‐ray diffraction and DSC were employed to examine the crystal structure and the crystallinity content. The as‐received material comprised a mixed structure of a predominately triclinic (α) form. DSC revealed that the material gave rise to two melting peaks. The compressive flow stress of Nylon 11 experienced a large increase at 1200 s^?1 and decreased at higher strain rates. The maximum level of the flow stress corresponded with a higher level of crystallinity and a structure mainly of a pseudohexagonal form. The subsequent drop in stress at higher rates was associated with a decrease in the crystallinity content and a mixed crystal structure, different from that observed in the as‐received material. After compression, the low melting peak disappeared and the material melted over an increased temperature range. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2031–2038, 2001     

Response of an HMX based explosive to dynamic loading by the Hopkinson split bar technique

The dynamic compression and localized adiabatic shear in samples of an HMX based explosive was studied using the split Hopkinson bar technique. Dynamic compression tests were performed at strain rates of (0.3–2.0) · 10³ s^?1. Fracture of the explosive samples was found to occur at stresses of 60–80 MPa. The behavior of HMX based samples was also studied in localized shear tests at different strain rates (200–2500 s^?1). The initiation of explosive transformations under the dynamic loads is discussed.     

The deformation behavior of polypropylene film under high strain rates

The relationship between stress and strain for polypropylene film was studied under strain rates from 0.13 to 5.21 s^?1 in order to study the deformation behavior of film under higher strain rates than previous studies. Uniform thickness was obtained in the strain rates from 2.08 to 5.21 s^?1 at 435 K, or from 2.08 to 3.13 s^?1 at 437 K. The temperature rise of film due to the generation of heat from plastic strain influenced the relationship between stress and strain, in particular, at high strain rates and low temperature. Material constants for the constitutive equation of film were determined using the measurements from 2.08 to 5.21 s^?1 at 435 K and from 2.08 to 3.13 s^?1 at 437 K. Film thicknesses during and after transverse direction stretching were successfully predicted by applying the material constants obtained. The authors concluded that the material constants should be determined by applying the stretching conditions, under which there is little or no effect from heat generation and under which film can be stretched uniformly in thickness. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Compressive mechanical properties of HTPB propellant at low,intermediate, and high strain rates

Low, intermediate, and high strain rate compression testing (1.7 × 10^?4 to 2500 s^?1) of the hydroxyl‐terminated polybutadiene (HTPB) propellant at room temperature, were performed using a universal testing machine, a hydraulic testing machine, and a split Hopkinson pressure bar (SHPB), respectively. Results show that the stress linearly increases with strain at each condition; the increasing trend of stress at a given strain with the logarithm of strain rate changes from a linear to an exponential form at 1 s^?1. By combining these characteristics, we propose a rate‐dependent constitutive model which is a linearly elastic component as a base model, then multiplied by a rate‐dependent component. Comparison of model with experimental data shows that it can characterize the compressive mechanical properties of HTPB propellant at strain rates from 1.7 × 10^?4 to 2500 s^?1. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43512.     

Melt spinning flow behaviour of high-density polyethylene blended with low-density polyethylene

ABSTRACT

The melt spinning flow behaviour of a high-density polyethylene (HDPE) blended with a low-density polyethylene (LDPE) was studied using a melt spinning technique in temperature ranging from 160 to 200°C and die extrusion velocity varying from 9 to 36?mm?s^?1. The results showed that the melt apparent extension viscosity of the blends was higher than those of the LDPE and HDPE; the melt apparent extension viscosity decreased with increasing temperature; while the melt apparent extension viscosity increased with increasing extension strain rate when the extension strain rate was lower than 0.2?s^?1, and then decreased; the melt apparent extension viscosity reached up to a maximum value when extension strain rate was about 0.2?s^?1; the relationship between the melt apparent extension viscosity and the LDPE weight fraction did not follow the mixing rule.     

Ultra-high creep resistant SiC ceramics prepared by rapid hot pressing

The high-temperature compression creep of additive-free β/α silicon carbide ceramics fabricated by rapid hot pressing (RHP) was investigated. The creep tests were accomplished in vacuum at temperature range 1500 °C–1750 °C and compressive loads of 200 MPa to 400 MPa. Under investigated condition the RHP ceramics possessed the lowest creep rate reported in the literature. The observed strain rates changed from 2.5 × 10^?9 s^?1 at 1500 °C and a lowest load of 275 MPa to 1.05 × 10^?7 s^?1 at 1750 °C and a highest load of 400 MPa. The average creep activation energy and the stress exponent remain essentially constant along the whole range of investigated parameters and were 315 ± 20 kJ?mol^?1, and 2.22 ± 0.17, respectively. The suggested creep mechanism involves GB sliding accommodated by GB diffusion and β?α SiC phase transformation.     

Constitutive modeling of polycarbonate during high strain rate deformation

A constitutive model is presented for large strain deformation of polycarbonate (PC) at high strain rates (above 10² s^?1). The proposed model considers the primary process (α) and the two secondary rate‐activated processes (β and γ). It is shown that the secondary transitions in the material affect the yield and post yield behavior of the material at high strain rates. The constitutive model has been implemented numerically into a commercial finite element code through a user material subroutine. The experimental results, obtained using a split Hopkinson pressure bar, are supported by dynamic mechanical thermal analysis (DMTA) and DSR (Decompose/Shift/Reconstruct) method. These are employed to gain understanding of the material transitions, and to further the linkages between material viscoelastic, yield, and stress–strain behavior. Comparison of model predictions with experimental data demonstrates the ability of model to capture the characteristic features of stress–strain curve of the material such as initial linear elasticity, global yield, strain softening, and strain hardening at very high strain rates (up to 10,000 s^?1). POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

Bulk and surface compression viscosities of high and low density polyethylenes

Bulk compression flow of high density polyethylene (HDPE) and low density polyethylene (LDPE) have been measured at temperatures of 423 to 463K, pressure up to 150 MPa, and bulk compression rates of about 1.0 to 200.00 × 10^?5 s^?1. Bulk and surface compression modulus of elasticity (L and K_s), longitudinal bulk compression viscosity (η_L), and surface compression viscosity (η) are described as a function of compression rates (k_v and k_s), compression deformations (k_v percent and K_s percent), and temperature (T). Bulk and surface compression flow activation energies are of the order of 40 to 100 KJ/mol and 3.84 KJ/mol, respectively.     

Strain-rate-dependent compressive and compression-shear response of an alumina ceramic

This paper assessed the microstructure and properties of CeramTec ALOTEC 98 SB alumina ceramic through microscopic characterization and mechanical experiments. The rate-dependent strength and failure response of an alumina ceramic were studied under both uniaxial compression and compression-shear loading. Under quasi-static uniaxial compression at rates of 10^?5 to 10³ s^?1, the strength had an average of 3393 ± 306 MPa, and at dynamic strain rates of 10² to 10³ s^?1, the strength ranged from 3763 to 4645 MPa. The CeramTec ALOTEC 98 SB alumina ceramic was found to have greater mechanical properties than other commercial alumina ceramics from the literature (i.e., AD-995). To monitor the strain field and the failure process of the alumina ceramic during testing, an ultra-high-speed camera coupled with digital image correlation (DIC) was used to visualize crack initiation and propagation processes, and obtain quantitative stress-strain information. A new data processing method was then proposed in this study to calculate the shear components for the compression-shear tests. Validation of the proposed method was confirmed by the shear strain obtained from the DIC analysis with the ultra-high-speed camera. Using the results obtained by the proposed model and the DIC analysis, new observations and understandings of failure mechanisms are obtained. (1) In compression-shear tests, the shear failure happens before complete failure, and shear behavior plays an important role during the failure process. (2) The equivalent peak stress (strength) of compression-shear test is smaller than the uniaxial compression one. (3) The directional cracks have weak influence on the compressive stiffness, but have a strong influence on the shear response.     

Experimental investigation on high strain rate compressive response of a ZrB2-SiC-G ceramic

The dynamic compression tests were conducted on a ZrB₂-SiC-graphite (ZrB₂-SiC-G) ceramic from the strain rate of 904–3136 s^–1 using the split Hopkinson pressure bar. The effects of strain rate on the compressive strength, critical strain, stress–strain relation, and fracture pattern were discussed from the experimental results. The results showed that the dynamic compressive response of this ZrB₂-SiC-G ceramic was obviously related to the strain rate at higher strain rates. At the strain rate of 3136 s^–1, the dynamic compressive strength, critical strain, and toughness of the ZrB₂-SiC-G ceramic increased to 1747 MPa, 0.0423, and 69.48 × 10⁶ J/m³, respectively. As the strain rate increased, the dynamic compressive strength and critical strain increased linearly, and the damage became more significant. Moreover, the energy absorption of the ZrB₂-SiC-G ceramic linearly increased with the strain rate, causing the ZrB₂-SiC-G ceramic fractured into numerous smaller fragments at higher strain rates.     

Rheology of polymer melts in high shear rate

Little is known of the rheology of polymer melts in the high shear rate up to 10⁶ s^?1 or more. A specially designed high-shear-rate rheometer was developed, by which the rheology of polymer melts for shear rates up to 10⁸ s^?1 can be investigated. Two non-Newtonian regions and a transition or the second Newtonian region were observed in the wide range of shear rates up to 10⁷ s^?1. The observed flow curves for various polymer melts are classified into three typical patterns. One is the flow curve typically shown of high-density polyethylene in which a clear second Newtonian region appears after the first non-Newtonian region. The second is the typical flow curve of polystyrene in which a “transition region” appears instead of the second non-Newtonian region. The third is the flow curve shown of acrylonitrile-styrene copolymer, which exhibits behavior between the two types. A generalized flow curve is proposed to explain the observed flow behaviors of various polymers over a wide range of shear rates. The flow behavior in high shear rate results from high orientation and scission of polymer molecules.     

The viscosity reducing effects of very low concentrations of a thermotropic copolyester in a matrix of HDPE

A thermotropic liquid crystalline polymer (TLCP), denoted as TLCP(1), was used as a processing aid for high density polyethylene (HDPE). The TLCP was a copolyester of hydroxybenzoic acid, hydroquinone and sebacic acid and was blended with HDPE to give 0.2, 0.5 and 2 wt% TLCP(1) blends. At a temperature of 185°C, when TLCP(1) is in the nematic regime, the processing window for HDPE was increased tenfold from a maximum shear rate of 100 s^?1 for pure HDPE to 1170 s^?1 for a 2 wt% TLCP(1) blend. There were large viscosity reductions of between 85% and 90% compared with the pure HDPE when the blended material was used at a wall shear stress value of approximately 2.4 × 10⁵ Pa. This viscosity reduction was attributed to the TLCP(1) droplets changing shape from spheres to slender fibrils under shear flows.     

Dynamic tensile strength of polyurea-bonded steel/E-glass composite joints

The dynamic tensile strengths of E-glass composite/polyurea and polyurea/steel interfaces within the E-glass composite/polyurea/AL-6XN stainless steel joint were measured using a laser spallation technique. Values of 370?±?20?MPa were obtained for the polyurea/composite interface while a much higher value of 486?±?20?MPa was obtained for the steel/polyurea interface. Because of the transient nature of the stress pulse, the strain rate changes continuously as the interface stress builds up. A peak strain rate of 5?×?10⁵?s^?1 was estimated. The effect of moisture on the tensile strength of the E-glass/polyurea interface was also examined. The effect was found to be minimal, with the tensile strength stabilizing at 320?±?25?MPa after 30?days of exposure to a 90%RH, 50?°C environment. When comparing the strengths of corresponding interfaces in an epoxy-bonded joint from a previous study, it was concluded that polyurea results in a much stronger and durable joint.     

Studies on high density polyethylene (HDPE) functionalized by ultraviolet irradiation and its application

The structure and properties of high density polyethylene (HDPE) functionalized by ultraviolet irradiation at different light intensities in air were studied by electron analysis, FTIR spectroscopy, contact angle with water, differential scanning calorimetry and mechanical properties measurement. The results show that oxygen‐containing groups such as C?O, C—O and C(?O)O were introduced onto the molecular chain of HDPE following irradiation, and the rate and efficiency of HDPE functionalization increased with enhancement of irradiation intensity. After irradiation, the melting temperature, contact angle with water and notched impact strength of HDPE decreased, the degree of crystallinity increased, and their variation amplitude increased with irradiation intensity. Compared with HDPE, the yield strength of HDPE irradiated at lower light intensity (32 W m^?2 and 45 W m^?2) increases monotonically with irradiation time, and the yield strength of HDPE irradiated at higher light intensity (78 W m^?2) increases up to 48 h and then decreased with further increase in irradiation time. The irradiated HDPE behaved as a compatibilizer in HDPE/polycarbonate (PC) blends, and the interface bonding between HDPE and PC was ameliorated. After adding 20 wt% HDPE irradiated at 78 W m^?2 irradiation intensity for 24 h to HDPE/PC blends, the tensile yield strength and notched Izod impact strength of the blend were increased from 26.3 MPa and 51 J m^?1 to 30.2 MPa and 158 J m^?1, respectively. Copyright © 2003 Society of Chemical Industry     

A correlation of Young's modulus with yield stress in oriented poly(vinyl chloride)

The variations of tensile and compressive yield stresses and of Young's modulus of oriented poly(vinyl chloride) sheet with direction and with degree of orientation, represented by birefringence, are shown. Young's modulus was calculated from elastic stiffness constants measured by an ultrasonic pulse method at 5MHz with estimated strain and strain rate amplitudes of 2 × 10^?5 and 100s^?1. Yield strains were about 5 × 10^?2 measured at strain rates of about 2 × 10^?2s^?1. Although the measuring conditions were so different there was found to be a close correlation between tensile yield stress and Young's modulus, the two quantities being connected by a simple linear relationship, as direction of measurement and degree of orientation were varied. Compressive yield stress did not correlate with Young's modulus, and changed little with direction or degree of orientation by comparison with tensile yield stress. The empirical linear relationship between tensile yield stress and Young's modulus, difficult to account for theoretically, might form the basis of a method for determining tensile yield stress ultrasonically.     

Uniaxial compression and combined compression‐and‐shear response of amorphous polycarbonate at high loading rates

We present results of a study conducted to better understand the yield and flow response of amorphous poly(bisphenol A carbonate), PC‐Lexan® (PC), under uniaxial compression and combined compression‐and‐shear impact loading. A split Hopkinson pressure bar (SHPB) is utilized to obtain nearly adiabatic uniaxial compression response of the PC in the strain‐rate range of 1000–2000 s^?1. Since temperature is expected to play an important role in governing the dynamic response of PC, nearly isothermal SHPB tests are also conducted and compared with the adiabatic response. In order to investigate the coupling of shear behavior and dilatation in PC at high loading rates, combined compression‐and‐shear plate impact experiments are conducted at strain‐rates in the range of 10⁵–10⁶ s^?1. In addition, novel plate impact experiments are conducted to better understand the evolution of the shearing resistance of PC in response to sudden alterations (drop) in hydrostatic pressure under extremely high shearing rates. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Uniaxial plastic deformation in the zirconia-based nanocrystalline ceramics containing a silicate glass

Nanocrystalline yttria-stabilized tetragonal zirconia polycrystal (nc-Y-TZP) powders coated with silicate based glasses were cold isostatically pressed and sintered near to the full density (98–99%). Two glasses with different compositions were used: 93 SiO₂–1 Na₂O–6 SrO (mol%) (designated as SNS glass) and 58 SiO₂–29 Al₂O₃–13 SrO (designated as SAS glass). Uniaxial compression tests of the pure (glass-free) nc-Y-TZP samples yielded strain rates as high as 2·10⁻⁴ s⁻¹ under 60 MPa at 1300 °C. Comparable strain rates were measured in the SNS glass-containing samples, with the maximum of 3·10⁻⁴ s⁻¹ at 1300 °C under a stress of 80 MPa (5 vol.% SNS glass content). Compression tests under 100 MPa exhibited relatively high strain rates of 5·10⁻⁴ and 10⁻⁴ at 1300 °C and 1200 °C, respectively, in the 15 vol.% SAS glass samples. The strain rates measured in the SAS glass-containing samples were achieved at temperatures lower by 100 °C compared to the similar strain rates in the glass-free and SNS glass-containing samples. The microstructure of the deformed samples was similar to that of samples before deformation, within which the ultrafine and equiaxed character of the grains was preserved. Clear evidence for cooperative grain boundary sliding was observed in the SAS glass-containing samples.     

Effect of holding time on high strain hysteresis loss of carbon black filled rubber vulcanizates

Hysteresis loss has been measured at constant stress and constant strain, at various holding times under tensile deformation of natural rubber (NR) and styrene-butadiene rubber (SBR) vulcanizates filled with various loadings of carbon black filler. The effects of temperatures (25°C to 150°C), strain rates (3.78 × 10^?5 sec^?1 to 210 × 10^?3 sec^?1) and strain levels (20% to 300%) have been studied. Hysteresis loss and hysteresis loss ratio increase with an increase in strain rate, filler loading, strain level and holding time. It decreases with an increase of temperature. However, higher hysteresis loss and hysteresis loss ratio are observed at constant stress than at constant strain. NR and SBR vulcanizates show similar behavior. Evidence has been produced for the existence of a distinct relaxation process that occurs within first 120 second of holding time at room temperature. This process becomes less important as the strain or the temperature is increased. However, at high temperature another distinct relaxation process has been observed. The activation energy has been found to be 66.3 kJ/mole for the rates at the higher holding time, while it has been found to be 17.3 kJ/mole for the rates at the lower holding time using the data of hysteresis loss at first cycle of 40 phr black filled NR vulcanizates.     

Performance of MWCNT/HDPE Nanocomposites at High Strain Rates

Performance of HDPE/MWCNT composite at high strain rate up to 10⁴ s^?1 was investigated in a split Hopkinson pressure bar. The results revealed that the incorporation of MWCNTs into HDPE can enhance the impact strength of HDPE. High strain rate impact has a significant influence on morphology, density, crystallinity and melting temperature of the composite. With increase in strain rate, the densities of both HDPE and HDPE/MWCNT composite decreased. The drop of the density of HDPE/MWCNT composite was quicker than that of HDPE density. This could be the reason that much more cracks were formed in the HDPE/MWCNT composite, which could result in high energy dissipation, during SHPB test. The corporation of MWCNTs did lead to the decrease in yield stress.