Viscoelastic behavior of HDPE polymer using tensile and compressive loading

Large containers for liquids, exposed to different static loadings, are mainly made of high-density polyethylene (HDPE). The viscoelastic response of HDPE under long-term tensile and compressive creep is investigated. Monotonic experiments under tension are performed over a wide range of strain rates. In these experiments, the transition in the damage mechanisms (development and propagation of contraction in the HDPE specimen) is analyzed. The monotonic tensile behavior of the HDPE is found to be nonlinear and depends on the strain rates. It is observed that both elastic modulus and plastic flow stress present an increase with displacement speed due to the viscoelastic behavior of HDPE. A similar observation can be made for monotonic compressive tests by developing a new experimental device that ensures accurate measurement of the strain. Such a device makes use of an extensometer of compressive displacement of the specimen. In addition, the long-term behavior of HDPE is evidenced through creep and relaxation tests at an imposed range respectively of lower stresses and strains. It is shown that the normalized curves, associated with these tests, can be represented by a single curve characterizing the compressive creep compliance or relaxation stresses versus time. The linearity of the viscoelastic behavior is confirmed within the linear domain of the monotonic compressive and tensile tests.     

纳米压痕法研究80Au/20Sn焊料蠕变应力指数

室温下通过恒加载速率/载荷纳米压痕法对铸态80Au/20Sn焊料进行了测试,得到了压痕试样的载荷—位移曲线.试验表明,加载速率对载荷一位移曲线影响显著,较低的加载速率产生了较大的位移变化;加载阶段,较低的加载速率导致了较大的压痕深度;载荷保持阶段,压痕深度的变化表明在该阶段发生了蠕变,较高的加载速率导致了较大的蠕变应变率.结果表明,基于压痕做功概念得到的蠕变应力指数与传统单轴拉伸或压缩蠕变测试得到的数值具有较好的一致性,该测试和分析方法可行.     

Uniaxial step loading test setup for determination of creep curves of oxidation-sensitive high strength materials in vacuum under tensile and compressive load

For long-term applications of components, such as in turbomachinery or automotive engineering, knowledge of creep behavior under increased load and temperature is of interest. Creep tests are commonly used to investigate the creep behavior of materials at a constant test temperature above room temperature under a constant force. The present work describes a so-called uniaxial step loading creep test setup and first results for a WC-Co hard metal under isothermal conditions at 700 °C in vacuum. Heating and temperature control within the tested specimen's gauge length were performed by induced eddy currents and a thermocouple, respectively. In contrast to conventional creep tests, the mechanical load is increased stepwise and the stress at each level is kept constant for 500 s. Displacement of the strain gauge markings was measured contactless with a laser extensometer. First tests were carried out for a WC-Co hard metal under compression and tensile loading. In order to avoid buckling of the high-strength material under compression, a special specimen geometry with non-constant specimen diameter was used. The minimum creep rate was determined for each applied tensile and compressive stress level. Under tensile load, minima of the creep rate were observed above a stress of 500 MPa that are interpreted as the secondary creep rates. Under compressive load, the respective creep rate minima were observed above a stress of −700 MPa.     

Al2O3,TiB2粒子增强铝基复合材料的动态压缩性能和高温蠕变性能

对ＴｉＯ２－Ａｌ－Ｂ和ＴｉＯ２－Ａｌ－Ｂ２Ｏ３体系制备的两种Ａｌ２Ｏ３和ＴｉＢ２原位粒子增强铝基复合材料进行了动态压缩试验和高温拉伸蠕变试验。动态压缩试验表明,随着应变速率的提高,复合材料的强度和初始加工硬化率明显增加。然而,复合材料中含有的条状Ａｌ３Ｔｉ对复合材料的动态机械响应基本没有影响。透射电镜观察表明,在高应变速率下两种复合材料强度和初始加工硬化率的明显提高可由复合材料基体中位错密度的显著     

Additive Manufacturing of Syntactic Foams: Part 2: Specimen Printing and Mechanical Property Characterization

High-density polyethylene (HDPE) and its fly ash cenosphere-filled syntactic foam filaments have been recently developed. These filaments are used for three-dimensional (3D) printing using a commercial printer. The developed syntactic foam filament (HDPE40) contains 40 wt.% cenospheres in the HDPE matrix. Printing parameters for HDPE and HDPE40 were optimized for use in widely available commercial printers, and specimens were three-dimensionally (3D) printed for tensile testing at strain rate of 10^?3 s^?1. Process optimization resulted in smooth operation of the 3D printer without nozzle clogging or cenosphere fracture during the printing process. Characterization results revealed that the tensile modulus values of 3D-printed HDPE and HDPE40 specimens were higher than those of injection-molded specimens, while the tensile strength was comparable, but the fracture strain and density were lower.     

Effect of nano-silver on microstructure,mechanical and tribological properties of cast 6061 aluminum alloy

Al6061 matrix with different amounts of nano-silver (1% and 2%) was produced by stir-casting method. Produced samples were characterized by hardness, tensile, compression and wear tests. The hardness of the specimens at room temperature was measured by Brinnell hardness testing machine. The magnitude of hardness increased evidently with the function of the mass fraction of the nano-Ag particle. The polished specimens were examined with an optical microscope. The fracture surfaces of tensile and compressive specimens were further examined by scanning electron microscopy. Wear mechanisms were discussed based on the scanning electron microscopy observations of worn surface and wear debris morphology. There is an increase in compressive strength, ultimate tensile strength, elongation and wear resistance of the Al-Ag composites compared with base alloy. The execution of stir-casting technique is relatively homogenous and fine microstructure which improves the addition of reinforcement material in the molten metal. The results show that Al6061-nano-silver which is the best combination of hardness can replace the conventional material for better performance and longer life.     

The role of grain boundaries in creep strain accumulation

To explain the seemingly complex results obtained when creep property measurements are described using power-law equations, it is commonly assumed that a transition from dislocation to diffusional creep processes occurs with decreasing applied stress. However, this assumption is negated by observations recorded for aluminium, copper and various particle-hardened alloys. Thus, all features of power-law behaviour for copper are predicted by new relationships based on normalization of the stress through the yield and ultimate tensile stresses determined from high-strain-rate tests at the creep temperatures. Moreover, the particle-free zones sometimes found on grain boundaries normal to the tensile axis exist prior to creep exposure, so do not provide definitive evidence for diffusional creep mechanisms. It is therefore proposed that the dominant dislocation processes differ only in detail as the test duration and temperature increase.     

Predicting creep strengths and lifetimes of creep resistant engineering alloys

The physical basis for predicting the long-term creep strengths and lifetimes at application temperatures using creep parameters determined from short-term creep tests is investigated for complex creep resistant engineering alloys. It is shown that the seemingly unpredictable stress and temperature dependence of minimum creep rate of such alloys can be rationalised using an approach based on the new power law creep equation that incorporate the tensile strength. This is demonstrated using the tensile and creep data measured for two completely different types of alloys: steel 11Cr-2W-0.4Mo-1Cu-Nb-V and Ni base superalloy 15Cr-28Co-4Mo-2.5Ti-3Al. For both alloys, the stress exponent n determined does not depend on temperature and activation energy of creep does not depend on stress. Consequently, it becomes possible to use the new power law creep equation in combination with the Monkman-Grant relationship to predict the long term creep rupture strengths and lifetimes and microstructure stability of the two alloys from short term creep test data. The implications of the results for creep mechanism identification and future microstructure analysis are discussed.     

Steady-state creep of a particulate SiC/6061 Al composite

The creep behavior of a 10 vol.% silicon carbide particulate reinforced 6061 Al composite produced by powder metallurgy (PM) has been examined by creep tests in both tension and compression at 400°C. The tensile creep data covering minimum creep rates of the orders 10⁻⁹ to 10⁻⁴/s show an apparent stress exponent n_app≈13, but a comparison with compressive creep data reveals that some high strain-rate data in tension are due to the transition to the tertiary stage. Analysis of the data is made only for the steady-state creep rate, together with that for an unreinforced PM 6061 Al alloy, by incorporating a threshold stress. This gives a stress exponent n=3 for the matrix alloy, whereas the composite data show such a trend that the n value gradually changes from 3 to 1 as the effective stress increases. A new method of steady-state creep data analysis is formulated by taking account of the interface-confined diffusional flow and thereby the finding above is reasonably assessed.     

Verhalten von Chemieapparate-Werkstoffen bei höheren Temperaturen

Behaviour of materials for chemical apparatus at higher temperatures Following an introductory survey of the essence and significance of static data (short-time and long-time tensile test at higher temperatures), the creep process associated with the latter is discussed and its effect on the service life of chemical apparatus indicated. On the basis of results of practical tests with nonferrous and ferrous metals, the statistical significance of conventional and desirable presentation results is discussed.     

Microstructure,tensile properties and compressive creep resistance of Mg-(5-8.5)%Sn-2%La alloys

The microstructure,tensile properties and compressive creep resistance of permanent-mould cast Mg-(5-8.5)%Sn-2%La (mass fraction) alloys were investigated.The results show that Mg-(5-8.5)%Sn-2%La alloys are all composed ofα-Mg phase, Mg_2Sn and Mg-La-Sn compounds.Compared with those of Mg-5%Sn binary alloy,the grain size and the content of Mg_2Sn compound in Mg-5%Sn-2%La alloy are decreased.With the increase of Sn content in Mg-(5-8.5)%Sn-2%La alloys,the content of Mg_2Sn compound increases,while that of...     

The high temperature tensile and compressive deformation characteristics of magnesia doped alumina

The mechanical characteristics of alumina have not yet been characterized completely in tension due in part to strain hardening accompanying grain growth and premature cavitation failure. Tensile tests were conducted on fine grained magnesia doped alumina over a range of strain rates, grain sizes and temperatures to evaluate the stress exponent, inverse grain size exponent and activation energy. Constant stress compression creep tests were also carried out under a similar range of experimental conditions. Extensive microstructural characterization after deformation indicated that there was considerable grain growth during deformation; however, the grains retained their initially equiaxed structure after significant deformation. Although a standard plot of strain rate versus stress indicated a stress exponent of ∼2, a complete analysis including the compensation of data for concurrent grain growth revealed that true stress exponent was ∼1, consistent with diffusion creep. It is argued that grain rearrangement processes accompanying grain growth will tend to mask the development of an elongated grain structure predicted by diffusion creep processes. In contrast to several ceramics with a significant amount of glassy phase, there is no significant difference between the elevated temperature tensile and compressive behavior of alumina.     

Impression Creep Behavior of 316LN Stainless Steel

Impression creep tests have been carried out at 923 K on 316LN SS containing 0.07, 0.14, and 0.22 wt.% nitrogen, under different applied stress levels. It was observed that the impression creep depth versus time curves were similar to the creep curves obtained from conventional uniaxial creep tests. The impression creep curves were characterized by a loading strain and primary and secondary creep stages similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep curves was absent. The steady-state impression velocity was found to increase with increasing applied stress. The equivalent steady-state creep rates calculated from impression velocities were found to be in good agreement with the steady-state creep rates obtained from conventional uniaxial creep tests. Equivalence between applied stress and steady-state impression velocity with uniaxial creep stress and steady-state creep rate, respectively, has been established based on the laws of mechanics for time-dependent plasticity. It was found that impression velocity was sensitive to the variation in nitrogen content in the steel; impression velocity decreased with increasing nitrogen content, and the results obtained in this study were in agreement with those obtained from uniaxial creep tests.     

Effect of tensile and compressive stresses on the radiation swelling and creep strain in austenitic steel Kh18N10T

Effect of tensile and compressive stresses on the radiation swelling, microstructure, and creep strain in austenitic steel Kh18N10T is considered. The gas-filled samples of a complex shape prepared from steel Kh18N10T were irradiated in a BOR-60 reactor for 2 years to a damaging dose of 15 dpa at a temperature of 420–450°C. In the shells of the irradiated samples, compressive and tensile stresses were created. Samples were also irradiated, in which these stresses practically were absent.     

Creep and damage investigation of advanced martensitic chromium steel weldments for high temperature applications in thermal power plants

Abstract

With the aim to increase base material creep strength and overcome the type IV cracking problem, a new design concept was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening through solid solution with precipitation strengthening by finely dispersed nitrides. In this work, uniaxial creep tests of the MARBN base material and welded joints have been carried out. The creep strength of the welded joints was analysed, and the evolution of creep damage was investigated. The creep tests of MARBN revealed increased strength of the base material of about +20% compared to the best commercially available 9Cr steel grade. At higher stress levels, the creep strength of crosswelds is between that of the MARBN base material and the conventional 9Cr base materials. Nevertheless, long term creep tests revealed a drop in creep strength of the MARBN welded joints. The underlying phenomena of crossweld creep behaviour are discussed in detail.     

Aging effects on the creep behavior of the near-alpha titanium alloy Ti-1100

During aging of the silicon-bearing, near-alpha titanium alloy Ti-1100 at the service temperature of 593 °C, the precipitation of Ti₃AI and silicide particles has been observed. The objective of this article is to determine the influence of these precipitates on creep behavior. Stress relaxation tests, with the advantage of needing only a short time to produce a complete creep curve, were used to determine the creep behavior of Ti-1100. These accelerated creep tests reduce the chance of metallurgical changes that could occur in the long time required to perform conventional creep tests. The creep results obtained from the stress relaxation tests were verified with those obtained from conventional monotonic creep tests. Aging of the material for 500 h at 593 °C, where full precipitation of Ti₃AI is assumed to be incomplete, resulted in a small decrease in creep resistance compared to the unaged condition. Specimens aged for 1000 h, which corresponds to the averaged condition in which the precipitation of Ti₃AI is expected to be completed, were also examined. It was observed that these specimens yielded creep resistance similar to that of the unaged condition. Material aging for 1000 h followed by a special heat treatment, which results in the dissolution of Ti₃AI particles, although not affecting the silicide precipitates, resulted in the poorest creep resistance of the aging conditions investigated. The major conclusion of this study is that the presence of Ti₃AI has a beneficial influence on the creep resistance of Ti-1100 alloy.     

两种不同变形方式对NiTi合金回复特性的影响

研究了马氏体再取向(MR)和应力诱发马氏体转变(SIM)两种不同变形方式对Ni50.2Ti49.8合金拉伸性能及预应变后加热回复特性的影响.结果表明:拉伸变形中,MR变形方式的应力平台结束时的应变值较SIM变形方式大.在拉伸预应变处于应力平台阶段时,两种变形方式在相同预应变后加热回复有相同的记忆能力;在拉伸预应变大于应力平台时,两种变形方式在相同预应变后加热回复,SIM变形方式的逆相变温度和回复应变略高于MR变形方式.     

Continuous extrusion and tensile strength of self-reinforced HDPE/UHMWPE sheet

A specially designed fish-tail shaped extrusion die was used to continuously extrude self-reinforcing sheet of HDPE/UHMWPE. The result indicated that under conventional extrusion conditions, the tensile strength of extruded sheet was comparable to conventional molded HDPE samples and was almost the same in both MD (machine direction) and TD (transverse direction) directions. While at temperatures slightly above the melting point of HDPE, compared with conventionally extruded sheet, above 6-fold and 3-fold improvements were obtained in the MD and TD direction for HDPE/UHMWPE (I) system in the optimum extrusion windows of 15–30 MPa extrusion pressure, the die temperature ranging from 126 °C to 137 °C and a rotation speed of 15 rpm. For HDPE/UHMWPE (III) system, in the optimum processing windows, that is, 15–30 MPa extrusion pressure, 131–139 °C die temperature and 15 rpm extruder rotation speed, about 5-fold and 2–3-fold improvements were obtained in the MD and TD direction compared with conventionally extruded sheet, respectively.     

Basic model for primary and secondary creep in copper

A model for primary and secondary creep is established. The starting point is a combination of basic models for tensile stress-strain curves and for secondary creep. The derived model can describe experimental creep strain curves for Cu 50 p.p.m. P in the temperature interval 75-250 °C with the same precision as the variation in the experimental creep strain curves for identical conditions. No fitting parameters are involved in this representation. The model has also been applied successfully to creep specimens with round notches, where multiaxial stress states are present. One main aim of deriving a basic creep model is to improve the accuracy of extrapolation. That this is possible has been demonstrated for creep tests with estimated secondary creep rates down to 5 × 10⁻²² s⁻¹. In conventional creep rupture testing strain rates down to 1 × 10⁻¹² s⁻¹ can be recorded. As a background, a summary of empirical methods for the extrapolation of creep strain data is given.     

Additive Manufacturing of Syntactic Foams: Part 1: Development,Properties, and Recycling Potential of Filaments

This work focuses on developing filaments of high-density polyethylene (HDPE) and their hollow particle-filled syntactic foams for commercial three-dimensional (3D) printers based on fused filament fabrication technology. Hollow fly-ash cenospheres were blended by 40 wt.% in a HDPE matrix to produce syntactic foam (HDPE40) filaments. Further, the recycling potential was studied by pelletizing the filaments again to extrude twice (2×) and three times (3×). The filaments were tensile tested at 10^?4 s^?1, 10^?3 s^?1, and 10^?2 s^?1 strain rates. HDPE40 filaments show an increasing trend in modulus and strength with the strain rate. Higher density and modulus were noticed for 2× filaments compared to 1× filaments because of the crushing of some cenospheres in the extrusion cycle. However, 2× and 3× filament densities are nearly the same, showing potential for recycling them. The filaments show better properties than the same materials processed by conventional injection molding. Micro-CT scans show a uniform dispersion of cenospheres in all filaments.