On creep test loading for soft polymers

The dead-weight loading speed in the creep test for soft polymers was investigated. The optimum dead-weight loading speed can be determined so as to assure instantaneous elastic deformation and, at the same time, also avoid dynamic effects. It is found that small dynamic effects are inevitable for soft polymers such as polypropylene even at optimum speed. The experimentally determined optimum dead-weight loading speed for polypropylene was 300 mm/min for 30 kg dead-weight at 16°C.     

Nanoindentation creep and glass transition temperatures in polymers

The nanoindentation creep behaviour of several different polymers has been investigated. The extent of creep ε is represented by the Chudoba and Richter equation: ε = ε_eln(ε_rt + 1), where t is the loading time and ε_e and ε_r are material constants. Creep was determined in this way for a variety of polymers at T_exper = 301.7 K. Some of the materials studied were far above, some far below and some near their glass transition temperatures T_g. The creep rate ε_r was plotted as a function of y = (T_g ? T_exper); a single curve was obtained in spite of a large variety of chemical structures of the polymers. The ε_r = ε_r(y) diagram can be divided into three regions according to the chain mobility. At large negative y values, the creep rate is high due to the liquid‐like behaviour. At large positive y values in the glassy region, the creep rate is higher than that in the negative y‐value region; the creep mechanism is assigned to material brittleness and crack propagation. In the middle y range there is a minimum of ε_r. These results can be related to glassy and liquid structures represented by Voronoi polyhedra and Delaunay simplices. The latter form clusters; in the glassy material there is a percolative Delaunay cluster of nearly tetrahedral high‐density configurations. The creep mechanism here is related to crack propagation in brittle solids. In the liquid state there is a different percolative Delaunay cluster formed by low‐density configurations, which, as expected, favour high creep rates. Copyright © 2007 Society of Chemical Industry     

Crazing limit of polymers in creep and stress relaxation

The appearance of the first visible damage in polymers in the form of crazes or microcracks may be assumed to be a sign of failure. Experimental investigations have shown that in uniaxial creep and stress relaxation experiments, under isothermal conditions, a certain time between the quasi-spontaneous loading and visible crazing is needed. This ‘incubation time’ is very strongly dependent on the magnitude of the quasi-spontaneously applied stress or induced strain. Based on the Reiner-Weissenberg theory of strength, simple relations allowing the prediction of crazing are developed. The agreement between theoretical computation and experiment is very good.     

Indentation creep of polymers. I. Experimental

An experimental setup is proposed to perform creep indentation tests on thermoplastics. An infrared (IR) lamp is used to locally heat the sample, and a flat punch made of tungsten carbide is loaded on the sample surface. In order to show the capability of this test in evaluating the creep properties, creep indentation tests were carried out on PA66 and HDPE sheets at different temperatures and creep loads. Dynamic mechanical analyses (DMA) and uniaxial creep tests were also performed at different temperatures to have a comparison with the indentation test results. Master curves were built for all the test results, and time shift factors were extracted. In all cases, the logarithm values of the shift factor were linearly dependent on the temperature. Small differences were found in the shift factors of the DMA and uniaxial test results, whereas high differences were observed in the case of the indentation results due to the nonuniformity of the temperature in the sample. However, indentation creep compliance is strictly correlated with tensile creep compliance, and equivalent tensile creep properties can be extracted. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Understanding of scratch-induced damage mechanisms in polymers

Following the ASTM and ISO test standards, a series of scratch tests were carried out on four categories of polymers: I) ductile and strong, II) ductile and weak, III) brittle and weak, and IV) brittle and strong. The scratch damage features were characterized by using a desktop scanner for scratch visibility assessment, and optical and electron microscopes for detailed damage mechanisms investigation. Various scratch damage mechanisms were identified for the different categories of polymers. The effect of testing rate on possible alteration of scratch damage mechanisms was also studied. The stress fields experienced by the polymer during scratch were determined using three-dimensional finite element methods modeling. It is found that both the material characteristics and the complex stress state exerted on the scratched surface are responsible for the various scratch damage mechanisms observed. A generalized scratch damage mechanism map for polymers is presented. The usefulness of the above understanding for designing scratch-resistant polymers is also discussed.     

Characterization of the thermomechanical behaviour of polymers: non-isothermal creep deflection processes

A non-isothermal creep process of a thermorheologically simple polymer is considered. For such a material, if the results of isothermal creep experiments are known, the creep compliance can be calculated as a function of time for any given thermal history by a suitable method. The results of creep deflection measurements on a sample of commercial polystyrene are reported. The data isothermally obtained at different temperatures are determined so as to obtain the master curve of the compliance as a function of time and the shift factor as a function of temperature. As an example, the above method is applied to predict the course of a laboratory test which is commonly used to determine the heat deflection temperature of thermoplastic polymers. The experimental results agree satisfactorily with the predicted values.     

Tensile yield energy in glassy polymers

An activation energy theory of yielding in glassy polymers has been proposed by Starita and Keaton. It predicts a linear relation between energy to yield and test temperature. This concept was tested and found valid for five amorphous polymers; poly(methyl methacrylate), polycarbonate, polyarylsulfone, polysulfone and poly(vinyl chloride), in uniaxial extension at temperatures from 25°C to the glass transition. For extenstion rates of 0.2 in/min, yield energy was found to go to zero at T_g, as commonly determined by thermodynamic methods like dilatometry or scanning calorimetry. The effect of other extension rates, plasticizer and molecular weight appears to affect only the intercept much as T_g is affected by the same changes. The slope, or the ratio of thermal to mechanical energy efficiency in overcoming the flow activation barrier, is largely unchanged.     

Description of creep of complex fibres made from rigid-chain polymers

The possibility of applying the temperature—time analogy principle for quantitatively describing creep of SVM fibre was demonstrated. An increase in the modulus of elasticity of SVM fibre was found in the region of nondestructive stress.     

Cushioning energy dissipation in foam polymers

Derivation of dynamic hysteresis loops (force deflection curves) of shock absorbing bushing materials by means of double integration of accelerometer signals is described. The significance of energy dissipation capability of expanded polymers with associated peak deceleration and maximal strain is discussed along with damping characteristics. A “Cushion Efficiency Factor” is introduced, in terms of the ratio of peak acceleration transmitted by a cushion to the energy dissipated per unit cushion area. Three types of expanded polymers commonly used as cushioning materials are evaluated accordingly. Expanded polyethylene (2,2 lb/cu ft), expanded polyurethane (2.1 lb/cu ft) and polystyrene filled polyurethane (1.7 lb/cu ft), all conditioned at 73°F and 50 percent RH. For each of these, two effective drop height tests were conducted, with heights of 12 in. and 42 in. Depending on the polymer type, peak product accelerations were between 33–38 g's in the 12 in. drops and 65–118 g's in the 42 in. drops, with corresponding maximal cushion strains of 0.15–0.20 and 0.39–0.47, respectively. Energy dissipating capabilities of polymers tested during first half cycle following drops were 148–179 lb in. and 477–708 lb in., respectively, depending on polymer type. This approach may facilitate optimal selection of cushioning materials for shock protection in packaging, as well as enable development of improved polymers for cushioning applications.     

An environmental instrument for measuring creep and stress relaxation in polymers

The construction and operation of an instrument for measuring tensile stress relaxation and creep, particularly of polymers, is described. The instrument is comparatively inexpensive to build and enables measurements to be carried out in vacuo or in a controlled atmosphere of gas or vapor. The design is based on principles used for some earlier stress relaxometers modified to enable characterization of samples having a very wide range of moduli either as stress relaxation or, additionally, as creep measurements. The instrument can therefore be used to evaluate material properties of hard plastics or of soft rubbers when exposed to selected environments.     

Non-linear creep of polymers under superimposed static and dynamic stress

The non-linear creep of polymeric materials under super-imposed static and dynamic stress is considered theoretically. The equation of state due to Green, Rivlin, and Spencer to-gether with the power law of time dependence for the kernel functions as suggested by Nakada is assumed to characterize the non-linear materials. Expressions for creep strain under constant static, oscillatory dynamic and superimposed static and dynamic stress are obtained in terms of the material constants and time dependent functions, called dynamic creep functions. It is shown that the creep strain due to dynamic stressing is damped as the number of stress cycles is increased. Damping is faster if the power law of time dependence is high. Expressions for the cumulative creep strain after multiple stress cycles are also obtained in terms of cumulative strain functions. All these functions are evaluated numerically at one thousand stress cycles. Finally, a special case of stress history is considered where the stress periodically reaches zero. It is shown that the ratio of the strains due to dynamic and static stressing can be characterized by the power law parameter when the mean stress is either very high or very low. Due to the slow damping when the power law parameter is small, the decrease of the strain ratio with number of cycles is slow compared to higher power law parameters.     

Mechanisms of energy absorption in the creep fracture of woven ceramic composites

A micromechanical model is developed which accounts for the energy absorbed in the creep deformation and fracture of a 2.5D SiC/C/SiC composite, representative of the new generation of non-oxide CMCs. The model quantifies the influence of the geometrical, mechanical and material parameters and, in particular, is very sensitive to the interfacial sliding stress. The effect of the sliding stress on the contribution of the different energy absorbing mechanisms in the creep fracture of CMCs is described. It is concluded that, for all testing conditions, most of the energy absorbed in the creep fracture is controlled by fibre–matrix debonding and fibre pull-out.     

Estimation of the surface free energy of polymers

A method for measuring the surface energy of solids and for resolving the surface energy into contributions from dispersion and dipole-hydrogen bonding forces has been developed. It is based on the measurement of contact angles with water and methylene iodide. Good agreement has been obtained with the more laborious γ_c method. Evidence for a finite value of liquid-solid interfacial tension at zero contact angle is presented. The method is especially applicable to the surface characterization of polymers.     

Effects of the printing parameters on short-term creep behaviors of three-dimensional printed polymers

Polylactic acid (PLA), despite its widespread use in three-dimensional (3D) printing technique, is lacking in the literature on creep behavior due to the printing parameters. Also, the potential use of carbon fiber-reinforced composites as 3D printing materials is remarkable as it improves mechanical properties of the produced parts. Therefore, it is important to find out the positive/negative effects of composite filaments on creep strength. The main purpose of this research is to examine the creep behaviors of PLA and PLA composite produced with 3D printer and to reveal the effects of the printing parameters on the short-term creep. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47564.     

A novel methodology to model the creep behavior of polymers at different temperatures

A new methodology for modeling the creep behavior of polymers at different temperatures, by using phenomenological constitutive models, is presented in this paper. The viscoelastic model is given by a combination of springs and dashpots and is used to describe the nonlinear response of polymers, and the viscoplastic formulation is given by a power-law equation. The approach proposed in this work is based on building master curves for different stress levels, and finding the dependency of the constitutive parameters with the temperature. After fitting the equations to the tensile creep tests at different temperatures, the final constitutive formulation is capable of modeling the behavior of polymers at any stress level and temperatures. Poly methyl metacrytale (PMMA) was used to investigate the accuracy of this proposal, and the results showed good agreement with the experimental data.