The impact fracture toughness of plasticized cellulose acetate and the relationship with the secondary loss transition

The impact fracture toughness of plasticized cellulose acetate has been determined by linear elastic fracture mechanics over the temperature range 20–70°C. A major peak in fracture toughness at ?10°C has been found to correspond to a secondary loss transition in the dynamic mechanical loss spectrum of plasticized cellulose acetate. At ?10°C, the fracture toughness is directly proportional to the log of the maximum strain rate.     

Effect of curing and heat treatment history on the dynamic mechanical response and the pore structure of hardened cement paste

Results for the dynamic mechanical response of hcp (hardened cement paste) specimens as a function of the curing-heat treatment history are reported. The temperature range investigated is from ?160°C to +100°C. In the temperature range from +25°C to +100°C, specimens cured at room temperature (R-cured) show a partially irreversible transition (reduction) in E. There are two low temperature transitions: the “adsorbate transition” between ?160°C and ?6-°C, and the “capillary transition” between ?50°C and 0°C. Both of these transitions are significantly affected by the curing-heat treatment history. Furthermore, both the E-modulus and the BET (water) surface area decrease as the severity of heat treatment increases.     

Mechanical and dielectric properties of bulky side chain poly(methacrylates). Analysis of the low frequency phenomena. 1: Poly(5-indanyl methacrylate)

This paper composes the dynamic mechanical and dielectric relaxation properties for Poly (5-indanyl methacrylate) (P51M), a polymer with a bulky side chain. Measurements were carried out from about ?100°C to near 250°C. A secondary loss peak was observed in dielectric measurements at room temperature (20°C at 0.1 Hz.), whereas nothing was resolved in the mechanical spectrum. A prominent α relaxation associated with the glass transition was also observed near 107°C at 0.1 Hz. in the dielectric and the mechanical spectra. Dipolar dielectric loss overlapped with conductivity at high temperatures and low frequencies. A new method to split the conductive, interfacial, and dipolar contributions to the spectrum is proposed.     

An investigation into the relationship between the impact performance of rotationally molded polyethylene products and their dynamic mechanical properties

In this paper, a study of the relationship between the impact performance of rotationally molded polyethylenes and their dynamic mechanical properties is carried out. A wide range of conventional linear low density polyethylene powders and met‐allocene catalyzed linear low density polyethylene powders were rotationally molded and tested. Instrumented falling weight impact tests were carried out over a temperature range of ?60°C to 20°C. Dynamic Mechanical Thermal Analysis (DMTA) tests were also carried out between ?100°C and 90°C, at a frequency of 100 Hz. Comparisons between the impact performance of metallocene catalyzed LLDPEs and Ziegler‐Natta LLDPEs are made. The transitions evident in the DMTA results are related to changes in impact performance with temperature. The beta transition is found to fall in the transition region between high impact performance at low temperatures and lower impact performance at ambient temperatures.     

The ductile-brittle transition of low-density polyethylene

The impact behavior of a low-density polyethylene was studied with an instrumented Charpy tester. A change from elastic or ductile response to brittle fracture was observed over a small temperature interval, usually within 1°C. This characteristic impact transition temperature (ITT) was highly sensitive to shallow, sharp notches. Whereas an unnotched test bar had a very low impact transition temperature of ?94°C, a razor cut with a depth of only 5 percent of the total thickness raised it to ?4°C. The impact transition temperature was effectively reduced by increasing the cooling rate during specimen preparation and by the addition of nonpolar liquids, On the other hand, impact properties were adversely affected by aging, annealing, and adding other thermoplastics.     

Temperature-dependent mechanical and long crack behavior of zirconium diboride–silicon carbide composite

Hot pressed ZrB2–20 vol.% SiC ultra-high temperature ceramic composites have been prepared for strength and fracture investigations. Two composites fabricated under differing hot pressing temperatures with (ZSB) and without (ZS) B4C sintering aids were selected for room temperature modulus of rupture (MOR) strength and single-edge-notch bend (SENB) fracture toughness experiments. Structure property relationships were examined for both composites. MOR and stiffness temperature dependence was also investigated up to 1500 °C. Long crack propagation studies were conducted up to 1400 °C using the double cantilevered beam geometry with half-chevron-notch initiation zones. Residual Boron-rich carbide maximum particle sizes were found to be strength limiting in ZSB billets while SiC controlled strength in ZS billets. Flexure strength decreased linearly with temperature from 1000 to 1500 °C with no visible plastic deformation prior to fracture. Similar stiffness decreases were observed with a transition temperature range of 1100–1200 °C. Long crack studies produced R-curves that show no significant toughening behavior at room temperature with some modest rising R-curve behavior appearing at higher temperatures. These studies also show the plateau toughness increases with temperature up to 1200 °C. This is supported by an observed transition from primarily transgranular fracture at room temperature to primarily intergranular fracture at high temperatures. Wake zone toughening is evident up to 1000 °C with K_R rise from 0.1 to 0.5 MPa√m. Beyond 1000 °C fracture mechanism transitions to include creep zone development ahead of crack tip with wake zone toughening vanishing.     

Dynamic mechanical and impact properties of PP/SEBS blend

Studies on impact behaviour of the blend of isotactic polypropylene (PP) with styrene-b-ethylene-co-butylene–b-styrene triblock copolymer (SEBS) in the composition range 0–25 wt % SEBS at three temperatures, viz., ambient, ?30°C, and ?190°C, are presented. Dynamic mechanical properties on a torsion pendulum in the temperature range ?100?100°C are also studied for this blend at various compositions. Scanning electron microscopic studies of the impact-fractured surfaces are presented to illustrate the differences in the mode of fracture at the three temperatures of impact tests. Choice of the three temperatures for impact tests was such that the effect of shear yielding mechanism of toughening of PP at ambient temperature remains suppressed at ?30°C, whereas at the lowest temperature (i.e., ?190°C) the elastomeric role of the inclusion SEBS is suppressed. The observed considerably large difference in impact toughening at ambient temperature and at ?;30°C seems not entirely accountable by the prevalence of shear yielding or crazing mechanisms in the respective temperature regions. A third mechanism, viz., viscoelastic energy dissipation, is invoked to account for the observed large difference of impact toughening at these two upper temperatures. Correlation of peak area of dynamic mechanical loss peaks occurring below the impact test temperature with the impact strength is also shown. This suggests greater significance of viscoelastic energy dissipation mechanism in the toughening of this blend at ambient temperature than at ?30°C.     

A dynamic mechanical study of the effect of chemical variations on the internal mobility of linear epoxy resins (polyhydroxyethers)

A series of polyhydroxyethers prepared from various bisphenols and their diglycidyl ethers has been investigated for dynamic mechanical behavior in a temperature range from ?180°C. up to their glass-rubber transition region, employing a torsion pendulum method for frequencies of about 0.1 hz and a vibrating-reed method for frequencies of about 100 hz. Glass-rubber transition temperatures ranged from 85 to 170°C. and are interpreted in terms of polarity, segment bulkiness, and packing capability of the polymer molecules. In most cases the activation energy of this transition was found to be extremely high, of the order of 500 kcal./mole. Fair consistency is found regarding the role played by methyl or chlorine substitution of the bisphenol rings, increasing bulkiness and steric hindrance, but reducing polar interaction. At least one and, in some cases, two secondary transitions were found well below the glass-rubber transition region. One is ascribed to a change in mobility of the glyceryl ether segments common to all nonesterified polymers of the series. While this is in general agreement with the findings of other authors, we found a spread in transition temperature values (?70 to ?102°C. at ≈ 1 hz) and activation energies which can be related to the detailed chemical structure of these polymers. With methyl substitution of the bisphenol rings of the highly polar polymers symmetry seems to be an important factor. In addition, another secondary transition appeared, which we ascribe to an onset of ring rotation.     

Zur Umwandlung der polymorphen Struktur von Polybuten-1 unter der Wirkung mechanischer Spannungen

Polybutene-1 crystallizes in the tetragonal form II during cooling of the melt. This form II is unstable and slowly transforms into the stable hexagonal form I. The rate of this crystal-crystal-transition can be considerably increased by applying mechanical stress to the sample. The effect of uniaxial tensile stresses on the II to I phase transition has been studied in a broad temperature range from ?196°C up to just below the melting point of the form II (ca. 115°C). The results can be summarized as follows:

• 1 Below the freezing temperature of the γ-process (?150°C) no Crystalline transformation can be observed. The polymer shows an “ideal” brittle fracture.
• 2 There is, furthermore a (macroscopic) brittle-like fracture at higher tempera-tures between ?150°C and ?70°C. On the fracture surface itself, however, a partial transformation from modification II to modification I has taken place during fracture. This can be considered as an evidence for a microscopic ductile fracture process.
• 3 Above ?70°C up to ?20°C necking occurs during stretching. Within the neck an almost quantitative transformation from I1 to I has taken place.
• 4 A t temperatures above the glass transition up to about 70°C a macroscopic homogeneous deformation of the samples is found. The amount of transformed material shows a complicated temperature dependence which can be explained by cooperative effects between the crystal-crystal transformation and the molecular mobility within the amorpheous regions on the basis of the relaxation behavior of this material.

     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.     

The mechanical behavior of polyarylsulfone

A detailed study of the fracture behavior of polyarylsulfone was conducted over a temperature range of ?175 to 120°C. Both fatigue crack propagation and fracture toughness tests were run as well as forced torsion pendulum tests to characterize the dynamic properties. The polymer exhibited a broad secondary loss peak and a high glass transition temperature at ?110 and 295°C, respectively. Fracture toughness, K_IC, and fatigue crack growth resistance were found to vary similarly with temperature, minima being observed near ?50°C. Below that temperature, both a rise in toughness and in fatigue resistance is associated with the broad secondary loss peak. The slopes of the log fatigue crack velocity (da/dN) vs log stress intensity range (ΔK) curves varied from 2.6 to 13.2. Since the equation da/dN = α(ΔK)ⁿ described all of the data, the log-log slope or exponent, n = ?ln(da/dN)?ln ΔK, was considered as a stress intensity sensitivity index with respect to fatigue behavior. This index was at a maximum near ?50°C, where the minimum in toughness occurred. A kinetic model was utilized to correlate the stress intensity sensitivity index and suggested that a single thermally activated mechanism controls the low temperature mechanical behavior of polyarylsulfone.     

Effect of molecular weight on brittle‐to‐ductile transition temperature of polyetherimide

The fracture and yield strength of polyetherimide was evaluated over a temperature range of 23 to 140°C for materials with number‐average (M_n) and weight‐average molecular weight (M_w) ranging from 15.6 to 22.8 and 36.6 to 52.3 kg/mol, respectively. The brittle‐to‐ductile transition temperature, where an equal probability exists that an impact will result in a brittle or ductile failure, was determined by evaluating the temperature at which fracture and yield strength are equal. The transition temperature decreased from 155 to 60°C with increasing molecular weight and provided a measure of relative ductility between material samples. As a case study, the practical impact strength of an injection‐molded food service tray was determined at 20°C and correlated with fracture strength as a function of molecular weight. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1666–1671, 2004     

Experimental Investigation on the Damping Behaviors of Epoxies with Different Epoxy Value

In this article, five different epoxies including a new kind of flexible epoxy having low glass transition temperatures (11 ～ 28°C) were prepared using polyamine as curing agent. Damping mechanical tests show that compared with other common available epoxies, the flexible epoxy has high loss factor over broad frequency and temperature range. Activation energy corresponding to glass transition process of different epoxies has been calculated from the temperature corresponding to tan δ_max values, obtained at different measurement frequencies. The maximum value of loss factor is 0.71 and the Tg varies from 11 to 28°C, indicating the flexible epoxy can be used as damping polymer materials in common temperature or frequency range.     

Continuous Gradient Temperature Raman Spectroscopy of Oleic and Linoleic Acids from −100 to 50 °C

We analyzed the unsaturated fatty acids oleic (OA, 18:1n‐9) and linoleic (LA, 18:2n‐3), and a 3:1 LA:OA mixture from ?100 to 50 °C with continuous gradient temperature Raman spectroscopy (GTRS). The 20 Mb three‐dimensional data arrays with 0.2 °C increments and first/second derivatives allowed rapid, complete assignment of solid, liquid, and transition state vibrational modes. For OA, large spectral and line width changes occurred in the solid state γ to α transition near ?4 °C, and the melt (13 °C) over a range of only 1 °C. For LA, major intensity reductions from 200 to 1750 cm^?1 and some peak shifts marked one solid state phase transition at ?50 °C. A second solid state transition (?33 °C) had minor spectral changes. Large spectral and line width changes occurred at the melt transition (?7 °C) over a narrow temperature range. For both molecules, melting initiates at the diene structure, then progresses towards the ends. In the 3:1 LA:OA mixture, some less intense and lower frequencies present in the individual lipids are weaker or absent. For example, modes assignable to C8 rocking, C9H–C10H wagging, C10H–C11H wagging, and CH3 rocking are present in OA but absent in LA:OA. Our data quantify the concept of lipid premelting and identify the flexible structures within OA and LA, which have characteristic vibrational modes beginning at cryogenic temperatures.     

Adhesive Fracture Energies of Some High Performance Polymers

A fracture mechanics approach was applied to determine the adhesive fracture energy of various high performance polymers. These polymers, including both thermosetting and thermoplastic materials, generally offer higher temperature capability than conventional epoxies. Double tapered cantilever beam specimens were used for fracture tests at both room temperatures and 225°C. The adhesive fracture energies of a tetrafunctional epoxy and a phthalocyanine resin were also determined at low temperatures. Adhesive fracture behavior of polymers at high temperatures was found to depend on polymer glass transition temperature, whereas at low temperatures it was related to secondary relaxation processes in the glassy state.     

High‐temperature tensile strength and fracture behavior of ZrB2‐SiC‐graphite composite

The tensile behavior of ZrB₂‐SiC‐graphite composite was investigated from room temperature to 1800°C. Results showed that tensile strength was 134.18 MPa at room temperature, decreasing to 50.34 MPa at 1800°C. A brittle‐ductile transition temperature (1300°C) of ZrB₂‐SiC‐graphite composite was deduced from experimental results. Furthermore, the effect of temperature on the fracture behavior of ZrB₂‐SiC‐graphite composite was further discussed by microstructure observations, which showed that tensile strength was controlled by the relaxation of thermal residual stress below 1300°C, and was affected by the plastic flow during 1300°C and 1400°C. At higher temperature, the tensile strength was dominated by the changes of microstructures.     

Dynamic mechanical analysis of rubber toughened epoxy resins

Dynamic mechanical analysis (DMA) was used to characterize cured epoxy resin formulations from ?150°C to temperatures above their α transitions. The resins were aromatic amine and aliphatic amine cured and were modified with carboxylterminated acrylonitrile-butadiene (CTBN) rubbers to improve their toughness, A DuPont 981 dynamic mechanical analyzer was used to measure the modulus and mechanical loss factor (tan δ) of the samples. Changes in the α and β transitions in the scan of tan δ as a function of temperature were related to changes in the formulation. Relations were also sought between changes in the DMA data and fracture and impact toughness of the cured formulations obtained using an instrumented impact test. Impact tests were performed at ?196°C and at room temperature. Results indicate that fracture toughness and the dynamic mechanical properties are affected by the amount of rubber, the compatibility of the rubber and epoxy, and changes in the curing agent stoichiometry.     

High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.     

Characterization of two polypentadienes of differing tacticity by physical methods

The preparation of isotactic and syndiotactic 1,4-polypentadienes with a cis content of at least 70%–75% using i-Bu₂AlH/Ti(i-OPr)₄ and AlEtCl₂/thiophene/Co(acetylacetonate)₂ catalysts, respectively, is reported. Physical characterization of the vulcanizates, prepared by a common recipe, involving infrared analysis, DTA, simple stress–strain and swelling measurements, and dynamic mechanical measurements over a frequency range of 2 decades and temperature range of ?60°C to +20°C indicated that no isomerization had taken place during vulcanization and that stereoregularity of the polymer chains affected the resultant cure: the isotactic form was found to have a greater crosslink density than the syndiotactic form. Master curves covering an extended frequency range were constructed from the reduced dynamic mechanical data and the calculated quantities—thermal expansion coefficients of free volume and the fractional free volumes at the glass transition temperatures—agree with the accepted values. Glass transition temperatures of the isotactic and syndiotactic polymers are ?37°C and ?42°C, respectively, and for their vulcanizates, ?33°C and ?40°C, respectively.     

Dynamic mechanical analysis and its relationship to impact transitions

Detailed, instrumented impact tests were carried out between about ? 100 and 60°C for flexible poly(vinyl chloride) (PVC), ethylene vinyl acetate (EVA), and polypropylene (PP) films. Secondary impact transitions in addition to the main transitions were detected for all three films, indicating that multiple impact transitions may be far more general in occurrence than commonly expected. Wide frequency (from 0.05 to 100 Hz) dynamic mechanical spectra of the same materials were also generated over similar temperature ranges. A new data treatment method was proposed whereby the material dissipation function was evaluated by summing the responses over broad frequency ranges of the loss modulus and the impulse spectrum. The dissipation function when plotted as a function of temperature was found to accurately (to within 3 to 5°C) predict the location of the main impact transitions for all three polymers. In addition, the existence and location of the secondary impact transitions for both PVC an PP were predicted. Both the functional form and the temperature match between the experiments and predictions strongly support the validity of the proposed method. However, some discrepancy remained in predicting the very low temperature (?65°C) impact transition for EVA.