The notch sensitivity of polymeric materials

Stress–strain tests were made on about five dozen polymeric materials using unnotched and notched specimens containing six different types of notches. Notches decrease the strength, but they decrease the elongation to break even more drastically in general. Notch sensitivity factors are defined for strength and for energy to fracture in such a manner that the greater the notch sensitivity factor, the greater is the effect of a notch relative to the unnotched material. The notch sensitivity factor for breaking (or yield) strength is not the same as the notch sensitivity factor for energy to fracture as measured by the area under the stress–strain curve. Brittle polymers and composites tend to have greater notch sensitivity factors for strength than ductile polymers. For brittle polymers, the notch sensitivity factor for energy to fracture tends to increase with the elongation to break of the unnotched polymer. Notches generally are more detrimental to ductile polymers than to brittle ones as far as the energy to fracture is concerned. For ductile polymers, the shape of the stress–strain curve is important in determining the sensitivity to notches. The ratio of the upper to lower yield strengths should be small for low notch sensitivity. It is desirable to have the breaking strength greater than the yield strength. Glass fibers and filler in ductile matrices increase the notch sensitivity for strength but decrease the sensitivity for energy to fracture relative to the unfilled polymer. Rubber–filled polymers have a reduced notch sensitivity for strength relative to the unfilled polymer, but the notch sensitivity for energy to fracture may be either increased or decreased, depending upon the system. The energy to fracture for notched specimens correlates better with Izod impact strength than does the energy to fracture for unnotched specimens. It is recommended that notched stress–strain specimens be routinely measured along with unnotched specimens.     

Fractography and failure mechanisms of particulate-filled thermoplastic polyester

Fractography has been used for the postfailure analysis of a filled thermoplastic polyester. The five fracture modes that were previously defined on the basis of macroscopic stress–strain behavior were distinguished by certain fractrographic features. These features were characteristic of the fracture mode and did not depend on filler type or filler content. The Mode A ductile fracture surface consisted of two regions: a pullout region of slower crack growth and a rosette region of faster crack growth. The Mode B ductile fracture surface contained only a ductile pull out texture. The Mode C quasi-brittle fracture surface exhibited secondary fracture features that sometimes included the herringbone pattern. The Mode D quasi-brittle fracture surface consisted of a stress-whitened dimple region and a brittle fracture region. The Mode E Fracture surface exhibited primarily the rough texture characteristic of brittle fracture. The failure mechanisms inferred from analysis of the fracture surfaces confirmed a microscopic failure model of the ductile-to-quasi-brittle transition in filled PETG that is based on the strain-hardening strength of the polymer ligaments between debonded filler particles. © 1994 John Wiley & Sons, Inc.     

Phenolic rigid organic filler/isotactic polypropylene composites. III. Impact resistance property

A novel phenolic rigid organic filler (KT) was used to modify isotactic polypropylene (iPP). The influence of KT particles on the impact resistance property of PP/KT specimens (with similar interparticles distance, 1.8 μm) was studied by notched izod impact tests. It was found that the brittle-ductile transition (BDT) of the PP/KT microcomposites took place at the filler content of about 4%, and the impact strength attains the maximum at 5% (with filler particles size of 1.5 μm), which is about 2.5 times that of unfilled iPP specimens. The impact fracture morphology was investigated by scanning electron microscopy (SEM). For the PP/KT specimens and the highdensity polyethylene/KT (HDPE/KT) specimens in ductile fracture mode, many microfibers could be found on the whole impact fracture surface. It was the filler particles that induced the plastic deformation of interparticles ligament and hence improved the capability of iPP matrix on absorbing impact energy dramatically. The determinants on the BDT were further discussed on the basis of stress concentration and debonding resistance. It can be concluded that aside from the interparticle distance, the filler particles size also plays an important role in semicrystalline polymer toughening. Keywords rigid organic filler, polypropylene, impact     

Characterization of the fracture properties of aragonite‐ and calcite‐filled poly(ε‐caprolactone) by the essential work of fracture method

Biodegradable polymers, blends, and composites are often investigated during tissue engineering studies, but their fracture properties, which are important mechanical engineering characteristics, are often disregarded or wrongly treated. In this study, essential work of fracture tests were performed on calcium carbonate filled poly(ε‐caprolactone), a very ductile polymer, to determine the effects of different filler shapes (calcite spheres and aragonite whiskers), sizes, and contents on the fracture parameters. Increasing the filler content caused stability problems during crack propagation, and this influenced the self‐similarity of the load–displacement response and resulted in the yielding point being missed. Moreover, the yielding‐related essential work of fracture and the energy dissipating during yielding were found to be almost independent of the filler content and thus could be indicators of matrix–filler adhesion. A shape effect of aragonite whiskers appeared during stable crack propagation; the motion of the particles and the friction on their surface slightly increased the dissipated energy quantum and resulted in a more oriented molecular structure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Mechanical properties of talc-filled polypropylene. Influence of filler content,filler particle size and quality of dispersion

Mechanical properties of polypropylene-talc composites are measured as a function of talc concentration up to 40 wt.-%, Young's modulus of filled polypropylene shows linear increase with talc concentration up to double the value of unfilled polymer. Yield stress and Charpy notch toughness decrease with increasing talc content below matrix level at the highest filler content. Composite ultimate tensile elongation and tensile impact strength decrease sharply beginning at the lowest filler concentration. The influence of the talc particle size on the mechanical properties, especially composite toughness, mentioned above, is investigated. Four type of talc were used. Notch toughness decreases according to a linear dependence with mean size of talc particles. Evaluating impact strength possible content of agglomerates of filler and other additions is necessary to be included: tensile impact strength gives slow linear dependence with increasing content of filler particles and/or agglomerates above about 10 μm. The influence of talc particle size on the toughness of filled polypropylene becomes strong if the rubber particles are present.     

Short- and long-term strength characteristics of particulate-filled cast epoxy resin

Particulate-filled thermosetting composites are widely used, yet little systematic work has been done on their long-term strength characteristics. In this study short-term tensile, flexural, and impact tests as well as tensile creep-rupture tests were made for unfilled and filled epoxy to clarify the effects of filler size, filler content, and temperature. Fillers used were silica, alumina particles, and glass beads. Test temperatures were varied from 25 to 110°C. As a result of short-term testing, it was found that the Petch relation held between strength and filler size if brittle fracture occurred, while a strength and filler size if brittle fracture occurred, while a strengthening effect existed when ductile fracture occurred. On creeprupture testing, a strengthening is observed with filler size and content for silica and glass beads. The Arrhenius plot of rupture time for various filler sizes and contents converges to a characteristic point corresponding to the glass transition temperature of the material. Using this relation, a modified Larson-Miller master rupture curve is proposed which can predict the long-term strength of particulate-filled thermosetting composites as functions of rupture time, temperature, filler size, and content.     

Toughenability of polymers

We demonstrate that all solid polymers are intrinsically brittle and will undergo a ductile to brittle fracture transition based on the nature of their bonding alone. The most effective way of avoiding a ductile to brittle transition is to reduce the plastic resistance to delay reaching the brittle strength which in unoriented polymers is governed by intrinsic cavitation. While a number of possibilities for this exist, the most widely used techniques involve incorporation of rubbery particles that can cavitate or rigid particles that can debond prior to plastic flow. In both approaches the continuous homo-polymer is transformed into a quasi-regular cellular solid that is much more capable of undergoing large local plastic flow by ligament stretching between cavitated particles and is less susceptible to the propagation of brittle cracks under the usual conditions of tensile straining. Under impact conditions, however, in a notched sample which concentrates the strain rate at the notch root, the plastic resistance of the stretching ligaments rises sharply due to two separate but related effects. First, by an increase in the shear modulus due to the high frequency nature of the Izod impact test to fracture, viewed as a quarter cycle oscillation, which directly elevates the flow resistance and second, by the further effect of increase due to the much increased plastic strain rate. At the notch root then, the plastically stretching and strain hardening ligaments are left with a much reduced capacity to strain further before the cavitation stress is reached. While rubber particle-modified polymers can still exhibit considerable toughening, rigid-particle-modified polymers suffer severely from clustering of rigid particles into super critical flaws that trigger brittle response, much like what is encountered in structural steels.Based on their known mechanical response in neat form six, semi-crystalline polymers have been analyzed in detail to evaluate their potential for toughening under impact conditions. The results correlate very well with the experimental findings.     

The effect of time and temperature on the mechanical behavior of epoxy composites

A crosslinked epoxy resin consisting of a 60/40 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirectional glass fibers and air (foams) were tested in tension, compression and flexure to determine the effect of time and temperature on the elastic properties, yield properties and modes of failure. Unidirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from 10^?4 to 10 in./in.-min and the temperature from ?1 to 107°C. Isotherms of tangent modulus versus strain rate were shifted to form master modulus curves. The moduli of the filled composites and the foams were predictable over the entire strain rate range. It was concluded that the time-temperature shift factors for tangent moduli and the time-temperature shift factors for stress relaxation were identical and were independent of the type and concentration of filler as well as the mode of loading. The material was found to change from a brittle-to-ductile-to-rubbery failure mode with the transition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. In the ductile region, an approximately linear relationship between yield stress and log strain is evident in all cases. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot that can be used to predict the yield stress of the composites at any temperature and strain rate in the ductile region. The time-temperature shift factors for yielding were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of stress. The compressive-to-tensile yield stress ratio was practically invariant with strain rate for the unfilled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and strain rate.     

Linear viscoelasticity of polymer melts filled with nano-sized fillers

The linear dynamic rheology of polymer melts filled with nano-sized fillers is investigated in relation to a proposed two phase model. A common principle is disclosed for nanofilled polymers exhibiting either fluid- or solid-like behaviors with increasing filler volume fraction. The bulky polymer phase far away from the filler inclusions in the nanocomposites behaves the same as in the unfilled case while its contribution to the composite modulus is enlarged due to strain amplification effect. The filler forms aggregates together with polymer chains absorbed on the filler surface, which is termed as the “filler phase” in the proposed model. The dynamics of the “filler phase” slow down with increasing filler concentration. The applicability of the proposed two phase model is discussed in relation to the well-known structural inhomogeneity of nanofilled polymers as well as the strain amplification and the filler clustering effects.     

Shear yield behavior of calcium carbonate–filled polypropylene

The rheological properties of calcium carbonate-filled polypropylene has been examined using a Rheometrics dynamic analyzer RDAII. The study included a steady shear test, a transient stress growth test, and a dynamic oscillatory shear flow. Yield behavior was observed in all kinds of rheological tests for highly filled compounds when the volume loading exceeded a critical value at about 20%. The empirical Cox-Merz rule, which is usually applicable to an unfilled polymer, was found to be invalid for highly filled compounds. The modified Cox-Merz rule, in which the apparent viscosity versus the shear rate is equal to the complex viscosity versus the frequency-amplitude in the nonlinear region, was found to be valid only for highly filled compounds. The viscosity and the apparent yield values appear to increase with increasing volume loading of filler particles. The surface treatment of fillers, which presumably reduces the interaction between filler particles and the extent of agglomeration, results in major viscosity reductions and decreases in apparent yield values. The yield values determined from various tests are not the same. The results are interpreted in terms of a system forming a filler network due to weak inter-particle forces. The yield stress resulting from the breakdown and recovery of the network is thus dependent on the characteristic time of the individual test.     

Influence of molecular weight on the fracture properties of aliphatic polyketone terpolymers

The influence of polymer molecular weight on the mechanical properties of aliphatic polyketones was investigated. The molecular weight varied from 100,000 to 300,000 g mol⁻¹. The crystallinity was found to be independent of polymer molecular weight, as was the glass transition temperature. The yield strength and stiffness of the aliphatic polyketone terpolymers were also found to be independent of molecular weight. The post yield behaviour showed strong dependency on polymer chain length. The draw stress was increased significantly with higher molecular weight material. The impact resistance was increased with molecular weight, resulting in ductile fractures with large energy consumption upon fracture. The brittle-to-ductile transition temperature was lowered with increasing chain length. The difference in material deformation was linked to the higher mechanical connectivity and more stable post yield behaviour of the polymers with an increased molecular weight.     

Wood-fiber reinforcement of styrene–maleic anhydride copolymers

Styrene–maleic anhydride (SMA) copolymers containing either 7 or 14% maleic anhydride were filled with either pine flour or dry-process aspen fiber from a medium density fiberboard (MDF) plant. Material properties of the filled and unfilled SMA plastics were compared with those of aspen-fiber-filled and unfilled polystyrene (PS). The fiber-filled SMA composites were equivalent or superior to unfilled SMA in strength, stiffness, and notched Izod impact strength. Filled PS composites outperformed or matched the performance of filled SMA composites in the parameters tested. Unnotched Izod impact strength of filled polymers was generally inferior to that of the unfilled polymers. Water absorption from a 90% relative humidity exposure, a 24-h soak, and a 2-h boil showed mixed results when compared to the unfilled polymers. Dynamic mechanical analysis showed no change in glass transition temperature (T_g) after the addition of filler for either SMA or PS composites. The presence of the anhydride functionality on the polymer backbone did not appear to improve the strength of the composite. No evidence was found for chemical bond formation between the SMA and wood fiber. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1567–1573, 1998     

Effect of Polymer Molecular Weight on Colloidal Reinforcement

The tear strength of several thermoplastic polymers filled with colloidal silica has been determined as a function of polymer molecular weight. There were two regimes of behaviour. Low molecular weight materials were embrittled by a small amount, around 10% volume fraction, of filler. High molecular weight polymers, on the other hand, were not degraded at low filler concentrations, and some thermoplastics were actually reinforced, that is toughened, by the colloidal particles with a peak in crack resistance at 12% volume fraction. There was found to be a characteristic transition in toughness, similar to that observed with environmental stress cracking, between the low and high molecular weight regimes. It was concluded that a polymer molecular weight above the transition value was a necessary but not sufficient condition for colloidal reinforcement.     

Filler surface treatment with hydrophobic esters

The use of fillers in polyolefin polymer composites has escalated in recent years in conjunction with the need for improved physical properties and the increased costs of petroleum-based polymers. Fillers, used as inert extenders and reinforcing agents, present problems when added to organic polymers that are different in chemical nature and physical form. Use of surface-treatment additives has been developed to overcome problems that originate in the interfacial region where the organic resin phase must wet out the inorganic filler being compounded. Achieving optimal physical and chemical properties in a filled compound, by the use of hydrophobic esters derived from castor oil as wetting and encapsulating agents, was evaluated. The hydrophobic esters improved the dispersion and distribution of filler particles throughout the organic medium. The surface treatment with the ester resulted in a lowering of viscosity and better ability to control the rheology of the compound, raise the extender filler loadings and an upgrading of mechanical properties of the filled resin. It was shown that the use of hydrophobic esters as a surface filler treatment resulted in increased tensile strength, higher impact strength and improved processing of filled polymer composites.     

Dynamic mechanical study of filled semi-interpenetrating polymer networks: Influence of γ-Fe2O3 on microphase structure

The preparation of filled two-component semi-interpenetrating polymer networks (semi-IPNs) is described and the results of an investigation of their morphology by means of dynamic mechanical spectroscopy are considered. The influence of an active dispersed filler (γ-Fe₂O₃) on the semi-IPNs phase structure is studied. A comparison is made between filled and unfilled semi-IPNs consisting of compatible or incompatible polymers. In the case of a semi-IPN of compatible polymers, the introduction of γ-Fe₂O₃ was observed to cause phase separation. With a two-phase semi-IPN the introduction of the filler enhanced the phase separation. The presence of two distinct peaks (the dynamic glass transition temperatures) corresponding to those of the two initial homopolymers shows the semi-IPN to have a two-phase structure.     

Fibre reinforcement of elastomers: nanocomposites based on sepiolite and poly(hydroxyethyl acrylate)

The reinforcement of poly(hydroxyethyl acrylate) provided by sepiolite particles has been analysed. Stress–strain measurements at equilibrium, as well as swelling experiments, were carried out on the unfilled and filled polymers. The state of dispersion of the filler was characterized by transmission electron microscopy. Moreover, an evaluation of chain orientation by infrared dichroism reveals a high orientation level in the filled system. The strong interaction between the two phases, combined with a good dispersion and a high shape factor of these clay particles, explain the improvement of the mechanical properties of the composites with respect to the pure polymer. Copyright © 2004 Society of Chemical Industry     

The ductile-to-quasi-brittle transition of particulate-filled thermoplastic polyester

The effects of various filler characteristics on the ductility of filled amorphous copolyester, Kodar 6763, have been examined. The five fillers in the study included two calcium terephthalates with different particle-size distributions and three calcium carbonates, also with different particle-size distributions. One of the calcium carbonate fillers had received a surface treatment. The increase in Young's modulus with increasing filler content was the same for all fillers and was satisfactorily described by Kerner's equation. The only filler to affect the yield stress was the surface-treated calcium carbonate; in this case, the decrease in yield stress was attributed to cracking and splitting of aggregated particles. A sharp drop in fracture strain was observed with increasing filler content. This ductile-to-quasi-brittle transition occurred when the fracture mode changed from fracture during strain-hardening of neck propagation to fracture during neck formation. The critical filler content of the ductile-to-quasi-brittle transition varied from one filler to another. A simple model qualitatively described the decrease in critical filler content with increasing breadth of the particlesize distribution and, in particular, with increasing volume percent of large particle in the distribution. © 1994 John Wiley & Sons, Inc.     

Statistical model for predicting the strength of solid-filled rubbers

The tensile strength of solid-filled rubbers is predicted by the statistical model proposed in this paper, by which the maximum area fraction of the solid filler in a representative cube is calculated. The minimum net cross-section area of the rubber matrix is thus obtained, and this is used to calculate the reduction in strength of the compound in comparison to that of the unfilled rubber. The prediction is then tested by experiments using hydroxyl-terminated polybutadiene (HTPB) filled with glass spheres. After curing, tensile specimens were cut from sheets and exposed to humidity-controlled environments so as to debond the filler particles from the rubber matrix, and were tested in tension. The experimental results were found consistent with the predictions.     

Ductility of filled polymers

The ductility of a calcium carbonate-filled amorphous copolyester PETG in a uniaxial tensite test was examined as a fiction other filler volume fraction. A ductile-to-quasibrittle transition occurred as the volume fraction of filler increased. This transition was from propragation of a stable neck through the entire gauge length of the specimen to fracture in the neck without propagation. The draw stress (lower yield stress) did not depend on the filler content and was equal to the draw stress of the unfilled polymer. It was therefore possible to use a simply model to predict the dependence of the fracture strain on the filler volume fraction. It was proposed that when the fracture strain decreases to the draw strain of the polymer the fracture mechanism changes and the fracture strain drops sharply. The critical filler content at which the fracture mode changes is determined primarily by the degree of strain-hardening of the polymer. © 1994 John Wiley & Sons, Inc.