Influence of molecular weight on the mechanical performance of a thermoplastic glassy polyimide

Mechanical testing of an advanced thermoplastic polyimide (LaRC^TM-SI) with known variations in molecular weight was performed over a range of temperatures below the glass transition temperature. The physical characterization, elastic properties and notched tensile strength were all determined as a function of molecular weight and test temperature. It was shown that notched tensile strength is a strong function of both temperature and molecular weight, whereas stiffness is only a strong function of temperature. A critical molecular weight (M _c) was observed to occur at a weight-average molecular weight (M _w) of 22000 g/mol below which, the notched tensile strength decreases rapidly. This critical molecular weight transition is temperature-independent. Furthermore, inelastic analysis showed that low molecular weight materials tended to fail in a brittle manner, whereas high molecular weight materials exhibited ductile failure. The microstructural images supported these findings.     

The effects of thermal pre-treatment and molecular weight on the impact behaviour of polycarbonate

Notched impact fracture experiments have been conducted on specimens of polycarbonate in three different conditions, (a) as received, (b) annealed at 125° C, and (c) electron beam irradiated to reduce the molecular weight. By consideration of the behaviour of a wide range of notch geometries four different failure modes were identified which were present in different proportions for each material. (1) Razor notched specimens failed in a completely brittle manner, well described by linear elastic fracture mechanics. (2) Small notch tip radii specimens failed in an apparently brittle manner through the formation of a single craze. (3) Some intermediate notch tip radii specimens failed in a predominantly brittle manner with small shear lips indicative of plane stress yielding. A fracture mechanics approach was used here, the measured toughness correlating with the extent of plane stress yielding. (4) Fully ductile failure was observed for large notch tip radii for all materials. It has been established that the embrittlement of polycarbonate caused by annealing is due to an increase in the yield stress, whereas that caused by reducing the molecular weight is due to a reduction of the crazing stress. In both cases, more specimens of intermediate notch tip radius are caused to fail in the low energy brittle mode designated (2) above. By varying the yield stress and crazing stress independently we have thus been able to distinguish clearly how both influence the brittle-ductile transition.     

含铜热轧低合金钢的韧脆转变研究

采用系列冲击试验和动态撕裂试验研究了屈服强度大于550MPa、含铜量为1%的热轧低合金钢板(板厚为6mm)的韧脆转变行为。结果表明,该钢具有很低的韧脆转变温度,ETT50和FATT50均低于-100℃。     

Effect of thermal treatments on the fracture behaviour of notched polyethylene terephthalate ribbons

The application of different thermal treatment procedures to thin polyethylene terephthalate (PET) sheets yields to microstructures of different molecular weight/ degree of crystallinity combination. As a consequence, variations in the mechanical properties, especially the fracture properties and the particular fracture mechanisms occur. This is demonstrated in this paper by measurements of elastic modulus, maximum stress, failure initiation energy, and total work to fracture of notched PET-ribbons. Failure mechanisms analysed by the use of optical and scanning electron microscopy vary between highly ductile via semi-brittle after crazing, to absolute brittle at very low stresses. The results are summarized in terms of a three-dimensional failure energy map divided into regions of particular failure behaviour for particular molecular weight/ degree of crystallinity combination. In addition, the typical values of material strength, defined as the product of resistance to damage initiation (maximum stress) and crack propagation (total work to failure) are given for each region. The optimum fracture resistance was achieved for PET material with moderately low molecular weight and moderately high degree of crystallinity.     

Fracture of notched polycarbonate under hydrostatic pressure

The brittle fracture behaviour and plastic deformation of round-notched polycarbonate bars subjected to three-point bending under hydrostatic pressure have been studied. Below a certain critical pressure, the brittle fracture initiated from an internal craze nucleated at the tip of the local plastic zone ahead of the notch rooT. The position of the nucleation of the craze receded from the tip of the notch with increasing applied pressure. When the pressure was increased over a critical value, general yielding occurred by passage of the plastic zone across the notched cross-section, that is, the brittle to ductile transition took place. A qualitative analysis of the stress distribution within the plastic zone explains that the brittle to ductile transition under hydrostatic pressure occurs when the general yield takes place before a critical stress for brittle crack propagation is reached.     

Unification of ductile failure and slow crack growth in an ethylene-octene copolymer

The time dependence of ductile and brittle failure of an ethylene-octene gas pipe resin was measured as a function of stress, notch depth and temperature. The ductile failure mode depends on the ligament stress and all the ductile data could be unified by normalizing the ligament stress with respect to the short-time yield point at each temperature. The brittle data were unified by correlating the stress and notch depth in terms of the stress intensity and by introducing a time-temperature shift. At each temperature there is a critical stress intensity above which the notch becomes blunted so that ductile failure ensues. The unified curve for the ductile failure is about the same for all polyethylenes. The unified curve for the brittle regime varies greatly with different polyethylenes where the primary difference is associated with the minimum time required to produce brittle behaviour for a common temperature and stress intensity.     

Condition for determining the static yield and fracture of a polycarbonate plate specimen with notches

The fracture behavior of notched flat bars of polycarbonate known as a ductile polymer has been studied for a wide range of notch geometries in slow tensile tests at room temperature. The sharply notched specimen fails in a brittle manner, and the bluntly notched specimen fails in a ductile manner. The predominant factors for determining the ductile-brittle transition of fracture mode are the notch root radius and the thickness of a specimen. The experimental results have been discussed in terms of the maximum stress at the root of the notch and the relative stress distribution near the notch root.     

A critical shear displacement criterion for ductile–brittle transition under mixed mode I and II loading

Abstract

Micromechanisms producing ductile and brittle damage operate in parallel at a crack tip. The dominant mode of failure depends upon which of the two (ductile or brittle) damage parameters first reaches its critical value. This has been shown by a study of ductile–brittle transition behaviour in HY100 steel under mixed mode I and II loading. The transition from ductile to brittle behaviour in HY100 steel was found to be affected by mixed mode I and II ratio (ratio of imposed tensile and shear loading) in a manner such that with increasing shear the transition temperature decreased. In the present paper, a criterion is proposed based on the shear strain ahead of a notch tip, to predict the fracture behaviour at any given temperature and mixed mode ratio.     

Statistical assessment of notch toughness against cleavage fracture of ferritic steels

The work is an initial effort on adopting a statistical approach to correlate the fracture behavior between a notched and a fracture mechanics specimen. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness, and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch brittleness of ferritic steels according to a recently proposed local approach model of cleavage fracture. The temperature independence of the 2 Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the 2 Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with 2 different microstructures.     

The effect of polymerization conditions and crystallinity on the mechanical properties and fracture of spherulitic nylon 6

The molecular and structural parameters controlling the mechanical properties, deformation and fracture of spherulitic nylon 6 have been investigated. The nylon was prepared by the anionic polymerization of ε-caprolactam and the polymerization conditions were varied to give samples having a range of spherulite diameter, molecular weight and degree of crystallinity. The tensile properties and fracture mode of the nylon varied considerably with degree of crystallinity and polymerization temperature. High crystallinity and low polymerization temperatures below 423 K gave a brittle material. Polymerization above 423 K resulted in a ductile material which showed a yield drop. In this material final fracture was preceded by the formation of inter and trans spherulitic cracks which coalesced to form a large cavity that led to final failure. In nylon having a low degree of crystallinity, fracture was fibrillar in nature and occurred by the ductile drawing of the material to strains greater than 250%.     

The transition from ductile to slow crack growth failure in a copolymer of polyethylene

The failures of ethylene-hexene copolymer single-edge notch tensile specimens were observed under a constant tensile load. The notch opening was measured against time over a range of stress. Three failure modes were observed: ductile, brittle and transitional. The microscopic changes at the notch tip were correlated with each of the modes of failure. Early in the test the ultimate mode of failure can be predicted from the microstructural changes in the notch. In the transition region, the lifetime increases as the stress increases because the blunting of the notch offsets the effect of the applied stress-stress field. The ductile failure is controlled by the macroscopic creep behaviour and the brittle failure occurs by slow crack growth that starts at a craze.     

Effects of creep ductility and notch constraint on creep fracture behavior in notched bar specimens

The creep rupture life of U-type notched specimens and smooth specimens has been calculated based on the ductility exhaustion damage model using stress-dependent creep ductility. Effects of creep ductility and notch constraint on creep fracture behaviour in notched bar specimens have been investigated. The results show that the U-type notch exhibits notch strengthening effect under a wide range of stress level and notch constraint condition (notch acuity) for creep ductile materials. The lower equivalent stress in notched specimens plays main role for reducing creep damage and increasing rupture life. The rupture life of notched specimens of creep brittle materials (with lower creep ductility) decreases with the increase in stress level and notch constraint. With increasing creep ductility and decreasing notch constraint, the degree of the notch strengthening effect increases. In creep life designs and assessments of high-temperature components containing notches, the material creep ductility, notch constraint and stress levels need to be fully considered.     

On combination of the equivalent material concept and J‐integral criterion for ductile failure prediction of U‐notches subjected to tension

The aim of the present research is to check the capability of the equivalent material concept (EMC) combined with the J‐integral failure criterion, called EMC‐J criterion, in predicting the load‐carrying capacity (LCC) of U‐notched ductile aluminium plates subjected to tension by considering the 2 moderate and large‐scale yielding regimes. For this purpose, first, a set of experimental results on LCC of 2 groups of thin U‐notched rectangular plates made of Al 7075‐T6 and Al 6061‐T6 are gathered from the recent literature. Then, because the Al 7075‐T6 and Al 6061‐T6 plates have ductile behaviour, EMC is employed to avoid performing elastic‐plastic failure analysis for LCC predictions. Up to now, different failure models in the context of the linear‐elastic notch fracture mechanics have been successfully utilized in combination with EMC for ductile failure prediction of notched members. However, this is the first time in this research that J‐integral, as a well‐known brittle failure criterion, is linked to EMC for predicting LCC of the U‐notched rectangular aluminium plates. Finally, it is shown that EMC‐J criterion can predict well the experimental results of tensile LCC.     

Determination of charpy transition temperature of ferritic steels using miniaturized specimens

Miniaturized specimen technology permits mechanical behaviour to be determined using a minimum volume of material. A method for obtaining the ductile-brittle transition temperature of ferritic steels was developed using a miniaturized notched bar test. Comparisons between conventional and miniaturized specimen ductile-brittle transition temperatures are encouraging. Fracture toughness values were calculated for the miniaturized notched specimens and compared with large-specimen data. The miniaturized specimen values were high, even after appropriate adjustments had been made. Further development may yield valid data when an optimum combination of specimen size, shape, and notch acuity is determined.The work was performed during employment of Battelle.     

Fracture at V-notches with contained plasticity

Fracture at V-notches with contained plasticity was studied. The tested material was a soft annealed tool steel, AISI O1, at −50°C. The material behaviour at this temperature was brittle for a standard fracture toughness test and ductile for a tensile test. Single-edge notched tension and three-point bend specimens were tested, with notch angles ranging from 0° to 140°. For the same notch angle, both brittle and ductile fracture occurred depending on the geometry. However, transferability between different geometries prevailed if the fracture was brittle.Fracture loci derived from simple point-stress or mean-stress criteria were fitted to the experimental results. The trend of the experimental results was followed well even though the fracture loci were derived under the assumption of linear elastic stresses. However, a fracture criterion based on the fracture toughness and a tension test alone will provide a conservative fracture locus.The effect of introducing a notch radius on the used fracture criteria is discussed within the context of linear elastic analysis. As expected, a mean-stress criterion is more sensitive to changes in the notch radius than a point-stress criterion. For both criteria, the sensitivity for changes in the notch radius is less for larger notch angles.     

Notch brittleness of ductile glassy polymers under plane strain

The brittle fracture behaviour of ductile glassy polycarbonate, poly(vinyl chloride) and poly(methyl methacrylate) notched bars subjected to plane strain tension has been studied at varying strain rates. Morphological observations of thin sections and fracture surfaces revealed that a disc-type craze was nucleated at the tip of the plastic zone which spread from the notch root. A slip-line theory modified so that the yield criterion is influenced by a hydrostatic stress component allows the calculation of the stress components at the elastic-plastic boundary, where the hydrostatic stress is highest, from the knowledge of the location of the fracture origin. An analysis of the data resulted in the conclusion that Orowan's analysis for notch brittleness is appropriate.     

The effect of fatigue pre‐cracking forces on fracture toughness

Testing procedures for the determination of the fracture toughness of a material by monotonic loading of fatigue pre‐cracked specimens are well established in standards such as BS 7448, BS EN ISO 15653, ISO 12135, ASTM E1820 and ASTM E1921. However, a review of these standards indicates a wide range of permitted fatigue pre‐cracking forces, whilst the underlying assumption in each standard is that the pre‐cracking conditions do not affect the fracture toughness determined. In order to establish the influence of different fatigue pre‐cracking forces on the fracture toughness, tests were carried out on specimens from an API 5L X70 pipeline steel. Single‐edge notch bend specimens of Bx2B geometry were notched through thickness and tested at temperatures of +20 °C, ?80 °C and ?140 °C to show the fracture behaviour in different regions of the fracture toughness ductile‐to‐brittle transition curve. Fatigue pre‐cracking was conducted on a high‐frequency resonance fatigue test machine over a range of pre‐cracking forces permissible within the various standards and beyond. The results showed that an excessively high pre‐cracking force can result in a significant overestimation of the value of fracture toughness for material exhibiting brittle behaviour, whilst very low fatigue pre‐cracking forces appeared to result in an increase in scatter of fracture toughness. A review of standards indicated that there was a possibility to misinterpret the intention of the ISO 12135 standard and potentially use excessively high pre‐cracking forces. Suggested clarifications to this standard have therefore been proposed to avoid the risk of overestimating fracture toughness.     

Competing risks models for fracture in the ductile to brittle transition temperature region

When fracture toughness testing is carried out over the ductile to brittle transition temperature region cleavage instability may be observed at the initiation of cracking or after some prior ductile crack growth. The amount of precleavage ductile crack growth increases with increasing temperature. At the lower test temperatures, it is possible to assume that all tests will result in cleavage instability. However, as the test temperature increases, at some limiting temperature, the failure mode during the final instability changes from cleavage to ductile. These two different types of behaviour can be accommodated in a statistical analysis which is based on the method of competing risks. A statistical approach is presented for the analysis of data by competing risks and a procedure is given for the estimation of the probability of cleavage failure.     

Change of fracture mode in Charpy toughness transition temperature range

Abstract

A transition layer of width 5 - 10 μm was found on the boundary between ductile and brittle fracture for Charpy V notch specimens in the transition temperature range of a structural steel having a microstructure of polygonal ferrite -pearlite. The fracture mode in the transition layer was shearing with occasional submicrometre dimples. From tensile tests on notched specimens, the cleavage fracture stress and flow stress by ductile decohesion were determined. Based on the experimental data and the assumption that the volume of metal involved in the plastic deformation during fracture was related to the volume of the dimples, it was deduced that the transition layer width represents the size of the plastic zone immediately before cleavage initiation. The crack opening displacement and the crack tip radius for the change of fracture mode were calculated.     

Ductile-brittle fracture transitions in polycarbonate

Test data is described for polycarbonate single edge notch specimens at a range of loading rates and two thicknesses at 20°C. The transition from ductile to brittle failure is shown to be both rate and thickness dependent. The effects may be described in terms of a plane stress and a plane strain fracture toughness coupled with a rate dependent yield stress and a plastic zone size to thickness ratio criterion. An increasing percentage of the incidence of brittle failure with rate rather than a sharp transition is noted and explained in terms of a distribution of toughness values introduced by variations in the notching method.