Influence of electroless nickel-plating on fracture toughness of pitch-based carbon fibre reinforced composites

Nickel-Pitch-based carbon fibres (Ni-PFs) were prepared by electroless nickel-plating to enhance fracture toughness of Ni-PFs reinforced epoxy matrix composites (Ni-PFs/epoxy). The surface properties of Ni-PFs were determined by scanning electron microscopy (SEM), X-ray photoelectron spectrometry (XPS), and X-ray diffraction (XRD). The fracture toughness of the Ni-PFs/epoxy was assessed by critical stress intensity factor (K_IC) and critical strain energy release rate (G_IC). The fracture toughness of Ni-PFs/epoxy was enhanced compared to those of PFs/epoxy. These results were attributed to the increase of the degree of adhesion at interfaces between Ni-PFs and matrix resins in the composites.     

Coated glass beads epoxy composites: influence of the interlayer thickness on pre-yielding and fracture properties

An elastomeric adduct based on a liquid rubber, an epoxy prepolymer and a liquid diamine has been prepared and deposited around glass beads reinforcing an epoxy matrix. The pre-yielding and fracture properties of such composites are studied and compared with those of untreated glass beads based composites. A linear dependence ofK _lc (critical stress intensity factor) versus volume fraction is obtained for untreated glass beads composites, whereas a maximum is reached at 20% volume fraction of filler for those with coated glass beads. Introduction of an elastomeric layer improves fracture toughness and the influence of interlayer thickness is studied. A maximum forK _lc is found for (e/r)=3% in connection with a strong decrease ofK/M ratio (work-hardening rate compression modulus) determined in the pre-yielding stage. The toughening mechanism is discussed primarily in terms of crack pinning and plastic deformation.     

Characterization of the paste-aggregate interfacial transition zone surface roughness and its relationship to the fracture toughness of concrete

Notched concrete beams containing varying amounts of pea gravel aggregate were tested under three-point bend, and their fracture toughness determined. The roughness of the region near the interface between the cement paste and the aggregate was evaluated by digitizing images from a confocal tandem scanning microscope. The average roughness of the paste was found to be related to the fracture parametersK _IC (critical stress intensity factor) and a _c (critical crack extension), as determined by the two-parameter fracture model. The roughness in the proximity of the paste-aggregate interface was generally higher than that of the paste far from the aggregate, and it decreased with the distance from the aggregate. This study indicates that aggregate particles increase the toughness of the cement paste portion of concrete, and that this is an important mechanism for toughening concrete.     

The application of fracture mechanics to the testing of wood

The problem of a mode I fracture toughness of wood is considered. After a short discussion of relevant literature the test results concerning mode I fracture on three types of wood and the obtained values of stress intensity factor, K _Ic, are discussed. The compressive and tensile strength of the wood fibres and flexural strength are also presented. A considerable variation of the stress intensity factor, K _Ic, has been found to depend on the wood species and the direction of taking specimens for tests. The character of a failure process and the obtained values of the stress intensity factor, K _Ic, were determined by interrelations of cohesion forces existing between particular components of the wood structure, and by anisotropy of the wood. Both the compressive and tensile strength tests performed along the fibres and the bending strength tests crosswise to the fibres have not confirmed the tendencies observed in the fracture toughness tests. The investigations performed show the usefulness of fracture mechanics for evaluation of the strength properties of wood. It is concluded that materials science must consider wood as a valuable and rewarding material upon which to focus research efforts.     

The effect of moisture content on the mechanical properties of a seed shell

The mechanical properties of the seed shells of the African mongongo nut, Schinziophyton rautenenii (Euphorbiaceae), were measured by compressive C-ring tests in an air-dry condition and also after soaking in distilled water. Young's modulus was found to be about 5 GPa and the fracture strength was 40–50 MPa, for both conditions. However, fracture toughness was affected significantly by moisture content. The critical stress intensity factor, K _IC, of air-dried specimens was 27% greater and the work of fracture, R, 69% greater than those of wet specimens. This difference corresponded well with microscopic observations of the complexity of the fracture surface. Viewed either by scanning electron microscopy or confocal microscopy, cracks in the wet shell deviated neatly around individual fibres, while cracks in air-dried shells either crossed individual fibres or ran obliquely across the outer layers of the secondary cell wall leaving a feathered appearance. It is proposed that the increase in toughness of shells which would be obtained from air-drying may help protect embryonic seed tissues from predation by larger animals (e.g. vertebrates such as rodents) after abcission from the parent plant.     

Determination of subcritical crack growth on glass in water from lifetime measurements on Knoop-cracked specimens

The crack growth behaviour of glass in water is investigated by use of lifetime measurements carried out with Knoop-damaged specimens in four-point bending tests. The method is outlined in detail and the crack growth law - exhibiting a threshold stress intensity factor K_ch = 0.35 MPam^1/2 - is determined. The initial and final crack size data as well as the fracture toughness are given by Weibull representations.     

Modeling of fiber toughening in cementitious materials using an R-curve approach

This paper presents the theoretical formulation describing the role of fibers in enhancing the fracture toughness of quasi-brittle cement based materials. The formulation is based on the well known R-curve approach which correlates the increase of the apparent fracture toughness of a material with the existence of a pre-critical stable crack growth region.By assuming that the critical crack length in plain matrix is a function of an initial crack length a ₀, a formulation for the R-curves has recently been derived and applied to predict the response of positive and negative geometry specimens of various sizes and materials. This approach is further applied to uniaxial tensile specimens containing various fiber types. Fiber reinforcement is modeled by means of applying closing pressure on crack surfaces resulting in closure of the crack faces and a decrease in the stress intensity factor at the tip of the propagating crack. Incorporation of these two factors in the energy balance equations for crack growth results in increases in both the slope and the plateau value of the R-curve representing matrix response. Enhancement in material response is shown to occur only if precritical crack growth exists, causing fibers to convert the stable cracking process into an increase in load carrying capacity of the material. Fracture response of fiber reinforced composites can be predicted up to the bend-over-point. The theoretical predictions are compared with the experimental results of cement-based composites containing unidirectional, continuous glass, steel or polypropylene fibers.     

Creep Crack Growth in Polyethylene Under Combined Mode I and Mode II Loading

Creep crack growth characteristics under various combined mode I and mode II loadings were studied using the compact tension shear (CTS) specimens of polyethylene. Creep crack growth rates da/dtunder combined mode I and mode II loading can be correlated with a single effective stress intensity factor K _Ieffderived from the combined — mode fracture toughness envelope. The steady state or constant crack growth rates which occupy the significant part of creep failure life increase with the increasing initial effective stress intensity factor.     

Fracture toughness of geopolymeric concretes reinforced with basalt fibers

The purpose of this work was to investigate the influence of the volumetric fraction of the fibers on the fracture toughness of geopolymeric cement concretes reinforced with basalt fibers. The values of fracture toughness, critical stress intensity factor and critical crack mouth opening displacement were measured on 18 notched beams tested by three-point bending. The a₀/h (notch height/beam height) ratio was equal to 0.2 and the L₀/h (distance between the supports/beam height) ratio was equal to 3.According to the experimental results, geopolymeric concretes have better fracture properties than conventional Portland cement. They are also less sensitive to the presence of cracks.     

Zusammenhänge zum Schlagzähigkeits‐ und Durchstoßverhalten thermoplastischer Natur‐/Langfaserverbunde

Correlations for the fracture work and falling weight impact properties of thermoplastic natural / long fibre composites The improvement of the fracture work is of special importance for the expansion of application fields in plastics construction parts of enhanced stress characteristics. An improvement of the toughness properties can be achieved by increased deformation ability. Fracture work and energy at the falling weight impact tests can be raised both by increasing of the strength and/or elongation of the reinforcing fibres and by reduction of the adhesion. The reduction of adhesion can be realized using no compatibilizers. But the increasing of the strength and/or elongation is restricted. The deformation ability of the resulting composite can be increased by addition of fibre components of higher strength respectively elongation to the natural and matrix fibres during manufacturing of the precursor. Significant improvements of the impact strength of such composites could be proved by impact bending tests and by measuring of strength and energy in breaking tests at shock load by falling weight tests. The results of both tests were compared to find out the origin of the improvement. It was found that the energy E_p needed for crack initiation has only a negligible effect on the improvement of the fracture work a_cU (Charpy impact strength). A significant higher effect on the fracture work a_cU was found for the energy E_r which will force the crack extension. So the increased crack retardation energy caused by the mechanical properties of the added fibres can be seen as the main origin of the improved toughness properties transition obtained by addition of fibres of high strength respectively elongation.     

An experimental analysis of fracture mechanisms of short glass fibre reinforced polyamide 6,6 (SGFR-PA66)

In the present work, toughness of unfilled polyamide 6,6 (PA66) and short glass fibre reinforced polyamide 6,6 (SGFR-PA66) was investigated. Digital image correlation (DIC) was used with a single camera for in-plane displacement field measurement and then strain computation. The results allowed to extract the resistance curve for the PA66 and critical stress intensity factors, K_Ic, for the SGFR-PA66 with three glass fibre contents (15%, 30% and 50% (wt)) and under room temperature (20 °C). The tests were carried out on single edge notched tension (SENT) specimens. The DIC technique allowed to precise the spatial distribution of the local strains in a defined region including the crack tip at different steps of the loading. Scanning electron microscopy observations illustrated different damage mechanisms occurring in the studied composites: matrix crack, fibre–matrix interface failure and fibres pull out.     

Short-rod elastic-plastic fracture toughness test using miniature specimens

The standard ASTM-E399 plane-strain fracture toughness (K _IC) test requires (1) the test specimen dimensions to be greater than a minimum size and, (2) fatigue precracking of the specimen. These criteria render many materials impractical to test. The short-rod elastic-plastic plane-strain fracture toughness test proposed by Barker offers a method of testing not requiring fatigue precracking and furthermore, it appears that test specimens smaller than that stipulated by ASTM can be used to obtain validK _IC values. In this study, the use of a modified miniature short-rod fracture toughness test specimen was investigated. Our miniature short-rod specimen is approximately 7 mm long and 4 mm diameter. These mini specimens are well suited for the purpose of testing biomaterials. The value of the minimum stress intensity factor coefficient (Y _m ^* ) for the mini short-rod specimens was determined experimentally using specimens machined from extruded acrylic rod stock. An elastic-plastic fracture toughness analysis using the mini specimens gave values ofK _IC for extruded acrylic (nominally PMMA) equal to 0.67 ± 0.06 MPa m^1/2. The problem of testing non-flat crack growth resistance curve materials (such as PMMA) using the short-rod fracture toughness test method is discussed. A modification to the test procedure involving the use of aY ^* value corresponding to a short crack length is suggested as a method of overcoming this difficulty.Nomenclature a crack length - a ₀ initial crack length - a ₁ length of the chevron notch on the mini short-rod specimen - a _m critical crack length — crack length atY _m ^* - C specimen compliance - C dimensionless specimen compliance = CED - D mini short-rod specimen diameter - E Young's modulus - K ₁ stress intensity factor - K _1C plane-strain fracture toughness - K _max fracture toughness calculated usingP _max - P load applied to the test specimen during a short-rod fracture toughness test - P _c load applied to the test specimen atY _m ^* - P _max maximum load applied to the specimen during a short-rod fracture toughness test - p plasticity factor - W mini short-rod specimen width - Y ^* stress intensity factor coefficient - Y _m ^* minimum of the stress intensity factor coefficient - dimensionless crack length =a/W - ₀ dimensionless initial crack length = ₀/W - ₁ dimensionless chevron notch length =a ₁/W - _m dimensionless critical crack length =a _m/W     

Effects of thickness and environmental temperature on fracture behaviour of polyetherimide (PEI)

The fracture behaviour of a polyetherimide (PEI) thermoplastic polymer was studied using compact tension (CT) specimens with a special emphasis on effects of specimen thickness and testing temperatures on the plane strain fracture toughness. The results show that the valid fracture toughness of the critical stress intensity factor, K _IC, and strain energy release rate, G _IC, is independent of the specimen thickness when it is larger than 5 mm at ambient temperature. On the other hand, the fracture toughness is relatively sensitive to testing temperatures. The K _IC value remains almost constant, 3.5 MPa in a temperature range from 25 to 130°C, but the G _IC value slightly increases due to the decrease in Young's modulus and yield stress with increasing temperature. The temperature dependence of the fracture toughness, G _IC, was explained in terms of a plastic deformation zone around the crack tip and fracture surface morphology. It was identified that the larger plastic zone and extensive plastic deformation in the crack initiation region were associated with the enhanced G _IC at elevated temperatures.     

Reinforcement of hydrated portland cement with high molecular mass water-soluble polymers

A small amount of polymer in water solution form (polyvinylpyrolidone or polyvinylalcohol) was added to a mix high silicate cement + amorphous silica, with reduced water to cement ratio. It was shown that as the molecular mass of the polymer is increased, the fracture stress and the stored energy at fracture of the specimens improved. The polymer induces an increase of the critical stress intensity factor (K _Ic ) (crack initiation). The fracture behaviour of the polymer modified cement paste beyond the elastic domain is also affected by PVA or PVP additions. The dissipated energy measured using the crack opening displacement CMOD was increased by a factor of two with 4wt% of PVP and by a factor of three with 3wt% of PVA, as a consequence of operative toughening mechanisms. The increase of mechanical properties is explained in case of PVP by crack interactions due to CSH microstructure modifications, and with PVA by crack bridging mechanisms as a consequence of dispersed polymer rich nodules in the hydrates phase.     

Effect of crosshead speed on the fracture toughness of Soda-lime glass, Al2O3 and Si3N4 ceramics determined by the surface crack in flexure (SCF) method

The fracture toughness of soda-lime glass, Al₂O₃ and Si₃N₄ specimens was measured by the surface crack in flexure method. For the soda-lime glass specimens, the fracture toughness was calculated from the initial crack size and flexure strength, and the value increased with increasing crosshead speed. This trend seems to be related to the difficulty in determining the critical crack size at fracture, since slow crack growth occurs during bending test. For the Al₂O₃ specimens, a halo region (stable crack growth region) was formed around the initial precrack during bending test. The halo size increased and the resultant flexure strength decreased with decreasing in the crosshead speed. The halo region, however, could not be observed in the Si₃N₄ specimens. Despite of the difference in the appearance of halo region, the fracture toughness of the Al₂O₃ and Si₃N₄ specimens was constant irrespective of the crosshead speed when the values were calculated with the critical crack sizes at fracture (halo incorporated crack sizes). The constant fracture toughness with the crosshead speed could be explained by the relation between the changes of halo size (thus critical crack size at fracture) and resultant flexure strength.     

A novel test method to measure the fracture toughness of ceramic balls used in bearings

A novel mechanical test has been developed to measure the fracture toughness of the silicon‐nitride (Si₃N₄) balls used in modern hybrid bearings. The ball is compressed diametrally between two hemispherical conforming dies, which causes the ball's equator to bulge, generating a tensile hoop stress. Under applied load, a precrack placed at the equator grows. To calculate the ball's fracture toughness at crack instability, finite‐element calculations of the applied stress field and an analytical solution for the stress intensity factor are used in the ‘two point plus semiellipse’ method. Tests of 16 Si₃N₄ balls and three soda‐lime glass balls gave fracture toughness measurements in good agreement with accepted published values. The new technique appears to be more accurate than the indentation technique used to measure the toughness of ceramics. As future work, the test can be extended to measure fatigue and stress corrosion properties for Si₃N₄ balls.     

Micro-defects Initiation Ahead of a Wedge-shaped Inhomogeneity

The problem of micro-defects that are initiated at the tip of a wedge-shaped inhomogeneity is investigated. Firstly, the elastic solution due to a screw dislocation near the tip of a semi-infinite wedge-shaped inhomogeneity is derived using the conformal mapping method. Then, a Mode III micro-crack initiated from the tip of the imhomogeneity is examined. The effects of the wedge angle, the crack location and the relative shear modulus of the inhomogeneity on the stress intensity factor of the micro-crack are studied. Finally, an interesting relationship between the critical stress intensity factor (K^*_III c for a line inhomogeneity and the fracture toughness of a crack (K_IIIc) in the same material is established for different shear modulus of the inhomogeneity.     

Study on cleavage fracture criteria of the quasi-brittle and micro-inhomogeneous materials

On the bases of recent achievements on the micro-mechanism of cleavage this paper analyses the inherent deficiencies of the stress intensity factor K _I which is used to evaluate the fracture toughness of quasi-brittle and micro-inhomogeneous materials. The K _I parameter can uniquely determine the field intensity ahead of a crack tip in the condition of elastic and small scale yielding (SSY). However, the K _I cannot uniquely determine the critical condition triggering the cleavage fracture in a quasi-brittle and inhomogeneous steel where the cleavage fracture process is not a direct extension of the precrack but is initiated at a variable distance from the precrack tip. The variable distances of cleavage initiations invoke varied critical values of K _I. On the bases of authors' experiments, this paper analyses the physical meaning of the local fracture stress _f, its stability and the feasibility to be used as an engineering parameter for assessing the fracture toughness.     

Rate-dependent fracture toughness of pure polycrystalline ice

A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (–20°C, –30°C and –40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10^–3 to 10^–1 s^–1. The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (K _IC) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of K _IC, the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10^–3 s^–1, the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen.     

Mode II fracture testing method for highly orthotropic materials like wood

In this paper a mode II fracture testing method has been developed for wood from analytical, experimental and numerical investigations. Analytical results obtained by other researchers showed that the specimen geometry and loading type used for the proposed mode II testing method results in only mode II stress intensity and no mode I stress intensity at the crack tip. Experiments have been carried out to determine mode II fracture toughness K _IIC and fracture energy G _IIF from the test data collected from both spruce (pice abies) and poplar (populus nigra) specimens. It was found that there existed a very good relation between fracture toughness K_IIC and fracture energy G _IIF when the influence of orthotropic stiffness E _II ^* in mode II was taken into account. It verified that for this mode II testing method the formula of LEFM can be employed for calculating mode II fracture toughness even for highly orthotropic materials like wood. In the numerical studies for the tested spruce specimen, the crack propagation process, stress and strain fields in front of crack tips and the stress distributions along the ligament have been investigated in detail. It can be seen that the simulated crack propagating process along the ligament is a typical shear cracking pattern and the development of cracks along the ligament is due to shear stress concentrations at the crack tips of the specimen. It has been shown that this mode II fracture testing method is suitable for measuring mode II fracture toughness K _IIC for highly orthotropic materials like wood.