Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant ΔK. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si₃N₄ grains within the crack wake.     

Fatigue crack growth behaviour of gamma base titanium aluminides under different loading conditions

Static and cyclic fatigue crack growth behaviour of gamma base titanium aluminides with three different microstructures were investigated. Influence of cyclic test frequency on fatigue crack growth behaviour was also studied at room temperature under a controlled humidity condition. The crack growth behaviour both under static and cyclic loading was strongly influenced by the microstructure. The threshold stress intensity and crack growth behaviour under cyclic loading were much inferior than that under static loading indicating the ‘true-cyclic fatigue’ effect exhibited in gamma base titanium aluminides. No significant effect of test frequency on the crack growth behaviour was observed for the equiaxed and duplex microstructure materials.     

Fracture toughness assessment of silicon carbide-based ceramics and particulate-reinforced composites

The fracture toughness of sintered silicon carbide (α-SiC) and silicon carbide reinforced with particulate titanium diboride (TiB₂/SiC) has been evaluated using specimens in bending containing chevron notches and through-thickness precracks at ambient and elevated temperatures in air and in vacuum. Fracture toughness values measured from through-thickness precracked test-pieces are lower at all test temperatures. The particulate reinforcement has been shown to toughen the matrix significantly at room temperature only. At the test temperature of 1200°C the difference in toughness between the two materials is reduced and increasing the temperature to 1600°C further reduces this difference, to the extent that the two materials have values of fracture toughness which are indistinguishable. This provides strong evidence that the dominant toughening mechanism in the composite is the effect of thermal residual stresses which are relieved as the temperature is increased. Fractographic observations suggest that the bonding between the SiC and TiB₂ particulate is relatively weak because interfacial decohesion of particles is observed at all test temperatures. Nevertheless, surface roughness measurements indicate that there may also be a contribution to the toughness from increased crack deflection in the composite material at room temperature only.     

Fatigue behaviour of 3-d SiC/SiC Composites

A tension–tension fatigue damage analysis was performed using 3-d silicon carbide fibre reinforced (orthogonal) silicon carbide matrix (SiC/SiC) composites. Two groups of SiC/SiC specimens were tested. The first group consisted of samples without any oxidation protective top layer coating, whilst the latter one contained samples covered with a well fitting, chemical vapour deposited (CVD) SiC system. This coating is necessary for the material to sustain high temperatures. Both the coated and uncoated material had a fibre volume fraction of about 36% equally distributed in three rectangular directions. Load control fatigue tests were conducted at room temperature. The fatigue life was found to decrease by increasing the cyclic stress level. A power-law equation is proposed, which correlates the applied maximum stress during the fatigue test with the number of cycles to failure. In general, the presence of the coating layer decreases the static strength of the material. However, the nominal maximum cyclic stress for which the endurance fatigue limit appeared, remained unaffected by the presence of the oxidation protective SiC coating. Microstructural examination has also been performed on the fractured specimens and it reveals some of the failure mechanisms of the composite that appeared under quasi-static and dynamic loading.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.     

Use of chevron notches for fracture toughness determination in brittle solids

Abstract

The chevron notch testpiece geometry has been used in three and four point bending to evaluate the toughness of silicon carbide and silicon carbide reinforced with particulate titanium diboride. The toughness values obtained do not appear to depend either on initial chevron notch depth m the range 0·2≤α_o≤0·4 or on chevron notch included angle in the range 40≤θ≤120°. This testpiece geometry allows relatively subtle effects of reinforcement in increasing the toughness of the monolithic material to be assessed with confidence. It also provides a convenient method of measuring toughness over a wide range of test temperatures.

MST/1595     

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m^1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al₂O₃-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.     

Subcritical crack-growth and lifetime behavior of glass and SiG under static load

Crack initiation and subcritical crack growth in glass sheet and SiC bar specimen under static loading were investigated to study the failure process. It has been demonstrated that the lifetime process of brittle materials involves three possible forms of crack growth: subcritical crack growth, partly subcritical crack growth and instantaneous fracture without subcritical crack growth. Curves of upsilon -K obtained in step-by-step static fatigue tests and in constant loading rate tests showed different trends for borosilicate glass sheet. alpha -SiC that is generally considered immune to mechanical fatigue effect and environmental attack was also tested under static loading and the lifetime was measured. The results showed that the threshold load to damage effect was over 80% of the initial strength for the SiC.     

Subcritical Crack-Growth and Lifetime Behavior of Glass and SiC under Static Load

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Mechanical and microstructural behaviour of a particulate reinforced steel for structural applications

Abstract

Low density, high modulus, and potentially improved wear resistance are the major benefits of a ferrous composite material. A BS S.156 (4%NiCrMo)gear steel reinforced with 15 vol.-% titanium diboride particles has been demonstrated for possible high performance structural applications. This composite has been produced by a powder metallurgy/mechanical milling processing route, to give a homogeneous distribution of fine reinforcement particles. The composite tensile strength was 90% of the matrix in the fully heat treated condition, with ductility of up to 6%. However, martensite formation and hence the hardenability was found to be suppressed in the composite. A fatigue study indicated that the titanium diboride particles did not appear to contribute to crack initiation in the composite.     

Low-cycle fatigue strength under step loading of a Si3N4 + SiC nanocomposite at 1350°C

The low-cycle fatigue behaviour of a hot pressed silicon nitride/silicon carbide nanocomposite and a reference monolithic Si₃N₄ have been investigated in 4-point bending at 1350°C in air using stepwise loading. The nanocomposite was prepared using 20% of SiCN amorphous powder as an additive, together with 5% yttria, to crystalline -silicon nitride powder. Two types of specimen have been tested, with and without a sharp notch (notch tip radius 10 m) at applied loads from 50 N with steps of 25 N and from 50 N with steps of 50 N, respectively. Five cycles have been performed at all applied load levels with an amplitude of 50 N for both types of specimen. The deflection of the specimens has been recorded up to specimen failure. The failure load of the unnotched nanocomposite was significantly higher than that of the monolithic material whereas for the notched specimens only a small difference has been found between the failure loads of the monolithic and the composite. Notched specimens of both materials exhibited a similar size of the slow crack growth area at catastrophic fracture, whereas for unnotched specimens the size of the slow crack growth area was significantly larger for the monolithic ceramic. The nanocomposite exhibits higher fatigue strength due to its higher resistance against stress corrosion damage and stress corrosion crack growth.     

Effect of Particulate Silicon Carbide on Cyclic Plastic Strain Response and Fracture Behavior of 6061 Aluminum Alloy Metal Matrix Composites

In this paper, the cyclic stress response and cyclic stress–strain response characteristics, cyclic strain resistance and low-cycle fatigue life, and mechanisms governing the deformation and fracture behavior of aluminum alloy 6061 discontinuously reinforced with silicon carbide (SiC) particulates are presented and discussed. Two different volume fractions of the carbide particulate reinforcement phase in the aluminum alloy metal matrix are considered. The composite specimens were cyclically deformed using fully reversed tension–compression loading under total strain-amplitude-control. The stress response characteristic was observed to vary with strain amplitude. The plastic strain-fatigue life response was found to degrade with an increase in carbide particulate content in the metal matrix. The fracture behavior of the composite is discussed in light of the interactive influences of composite microstructural effects, cyclic strain amplitude and concomitant response stress, deformation characteristics of the composite constituents and cyclic ductility.     

FATIGUE LIFE PREDICTION OF NOTCHED COMPOSITE COMPONENTS

Abstract— The local stress/strain approach has been used to predict the fatigue lives of notched composite components. The method was based on a microstress analysis and the application of a multiaxial fatigue parameter incorporating the alternating strain components on the critical plane. This parameter was able to correlate the fatigue lives obtained under a variety of multiaxial loading and geometrical configurations, enabling a generalized fatigue life curve to be determined on the basis of limited experimental data.
The ability of the multiaxial fatigue parameter to relate the fatigue behaviour of composites was illustrated by predicting the locations of crack initiation sites in a unidirectional silicon carbide fibre reinforced titanium plate containing a circular hole tested under constant amplitude cyclic loading. The same approach was also successfully employed to predict the fatigue lives of graphite reinforced epoxy composite tubes with circular holes tested under several combinations of cyclic tension and torsion.     

Erosive wear behaviour of aluminium based composites

Al-MMCs reinforced with short fibres or particles of ceramics such as alumina, titanium diboride and silicon carbide result in composites of high specific strength and stiffness, suitable for advanced engineering applications such as in the aerospace and automotive industries. This paper studies the erosion wear behaviour of Al-based composites reinforced with alumina-fibre and in-situ TiB₂ particles using a water/SiC particles slurry jet. From the results of our experiment, the erosion resistance of reinforced Al-MMCs depends on that of the Al-alloy and the reinforcing ceramics, as well as on the bonding strength between the matrix and ceramic fibres or particles. Some design strategies to enhance the erosion resistance of Al-MMCs reinforced with short fibres and particles have been discussed.     

A new testing method to obtain mode II fatigue crack growth characteristics of hard materials

Flaking type failure in rolling‐contact processes is usually attributed to fatigue‐induced subsurface shearing stress caused by the contact loading. Assuming such crack growth is due to mode II loading and that mode I growth is suppressed due to the compressive stress field arising from the contact stress, we developed a new testing apparatus for mode II fatigue crack growth. Although the apparatus is, as a former apparatus was, based on the principle that the static K_I mode and the compressive stress parallel to the pre‐crack are superimposed on the mode II loading system, we employ direct loading in the new apparatus. Instead of the simple four‐point‐shear‐loading system used in the former apparatus, a new device for the application of a compressive stress parallel to the pre‐crack has been developed. Due to these alterations, mode II cyclic loading tests for hard steels have become possible for arbitrary stress ratios, including fully reversed loading (R=?1); which is the case of rolling‐contact fatigue. The test results obtained using the newly developed apparatus on specimens made from bearing steel SUJ2 and also a 0.75% carbon steel, are shown.     

STATIC AND CYCLIC FRETTING FATIGUE BEHAVIOUR OF SILICON NITRIDE

A cyclic fretting fatigue test machine was constructed. The piezoelectric bimorphs were used as actuators for cyclic loading and fretting motion at the resonance frequency of the system. Fretting fatigue tests under static and cyclic loading conditions were carried out using HIP-sintered silicon nitride. From the experimental results, it was found that fretting fatigue strengths under the two test conditions were identical and hence the effect of cyclic loading on fretting fatigue strength of silicon nitride was almost negligible. A fretting crack initiated in a very early stage of the fatigue life at the position of the maximum frictional stress in the contact area. Fretting fatigue life prediction based on fracture mechanics was also carried out. The predicted lives were in good agreement with the experimental results.     

The effect of cyclic creep on grain-boundary cavitation in ceramics

Small-angle neutron scattering has been used to characterize the cavity distribution in crept samples of a hot-pressed silicon carbide, which contained a thin continuous amorphous phase, and a sintered alumina, which contained no amorphous phase. The compression creep experiments were performed under cyclic loading at 1600° C and a frequency of 0.33 Hz. Comparison of the cavitation rates under cyclic loading with previously measured rates under static loading indicates that cavitation in the silicon carbide was unaffected by the cyclic loading, while the cavity volume fraction and the cavity size in the alumina were slightly increased by the cyclic loading. The results suggest that 0.33 Hz is too slow a frequency to affect the stress distribution and thus cavitation in the glassy phase containing silicon carbide, but it is rapid enough to accelerate cavitation in the absence of a glassy phase. This hypothesis is supported both by experimental results from other ceramic and metal systems and by calculations of characteristic stress relaxation times.     

On numerical analysis of damage evolution in cyclic elastic-plastic crack growth problems

Structures subjected to severe cyclic loading may fail due to low cycle fatigue. During the latter part of the fatigue life the crack growth rate may increase due to crack growth from static failure modes. This was investigated numerically by Skallerud and Zhang ( Int. J. Solids Struct. 34, 3141–3161, 1997) for a butt-welded plate with a circular crack growing from the centre of the weld. The weld material was slightly overmatching, and for simplicity, base material properties were employed in the finite element model. The predicted crack growth rate was significantly underpredicted in the early part of crack growth. In the present investigation, more detailed material modelling was used, and some metallurgical aspects were addressed. The fatigue part of the crack growth was determined by using the computed cyclic J -integral, and the static mode crack growth from ductile tearing is determined from computations accounting for void nucleation/growth/coalescence by means of a modified Gurson–Tvergaard model.     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.     

Fatigue strength of crack-healed Si3N4/SiC composite ceramics

Si₃ N₄ /SiC composite ceramics were sintered and subjected to three-point bending on specimens made according to the appropriate JIS standard. A semi-circular surface crack of 110 μm in diameter was made on each specimen. By using three kinds of specimen (smooth, cracked and crack healed), crack-healing behaviour, cyclic and static fatigue strengths were determined systematically at room temperature and 1000 °C. The main conclusions are as follows: (i) Si₃ N₄ /SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens showed similar cyclic and static fatigue strengths as smooth specimens, this being caused by crack healing; (iii) crack-healed zones had a sufficient fatigue strength and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.