INITIATION AND GROWTH OF SMALL CRACKS IN DIRECTIONALLY SOLIDIFIED MAR-M247 UNDER CREEP-FATIGUE PART II: EFFECT OF ANGLE NETWEEN STRESS AXIS AND SOLIDIFICATION DIRECTION

This paper deals with the effect of anisotropy on fracture processes of a directionally solidified superalloy, Mar-M247, under a push–pull creep-fatigue condition at high-temperature. Three kinds of specimen were cut from a cast plate such that their axes possess angles of 0°, 45° and 90° with respect to the 〈001〉 orientation that is aligned parallel to the solidification direction (also to the grain boundaries and primary dendrite axis); these specimens being denoted the 0° specimen, the 45° specimen, and the 90° specimen, respectively. The tests were conducted at 1273  K (1000 °C) in air under equal magnitudes of the range of a Δ J -related parameter, Δ W _c , which represents the driving force for crack growth in creep-fatigue. Although the grain boundaries are macroscopically parallel to the solidification direction, they are wavy or serrated microscopically. Small cracks nucleate along parts of the grain boundaries perpendicular to the stress axis in all specimens. The 90° specimen has the shortest crack initiation life and the 0° specimen has the longest. In the 90° and 45° specimens, intergranular cracks continue to nucleate and a main crack is formed along the grain boundary due to the frequent coalescence of small cracks. In the 0° specimen, cracks grow into the grain, and transgranular cracks coalesce along the primary dendrite or grain boundary. The 0° specimen exhibits the slowest crack growth rate and the 90° specimen the fastest. These differences in the initiation and growth behaviour of small cracks cause the longest failure life in the 0° specimen and the shortest in the 90° specimen.     

Effect of heat treatment on fatigue crack growth in chain steels

Abstract

The influence of tempering temperature, stress ratio, and prior strain on fatigue crack propagation in a low-alloy chain steel has been investigated. At small stress ratios (R=0·1) tempering above 400°C is beneficial, resulting in higher threshold levels and slower growth rates in the initial growth regime. Thereafter, crack growth is independent of tempering temperature, as it is over the entire growth period under a high mean stress (R=0·5). Prior strain produces a slower growth and higher thresholds at R= 0·1. Intergranular fracture is common and is a function of stress intensity range and tempering temperature. It is concluded that residual stress effects, rather than microstructural effects, account for the experimental observations. In particular, the existence of a tensile residual stress during initial growth and a crack closure stress greater than the minimum applied stress level are proposed.

MST/672     

Short fatigue cracks in austempered ductile cast iron (ADI)

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 10⁷ cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit.     

EFFECTS OF TEMPERATURE, CYCLIC FREQUENCY AND ENVIRONMENT ON FIBRE DEGRADATION AND FATIGUE CRACK GROWTH RESISTANCE IN A FIBRE-REINFORCED TITANIUM METAL-MATRIX COMPOSITE UNDER DISPLACEMENT-RANGE LOADING

The fatigue crack growth resistance of a [0/90°]_2s cross-ply SCS6 fibre-reinforced Ti–6Al–4V alloy metal-matrix composite has been assessed under displacement range control (i.e. under load shedding conditions with crack extension) to investigate potential fibre degradation and the process of crack extension at room temperature, and at 450°C, in air and in vacuum. Attention is focused on initial conditions that will promote crack arrest at room temperature. Under the test conditions employed here, regions of crack growth can occur where the applied nominal stress intensity factor range (ΔK) is relatively constant. This 'constant'ΔK range is the result of a fortuitous balance between the particular test-piece geometry, loading conditions utilized, matrix crack growth and the rate of fibre fracture. It allows the influence of environment, cyclic frequency and temperature on fatigue crack growth resistance to be analysed more easily than for tests carried out under load control.
The crack growth rate remained almost constant but with some steep local retardations in growth rate in the constant ΔK region at a temperature of 450°C, while crack arrest occurred at room temperature for the same initial ΔK. The average crack propagation rate in this 'constant ΔK region' at a temperature of 450°C in air was much greater than that at a temperature of 450°C in vacuum. This indicates that environment plays an important role in the process of fibre degradation. The effect of cyclic frequency is saturated at a frequency of less than 1  Hz. The process of crack growth at various frequencies is also discussed.     

EFFECT OF TEMPERATURE ON FATIGUE CRACK GROWTH IN THE POLYMER ABS

Abstract— Fatigue crack growth in a commercial grade ABS over the temperature range - 50°C to 80°C has been studied. An Arrhenius type relationship between fatigue crack growth rate and absolute temperature was found to describe the experimental data. At Δ K = 1 MPa√m, the activation energy for crack growth in the temperature range −50°C to 19°C is 3.47 kJ/mole and in the temperature range 30°C to 80°C it is 19.63 kJ/mole. The two different activation energies were found to be associated with the roughness of the fracture surfaces. The roughness of the fracture surfaces is discussed in relation to modes of fatigue crack growth. In the low temperature range (− 50°C to 19°C) the fracture surfaces were found to be rather coarse, whereas in the high temperature range (30°C to 80°C) they were found to be somewhat smooth. These different roughnesses were deduced to be due to different modes of crack branching influenced by crazing. A "stress intensity factor"-biased Arrhenius equation for fatigue crack growth successfully predicts growth rates at various temperatures.     

STATISTICAL INVESTIGATION OF THE EFFECT OF LABORATORY AIR ON THE FATIGUE BEHAVIOUR OF A CARBON STEEL

In order to clarify the effect of the atmospheric conditions on fatigue damage, rotary bending fatigue tests were carried out on smooth specimens of a normalized 0.37% carbon steel in controlled laboratory air. The air conditions used in the tests were moist air at 20 °C, moist air at 35 °C and dry air at 35 °C. The influence of atmosphere on crack initiation and propagation behaviour was investigated in detail based on successive observations of the surface. Experimental results showed that the fatigue life was superior at 20 °C compared to 35 °C by a factor of 2, while the effect of moisture was small compared to that of temperature. The statistical investigation of crack initiation and propagation behaviour indicated that the temperature strongly affects the crack initiation process; conversely, moisture plays an important role in the propagation process of cracks smaller than 0.3  mm. Moreover, the distribution characteristics of crack initiation life, crack propagation life, fatigue life and crack growth rate were analysed by assuming either a Weibull distribution or a log-normal distribution.     

The effect of heat treatment on the toughness, hardness and microstructure of low carbon white cast irons

The effect of destabilisation and subcritical heat treatment on the impact toughness, hardness, and the amount and mechanical stability of retained austenite in a low carbon white cast iron have been investigated. The experimental results show that the impact energy constantly increases when the destabilisation temperature is raised from 950°C to 1200°C. Although the hardness decreases, the heat-treated hardness is still greater than the as-cast state. After destabilisation treatment at 1130°C, tempering at 200 to 250°C for 3 hours leads to the highest impact toughness, and secondary hardening was observed when tempering over 400°C. The amount of retained austenite increased with the increase in the destabilisation temperature, and the treatment significantly improves the mechanical stability of the retained austenite compared with the as-cast state. Tempering below 400°C does not affect the amount of retained austenite and its mechanical stability. But the amount of retained austenite is dramatically reduced when tempered above 400°C. The relationship between the mechanical properties and the microstructure changes was discussed.     

A PRELIMINARY STUDY ON CRACK HEALING BEHAVIOUR OF Si3 N4 /SiC COMPOSITE CERAMICS

Ceramic (Si3 N4 /SiC) composites have been produced by sintering. From the sintered block, three point bend specimens were cut out. A semi-circular crack was made on the centre of the tension surface of the test specimen with the aid of a Vicker's indenter. The diameter of the semi-circular crack was about 60–70  μm. The specimens were subsequently heat treated at 1300°C for 1  h in vacuum, nitrogen or air. The bending strength was measured at room temperature, 800 and 1000°C. The specimens heat treated in air recovered considerably their bending strength at 1000°C, several specimens failed at a location different from the healed crack and their average strength showed the same value of that for smooth specimens. From these results, it is concluded that the recovery in bending strength of a cracked specimen was caused by crack healing.     

POST WELD HEAT TREATMENT EFFECTS ON FATIGUE CRACK PROPAGATION IN AN ELECTRON BEAM WELDMENT

This paper studies the effect of two post-weld heat treatment processes on the fatigue behaviour of an electron beam weldment in 9 mm AISI 4130 steel. Electron beam tempering, in a vacuum chamber, immediately after welding and a traditional furnace tempering treatment were compared. Fatigue crack propagation resistance was assessed by a linear elastic fracture mechanics analysis. The resistance to fatigue crack growth was improved with post weld heat treatment due to residual stress relief and the existence of a toughened tempered microstructure. The specimens with an electron beam post-weld heat treatment showed better fatigue properties than those of furnace-treated specimens. An electron beam post-weld heat treatment causes the fatigue crack growth rate to decrease with increasing energy input and decreasing micro-dot-pattern width. For a furnace post-weld heat treatment, the fatigue crack growth rate decreases with increasing tempering temperature.     

Mixed-mode crack growth from an unusual source

Out-of-plane crack growths were observed in compact tension specimens under static and quasi-static loading conditions at 650 °C in a nickel based alloy U720Li. The apparent mixed-mode crack growth behaviour may be explained based on a stability argument. The actual occurrence of the unstable crack growth may depend also on other factors such as the crack growth mechanism, grain size and loading mode.     

ELEVATED TEMPERATURE FRACTURE OF PARTICULATE-REINFORCED ALUMINUM PART I: FRACTURE TOUGHNESS

Abstract—The plane-strain initiation and growth fracture toughnesses of powder-metallurgy-processed, SiC particulate-reinforced 2009 plate were measured at temperatures from 25°C to 316°C. Initiation toughness from electrical potential monitoring ( K _JICi) is 18 MPa°m at 25°C, and is nearly constant to 220°C before decreasing sharply to 6 MPa°m at 316°C. Growth toughness, given by the tearing modulus ( T _R), is less than 3 from 25°C to 125°C, and increases dramatically above 200°C. The magnitude and temperature dependence of initiation toughness depend on detection of the critical fracture event. Standard measures of toughness K _IC and K _JIC exceed K _JICi and increase to a plateau with increasing temperature. The fracture mode for the composite is microvoid nucleation, growth and coalescence at all temperatures. Void nucleation is associated with SiC; such particles both crack and create stress and plastic strain concentrations that rupture the interface or adjacent matrix, particularly at corners. Matrix plasticity and cavitation increase with increasing temperature. Void growth is regular at all temperatures, but limited by adjacent SiC particles. Both K _JICi and T _R are governed by the temperature-dependent crack-tip plastic stress and strain fields, and the intrinsic damage resistance of the composite microstructure.     

Strength and toughness of sintered plus forged T1 high speed steel

The stresses for macroscopic plastic flow and critical stages of fracture, fracture toughness and hardness of sintered plus forged T1 high speed steel were determined. The results are compared to similar data for sintered, sintered to closed porosity plus hot isostatically pressed and electroflux refined (EFR) alloys of comparable composition. EFR meltstock, with addition of 0.6 wt% Mo, was water-atomized in a 200 kg unit which incorporated ceramic filters and an argon shroud to ensure maximum cleanliness. The powder was sieved, <125 μm, vacuum annealed, blended, isostatically compacted and vacuum sintered and hot forged to produce a 300 kg billet. Mechanical properties were determined in four-point bending of heat-treated beam specimens. Most samples showed evidence of macroscopic plastic flow, up to ∼1%, beyond a stress of ∼1.8 GPa, σY. Using surface replica microscopy, crack nucleation was detected at stresses σN, between 0.5 and 0.9 σY, and subcritical short crack growth, at stresses generally larger than σY. Fracture, from crack nuclei associated (only) with fractured M6C carbides, took place at stresses, σF, in the range 1.4 to 3.0 GPa Macroscopic fracture toughness, KIC, was in the range 17–24 MPa m1/2 and, like σN and σF, appeared to depend sensitively on the tempering temperature. The most attractive combination of properties, for the overtempered, 580°C, structure at HV50 ∼750 appears to be: σY≈1.9 GPa, σF≈2.8 GPa, KIC≈23 MPa m1/2. These values are comparable to those for EFR aerospace quality T1 high speed steel. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical Response and Energy Dissipation Analysis of Heat-Treated Granite Under Repeated Impact Loading

The mechanical behaviors and energy dissipation characteristics of heat-treated granite were investigated under repeated impact loading. The granite samples were firstly heat-treated at the temperature of 20°C, 200°C, 400°C, and 600°C, respectively. The thermal damage characteristics of these samples were then observed and measured before impact tests. Dynamic impact compression tests finally were carried out using a modified split-Hopkinson pressure bar under three impact velocities of 12 m/s, 15 m/s, and 18 m/s. These test results show that the mineral composition and the main oxides of the granite do not change with these treatment temperatures. The number of microcracks and microvoids decreases in the sample after 200°C treatment. The mechanical properties of a sample after 600°C treatment were rapidly deteriorated under the same impact velocity. The average of peak stress is much smaller than those after 20°C, 200°C and 400°C treatments. The heat-treated samples have an energy threshold each. When the dissipated energy of a sample under a single impact is less than this threshold, the repeated impacts hardly lead to further damage accumulation even if its total breakage energy dissipation (BED) density is large. Under the same number of repeated impacts, the cumulative BED density of a sample after 600°C treatment is the largest and its damage evolves most quickly. The total BED density of the sample after 200°C treatment is the highest, which implies that this sample has better resistance to repeated impact, thus having less crack initiation and growth.     

Mechanical Properties Measurement of PECVD Silicon Nitride after Rapid Thermal Annealing Using Nanoindentation Technique

Abstract:  This paper presents the results of mechanical characterisation of residual stress, elastic modulus, hardness and fracture toughness of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films subjected to rapid thermal annealing (RTA), processed between 200 and 800 °C. Additional tensile residual stresses were generated during the RTA period and the stress reached peak values after a 400 °C RTA process. On the other hand, nanoindentation testing revealed that both the modulus and hardness varied significantly with different RTA temperatures. Finally, the fracture toughness of the nitride was estimated to be 1.33 MPa √m based on a series of Vickers micro-indentation tests and it can be enhanced by the RTA process. These results should be useful for microelectromechanical systems (MEMS) or integrated circuit (IC) structure fabrication as regards maintaining the structural integrity and improving fabrication performance.     

Subcritical crack growth of trip steels in air under static loads

Pre-fatigued compact tension fracture toughness specimens of TRIP steel were held at constant loads at 25°C in 40 per cent r.h. air. Testing was done using an MTS closed loop universal test machine in the load control mode and the displacement was monitored as a function of time using a clip-on gauge and a strip chart recorder. Subcritical crack growth was observed and the experimental data was used to obtain a correlation between stress intensity and the rate of crack growth. The curves usually exhibited three distinct regions, including a plateau of stress intensity insensitive constant crack growth rates which have been observed by other investigators for titanium alloys and high strength steels. Based on comparisons with other investigators, the mechanism of subcritical flaw growth was tentatively identified as being due to hydrogen embrittlement. Fracture was observed to follow austenite grain boundaries and it was hypothesized that the austenite → martensite transformation sensitizes them to hydrogen by causing a large strain accumulation to be accommodated at the boundaries resulting in a large dilatation. Metallography revealed that the crack growth rate decreased as the strain-induced martensite increased and this was attributed to crack tip blunting by plastic deformation due to the invariant shear of the transformation. There are thus two apparently competing processes in the subcritical flaw growth of TRIP steels. Fractography showed that numerous fracture micromechanisms were operative. The subcritical crack growth characteristics compared favorably with other high strength steels tested under almost equivalent conditions and an apparent threshold for subcritical growth in air was determined to occur at about 56 per cent of K_IC.     

Conditions of invariance of corrosion crack resistance characteristics

Conclusions It has been established that for a number of materials the condition of invariance of fracture toughness depends not only upon the type of material but also upon its structure.For a high-strength steel with a martensitic structure the kinetics of subcritical crack growth and also the parameter K_Iscc are sensitive to the original austenitic grain size. Heat treatment for coarse grains has a favorable influence on the corrosion crack resistance of such a steel. The creation in coarse-grained steel of serrated austenitic grain boundaries leads to an additional increase in resistance to corrosion crack growth.The generally accepted criterion of invariance of fracture toughness Eq. (1) is unsuitable as a condition guaranteeing no change in corrosion crack resistance parameters. The value of the coefficient A_c in Eq. (2), which characterizes the condition of obtaining the parameter K_Iscc independent of sample thickness, is determined by the structure of the material and for the systems considered is more than 500.Corrosion cracks in steel with a martensitic structure may have a complex morphology dependent upon the subcritical crack growth mechanism, the size of the austenitic grains, and the form of their boundaries. In contrast to fine-grained and overheated steel, for which intergranular subcritical crack growth is characteristic, in steel with serrated grain boundaries the subcritical crack growth mechanism is more complex. It was also observed that in the center layers of thick samples there is primarily transgranular failure replaced by intergranular at the transition to the surface layers.To determine the effective stress intensity factor at the tip of a corrosion crack with a complex trajectory, a method based on determining the pliability of a sample with a crack propagating in a curved trajectory was found to be effective.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 17, No. 3, pp. 24–33, May–June, 1981.     

ACCELERATION OF CRACK GROWTH UNDER INTERMITTENT OVERLOADING AT ELEVATED TEMPERATURE

Abstract— Acceleration of crack growth by intermittent overloading was investigated at 650°C by using specimens of different thickness made of Type 304 stainless steel. When the hold time of overload was very short (∼20s), the crack growth rate was significantly accelerated (20–50 times) and the fracture surface morphology showed extremely ductile transgranular fracture by glide plane decohesion or microvoid coalescence, suggesting significant recovery of the material. In the thinner plate specimens, the crack growth rate under intermittent overloading was correlated well with the modified J -integral, i.e. J ( C %) and agreed with the growth rate of static creep cracks in a J versus d a /d t diagram. In the thicker plate specimens, however, this is not the case and the growth rate was about 20% of that in the thinner plate specimens in the J diagram. Transgranular fatigue type crack growth appeared in the low growth rate region.     

The temperature dependence of the fracture toughness and acoustic emission of polycrystalline alumina

The technique of acoustic emission has been shown to be suitable for the monitoring of fracture-toughness tests over a range of temperatures. Commercial polycrystalline alumina has been tested at temperatures up to 1000° C to determine the effect of microstructure and impurity content on fracture toughness and acoustic emission. For a given alumina there was no significant variation in acoustic response or fracture toughness up to 650° C. The emissions observed prior to fracture in this temperature range were attributed to subcritical crack growth. The number of emissions depended on the amount of subcritical crack growth, the grain size, and the presence and amount of porosity. Above 650° C the fracture behaviour changed due to the flow of a grain-boundary glassy phase. This was associated with a peak in the temperature dependence of the apparentK _IC and was accompanied by a large number of acoustic events of low amplitude and low pulse width. At these elevated temperatures the extent of grain-boundary glassy flow, and hence the acoustic response, increased with decreasing grain size and increasing impurity content.     

Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr-1 Mo pressure vessel steel

Tests were performed on a 2¼  Cr–1  Mo steel to measure the fracture toughness at initiation, K _Ic and at arrest, K _Ia . The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure K _Ia : (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to −196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (∼500  m  s⁻¹ ) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic–plastic finite element method to determine K _Ia values. These tests reveal a greater temperature shift (∼100 °C) between K _Ic and K _Ia in 2¼  Cr–1  Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼  Cr–1  Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism     

A new approach to the subcritical cracking of ceramic fibers

A new modeling approach to subcritical crack propagation (i.e. static fatigue) in ceramics is introduced and applied to the case of ceramic fibers. This approach relies on classical fracture mechanics, in which the zone near the crack’s tip is modified by the environment (particularly through oxidation). Chemical reactions and diffusion are introduced naturally, which makes the model highly dependent on the environmental conditions, such as partial oxygen pressure. In some cases, the result comes down to the widely-used Paris-like subcritical crack propagation law. For the general case, a two-parameter model calibrated and validated up to 1000 °C for Hi-Nicalon fibers is presented.