ELEVATED TEMPERATURE FRACTURE OF PARTICULATE-REINFORCED ALUMINUM PART II: MICROMECHANICAL MODELLING

Abstract Micromechanical fracture-toughness models are applied to experimental results for a metal-matrix composite (2009/SiC/20p-T6) to understand the temperature dependencies of toughness and fracture mechanisms, as well as to test quantitatively a continuum fracture-mechanics approach. Models which couple the crack-tip strain field, characteristic fracture-process distance and measured intrinsic micro void-fracture resistance predict the temperature dependencies of fracture-initiation (K_JICi) and crack-growth (T_R) toughnesses from 25°C to 316°C. The temperature dependencies of K_JICi and T_R result from the interplay between the fracture resistance and the crack-tip strain field, each being temperature-dependent. Strain-based models are equally valid for void nucleation- or growth-controlled fracture. A scenario for fracture is nucleation-controlled damage within Sic-particle clusters, corresponding to K_JICi, followed by cluster-damage growth to coalescence under increasing stress intensity. Void growth is stabilized increasingly at elevated temperatures.     

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     

DETERMINATION OF THE ENERGETIC PARAMETERS CONTROLLING CLEAVAGE FRACTURE INITIATION IN STEELS

Abstract— The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies γ_pm and γ_mm, which define the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the γ_pm value remains constant around 7 J/m², it has been observed that γ_mm is dependent on temperature. At −196°C the value is lower than 50 J/m² and at room temperature it is higher than 200 J/m². This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.     

DYNAMIC CRACK PROPAGATION AND CRACK ARREST BEHAVIOUR IN RELATION TO BRITTLE INTERGRANULAR AND CLEAVAGE FRACTURE

Abstract— Conventional mechanical tests and fracture mechanics experiments were carried out at – 196°C on a low alloy steel (A508 class 3) which was investigated under two different conditions: (i) a reference condition in which the failure mode was cleavage and (ii) an embrittled condition in which the fracture mode was either partly or predominantly intergranular fracture. These experiments, performed with a new specimen geometry, a ring specimen instrumented to measure also the crack velocity, were used to determine the fracture toughness at crack initiation ( K _Ic) and at crack arrest ( K _Ia). It is confirmed that the reduction in K _Ic measured in the embrittled material is associated with the appearance of intergranular fracture. It is also shown that K _Ia, determined by a static analysis decreases rapidly with crack velocity when the fracture mode is predominantly cleavage. On the other hand, K _Ia, corresponding to intergranular fracture seems to be much less dependent on crack speed. This difference in the sensitivity of both modes of brittle fracture to crack velocity is briefly discussed.     

NON-ISOTHERMAL FATIGUE CRACK GROWTH IN HASTELLOY-X

Abstract— Non-isothermal fatigue crack growth tests were performed on Hastelloy-X single edge notch specimens in which strain and temperature were varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 925°C. The crack growth rates were first reported as a function of the strain intensity factor (δ K _ε) derived from a crack compliance analysis. Out-of-phase (ε_max at T _max) cycling showed faster crack growth rates than isothermal or in-phase (ε_max at T _max) cycling under elastic straining. Under fully plastic cycling, the opposite results was observed, i.e. crack growth rates under isothermal cycling are faster than under TMF cycling. On a δ K _ε-basis, a strain range effect was observed. All the results were rationalized using a corrected stress-intensity factor (δ K _eff) computed from the actual load, the closing bending moment caused by the increase compliance with crack length, and with the effective opening stress. Each mode of fracture was found to be characterized by a unique crack growth rate vs δ K _eff curve. On a δ K _eff-basis, the isothermal crack growth rates at T _min and T _max provide an upper and a lower bound for the TMFCG rates. The effectiveness of δ K _eff to correlate crack growth rates under fully plastic cycling is discussed in detail.     

SIZE, THICKNESS AND GEOMETRY EFFECTS ON TRANSITION FRACTURE

Abstract— Transition fracture toughness was studied to look at the effect of size, thickness and geometry. Size effects were studied on six sets of data collected from the literature in which proportionally sized compact specimens of various steels were tested. Thickness effects were studied on tests of compact specimens of constant thickness and varying planar dimensions. Tests were conducted on a pressure vessel steel at a constant thickness of 20 mm where planar dimensions were increased so that thickness constraint was decreased. Geometry effects were studied on tests from a center cracked tension specimen geometry. Initially all of the data from the tests were included in the study; none were eliminated due to a size or other validity requirement. Then two validity requirements, the K _Ic and the Anderson-Dodds size requirements, were imposed to study their effect on the data.
The results showed that a smaller specimen size does not necessarily result in higher toughness. Rather, the smallest size often gave the lowest values of toughness. Loss of thickness constraint tended to increase toughness but not very much; it may not increase at all at the lower temperatures. The center cracked tension geometry appeared to have a lower constraint. These specimens showed an increase in toughness which is similar to that observed on a compact specimen for a change of temperature from −90°C to −60°C. Imposing a size validity requirement eliminated much of the fracture toughness data in the transition and influenced the distribution of data. Validity size criteria should be avoided if possible, especially for a scientific study.     

CYCLIC LOADING AND FRACTURE TOUGHNESS OF STEELS

Abstract— The paper considers the effect of cyclic loading and loading rate upon fracture toughness characteristics of steels at room and low temperatures. It is shown that fracture toughness of a low-alloy ferrite-pearlite steel with 0·1% C (steel 1) and for 15G2AFDps steel of the same class (steel 2) are 2 to 2·5 times lower under cyclic loading (50 and 0·5 Hz) and dynamic loading (= 1·5 × 10⁶MPa √m s⁻¹) than under static loading (= 0·6 to 9 MPa √m s⁻¹). For quenched and low-tempered 45 steel at 293 K and for armco-iron at 77 K fracture toughness characteristics do not depend on the loading condition. Macro- and micro-fractographic investigations revealed a correlation between the plastic zone size and the length of brittle fracture areas which are formed in steels 1 and 2, and in armco-iron during unstable propagation of the fatigue crack. Dependence of the decrease of the critical stress intensity factor under cyclic loading on the number of load cycles are obtained for repeating ( R = 0) and alternating bending ( R =−1) of specimens with a crack. A model for the transition from stable to unstable crack propagation is proposed involving crack velocity in the zone ahead of the crack tip damaged by cyclic plastic deformation. A new approach is suggested to the classification of materials on the basis of the sensitivity of fracture toughness characteristics to cyclic conditions of loading.     

AN ENGINEERING FRACTURE PARAMETER FOR NON-J-CONTROLLED CRACK GROWTH

Abstract— An engineering fracture parameter (CTOA) is studied under non- J -controlled growth. Based on the asymptotic solution of the crack tip field and the concept of the ₁ integral, an analytical relation linking the crack tip opening angle (CTOA) and the ₁ integral is established. A numerical investigation of the process of crack growth is carried out by the finite element method. The whole field solution, fracture parameters and indeterminate parameters are determined by FE analysis. An experimental investigation on a ductile structural steel has also been conducted and the crack resistance curve (CTOA)_R is obtained. The numerical and experimental results validate the theoretical analysis and show that they are consistent. It can be concluded that CTOA is always constant during steady crack propagation.     

PROBABILISTIC ASPECTS OF CLEAVAGE CRACK INITIATION SITES AND FRACTURE TOUGHNESS

Abstract— Fracture toughness tests were performed in the ductile-brittle transition temperature range using 110 specimens of the three-point bend and CT types. Probabilistic characteristics of fracture toughness and cleavage crack initiation sites were analysed in detail, together with the fibrous crack shape, from which the plane strain region in the specimen was deduced. The criterion for obtaining plane strain at the mid plane of the specimen was established as: B ≤ 0.004{ K _c( J )/σ _y }²+ 0.01. The thickness effect of cleavage fracture toughness for the specimen satisfying this equation is mainly caused by the statistical distribution of the weakest points ahead of the crack front (the Weibull volume effect).     

Predicting creep crack initiation in austenitic and ferritic steels using the creep toughness parameter and time-dependent failure assessment diagram

Methods for evaluating the creep toughness parameter,   K ^mat _c   , are reviewed and   K ^mat _c   data are determined for a ferritic P22 steel from creep crack growth tests on compact tension, C(T), specimens of homogenous parent material (PM) and heterogeneous specimen weldments at 565 °C and compared to similar tests on austenitic type 316H stainless steel at 550 °C. Appropriate relations describing the time dependency of   K ^mat _c   are determined accounting for data scatter. Considerable differences are observed in the form of the   K ^mat _c   data and the time-dependent failure assessment diagrams (TDFADs) for both the 316H and P22 steel. The TDFAD for P22 shows a strong time dependency, but is insensitive to time for 316H. Creep crack initiation (CCI) time predictions are obtained using the TDFAD approach and compared to experimental results from C(T) specimens and feature components. The TDFAD based on parent material properties can be used to obtain conservative predictions of CCI on weldments. Conservative predictions are almost always obtained when lower bound   K ^mat _c   values are employed. Long-term test are generally more relevant to industrial component lifetimes. The different trends between long- and short-term CCI time and growth data indicate that additional long-term test are required to further validate the procedure to predict the lifetimes of high temperature components.     

DYNAMIC FATIGUE AND COMPUTER SIMULATION OF THE BEHAVIOUR OF Y2O3-ZrO2 CERAMICS

Abstract— There is a critical stress rate K _c for each of the three ZrO₂ ceramics tested. Fracture is controlled by the materials polycrystal fracture toughness, K ^c_p, if the stress rate is less than K _c. Otherwise, fracture is controlled by the single crystal fracture toughness, K ^c_s. The crack growth parameters determined by dynamic fatigue experiments can only represent macrocrack growth behaviour although the fracture of specimens in experiments originates from small surface flaws.     

MORPHOLOGICAL STUDIES RELATING TO THE FRACTURE STRESS AND FRACTURE TOUGHNESS OF SILICON NITRIDE

Abstract— Microstructure and mechanical properties of HP (Hot Pressed), HP/GP (Gas Pressed), and HP/HIP (Hot Isostatic Pressed)—Si₃N₄ are studied using scanning electron microscopy, bending tests and the indentation fracture method. The grain diameter distribution is analyzed to clarify the relationship between microstructure and mechanical properties; and also the bending strength and fracture toughness. It is shown that bending strength increases with decreasing grain diameter. The results also show that a Hall—Petch type of relationship is obtained between grain diameter and fracture strength. The fracture toughness shows a linear relationship with , where σ_F= bending strength, β= a proportionality factor and d _a= average grain diameter, and is closely related to the aspect ratio of Si₃N₄ grains. It is concluded, from the morphological analysis, that a microstructure composed of Si₃N₄ grains, with both a small grain diameter and a large aspect ratio, is effective in improving both the fracture strength and fracture toughness.     

FRACTURE TOUGHNESS OF BAINITIC NODULAR CAST IRON

Abstract— The fracture toughness of bainitic ductile iron transformed at various austempering temperatures and austempering times was evaluated by using compact tension specimens and compared with the fracture toughness of bulleye casting structure. Using Scanning Electron Microscopy, the mechanism of the fracture mode can be understood by observing the fracture surface. An X-ray diffractometer was used to determine the volume fraction of retained austenite. From the results of fracture toughness properties, it can be concluded that the most suitable austempering temperature of the material used in the present study is from 300 to 350°C.     

ELASTIC-PLASTIC FRACTURE MECHANICS FOR INITIATION AND PROPAGATION OF NOTCH FATIGUE CRACKS

Abstract Initiation and propagation are considered to be controlled by the extent of total plastic shear deformation φ. Crack initiation and crack propagation occur when φ, exceeds a critical threshold value which can be equated to threshold conditions determined from linear elastic fracture mechanics analyses. When a crack is in a plastically deformed zone φ_t=φ _p +φ _e . where φ _p is the component of φ _t due to notch bulk plasticity and φ _e , is the component of φ _t due to a linear elastic fracture mechanics (LEFM) analysis of the crack tip plasticity field.
When cracks initiate at notch roots φ _t > φ_th. As the crack propagates in the notch plastic zone the rate of decrease of v _p will be different from the rate of increase of φ _e and it is possible for φ _t to decrease to a level below φ_th thereby creating a non-propagating crack.     

DYNAMIC FRACTURE STUDY ON CrNiMoV ALLOY STEEL UNDER DIFFERENT HEAT TREATMENTS AND IMPACT RATES

Abstract— Two methods are employed, namely the Charpy impact test and a strain gauge technique, for the determination of the dynamic fracture toughnesses of CrNiMoV27 and CrNiMoV45 alloy steels under various impact loading rates and heat treatments. The results show that the dynamic fracture toughness K _Id is significantly affected by impact velocity and that tempering has a greater influence on K _Id than quenching. It is also shown that the alloy containing a greater amount of vanadium yields consistently a higher value of K _id.     

基于断裂力学的高强度钢材梁柱节点受力性能分析

为了研究国产Q460C高强度结构钢材梁柱节点的断裂行为,该文基于断裂力学理论,计算了Q460C高强度钢材焊缝及热影响区材料的断裂韧性,并且采用三维有限元断裂模型,以I型裂纹尖端应力强度因子K_I和J积分为定量的评价指标,分析了焊缝及热影响区不同长度的裂纹对梁柱节点断裂韧性的需求。弹性分析表明,K_I沿梁翼缘宽度方向呈W形分布,最大值出现在翼缘中心,且与名义弯曲应力呈线性关系,而焊根裂纹的断裂韧性需求比热影响区裂纹更高。弹塑性分析表明,J_I最大值出现在翼缘的边缘,热影响区裂纹的断裂韧性需求比焊根裂纹更高。研究结果表明,Q460C高强度钢材梁柱节点的断裂由焊根裂纹控制,断裂承载力与梁全截面塑性承载力相近,临界转角小于0.02rad,因此建议通过改善焊接工艺或局部构造来保证节点拥有足够的转动能力。     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

THE EFFECT OF SPECIMEN SIZE ON THE CTOD R-CURVE BEHAVIOUR OF A TITANIUM ALLOY

Abstract— —This paper presents preliminary results from a large experimental programme to study geometry and size effects in CTOD R-curves. The results presented were obtained from unloading compliance R-curve tests performed at room temperature on different sized single-edge-notch bend specimens made from Ti–3Al–2V alloy. The crack growth resistance was measured in terms of both the conventional CTOD, δ₀, (i.e. as defined in BS 5762) and CTOD corrected for crack growth, δ_R . It was found that the δ₀ and δ_R R-curves were independent of specimen size up to crack extensions corresponding to approximately 10–15% of the original uncracked ligament. Also, after crack extensions of 30–40% of the initial ligament the δ₀ and δ_R R-curves exhibited well defined upswings. The upswing in the CTOD R-curves is thought to be a result of a reduction in crack tip constraint.     

FRACTURE BEHAVIOUR OF UNTREATED AND SILANE-TREATED TALC-FILLED POLYPROPYLENE COMPOSITES

Abstract— Fracture behaviour of injection-moulded polypropylene filled with silane-treated talc was studied as a function of filler volume fraction (0–20%) and compared to that of polypropylene filled with untreated talc. High-rate tests (0.57 m/s) on SENB specimens were carried out using an instrumented Charpy impact pendulum, and linear elastic fracture mechanics (LEFM) was applied to calculate the fracture parameters, K _C and G _C. It was found that moderate fractions of talc which were added to the polypropylene matrix increased the fracture toughness of the composite independent of the talc surface treatment. This general improvement seems to be due to the peculiar orientation of the talc platelets in the injection-moulded specimens. The fracture behaviour of the composites was also studied at low strain rate (1 mm/min) by tests on J -integral type specimens with the same SENB geometry. In this case, the composites with silane-treated talc presented poor J -integral values compared to those of the samples with untreated talc. This was attributed to a reduction of the plastic zone at the crack tip, since the improved coupling between the talc platelets and matrix increased the yield strength of the composite. All the results are explained on a basis of morphological and microstructural details.     

Fracture behaviour of PC/ABS resin under mixed-mode loading

Fracture behaviour of polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) under mixed-mode loading conditions was studied for several weight fractions of PC and ABS. Mode I and mixed-mode fracture tests were carried out by using compact–tension–shear specimens. At a certain value of mixed-mode loading ratio K _II / K _I a crack of the shear type will initiates at the initial crack tip. Fracture toughness increases under mixed-mode loading with an increase in the mode II component, whereas it reduces with the appearance of a shear-type fracture. Fracture toughness and the appearance of a shear-type fracture depends on the blending ratio of PC and ABS. The transition to shear-type fracture occurs at lower value of K _II / K _I for resins with higher fracture toughness.