Fatigue crack growth in thin section type 316 stainless steel

Fatigue crack growth under cyclic tensile stress was measured in thin ($\text{< 1}\text{mm}$) specimens of type 316 austenitic stainless steel by means of direct optical observation. The effect of varying the maximum applied stress and the load ratio was investigated, and crack growth rate data were correlated with the stress intensity factor range. Fracture surfaces were studied using optical and scanning electron microscopy in order to ascertain the fracture plane orientation and the crack growth mechanisms. The intent of the work was to establish whether fatigue tests on reduced size, thin section specimens, where these are required by experimental exigencies, could provide meaningful crack growth data. The validity of the linear elastic fracture mechanics approach to fatigue crack growth in such specimens is demonstrated. Some reservations remain however as to the applicability of thin specimen data to crack growth in bulk material because the transition from normal to slant fracture plane orientation, with consequent reduction in crack growth rate, takes place at an earlier stage in thinner section material.     

THE EFFECT OF SPECIMEN SIZE ON THE J 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 J 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 the standard fracture resistance J (i.e. not corrected for crack growth), J corrected for crack growth, and the J modified parameter proposed by Ernst. It was found that the best agreement was exhibited by the R-curves based on the standard fracture resistance J , which displayed reasonable size independence up to, and in many cases beyond crack growths corresponding to 50% of the initial uncracked ligament.     

Determination of fracture toughness from thin side-grooved specimens

Methods of determining fracture toughness from specimens of thickness lower than that required by ASTM Standard, E399 were studied using aluminum and titanium alloy specimens. In thin specimens in which crack growth initiation is clearly marked by a sudden change in the slope of the load-displacement curve, the stress intensity at the crack growth initiation point was found to be the same as the standard fracture toughness value. Crack growth initiation was more easily identifiable in the aluminum alloys than in the titanium alloy, although the latter was more brittle. Side grooves enable identification of crack growth initiation in thinner specimens, reducing considerably the thickness requirement for fracture toughness testing. A nearly straight crack front was found to be essential for obtaining reproducible results. Sharp and deep side grooves produced fatigue cracks leading at the edges.     

Study of fracture toughness evaluation of FRP

Using the fibre reinforced plastics (FRP) laminates consisting of glass chopped strand mat and unsaturated polyester resin, experiments were conducted under various conditions in order to determine the fracture toughness for crack instability. Crack growth was judged not by cracking of the resin matrix but by break of the glass fibres. The crack front was considered to be located in the section which was cracked through the 90% of the specimen thickness. Crack extension resistance (R-curves) thus obtained did not significantly vary with specimen thickness and initial crack length, but depended greatly on specimen configurations, compact tension (CT) and centre-cracked tension (CCT) specimens. The R-curve for a CT specimen was steeper than the one for a CCT specimen, which is quite contrary to the tendency for metals. It was deduced that the instability fracture toughness calculated from the maximum load on a load-deflection diagram, K _max, was scarcely affected by specimen thickness, initial crack length and specimen geometry (i.e. loading configuration), and therefore could be regarded as a material constant of the FRP used.     

Experimental investigation on low cycle fatigue and fracture behaviour of a notched Ni‐based superalloy at elevated temperature

In order to investigate the effects of stress concentration on low cycle fatigue properties and fracture behaviour of a nickel‐based powder metallurgy superalloy, FGH97, at elevated temperature, the low cycle fatigue tests have been conducted with semi‐circular and semi‐elliptical single‐edge notched plate specimens at 550 and 700 °C. The results show that the fatigue life of the notched specimen decreases with the increase of stress concentration factor and the fatigue crack initiation life evidently decreases because of the defect located in the stress concentration zone. Moreover, the plastic deformation induced by notch stress concentration affects the initial crack occurrence zone. The angle α of the crack occurrence zone is within ±10° of notch bisector for semi‐circular notched specimens and ±20° for semi‐elliptical notched specimens. The crack propagation rate decreases to a minimum at a certain length, D, and then increases with the growth of the crack. The crack propagation rate of the semi‐elliptical notched specimen decelerates at a faster rate than that of the semi‐circular notched specimen because of the increase of the notch plasticity gradient. The crack length, D, is affected by both the applied load and the notch plasticity gradient. In addition, the fracture mechanism is shown to transition from transgranular to intergranular as temperature increases from 550 to 700 °C, which would accelerate crack propagation and reduce the fatigue life.     

Environmental stress cracking of PVC and PVC-CPE

The fracture toughness of Polyvinylchloride (PVC) and PVC modified with 10% chlorinated polyethylene (PVC-CPE) was studied in vapour and in liquid environments by crack growth measurements on single-edge notch specimens under three-point bending at 23°C. In addition, some results obtained in air at lower temperatures are presented. The fracture toughness is quantified by a stress intensity factor leading to failure after a given loading period. It is shown that for a given slow crack growth rate at 23 °C, the environment hardly affects the fracture toughness of PVC. In contrast, the slow crack growth in PVC-CPE at 23 °C is accelerated by the presence of benzene vapour, n-octane/benzene mixtures and gas condensate. A decrease in temperature results in an increase in fracture toughness, both for PVC and for PVC-CPE. A Dugdale model to describe the craze ahead of the crack was used to analyse the observed changes in fracture toughness.     

On the relation between micro- and macroscopic fatigue crack growth rates in aluminum alloy AMS 7475-T7351

This paper discusses the relationship between striation spacing, i.e., the microscopic crack propagation rate, as measured in postmortem fractographic inspection of fatigue fractured surfaces, and the macroscopic crack propagation rate, i.e., da/dN, as monitored during fatigue crack growth tests. Compact tensile specimens C(T) in prevalent plane-strain conditions were extracted in LT orientation from the center of a 2-in. thick rolled plate of a SAE-AMS 7475-T7351 Al alloy. Testpieces were fatigue tested according to ASTM-E647 standard, at room temperature in a servo-hydraulic closed-loop MTS testing machine operating with the unloading elastic compliance technique. da/dN-ΔK data points were collected in the Paris’ law validity region, with crack growth rates typically ranging from 0.18 to 2.02 μm/cycle. Topographical survey was conducted on the test specimen fracture surfaces in a scanning electronic microscope in order to determine striation spacing created during the fatigue test. Macro- and micro-crack growth rates were compared and good correlation have been obtained for the data within the range of ΔK assessed in the study. Results of crack growth rates have been quantitatively evaluated in terms of fatigue life estimation.     

AN EXPERIMENTAL PARAMETER Jmax IN ELASTIC-PLASTIC FATIGUE CRACK GROWTH

Abstract— Imitating Garwood's 3-parameter technique, an experimental parameter J _max was introduced to predict fatigue crack growth rate (d a /d N ) over a wide range including small scale yielding and large scale yielding. It was found that for a Δ K -increasing fatigue test condition, J _max is a valid parameter. A significant crack growth acceleration, caused by a transition of fracture mechanism, occurs when J _max= J _IC The fracture mechanism involving striation formation when J _max < J _IC becomes ductile tearing when J _max > J _IC Equations to predict the effect of stress-ratio on J _max as well as on d a /d N are given.     

2(1/4)Cr-1 Mo钢的短裂纹疲劳断裂特征

本文用弹塑性断裂力学分别研究了在空气和氢中 ,压力容器器壁的承载材料 2 ( 1 / 4 )Cr— 1 Mo钢的短裂纹低频疲劳特性 ,结果表明 :都可用 da/ d N =C(ΔJ) n 的关系式计算上述两环境中的短裂纹扩展速率 .裂纹呈穿晶扩展 .氢明显增加了 2 ( 1 / 4 ) Cr— 1 Mo钢的断裂敏感性     

Bifurcation and propagation of a mixed-mode crack in a ductile material

The bifurcation and the propagation of a 2-D mixed-mode crack in a ductile material under static and cyclic loading were investigated in this work. A general methodology to study the crack bifurcation and the crack propagation was established. First, for a mixed-mode crack under static loading, a procedure was developed in order to evaluate the fracture type, the beginning of the crack growth, the crack growth angle and the crack growth path. This procedure was established on the basis of a set of criteria developed in the recent studies carried out by the authors [Li J, Zhang XB, Recho N. J-M^p based criteria for bifurcation assessment of a crack in elastic-plastic materials under mixed mode I-II loading. Engng Fract Mech 2004;71:329-43; Recho N, Ma S, Zhang XB, Pirodi A, Dalle Donne C. Criteria for mixed-mode fracture prediction in ductile material. In: 15th European conference on fracture, Stockholm, Sweden, August 2004]. A new criterion, by combining experimentation and numerical calculation, was developed in this work in order to predict the beginning of the crack growth. Second, in the case of cyclic loading, the crack growth path and crack grow rate are studied. A series of mixed-mode experiments on aluminium and steel specimens were carried out to analyse the effect of the mixed mode on the crack growth angle and the crack growth rate. On the basis of these experimental results, a fatigue crack growth model was proposed. The effect of the mixed mode on the crack growth rate is considered in this model. The numerical results of this model are in good agreement with the experimental results.     

DUCTILE TEARING SIMULATION BASED ON A LOCAL ENERGETIC CRITERION

It is well known that the tearing resistance curve J–Δa is not a material property and that probably the energy dissipation rate is preferable to the integral J to characterize crack growth. The G parameter represents the energy dissipated in plasticity and fracture and, under certain conditions, this parameter could be used directly as a critical value for a criterion based on an energy release rate calculated near the crack tip to simulate propagation. Indeed, we see that a local energy release rate could be calculated near the crack tip to take account only of that portion of energy which participates in the fracture.
We applied this approach to simulate the crack growth for CT specimens with and without side grooves in a 20  MnMoNi 55 ferritic steel, and compared the results with Rousselier's model.     

High cycle fatigue characteristics of 2124-T851 aluminum alloy

The fatigue crack growth rate, fracture toughness and fatigue S-N curve of 2124-T851 aluminum alloy at high cycle fatigue condition were measured and fatigue fracture process and fractography were studied using optical microscopy (OM), X-ray diffraction (XRD) technique, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that at room temperature and R = 0.1 conditions, the characteristics of fatigue fracture could be observed. Under those conditions, the fatigue strength and the fracture toughness of a 2124-T851 thick plate is 243 MPa and 29.64 MPa · m^1/2, respectively. At high cycle fatigue condition, the higher the stress amplitude, the wider the space between fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area, and the greater the ratio between the intermittent fracture area and the whole fracture area.     

WSM50C钢的裂尖塑性损伤与断裂

本文利用一组紧凑拉伸(CT)试样。测定了WSM50C高炉钢板材料的J_(?)阻力曲线和断裂韧性J_(IC),并采用液氮冲剖试样技术,在扫描电镜下研究了裂纹尖端附近材料塑性损伤与断裂的细观力学行为。观测研讨了内部空穴的演化规律。结果表明,裂尖附近的塑性损伤与断裂可以用细观空穴扩张比参数R/R_0来描述。当J相似文献   

A study of fatigue crack growth of 7075-T651 aluminum alloy

Both standard and non-standard compact specimens were employed to experimentally study the crack growth behavior of 7075-T651 aluminum alloy in ambient air. The effects of the stress ratio (R), overloading, underloading, and high–low sequence loading on fatigue crack growth rate were investigated. Significant R-ratio effect was identified. At the same R-ratio, the influence of specimen geometry on the relationship between crack growth rate and stress intensity factor range was insignificant. A single overload retarded the crack growth rate significantly. A slight acceleration of crack growth rate was identified after a single underload. The crack growth rate resumed after the crack propagated out of the influencing plastic zone created by the overload or underload. A parameter combining the stress intensity factor range and the maximum stress intensity factor can correlate the crack growth at different stress ratios well when the R-ratio ranged from −2 to 0.5. The parameter multiplied by a correction factor can be used to predict the crack growth with the influence of the R-ratio, overloading, underloading, and high–low sequence loading. Wheeler’s model cannot describe the variation of fatigue crack growth with the crack length being in the overload influencing zone. A modified Wheeler’s model based on the evolution of the remaining affected plastic zone was found to predict well the influence of the overload and sequence loading on the crack growth.     

Effects of plate thickness on fatigue fracture behaviors of an aluminum alloy (Al‐Cu‐Mg)

High cycle fatigue properties of 2124 aluminum alloy plates with different thickness were investigated by determining fatigue S?N curves, fatigue crack growth rates and fracture toughness of 2124‐T851 aluminum alloy plates with the thickness of 30 mm, 40 mm and 55 mm, respectively. Fatigue fracture behaviors of alloy plates were also analyzed and discussed using scanning electron microscope morphology observation, energy spectrum analysis, X‐ray diffraction phase analysis and transmission electron microscopy observation in this paper. The results indicate that plate thickness affects the comprehensive fatigue properties of 2124 aluminum alloy plates. Thinner plate achieves better comprehensive fatigue properties. Due to the different amount of deformation during hot rolling, the variation of microstructure of alloy plates with different thickness mainly concentrates on the difference of grain sizes, substructure and volume fraction of grain boundaries. The thinner the plate, the smaller the grain sizes and therefore the thinner plate produces a higher volume fraction of grain boundaries and substructure, and a greater resistance to fatigue crack growth, thus thinner plate exhibits better fatigue properties.     

Structure,fracture toughness,and fatigue of two aluminium matrix compositesproduced by vertical squeeze casting technique

Abstract

Composite materials produced from ceramic reinforcement of aluminium alloys have some properties that are better (higher modulus and strength, lower thermal expansion coefficient and density, and good creep and wear resistance) than those of the conventional monolithic aluminium alloys. However, they have a poor fracture toughness. The aim of the present work was to characterise the structure and mechanical properties of two different aluminium matrix composites (AS9C1G/20%(Al₂O₃-SiO₂) and 2014/20%(Al₂O₃-SiO₂)) manufactured using the vertical squeeze casting technique. Tensile, plane strain fracture toughness, and fatigue crack growth rate tests were carried out. In particular, the influence of specimen geometry on the toughness tests was examined. It was found that chevron notched short bar specimens gave toughness values ～ 40% higher than other types of specimens. Fatigue crack growth rate data were interpolated using some semiempirical models. An accurate metallographic investigation of both the structures and the fatigue fracture surfaces was carried out using optical microscopy and energy dispersive spectroscopy with SEM.     

Life extension technology for steam pipe systems—I. Development of material properties

Material properties of A106B low-carbon steels were developed for life prediction analyses of steam pipes operated at elevated temperatures but in the sub-creep temperature range. Tensile, fracture toughness, fatigue crack growth rate and low-cycle fatigue properties were obtained on the piping steel at 24°C (75°F) and 288°C (550°F). The latter temperature corresponded to the highest operating temperature of nuclear plant steam piping. Increasing the test temperature from 24°C (75°F) to 288°C (550°F) decreased the yield strength and fracture toughness of the steel. Fatigue crack propagation rate properties at 24°C (75°F) and 288°C (550°F) were found to be comparable.

In the low-cycle fatigue tests, below a strain amplitude level of approximately 0.5%, cyclic softening was observed, while at higher strain levels, cyclic hardening was present. Based on the results of tensile and incremental-step fatigue testing, the strain-life curve was predicted. The predicted strain-life curve was found to be in agreement with the experimental result.

The fracture surfaces of fracture toughness specimens showed ductile fracture, while striations were observed on those of fatigue crack growth specimens. Fatigue striations were also observed on the fracture surfaces of low-cycle fatigue specimens. Fatigue initiation was associated with inclusions. It was shown that plastic straining in A106B steel could be detected by acoustic emission and by monitoring the eddy current response. These nondestructive evaluation techniques exhibit possibilities for in-situ monitoring of fatigue deformation.

While the development of material properties for the life prediction assessment of steam pipes is included in Part I of this paper, the establishment of a quantitative life prediction methodology and inspection criteria is contained in Part II. The developed life prediction methodology quantifies the effects of operating parameters on the remaining life of steam pipes using the material properties obtained in Part I.     

Crack growth characteristics of carbon nanotube-based polymer composites subjected to cyclic loading

This paper presents the characterization of crack growth in carbon nanotube (CNT)-based polymer composites under fatigue loading. Fatigue crack growth tests were performed on single-edge cracked plate specimens of CNT/polycarbonate composites at room temperature and liquid nitrogen temperature (77 K). An elastic–plastic finite element analysis was also conducted to determine the J-integral range. The crack growth rate data were expressed in terms of the J-integral range, and the effect of nanotube addition on the fatigue crack growth behavior was examined. In addition, possible mechanisms of the crack growth in the nanocomposites are discussed based on microscopic observations of the specimen fracture surfaces.     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part II: influence of hydrogen trapped by inclusions

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness R _a increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: R _a ≅ C Δ K ²_I holds. At the border of a fish-eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish-eye. The height of the stretch zone is approximately a constant value around the periphery of the fish-eye. If we assume that a fatigue crack grows cycle-by-cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish-eye, we can correlate the crack growth rate d a/ d N , stress intensity factor range Δ K _I and R _a for SCM435 steel by the equation
   
and by d a/ d N proportional to the parameter R _a .
Integrating the crack growth rate equation, the crack propagation period N _p2 consumed from the edge of the ODA to the border of the fish-eye can be estimated for the specimens which failed at N _f > 10⁷. Values of N _p2 were estimated to be ∼1.0 × 10⁶ for the specimens which failed at N _f ≅ 5 × 10⁸. It follows that the fatigue life in the regime of N _f >10⁷ is mostly spent in crack initiation and discrete crack growth inside the ODA.