The influence of pearlite fraction on fracture toughness and fatigue crack growth in nodular cast iron

Fracture toughness and fatigue crack growth data of four nodular cast irons with different pearlite fractions are studied. The influence of temperature on fracture toughness is also investigated. Fracture surfaces are observed using a scanning electron microscope to correlate fracture toughness with the fracture surface and to understand the mechanism of crack growth at different stress intensity factor ranges. It is shown that the upper shelf fracture toughness increases with pearlite fraction. The existence of nodular void regions on the fracture surface plays an important role in fracture toughness. The fatigue crack growth rate is less sensible to stress intensity factor range when pearlite fraction increases.     

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions. It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low-temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature. An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.     

FRACTURE TOUGHNESS OF NODULAR GRAPHITE CAST IRONS UNDER STATIC, IMPACT AND CYCLIC LOADING

Abstract— The paper deals with the fracture toughness of nodular cast irons of differing microstructure and manganese content. Fracture toughness was determined for static and impact loading in the temperature range from 77 to 523 K. The dependences of fracture toughness on manganese content and microstructure were obtained.
Fatigue crack growth rate curves were obtained at room temperature and a correlation between the parameters characterizing fatigue crack growth rate was found.
Fractographic studies revealed an important role of graphite inclusions in the process of fracture. On the one hand, as stress raisers they favour the nucleation of microcracks while on the other hand, they cause local retardation of fatigue crack growth probably due to the residual compressive stress surrounding them. In the mid-range of the fatigue crack growth rate curve a correlation was found between fatigue striation (or blocks of striations) spacings and the crack growth rate.     

Fatigue and fracture toughness of aluminium alloys reinforced with SiC and alumina particles

The mechanisms and fracture mechanics of fatigue crack initiation and the growth of small and large fatigue cracks and fracture toughness are reviewed in this paper. It is concluded that: (1) there are many factors which can affect fatigue crack initiation, some of which are understood; (2) small and large fatigue cracks can be correlated with stress intensity factor if closure is excluded; and (3) fracture toughness is mainly related to matrix plasticity but is strongly influenced by particle characteristics.     

Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti‐6Al‐4 V

Experimental investigation was conducted to evaluate the fracture toughness and fatigue crack growth characteristics in selective laser‐melted titanium 6Al‐4 V materials as a follow‐on to a previous study on high cycle fatigue. For both the fracture toughness and crack growth evaluation, the compact tension specimen geometry was used. It was found that the fracture toughness was lower than what would be expected from wrought or cast product forms in the same alloy. This was attributed to the rapidly cooled, martensitic microstructure, developed in the parts. At low stress ratios, the crack growth rates were faster than in wrought titanium but became comparable at higher ratios. The fracture toughness appears to be higher when the crack is oriented perpendicular to the build layers. The difference in the average threshold and critical stress intensity values for the crack growth results for the three orientations was within the scatter of the data, so there was essentially no difference. The same was true for the empirically derived Paris Law constants. Residual stresses were likely to have overshadowed any variation in crack growth because of microstructural directionalities associated with build orientation.     

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.     

An investigation of the effects of mix strength on the fracture and fatigue behavior of concrete mortar

This paper examines the effects of mix compressive strength (30, 35 and 40 MPa) on the fracture initiation toughness, resistance-curve behavior and fatigue crack growth behavior of concrete mortar. The fracture initiation toughness and the resistance-curve behavior are shown to increase with increasing mix strength. The observed resistance-curve behavior is then attributed largely to the effects of ligament bridging, which are predicted using small- and large-scale bridging models. In contrast, the fatigue crack growth resistance is shown to decrease with increasing mix strength. An extended multiparameter framework was used for the modeling of fatigue crack growth. Finally, the implications of the results are discussed for the design of concrete mixtures with attractive combinations of strength, fracture toughness and fatigue crack growth resistance.     

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al₂O₃-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al₂O₃-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.     

Bruchzähigkeit und Ermüdungs-Rißwachstum von Gußeisen

Fracture Toughness and Fatigue Crack Growth of Cast Irons Fracture toughness, elastic moduli and fatigue crack growth rates in air and in vacuum were measured for 17 different cast irons. The graphite shape in the cast irons varied from flakes to nodules, the matrix varied from ferrite to pearlite to martensite. In the fatigue crack growth rate tests, using fracture mechanics methods, it was observed that the fatigue crack growth rate increases significantly as the cyclic stress intensity range approaches the critical value for stable crack growth. This phenomenon was used to determine the fracture toughness of the cast irons. Such toughness data agree well with literature data on the fracture toughness of cast irons. An extensive review of the effects of strength on the fracture toughness of commercial cast irons is presented. In cast irons with flake graphite, cyclic loading results in a reduced modulus of elasticity. This is attributed to the rupture of the graphite flakes under cyclic loading.     

Some Peculiarities of Fatigue Crack Growth at Various Stages of Its Development

The author considers some peculiarities of fatigue crack growth in metals at the stages of its initiation and initial development, and stable and unstable growth that precedes final fracture. It is shown that at the stage of initial growth of fatigue cracks, the stress state, nonlocalized fatigue damage that precedes initiation of the main fatigue crack, residual surface stresses, surface manufacturing and in-service defects, and contact interactions are the factors that determine the crack paths. Stable growth of a fatigue crack is primarily determined by the stress-strain state of a structure as a whole and by the stress-strain state at the crack tip with allowance for its variation due to crack propagation, which is evaluated by the criteria of fracture mechanics. The author also studied peculiarities of fatigue crack development in compressor blades of marine gas turbines. It is shown that for embrittled steels, when fatigue cracks develop under plane strain conditions, final fracture occurs at very small crack sizes. In this case, the characteristics of fatigue fracture toughness are appreciably lower than the static values. The paper also considers peculiarities of unstable fatigue crack propagation.     

A unified theory of fatigue crack growth: a statistical approach

Treating the local fracture toughness of a material as a random value, a general relationship between the applied fracture parameter and the stable crack growth distance is developed. The result is applied to a study of the fatigue crack growth and a general expression connecting fatigue crack growth rate and the applied loading is rendered. Several empirical fatigue crack growth models can be derived on the basis of this unified view, and the valid ranges of these models are established. The conclusions are found to be in agreement with experimental evidence.     

Bifurcation analysis of the Hobson short fatigue crack growth law

A bifurcation analysis of the Hobson short fatigue crack growth law is presented. The analysis reveals that, although the growth law is non‐linear, it contains no bifurcation points.     

Fracture testing and results for a Ti-6A1-4V alloy at liquid helium temperature

This paper discusses fracture toughness and fatigue crack growth testing at liquid helium temperature, 4 K (?452°F). The design and performance of a fracture testing cryostat and associated instrumentation are described. Fracture toughness and fatigue crack growth data for Ti-6A1-4V from room temperature to 4 K are presented.     

铸造、锻造和粉末冶金TC4钛合金损伤容限行为对比研究

目的 研究影响铸造、锻造和粉末冶金TC4钛合金的损伤容限行为差异的主要因素。方法 分别从裂纹尖端塑性变形行为、二次裂纹及断口表面粗糙度3个方面对比,分析造成3种成形方法制备的TC4钛合金的断裂韧性和疲劳裂纹扩展速率差异的原因。结果 铸造TC4钛合金断裂韧性优于锻造和粉末冶金TC4钛合金,主要是因为新产生的裂纹面积大,消耗更多断裂能量。铸造TC4钛合金疲劳裂纹扩展速率低于锻造、粉末冶金TC4钛合金,其主要原因为曲折的裂纹路径和断面粗糙度诱发裂纹闭合效应以及长而深的二次裂纹。结论 3种成形方法制备TC4钛合金损伤容限行为差异的主要原因是断裂形成了不同裂纹路径形貌。     

7075T73510合金断裂韧性和疲劳裂纹扩展

研究了７０７５Ｔ７３５１０合金的断裂韧性和疲劳裂纹扩展行为，分析不同方向性能差异的原因。结果表明：合金的断裂韧性依赖于取向。Ｌ－Ｔ方向的断裂韧性值远大于Ｔ－Ｌ方向。而其疲劳裂纹扩展速率对应力比更敏感，无论Ｔ－Ｌ还是Ｌ－Ｔ方向，Ｒ＝０．６时的ＦＣＧＲ值是Ｒ＝０．１时的二倍左右。在相同应力比和较小的△Ｋ下，两个方向的ＦＣＧＲ值基本无差异。文中认为：呈带分或串状分布的杂质相对Ｔ－Ｌ向怕裂韧性影响较大。     

Temperature effects on fatigue crack growth in polycarbonate

The fatigue behaviour of a high strength thermoplastic, polycarbonate, has been investigated as a function of temperature. Fatigue crack growth properties were measured in the temperature range of 100 to 373 K and were analysed using a fracture mechanics approach. Fatigue behaviour was found to be related to the fracture toughness of the material. This correlation with fracture toughness was used to develop an empirical model based on the toughness for describing the effect of temperature on fatigue crack growth, and to consider fatigue in terms of the secondary losses of the polymer.     

The influence of stress intensity and microstructure on fatigue crack propagation in ferritic materials

New and published fatigue crack growth data for a wide range of steels have been categorized in terms of different growth mechanisms, namely striation formation, microcleavage, void coalescence and intergranular separation. General principles emerged concerning the influence of mean stress, specimen thickness, flow stress and toughness on rates of fatigue crack propagation through their effect on growth mechanism.

Crack propagation rates associated with striation formation were insensitive to changes in mean stress (except at very low stress intensities) and specimen thickness. Increase in flow stress resulted in a small decrease in growth rate, although the path of a crack through complex structures like welds was, nevertheless, strongly influenced by plastic relaxation. Crack propagation rates increased when deformation led to net-section yielding (general yielding) and the increase was related to specimen thickness and geometry. It has been shown that simple relationships between the rate of propagation and alternating stress intensity are adequate for describing fatigue crack growth by the striation mechanism.

Departures from exclusively striation formation to include micro-cleavage, void coalescence or intergranular separation were found to result in accelerated growth rates. Where growth occurred by combined striation formation and microcleavage, the increase in fatigue crack growth rate was dependent on the maximum tensile stress and hence on the mean stress and specimen thickness. Similarly, when fatigue involved the void coalescence mechanism the rate was increased by raising the mean stress. The role of microstructure and fracture toughness in promoting the different growth mechanisms is discussed. Modification of the simple growth law is necessary in order to describe the observed results.     

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.     

Fracture toughness and fatigue crack growth rate properties in wire + arc additive manufactured Ti‐6Al‐4V

This paper presents an experimental investigation of the fracture and fatigue crack growth properties of Ti‐6Al‐4V produced by the Wire + Arc Additive Manufacture (WAAM®) process. First, fracture toughness was measured for two different orientations with respect to the build direction; the effect of wire oxygen content and build strategy were also evaluated in the light of microstructure examination. Second, fatigue crack growth rates were measured for fully additive manufactured samples, as well as for samples containing an interface between WAAM® and wrought materials. The latter category covers five different scenarios of crack location and orientation with respect to the interface. Fatigue crack growth rates are compared with that of the wrought or WAAM® alone conditions. Crack growth trajectory of these tests is discussed in relation to the microstructure characteristics.     

Nature of scatter of fracture toughness in static loading

The previously proposed model of unstable fatigue crack growth is used to explain a large (in comparison with other mechanical characteristics) scatter of static fracture toughness for 15Kh2MFA and 15Kh2NMFA steels at temperatures below the tactile-brittle transition temperature. The results show that for the materials for which K_fc ¹ < K_Ic the critical stress intensity factor K_Ic depends on the specific energy of inelastic strain W at the tip of the initial fatigue crack in its formation stage. The value of W is a function of the number of load cycles (in the conditions with a constant range of the stress intensity factor K) as a result of irregular fatigue crack growth. Here K_fc ¹ is the minimum cyclic fracture toughness. A method is proposed of evaluating the minimum fracture toughness of the material in static loading based on inspection of the process of irregular fatigue crack growth in the stage of crack initiation.Translated from Problemy Prochnosti, No. 2, pp. 10–16, February, 1990.