The flexural strength and fracture toughness of reactive powder concrete in response to changing fibre size and pre‐setting pressure

In this study, steel fibre mixtures having different compositions of fibres with various sizes were used in reactive powder concrete. The mixture composition, which would present the best performance during the flexural tests, was determined. Pre‐setting pressure tests at six different magnitudes (0, 5, 10, 15, 20, 25 MPa) were conducted on the freshly prepared reactive powder concrete mixture having the determined mixture composition. The 30‐mm‐long steel fibres mixed at a ratio of 4.5% provided the highest flexure strength and the fracture toughness during these tests. The highest pre‐setting pressure was attained at 25 MPa and the flexural strength of the sample increased by 117% reaching 28.07 MPa at that pre‐setting pressure.     

Effects of divider orientation split‐out on fracture toughness

The Split‐out phenomenon is a sudden instability, which takes place near the crack tip, and is generated by a rapid growth and arrest of a crack at the divider orientation. This phenomenon is related to stress triaxiality in the front of the crack tip when the structure is under plane strain conditions, plus the existence of weak interfaces oriented normal to the thickness direction. The weak interfaces can be generated as a result of some metallurgical process such as hot‐rolling lamination in materials with a high level of impurities, as well as steels where the structure results in a strong banding of ferrite and pearlite. When a cracked structure is loaded, the high σ_z stresses related to the plain strain state can lead to a split‐out brought about by delamination of the weak interfaces. During a fracture toughness test, this is noticed as a drop in the load‐displacement record, which is similar to that produced by the well‐known pop‐in instability in welded joints. Although the load drops are very similar in both phenomena, their etiologies are completely different, since the pop‐in is produced by an unstable crack growth and its ulterior arrest, due to local brittle zones located generally at the heat affected zone (HAZ) at or near a welded joint. As the split‐out has been less studied than the pop‐in, and due to the similarities between both load‐displacement records, it is very common to consider both instabilities under the same failure acceptance criteria. In this way, the split‐out is usually treated as a critical event, and therefore, many materials are rejected when split‐outs occur in fracture testings. However, when a split‐out occurs, the σ_z stress is drastically reduced, leading to a condition close to a plane stress situation (in fact, the σ_z stress reduces to 0 at the crack surfaces of the split‐out). The aim of this work was to study the effect of the split‐out in the fracture toughness of hot laminated steels working in the upper‐shelf region. In order to achieve this, the fracture toughness was evaluated by means of the resistance curve J‐R of the material, using a single specimen test method. The occurrence of load drop and displacement increase in the test record are explained based on a constant main plane crack length hypothesis.     

Fatigue fracture toughness of metals

The paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.     

Determination of the fracture properties of metallic materials using pre‐cracked small punch tests

In this paper, the use of pre‐cracked small punch test (p‐SPT) miniature specimens to obtain the fracture parameters of a material is presented. The geometry of the specimens used was square of 10 × 10 mm with a thickness of 0.5 mm. An initial crack‐like notch was created in the SPT specimens by means of a laser micro‐cutting technique. In order to obtain the fracture parameters from p‐SPT specimens three different approaches have been considered here. The first approach is based on the crack tip opening displacement concept, the second is based on the measure of the fracture energy using the area under the load–displacement curve for different crack sizes, and the third approach is based on the direct numerical simulation of the p‐SPT specimen and the numerical calculation of the J‐integral. In order to study the crack initiation in these p‐SPT specimens, several interrupted tests and the subsequent scanning electron microscope analysis have been carried out. The results indicate that p‐SPT specimens can be used as an alternative method for determining the fracture properties of a material in those cases where there is not enough material to undertake conventional fracture tests. For these p‐SPT specimens, the multi‐specimen method for the determination of the fracture energy is the most promising approach. The results indicate that this small specimen size allows the value of the material toughness, under low constraint conditions to be obtained.     

Precracking high speed steel for fracture toughness testing

Fracture toughness testing of high speed steel, which has a high fatigue strength and low fracture toughness, is a problem because fatigue cracks are difficult, or impossible, to initiate at a maximum fatigue stress intensity of $\text{0.7 K}{}_{\mathrm{<mtext>IC</mtext>}}$, as specified. A method of initiation by the use of an electric pen and subsequent fast propagation by fatigue has been studied and a procedure developed to give accurate, reproducible values of $\text{K}{}_{\mathrm{<mtext>IC</mtext>}}$ on subsequent fracture toughness testing.     

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.     

Fracture toughness and growth of short and long fatigue cracks in ductile cast iron EN‐GJS‐400‐18‐LT

Heavy components of ductile cast iron frequently exhibit metallurgical defects that behave like cracks under cyclic loading. Thus, in order to decide whether a given defect is permissible, it is important to establish the fatigue crack growth properties of the material. In this paper, results from a comprehensive study of ductile cast iron EN‐GJS‐400‐18‐LT have been reported. Growth rates of fatigue cracks ranging from a few tenths of a millimetre (‘short’ cracks) to several millimetres (‘long’ cracks) have been measured for load ratios R=?1, R= 0 and R= 0.5 using a highly sensitive potential‐drop technique. Short cracks were observed to grow faster than long cracks. The threshold stress intensity range, ΔK_th, as a function of the load ratio was fitted to a simple crack closure model. Fatigue crack growth data were compared with data from other laboratories. Single plain fatigue tests at R=?1 and R= 0 were also carried out. Fracture toughness was measured at temperatures ranging from ?40 °C to room temperature.     

Experimental investigation and life prediction of hot corrosion pre‐exposure on low‐cycle fatigue of a directionally solidified nickel‐base superalloy

Low‐cycle fatigue tests were conducted on the directionally solidified nickel‐base superalloy DZ125 at 850 °C in the unexposed and exposed specimens for 2, 15, 25 and 50 h in hot corrosion environment. The pre‐exposed specimen exhibited a lower life than unexposed specimens. Fatigue cracks in the unexposed specimens are initiated from defects near the surface, while the cracks of exposed specimens preferentially occur on the surface. Hot corrosion damage in fatigue life was found to be associated with the reduction of the bearing area. A novel life prediction methodology based on continuum damage mechanics was proposed to predict the experimentally observed decrease in low‐cycle fatigue life with increasing prior exposure time.     

Microstructure‐texture‐fracture toughness property correlation in annealed Al‐6Mg alloy with minor scandium and zirconium additions

The effect of minor addition of scandium and zirconium on the fracture toughness (FT) behaviour of aluminium‐6 wt% magnesium alloy is studied. Texture measurement and transmission electron microscopy have revealed that the evolution of texture in cast alloys after annealing is decided by the morphology and character of the precipitates. It is further demonstrated that the said minor addition influences the FT behaviour of Al‐6Mg alloy by manipulating in‐plane anisotropy as decided by the precipitate morphology. Textural situation in annealed state has also been related to the FT behaviour.     

A new method for determining the fracture toughness of main pipeline steels

One of the fundamental aims of fracture mechanics is to define fracture toughness K_IC of a material. Hence, the ASTM E399 standard was developed. However according to the standard, large‐sized specimens are required to determine the fracture toughness of low alloy carbon steels. ASTM E1921 standard was developed on the fracture toughness of ferritic steels. In this study, a new method was proposed to determine the fracture toughness of ferritic steels. The purpose of the present paper is to compare the results of the method with the experimental results. Two steels that are used in gas and oil main pipelines were investigated in this study.     

The relationship between fatigue fracture parameters during selfsimilar fatigue crack growth

The relationship between cyclic fracture toughness and the parameter n during selfsimilar fatigue crack growth has been investigated for SS41 and SM41A steels. The conditions under which selfsimilar growth is realized are also analysed.     

Unified correlation of in‐plane and out‐of‐plane constraints with fracture toughness

In this paper, the specimens with different geometries and loading configurations were used to study the unified correlation of in‐plane and out‐of‐plane constraints with fracture toughness by using numerical simulation method. The results show that the unified constraint parameter A_p which was defined on the basis of the areas surrounded by the equivalent plastic strain isolines ahead of crack tip can characterise both in‐plane and out‐of‐plane constraints induced by different specimen geometries and loading configurations. A sole linear relation between the normalised fracture toughness J_IC/J_ref and was obtained. The J_IC/J_ref ‐ line is a unified correlation line of in‐plane and out‐of‐plane constraints with fracture toughness of a material, and the constraint dependent fracture toughness of a material can be determined from the unified correlation line. The results also demonstrate that the out‐of‐plane constraint effect is related to the in‐plane constraint effect, and there exists interaction between them. The higher in‐plane constraint strengthens the out‐of‐plane constraint effect, whereas the lower in‐plane constraint is not sensitive to the out‐of‐plane constraint effect.     

The effect of seawater on fracture mode transition in fatigue

The fracture mode transition for two steels fatigued in air and seawater was investigated by measuring the crack growth rates and by examining the fractured surfaces using scanning electron microscopy. It was found that the transition from normal to shear fracture mode did not always occur in seawater under the same fatigue conditions as in air. Three possible mechanisms, based on the effective stress intensity factor range, the threshold stress intensity factor range and environmentally assisted brittle fracture, are proposed to account for this behaviour and their validity is discussed.     

A modified specimen for evaluating the mixed mode fracture toughness of adhesives

This article introduces a specimen geometry that allows the separation of fracture energy release rates G _I and G _II in adhesively joined beams made of disparate materials. The analysis is based upon a Green's functions formulation for shear deformable beams and circumvents the need to employ finite element computations. The current method results in a system of non-singular integral equations, that can be discretized and reduced to a system of algebraic equations which may be solved by common numerical techniques. The analysis accounts for the dimensions and properties of the adhesive and provides results for a wide range of G _I, G _II and their ratio. Those results agree with finite element computational values to within less than 4%.     

The effect of basalt fibres on fracture toughness of asphalt mixture

This paper deals with the effect of basalt fibres on fracture toughness of asphalt mixture. For this purpose, basalt fibres with three different contents (i.e., 0.1%, 0.2%, and 0.3% by weight of asphalt mixture) and lengths (ie, 4, 8, and 12 mm) are incorporated into asphalt mixture to prepare fibre‐reinforced asphalt mixtures. Fracture tests are then carried out on these mixtures under four different modes of loading (i.e., pure mode I, pure mode II, and two mixed modes of I/II) using semicircular bend (SCB) specimens. The results exhibit that the fracture toughness increases with the enhancement of the fibre content. In addition, increase in the length of basalt fibre results in reduction of the fracture toughness of asphalt mixture. However, the asphalt mixture containing 0.3% of basalt fibres with the length of 4 mm shows the highest fracture toughness compared with other mixtures. It is also found that the basalt fibre improves mode I fracture toughness of asphalt mixtures more significantly than mode II one. Statistical analysis is also performed on the experimental data. Analysis of ANOVA demonstrates that all the three factors investigated in this study (i.e., length of basalt fibre, content of basalt fibre, and mode of loading) have significant influence on the fracture toughness of asphalt mixtures.     

The effect of pit size and density on the fatigue behaviour of a pre‐corroded martensitic stainless steel

UNS S17400 steel is used in turbines for the aerospace and utility industries. While it is generally corrosion resistant, it is susceptible to pitting when exposed to aqueous chloride environments. Effects of pitting characteristics, such as depth, width, and local density on fatigue life, have been studied in this work to better inform criteria for component replacement or repair. While pit depth correlates well with cracking, the deepest pit never initiated the crack that ultimately led to failure. The clustering of pits, or local pitting density, also correlated well with crack initiation location; however, the densest region of pitting was not always the location where cracking occurred. There is likely no single metric that directly correlates pitting with fatigue cracking, rather there is a combination of pitting characteristics that ultimately lead to cracking. The results from this work suggest that pit depth and local pitting density are among the more important metrics.     

The effect of PVDF nanofibers on mode-I fracture toughness of composite materials

In this study, the fracture behavior of carbon/epoxy laminates interleaved by polyvinylidene fluoride (PVDF) nanofibers is investigated. For this aim, a mode-I fracture test is conducted on virgin and modified laminates. Unlike the results of other studies, it is shown that PVDF nanofibers can increase mode-I fracture toughness (G_I) noticeably in a specific situation. The results show that G_I is enhanced about 43% and 36% in initiation and propagation stages of the fracture, respectively, using PVDF nanofibers. The morphology of the fractured surface is also presented for investigating the mechanism of toughening.     

Using torsional bar testing to determine fracture toughness

A new method to determine fracture toughness K _IC of materials is introduced. A round-rod specimen having a V-grooved spiral line with a 45° pitch is tested under pure torsion. An equibiaxial tensile/compressive stress state is effectively created to simulate conventional test methods using a compact-type specimen with a thickness equivalent to the full length of the spiral line. K _IC values are estimated from the fracture load and crack length with the aid of a three-dimensional finite element analysis. K _IC of 7475-T7351 aluminium is estimated to be 51.3 MPa √m, which is higher than the vendor's value in the TL orientation by ∼0.8% and higher than 0.5T compact tension (CT) value by 6%; A302B steel yields 54.9 MPa √m being higher than CT test value by ∼2%. Good agreement between the K _IC values obtained by different methods indicates the proposed method is sound and reliable.     

铝镁合金平面应力断裂韧性测试研究

为测试铝镁合金平面应力的断裂韧性,根据HB 5261-1983《金属板村KR曲线试验方法》要求,在MTS-810试验机上,对铝镁合金中心裂纹拉伸试样(CCT)进行裂纹扩展阻力曲线KR与平面应力断裂韧性KC的测试;分析屈曲、裂纹长度的修正以及KR曲线有效数据点等因素的影响.测试结果表明:铝镁合金材料的平面应力断裂韧性KC较平面应变断裂韧性KIC高40％.     

Microstructure-related fracture toughness and fatigue crack growth behaviour in toughened, anhydride-cured epoxy resins

Fracture toughness and fatigue crack propagation (FCP) of plain and modified anhydride-cured epoxy resin (EP) were studied at ambient temperature. Liquid carboxyl-terminated acrylonitrile-butadiene (CTBN) and silicon (SI) rubber dispersions were used as tougheners for the EP. The morphology of the modified EP was characterized by dynamic mechanical analysis (DMA) and by scanning electron microscopy (SEM). The fracture toughness, K_c, of the compositions decreased with increasing deformation rate. K_c of the EP was slightly improved by CTBN addition and practically unaffected by incorporation of the SI dispersion when tests were performed at low cross-head speed, v. Both modifiers improved K_c at high v, and also the resistance to FCP, by shifting the curves to higher stress intensity factor ranges, ΔK, by comparison with the plain EP. It was established that both fracture and fatigue performance rely on the compliance, J_R, at the rubbery plateau, and thus on the apparent molecular mass between crosslinks, M_c. The failure mechanisms were less dependent upon the loading mode (fracture, fatigue), but differed basically for the various modifiers. Rubber-induced cavitation and shear yielding of the EP were dominant for CTBN, whereas crack bifurcation and branching controlled the cracking in SI-modified EP. The simultaneous use of both modifiers resulted in a synergistic effect for both the fracture toughness at high deformation rate and the FCP behavior.