Fracture toughness is usually used as a generic term for measures if materials resistance to expansion of a crack in engineering applications such as petrochemical industries and oil and gas pipelines. The concept of fracture toughness received profound importance in engineering design and applications, after the destruction of liberty ships in the World War II. In this paper, fracture toughness test methods, evaluation and standardization of metal materials are reviewed based on linear elastic and elastic-plastic fracture mechanics. The general principles and methods of testing developed by the American Society for Testing and Materials are explained. The main parameters of fracture mechanics such as elastic energy release rate (G), stress intensity factor (K), J integral, crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) are defined. Finally, the scientific and computational foundations of the two factors (K) and CTOD, as the most widely used industrial factors will be examined in detail.

In this research, the failure behavior and toughness mechanism of ST44 sheet in the plane stress state have been investigated using the essential work of fracture method. In this research, ST44 steel sheet was used to make samples. To obtain the elastic-plastic mechanical properties of ST44 sheet, tail-shaped specimens were used for tensile testing. Then tensile samples with two deep edge grooves (DENT) were made to perform the essential work of fracture test. The samples were cut by laser in dimensions of      180 × 60 × 2 mm. In the middle part of each sample, two notches with a length of 9 to 19 mm were created, and the distance between the tips of these two notches was the length of the ligament. After performing the tensile test on the samples, the area under the force-displacement graph was obtained for each sample and with the help of regression, a line was drawn from the obtained points. The width obtained from the origin of the equation of the line is the specific essential work of fracture and is equal to 554.92 kJ/m2. In order to calculate the critical opening of the crack tip and the critical opening angle of the crack tip, the maximum displacement diagram obtained from the force-displacement plot was drawn. From the drawn plot, the critical opening of the crack tip for ST44 sheet is equal to δc=2.5285 mm and the critical opening angle of the crack tip is equal to Ψc=0.33 rad.

Fast fracture is one of the major failure modes in brittle polymers. Generally, crack growth in these polymers may occur under combined tensile-shear loading. However, the fracture test data reported previously for some dental restorative resins can not be predicted by using the available fracture criteria. In this paper, a modified fracture criterion is used for estimating the fracture load for the mentioned resin samples. It is shown that by considering a more accurate description for the crack tip stresses and also by accounting for the craze zone effects in front of the crack tip, the modified criterion presents significantly better predictions for the fracture resistance in such brittle polymers.

The effects of carbon equivalents of 2.3, 3.3 and 4.3% on the microstructure, hardness and fracture toughness of a low alloyed ductile cast iron in as cast, annealed, normalized and austempered conditions have been investigated. Fracture toughness was measured using a plain-strain fracture toughness (kIc) test and also J-Integral approach. The results show that the pearlite content increases as the CE decreases from 4.3 to 2.3% The nodularity and nodule count also decrease with decreasing the CE%. The annealed samples show a predominantly ferritic structure. Iron with 2.3 CE% shows pearlitic microstructure. The austempered microstructure contains bainitic ferrite and retained austenite at all carbon equivalents. The fracture toughness value (kQ and kIc) of ductile iron increases with increasing the CE% in all conditions and a maximum value is obtained in austempered condition. The hardness decreases with increasing CE% and an maximum value of 526 HV50 can be obtained at 2.3 CE% in a austempered condition. The SEM study shows that all the fractures of austempered specimens are ductile in nature whilst all that of normalized condition are brittle. Fracture behavior of 4.3 CE% ductile iron in as cast and annealed conditions are ductile where those of 2.3% are brittle. The iron with 3.3% shows a brittle fracture in as cast condition and a ductile fracture in annealed condition. The results show that J. integral approach can be used in measuring of kIc, particulary if the kIc values can not be achieved.

Fracture toughness is one of the most important properties of concrete that controls the conditions for crack propagation and ultimately concrete failure. This research uses the Brazilian disk test to prediction of crack propagation and fracture toughness in ordinary concrete samples without micro-silica and lime powder and ordinary concrete samples containing micro-silica and lime powder has been investigated. Micro-silica replaces 10% by weight of cement and limestone powder replaces 5% by weight of cement. The crack propagation process was investigated from pre-existing cracks in the specimens as well as fracture toughness in modes I, II and hybrid mode I-II. Fracture toughness tests have been performed on Brazilian disk specimens at angles of 0, 15, 28.83, 45, 60, 75 and 90 degrees relative to the pre-existing crack direction. After laboratory studies, it was found that the onset of fin cracks at angles less than 60 degrees (0

One of the most important applications of nanomaterials in polymer resins is to achieve high fracture toughness even in small volume fractions of filler nanoparticles. Such performance is related to energy released through damage mechanisms that occur at the nano scale. Among these mechanisms, nanoparticle surface separation is more important. In the present work, considering the hierarchical structure of nanocomposites, it has been tried to investigate the effect of debonding of graphene nanoparticles from the surrounding resin on the fracture toughness of the first mode of the presented epoxy/graphene nanocomposite using a multi-scale method. Therefore, a representative volume element has been selected and using the data available in the experimental works of other researchers, the effect of several parameters such as Young's modulus, weight fraction and dimensions of nanographene was investigated. Finally, it was observed that the fracture toughness has a direct relationship with the Young's modulus of the nanocomposite. Also, the smaller the dimensions of the nanoparticles used, the greater the improvement in the fracture toughness.

In recent years, the application of composite materials in aircraft and space vehicle structural components has become increasingly popular. Preventing failure of composite structural systems has been an important issue in engineering design. Due to the lack of through-the-thickness reinforcement, structures made from laminated composite materials and adhesively bonded joints are highly susceptible to failure caused by interfacial crack initiation and growth. The delamination phenomenon in a laminated composite structure may reduce the structural stiffness and strength and redistribute the load in a way that the structural failure is delayed or may lead to structural collapse. As the most application of these materials is in space structures, the failure of a composite structure component can lead to a catastrophic failure of the structure as whole, thus, an accurate quantitative measurement and understanding of damage and determination of remaining useful service life plays a vital role in prediction of failure of such structures. To do so, first, the fracture properties of material should be determined. To calculate these properties, there are several experimental methods. Among them, the modified Arcan test is one of the most apposite methods for in-plane mixed-mode loading condition. In this investigation, a new specimen has been introduced to reduce cost and problems concerning utilization of adhesively. The critical fracture parameters of carbon-epoxy woven laminated composite material, which is of paramount materials in industry, in room temperatures is calculated both with experimental and numerical methods. These parameters include the critical stress intensity factors and critical strain energy release rate on crack tip. The geometric correction factors for modified Arcan test are calculated with a 2D finite element analysis. Finally, the values of critical fracture properties are calculated using these geometric modification factors and critical loads generated via experimental procedure.