Brittle Fracture Tests Research Articles

On brittle fracture resistance determination of chromium stainless steel irradiated in ion accelerator

The possibility of brittle fracture resistance (BFR) determination is investigated with mechanical testing for ion-irradiated chromium stainless steel. To consider the ion irradiation effect on material properties disc specimens in initial and irradiated states were tested by mechanical loading until brittle fracture. The micro- hardness of specimens before and after ion irradiation was also determined. It was found that the microhardness of ion-irradiated steel is higher than the microhardness of steel in initial state. However, critical loads for brittle fracture of disc specimens are practically equal for initial and ion-irradiated states. The performed calculations, SEM investigations and also analysis of mechanisms of cleavage microcrack nucleation in disc specimen surfaces allowed us to explain the brittle fracture test results. As fracture tests do not allow us to estimate BFR, it is proposed to estimate BFR by microhardness measurement results.

A modified MERR model for predicting mode II fracture initiation angle considering boundary constraint effect

AbstractDuring the service process of brittle materials like marble, soda‐glass, and acrylic, mode II fracture is usually a typical failure mode. The fracture crack initiation, which is a critical factor affecting mode II fracture behavior, is an essential issue for understanding the fracture mechanism and determining fracture toughness. This paper proposed a modified maximum energy release rate model to predict the mode II fracture initiation angle through superimposing two idealized boundary conditions, remote loading, and crack‐face traction. The degree of the boundary constraint effect in finite‐size specimens was quantified by introducing the dimension‐ratio parameter. As experimentally validating through tension‐loading shear fracture tests of brittle poly(methyl methacrylate) (PMMA), the proposed model was demonstrated to be a good prediction of the mode II fracture initiation angle. The applicability of the proposed model was also verified with other brittle materials and fracture test techniques in references. The findings provide an insightful understanding of the mode II fracture behavior of brittle materials.

Mixed‐mode brittle fracture test of polymethylmethacrylate with a new specimen

AbstractA new test configuration called Holed‐Cracked Square Plate (HCSP) is proposed to investigate I/II mixed‐mode fracture of brittle and quasi‐brittle materials. This specimen is a square plate containing a central hole with two radial cracks emanating from its circumference. The finite element method was used to calculate mode I and mode II stress intensity factors and T‐stress solutions for various crack lengths and hole diameters. The numerical results show that a full range of fracture mode mixities can be realized by changing the orientation angle of two radial cracks, while maintaining a very simple specimen geometry and loading requirement. A series of fracture tests were conducted on polymethylmethacrylate (PMMA) to study the practical capabilities of the HCSP specimen. These yielded fracture toughness values that are consistent with other experimental results. Other observed quantities including fracture initiation angles and fracture resistance also agree very well with mixed‐mode fracture theories.

A weakest link model for multiple mechanism brittle fracture — Model development and application

A multiple mechanism weakest link model for intergranular and transgranular brittle fracture is developed on the basis of experimental observations of a thermally aged low alloy steel. The model development is carried out in tandem with micro mechanical analysis of grain boundary cracking using crystal plasticity modeling of polycrystalline aggregates with the purpose to inform the weakest link model. The fracture modeling presented in this paper is carried out by using a non-local porous plastic Gurson model where the void volume fraction evolution is regularized over two separate length scales. The ductile crack growth preceding the final brittle fracture is well predicted using this type of modeling. When applied to the brittle fracture tests, the weakest link model predicts the fracture toughness distribution remarkably well, both in terms of the constraint and the size effect. Included in the study is also the analysis of a reference material.

Effect of Impact Energy in ESSO Test (Part 2: Proposal of Validity Criteria of Impact Condition in ESSO Test by FEM Crack Propagation Analysis

Abstract Based on the fundamental knowledge revealed in Part 1 of the present work, crack propagation analysis is carried out in Part 2 of the present work to the establish validity criteria for impact conditions. In cases where there is excessive impact energy in a brittle fracture test in the evaluation of crack arrest toughness, the impact energy gives conservative test results. In this research, numerical simulation was systematically conducted in order to clarify the effects of impact energy. Dynamic elastic finite element method Finite Element Method (FEM) analysis, which was validated by actual dynamic strain data in Part 1, was employed. The FEM analysis revealed that the impact effect reached its maximum immediately after impact and gradually decreased thereafter. However, the increase of the dynamic stress intensity factor Kd, with respect to the impact, remains significant in cases where applied stress is low and impact energy is high. The judgment of whether the impact had an effect on the results was made by calculating the ratio of the values of Kd both for cases where impact was considered and where it was not. The threshold condition from which the evaluated value should be corrected was set to 1.5. The specified range could be determined without considering the effect of impact by using the dimensionless ratio of the quantity impact energy Ei/(Es + Et).

Size effects on brittle fracture of Brazilian disk samples containing a circular hole

Brittle fracture tests are carried out on Brazilian disk specimens containing a circular hole. The experimental campaign involves two different polymeric materials, Polymethyl-methacrylate (PMMA) and general-purpose Polystyrene (GPPS), respectively. Keeping fixed the disk diameter, five different hole sizes are tested for each material sample, thus allowing a full description of the related size effects. The recorded failure stresses are compared with theoretical predictions by the coupled Finite Fracture Mechanics criterion, which is originally implemented for the geometry under investigation by means of analytical relationships for the stress field and the stress intensity factor available in the Literature. The agreement is generally satisfactory, except from very small holes where some nonlinear effects influenced the failure mechanisms.

Combined tension–shear fracture analysis of V-notches with end holes

In the present research, two stress-based failure criteria were proposed to predict brittle fracture in components containing V-notches with end holes (VO-notches) under mixed mode I/II loading. The first criterion, called VO-MTS, was an extension of the maximum tangential stress (MTS) criterion, and the second one, called VO-MS, was developed based on the mean stress (MS) failure concept. Two different groups of critical distances were utilized in the predictions. The first group was equal to the critical distances for sharp crack, and the second one was computed by using the mode I fracture test results on notched specimens. To verify the criteria, the theoretical fracture curves were compared with numerous experimental results gathered from 108 new brittle fracture tests performed on the Brazilian disk specimens weakened by central V-notches with end holes and made of PMMA under mode I and mixed mode I/II loadings. It was found that both the criteria provide very good predictions to the experimental results for different mode mixity ratios. Also, found in this research was that the curves are almost independent of the critical distance groups, meaning that one can simply utilize the critical distances of sharp crack in both the VO-MTS and the VO-MS criteria without requiring performing mode I VO-notch fracture experiments.

Fracture assessment of VO-notches under mode II loading: Experiments and theories

In the present investigation, two brittle fracture criteria were suggested to predict the onset of brittle fracture in engineering components weakened by V-notches with end holes (VO-notches) under pure mode II loading. These criteria, called VO-MTS and VO-MS were developed based on the two well-known failure concepts of the maximum tangential stress (MTS) and the mean stress (MS), respectively. In order to evaluate the accuracy of the criteria, 72 new brittle fracture tests were conducted at room temperature on the new disk type specimen containing a central slit, called the VO-notched Brazilian disk (VO-BD) specimen, made of PMMA. First, the experimental fracture loads were converted to the corresponding mode I and mode II notch fracture toughness (NFT) values and the important ratio mode II NFT/mode I NFT was experimentally achieved. Then, the theoretical values of the ratio were predicted by means of the VO-MTS and the VO-MS criteria. It was found that both the criteria could provide very good predictions to the experimental results of the ratio mode II NFT/mode I NFT, as well as those of the fracture initiation angle. Also, found in this research was that one can utilize the critical distances of sharp crack in the fracture predictions of VO-notches.

CHARACTERIZATION OF FRACTURE IN Fe3Al-BASED ALLOY USING THE SMALL PUNCH TEST

The small punch tests at constant deflection rate were performed on two Fe 3 Al-based alloys with additions of niobium and carbon. The purpose was to study the effect of carbides on ductility and fracture behaviour. Three main quantities were evaluated: maximum force, deflection at maximum force and fracture energy. Temperature dependence of these quantities was fully corresponding to the microscopic image of the fractured specimens. Radial cracks were observed at low temperatures, the contraction of the disc thickness was negligible and the fracture surface was characterized by cleavage facets. On the other hand, a circumferential crack clearly separated spherical cap from the rest of specimen at the highest temperatures. Reduction of the specimen thickness was observed as well as the volume analogical to necking in tensile test. Fracture surface had a dimpled appearance. It was proved that both the fracture energy and the deflection at maximum load were greater in the alloy with the greater carbon and that the ductility was not reduced by the presence of niobium carbides. lower t� tap�qP$ue slow pulling rate. Microstructure analyses showed the composition of the surface zone was formed by ferrite and pearlite. Coarser ferrite was seen in the surface zone with the higher slab pulling rate and higher cooling rate in the secondary cooling zone. The surface zone microstructure was polyedric for the lower cooling rate and sporadically nonpolyedric with needle-like, or acicular ferrite for faster cooling. Brittle fracture test pieces showed fracture surfaces with transcrystalline cleavage facets (TCF) regardless of the applied cooling rate. With lower cooling rates, smooth facets of intercrystalline decohesion (FID) were identified too, but at less than 0.1%. With faster cooling they showed up in a few isolated cases only. The occurrence of dimpled transcrystalline ductile fractures (DTDF) was generally low. It was confirmed that the morphology of forced fractures was influenced by the cooling rate via the produced microstructure. The embrittlement of the tested samples was assisted by clusters and single particles. They were identified using EDX as based on Al, or combined with Ti, Nb nitrides, or carbide and sulphide eutectics, or inclusions ordered in rows in the ferrite network. Since the occurrence of intercrystalline fractures was low with faster cooling and high slab pulling rate, distinctive suppression of segregation can be assumed for this technology, if compared to slow cooling and low slab pulling rate.

INFLUENCE OF COOLING RATE ON TiNb MICROALLOYED STEEL SLAB SURFACE ZONE FRACTURE MORPHOLOGY

The aim of this work was to analyze the influence of technology on the morphology of fractures and Charpy impact toughness in the TiNb microalloyed steel slab surface zone. The slab was made by continuous casting using different cooling rates in the secondary cooling zone (2 cooling rates were selected for testing) and 2 slab pulling rates 0.5 m/min and 0.8 m/min. It turned out that, with a higher slab pulling rate for both cooling rates applied, the impact toughness was generally lower than that with the slow pulling rate. Microstructure analyses showed the composition of the surface zone was formed by ferrite and pearlite. Coarser ferrite was seen in the surface zone with the higher slab pulling rate and higher cooling rate in the secondary cooling zone. The surface zone microstructure was polyedric for the lower cooling rate and sporadically nonpolyedric with needle-like, or acicular ferrite for faster cooling. Brittle fracture test pieces showed fracture surfaces with transcrystalline cleavage facets (TCF) regardless of the applied cooling rate. With lower cooling rates, smooth facets of intercrystalline decohesion (FID) were identified too, but at less than 0.1%. With faster cooling they showed up in a few isolated cases only. The occurrence of dimpled transcrystalline ductile fractures (DTDF) was generally low. It was confirmed that the morphology of forced fractures was influenced by the cooling rate via the produced microstructure. The embrittlement of the tested samples was assisted by clusters and single particles. They were identified using EDX as based on Al, or combined with Ti, Nb nitrides, or carbide and sulphide eutectics, or inclusions ordered in rows in the ferrite network. Since the occurrence of intercrystalline fractures was low with faster cooling and high slab pulling rate, distinctive suppression of segregation can be assumed for this technology, if compared to slow cooling and low slab pulling rate.

Fracture theory based on the concept of characteristic fracture length of materials

Fracture theory is a classic, but not a well-dealt with, difficulty in solid mechanics. This paper has proposed the concept of characteristic fracture length of materials from the fact that fracture happens with area failure rather than point failure in materials. A unified theory is then proposed, which can be applied both to smooth and defected materials (whether with micro or macro defects). Brittle fracture tests with specimens of different sizes of holes are carried out to examine the fracture theory. It is found that the fracture stresses obtained by experiments agree well with those predicated by the presented fracture theory. Though the brittle fracture is the focus of the paper, the concept of characteristic length can be easily extended to fatigue or other failure problems.

Evaluation of Residual Stress Effects on Brittle Fracture Strength Based on Weibull Stress Criterion

Abstract This paper studies the method for estimating the residual stress effects on brittle fracture of structural steel based on the Weibull stress criterion. First, brittle fracture tests were conducted for four-point bend notched specimens with and without compressive preloading. It is shown that the compressive preloading apparently decreases the critical crack tip opening displacement (CTOD). The critical CTOD of the preloaded specimen can be predicted from monotonically loaded test results based on the Weibull stress criterion. It has been found that the Weibull stress is an effective fracture parameter for brittle fracture initiation under tensile residual stress. Second, using wide plate specimens with and without welding residual stress subjected to uniform tension and three-point bend fracture toughness specimens, welding residual stresses effect on the brittle fracture strength is investigated. Experiments show that the critical CTOD of wide plate with welding residual stress is significantly smaller than that without residual stress. Constraint loss effect on CTOD of wide plate can be assessed by the equivalent CTOD ratio β. β is defined as β=δ3PB/δWP, where δ3PB and δWP are CTODs of the standard fracture toughness specimen and wide plate, respectively, at the same level of the Weibull stress. Fracture assessment result using β is shown within the context of failure assessment diagram. An excessive conservatism observed in the conventional procedure (β=1) is reasonably reduced by applying the equivalent CTOD ratio β.

溶接残留応力場中のき裂からの脆性破壊発生評価におけるワイブル応力仮説による塑性拘束補正方法

This paper studies the method for estimating the welding residual stress effect on brittle fracture of structural steel based on the Weibull stress criterion. Using wide plate (WP) specimens with and without welding residual stress, the brittle fracture tests have been conducted at -75°C. The material used is a high strength steel of 780MPa class. Welding residual stress significantly decreases the fracture force and the critical CTOD of the wide plate with welding residual stress at the onset of brittle fracture initiation. And it has been shown that the critical CTOD of the WP specimens can be predicted from fracture test results of the 3-point bend specimens without residual stress based on the Weibull stress criterion. The present paper proposes the assessment method of constraint loss effects on CTOD of wide plate with residual stress based on the equivalent CTOD ratio, βr under residual stress field. The equivalent CTOD ratio, βr is defined as the ratio, δ/δrWP,whereδandδrWPare CTODs of the standard fracture toughness specimen and wide plate with welding residual stress, respectively, at the same level of the Weibull stress.Fracture assessment results using βr are shown within the framework of failure assessment diagram (FAD). An excessive conservatism observed in the conventional procedure is reasonably reduced by applying the equivalent CTOD ratio, βr.

Analysis of Premature Low-Temperature Cracking in Three Ontario, Canada, Pavements

An analysis of premature low-temperature cracking in three Ontario, Canada, pavements is documented and discussed. Two of the contracts consisted of a 50-mm single lift of hot-mix asphalt on recycled asphalt pavement; the third contract was a single lift overlay on a previously cracked pavement. All three pavements exhibited extensive transverse cracking during their first winter. The binders used in each of the surface courses were evaluated for possible deviations from specification requirements and for their susceptibility to cracking as a result of excessive reversible aging during cold storage. The reversible aging tendencies, as indicated by bending beam rheometer (BBR) tests after extended periods of conditioning, showed that although all three binders met the specification, they lost a considerable amount from their grade temperature as determined according to AASHTO M320 criteria. Fracture tests on the binders showed that properties in the brittle state were all considered to be poor, yet typical for unmodified materials. Fracture properties in the ductile state for the binder from the one overlay contract indicated that low resistance to ductile failure could explain the observed distress. The experience has further confirmed the need to develop an extended BBR test protocol as well as brittle and ductile fracture tests to exclude binders that will be susceptible to these types of failures for future paving contracts.

Analysis of Premature Low-Temperature Cracking in Three Ontario, Canada, Pavements

An analysis of premature low-temperature cracking in three Ontario, Canada, pavements is documented and discussed. Two of the contracts consisted of a 50-mm single lift of hot-mix asphalt on recycled asphalt pavement; the third contract was a single lift overlay on a previously cracked pavement. All three pavements exhibited extensive transverse cracking during their first winter. The binders used in each of the surface courses were evaluated for possible deviations from specification requirements and for their susceptibility to cracking as a result of excessive reversible aging during cold storage. The reversible aging tendencies, as indicated by bending beam rheometer (BBR) tests after extended periods of conditioning, showed that although all three binders met the specification, they lost a considerable amount from their grade temperature as determined according to AASHTO M320 criteria. Fracture tests on the binders showed that properties in the brittle state were all considered to be poor, yet typical for unmodified materials. Fracture properties in the ductile state for the binder from the one overlay contract indicated that low resistance to ductile failure could explain the observed distress. The experience has further confirmed the need to develop an extended BBR test protocol as well as brittle and ductile fracture tests to exclude binders that will be susceptible to these types of failures for future paving contracts.

Master Curve analysis of inhomogeneous ferritic steels

The basic Master Curve (MC) method for analysis of brittle fracture test results is intended for macroscopically homogeneous ferritic steels only. In reality, the steels in question are seldom fully macroscopically homogeneous. The materials' toughness may depend on the specimen location in the sample. Inhomogeneity may be deterministic or random (or a mixture of both) in nature. Deterministic inhomogeneity can be accounted for, provided that the specimen extraction histories are known and enough specimens are tested. Random inhomogeneity is much more difficult to handle. The structural integrity assessment procedure SINTAP contains a lower tail modification of the MC analysis. This enables conservative lower bound type fracture toughness estimates also for inhomogeneous materials. The problem is that the SINTAP method, does not provide information of the tougher material. Therefore, a probabilistic description of the complete material is not possible. In this paper, a new comparatively simple extension of the MC is introduced for inhomogeneities governed by two separate MC distributions. The extension is shown to be extremely efficient in describing e.g. weld heat-affected zone (HAZ) data. In addition, a simple method for the analysis of random inhomogeneous material consisting of mixed data is presented. The method is also applicable for data sets including several different materials.

Brittle Fracture Tests of Cask Materials for Transporting Radioactive Materials

AbstractThe IAEA Regulations for the Safe Transport of Radioactive Material stipulates that Type B(U) packages shall be designed for an ambient temperature range from −40°C to +38°C. Guidelines for the safe design of shipping packages against brittle fracture were provided by the IAEA TECDOC-717 in 1993. This report discusses the applicability of the actual method for brittle fracture assessment to Type B transport cask materials used in Japan. The necessity of brittle fracture assessment at low temperature was estimated for each material of Type B transport casks used in Japan and the applicability was investigated. Dynamic fracture toughness tests were conducted and KId (JId), and RTNDT values of low Mn carbon steels (SA350 Gr. LFI Modify and SA516 Gr. 70 material used in Type B transport cask body) were obtained. All the obtained KId (JId) values were over the ASME KIR curve. It was then confirmed that an easier and conventional RTNDT-KIR curve method prescribed in ASME Code Section III can be substitu...

Effect of Flaws on Fatigue Strength of Cast Materials

As material flaws have great influence on fatigue strength of cast materials components, it is important to establish fatigue strength evaluation method of cast materials with flaws.In this study, concept of allowance rate was introduced to evaluate fatigue strength of cast steel with round-shaped flaws. The results obtained were summarized as follows : (1) A fracture mechanics approach was made to estimate fatigue limit of cast steel with a round-shaped flaw, regarding the flaw as an elliptical notch having an intrinsic crack, which is material constant, at the root. By this method, the allowance rate of flawed body againtst cracked one can be estimated.(2) The allowance rate can also be obtained experimentally by brittle fracture test of notched body at an extremely low temperature.(3) The concept of allowance rate was verified by fatigue test of specimen with a notch and with an actual casting flaw.

Evaluation of the effect of a sharp notch on the fatigue and static strength by the application of an equivalent stress-intensity factor.

A new method is proposed to evaluate the strength of a sharply notched structure with a large notch angle. A new parameter, Ke which is to be called the equivalent stress-intensity factor and which prescribes the singularity of stress distribution near the notch tip, is defined to evaluate the effect of a sharp notch on the fatigue and static strength. This method enables one to predict the strength of a notched structure by using the well-known parameters ΔKth and KIC. Fatigue tests of HT60, SS41 and a brittle fracture test of an acrylic plate were conducted and the applicability of the parameter was verified.

Dependence of “True” Surface Energy of Soils on Air Entry Pore Size and Chemical Constituents

AbstractStability and strength of soil structure depend on soil constituents and the manner in which they interact with each other. Some constituents, such as CaCO3, organic matter (OM), and Fe oxide act as cementing agents. Soil structure strength dependence on those soil constituents was studied. Apparent and “true” surface energy of a soil was defined as the amount of energy required to produce a new unit of surface area or propagate a crack, respectively. These energy values were determined by modifying Sack's (1946) and Griffith's (1924) theory to data obtained from brittle fracture tests on molded soil samples from 16 soils varying in clay, OM, CaCO3, and Fe2O3 contents. A negative linear relationship between the apparent surface energy and the pore radius at air entry was found. No distinct relationship was found between the “true” surface energy of the soils and the contents of CaCO3 and Fe oxide. An inverse relationship was found between “true” surface energy and cation exchange capacity. Possible new research directions are discussed in view of the data presented.