Plastic Fracture Mechanics Research Articles

Viscoplastic crack initiation and propagation in crosslinked UHMWPE from clinically relevant notches up to 0.5mm radius.

Highly crosslinked UHMWPE is now the material of choice for hard-on-soft bearing couples in total joint replacements. However, the fracture resistance of the polymer remains a design concern for increased longevity of the components in vivo. Fracture research utilizing the traditional linear elastic fracture mechanics (LEFM) or elastic plastic fracture mechanics (EPFM) approach has not yielded a definite failure criterion for UHMWPE. Therefore, an advanced viscous fracture model has been applied to various notched compact tension specimen geometries to estimate the fracture resistance of the polymer. Two generic crosslinked UHMWPE formulations (remelted 65kGy and remelted 100kGy) were analyzed in this study using notched test specimens with three different notch radii under static loading conditions. The results suggest that the viscous fracture model can be applied to crosslinked UHMWPE and a single value of critical energy governs crack initiation and propagation in the material. To our knowledge, this is one of the first studies to implement a mechanistic approach to study crack initiation and propagation in UHMWPE for a range of clinically relevant stress-concentration geometries. It is believed that a combination of structural analysis of components and material parameter quantification is a path to effective failure prediction in UHMWPE total joint replacement components, though additional testing is needed to verify the rigor of this approach.

Evolution of the texture and microstructure in a nickel based superalloy during thermo-mechanical fatigue (TMF), using a modified integrated model and experimental results

Microstructure & texture evolutions in Hastelloy X superalloy during thermo-mechanical fatigue (TMF) were investigated in this research. For this purpose TMF tests were performed in two different ways using linear elastic fracture mechanics (LEFM) and elastic plastic fracture mechanics (EPFM). These methods can produce small and large scale yielding in plane stress mode, respectively. On the base of data obtained in these tests a modified integrated model for prediction and simulation of texture evolution has been proposed. This model consists of a mechanical and a thermal part relating variations of stress and temperature occur during TMF tests. For the mechanical part of the model, a modified Taylor method based on strain partitioning in the grains orientations was used; and for the thermal part of the model, an integrated phased field and Monte Carlo models with harmonic Fourier approach for orientations distribution function (ODF) was used. The effects of Twin boundaries and dispersed secondary phase particles were also considered in the model. The validation tests show a good correlation between experimental results and the proposed model for texture development during TMF. To predict the initiation of unstable crack growth up to failure of the specimen during TMF, a new parameter in the form of reduced critical ODF value has also been introduced in this research.

Effects of recycled HDPE and nanoclay on stress cracking of HDPE by correlatingJcwith slow crack growth

The effects of recycled high density polyethylene (HDPE) and nanoclay on the stress crack resistance (SCR) of pristine HDPE were evaluated using the Notched Constant Ligament Stress (NCLS) test. The test data were analyzed by both linear elastic fracture mechanics (LEFM) and elastic plastic fracture mechanics (EPFM). The LEFM approach uses the stress intensity factorKto define the two failure mechanisms: creep and slow crack growth (SCG). In contrast, using theJ‐integral in EPFM, which emphasizes the nonlinear elastic‐plastic strain field at the crack‐tip, revealed a short‐term failure stage prior to the creep failure. In this article, a power law correlation between the fracture toughnessJcand SCG was found under a plane‐strain condition. Increasing recycled HDPE content lowered the SCG resistance of pristine HDPE by decreasingJc. Adding nanoclay up to 6 wt% also decreasedJcwhile simultaneously, lowering the stress relaxation of nanocomposites, leading to longer SCG failure times at lowJvalues. POLYM. ENG. SCI., 58:1471–1478, 2018. © Published [2017]. This article is a U.S. Government work and is in the public domain in the USA.

Ductilisation of tungsten (W): Tungsten laminated composites

Here we elucidate the mechanisms of plastic deformation and fracture of tungsten laminated composites. Our results suggest that the mechanical response of the laminates is governed by the plastic deformation of the tungsten plies. In most cases, the impact of the interlayer is of secondary importance.Severely cold-rolled ultrafine-grained tungsten foils possess exceptional properties in terms of brittle-to-ductile transition (BDT), toughness, and tensile ductility. The motivation for investigating laminated composites is to determine whether a bulk material can be made that retains the ductility of the thin tungsten foils.In this paper we analyse W-AgCu, W-Cu, W-V, and W-Pd laminates in their as-produced and annealed conditions (e.g. 10, 100 and 1000h at 1000°C (1273K) in vacuum). The analyses comprise (i) the mechanical characterisation by means of three-point bending (damage tolerance), Charpy impact (BDT), and tensile tests (total elongation to fracture) as well as (ii) the in-depth analyses of the microstructure by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Auger electron spectroscopy (AES).W-Cu laminates (60vol% W) show 15.5% total elongation to fracture in a tensile test at room temperature. Furthermore, the BDT of tungsten laminated composites occurs at a temperature that is several hundreds of Kelvin lower than the BDT temperature of the pure tungsten bulk counterparts.Finally, we present the successful fabrication of a 1000mm long W-Cu laminated pipe and show its high heat flux performance. Fabrication studies of high heat flux components made of tungsten laminates, in which the laminates are used either as heat spreaders or structural pipes, are presented.

Elastic-plastic fracture mechanics approach for stress corrosion cracking of nickel aluminium bronze under ammonia-containing artificial seawater

With the growth of urbanization and industries, the seawater near coastal areas has become polluted, and the nickel aluminium bronze components around coastal areas are affected by ammonia-containing seawater. Unfortunately, the influence of the ammonia concentration in seawater on the stress corrosion cracking of thin nickel aluminium bronze components with large plastic zones at the defects has not been evaluated before. In the present work, stress corrosion cracking experiments on nickel aluminium bronze components under artificial seawater and ammonia-containing artificial seawater were conducted using a four-point bending technique. The elastic–plastic fracture mechanics parameter ( J-integral) was evaluated using finite element analysis. The J-integral successfully characterized the crack growth rate under the present corrosive environments. Stress corrosion cracking was possible under both artificial seawater and ammonia-containing artificial seawater. The threshold J-integral for susceptibility to stress corrosion cracking ( JSCC) and fracture toughness ( JC) was the highest for stress corrosion cracking under artificial seawater and decreased as the amount of ammonium hydroxide added to the artificial seawater increased.

Improvement of mechanical properties and fracture toughness of low SFE Cu-Al alloy through microstructural modification by multiaxial cryoforging

Aim of the present study is to investigate mechanical properties and fracture toughness of low SFE ultrafine grained (UFG) Cu-4.5wt%11Al content is in weight % until unless stated. Al alloy processed through multiaxial cryoforging followed by short-annealing. The homogenized annealed samples (800°C, 4h) were cryoforged for 5, 9 and 11 cycles. The 11-cycles cryoforged sample showed more than 10 times improvement in the yield strength (YS) (818MPa) as compared to that of the homogenized annealed sample (76MPa). However, the ductility decreased to 10.1% elonagtion due to suppression of dynamic recovery and recrystallization during cryoforging. Cryoforging followed by short-annealing (225–300°C for 20min) was found to enhance the ductility without affecting its YS. Fracture toughness study has been carried out through analysis of apparent fracture toughness (KQ) using linear elastic fracture mechanics, equivalent energy fracture toughness (Kee) and J-integral values using elastic plastic fracture mechanics from 3-point bend test data. The improvement of the YS with significant ductility as well as fracture toughness is ascribed to the formation of ultrafine grains & rhombic blocks (due to the intersection of nanotwins close to 90°), mutual crossing & misorientation of microshear bands and multisystem deformation twins. The low SFE of the alloy is found to play the pivotal role to develop such microstructural features.

Reconciling fracture toughness parameter contradictions in thin ductile metal sheets

AbstractThe well‐known trade‐off between strength and fracture toughness in bulk specimens is often used to explain the low fracture toughness of very thin ductile face‐centred cubic metal specimens, but this interpretation contradicts the relative length scales of thickness‐dependent strength and thickness‐dependent fracture toughness. This study uses the concept of similitude to demonstrate that linear elastic fracture mechanics analysis of 25.4 μm thick annealed aluminium is invalid, although the resulting fracture toughness measurements fit well with the existing literature and idea of a strength/fracture toughness trade‐off. Similarly, an elastic plastic fracture mechanics analysis is sensitive to out‐of‐plane deformation that cannot be practically eliminated or corrected for with a model. However, a plastic collapse analysis using a critical net section stress criterion is demonstrably valid by the concept of similitude, is insensitive to out of plane deformation, and agrees with the evidence of extensive plasticity in the fracture surfaces.

Dynamic Loading of Carrara Marble in a Heated State

Useable land is a finite space, and with a growing global population, countries have been exploring the use of underground space as a strategic resource to sustain the growth of their society and economy. However, the effects of impact loading on rocks that have been heated, and hence the integrity of the underground structure, are still not fully understood and has not been included in current design standards. Such scenarios include traffic accidents and explosions during an underground fire. This study aims to provide a better understanding of the dynamic load capacity of Carrara marble at elevated temperatures. Dynamic uniaxial compression tests are performed on Carrara marble held at various temperatures using a split-Hopkinson Pressure Bar (SHPB) setup with varying input force. A customized oven is included in the SHPB setup to allow for testing of the marble specimens in a heated state. After the loading test, a three-wave analysis is performed to obtain the dynamic stress–strain curve of the specimen under loading. The fragments of the failed specimens were also collected and dry-sieved to obtain the particle size distribution. The results reveal that the peak stress of specimens that have been heated is negatively correlated with the heating temperature. However, the energy absorbed by the specimens at peak stress at all temperatures is similar, indicating that a significant amount of energy is dissipated via plastic deformation. Generally, fragment size is also found to show a negative correlation with heating temperature and loading pressure. However, in some cases this relationship does not hold true, probably due to the occurrence of stress shadowing. Linear Elastic Fracture Mechanics has been found to be generally applicable to specimens tested at low temperatures; but at higher temperatures, Elastic–Plastic Fracture Mechanics will give a more accurate prediction. Another contribution of this study is to show that other than the peak stress of the rock failure type, the strain history experienced by the rock during impact and the post-impact fragment size distribution are also significant distinguishing features of damage caused by dynamic loading on heated rocks.

Short-Term Stable Crack Propagation through Polyolefin Single- and Bilayered Structures - Influence of Welding, Composition and Direction of Crack Propagation

The overall stable crack initiation and propagation behaviour of specimens cut from plastic pipes that were composed of different polyolefin materials were investigated using concepts of elastic–plastic fracture mechanics including the crack propagation kinetics. The effect of specimen shape, orientation, welding, lading rate, composition/microstructure and direction of crack propagation on the crack resistance (R) behaviour of these materials has been thereby assessed.

Predicting Fracture in Civil Engineering Steel Structures: State of the Art

Fracture is an extreme limit state in steel structures, often precipitating structural failure or serious loss of function. Methods to predict fracture in civil structures include traditional approaches developed in other disciplines (mechanical or aerospace engineering) subsequently adopted in structural engineering, as well as approaches to characterize earthquake-induced fractures originating within civil engineering. Developed over nearly six decades, the state of the art is composed of theories and models that address fracture over multiple scales and are targeted toward disparate application scenarios. The paper examines these approaches from a structural engineering standpoint, considering trade-offs in accuracy and expense, while identifying areas for improvement. Traditional approaches (including linear elastic and elastic plastic fracture mechanics) are presented, followed by newer local approaches that are better suited for scenarios where traditional approaches are inapplicable. By simulating micromechanisms such as microvoid growth as well as granular cleavage, local approaches address fracture under large-scale yielding, ultralow-cycle fatigue (which occurs during earthquakes), and low-stress triaxiality, all of which are important in civil structures. The physical basis for these approaches is outlined, with a summary of best practices for calibration and application. However, these local approaches have limitations as well, and often require substantial resources for successful implementation. With this background, optimal fracture assessment strategies are outlined for common structural scenarios, considering accuracy and cost. Limitations of the entire fracture modeling framework are summarized because they pertain to mainstream adoption within structural engineering research and practice. As the profession moves toward accurate performance characterization, it is anticipated that research will accelerate to overcome these limitations.

Numerical simulation of elastic plastic fatigue crack growth in functionally graded material using the extended finite element method

ABSTRACTIn the present work, the extended finite element method has been used to simulate the fatigue crack growth problems in functionally graded material in the presence of holes, inclusions, and minor cracks under plastic and plane stress conditions for both edge and center cracks. Both soft and hard inclusions have been implemented in the problems. The validity of linear elastic fracture mechanics theory is limited to the brittle materials. Therefore, the elastic plastic fracture mechanics theory needs to be utilized to characterize the plastic behavior of the material. A generalized Ramberg-Osgood material model has been used for modeling purposes.

A simple intrinsic measure for rapid crack propagation in bimodal polyethylene pipe grades validated by elastic–plastic fracture mechanics analysis of data from instrumented Charpy impact test

Conventional test procedures, such as the S4 test to analyze the resistance against rapid crack propagation (RCP) of plastic pipe materials are characterized by usage of a lot of material, are far from saving of time and they are‐in need of special experimental set‐ups. Therefore, in the last decade, small‐scale accelerated reliable tests (SMART) are developed ‐ worldwide to overcome the disadvantage of such conventional tests. In this article, fracture mechanics based analysis of instrumented Charpy impact test data for a set of bimodal high‐density polyethylene pipe grades are compared with data of the conventional Charpy impact test. From this comparison the Charpy impact strength at −30°C comes forth as a robust reproducible measure of the resistance to RCP and it is therefore proposed as a SMART method to rank materials with respect to RCP resistance. POLYM. ENG. SCI., 57:13–21, 2017. © 2016 Society of Plastics Engineers

Probabilistic elastic-plastic analysis of repaired cracks with bonded composite patch

The objective of this work was to evaluate the ductile cracked structures with bonded composite patch used in probabilistic elastic plastic fracture mechanics subjected to tensile load. The finite element method is used to analyze the stress intensity factors for elastic case, the effect of cracks and the thickness of the patch (<i>e_r</i>) are presented for calculating the stress intensity factors. For elastic-plastic the Monte Carlo method is used to predict the distribution function of the mechanical response. According to the obtained results, we note that the stress variations are important factors influencing on the distribution function of (J/Je).

Fracture mechanics assessment of thermal aged nuclear piping based on the Leak-Before-Break concept

The Leak-Before-Break (LBB) concept has been accepted to design the primary piping system of the pressurized water reactor (PWR). Due to thermal aging of long term operation, the cast stainless steels (CSSs) which are used for the primary piping of PWR, suffer a significant loss of fracture toughness, and as a consequence the safety margin of the thermal aged pipe decreases. Therefore, the aged piping should be analyzed and validated by the LBB concept. In this paper, elastic–plastic fracture mechanics (EPFM) assessments of the thermal aged piping are presented according to the LBB concept. The critical break size of crack unstable tearing is calculated by the EPFM method. The crack driving force diagram (J–a diagram), the stability assessment diagram (J–T diagram) and a numerical method are applied to calculate the critical crack size of crack break. The effects of thermal aging on the plastic limit load, J–T diagram, critical crack size of the EPFM and the critical failure mode are studied. The results show that the thermal aging effect decreases the maximum allowed J-integral at a certain ductile tearing modulus by more than 50% and it increases the flow stress and plastic limit load by 11.78%. The results based on the J–T diagram are about 40% conservative than those based on the direct numerical method for the high loading case. For the thermal aged piping, it is important to consider the competition failure modes between plastic collapse and unstable ductile tearing.

Mechanisms and modeling of low cycle fatigue crack propagation in a pressure vessel steel Q345

The fatigue process near crack is governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic–plastic fracture mechanics, we explore the cyclic J-integral as breakthrough point, an analytical model is presented in this paper to determine the CTOD for cracked component subjected to cyclic axial in-plane loading. A simple fracture mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN). In order to validate the model and to calibrate the model parameters, the low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel. The effects of stress ratio and crack closure on fatigue crack growth were investigated by elastic–plastic finite element stress–strain analysis of a cracked component. A good comparison has been found between predictions and experimental results, which shows that the crack opening displacement is able to characterize the crack tip state at large scale yielding constant amplitude fatigue crack growth.

Mechanisms of plastic instability and fracture of compressed and tensile tested Mg-Li alloys investigated using the acoustic emission method

The results of the investigation of both mechanical and acoustic emission (AE) behaviors of Mg4Li5Al alloy subjected to compression and tensile tests at room temperature are compared with the test results obtained using the same alloy and loading scheme but at elevated temperatures. The main aim of the paper is to investigate, to determine and to explain the possible influence of factors related with enhanced internal stresses such as: segregation of precipitates along grain boundaries or solute atoms along dislocations (Cottrell atmospheres) or dislocation pile-ups at grain boundaries which create very high stress concentration leading to fracture. The results show that the plastic instabilities are related to the Portevin–Le Châtelier phenomenon (PL effect) and they are correlated with the generation of AE peaks. The fractography of breaking samples was analyzed on the basis of light (optical), TEM and SEM images.

The probabilistic structural integrity assessment of reactor pressure vessels under pressurized thermal shock loading

The pressurized thermal shock (PTS) event poses a potentially significant challenge to the structural integrity of the reactor pressure vessel (RPV) during the long time operation (LTO). In the USA, the “screening criteria” for maximum allowable embrittlement of RPV material, which forms part of the USA regulations, is based on the probabilistic fracture mechanics (PFM). The FAVOR software developed by Oak Ridge National Laboratory (ORNL) is used to establish the regulation. As the technical basis of FAVOR is not the most widely-used and codified methodologies, such as the ASME and RCC-M codes, in most countries (with exception of the USA), proving RPV integrity under the PTS load is still based on the deterministic fracture mechanics (DFM). As the maximum nil-ductility-transition temperature (RTNDT) of the beltline material for the 54 French RPVs after 40 years operation is higher than the critical values in the IAEA-TECDOC-1627 and European NEA/CSNI/R(99)3 reports (while still obviously lower than the “screening criteria” of the USA), it may conclude that the RPV will not be able to run in the LTO based on the DFM. In the FAVOR, the newest developments of fracture mechanics are applied, such as the warm pre-stress (WPS) effect, more accurate estimation of the flaw information and less conservation of the toughness (such as the three-parameter Weibull distribution of the fracture toughness). In this paper, the FAVOR software is first applied to show both the methodology and the results of the PFM, and then the limits in the current FAVOR software (Version 6.1, which represents the baseline for re-assessing the regulation of 10 CFR 50.61), lack of the impact of the constraint effect, ductile–tearing initiation failure analysis, elastic–plastic fracture mechanics analysis, safety assessment of the crack front interface points and interaction among multiple cracks, are also discussed.

Estimation of Constraint Factor on the Relationship Between J Integral and CTOD for Offshore Structural Steel Weldments

Offshore structures are exposed to severe operating conditions because energy resource development has recently extended toward deeper seabed and lower temperature regions. Hence, fracture toughness evaluation for very thick and high strength steels is one of the most important parameters required for the structural integrity assessment of offshore structures. Fracture toughness is known as a property which describes the ability of a material containing a crack to resist unstable brittle fracture. Crack tip opening displacement (CTOD) and J integral are the most commonly employed parameters as fracture criteria in elastic plastic fracture mechanics (EPFM). There have been extensive research efforts to clarify the relationship between CTOD and J integral in elastic plastic regime. Plastic constraint factor (PCF) in the relationship between CTOD and J integral can serve as a parameter to characterize constraint effects in fracture involving plastic deformation. In this regard, the characteristics of the PCF are of significant importance in EPFM analysis. In this study, we evaluated fracture toughness of American Petroleum Institute (API) 2 W Gr. 50 steel in terms of CTOD in various temperatures using single edge notched bend (SENB) specimens. Test specimens are fabricated by submerged arc welding (SAW) and flux cored arc welding (FCAW). In addition, CTOD values are compared to absorbed impact energy with respect to the weld metal (WM) and heat affected zone (HAZ). Then, we investigated PCFs with respect to several regions of the weldment at various temperatures. Experimental values of PCFs were calculated and then compared against the predicted values according to the American Society for Testing and Materials (ASTM) standard. CTOD values of WM by SAW is found to be about three times higher than that of FCAW at −10 °C, and CTOD values calculated by the ASTM standard are approximately 30% lower than the CTOD according to British Standard (BS). In addition, the maximum of 40% discrepancy is observed in PCFs obtained between the experiment and the predicted values according to the ASTM standard. This may lead to too conservative fracture toughness estimation for the welded joints of API 2 W Gr. 50 steel when using PCF by ASTM. Based on the accurate estimated PCF values obtained from this study, it is believed that rational fracture design of offshore structures is possible.

Crack closure effects during low cycle fatigue propagation in line pipe steel: An analysis with digital image correlation

Fatigue crack growth was investigated in a line pipe steel. Severe straining conditions, like those experienced by pipelines, were considered during the experiments. Crack closure effects were investigated during the tests by adopting an innovative technique based on digital image correlation. Experimentally measured crack closure levels were implemented in a crack propagation model based on elastic–plastic fracture mechanics. A modified formulation of ΔJeff, necessary to accurately describe crack propagation driving forces, is presented and employed to assess fatigue life in presence of high plastic strains.

The surface-forming energy release rate based fracture criterion for elastic–plastic crack propagation

The J-integral based criterion is widely used in elastic–plastic fracture mechanics. However, it is not rigorously applicable when plastic unloading appears during crack propagation. One difficulty is that the energy density with plastic unloading in the J-integral cannot be defined unambiguously. In this paper, we alternatively start from the analysis on the power balance, and propose a surface-forming energy release rate (ERR), which represents the energy available for separating the crack surfaces during the crack propagation and excludes the loading-mode-dependent plastic dissipation. Therefore the surface-forming ERR based fracture criterion has wider applicability, including elastic–plastic crack propagation problems. Several formulae are derived for calculating the surface-forming ERR. From the most concise formula, it is interesting to note that the surface-forming ERR can be computed using only the stress and deformation of the current moment, and the definition of the energy density or work density is avoided. When an infinitesimal contour is chosen, the expression can be further simplified. For any fracture behaviors, the surface-forming ERR is proven to be path-independent, and the path-independence of its constituent term, so-called Js-integral, is also investigated. The physical meanings and applicability of the proposed surface-forming ERR, traditional ERR, Js-integral and J-integral are compared and discussed. Besides, we give an interpretation of Rice paradox by comparing the cohesive fracture model and the surface-forming ERR based fracture criterion.