Fracture Mechanics Approach Research Articles

A comparison of different experimental approaches in the determination of dynamic fracture toughness (J1d) behavior for RHA steel cost sustainable MMAW weldments

In the present investigation, the low hydrogen ferrite consumable weldment (weldment-1) and austenitic stainless steel consumable weldment (weldment-2) of rolled and homogenous armor steel (RHA) have been prepared with manual metal arc welding (MMAW). Further, the influence of weldments (weldment-1 and weldment-2) microstructure on the dynamic fracture toughness (J1d), strength, ductility and hardness has been evaluated and compared. The three different approaches have been utilized to estimate J1d for RHA weldments. Further, all three approaches’ outcomes have been compared with the elastic-plastic fracture mechanics (EPFM) approach through Jp-R curve. The results revealed that weldment-1 (yield strength 504.24 MPa and ultimate tensile strength 763.03 MPa) exhibited 26.93 and 25.07% more yield and tensile strength respectively than weldment-2 (yield strength 384.55 MPa and ultimate tensile strength 593.05 MPa). Further, the elongation of weldment-2 (elongation 42%) is 26.72% more than weldment-1 (elongation 32.10%). The joint efficiency and fusion zone hardness of weldment-1 are 5.85 and 57.56%, respectively more than weldment-2. The EPFM approach has good agreement with the correction factor approach (approach 2) and the compliance change rate approach (approach 3) with less than 7% difference in the estimation of dynamic fracture toughness of base metal and weldments (weldment-1 and weldment-2). Further, the percentage difference between maximum load point approach (approach 1) and the EPFM approach is more than 15% difference. The fractographic investigation confirms the ductile fracture in all under-investigated materials.

Evaluation of the Reliability of Computational Fracture Mechanics Approaches for Compact Tension Specimen

Evaluation of the Reliability of Computational Fracture Mechanics Approaches for Compact Tension Specimen

Relevance of monitoring residual stresses and the need to reduce them

The objective of the study is to show the significant impact of taking into account the actual values of residual stresses on the strength and resource of safe operation of nuclear power facilities. Generally accepted approaches of strength physics and fracture mechanics were used to determine strength and service life. The degree of influence of residual stresses on the strength and service life of pipeline elements in the case of a cyclically acting load, under static load, when there is a danger of brittle fracture, and in the case of corrosion is investigated. The objects of the study were pipelines of nuclear power plants. It is noted that the current state of science and technology has made it possible to create a technique for non-destructive testing of mechanical stresses acting in metal structures based on the use of the acoustoelasticity method. Currently, on the basis of this method, a method for measuring residual welding and installation stresses in pipelines has been developed and certified for use in the field of atomic energy use. The measurement technique provides for the determination of membrane and bending stresses in annular sections located on rectilinear sections of pipelines. The values of stresses in the structural elements of pipelines and residual welding stresses in the deposited metal of welded joints are determined using ratios based on the use of the balancing principle. The relevance of introducing into the procedure for monitoring the technical condition of pipelines and equipment of nuclear power plants the procedure for determining the actual level of residual stresses, which significantly increases the reliability of the assessment of their strength and service life, is shown. It is proved that in order to implement the procedure for determining the actual level of residual stresses, it is necessary to use a non-destructive testing system for residual welding and installation stresses using the acoustoelasticity method based on a measurement technique that allows determining the actual values of residual stresses with established error characteristics.

Towards the realization of composite metastructures: A failure analysis of connections

Metastructures hold significant potential for applications such as adaptive structures and soft robotics. Architectures of fiber-reinforced polymer metastructures may relate to modular arrangement of straight and curved laminates, with their connections to resemble perfect cracks, thus susceptible to delamination. This study investigated geometrical effects on the load-carrying capabilities of these connections upon a global tensile deformation, as well as lean modeling tools to facilitate the development of architected composite metastructures. Numerical fracture mechanics approach on different connection geometries and thicknesses showed that connection delamination is a critical failure mode, but crack-driving-force has low dependence on connection shape for given ligament thickness (and stiffness). Adopted analytical models could capture either moment or force-driven delamination failure, while the intermediate regime necessitates numerical tools. First-ply failure may precede depending on shape and ligament stiffness. These trends were also verified on an exemplary rotating chiral composite geometry. Furthermore, interface load-carrying capability improvements were studied via design considerations including connection filler material and element variable thickness. Indicatively, the latter showed a 157 % increase in bending deflection (and global deformations), while reducing crack driving force by 38 % for a given load case. The conducted analysis offers valuable insights into the design of lightweight, load-carrying composite metastructures.

Peeling pressure-sensitive adhesive elastica from elastica with pinned and roller ends

Quasi-static peeling of a horizontal elastica (the tape) from another horizontal elastica (the beam) supported by a pinned end and a roller end is analyzed. Bending resistance is assumed to dominate the behavior, and large deflections and rotations are allowed. The tape is shorter than the beam and is pulled upward at a constant angle with the horizontal. A transversality (debonding) condition is derived for peeling, based on the common fracture mechanics approach. Displacement control is considered. The associated force exhibits its peak value at the onset of peeling. The vertical deflection of the pulled end of the tape also is of interest, especially when the tape detaches from the beam. Equilibrium curves are determined, and the effects of the following parameters on the peak force and the detachment deflection are investigated: the relative thicknesses and moduli of elasticity of the tape and the beam; the relative lengths of the tape and the beam; the angle of pulling; and the nondimensional work of adhesion. The system may be used as a model of peeling bandages, medical patches, biosensors, and other wearable devices from skin. Other possible thin flexible substrates include fabric, paper, leather, rubber, and plastics.

Relation of craze to crack length during slow crack growth phenomena in high‐density polyethylene

AbstractThe craze‐crack mechanism occurring in high‐density polyethylene (HDPE) causing slow crack growth and environmental stress cracking is investigated in detail with respect to the relation of crack length and the related craze zone. This is essential for the understanding of the resulting features of the formed fracture surface and their interpretation in the context of the transition from crack propagation to ductile shear deformation. It turns out that an already formed craze zone does not inevitably result in formation of a propagating crack, but could also undergo ductile failure. For the examination, the full notch creep test (FNCT) was employed with a subsequent advanced fracture surface analysis that was performed using various imaging techniques: light microscopy, laser scanning microscopy, scanning electron microscopy, and X‐ray micro computed tomography scan. FNCT specimens were progressively damaged for increasing durations under standard test conditions applying Arkopal, the standard surfactant solution, and biodiesel as test media were used to analyze the stepwise growth of cracks and crazes. From considerations based on well‐established fracture mechanics approaches, a theoretical correlation between the length of the actual crack and the length of the preceding craze zone was established that could be evidenced and affirmed by FNCT fracture surface analysis. Moreover, the yield strength of a HDPE material exposed to a certain medium as detected by a classic tensile test was found to be the crucial value of true stress to induce the transition from crack propagation due to the craze‐crack mechanism to shear deformation during FNCT measurements.Highlights Progress of crack formation in high‐density polyethylene is analyzed by different imaging techniques Determined growth rates depend on distinction between craze zone and crack The ratio of the present crack to the anteceding craze zone is validated theoretically The transition from crack propagation to ductile shear deformation is identified An already formed craze zone may still fail by ductile mechanisms

Modelling short crack growth under creep-fatigue interaction for different dwell times

The creep-fatigue interaction (CFI) short crack growth behavior of 316H steel under different dwell times at 550℃ was investigated in this work. The dwell time effect of the CFI short crack growth behavior is analyzed systematically, the results revealed that compared to the long cracks, the short cracks is more sensitive to the creep damage effect during the dwell period. Meanwhile, a CTOD-based CFI short crack growth rate predication model is proposed by introducing the creep effect during the dwell time. Compared with conventional fracture mechanics approach, the CTOD-based approach demonstrated a superior capability in providing a unified prediction result for the creep-fatigue interaction in short crack growth rates under different dwell times.

Numerical fracture toughness evaluation of SS316L CT specimens using an Elastic Plastic Fracture Mechanics (EPFM) approach

In this study, fracture toughness was determined using an elastic-plastic fracture mechanics approach via numerical simulation. The material chosen for study is austenitic stainless steel 316L, which is extensively used in the nuclear industry for core internal structures. Three different SS316L conditions were considered for the analysis, including welded and post-weld heat-treated conditions aged at 300 °C for 10,000 hours (over-aged) and at 400 °C for 5,000 hours (peak-aged). A compact tension (CT) specimen of standard dimensions, which was taken from ASTM E1820–01, was used to perform the analysis. The J-integral value that is obtained for those three conditions was compared with the base material, which was taken from the open literature. It was found that theJ-integral values of SS 316L for welded conditions were lower than the base material. Also, J-integral values of SS 316L for the two post-weld heat-treated conditions were found to be greater than the weldments but less than the base material. It can also be noted that the values obtained for the SS316L weldments and the post-weld heat-treated sample aged at 335 °C for 10,000 hours were notably similar.

On Cyclic-Fatigue Crack Growth in Carbon-Fibre-Reinforced Epoxy-Polymer Composites.

The growth of cracks between plies, i.e., delamination, in continuous fibre polymer matrix composites under cyclic-fatigue loading in operational aircraft structures has always been a very important factor, which has the potential to significantly decrease the service life of such structures. Whilst current designs are based on a 'no growth' design philosophy, delamination growth can nevertheless arise in operational aircraft and compromise structural integrity. To this end, the present paper outlines experimental and data reduction procedures for continuous fibre polymer matrix composites, based on a linear elastic fracture mechanics approach, which are capable of (a) determining and computing the fatigue crack growth (FCG) rate, da/dN, curve; (b) providing two different methods for determining the mandated worst-case FCG rate curve; and (c) calculating the fatigue threshold limit, below which no significant FCG occurs. Two data reduction procedures are proposed, which are based upon the Hartman-Schijve approach and a novel simple-scaling approach. These two different methodologies provide similar worst-case curves, and both provide an upper bound for all the experimental data. The calculated FCG threshold values as determined from both methodologies are also in very good agreement.

Effect of Temperature and Ageing on Fracture Toughness of Alloy 617M

In recent years, Advanced Ultra Super Critical (AUSC) plants have used Ni-Cr-Co-Mo based alloy 617M for high-temperature components like steam piping and rotors, where material resistance against fracture after prolonged exposure in a hostile environment is a matter of concern to the designers. To this end, fracture toughness characterization of the material in as-received and aged conditions using an elastic-plastic fracture mechanics (J-R curves) approach can provide a significant input towards the component integrity assessments at service conditions. In this study, J-R curves of the alloy 617M have been determined for the as-received and differently aged (up to 20000 h at 710 °C) conditions using compact tension (CT) specimens following ASTM E 1820-13. Repeated tests have been carried out at temperatures 27 °C, 650 °C, 710 °C and 750 °C. The fracture toughness (JIC) thus determined, has shown a declining trend with test temperatures. This attributes to the ease of the crack initiation process owing to the γ′ induced matrix hardening and subsequent ageing-inducing precipitations of M23C6 carbides at the grain boundaries.

Telephone cord blister formation in solvent swollen elastomer films.

Video imaging was used to study large-slope folded telephone cord blister formation in solvent swollen films of polydimethylsiloxane (PDMS) elastomers. Chlorobenzene, chloroform, heptane and toluene were used to swell PDMS films with thickness values in the range 15 μm < h < 223 μm supported on glass substrates. Measurements of the blister width, corrugation wavelength and blister growth speed were studied as a function of the film thickness for all four solvents. Modified theories of buckling were shown to accurately predict the film thickness dependence of the width and corrugation wavelength and a modified fracture mechanics approach was shown to reproduce the non-monotonic thickness dependence of the blister growth rates. Two critical thickness values were identified for telephone cord blisters formed on solvent swollen PDMS films-one corresponding to a lower critical thickness for blistering and the other corresponding to a peak in the thickness dependent blister growth rates. Blister formation is shown to be consistent with the existence of mixed modality in the growth of the crack tip at the film-substrate interface. Both critical thickness values are shown to depend upon the strength of the adhesive interactions between the film and the substrate and the Young's modulus of the films. A simple method of patterning surfaces with telephone cord blisters is also introduced.

Fracture and Toughening of Mycelium-based Biocomposites

This study presents a combined experimental and analytical study of the fracture behavior and toughening mechanisms of bioprocessed mycelium-based biocomposites. The composites comprise hemicellulose hemp ducts (as nutritional and reinforcing components) intertwined with increasing weight percentages of laterite particles. Single-edge notched fracture experiments and in-situ observations of crack growth were used to explore the effects of varying proportions of laterite on the composite resistance-curve behavior. The toughening mechanisms, fracture modes, and crack-microstructure interactions were also elucidated. Since crack-bridging and crack-deflection were observed to be the dominant toughening mechanisms, they were modeled using fracture mechanics approaches. Crack-bridging was shown to dominate the toughening at lower weight fractions of laterite (0–20 wt%). However, as the laterite content increases (20–40 wt%), a combination of crack-bridging and crack-deflection was observed. Finally, at higher laterite weight fractions (>40 wt%), crack-tip shielding occurred primarily via crack deflection. The fracture mechanics predictions of resistance-curve behavior are shown to be consistent with the experimental measurements. The results suggest that mycelium-based and mycelium-laterite composites can be engineered with tunable fracture toughness. The implications of the results are also discussed for the development of sustainable building materials.

Durability and Damage Tolerance Analysis Approaches for Wind Turbine Blade Trailing Edge Life Prediction: A Technical Review

The size of wind turbine blades is increasing rapidly, and they are being installed in remote offshore locations. Consequently, it is essential to focus on improving the design and maintenance procedures in the blade industry to meet the growing demand. Of particular concern is the long-term operational performance of the wind turbine blade trailing edge. In this paper, we discuss the application of durability and damage tolerance analysis (DADTA) approaches to trailing edge service life prediction. DADTA is mandated in the aerospace sector to support airworthiness certification and to provide an updated life prediction of the structure based on the different stages of their service life. The DADTA framework has two main parts: durability and damage tolerance analysis. The durability part uses a structural fatigue approach based on a damage accumulation method during the initial design phase to predict the lifespan of a structure without defects. On the other hand, the damage tolerance analysis part uses a fracture mechanics approach and a damage growth method to update the lifespan prediction of a structure during the operation stages. This is achieved by utilizing sensors and inspection data as inputs while the structure is in service. Both these methods are comprehensive and have merits; however, their broad adoption in the wind turbine blade industry is still lacking. The current paper provides an extensive review of these methods and shows how these can be applied to the wind turbine blade industry, specifically for predicting the structural design life of the trailing edge of composite wind turbine blades. The review includes (a) defining wind turbine trailing edge failure modes, (b) trailing edge design procedures, and (c) a detailed discussion of the application of durability and damage tolerance analysis for trailing edge life prediction. Overall, this review paper would be of special interest to blade designers and would guide researchers and engineers interested in life prediction methodologies based on DADTA approaches for wind turbine blades.

Examination of cryogenic durability in self-consolidating concretes through mechanical and fracture mechanics approaches

Thermal shocks under extreme conditions, such as leakage in concrete-enclosed tanks, which are a popular choice for cryogenic liquid storage, may lead to undesirable effects on the structures. In this study, the effects of cyclic cryogenic exposures have been investigated using both mechanical and fracture mechanics approaches. To ensure durable concrete, self-consolidating concretes have been chosen in the mix design. While previous studies have primarily investigated the effects of a single cryogenic exposure on conventional concrete, this study focuses on repetitive exposure to cryogenic effects on self-consolidating concretes. Furthermore, the self-consolidating concretes were subjected to two different curing conditions: air curing and water curing, before cryogenic processing, in order to consider real application conditions and laboratory circumstances. After the first and fifth cryogenic exposures, fundamental engineering properties and fracture mechanics parameters were evaluated. To determine the fracture mechanics parameters, beams of three different dimensions were produced, with three different notch lengths for each dimension. The dynamic modulus of elasticity, ultrasonic pulse velocity, compressive strength, and bending strength of self-consolidating concretes were examined after 0, 1, and 5 cycles of cryogenic exposure. As a result, the air-cured samples exhibited greater resistance to cryogenic cycles than the water-cured samples. The KIC and CTODC values increased after one cycle of cryogenic exposure, as noted in the literature. However, the results obtained in this study demonstrate a decrease in KIC and CTODC values after five cycles compared to one cycle of cryogenic exposure.

Fracture mechanics approach to fatigue crack initiation at uniaxially loaded notches

AbstractA fracture mechanics model is developed to simulate cracking events that may have taken place at the root of a notch in a uniaxially loaded plate made of a two‐phase alloy during the fatigue crack initiation life. The roughness of the notch surface resembles microcracks of different sizes and locations at the notch root along the plate thickness, where material grains of different phases, sizes, and strengths are distributed at random. Potential notch surface cracking activities are anticipated during loading cycles until the emergence of a through‐thickness crack that propagates from the notch root on the plate surface along its width. Thus, the duration of the experimentally defined fatigue crack initiation at the notch root can be calculated. The model is applied to previously published experimental data on symmetrically U‐shaped ferritic‐pearlitic steel notched plates subjected to constant amplitude cyclic uniaxial stresses with different stress concentration factors. Plates with a random configuration were virtually tested. The comparison of experimental results with corresponding predictions validates the model.

A Combined Numerical–Analytical Study for Notched Fatigue Crack Initiation Assessment in TRIP Steel: A Local Strain and a Fracture Mechanics Approach

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies.

Fatigue life prediction of welded joints at sub-zero temperatures using modified Paris–Erdogan parameters

The fatigue life of welded steel components is usually determined by the weldment; in these cases, fracture mechanical approaches are widely used for their prediction. Ferritic steels are known to have a fatigue strength that is dependent on temperature. Therefore, this study evaluates fatigue tests of cruciform joints and transverse stiffeners at different sub-zero temperature levels regarding fatigue life. Simultaneously, the stress intensity factors over the crack length are calculated for the individual experiments using analytical solutions. Then, using the Paris–Erdogan relation with temperature- and material-specific C and m parameters as well as tabular values, the fatigue lives are calculated with analytical solutions and compared with the experimental results. It is shown that the prediction accuracy is significantly increased for the sub-zero temperature range by using temperature-adjusted Paris–Erdogan parameters, as long as the temperature is above the fatigue transition temperature.

Using an Interlayer to Toughen Flexible Colorless Polyimide-Based Cover Windows

Colorless polyimide (PI)-based flexible cover windows are a critical component of flexible electronics to protect devices from unwanted chemical and mechanical damage. The integration of flexible colorless PI-based windows into electronics applications is limited by the embrittlement of some colorless PI films when they are coated with hard coats. Here, we investigate the embrittlement mechanism of hard-coated colorless PI films and the role of interlayers in toughening the colorless PI-based cover windows for flexible electronics applications. A fracture mechanics approach combined with finite element analysis (FEA) models is employed to compute fracture strain, εc, for different crack cases in the bilayer (hard coated colorless PI) and trilayer (with an additional interlayer) cover windows. For the model inputs and validation, the material properties of the cover windows are characterized. We show that the embrittlement is attributed to the fracture behavior of the cover windows, and placing a ductile interlayer increases the εc of colorless PI films. Using the fracture analysis as a design guide, we fabricate a trilayer cover window with an acrylic thermoset interlayer and demonstrate an improvement of the εc of the colorless PI cover window by ~42%. We believe our analysis provides insights into design guides for mechanically robust cover windows using colorless PI films and flexible HCs for emerging flexible electronics.

Fatigue life evaluation for ring-welded lap joints of stainless steel based on fracture mechanics approach

This study aims to evaluate the fatigue life of ring-welded lap joints of stainless steel with various geometries. It involves welding morphologies observations, static and fatigue tests under tensile-shear loading, and fatigue life evaluation based on fracture mechanics approach. Test results show that the interfacial failure occurs for each specimen under the static loading, and the static strength increases with an increase in weld area, as well as the degree of bending deformation during the test. In addition, three failure modes were identified under fatigue loadings, with crack propagation from the weld root to the weld metal surface accounting for over 90% of the fatigue life for most specimens. Taking into account the fatigue failure modes, a fracture mechanics-based method was used to unify the Fa-Nf curves of ring-welded lap joints with different geometries. This unified curve considers the effects of specimen geometries, crack length, loading ratio, and bending deformation. The analysis demonstrates that the fatigue life of ring-welded lap joints increases with an increase in the thickness of the sheet with a hole and the hole diameter, while decreases with the increasing loading ratio and bending deformation. By utilizing this method, the predicted fatigue life of various ring-welded lap joints can be controlled within a range that is four times of the measured values.

Comparison of the continuum damage and fracture mechanics in fatigue assessment of components containing residual stresses

In the present study, the fatigue crack growth life of standard and smooth notched specimens containing residual stresses are evaluated based on the continuum damage and fracture mechanics approaches. It is indicated that fatigue crack growth lives assessed by the continuum damage model are closer to the experimental results, with a difference of <10% in the specimens containing residual stresses. The total stress intensity factor range is used to consider the effects of residual stresses in the estimation of fatigue crack growth life based on the fracture mechanics. It is found that in existing residual stresses, the stress intensity factor K is not a proper parameter to evaluate crack tip zone. Fatigue crack initiation life is experimentally obtained based on assessing variations of the slope of unloading curve in the force-displacement diagram of fatigue experiments. The cycle including a change in the slope is considered as fatigue crack initiation event. Isotropic and kinematic hardening parameters of ASTM A516 pressure vessel steel are considered in finite element analyses. Tensile residual stresses are introduced into the notched specimens by employing four-point-bending with a magnitude of around 90% of the yield stress of the material. The results indicate that at least 65% of the fatigue life of the specimens is decreased due to the presence of tensile residual stresses. Tensile residual stresses are measured using the hole-drilling technique.