Fracture Mechanics Methodology Research Articles

A framework for predicting top-down crack initiation and growth for flexible pavements

ABSTRACT Top-down cracking (TDC) is recognized as a significant form of distress in asphalt pavements. While the NCHRP project 1-52 introduced a mechanistic-empirical (ME) predictive model for TDC, this model was found to be less accurate in certain traffic conditions—overpredicting crack initiation in sections with medium to low traffic and underpredicting in high-traffic sections. To enhance the precision of TDC predictions, a new fracture-mechanics-based framework has been developed. This improved framework employs a Hot Mix Asphalt (HMA) fracture mechanics methodology, resulting in a marked enhancement in the accuracy of predicting crack initiation times when compared to the original sub-model from NCHRP 1-52. The new framework also incorporates a novel aging model for the asphalt mixture, ensuring more precise inputs for the crack propagation model. The propagation prediction sub-model has been refined by implementing a J-integral-based Paris’ law developed in the NCHRP 1-52 project. The efficacy of this updated framework was rigorously tested on eighteen diverse pavement sections across various geographic locations, with well-documented performance history and material properties. The outcomes indicate that the predicted initiation and propagation of TDC closely align with actual field observations, demonstrating the robustness and improved reliability of the new model.

Methodology for local ageing and damage development characterization of solar glass/encapsulant interfaces under superimposed fatigue stresses and environmental influences

In this paper, a novel fracture mechanics test-setup was developed, implemented and applied for assessment of the fatigue delamination behavior of glass/encapsulant laminates under superimposed mechanical stresses and environmental influences. Double cantilever beam (DCB) specimens with glass substrates were prepared using ethylene vinyl-acetate copolymer (EVA) or polyolefin elastomer (POE) encapsulants. Displacement-controlled fatigue tests were conducted at 80 °C under dry (5% rh) and humid (80% rh) conditions. The damage zone ahead of the delamination crack tip was monitored with a topside mounted camera system. A superior delamination resistance of POE based double glass laminates was ascertained. Under less service-relevant, hot-dry conditions, the glass laminates exhibited a mm-sized damage process zone ahead of the crack tip, in which void nucleation and fibril formation took place. While the POE laminate failed also in a cohesive manner within the encapsulant at 80 °C and 80% rh (i.e., hot-humid), the crack was propagating in the glass/EVA laminate close to the interface in a brittle manner. Local ageing mechanisms were confirmed for the fractured EVA laminates by XPS. The superposition of mechanical and thermal stresses along with humidity resulted in sodium ion migration presumably promoting deacetylation of EVA. Interestingly, the analyzed degradation products were in good agreement with data obtained for damp heat aged glass/foil PV modules characterized subsequently by monotonic delamination testing. The main advantage of the novel environmental fracture mechanics methodology is the high acceleration factor (50 to 100x faster) compared to conventional ageing and delamination testing of PV modules. • Novel concept for highly accelerated delamination testing of PV module interfaces. • Superimposed mechanical fatigue stresses and environmental influences. • Service-relevance confirmed for glass/EVA interface under hot-moist conditions. • Potential acceleration factor of 50–100 due to local ageing effects. • High-throughput screening of novel material formulations & interfaces.

Fatigue life assessment of polyamide 12 processed by selective laser sintering. Damage modelling according to fracture mechanics

PurposePolyamide 12 (PA12) processed by the additive manufacturing technique of selective laser sintering (SLS) is acquiring a leading role in cutting-edge technological sectors pertaining to transport and biomedical among others. In many of these applications, design requirements must ensure fatigue structural integrity. One of the characteristic features of these SLS PA12 is the layer-wise structure that may influence the mechanical response. Therefore, this paper aims to assess the fatigue life behavior of PA12, focusing on the effect of the load direction with respect to the load orientation.Design/methodology/approachWith the aim of analyzing the effect of the load direction with respect to the layer wise structure, fatigue tests on plain samples of SLS PA12 were carried out with the load applied parallel and perpendicular to the layer planes. The S-N stress life curves and the fatigue limit at 106 cycles were determined at room temperature and at a stress ratio of 0.1. The fracture surfaces were inspected to evaluate the damage evolution, modeled via the fracture mechanics methodology to obtain the fracture parameters.FindingsThe fatigue resistance was better when the load was applied parallel than when was applied perpendicularly to the layered structure. The analysis of the postmortem specimens evidenced three regions. The inspection of the fatigue macro crack growth region revealed that crazing was the mechanism responsible of nucleation and growth of damage till a macroscopic crack was generated, as well as of the consequent crack advancement. The calculated fracture parameters computed from the application of the fracture mechanics approach were similar to those obtained from standardized fracture tests, except when the stress levels were close to the yield strength.Originality/valueThe fatigue knowledge of polymers, and especially of polymers processed via additive manufacturing techniques, is still scarce. Therefore, the value of this investigation is not only to obtain fatigue data that could be used for structural design with SLS PA12 materials but also to advance in the knowledge of damage evolution during the fatigue process.

On the behaviour of small fatigue cracks emanating from corrosion pits: Part I – The influence of mechanical factors

The initiation and propagation behaviour of cracks emanating from corrosion pits in a high strength low alloy steel is investigated by testing pre-pitted specimens in the high cycle fatigue regime in an air environment. This was complemented with finite element analysis of electrochemically generated pit profiles. Fatigue cracks were found to initiate from the mouth of pits due likely to strain localisation at this region. Crack initiation lifetime and fatigue endurance decreased with increasing pit depth. Fatigue behaviour of the steel was found to be crack initiation dominated. A fracture mechanics methodology that provides a satisfactory estimation of critical threshold stresses and threshold stress intensity factor required for fatigue failure from corrosion pits is discussed.

Fracture mechanics for fatigue design of metallic components and small defect assessment

The use of fracture mechanics based methodologies to assess the influence of defects on the high cycle fatigue resistance of mechanical components has become increasingly important for industrial applications and design. This presentation introduces the most commonly used fracture mechanics methods, emphasizing the assumptions on which they are based and analyzing their potential in predicting the influence of the main geometrical, mechanical and microstructural variables involved in the definition of the fatigue resistance of the components. Underlying hypotheses are discussed thoroughly and possible directions for future research are suggested. Applications examples on different configurations with small defects are presented. Finally, the potential of the fracture mechanics methodologies is analyzed when proposing simplifications suitable for use in the development of design documents for engineering applications.

Application of fracture mechanics for the characterization of poly(vinyl chloride) pipes. II. Evaluation of a ring‐type specimen for fracture mechanics testing

AbstractAn unplasticized poly(vinyl chloride) (U‐PVC) pipe sample used in infrastructure applications in Brazil (nominal diameter DN 100, outside diameter 110 mm) was evaluated according to different fracture mechanics methodologies, including essential work of fracture (EWF) and other fracture toughness parameters such as the stress intensity factor and plane strain energy release rate . This pipe sample was also tested for the quality of processing (degree of gelation) via differential scanning calorimetry (DSC) and tensile strength. The comparative evaluation of different specimen configurations—curved specimens in three‐point bending (CTPB) and full rings in tension (SNRT), in thicknesses varying from 5 to 30 mm, showed initial evidence of the suitability of ring‐type specimens for the evaluation of EWF and . Results also indicate that the full ring geometry, at least in the present experimental setup, presents some drawbacks probably due to the storage of large amounts of elastic energy throughout the test. This fact leads to relevant deviations in both load–displacement behavior and results for strain energy release rate (), and the results found here will guide future research using full and split rings in different loading modes and improved experimental setups. It was also confirmed that the value of , determined by a modified Charpy test, is independent of the type of specimen tested, as long as the test mode and specimen width are the same. Both the experimental value of and the estimated value for the plane strain stress intensity factor () showed excellent agreement with values reported for other U‐PVC compositions.

Field Investigation and Rapid Deterioration Analysis of Heavy Haul Corrugation

In this study, the rapid growth of corrugation caused by the bad quality of grinding works and their wavelength, depth, and evolution processes are captured through field measurements. The residual grinding marks left by poor grinding quality lead to further crack accumulation and corrugation deterioration by decreasing plastic resistance in rails. In this case, the average peak-to-peak values of corrugation grow extremely fast, reaching 1.4 μm per day. The finite element method (FEM) and fracture mechanics methodologies were used to analyze the development and trends in rail surface crack deterioration by considering rails with and without grinding marks. Crack propagation trends increase with residual grinding marks, and they are more severe in circular curve lines. To avoid the rapid deterioration of rail corrugation, intersections between grinding marks and fatigue cracks should be avoided.

Criteria for crack path deviation in adhesive layer of bi-material DCB specimen

An alternative to traditional fracture mechanics methodology to predict direction for crack propagation in the adhesive layer of bonded stiff materials is demonstrated. The approach is based on the analysis of the location of maximum of the hoop stress in relation to the existing crack tip. Such method is very convenient and fast as it does not require a lot of computational resources and is easy to implement compared to other known numerical methods dealing with similar problems (e.g. X-FEM). The method is validated by fracture mechanics approach using energy release rate to predict crack propagation direction. The verification is done by using bi-material DCB specimen with relatively thick adhesive layer as an example.After proving the applicability of the maximum hoop stress criterion the parametric study on factors affecting crack propagation in the adhesive layer is carried out. Such parameters as bending stiffness of beams, thickness of the adhesive layer, distance to the bond-line, length of the initial pre-crack are analyzed.

Peridynamics-based simulation of semi-circular bending (SCB) testing

Since most asphalt pavement distresses are directly related to material fracture resistance, characterizing its properties has been extensively studied through laboratory testing and numerical simulation during the past decades. Semi-circular bending (SCB) test is a widely used laboratory test to determine the fracture energy of asphalt mixtures, while numerical simulation based on finite element method (FEM) and extended FEM (XFEM) has been performed to investigate the propagation of cracking development. In this study, peridynamics, a new fracture mechanics methodology that extends classical continuum mechanics to discontinuous material responses, is implemented to simulate crack propagation during the SCB testing process. The numerical peridynamics analysis results are compared with laboratory SCB results and it is found that the paths of simulated crack development match lab observations well. The arch effect resulting from the geometry of SCB samples is analyzed and the energy transformation during the cracking process is quantified. Two loading peaks are observed during the simulation from the load verse load line displacement (P–u) curves. The energy absorption in the SCB test can be further divided into four different distinctive energy transfer phases. It is anticipated that this work could promote the in-depth understanding of crack propagation for the SCB test and better evaluate fracture resistance of asphalt mixtures.

Failure of a pre-cracked epoxy sandwich layer in shear

Adhesive joints are frequently used in automotive, maritime and construction applications, yet joint reliability remains a concern. The purpose of this study is to develop a fracture mechanics methodology for the failure of an elastic-brittle lap-shear joint comprising a thick adhesive layer (an epoxy, of thickness on the order of 10 mm) sandwiched between thick steel adherends. This configuration is of practical application to ship-building, such as the bonding of a superstructure to the underlying hull. A modified thick-adherend shear test (TAST) is designed, and specimens are fabricated, with a range of interfacial pre-crack length and a range of adhesive layer thickness. The failure of samples with no pre-crack is governed by a critical value of corner singularity, whereas the failure of samples with long pre-cracks is governed by a critical value of interfacial stress intensity factor. Predictions for the dependence of failure load upon layer thickness and pre-crack length are obtained by analysing the elastic stress state for both a corner singularity and for an interfacial crack. Both the experimental results and the theoretical framework are useful in the design and fabrication of reliable lap-shear joints that comprise a thick elastic adhesive layer.

Ductile fracture properties of 16MND5 bainitic forging steel under different in-plane and out-of-plane constraint conditions: Experiments and predictions

Bridging the gap between small-scale laboratory specimens and engineering full size engineering structures plays a critical role in integrity assessment of reactor pressure vessels (RPVs). In the present work, in-plane and out-of-plane constraint effects are both considered to determine ductile fracture properties under various low constraint conditions. The fracture behavior of 16MND5, a bainitic forging steel was investigated using clamped single edge notched tension (SENT) specimens with different crack sizes, specimen thicknesses and span lengths. Various fracture properties, including R-curves (J-integral, CTOD) and initiation fracture toughness (JSZW, J0.2, CSZW and C0.2) were determined. The results showed that the fracture properties are highly dependent on the in-plane (crack depth) and out-of-plane (specimen thickness) constraint conditions. The lower in-plane and out-of-plane constraint levels will introduce higher fracture properties. To further understand and quantify the impact of in-plane and out-of-plane constraint effects, the finite element method (FEM) and fracture surface analysis were conducted. Constraint dependent R-curves and initiation fracture toughness are developed for 16MND5 steel using a newly developed three-parameter fracture mechanics methodology. In particular, two independent constraint parameters, one for in-plane and one for out-of-plane, were used to quantify the 3D constraint effect. Finally, the present method was used to predict fracture properties for other specimens or low-constraint RPV cracked structures with different in-plane and out-of-plane constraint conditions.

Application of fracture mechanics for the characterization of PVC pipes. I. Evaluation of the applicability of the EWF technique in specimens produced directly from pipes

AbstractUnplasticized (U‐PVC) and modified (M‐PVC) pipes used in irrigation and infrastructure applications in Brazil (nominal diameter DN 300, outside diameter 326 mm), were evaluated according to various fracture mechanics methodologies, including essential work of fracture (EWF), C‐ring toughness and fracture toughness parameters (stress intensity factor KC and energy release rate GC, both at plastic collapse). These pipes were also evaluated for the quality of processing (degree of gelation) via DSC and tensile strength. Results show that the differences in behavior against fracture propagation are sensitive, depending on the technique, when comparing the results between the different types of pipes, opening up new possibilities for testing this product. This possibility is particularly interesting in the case of the evaluation of the degree of gelation in industrial process controls, since the dichloromethane (DCMT) solvent immersion method still in use tends to be replaced in the future by alternative methodologies due to chemical exposure factors.

Effect of blend composition and related morphology on the quasi-static fracture performance of LLDPE/PP blends

In the present work, the effect of composition and related morphology on the fracture behavior of LLDPE/PP blends was thoroughly investigated. Fracture behaviors evaluated under quasi-static loading conditions and different fracture mechanics methodologies were applied to assess fracture toughness depending on the materials behavior. For pure PP and 2575 blend, J at instability was chosen whereas for blends which exhibited completely ductile behavior (such as LLDPE, 7525 and 5050), the EWF methodology was used. Fracture mechanisms were elucidated with the aid of scanning electron microscopy, and results correlated with blends morphology. It was observed that fracture properties are mostly dominated by the majority component properties. In addition, for the 5050 blend, the presence of a co-continuous morphology is responsible for the high scatter of experimental data obtained.

Fatigue life assessment of a porous casting nickel-based superalloy based on fracture mechanics methodology

Inherent defects induced in casting process seriously affect mechanical performance and fatigue properties of the material. Micro-structural investigations reveal that the grain size and dendrite microstructure are sensitive to casting processes and change materials property. The fatigue performance of the material decreases with porosity and is characterized by distributed voids on grain boundaries. Fatigue cracks nucleated from casting defects dominate the whole fatigue failure process. A developed fatigue micro-crack model based on Paris’ law can be used to calibrate low cycle fatigue property of the casting material with different microstructures and agrees with experimental results well.

Shear bond strength vs interfacial fracture toughness — Adherence to CAD/CAM blocks

ObjectiveTo compare shear bond strength (SBS) and interfacial fracture toughness (IKIC) results when assessing the effect of surface roughness and thermocycling on the adherence of a resin composite luting agent (RCLA) to a CAD/CAM resin composite block (RCB). MethodsTetric CAD HT along with the recommended bonding system, Adhese Universal and Variolink Esthetic LC, were used. Surface roughness was achieved with 600/320/60 grit SiC papers. Samples were stored 24h in 37°C water or thermocycled 10000× (5°C–55°C) prior to testing. Results were analyzed by univariate ANOVA and Scheffé modified t-tests (α=0.05). Fractured specimens were viewed with a stereo microscope and selected specimens with a scanning electron microscope. ResultsSBS results showed a significant difference between the 60 grit group and the other groups, both after 24h and thermocycling. A large number of SBS samples showed cohesive fracture or subsurface damage in RCB. Thermocycling led to a significant decrease in SBS in all groups. IKIC results showed no significant differences due to surface preparation after 24h storage in 37°C. After thermocycling, there was a significant difference between the 60 and the 600 grit groups. All KIC samples fractured adhesively at the RCB surface. KIC of the RCLA was significantly higher than IKIC of all groups. SignificanceThe results endorse the use of fracture mechanics methodology for the assessment and characterization of adherence, while identifying difficulties in its implementation. The results suggest also that adherence to CAD/CAM RCB may be limited by the strength of the resin composite block — adhesive interface.

"CAD-on" Interfaces - Fracture Mechanics Characterization.

To apply fracture mechanics methodology to determine the interfacial fracture toughness of the interfaces present in "CAD-on" crowns consisting of CAD/CAM milled lithium disilicate veneers glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal framework. The notchless triangular prism specimen fracture toughness test was used to determine interfacial fracture toughness. Four groups, each consisting of (6 × 6 × 6 × 12) mm prisms (n = 22), were produced. Half-size [(6 × 6 × 6 × 6) mm] specimens of IPS e.max CAD and IPS e.max ZirCAD were approximated under vibration with Crystal Connect fusing glass and sintered according to manufacturer's guidelines to obtain the following three interfaces: (1) e.max CAD/Crystal Connect/e.max CAD (Group I); (2) Zir CAD/Crystal Connect/Zir CAD (Group II); and (3) Zir CAD/Crystal Connect/e.max CAD (Group III). For Group IV (control, based on the "press-on" veneering technique), half-size [(6 × 6 × 6 × 6) mm] IPS e.max ZirCAD prisms were coated with ZirLiner and pressed with IPS e.max ZirPress ingots to obtain (6 × 6 × 6 × 12) mm prisms. All specimens were tested using a computer controlled material testing machine. Results were analyzed with one-way ANOVA, Scheffé multiple means comparisons (α = 0.05) and Weibull statistics. All fractured surfaces were characterized with a light microscope. Selected fractured surfaces were characterized under a scanning electron microscope. All experimental groups demonstrated a cohesive mode of failure in the fusing glass layer. The number and size of defects appeared to correlate with the variability of fracture toughness values. There were no significant differences between the fracture toughness of the "CAD-on" interfaces (p = 0.052). The results suggested that the fracture toughness of Crystal Connect limited the interfacial fracture toughness values. The "CAD-on" fracture toughness value (Group III) was significantly greater than that of the ZirPress "press-on" control (Group IV) (p < 0.001). The "CAD-on" process results in stronger bonding between veneer and framework, compared to conventional veneering. The clinical use of "CAD-on" crowns could therefore be advocated. The selection of any restorative material requires a thorough analysis of advantages, limitations and results from clinical studies to inform the clinical decision in a case-by-case approach.

Three-dimensional assessments of crack tip constraint

Crack tip constraint can be used as a means to relate the stresses at the crack tip to a measure of fracture toughness of a material. Current technique to determine crack tip constraint demonstrated in structural integrity standards are based on two-dimensional plane strain approach. This paper explores the possibility of extending the current two-dimensional crack tip constraint solutions to three-dimensional crack problems through an investigation of the asymptotic structure of three-dimensional crack tip stress fields under elastic-plastic non-hardening responses. Analysis was based on three-dimensional boundary layer formulations, combined with full-field solutions of single edge notched bend bars and centre cracked tension panels having a range of thicknesses. The fields along the crack front were examined as a function of load level and thickness. From the results, it is demonstrated that, at the crack tip (r = 0), a family of asymptotic fields develop which feature a constant stress sector directly at the crack tip. Within this sector the fields differ hydrostatically while being similar in respect of the maximum stress deviator. At the intersection with the free surface, an elastic perfectly-plastic corner field which differs from the plane stress field is shown to develop. It is shown that in these fields the level of constraint is parameterised by J/zσo, where z is the distance from the free surface. J/zσo unifies both the change in the maximum principal stress and the mean stress, as a function of thickness and deformation level from all sections of different geometry and thickness specimens. An explicit expression has been developed which demonstrated the constraint loss due to in-plane and out-of plane crack tip constraint effects, allowing a two-parameter fracture mechanics methodologies in a modified form to be extended to fully three-dimensional fields.

Engineering critical assessment for a sandwich pipe field joint

This paper seeks to apply a combination of techniques with the aim of outlining a finite element (FE) based methodology for carrying out Engineering Critical Assessment on the swaged weld for J-lay installation. The critical potential defect position during installation is identified and its severity is evaluated using the Stress Concentration Factor (SCF). Closed-form parametric equations for quantifying the geometric SCF as a function of the swaged weld dimensions are derived using large-scale parametric studies and statistical analysis for the joint under tension. The maximum allowable defect size for a swaged weld under potential installation loadings is evaluated by two proposed FE-based fracture mechanics methodologies. In the absence of tearing resistance data, the influence of the filler resin stiffness, loading type and material response on the acceptability of a defect size is studied and the conservative nature of brittle fracture design for the fracture assessment of carbon steel pipelines with significant ductility is illustrated.

A computational investigation of length-scale effects in the fracture behaviour of a graphene sheet using the atomistic J-integral

The overarching objective of this paper is to investigate the validity of application of continuum-based linear elastic fracture mechanics (LEFM) methodology, which is often employed by researchers to model fracture processes at the “discrete” atomic scale. The potential sources of error in the application of LEFM at the nanoscale are: (a) Length-scale effects, (b) non-local effects due to long range inter-atomic forces, and (c) entropic effects due to random thermal motion of atoms. The material selected for this study is monolayer graphene, primarily because extensive data, both experimental and analytical, already exist for this material in the literature for model validation. Further, an atomistic J-integral is implemented as a nano-scale fracture metric to investigate flaw-tolerance at the nanoscale reported by many researchers, and to develop a methodology to predict the initiation fracture toughness of the material. For this purpose, a bond-order based potential (ReaxFF) available in LAMMPS molecular dynamics (MD) software is utilized to accurately pinpoint bond separation. Predictions obtained using the atomistic J are compared with LEFM predictions for the case of a single (zig-zag) graphene sheet with a center-crack under tensile loading at room temperature, and show significant deviation from LEFM for crack lengths below a certain threshold.

Probabilistic Fracture Mechanics for Analysis of Longitudinal Cracks in Pipes Under Internal Pressure

In this paper, we present a probabilistic fracture mechanics methodology to analyze elastic and elastic–plastic fracture of semi-elliptical longitudinal cracks in pipes under internal pressure. Numerical results are acquired using three-dimensional finite element simulations. Analytical expressions are proposed with unknown coefficients obtained by nonlinear fitting to the numerical results. For the elastic case, results of the shape function using the newly proposed expression are found to be in a good agreement with those found in the literature. In the elastic–plastic case, estimates of the J-integral are presented for various ratios including crack depth to pipe thickness (a/t), reference stress to material yield stress (σref/σy) and mean pipe radius to its thickness (Rm/t). It is found that the range of applicability of the proposed expressions is extended even beyond those found in the literature. Finally, failure probability is accessed by a statistical analysis for uncertainties in loads and material properties, and structural reliability. The probability density function is estimated by the Monte Carlo Method. It is shown from the present results that the crack size is an important factor influencing the distribution function of (J/Je), failure and reliability rates.