Crack Initiation Fracture Research Articles

Bone collagen network integrity and transverse fracture toughness of human cortical bone.

Greater understanding of the determinants of skeletal fragility is highly sought due to the great burden that bone affecting diseases and fractures have on economies, societies and health care systems. Being a complex, hierarchical composite of collagen type-I and non-stoichiometric substituted hydroxyapatite, bone derives toughness from its organic phase. In this study, we tested whether early observations that a strong correlation between bone collagen integrity measured by thermomechanical methods and work to fracture exist in a more general and heterogeneous sampling of the population. Neighboring uniform specimens from an established, highly characterized and previously published collection of human cortical bone samples (femur mid-shaft) were decalcified in EDTA. Fifty-four of the original 62 donors were included (26 male and 28 females; ages 21-101 years; aging, osteoporosis, diabetes and cancer). Following decalcification, bone collagen was tested using hydrothermal isometric tension (HIT) testing in order to measure the collagen's thermal stability (denaturation temperature, Td) and network connectivity (maximum rate of isometric tension generation; Max.Slope). We used linear regression and general linear models (GLMs) with several explanatory variables to determine whether relationships between HIT parameters and generally accepted bone quality factors (e.g., cortical porosity, pentosidine content [pen], pyridinoline content [pyd]), age, and measures of fracture toughness (crack initiation fracture toughness, Kinit, and total energy release/dissipation rate evaluated at the point of unstable fast fracture, J-int) were significant. Bone collagen connectivity (Max.Slope) correlated well with the measures of fracture toughness (R2 = 24-35%), and to a lesser degree with bound water fraction (BW; R2 = 7.9%) and pore water fraction (PW; R2 = 9.1%). Significant correlations with age, apparent volumetric bone mineral density (vBMD), and mature enzymatic [pyd] and non-enzymatic collagen crosslinks [pen] were not detected. GLMs found that Max.Slope and vBMD (or BW), with or without age as additional covariate, all significantly explained the variance in Kinit (adjusted-R2 = 36.7-49.0%). Also, the best-fit model for J-int (adjusted-R2 = 35.7%) included only age and Max.Slope as explanatory variables with Max.Slope contributing twice as much as age. Max.Slope and BW without age were also significant predictors of J-int (adjusted-R2 = 35.5%). In conclusion, bone collagen integrity as measured by thermomechanical methods is a key factor in cortical bone fracture toughness. This study further demonstrates that greater attention should be paid to degradation of the overall organic phase, rather than a specific biomarker (e.g. [pen]), when seeking to understand elevated fracture rates in aging and disease.

Influence of Replacement Level of Recycled Coarse Aggregate on Fracture Properties of Recycled Aggregate Concrete

recycled aggregate concrete made with recycled aggregate was one of the effective ways to reuse the construction and demolition waste. Due to the weak complex interface transition zone of recycled aggregate concrete, the defect had great effect on the bearing capacity and durability of recycled aggregate concrete. The fracture performance of recycled aggregate concrete was studied, which was of great practical significance and engineering value. The wedge splitting test of 200mm × 200mm × 230mm specimens was carried out using the fatigue testing machine (MTS). The elastic modulus of the specimens were measured and calculated by strain gauge and dynamic resistance strain gauge, and the elastic modulus was about 4GPa lower than that of standard test. Based on double-K fracture model of recycled aggregate concrete, the crack initiation fracture toughness and the unstable fracture toughness was obtained. The test results show that the replacement level of recycled aggregate is not obvious to the crack initiation fracture toughness, and the unstable fracture toughness is lower than that of natural concrete. Using the exponential function to fit the tail of the PH-COMD curve of the wedge splitting test, it is found that the fracture energy of the recycled aggregate concrete is not reduced with the increase of the replacement level of the recycled aggregate.

Comparison of different surface treatments of carbon fibers used as reinforcements in epoxy composites: Interfacial strength measurements by in-situ scanning electron microscope tensile tests

In-situ characterization of the fiber/matrix interfacial failure behavior at microscopic scale is important to optimize the fiber surface treatment and to design high performance composites. In this study, in-situ tensile tests in scanning electron microscope (SEM) were used to investigate the interfacial adhesion strength of epoxy composites reinforced by four kinds of carbon fibers (CF)—raw CF, desized CF, carbon nanotube-grafted CF (CNT-CF) and oxidized CNT-CF. The crack initiation position and fracture failure mode were well recorded. The strains and the interfacial adhesion strength were obtained for these four kinds of composites. It was found that the interfacial strength decreased from 53 MPa to 48 MPa after removing the sizing on carbon fiber surface. However, by grafting CNTs on the CF surface, the interfacial strength reached 55 MPa and was further increased to 58 MPa after a simple thermal oxidation treatment. Moreover, energy dispersion X-ray analysis (EDX) was carried out using scanning transmission electron microscopy (STEM). The EDX mapping demonstrated that oxygen aggregated at the interfaces of raw CF/epoxy and oxidized CNT-CF/epoxy. Thus, a combination of CNT grafting with chemical functionalization should be necessary to achieve high performance carbon fiber reinforced polymer composites.

Mechanical properties in double pulse resistance spot welding of Q&P 980 steel

The weldment consisted of four zones: fusion zone, partially melted zone, heat affected zone and base metal. Liquation film, liquated grain boundaries, liquation crack and softening zone were observed in the partially melted zone. Compared to single pulse resistance spot welding (RSW), the double pulse welding with a higher secondary current led to an enhancement in tensile-shear strength and a desirable failure mode, while a medium level value improved cross-tensile strength and ductility ratio. The partially melted zone acted as a crack sensitive area due to P segregation, causing a distinct partial thickness-partial pullout failure in the tensile-shear tests and pullout interfacial failure in the cross-tensile tests. Liquation crack, referred as a secondary crack initiation, further promoted crack propagation in these failures. The partial thickness-partial pullout failure can be divided into three stages: crack initiation, crack propagation and instantaneous fracture. The fracture mechanism were quasi-cleavage fracture during crack propagation. Then, the instantaneous fracture is occurred with ductile characteristic.

The mixed-mode fracture resistance of C 50/60 and its suitability for use in precast elements as determined by the Brazilian disc test and three-point bending specimens

The mixed mode fracture behaviour of C 50/60 concrete was studied both experimentally and theoretically using two different test configurations (three-point bending and Brazilian disc tests with various crack lengths). Different fracture behaviour in mode I was observed for the tested specimens. The level of crack tip constraint in the test-specimens (which relates to the stress intensity factors (SIF or K) and the T-stress) was used as the main parameter for predicting the mixed mode fracture results based on a two parameter (K-T) linear elastic fracture theory. To predict the crack initiation angles and fracture resistances, the generalized maximum tangential stress (GMTS) criterion was employed. The GMTS simultaneously involves the effects of the mixed mode SIFs, the T-stress and a critical distance rC. For each test configuration, a fracture initiation direction, a fracture toughness and mixed mode fracture resistance curves were obtained throughout the entire range of the mixed mode I/II loading conditions. It was observed that for all specimens the loss of the crack tip constraint increases the mixed mode fracture resistance and decreases the fracture initiation direction and vice versa.

Tensile fatigue behavior of single carbon nanotube yarns

In order to understand the tensile fatigue characteristics of single carbon nanotube yarn (CNTY), experiments of fatigue loading and residual strength after different fatigue cycles were conducted. Results show that the tensile fatigue limit of the CNTY is ~ 68% of ultimate tensile strength (UTS). SEM figures show a typical fatigue process including crack initiation, crack propagation, and sudden fracture. Helix angles on the surface of CNTY decreased when the yarn underwent a certain number of tension–tension fatigue loading cycles, and the yarn was increasingly strengthened. Specifically, the strength and modulus of CNTY were increased by 21% and 468%, respectively, when the yarn was subjected to a 105 fatigue cycles at 68% UTS. The increase in residual specific strength after cyclic loading was found out to be the reason for the inflection point of the S–N curve. However, there were gaps between the surface layer and inner layer in the yarn. Cracks initiated along the gaps by shear force and friction during cyclic loading. Then, the failure of the inner CNT layers was caused by stress concentration at one of the relatively large cracks. A sudden fracture of the CNTY occurred eventually.

A study of size effects in bioinspired, “nacre-like”, metal-compliant-phase (nickel-alumina) coextruded ceramics

Coextrusion has been shown to be a viable processing route for the fabrication of bioinspired ceramic materials that exhibit improved damage tolerance. This provides one of the few examples of the synthesis of “nacre-like” ceramic hybrid structures with “brick-and-mortar” architectures using a method that includes the “mortar” (i.e., the metallic or polymeric compliant phase) during the entire fabrication process, instead of infiltrating it into a pre-fabricated ceramic scaffold. In this study, we examine how manipulation of filament size can lead to improved mechanical performance in the resulting biomimetic ceramics by reduction of the brick size and mortar thickness while still maintaining a model high volume fraction (∼90 vol.%) ceramic containing a metallic compliant phase. Specifically, we synthesized model brick-and-mortar alumina hybrid structures (Al2O3/Ni) containing small volume fractions (<10%) of nickel which we made by the coextrusion of alumina and nickel oxide in a thermoplastic (polyethylene-ethyl acrylate) suspension. Flexural strength and crack-initiation fracture toughness values were used to compare the performance of various ceramic brick sizes, with full crack-growth resistance-curves (R-curves) measured and compared to similar bioinspired ceramics to ascertain how brick size and mortar thickness affect the final mechanical performance. It was found that even though these structures are significantly coarser than those made with other processing methods, they still exhibit comparable crack-growth resistance, despite their lower strength, as well as improving R-curve behavior that tracks closely with brick size. Indeed, these structures display some of the highest fracture toughness values of any high-volume fraction alumina with a metallic compliant phase to date. Toughening was found to be induced by marked crack deflection as the crack path followed the metallic “mortar” phase, coupled with significant crack bridging and “brick” pull-out in the image of the toughening mechanisms seen in nacre.

Experimental and Numerical Analysis of Fracture in 41Cr4 Steel – Issues of the Stationary Cracks

This paper analyzes the process of fracture in 41Cr4 steel on the basis of experimental and numerical data obtained for non-propagating cracks. The author’s previous and latest experimental results were used to determine the apparent crack initiation moment and fracture toughness for the material under plane strain conditions. Numerical simulations were carried out to assess changes in the J-integral, the crack tip opening displacement, the size of the plastic region and the distribution of stresses around the crack tip. A complex numerical analysis based on the true stress-strain curve was performed to determine the behavior of 41Cr4 steel under increasing external loads.

Fatigue strength of notched specimens made of Ti-6Al-4V produced by Selected Laser Melting technique

As an Additive Manufacturing (AM) method, Selective Laser Melting (SLM) allows fabrication of metallic components of any desired geometry with densities greater than 99.5%. This method is usually used for geometrically optimized components with complex geometries including various kinds of notches. Hence, it is important to have a clear information about the fatigue behavior of notched components made by SLM method. This paper evaluates the fatigue behavior of Ti-6Al-4V blunt V-notched and circular notched samples produced by SLM. The fatigue results were then compared with those of the corresponding smooth samples. The fracture surfaces of the tested samples were evaluated using Environmental Scanning Electron Microscopy (ESEM) and the crack initiation points and fracture mechanisms were identified.

Research on crack initiation mechanism and fracture criterion of rock-type materials under compression–shear stress

With the focus on the failure of single-cracked rocks under compression–shear stress, the form and the critical conditions of crack initiation by adopting theoretical derivation and numerical exper...

A Case Study on Fatigue Failure of a Transmission Gearbox Input Shaft

In this paper, a root cause analysis of premature failure of a gearbox input shaft, manufactured of AISI 1045-H, was performed through standard procedures for failure analysis. Shaft failed on cross oil hole through a helical fracture and therefore did not meet bogie 100,000 cycles during the verification with 10 Hz frequency cyclic testing. The fracture in the oil hole implied evidence of fatigue (i.e., beach marks on the fracture surface were clearly visible). Prior to improving the fatigue life and suggesting required remedial actions, mechanism of failure has to be understood, especially the initiating point of cracking. To this end, chemical analysis, microstructural characterization, fractography, hardness measurements, and finite element simulation were used to assess the nature of fracture in detail. The fractography analysis showed that fatigue beach marks originate from transition zone of the case on the cross oil hole. This is possibly due to the fact that torsional strength in this area is lower than torsional fatigue strength which leads to fatigue crack initiation, crack growth, and final fracture. At the end of this paper, proper remedial actions have been proposed.

Effect of Shrinkage Porosity and Degenerated Graphite on Fatigue Crack Initiation in Ductile Cast Iron

Heavy-section ductile iron castings solidify under low cooling rates, giving the risk to form defects like non-metallic inclusions, microporosities and degenerated shapes of graphite that have a negative effect on fatigue resistance, favoring the crack nucleation. The aim of this work is to precisely identify the defects that most affect the fatigue behavior in heavy-section castings, in order to guide the foundry to take the proper countermeasures. For this purpose, fatigue rotating bending tests were carried out on specimens machined from small-scale samples opportunely cast to reproduce long solidification times. The fracture surface of broken samples were investigated by means of Scanning Electron Microscopy in order to identify crack initiation points and fracture mechanisms. Shrinkage porosities and spiky graphite were found to play the most important effect on crack nucleation, lowering the fatigue resistance of the castings, while chunky graphite just behaved as a preferential path for crack propagation.

Fracture properties of concrete specimens made from alkali activated binders

The aim of this paper is to quantify crack initiation and other fracture properties – effective fracture toughness and specific fracture energy – of two types of concrete with an alkali activated binder. The beam specimens with a stress concentrator were tested in a three-point bending test after 28, 90, and 365 days of maturing. Records of fracture tests in the form of load versus deflection (P–d) diagrams were evaluated using effective crack model and work-of-fracture method and load versus mouth crack opening displacement (P–CMOD) diagrams were evaluated using the Double-K fracture model. The initiation of cracks during the fracture tests for all ages was also monitored by the acoustic emission method. The higher value of monitored mechanical fracture parameters of concrete with alkali activated blast furnace slag were achieved with substitution blast furnace slag by low calcium fly ash in comparison with substitution by cement kiln dust.

RETRACTED ARTICLE: Fatigue Assessment of Ti–6Al–4V Circular Notched Specimens Produced by Selective Laser Melting

Additive manufacturing (AM) offers the potential to economically produce customized components with complex geometries in a shorter design-to-manufacture cycle. However, the basic understanding of the fatigue behavior of these materials must be substantially improved at all scale levels before the unique features of this rapidly developing technology can be used in critical load bearing applications. This work aims to assess the fatigue strength of Ti–6Al–4V smooth and circular notched samples produced by selective laser melting (SLM). Scanning Electron Microscopy (SEM) was used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Despite the observed fatigue strength reduction induced by circular notched specimens compared to smooth specimens, notched samples showed a very low notch sensitivity attributed both to hexagonal crystal lattice characteristics of tempered alpha prime grains and to surface defects induced by the SLM process itself.

Fatigue Assessment of Ti–6Al–4V Circular Notched Specimens Produced by Selective Laser Melting

Additive manufacturing (AM) offers the potential to economically produce customized components with complex geometries in a shorter design-to-manufacture cycle. However, the basic understanding of the fatigue behavior of these materials must be substantially improved at all scale levels before the unique features of this rapidly developing technology can be used in critical load bearing applications. This work aims to assess the fatigue strength of Ti–6Al–4V smooth and circular notched samples produced by selective laser melting (SLM). Scanning Electron Microscopy (SEM) was used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Despite the observed fatigue strength reduction induced by circular notched specimens compared to smooth specimens, notched samples showed a very low notch sensitivity attributed both to hexagonal crystal lattice characteristics of tempered alpha prime grains and to surface defects induced by the SLM process itself.

Subcritical crack growth and crack tip driving forces in relation to material resistance

AbstractBasic concepts, related to the crack tip driving forces in relation to the material resistance, are analyzed for the elastic and elastic-plastic crack growth condition. This defines the crack initiation and growth conditions, as well as for crack arrest. Environment provides an additional driving force, thereby reducing the mechanical driving force required for the crack to grow. It is shown that (a) crack initiation and its growth are inseparable and (b) the magnitude of the applied and/or internal stresses; their gradients are also important for initiation and continuous growth of a crack. Elastic-plastic crack growth is also analyzed using the discrete dislocation models. The results show that its behavior is similar to that of an elastic crack. These concepts are valid for all subcritical crack growth. Mechanical and mechanical equivalent of chemical forces are defined for estimating the life prediction of a component in service. Failure diagrams are defined based on the extension of classical Kitagawa-Takahashi diagram that bridges the behavior of smooth and fracture mechanics specimens. Connections between crack initiation, growth, arrest, and overload fracture are established via these failure diagrams. Application of these diagrams for engineering components in service is outlined for diagnostic and prognostic purposes.

Fatigue strength of blunt V-notched specimens produced by selective laser melting of Ti-6Al-4V

Selective Laser Melting (SLM) process is an Additive Manufacturing (AM) technique that allows producing metallic parts of any kind of geometry with densities greater than 99.5%. Complex shapes lead however to notches with different radii of curvature that may reduce load bearing capacities. This work is aimed to assess the fatigue strength of Ti-6Al-4V blunt V-notched samples produced by SLM. Results were compared with those of the corresponding smooth samples and Environmental Scanning Electron Microscopy (ESEM) have been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Finally, the strain energy density approach was used to evaluate the critical radius value. Despite the observed fatigue strength reduction induced by the notch, samples showed a sufficient low notch sensitivity that it was not possible to define a critical radius for the material analysed.

Mechanical and fatigue properties of pearlitic ductile iron castings characterized by long solidification times

Abstract In this paper, the fatigue behaviour of heavy section pearlitic ductile iron castings has been investigated. The inoculation treatment has been changed for each casting in order to investigate its influence on the mechanical and fatigue properties of the materials. Tensile tests and axial fatigue tests under nominal ratio R = 0.01 have been performed on specimens taken from the core of casting components characterized by long solidification times. Scanning Electron Microscopy has been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Metallographic analyses have been carried out to measure nodule count and nodules dimensions and to identify matrices structures. It has been found that fatigue behaviour is strongly influenced by defects, such as microshrinkages or degenerated graphite particles near to specimens' surface. It has been also found that inoculation process influences the microstructure and the fatigue resistance of heavy section pearlitic ductile iron castings.

Fracture Behavior Investigation of a Typical Sandstone Under Mixed-Mode I/II Loading Using the Notched Deep Beam Bending Method

The brittle fracture behavior of rocks under mixed-mode loading is important in rock engineering. First, a new configuration called the notched deep beam (NDB) specimen was introduced for the fracture testing of rock materials under mixed-mode I/II loading, and a series of finite element analyses were performed to calibrate the dimensionless fracture parameters (i.e., Y I, Y II and $$T^{*}$$ ). The results showed that an NDB specimen subjected to three-point bending is able to generate pure mode I loading, pure mode II loading, and any mixed-mode loading in between. Then, several NDB specimens made of sandstone were used to investigate the brittle fracture behavior of rock under mixed-mode I/II loading. The fracture surfaces were theoretically described using a statistical method, and the results indicated that all the fracture surfaces generated under different mixed-mode loading were statistically identical; to some extent, these results experimentally showed that only tensile fracture occurs under mixed-mode I/II loading. The obtained fracture strengths were then analyzed using several brittle fracture criteria. The empirical criterion, maximum energy release rate criterion, generalized maximum tangential stress (GMTS) criterion, and improved R-criterion accurately predicted the fracture strength envelope of the sandstone. Finally, based on the concepts of point stress and mean stress, the micro-crack zones (MCZs) under different mixed-mode loading were theoretically estimated based on the MTS and GMTS criteria. The critical radius of MCZ in the crack propagation direction was not a constant for all mixed-mode loading conditions regardless of whether the T-stress was considered. This result suggests that the size of the core region used to predict the crack initiation direction and fracture strength based on the GMTS criterion should be chosen more carefully.

Investigation of fatigue cracks on RAP mixtures containing Sasobit and crumb rubber based on fracture energy

Fatigue cracking, rutting and low temperature cracking are three major distresses of asphalt mixtures. In last years, different methods based on fracture mechanic have been developed to measure performance of asphalt mixtures due to cracking. In this study, fracture mechanic method developed by Roque et al. has been employed to evaluate fatigue cracking performance of modified asphalt mixtures. Samples containing Reclaimed Asphalt Pavement (RAP) with different percentages of Sasobit (as a Warm Mix Asphalt additive) and crumb rubber were built. Resilient Modulus, static creep and Indirect Tensile (IDT) tests also conducted on samples and Resilient Modulus, creep compliance and Indirect Tensile Strength were measured. Based on fracture mechanic, Dissipated Creep Strain Energy (DCSE) was calculated for each sample, and fatigue strength of mixtures were investigated. The results indicated that the use of RAP, crumb rubber and Sasobit simultaneously caused desirable fatigue performance of modified mixtures. These mixtures had better Resilient Modulus and more loading capacity, as well as, lower cumulative damage rates rather than virgin sample. The ITS results also indicated more appreciable improvement in crack initiation threshold, Fracture Energy (FE) and Energy Ratio (ER).