Onset Of Fracture Research Articles

Risk assessment of vertebral compressive fracture using bone mass index and strength predicted by computed tomography image based finite element analysis

BackgroundA main purpose of osteoporosis diagnosis is to evaluate the bone fracture risk. Some bone mass indices evaluated using bone mineral density has been utilized clinically to assess the degree of osteoporosis. On the other hand, Computed tomography image based finite element analysis has been developed to evaluate bone strength of vertebral bodies. The strength of a vertebra is defined as the load at the onset of compressive fracture. The objective of this study was therefore to propose a new feasible method to combine the advantages of the two osteoporotic indices such as the bone mass index and the bone strength. MethodsThree-dimensional finite element models of 246 vertebral bodies from 88 patients were constructed using the computed tomography images. Finite element analysis was then conducted to evaluate their strength values. The Pearson's correlation analysis was also conducted between the vertebral strength and bone mass indices. FindingsIt was found that relatively weak positive correlations existed between the strength and the bone mass indices. A new assessment method was then proposed by combining the strength and the bone mass index. “high risk zone” corresponding to low strength with normal bone mass was found from the assessment method. InterpretationSinge bone mass index cannot predict the fracture risk with high standard. The results of fracture risk assessment conducted by the new method clearly indicated the necessity and effectiveness to take both the strength and the bone mass index into account.

A framework of modelling slip-controlled crack growth in polycrystals using crystal plasticity and XFEM

Short cracks tend to develop at high and irregular rates compared to macroscopic cracks, making the prediction of fatigue life a challenging task. In this work, a numerical framework combining crystal plasticity model and the Extended Finite Element Method (XFEM) is applied to study the slip-controlled short crack growth in a polycrystal superalloy RR1000. The model is calibrated from experiments and used to evaluate short crack growth paths and rates. Two fracture criteria are used and compared: the onset of fracture is controlled by the total and individual cumulative shear strain respectively, and the crack grows either perpendicular to the direction of maximum principal strain or along crystallographic directions.

The effects of machining residual stresses on springback in deformation machining bending mode

Hybrid machining-forming technology is a combination of two thin-structural machining and incremental forming manufacturing processes. A group of complex geometry parts in which a thick region is connected to a thin-wall region can be manufactured through this technology with certain advantages. The parts made by this technology require less raw material compared to the ones created by means of machining. The major problem with this technology is the dimensional and geometric inaccuracy of its products which is mainly due to springback. The main purpose of this research was to study the effects of machining parameters and residual stresses induced by the machining primary stage on the subsequent springback after the forming stage. It was found by experiments that the parameters of cutting speed, axial depth of cut, mode of milling, and milling path had a minor effect on springback. However, the workpiece fracture during the forming stage was observed to be sensitive to the prior machining feed rate. Both finite element simulations and experimental results confirmed that the compressive machining residual stresses increased with an increase in the machining feed rate. The compressive residual stresses postponed the onset of fracture at the workpiece lower end during the forming stage. Therefore, we could approach the forming tool closer to the bottom of the wall during forming and, as a result, springback decreased considerably.

Fracture mechanics analysis of delamination along width-varying interfaces

The subject of controlled interface fracture is gaining considerable attention and goes hand-in-hand with ‘smart’ and ‘on-demand’ material designs and ‘metamaterial’ approach. To further unlock the potential behind geometrical enhancements, an effective crack tip forces approach is used to derive the strain energy release rate and the critical fracture onset force for layered and laminated materials of arbitrary shapes. The analytical formulation allows prediction of delamination onset forces as a function of laminate cross-section geometry at the crack front. Here, we focus on width-varying geometries with the choice motivated by the recent use of composite, patch-alike, multilayer material systems and repairs. The theoretical model is successfully verified by comparison with experimental data from width-varying carbon fibre laminates and numerical results. The proposed theoretical and experimental approaches are beneficial for predicting and controlling fracture and can prove useful to motivate new designs for multilayer and laminated material by indicating the relationship between the material geometry and the delamination onset force.

Influence of experimental boundary conditions on the calibration of a ductile fracture criterion

Hybrid experimental–numerical approach is the commonly used technique to identify the material parameters of ductile fracture criteria. In this work, attention is paid to the precise definition of the Finite Element (FE) model of fracture tests by applying real boundary conditions coming from the local displacement field. The aim is to investigate the effect of the way to reproduce experimental boundary conditions on the accuracy of the hybrid experimental–numerical approach. A stress triaxiality and Lode angle based ductile fracture criterion is used to predict the onset of ductile fracture for AA6061-T6 aluminum alloy sheets. To calibrate this criterion, a series of fracture tests is carried out up to fracture. Notched tensile specimens with different radius, tensile specimens with a central hole and shear specimens are used to cover a wide range of stress states. Strain distribution over the surface is measured with digital image correlation technique. Two numerical models are defined for each geometry type, a first one constrained with real experimental boundary conditions and a second one with ideal boundary conditions, to predict stress and strain distributions. By comparing results of both approaches with experiments, it is shown that the model constrained by local experimental boundary conditions provides a better agreement with the experiments than the simplified model does, which underlines the sensitivity of the hybrid method to the adopted boundary conditions. Furthermore, several fracture test combinations are used to calibrate the ductile criterion and the effect of the calibration tests is discussed.

Process Condition Diagram Predicting Onset of Microdefects and Fracture in Cold Bar Drawing

This paper presents a process condition diagram (PCD) that not only identifies conditions under which materials fracture during bar drawing, but also infers the presence or absence of microdefects such as microvoids and microcracks in the drawn material as accumulative damage changes owing to the die semi-angle and reduction ratio. The accumulative damage values were calculated by finite element (FE) analysis. The critical damage values were determined by performing a tensile test using a smooth round bar tensile specimen and performing FE analysis simulating the tensile test. High alloy steel with a 13 mm diameter was used for the draw bench testing in a wide range of drawing conditions. Scanning electron microscopy (SEM) analysis was performed to verify the usefulness of the PCD. SEM images showed that the accumulative damage roughly matched the size of microvoids around the non-metallic inclusions and the creation of microcracks, which eventually led to fractures of material being drawn. Hence, utilizing the proposed PCD, a process designer can design drawing conditions that minimize the occurrence of microdefects in the material being drawn while maximizing the reduction ratio.

Multi-material adhesive joints with thick bond-lines: Crack onset and crack deflection

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines (≈10 mm) under global mode I loading. The role of adherend-adhesive modulus-mismatch and pre-crack length are scrutinized. The parameters controlling the crack path directional stability are also discussed. Single-material (i.e. steel-steel and GFRP-GFRP) and bi-material (i.e. steel-GFRP) double-cantilever beam joints bonded with a structural epoxy adhesive are tested. The joints are modelled analytically, considering a beam on elastic-plastic foundation, to include characteristic length scales of the problem (e.g. adhesive thickness, plastic zone) and numerically using Finite Element Model. An empirical relation, in terms of geometrical and material properties of the joints, that defines the transition between non-cohesive and cohesive fracture onset is found. Above a specific pre-crack length the stress singularity at pre-crack tip rules over the stress singularity near bi-material corners, resulting in mid-adhesive thickness cohesive fracture onset. However, the cracking direction rapidly deflects out from the adhesive layer centre-line. Positive T-stress along the crack tip is found to be one of the factors for the unstable crack path.

Vacuum-assisted elevation of pediatric ping-pong skull fractures: a case series and technical note.

The management of children with ping-pong skull fractures may include observation, nonsurgical treatments, or surgical intervention depending on the age, clinical presentation, imaging findings, and cosmetic appearance of the patient. There have been 16 publications on nonsurgical treatment using negative pressure with various devices. Herein, the authors report their experience with vacuum-assisted elevation of ping-pong skull fractures and evaluate the variables affecting procedural outcomes. The authors performed a retrospective chart review of all ping-pong skull fractures treated via vacuum-assisted elevation at the Children's Hospital of Wisconsin between 2013 and 2017. Data collected included patient age, head circumference, mode of injury, time to presentation, imaging findings, procedural details, treatment outcomes, and complications. Four neonates and 5 infants underwent vacuum-assisted elevation of moderate to severe ping-pong skull fractures during the study period. Modes of injury included birth-related trauma, falls, and blunt trauma. All patients had normal neurological examination findings and no evidence of intracranial hemorrhage. All fractures were deemed severe enough to require elevation by the treating neurosurgeon. All fractures involved the parietal bone. Skull depressions ranged from 23 to 62 mm in diameter and from 4 to 14 mm in depth. Bone thickness ranged from 0.6 to 1.8 mm. The time from fracture to intervention ranged from 7 hours to 8 days. The Kiwi OmniCup vacuum delivery system was used in all cases. Negative pressures were increased sequentially to a maximum of 500 mm Hg. A greater number of sequential vacuum applications was required for patients with a skull thickness greater than 1 mm at the site of depression and for those undergoing treatment more than 72 hours from fracture onset. Successful fracture elevation was attained in 7 of 9 patients. Two patients required subsequent surgical elevation of their fractures. Postprocedure imaging studies revealed no evidence of complications. Increasing bone thickness and time from fracture onset to intervention appeared to be the greatest limiting factors to the successful elevation of moderate to severe ping-pong fractures via this vacuum-assisted approach. This procedure is a well-tolerated option that should be considered prior to performing an open repair in cases deemed to require fracture elevation. Future efforts will focus on larger-volume studies to better delineate inclusion and exclusion criteria, and volumetric analysis for better fracture-to-suction device customization.

Fracture analyses of surface asperities during sliding contact

Fracture of interlocking asperities during sliding contact plays a central role in determining friction and wear of brittle materials. The local force to break surface asperities and subsurface crack propagation directly affect the frictional force, wear rate and evolution of surface roughness. Here, we study the fracture failure of an isolated asperity under a lateral contacting force using a systematic set of experiments and numerical simulations. Combining the generalized elastic fracture mechanics and the Euler-Bernoulli beam theory, we also develop an analytical model to predict the critical force at the onset of asperity fracture as a function of the asperity's shape and material properties. The developed model can be used to develop physics-based friction and wear models for brittle solids.

Berkovich nanoindentation of Zr55Cu30Al10Ni5 bulk metallic glass at a constant loading rate

Berkovich nanoindentation experiments were conducted on Zr55Cu30Al10Ni5 bulk metallic glass under a constant loading rate. Indentation hardness decreased linearly with contact depth, which defied the direct application of conventional indentation size effect model for crystalline material. Elastic modulus remained invariant under small loads, but decreased linearly under large loads. The scaling relationships between indentation variables such as contact depth, permanent depth, the maximum indentation displacement, plastic energy, the total work of indentation and their ratios were investigated. The area of shear band circle, the projected contact area at the maximum load, and the residual projected area were all found to be proportional to one another. A new methodology to determine fracture toughness was proposed based on the total work of indentation, provided that the critical indentation displacement at the onset of fracture corresponds to zero value of elastic modulus extrapolated from curve fitting.

Experimental investigation of scatter in fracture toughness data of SA516Gr.70 steel in the ductile-to-brittle transition regime for high rate of loading using split Hopkinson pressure bar test setup

Ferritic grade steels exhibit ductile to brittle transition in the sub-zero temperature regime and the fracture toughness data show scatter and temperature dependence. The fracture toughness variation also depends upon rate of loading in addition to neutron irradiation, specimen geometry and loading configuration. The material SA516Gr.70 steel is widely used in nuclear industry as material of transportation cask, vessels and piping etc. The study of the effect of rate of loading on the fracture toughness transition curve in the ductile-to-brittle transition regime of this material is important for designers and safety analysts. In this work, the split Hopkinson pressure bar test technique has been used to carry out fracture tests on sub-sized single-edged notched bend specimens in the temperature range of −100 to −60 deg. C by loading the specimens at a very high loading rate of 700 m/min. This loading rate is at least 6 orders of magnitude higher than the rates (i.e., fraction of mm/min) employed for quasi-static fracture tests. The strain pulses, during high rate of loading, have been captured through strain gages located in the incident and transmission bars of the test setup. The load–displacement data for the specimens have been evaluated from the strain signal and these have been used to evaluate fracture toughness for onset of cleavage fracture in terms of KJc. The scatter and temperature dependence of the data have also been modelled through Weibull’s statistical distribution and master curve bounds according to ASTM E1921. From the results of fracture experiments, it was observed that the fracture transition temperature T0 increased significantly for the high rate of loading when compared to quasi-static data of similar materials in literature. This observation has been explained in terms of micro-mechanism of the cleavage fracture in the process zone ahead of crack-tip which changes due to rate of loading. This study indicates that the rate of loading is an important parameter in the design and integrity analysis of ferritic grade steel components in addition to other parameters such as specimen geometry and loading conditions.

Strain-resilient electrical functionality in thin-film metal electrodes using two-dimensional interlayers.

Flexible electrodes that allow electrical conductance to be maintained during mechanical deformation are required for the development of wearable electronics. However, flexible electrodes based on metal thin-films on elastomeric substrates can suffer from complete and unexpected electrical disconnection after the onset of mechanical fracture across the metal. Here we show that the strain-resilient electrical performance of thin-film metal electrodes under multimodal deformation can be enhanced by using a two-dimensional (2D) interlayer. Insertion of atomically-thin interlayers — graphene, molybdenum disulfide, or hexagonal boron nitride — induce continuous in-plane crack deflection in thin-film metal electrodes. This leads to unique electrical characteristics (termed electrical ductility) in which electrical resistance gradually increases with strain, creating extended regions of stable resistance. Our 2D-interlayer electrodes can maintain a low electrical resistance beyond a strain in which conventional metal electrodes would completely disconnect. We use the approach to create a flexible electroluminescent light emitting device with an augmented strain-resilient electrical functionality and an early-damage diagnosis capability.

A combined stress/energy-based criterion for mixed-mode fracture of laminated composites considering fiber bridging micromechanics

A combined stress/energy-based (CSE) criterion is developed to predict mixed mode I/II fracture behavior in laminated composites by taking into account the effect of fiber bridging toughening. Following the proposed criterion, first, the absorbed energy resulting from fiber bridging micro-mechanisms is obtained and translated into a defined effective critical distance (i.e., the size of the crack tip micro-cracking zone). Then, the maximum principal stress is evaluated at the calculated effective critical distance around the crack tip to predict the onset of fracture. The CSE criterion is employed, along with other criteria, to predict experimental data reported in the literature on the onset of mixed mode I/II fracture in laminated composites and wood species (i.e. orthotropic composites). A higher correlation is found between the theoretical results predicted by the CSE criterion and the test data, as compared to other criteria. The stress-based formulation of the CSE criterion offers more computational simplicity as compared to the energy-based criteria while providing an acceptable accuracy and taking into account the effects of absorbed energy resulting from the micromechanical fiber bridging in the solution.

Essential work of fracture assessment of acrylonitrile butadiene styrene (ABS) processed via fused filament fabrication additive manufacturing

Experiments and finite element (FE) calculations were performed to study the raster angle–dependent fracture behaviour of acrylonitrile butadiene styrene (ABS) thermoplastic processed via fused filament fabrication (FFF) additive manufacturing (AM). The fracture properties of 3D-printed ABS were characterized based on the concept of essential work of fracture (EWF), utilizing double-edge-notched tension (DENT) specimens considering rectilinear infill patterns with different raster angles (0°, 90° and + 45/− 45°). The measurements showed that the resistance to fracture initiation of 3D-printed ABS specimens is substantially higher for the printing direction perpendicular to the crack plane (0° raster angle) as compared to that of the samples wherein the printing direction is parallel to the crack (90° raster angle), reporting EWF values of 7.24 kJ m−2 and 3.61 kJ m−2, respectively. A relatively high EWF value was also reported for the specimens with + 45/− 45° raster angle (7.40 kJ m−2). Strain field analysis performed via digital image correlation showed that connected plastic zones existed in the ligaments of the DENT specimens prior to the onset of fracture, and this was corroborated by SEM fractography which showed that fracture proceeded by a ductile mechanism involving void growth and coalescence followed by drawing and ductile tearing of fibrils. It was further shown that the raster angle–dependent strength and fracture properties of 3D-printed ABS can be predicted with an acceptable accuracy by a relatively simple FE model considering the anisotropic elasticity and failure properties of FFF specimens. The findings of this study offer guidelines for fracture-resistant design of AM-enabled thermoplastics.Graphical abstract

Optimizing film thickness to delay strut fracture in high-entropy alloy composite microlattices

Incorporating high-entropy alloys (HEAs) in composite microlattice structures yields superior mechanical performance and desirable functional properties compared to conventional metallic lattices. However, the modulus mismatch and relatively poor adhesion between the soft polymer core and stiff metallic film coating often results in film delamination and brittle strut fracture at relatively low strain levels (typically below 10%). In this work, we demonstrate that optimizing the HEA film thickness of a CoCrNiFe-coated microlattice completely suppresses delamination, significantly delays the onset of strut fracture (∼100% increase in compressive strain), and increases the specific strength by up to 50%. This work presents an efficient strategy to improve the properties of metal-composite mechanical metamaterials for structural applications.

Investigations into Enhanced Formability of AA5083 Aluminum Alloy Sheet in Single-Point Incremental Forming

In this article, formability of aluminum alloy AA5083-sheet in single point incremental forming (SPIF) is investigated through forming limit curves (FLCs) and maximum formable wall angle considering different forming parameters and conditions. Theoretical FLCs were predicted for SPIF and conventional forming utilizing deformation instability and Marciniak-Kuczynski methods, respectively, and validated by experiments. SPIF was found to give better formability compared to the conventional one in terms of the limit strain values from varying plane strain to equi-biaxial stretching modes of deformations. Groove depth at the onset of fracture in incremental sheet forming test was observed to be more for higher forming speed, i.e., at higher tool rotational speed and feed and for lower incremental depth. The maximum formable wall angle was improved for lower step depth but not significantly increased for higher forming speed. The forming limit strains and maximum forming wall angle were found to increase for incremental forming at elevated temperature. Microstructure studies revealed grain refinement in the deformed sheet in SPIF forming, and microhardness values in the deformed sheets were observed to increase for incrementally formed parts compared to that of the as-received sheet.

Acoustic emissions in vertebral cortical shell failure

Understanding the initiation of bony failure is critical in assessing the progression of bone fracture and in developing injury criteria. Detection of acoustic emissions in bone can be used to identify fractures more sensitively and at an earlier inception time compared to traditional methods. However, high rate loading conditions, complex specimen-device interaction or geometry may cause other acoustic signals. Therefore, characterization of the isolated local acoustic emission response from cortical bone fracture is essential to distinguish its characteristics from other potential acoustic sources. This work develops a technique to use acoustic emission signals to determine when cortical bone failure occurs by characterization using both a Welch power spectral density estimate and a continuous wavelet transform. Isolated cortical shell specimens from thoracic vertebral bodies with attached acoustic sensors were subjected to quasistatic loading until failure. The resulting acoustic emissions had a wideband frequency response with peaks from 20 to 900 kHz, with the spectral peaks clustered in three bands of frequencies (166 ± 52.6 kHz, 379 ± 37.2 kHz, and 668 ± 63.4 kHz). Using these frequency bands, acoustic emissions can be used as a monitoring tool in biomechanical spine testing, distinguishing bone failure from structural response. This work presents a necessary set of techniques for effectively utilizing acoustic emissions to determine the onset of cortical bone fracture in biological material testing. Acoustic signatures can be developed for other cortical bone regions of interest using the presented methods.

The association between gallstone disease (GSD) and hip fracture: a nationwide population-based study

ABSTRACT Objectives: Despite the high prevalence of gallstone disease (GSD), the shared risk factors of GSD and osteoporosis, and the known association between hip fracture and hepatobiliary diseases, the association between hip fracture and GSD is not currently clear. Therefore, we performed a nationwide population-based study to investigate the association between GSD and hip fracture to determine the impact of cholecystectomy on the risk of fracture. Methods: In this study, we assessed all subjects in the longitudinal health insurance database (LHID) between 2000 and 2011, excluding those subjects aged >20 years old and those with a previous history of hip fracture (ICD-9-CM 820). Among those that were included, subjects with at least two or more outpatient visits or with one record of hospitalization under the coding of GSD (ICD-9-CM code: 574) were allocated to the GSD cohort. The remaining subjects were designated to the control cohort. All participants were followed till the onset of hip fracture, withdrawal from the NHI, or the end of 2013. Results: We found that the cumulative incidence of hip fracture was higher in the GSD cohort than in the control cohort (log-rank test: p-value < 0.01). After adjustment, the GSD patients had a 1.21-fold risk of hip fracture compared to control subjects (aHR = 1.21, 95% CI = 1.21–1.30). Comparison between those subjects without GSD and those without cholecystectomy revealed that the risk of hip fracture was higher among GSD patients that had not undergone cholecystectomy (aHR = 1.17, 95% CI = 1.06–1.29) or those that had undergone cholecystectomy (aHR = 1.22, 95% CI = 1.06–1.41). Conclusion: Based upon these results, we concluded that GSD was associated with an increased risk of hip fracture regardless of whether the patient had undergone cholecystectomy.

Impact of paravertebral muscle in thoracolumbar and lower lumbar regions on outcomes following osteoporotic vertebral fracture: a multicenter cohort study.

Paravertebral muscle (PVM) is an important component of the spinal column. However, its role in the healing process after osteoporotic vertebral fracture (OVF) is unclear. This study aimed to clarify the effect of PVM in thoracolumbar and lower lumbar regions on OVF clinical and radiological outcomes. This was a multicenter prospective cohort study from 2012 to 2015. Patients ≥ 65 years old who presented within 2 weeks after fracture onset were followed up for 6 months. PVM was measured at the upper edge of the L1 and L5 vertebral body in the magnetic resonance imaging (MRI) T2-axial position at registration. The cross-sectional area (CSA), relative CSA (rCSA), and fat infiltration percentage (FI%) were measured. Severe vertebral compression, delayed union, new OVF, and remaining low back pain (LBP) were analyzed. Among 153 patients who were followed up for 6 months, 117 with measurable PVM were analyzed. Their average age was 79.1 ± 7.2 years, and 94 were women (80.3%). There were 48 cases of severe vertebral compression, 21 delayed unions, 11 new OVF, and 27 remaining LBP. Among all poor prognoses, only the FI% of the PVM was significantly associated with new OVF (p = 0.047) in the thoracolumbar region and remaining LBP (p = 0.042) in the lumbar region. The occurrence of additional OVF in the thoracolumbar region and remaining LBP in the lumbar region was significantly related to the FI% of the PVM. Physicians should be aware that patients with such fatty degeneration shown in acute MRI may require stronger treatment.

Mixed mode I/II fracture criterion to anticipate cracked composite materials based on a reinforced kinked crack along maximum shear stress path

In this paper, a fracture criterion for predicting the failure of the cracked composite specimens under mixed mode I/II loading is provided. Various tests performed on composite components reveal that cracks always grow along the fibers in the isotropic media. Using a new material model called reinforcement isotropic solid (RIS) concept, it is possible to extend the isotropic mixed mode fracture criteria into composite materials. In the proposed criterion, maximum shear stress (MSS) theory which is widely used for failure investigation of un-cracked isotropic materials will be extended to composite materials in combination with RIS concept. In the present study, cracks are oriented along the fibers in the isotropic material. It is assumed that at the onset of fracture, crack growth will be in a path where the shear stress has the highest value according to the MSS criterion. Investigating the results of this criterion and comparing with the available experimental data, it is shown that, both the crack propagation path and the moment of crack growth are well predicted. Available mixed mode I/II fracture data of various wood species are used to evaluate and verify the theoretical results.