Rapid Crack Growth Research Articles

Mechanical and Compositional Implications of Gallium Ion Milling on Epoxy Resin.

Focused Ion Beam (FIB) is one of the most common methods for nanodevice fabrication. However, its implications on mechanical properties of polymers have only been speculated. In the current study, we demonstrated flexural bending of FIB-milled epoxy nanobeam, examined in situ under a transmission electron microscope (TEM). Controllable displacement was applied, while real-time TEM videos were gathered to produce morphological data. EDS and EELS were used to characterize the compositions of the resultant structure, and a computational model was used, together with the quantitative results of the in situ bending, to mechanically characterize the effect of Ga+ ions irradiation. The damaged layer was measured at 30 nm, with high content of gallium (40%). Examination of the fracture revealed crack propagation within the elastic region and rapid crack growth up to fracture, attesting to enhanced brittleness. Importantly, the nanoscale epoxy exhibited a robust increase in flexural strength, associated with chemical tempering and ion-induced peening effects, stiffening the outer surface. Young’s modulus of the stiffened layer was calculated via the finite element analysis (FEA) simulation, according to the measurement of 30 nm thickness in the STEM and resulted in a modulus range of 30–100 GPa. The current findings, now established in direct measurements, pave the way to improved applications of polymers in nanoscale devices to include soft materials, such as polymer-based composites and biological samples.

Damage accumulation during high temperature fatigue of Ti/SiCf metal matrix composites under different stress amplitudes

The damage mechanisms and load redistribution taking place under high temperature (350 °C), high cycle fatigue (HCF) of TC17 titanium alloy/unidirectional SiC fibre composites have been investigated in situ using synchrotron X-ray computed tomography (CT) and X-ray diffraction (XRD) under two stress amplitudes. The three-dimensional morphology of the fatigue crack and fibre fractures has been mapped by CT. At low stress amplitude, stable growth occurs with the matrix crack deflecting by 50-100 µm in height as it bypasses the bridging fibres. At higher stress amplitude, loading to the peak stress led to a burst of fibre fractures giving rise to rapid crack growth. Many of the fibre fractures occurred 50-300 µm above/below the matrix crack plane during rapid growth, contrary to that in the stable growth stage, leading to extensive fibre pull-out on the fracture surface. The changes in fibre loading, interfacial stress, and the extent of fibre-matrix debonding in the vicinity of the crack have been mapped over the fatigue cycle and after the rapid crack growth by XRD. The fibre/matrix interfacial sliding extends up to 600 µm (in the stable-growth zone) or 700 µm (in the rapid-growth zone) either side of the crack plane. The direction of interfacial shear stress reverses over the loading cycle, with the maximum frictional sliding stress reaching ~55 MPa in both regimes. In accordance with previous studies, it is possible that a degradation in fibre strength at elevated temperature is responsible for bursts of fibre fracture and rapid crack growth under higher stress amplitude.

Structural health monitoring of adhesive joints under pure mode I loading using the electrical impedance measurement

The addition of Multi-wall carbon nanotubes (MWCNTs) or functionalized-graphene nanoplatelets (f-GNPs) into the epoxy resin create an electrical conductive networks which can be employed for structural health monitoring (SHM) in sensitive adhesive joints. The opening fracture mode (Mode I) loading condition, as one of the important fracture modes in adhesive joints was precisely evaluated in this study. The study aims to investigate the crack extension in the adhesive joint (Fracture mechanics) under mode I loading by both smart adhesives and proper measurement technique. In order to provide high sensing capability in the adhesive joint, several aluminum to aluminum adhesive joints were prepared by dispersion of 2.5 wt% MWCNTs or 12 wt% f-GNPs into the epoxy adhesive, respectively. Considering the electrical impedance changes simultaneously with the mechanical response of the adhesive joints, novel outcomes were obtained. Almost linear trend has been observed in the impedance curve for the strain value of 0.57. During crack growth, the conductive network in epoxy matrix near the crack tip would be break and thus the electrical impedance would be increased. In addition, the electrical impedance curve has exhibited several sharp increases, which are attributed to the rapid crack growth.

An Energy-Based Analysis for Machining Novel AZ91 Magnesium Composite Foam Dispersed With Ceramic Microspheres

Abstract Metal syntactic foams are a novel grade of materials that find potential applications in the manufacture of lightweight structural components and biomedical applications. For these materials to be inducted into industrial applications, it becomes imperative to study their machining behavior. In this article, for the first time in the literature, machining characteristics of AZ91 magnesium foam reinforced with thin-walled hollow alumina ceramic microspheres being studied. Through cutting experiments, it is found that finer the size of hollow microspheres and higher their volume fraction, higher was the magnitude of cutting forces recorded. The failure mechanisms that constituted chip formation during cutting AZ91 foam has been explicated through a mechanistic cutting force model. The proposed force model takes into account key hollow alumina microsphere properties such as wall thickness-to-diameter ratio, average microsphere size, and volume fraction. The scanning electron microscopic (SEM) analysis showed two key modes of failure during cutting metallic foams. Microsphere bursts and fractures control matrix plastic deformation through an effective load transfer mechanism. The transverse matrix cracks, which are initiated as a result of induced shear stress, promote the propagation of longitudinal adhesive cracks. This rapid crack growth takes place along the direction of maximum energy release rate, thus weakening the interfacial strength and reducing effective load transfer. This leads to microsphere separation, and further matrix densification due to the collapse of microsphere cavities leads to chip separation. The developed mechanistic model was in better agreement with experimental results.

Characterization of ceramics based on laser speckle photometry

Abstract. Advanced ceramic components are frequently used in industrial applications. As a brittle material, ceramic reacts very suddenly to excessively high stresses. Existing defects lead to rapid crack growth followed by spontaneous destruction. This leads to a functional failure of the entire component. It is therefore important to develop innovative techniques to ensure a good quality condition of ceramic products. Laser speckle photometry (LSP) is an optical nondestructive testing method. It is based on the dynamic analysis of time-resolved speckle patterns that are generated by an external excitation. In this paper, we will present two investigations on ceramic components using the LSP technique. One is the nondestructive stress characterization on ceramic surfaces, and the other is the defect detection on ceramics components. The aim is to improve the quality and safety control of ceramic production in the challenging industrial field. Preliminary results have shown the potential of the LSP sensor system for the nondestructive characterization of ceramics in terms of stress monitoring and surface defect detection.

Study on dynamic mechanical properties of phosphate rock through experimental tests and mesoscale simulation

Dynamic characteristics of rock masses are an important part of the safety and stability analysis in rock mass engineering. The split Hopkinson pressure bar (SHPB) tests on the phosphate rock in Yichang are conducted, combined with the Split-desktop digital image method and mesoscopic discrete element method (DEM). Both macroscopic and mesoscopic characteristics of the phosphate rock have been studied from such perspectives as stress and strain, fractal characteristics of fragment size, and the spatial and temporal evolution of cracks. The research results show that the dynamic strength of the phosphate rock presents obvious strain rate effect, but the dynamic elasticity modulus is not so sensitive to the strain rate. There is a great correlation between the failure degree of rock specimens and the strain rate. The fractal characteristics of specimen fragments can be used to quantitatively evaluate the rock failure degree under impact loading. According to the characteristics of crack propagation, the rock failure process under the impact can be divided into four stages, namely the elastic crack-free stage, the crack initiation stage, the rapid crack growth stage, and the slow crack development stage. Moreover, with the increase of impact loading, micro-cracks are activated in large quantities. The specimens gradually change from the local damage mode to the axial splitting failure mode and to the crushing failure mode, and in this process, the number of tensile cracks takes an absolute advantage compared with the number of shear cracks.

Analytical approach for low and high cycle bending fatigue life prediction of carburized gear steel specimens

Bending fatigue is one of the most common failure modes in spur gears. In carburized spur gears, subsurface failure initiation is characteristic for the high cycle fatigue regime, which is rather hard to detect and may cause rapid crack growth and complete failure of the tooth. Experimental testing of specimens made of the same material under comparable loading conditions is often used to gain insight into actual fatigue behavior of the component. Furthermore, in the absence of definite fatigue data of the specimens, various methods are used for estimating fatigue parameters. In this paper, analytical model for bending fatigue life prediction of carburized gear steel specimens based on strain-life approach is proposed. Multilayer method and hardness method are used for estimating strain-life fatigue properties. Rule of mixture is employed to find average cyclic stress-strain curves of carburized specimens. An approach for conversion of axial to bending fatigue data by utilizing Neuber’s rule and modifying factors is suggested. Initial validation of the analytical model is carried out against experimental results from the literature of two types of specimens with different carburizing depths. Good correlation between the predicted and experimental data is observed for both types of specimens, with nearly all data points falling within the scatter factor of three.

Failure analysis of AA8011-pultruded GFRP adhesively bonded similar and dissimilar joints

In this work, a comparative failure analysis of aluminum (AA8011/AA8011) and glass fiber reinforced polyester (GFRP/GFRP) based similar and dissimilar joints is presented. The GFRP is prepared using pultrusion technique. Single lap joints are prepared by using Araldite R2011 epoxy as an adhesive. The lap joints are then tested under tension to estimate the average shear strength of the assembly. It is observed that the average bond strength of AA8011/AA8011 is lesser than that of the GFRP/GFRP joint. The failure of similar joints occurred by fracture within the adhesive. The dissimilar joint is failed predominantly by interface debonding. Further, a detailed three dimensional stress analysis of the joints is carried out using finite element method (FEM). The damage analysis of adhesive layer is carried out by coupling FEM with cohesive zone model (CZM). The stress, damage distributions and failure mechanisms are compared for similar joints in detail. A failure mechanism is proposed for AA8011/AA8011 type joint that favours a rapid crack growth in the adhesive after crack initiation, which is responsible for lesser bond strength. The increase in overlap length has positive effect that the peak load increases proportionally with overlap length.

Theoretical analysis of fatigue failure in mechanically fastened Fibre Metal Laminate joints containing multiple cracks

Mechanically fastened joints are susceptible to the presence of multiple-site damage (MSD) cracks in the critical fastener row. Different from the MSD growth in joints consisting of metallic substrates, the two coupled metal crack growth and interfacial delamination propagation failure mechanisms in Fibre Metal Laminates (FMLs) make the prediction of fatigue behaviour in FML joints with MSD scenario burdensome and impractical when considering all factors influencing the fatigue performance. This paper presents a theoretical study on the MSD crack growth behaviour in mechanically fastened FML joints with a focus of modelling the effects of bearing and bypass loads. The proposed model in this paper is built upon analytical models dealing with MSD growth in flat FML panels and single crack growth in FML panels subjected to a combined tension-pin loading case. This model would be particularly useful for symmetric FML joints where no secondary bending effects present. A deliberately designed symmetric FML joint was tested to validate the proposed model. The model captures the rapid crack growth in the vicinity of fastener holes due to bearing stresses and crack acceleration due to the interaction of cracks. It is identified that the load redistribution between intact fastener rows and the cracked fastener row accelerates crack growth with crack length. The effects of secondary bending stresses in FML joints on the crack growth behaviour is extensively discussed. The performance of the proposed model for single lap FML joints is also examined using test data from open literature. It is found that the proposed model provides a conservative prediction for the tested single shear lap FML joint from open literature.

Study on Fatigue Degradation Behavior of a Cracked Rotor Subjected to Lateral Vibration

Fatigue crack in a rotary shaft is a common failure observed in rotor systems. Since vibration of the shaft causes alternating fatigue loads, the crack propagates slowly. Meanwhile, the propagating crack may cause nonlinear or unstable vibration of the rotor system. In fact, growth of the crack and vibration of the shaft are coupled with each other. Hence, it is necessary to study the fatigue degradation behavior of the cracked rotor accounting for this coupling effect. In this paper, a coupling model of rotor vibration and crack growth is established through dynamic theory and fracture mechanics, and a sequential iterative procedure is proposed to solve the coupling model. Then, the competing degradation failure mode of the cracked rotor is analyzed with regard to the rapid crack growth failure and the unstable vibration failure. And degradation measures are proposed based on the competing degradation failure criterion. At last, degradation behaviors with the coupling effect of nonlinear vibration behavior and multiple parameters including rotation speed, unbalance eccentricity and orientation angle, and damping are investigated by numerical simulation. The results indicate that nonlinear vibration behavior and multiple parameters have considerable influence on the degradation behaviors, which present complex regularity. The findings are of significance to guide the safety design of the rotor system for long time operation and help to the further research on prognostics and lifetime prediction.

Study of magnetic field distribution in anisotropic single twin-boundary magnetic shape memory (MSM) element in actuators

Magnetic shape memory effect exhibited by certain alloys at room temperature is known for almost 20 years. The most studied MSM alloys are Ni-Mn-Ga alloys which exhibit up to 12% magnetic field-induced strain (change in shape) depending on microstructure. A multibillion cycle operation without malfunction along with their “smart” properties make them very promising for application in electromagnetic (EM) actuators and sensors. However, considerable twinning stress of MSM crystals resulting in magneto-mechanical hysteresis decreases the efficiency and output force of MSM actuators. Whereas twinning stress of conventional MSM crystals has been significantly decreased over the years, novel crystals with Type II twin boundaries (TBs) possess even lower twinning stress. Unfortunately, the microstructure of MSM crystals with very low twinning stress tends to be unstable leading to their rapid crack growth. Whilst this phenomenon has been studied experimentally, the magnetic field distribution in anisotropic single twin-boundary MSM elements has not been considered yet. This paper analyses the magnetic field distribution in two-variant single twin-boundary MSM elements and discusses its effects on magnetic field-induced stress acting on the twin boundary.

Low-cycle fatigue properties and life prediction of Al-Si piston alloy at elevated temperature

The influences of temperature on the microstructure evolution, tensile properties, especially low-cycle fatigue (LCF) behaviors and damage mechanisms of Al-Si piston alloy have been investigated in this paper. The results show that the alloy exhibits cyclic softening at high-temperature. Fatigue cracks usually initiate from primary silicon phase and preferentially grow along particles in a slightly zigzag path at relatively low temperature. With temperature increasing, however, the ductile tearing fracture through micro-cracks can be found. In order to evaluate the fatigue life, considering the temperature and loading conditions, a comprehensive 3-parameter model based on hysteresis energy has been proposed; at a constant temperature the fatigue life can be controlled by two parameters, i.e., the intrinsic fatigue toughness W0 (the resistance to crack propagation) and the fatigue cracking exponent β (the resistance to fatigue cracking), which dominate the LCF damage mechanisms (from fatigue-induced particle cracking to rapid fatigue crack growth). For the current Al-Si alloy, the combined effect of W0, β and temperature T can lead to an optimal fatigue life at a critical temperature. This model provides a new clue for optimizing and designing the high-temperature materials.

In situ observation of ultrasonic cavitation-induced fragmentation of the primary crystals formed in Al alloys

The cavitation-induced fragmentation of primary crystals formed in Al alloys were investigated for the first time by high-speed imaging using a novel experimental approach. Three representative primary crystal types, Al3Ti, Si and Al3V with different morphologies and mechanical properties were first extracted by deep etching of the corresponding Al alloys and then subjected to ultrasonic cavitation processing in distilled water. The dynamic interaction between the cavitation bubbles and primary crystals was imaged in situ and in real time. Based on the recorded image sequences, the fragmentation mechanisms of primary crystals were studied. It was found that there are three major mechanisms by which the primary crystals were fragmented by cavitation bubbles. The first one was a slow process via fatigue-type failure. A cyclic pressure exerted by stationary pulsating bubbles caused the propagation of a crack pre-existing in the primary crystal to a critical length which led to fragmentation. The second mechanism was a sudden process due to the collapse of bubbles in a passing cavitation cloud. The pressure produced upon the collapse of the cloud promoted rapid monotonic crack growth and fast fracture in the primary crystals. The third observed mechanism was normal bending fracture as a result of the high pressure arising from the collapse of a bubble cloud and the crack formation at the branch connection points of dendritic primary crystals. The fragmentation of dendrite branches due to the interaction between two freely moving dendritic primary crystals was also observed. A simplified fracture analysis of the observed phenomena was performed. The specific fragmentation mechanism for the primary crystals depended on their morphology and mechanical properties.

Analysis of fatigue crack propagation behaviour in SiC particulate Al2O3 whisker reinforced hybrid MMC

The fatigue crack propagation behaviour of a cast hybrid metal matrix composite (MMC) was investigated and compared with the crack propagation behaviour of MMC with Al2O3 and Al alloy in this article. Three dimensional (3D) surface analysis is carried out to analyze the crack propagation mechanism. All three materials clearly show near threshold and stable crack growth regions, but the rapid crack growth region is not clearly understood. The crack propagation resistance is found higher in hybrid MMC than that of MMC with Al2O3 whisker and the Al alloy in the low ΔK region. The crack propagation in the hybrid MMC in the near-threshold region is directed by the debonding of reinforcement-matrix followed by void nucleation in the Al alloy matrix. Besides, the crack propagation in the stable- or midcrack-growth region is controlled by the debonding of particle-matrix and whisker-matrix interface caused by the cycle-by-cycle crack growth along the interface. The transgranular fracture of the reinforcement and void formation are also observed. Due to presence of large volume of inclusions and the microstructural inhomogeneity, the area of striation formation is reduced in the hybrid MMC, caused the unstable fracture.

Mechanical properties and failure mechanism of cemented red clay under chemical erosion environment

This paper presents an experimental investigation on the strength behaviour of red clay improved by cement, considering initial water content (ω), cement content and curing pH levels. Along with the unconfined compressive tests, acoustic emission signals were detected to investigate the compression failure. It was shown that unconfined compressive strength (UCS) of the cement red clay rose first and fell later with water content increasing, and maximum UCS was reached at ω = 75%. As cement content increased from 15% to 30% under water content of 75%, UCS of blocks increased from 1.03 MPa to 6.09 MPa. Stress-strain curves of blocks curing in acid condition were divided into four stages: microcracks closure, elastic deformation and stable crack propagation, rapid crack growth and final failure. Acoustic emission signals detected in compression process showed that a large number of microcracks occurred in the second stage with more in the third stage.

Dynamic stress intensity factors for suddenly loaded structures using enriched finite elements

In this paper, an efficient finite element formulation for stationary cracks subjected to dynamic impact loading is presented. For impact problems, where wave propagation effects dominate, the onset of rapid crack growth is strongly influenced by inertia effects, including stress wave reflections from geometric boundaries. Dynamic stress intensity factors generally attain maximum values that can be many times greater than their static counterparts. Because of this, there is a strong motivation for developing efficient computational techniques to evaluate this type of engineering problem in a relatively straightforward manner. In order to analyze the dynamic stress intensity factor problem efficiently, a computational technique that does not require a special crack tip mesh is of considerable value. The enriched finite element approach is shown to be a practical and effective technique for obtaining dynamic stress intensity factors, especially for cracks located on bimaterial interfaces, where there is inherent coupling between the mode I and mode II stress intensity factors. The enriched crack tip element approach utilizes the analytic asymptotic crack tip fields to directly compute the stress intensity factors. In this paper, fracture problems known to have two different types of crack tip singularities, subjected to elastic wave propagation effects during impact loading, are given as examples.

Synthesis <i>In Situ</i> Composite TiAl-Based with Particulate Al<sub>2</sub>O<sub>3</sub> Reinforcement by Powder Metallurgy Route

A major effort has been made over last 20 or so years to introduce TiAl-based alloys into the market-place as engineering component. Recently, titanium aluminide based composites are competitive candidate for aerospace use due to the favorable properties of matrix material, such as low density, high specific strength and relative good properties at elevate temperature [1-4]. The amount of aluminum in titanium alloys exceeds that used in conventional alloys and can range from 10 to 48at%. This concentration of aluminum allows the formation of anin-situalumina reinforcement which is responsible for the excellent oxidation, sulfidation and carburization resistance at temperatures of 1000°C and higher. However, their brittleness and rapid crack growth rate at low to intermediate temperature hinders their application/3/. Nevertheless, nanostructure of monolithic TiAl is unstable at elevate temperatures which deteriorate the high temperature properties. In order to improve the high temperature strength of intermetallic, ceramic particles can be utilized as reinforcements [4, 5]. Recently, in-situ techniques have been utilized to fabricate TiAl-Al2O3composite through displacement reaction between TiO2and Al in planetary ball milling and subsequence heat treatment. The knowledge of reaction characteristic in the Al-TiO2system is great importance to optimize the processing/4, 6/. The chemical compatibility with the iron aluminide matrix at temperature above 1000°C is an important factor for the selection of reinforcements because extreme interfacial reaction during processing results in the degradation of mechanical properties [3, 4].

Determination of critical strain for rapid crack growth during tensile deformation in aluminide coated near-α titanium alloy using infrared thermography

Determination of strain for initiation of rapid crack growth is vital for designing coated components for aerospace applications. Knowledge of such strain is useful for prevention of catastrophic failure of coated components. In the present study this critical strain has been determined during tensile deformation of aluminide coated near-α titanium alloy using infrared thermography. A single step on-line method of determination of stored energy change as a function of true plastic strain, that incorporates conduction heat loss correction in a simple way, has been used to determine such strain level. It is shown that beyond a strain level the crack ensemble that form becomes unstable and further straining leads to rapid crack growth in the coating that penetrates into the substrate material. This manifests in an unprecedented trend in the stored energy change that has been identified with the strain for inception of rapid crack growth.

Simplification of Linear Amplitude Sweep Test and Specification Parameter

The linear amplitude sweep (LAS) test has recently been approved by AASHTO as a provisional standard and is currently being considered for specification of asphalt binder fatigue resistance. The LAS test is a cyclic torsion test conducted in the dynamic shear rheometer (DSR) that uses increasing loading amplitudes to accelerate damage. The LAS test has been proposed as a surrogate to the time sweep (TS) test, which is a conventional fatigue test consisting of repeated cyclic loading at constant amplitude. The TS test is considered unsuitable for specification because of the uncertainty in the test duration required to induce and quantify fatigue. Recent ruggedness testing efforts have shown that the LAS test cannot be conducted by all DSRs. This study evaluates a simpler amplitude ramping procedure that resolved the difficulties encountered with some rheometers. Additionally, a simple specification parameter is introduced to offer a practical approach to define damage tolerance. The definition of failure is based on a more recent study to explain the mechanisms of fatigue damage under TS testing in the DSR. On the basis of a comparison between TS and LAS test results, this paper demonstrates that the damage evolution in the two tests differs, thus indicating that the LAS test is very complex and is better referred to as a damage tolerance test rather than a true fatigue test. With the simplified analysis approach for the modified LAS procedure, a critical cracking length signifying the start of rapid crack growth is proposed for specification of binder damage tolerance at intermediate temperature.

Distribution of microcrack lengths in bone in vivo and in vitro

It is well known that bone contains small cracks; in vivo these microcracks are constantly growing and being repaired. Too rapid crack growth leads to stress fractures or fragility fractures. In vitro, changes occur in this population of microcracks when subjected to cyclic loading up to and including failure. Normally, the only parameters reported from such investigations are the number density of cracks and their average length. In the present work we examined the microcrack population in more detail. We analysed ten different sets of experimental data including in vivo and in vitro microcracks, plus two theoretical simulations. We showed for the first time that the distribution of crack lengths can be described using the two-parameter Weibull equation. The values of the two constants in the equation varied depending on bone type/species and showed consistent trends during in vitro testing. This is the most detailed study to be conducted on microcrack populations in bone; the results will be useful in future studies including the development of theoretical models and computer simulations of bone damage and failure.