Crack Initiation Strain Research Articles

Effects of Microstructure Characteristics of Cortical Bone on its Microcrack Propagation

Scanning electron microscope (SEM) was used to observe and analyze the microstructure of the cross section of cortical bone. The observation results illustrated that the cortical bone is composed of cylindrical osteons and interstitial bone between osteons, and the osteon are unevenly distributed. Based on the microstructure characteristics of cortical bone, three types of cortical bone mesoscopic analysis models were established. Then, the extended finite element method (X-FEM) was used to simulate the microcrack propagation process in bone. The simulate results show that the crack initiation strain of the two-phase model is 19.1% larger than that of the single-phase model, and the three-phase model is 57.8% larger than that of the two-phase model, which demonstrated that the osteons and cement line can significantly enhance the crack initiation strain of bone. In addition, under the same boundary conditions, the model with cement line can effectively change the propagation path of microcrack and prevent the propagation of crack. Therefore, the cement lines in cortical bone can effectively increase the fracture resistance of bone and enhance the fracture toughness of cortical bone.

Anisotropic characteristics of crack initiation and crack damage thresholds for shale

The inherent anisotropy is the most important structural feature of shale. Most research on the mechanical behavior of shale has focused on its anisotropic strength, deformation and failure patterns. Few studies have examined the anisotropic micro-failure mechanism and failure process of shale. To investigate the anisotropic crack initiation mechanism and evolution of shale during the progressive rock failure process, two stress thresholds (i.e., crack initiation stress (σci) and crack damage stress (σcd)) and two strain thresholds (i.e., crack initiation strain and crack damage strain) of shale under various bedding layer orientations and confining pressures are identified on the basis of the crack volumetric strain. The anisotropic characteristics of the four thresholds are described. The crack initiation stress can be seen as independent of the bedding layer orientation, but the other thresholds are strongly dependent on the bedding layer inclined angle. The physical mechanism of this phenomenon is analyzed in detail. The anisotropic features of the mutual relationships among the peak stress (σc), σci and σcd are presented. The ratios σci/σc and σci/σcd at confining pressures greater than 30 MPa and the ratio σcd/σc increase first and then decrease with the increasing inclination angle. A high confining pressure constraints shale from entering the unstable crack propagation stage but has little effect on preventing crack propagation once it has entered this stage. The anisotropic characteristics of these thresholds can provide meaningful information for understanding the anisotropic deformation and failure mechanism of shale.

Micro-Crack Induced Buckypaper/PI Tape Hybrid Sensors with Enhanced and Tunable Piezo-Resistive Properties

Piezoresistive properties play a vital role in the development of sensor for structural health monitoring (SHM) applications. Novel stable crack initiation method (SCIM) is established to improve the gauge factor (GF) with maximum achievable working strain region for PI tape enabled buckypaper hybrid sensors. Cracks are generated by applying strain rate-controlled tension force using dynamic mechanical analyzer (DMA). The sensor has been cycled in tension to characterize GF with crack opening. It is determined experimentally that GF increases with increasing crack opening and crack becomes unstable when opening increases above 8 µm. Tremendous improvement in GF has been observed which improved from single-digit to several hundreds. The highest GF obtained so far is ~255, showing 75 times improvement compared with the ones without the SCIM implementation. The crack initiation strain (CIS) is characterized by sonication and centrifugation time. It is determined experimentally that the maximum CIS of 3.5% can be achieved with sonication time of 40 min and increasing centrifuge time has an in-significantly dropping effect on CIS. Excellent stability/reproducibility has been proved/demonstrated on SCIM implemented sensors through a rigorous 12,500 tensile cycle test on DMA. The performance of sensor is practically demonstrated in tension and bending on glass fiber reinforced polymer (GFRP) structures.

Crack evolution behavior of rocks under confining pressures and its propagation model before peak stress

The understanding of crack propagation characteristics and law of rocks during the loading process is of great significance for the exploitation and support of rock engineering. In this study, the crack propagation behavior of rocks in triaxial compression tests was investigated in detail. The main conclusions were as follows: 1) According to the evolution characteristics of crack axial strain, the differential stress-strain curve of rocks under triaxial compressive condition can be divided into three phases which are linear elastic phase, crack propagation phase, post peak phase, respectively; 2) The proposed models are applied to comparison with the test data of rocks under triaxial compressive condition and different temperatures. The theoretical data calculated by the models are in good agreement with the laboratory data, indicating that the proposed model can be applied to describing the crack propagation behavior and the nonlinear properties of rocks under triaxial compressive condition; 3) The inelastic compliance and crack initiation strain in the proposed model have a decrease trend with the increase of confining pressure and temperature. Peak crack axial strain increases nonlinearly with the inelastic compliance and the increase rate increases gradually. Crack initiation strain has a linear relation with peak crack axial strain.

Effect of Applied Stress on T91 Steel Performance in Liquid Lead at 400 °C.

The environmental performance of structural materials (e.g., corrosion and environmental assisted cracking) is one of the critical issues being addressed in the development of Generation IV fast reactors. This work aims to support the study of the performance of the ferritic-martensitic steel T91 in liquid lead, under close to operation conditions, in order to assess its suitability for an application in lead fast reactors. T91 steel was tested in air and liquid lead at 400 °C using static and slow loading regimes. Applied stresses were chosen to be slightly above the yield strength in order to evaluate the threshold stress and strain for crack initiation. Three-point bending static exposure and constant extension tests of tapered specimens were performed. Post-test surfaces and cross sections of specimens were observed using scanning electron microscopy techniques in order to detect cracks and to analyze oxide layers. The effect of strain rate of the oxide layer cracking was observed. In conclusion, T91 was not susceptible to liquid metal embrittlement, a special case of environmentally assisted cracking under the testing conditions. The cracking conditions are discussed based on previous experience.

Loading rate effect on crack velocity in ultra-high-performance fiber-reinforced concrete

Loading rate effect on crack propagation in ultra-high-performance fiber-reinforced concrete (UHPFRCs) was investigated using a pre-notched three-point bending specimen in an improved-strain energy frame impact machine (I-SEFIM) and image processing techniques. The crack velocity of up to 984 m/s and the crack initiation strain rate of up to 271 s−1 were observed. Crack velocity in UHPFRCs increased as the applied strain rate increased. Fiber reinforcements significantly affected on the crack velocity in the UHPFRC at static rates, but slightly did at high strain rates. There is a strong correlation between the strain-rate sensitivity and the dynamic crack growth characteristics of UHPFRCs.

Development of Fatigue Crack Growth Prediction Model in Reactor Coolant Environment

In order to conduct effective and rational maintenance activity of components in nuclear power plants, it is proposed to manage fatigue degradation based on crack size corresponding to an extent of cumulative fatigue effect. This study is aimed at developing a prediction model for fatigue crack growth in simulated reactor coolant environment. In order to investigate influence of reactor coolant environment on crack initiation and crack growth, two-step replica observations were conducted for environmental fatigue test specimens (type 316 stainless steel) subjected to three kinds of strain range. Crack initiation, growth, and coalescence were observed in the experiments. It is clarified that crack coalescence is one of the dominant factors causing fatigue life reduction, and fatigue life reduction depends on crack size and distance of two coalescing cracks. Then, a model was developed for predicting statistical crack initiation and growth behavior. The relationship between dispersion of crack initiation life and strain range was approximated by the Weibull model to predict crack initiation. Then, the statistical crack growth was modeled using the relation of crack growth rate and strain intensity factor. Furthermore, the crack coalescence was taken into account to the crack growth prediction considering the distance between two cracks. Finally, the crack growth curve, which is the relationship between crack size and operation period, was derived through Monte Carlo simulation with the developed model. The crack growth behavior and residual life in the simulated reactor coolant environment can be reviewed by the crack growth curve obtained with crack initiation, and the growth model developed was compared with the fatigue test results.

Effect of Heat Input on the Tensile Damage Evolution in Pulsed Laser Welded Ti6Al4V Titanium Sheets

The present paper is focused on studying the effect of heat input on the tensile damage evolution of pulsed Nd:YAG laser welding of Ti6Al4V alloy under monotonic loading. To analyze the reasons that the tensile fracture site of the pulsed-laser-welded Ti6Al4V sheet joints changes with the heat input under monotonic loading, the microstructure of the sample with different nominal strain values was investigated by in situ observation. Experiment results show that the tensile ductility and fatigue life of welded joints with low heat input are higher than that of welded joints with high heat input. Under tensile loads, the critical engineering strain for crack initiation is much lower in the welded joint with high heat input than in the welded joints with low and medium heat input. And the microstructural damage accumulation is much faster in the fusion zone than in the base metal for the welded joints with high input, whereas the microstructural damage accumulation is much faster in the base metal than in the fusion zone for the welded joints with low input. Consequently, the welded joints fractured in the fusion zone for the welds with high heat input, whereas the welded joints ruptured in the base metal for the welds with low heat input. It is proved that the fine grain microstructure produced by low heat input can improve the critical nominal strain for crack initiation and the resistance ability of microstructural damage.

Bending strength of adhesive joints in microelectronic components: Comparison of edge-bonding and underfilling

The bending strength of underfilled and edge-bonded ball grid array (BGA) microelectronic packages assembled on printed circuit boards (PCBs) was compared using double cantilever beam (DCB) specimens. All specimens with fillets of the same size and shape failed at the same load, with cracks initiating and propagating within the PCB. This was consistent with measurements of the crack initiation strain energy release rate for PCB interfacial failure, which was significantly smaller than that of cohesive failure within the adhesives. Finite element analysis (FEA) indicated that the stress state in the PCB near the PCB-fillet interface in both underfilled and edge-bonded specimens was only a function of the adhesive fillet size and shape, and independent of the extent of the adhesive layer between the PCB and the BGA, and independent of the adhesive mechanical and thermal properties over the broad range of properties of the tested adhesives. This explained why decreasing the fillet curvature in edge-bonded specimens produced a significant increase in the joint strength. The crack path in the PCB of the edge-bonded specimens was found to change with the adhesive cure temperature; however, this had a negligible effect on the failure load.

Crack propagation speed in ultra high performance concrete (UHPC)

This paper investigates crack speed in ultra-high performance concrete (UHPC) using pre-notched three-point bending specimens. The experimental parameters are fiber volume fraction and rate of loading. A hydraulic servo-controlled testing machine is used to apply lower notch tip strain rates, in the range of 0.025–1.01/s, while a newly developed impact testing system is used to achieve higher notch tip strain rates, ranging from 6.8 to 41.11/s. A high-speed camera is used to record images of the UHPC specimens during testing. Notch tip strain and crack speed are computed from the images, which show that crack speed increases asymptotically as the crack initiation strain rate increases. Crack speeds of up to 514m/s were achieved at the lower notch tip strain rates and up to 1454m/s for the higher notch tip strain rates. The achieved relationships are incorporated into a recently proposed crack-velocity dependent dynamic fracture model. The model is validated using published experimental data and used to show that, like conventional concrete, the strain rate sensitivity of UHPC is strongly associated with the characteristics of dynamic crack growth.

Effect of microstructure on oxygen rich layer evolution and its impact on fatigue life during high-temperature application of α/β titanium

The near alpha titanium alloy, Ti-6424S, is utilized in many critical high-temperature aerospace components due to its unique properties. However, oxygen ingress during elevated-temperature exposure induces formation of a subsurface brittle oxygen-rich layer (ORL), resulting in a deterioration of mechanical performance. This paper, for the first time, establishes the effect of the underlying microstructure on the formation and evolution of the ORL in α/β titanium alloys. In addition, models were developed to predict (i) the evolution of ORL as a function of the material microstructure, (ii) the effect of ORL on the critical strain for in-service crack initiation, and (iii) estimates of fatigue life of components made from a specific microstructure during in-service high temperature exposure and formation of ORL. In particular, five different microstructures were produced by tailored heat-treatments and thermally exposed at 650 °C up to 420 h. The base metal and the ORL were quantified using microhardness indentations, optical microscopy, and scanning electron microscopy (electron backscatter diffraction (EBSD), backscattered electron (BSE), and secondary electron (SE) imaging). The effective diffusion coefficients (Deff) for each microstructure were calculated and then integrated into a critical strain model to predict crack initiation strain as a function of exposure time. The predicted ORL thickness was used to estimate fatigue life using experimentally measured crack growth data. The largest Deff coefficient was observed in a colony microstructure, while a basketweave microstructure showed the smallest Deff. For several bimodal microstructures, Deff was noted to increase with increasing area fraction of secondary alpha colonies.

Transition between mild and severe wear in titanium alloys

The mild to severe wear transition of two titanium alloys, Ti–10V–2Fe–3Al (a near-β titanium alloy) and Ti–6Al–4V (an α+β titanium alloy) was investigated and the correlation between the microstructure and wear micromechanisms was studied. Mild wear was dominated by oxidation via the formation and spallation of mechanically mixed layers (MMLs). The near-β alloy exhibited lower wear compared with α+β alloy in the mild wear region due to more stable MMLs as evidenced by 16% higher thickness of layer. Severe wear was characterized by strain localization and subsurface crack growth. Ti–6Al–4V showed lower transition load and lower resistance to unstable plastic deformation. However, Ti–10V–2Fe–3Al exhibited higher critical shear strain for subsurface crack initiation.

Prestress Driven Improvement in Fracture Behavior of in Situ Sputtered Zinc Oxide Thin Films on Stretched Polymer Substrates.

Flexible electronic devices need to survive bending or stretching operation without mechanical failure. If inorganic thin films are involved in the device structure, the evolution of cracks is a major challenge to overcome. Here, we report a novel way to substantially improve the fracture behavior of films that are based on intentional utilization of residual stress on the films by in situ sputtering on a stretched polymer substrate. The in situ sputtering combined with a stabilization stage yielded ZnO:Al thin films with a nearly 2-fold improvement in crack initiation strain, which indicates greater resistance to bending. The critical strain of the optimal ZnO:Al films was ∼1.83%, which is a significant improvement compared to the current tolerance value of ∼1%. This was accompanied by a ∼300% improvement in fracture energy. We attributed the improved fracture behavior to the presence of residual compressive stresses, which creates a barrier for crack formation by acting opposite to the applied bending strain.

Crack initiation stress and strain of jointed rock containing multi-cracks under uniaxial compressive loading: A particle flow code approach

The ratio of crack initiation stress to the uniaxial compressive strength (S CI,B/S UC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength $$\left( {S_{A,S_{CI,B} } /S_{A,S_{UC,B} } } \right)$$ were studied by performing numerical stress analysis on blocks having multi flaws at close spacing’s under uniaxial loading using PFC3D. The following findings are obtained: S CI,B/S UC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°, $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found between $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ and β; however, the average $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ seems to slightly decrease with increasing k. The variability of $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ is found to increase with k. Based on the cases studied in this work, $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ ranges between 0.3 and 0.5. This range is quite close to the range of 0.4 to 0.6 obtained for S CI,B/S UC,B. The highest variability of ± 0.12 for $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ is obtained for k=0.8. For the remaining k values the variability of $$S_{A,S_{CI,B} } /S_{A,S_{UC,B} }$$ can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of S CI,B/S UC,B.

Loading rate effect on crack velocities in steel fiber-reinforced concrete

This paper presents the recent experimental results aimed at studying the loading rate effect on the crack velocity in steel-fiber reinforced concrete. Three-point bending tests were performed by adopting either a servo-hydraulic testing machine or a drop-weight impact instrument. Four strain gauges glued along the ligament of the beam were used to measure the crack velocity and, the fifth one was bonded right in front of the notch root for detecting the crack initiation strain rate. Six different loading rates were applied, from 10−3 mm/s to 103 mm/s, i.e., a low loading rate range (3.33 × 10−3 mm/s, 0.10 mm/s and 3.33 mm/s) and a high loading rate range (8.85 × 102 mm/s, 1.77 × 103 mm/s and 2.66 × 103 mm/s). At quasi-static loading condition, 3.33 × 10−3 mm/s, the crack velocity almost keeps constant at 10−4 m/s, while the main crack propagates with decreasing velocity at other loading rates. Furthermore, the crack initiation strain rate is proportional to the loading rate, varied from 10−6/s to 101/s when the loading rate is less than 1.77 × 103 mm/s.

J0430102 引張負荷を受ける2-D溶射補修材の有限要素解析([J043-01]厚膜形成技術と厚膜の機械特性評価(1),機械材料・材料加工部門)

In this study, capability and reliability of advanced repairing technology based upon thermal spraying technique were examined. In order to achieve this purpose, residual stress and the critical strain up to the delamination in the part were examined experimentally and identified by numerical analysis. For the tensile testing, the repairing geometry was assumed to be four types of the cutting angle (20, 30, 40, 50deg) under the same depth and length. Afterward, we carried out FEM analysis about the same condition as examination. As the results, crack initiation strain and delamination strain were increased with cutting angle in the repaired part. On the other hand, residual stress was almost constant in compressive side. FEM analysis revealed that the crack initiates at the corner in the repairing part and propagates along the interface.

The effect of welding conditions on solidification cracking susceptibility of type 310S stainless steel during laser welding using an in-situ observation technique

Solidification cracking occurs easily at high welding speeds, and should therefore occur more easily during laser welding. Both the solidification behavior and thermal strain change depend on the welding speed, and therefore, the critical strain for solidification cracking must be measured to clarify the factors influencing the solidification cracking susceptibility. However, the critical strain required for solidification cracking under high welding speed conditions has not yet been determined. The aim of this work was to investigate the effect of welding speed on the solidification cracking susceptibility of Type 310S stainless steel. U-type hot cracking tests were conducted using a developed in-situ observation technique with high-speed camera, and the critical strain for solidification cracking was evaluated quantitatively. The critical strain for solidification crack initiation decreased with increasing welding speed. The distribution of residual liquid depended on the microstructure, and the morphological distribution of the residual liquid changed from a droplet to a thin film with increasing welding speed. The transition in distribution morphology of the residual liquid implies the material is susceptible to solidification cracking.

Failure process analysis of spalling failure—Comparison of laboratory test and numerical modelling data

This paper presents a comparison of laboratory tests and numerical modelling results for evaluation of spalling failure in hard rock. The aim of this paper was to evaluate if the failure and deformation (i.e., strain at crack initiation, crack damage and peak strength stages) process in the zone of spalling failure of two real excavations can be predicted by using strain data from rocks tested in laboratory. Data such as axial strain and lateral strain quantities from uniaxial compressive laboratory test performed on Fennoscandian hard rocks were used. Numerical modelling of these excavations using Phase2 was conducted. Volumetric strain and maximum shear strain were chosen as indicators of the depth and shape of the spalling failure developing in the walls and roof of the excavations. Quantities within the spalling failure zone such as volumetric strain, maximum shear strain, and major and minor principal strain, were calculated, and compared to the failure-deformation data of limestone and quartzite (from laboratory tests). This paper shows that the failure process of hard rock excavations can be predicted by using laboratory test data. This paper further demonstrated that the onset of cracking calculated from laboratory tests can be related to the in situ crack initiation strain.

結合力要素法による薄層CFRP直交積層板のき裂進展抑制メカニズムの検討

In this study, the crack suppression mechanism in the thin-ply laminate was discussed by the mesoscopic finite element analysis. The intraply crack extension in the thin ply 90° layer, which has the thickness from 40 to 150 μm, was simulated with the cohesive element. In addition, the effect of initial defect on the crack extension behavior was also discussed. The crack extension behavior showed good agreement with the experimental results in any ply thickness for 6 J/m2 was applied to the cohesive energy. The crack initiation strain was not influenced by the existence of the initial defect up to 10 μm length. However the crack initiation strain was not influenced by the ply thickness, the crack penetration strain tended to increase with decrease of ply thickness.

Ni基超合金Inconel 718の固相線温度近傍における力学特性取得に関する検討

Nickel-based superalloys have been applied to gas turbine and aircraft jet engine parts due to superior high temperature strength and corrosion resistance. However, casting defect such as solidification cracking often occurs. In order to increase productivity of precision casting and various shape casting processes, predicting the occurrence of solidification cracking by using CAE(Computer Aided Engineering) should be essential. Therefore, it is necessary to obtain mechanical properties in the state of solid-liquid coexistence. In this study, we try to get high temperature (around solidus temperature) mechanical properties of the nickel-based superalloy such as Inconel 718 by using originally developed tensile test. In the previous reports, flat distribution of temperature in the gage length, and crack initiation strain were not ensured. On the other, the developed device ensured within 3℃ of the temperature distribution in the gage length as to the Inconel 718. Tensile strain was measured by using in-situ observation of marker on the surface of the specimen. As a result, Stress-Strain curve, crack initiation stress, and crack initiation strain of Inconel 718 were obtained.