Microcrack Growth Behavior Research Articles

Investigation of surface micro-crack growth behavior of asphalt mortar based on the designed innovative mesoscopic test

The objective of this study is to design an innovative mesoscopic test for characterizing the damage evolutions and fracture performances of the asphalt mortar. Based on the SEM-Servopulser devices and improved specimens, the pure mode I cracking tests were conducted successfully under scanning electron microscope (SEM). It can have a real-time online observations and records of the surface micro-crack behaviors and evolutions under different loads. And it is the first time in road engineering to capture the dynamic micro-behavior and micro-structure of the bituminous materials in such high accuracy (μm level), especially the synchronous recording when applying loads without stopping on-going tests. This proposed method will have significant meaning in revealing the fracture mechanism of the bituminous materials since the traditional specimen sizes and the self-healing property of the asphalt hinder the further micro-scale investigations in this field. With the help of the pioneering designed tests, the damage variable based on the micro-crack density was proposed and was verified by the experimental results. It is found that the interface failure dominated during the overall process in pure mode I cracking tests, which should be paid more attention on in the future material design.

Microcrack Growth Behavior and CWFS Criterion Parameters Optimization of Granite with PFC

Abstract Based on the physical and mechanical parameters obtained in laboratory tests, along with the section scanning of granite specimens, a microgeometric model of the granite particles was established in the Particle Flow Code (PFC) programs. The loading codes were programed and adjusted using the Fish language to simulate both uniaxial and triaxial compression tests. The micromechanical behavior and the evolution of cracks from the microcosm to the macrocosm of the granite were obtained under different loading conditions based on the complete stress-strain curves, Acoustic Emission (AE) monitoring results and crack number obtained from the tests, and PFC simulation. On this basis, the Cohesion Weakening and Friction Strengthening (CWFS) criterion model parameters of the granite were optimized and verified using the stress-strain curves, the relationship between the cracks, friction energy and plastic strain, and Fast Lagrangian Analysis of Continua simulation. The optimized CWFS criterion parameters of initial cohesion, residual cohesion, initial friction angle, and residual friction angle, as well as the critical plastic strain ϵcp and ϵfp of the granite, are 23 MPa, 4.3 MPa, 0°, 46.3°, 0.0015, and 0.0037, respectively. This provides a scientific basis for the establishment of a rock constitutive model during the transfer from open pit to underground mining in the Xingshan Iron Mine in China.

Influence of nano-silica on the failure mechanism of concrete specimens

Failure of basic structures material is usually accompanied by expansion of interior cracks due to stress concentration at the cracks tip. This phenomenon shows the importance of examination of the failure behavior of concrete structures. To this end, 4 types of mortar samples with different amounts of nano-silica (0%, 0.5%, 1%, and 1.5%) were made to prepare twelve 50x50x50 mm cubic samples. The goal of this study was to describe the failure and micro-crack growth behavior of the cement mortars in presence of nano-silica particles and control mortars during different curing days. Failure of mortar samples under compressive strength were sensed with acoustic emission technique (AET) at different curing days. It was concluded that the addition of nano-silica particles could modify failure and micro-crack growth behavior of mortar samples. Also, monitoring of acoustic emission parameters exposed differences in failure behavior due to the addition of the nanoparticles. Mortar samples of nano-silica particles revealed stronger shear mode characteristics than those without nanoparticles, which revealed high acoustic activity due to heterogeneous matrix. It is worth mentioning that the highest compressive strength for 3 and 7 test ages obtained from samples with the addition of 1.5% nano-silica particles. On the other hand maximum compressive strength of 28 curing days obtained from samples with 1% combination of nano-silica particles.

Effect of helium on fatigue crack growth and life of reduced activation ferritic/martensitic steel

The effects of helium on the fatigue life, micro-crack growth behavior up to final fatigue failure, and fracture mode under fatigue in the reduced activation ferritic/martensitic steel, F82H IEA-heat, were investigated by low cycle fatigue tests at room temperature in air at a total strain range of 0.6–1.5%. Significant reduction of the fatigue life due to helium implantation was observed for a total strain range of 1.0–1.5%, which might be attributable to an increase in the micro-crack propagation rate. However, the reduction of fatigue life due to helium implantation was not significant for a total strain range of 0.6–0.8%. A brittle fracture surface (an original point of micro-crack initiation) and a cleavage fracture surface were observed in the helium-implanted region of fracture surface. A striation pattern was observed in the non-implanted region. These fracture modes of the helium-implanted specimen were independent of the strain range.

Effect of specimen shape on micro-crack growth behavior under fatigue in reduced activation ferritic/martensitic steel

The effect of specimen shape on the micro-crack growth behavior of F82H was investigated by the low cycle fatigue test at room temperature in air at the total strain range of 0.4–1.5% using three types of the miniature hourglass-type specimens with different stress concentration factors. In case of the high strain range conditions above 0.8%, almost no effect of the specimen shape on the fatigue life, micro-crack initiation life and the crack growth rate was observed. In case of the low strain range conditions below 0.6%, the effect of specimen shape on the fatigue life was observed. The fatigue life was considered to be dependent on the stress concentration factor. Based on the micro-crack growth analysis, the difference of the fatigue life under the low strain range conditions was attributed not to the crack growth rate but to the micro-crack initiation life dependent on the specimen shape.

Micro-crack growth behavior and life in high temperature low cycle fatigue of blade root and disc joint for turbines

Low cycle fatigue tests were carried out at a temperature of 600 °C using a component specimen of 12%-Cr steel, which simulates a blade root and disc joint for turbines. The growth behavior of micro-cracks in the joint region of the specimens was investigated to clarify the damage mechanism of blade-root joints used in high temperature environments and to improve life assessment methods using finite element analysis. Micro-crack growth behavior similar to that in smooth bar specimens was observed in the specimens tested under conditions of relatively high total strain. Micro-cracks initiation was observed at the notch region of the specimens at an early stage. The crack growth rate increased with surface crack length. The fatigue life of the component specimens under this condition was similar to that of smooth bar specimens. Meanwhile, the component specimens tested under conditions of relatively low total strain showed a different growth behavior. No cracks were observed at the notch region and some micro-cracks were initiated at the edge of the contact region of the specimens in the early stages. Almost no increase in the crack growth rate was observed. Life of the component specimens under this condition was shorter than that of the smooth bar specimens. This might be attributed to fretting fatigue at the contact edge and to mean stresses.

Microcrack growth and fatigue behavior of a duplex stainless steel

The kinetics of microcrack growth during cycling has been studied in a S32205 duplex stainless steel in the as-received and aged (100 h at 475 °C) conditions. Cylindrical specimens with a shallow notch were subjected to a constant plastic strain range of 0.3% in both thermal conditions. The characteristic features of surface damage and crack growth showed striking differences in microcrack density, nucleation location and propagation rate between the two thermal conditions even though the fatigue lives are comparable. In the as-received material, microcrack density is low and they nucleate mainly at grain and phase boundaries or second-phase particles. In the aged condition, slip markings first appear in the ferritic phase and they are the preferred site for microcrack nucleation. Crack propagation takes place along slip markings in adjacent grains for crack lengths less than 100 μm. A comparison between fatigue life and the relevant parameters of a microcrack growth law was made.

1337 ラチェット疲労における微小き裂の成長挙動と寿命評価(S21-6 ステンレス鋼とチタン合金の疲労,S21 金属材料の疲労特性と破壊機構)

Micro-crack growth behaviors in ratcheting fatigue tests were investigasted to discuss the fatigue life reduction mechanism in ratcheting conditions. Ratcheting fatigue tests for 316FR stainless steel were conducted at 550C in order to discuss the design criteria for the commercialized fast reactor system. Fatigue lives decrease with increasing the accumulated strain, but the life reduction was negligible when the accumulated strain was less than 2.2%. Micro-crack growth behavior can be devided into initiation and growth stage. The crack initiation length was approximately 0.2mm. The initiation life was almost negligible when the fatigue life decreased by ratcheting. The micro-crack initiation life shortening was considered as the mechanism of fatigue life reduction in ratcheting conditions.

銅合金のき裂発生および微視的き裂の伝ぱ挙動(金属組織と強度評価 OS.1)

銅合金のき裂発生および微視的き裂の伝ぱ挙動(金属組織と強度評価 OS.1)

Investigation of surface fatigue microcrack growth behavior of cast Mg–Al alloy

To understand the fatigue microcrack growth mechanisms of a cast AM50 alloy, fatigue microcrack growth experiments were carried out with online observation using a scanning electron microscope (SEM). The results concerning the fatigue microcrack propagation under higher stress levels ( σ max=0.86−1.00 σ 0.2) displayed that microcracks propagate mainly along grain boundaries or interdendritic regions of α-Mg grains. Fatigue crack initiation also occurs on the grain boundaries near the root of a notch because of stress concentration, and fatigue crack initiation rarely occurs at Mg 17Al 12 (β-phase) regions within the α-grains. The fatigue crack propagates microscopically in a zigzag manner. A method for estimating the fatigue microcrack growth rate was proposed in terns of the crack tip opening displacement (CTOD).

Ｎｉ基超合金Ｈａｓｔｅｌｌｏｙ‐Ｘ溶接材の高温多軸低サイクル疲労における微小き裂発生と進展挙動

Tensile-torsional-combined biaxial low-cycle fatigue tests on welded tubular specimens of the Ni-base superalloy Hastelloy-X were carried out and the micro-crack growth behavior in the weldments was investigated with the aim of improving life assessment methods for high-temperature components. Welded hollow cylindrical specimens of two types were prepared, one welded in the axial direction and the other welded in the circumferential direction. Straincontrolled low-cycle fatigue tests were carried out at 700°C. Fatigue lives of the welded specimens were about one-half that of the base metal specimens and the corresponding fatigue strength reduction factor was about 1.4. Microcracks also observed by using a replication technique during occasional interruptions of the tests. In both weld and base metal, the initiation of micro-cracks was observed in the early stage of life, but the initiated lengths of the microcracks in the weld metal was about 0.5mm while the equivalent figure for base metal was about 0.1mm. The Grain boundary oxidation would strongly affect these crack initiations. The crack growth life from 0.5mm to failure in the base metal specimen almost coincided with the failure life in the welded specimen. Therefore, the reduction of the fatigue life of welded specimen is because the initiated lengths of micro-cracks in the weldment are greater the lengths in the base metal. The maximum principal strain was confirmed as a good parameter for the evaluation of crack growth rates for both weldments and base metal. This result shows that the fatigue strength reduction is not due to the strain concentration at the weld. The initiated lengths of micro-cracks affect the fatigue life of weldments of Hastelloy-X.

Micro-Crack Initiation and Growth Behavior and Life Prediction under Creep-Fatigue Conditions for Low-Carbon/Nitrogen-Added SUS316.

Damage evaluation method for 316FR stainless steel based on micro crack growth behavior has been investigated to improve the life prediction method for high temperature plant. Micro-crack growth was observed in the early stage of life in the coarse grain material even at lower strain ranges. On the other hand, crack growth started in the later stage of the life at lower strain ranges in the fine grain material. The initiation life of 0.2mm length micro-crack could be evaluated by a creep-fatigue damage estimation method using the ductility exhaustion method. In this evaluation, initiation life of 0.2-mm-length micro crack in pure fatigue and creep test was used to calculate the fatigue and creep damage. Crack growth life was also able to predict by a crack growth analysis based on nonlinear fracture mechanics. The failure life of the specimen was determined by the summation of initiation and growth life and this prediction was confirmed to give a reasonable accuracy. Micro-crack initiation and growth is the main mechanism of damage in creep fatigue and lives of real components strongly depend on their allowable crack size. Therefore, the life prediction method will be rationalized by relating the life or damage evaluating in the method with micro crack length as mentioned in this study.

はんだにおける非弾性挙動と疲労き裂の発生・進展

Thermal cycling of an electronic assembly produces repeated inelastic strains in solder joints, which may cause thermal fatigue cracks to initiate and grow in a solder. The stress-strain relationship, creep deformation and stress relaxation in 37Pb63Sn, 50Pb50Sn and 90Pb10Sn were measured at various temperatures. Micro-crack initiation and growth behavior in plain specimens of the solders were observed under low-cycle fatigue and creep-fatigue at elevated temperatures. Even at room temperature, strain rate dependency of stress, creep deformation and stress relaxation in the solders were noticeable. Therefore, elements joined by solder are considered expanding freely at elevated temperatures, which results in a large shear strain in solders. Micro-cracks initiated at an early stage and their growth rates dominated the fatigue life of the specimen. The fatigue crack growth rate in 50Pb5OSn was much lower than in 90Pb10Sn and 37Pb63Sn. A slow tensile and fast compressive strain-wave caused a higher crack growth rate than a slow tensile and slow compressive one at elevated temperatures. Cracks 0.1 mm long existed at from 1/20 to 1/10 of the failure life of plain specimens. Above this size the fatigue crack growth rate had a good relationship with the strain intensity factor in different strain ranges. The strain intensity factors of cracks in die bondings were calculated using a low elastic modulus value for the solder. Also, the fatigue crack growth behaviors were calculated using the strain intensity factor values and fatigue crack growth rate of the solders at the maximum temperature of thermal cycling. This simulation gave a good approximation to the measured crack behavior in the thermal fatigue of the die bondings.

341 蒸気タービンフォーク型翼植込部の低サイクル疲労寿命評価(GS-7 疲労(2))

A new evaluation method was developed for low-cycle fatigue life for fork-type root attachments of long steam turbine blades. In this study, low-cycle fatigue life was estimated by calculating the micro crack growth life under a steep strain distribution, using the micro-crack growth rate, which is proportional to the effective crack length according to non-linear fracture mechanics. To verify this evaluation method, low-cycle fatigue tests on pin joint models were conducted, in which the crack growth behavior at the inner surface of the pin hole was observed by interrupting the tests. The estimated results for micro-crack growth behavior agreed well with the measured results. To obtain the local strain range distribution for fork-type root attachments, a two-step FEM analysis was proposed, including three-dimensional analysis of a whole-blade model and two-dimensional elastic-plastic contact analysis of a single blade-fork model.

Diffusive Crack Growth at a Bimaterial Interface

The high temperature microcrack growth behavior along a planar interface between two elastic dissimilar media is investigated with an aim at estimating service life of advanced ceramic composites under creep-rupture conditions. The crack is assumed to grow along the interface normal to a remote applied tensile stress via a coupled surface and grain-boundary diffusion under steady-state creep conditions. The crack-tip conditions were first derived from the asymmetric tip morphology developed by surface self-diffusion. The governing integro-differential equation containing the unknown tensile stress distribution along the interface ahead of the moving crack tip was derived and it was found that a new length parameter exists as a scaling factor for the interface for which the solution becomes identical to that of the single-phase media when plotted on the nondimensional physical plane. In contrast to the elastic stress solution which shows singularity at the tip and oscillatory character away from the tip, the creep stresses have a peak value away from the tip due to a wedging effect and interfacial sliding eliminates stress oscillation resulting in a decoupling between mode I and mode II stress fields. This stress solution ties the far-field loading parameter to the crack-tip conditions in terms of the unknown crack velocity to give a specific V-K functional relationship. It was shown that a stress exponent of 12 in the conventional power-law crack growth emerges at higher applied stress levels. An analysis on energy balance shows that the energy release during crack growth amounts to the J-integral which derives mostly from work done by “wedging,” not from strain energy loss. A constraint on interfacial diffusivities of the two species was found and its implications on possible microstructural developments were discussed.

ＳＥＭ内クリープ疲労試験による微視的損傷過程の連続観察

Creep-fatigue damage occurs gradually in high temperature components and structures. It is necessary to clarify the micro-damage evolution mechanism under creep-fatigue conditions for the quantitative evaluation of creep-fatigue damage in local portions of the components. In order to examine the relation between damage evolution and change of microstructure, such as creep cavities and surface microcracks, strain controlled creep-fatigue tests were performed and interrupted at several damage levels. The creep-fatigue tests were also performed under a low stress level by the high temperature fatigue machine combined with a scanning electron microscope (SEM), and the microcrack initiation and growth behavior were continuously observed by the SEM during the tests.It was found that even though many cavities were initiated and grew at the internal grain boundaries of the specimens during the strain controlled creep-fatigue tests, the failure life was governed by the propagation of surface cracks. On the other hand, microcracks of about one grain size order initiated mainly on the grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the microcracks was slow and at random by the influence of microstructure. The microcracks gradually opened to the loading direction with increasing number of cycles and coalescening occurred to grow. From the experiments, the creep-fatigue damage extension mechanism was divided into the Fc type, where the main cracks on the surface propagated through damaged grain boundaries, and Cf type, where damage evolution was characterized by opening and coalescening of the microcracks.

Phenomenological aspects of fatigue life and fatigue crack initiation in high strength alloys at cryogenic temperature

Based on the S N (stress cycling) data of Ti6Al4V alloys and high CrNi steels (JN1 and YUS170) at 4, 77 and 293 K, the fatigue strength and crack initiation behavior in the long-life range were investigated in this study. Ti6Al4V rolled alloys and YUS170 steel exhibited a good fatigue strength in the long-life range at cryogenic temperatures. In Ti6Al4V forged alloys and JN1 steel, however, there was a rather sharp drop in the fatigue strength at 4 and 77 K between 10 6 and 10 7 cycles. Subsurface crack initiation occurred in the long-life range for each material, and the location of its site was determined. The size of the subsurface crack initiation site obviously increased for Ti6Al4V forged alloys and JN1 steel as the number of cycles increased. We then proposed that the fatigue strength in the long-life range for the present alloys is related to the microcrack growth behavior in their microstructure.

Crack Growth Behavior under Multiaxial Low Cycle Fatigue at High Temperature for 1CrMoV Steel.

Microcrack growth behavior under axial and torsional loading at 550°C was observed. Cracking behavior was correlated with the results of microdamage observations using several strain parameters. From the microdamage observation, micro-cracking of oxide film at the crack tip in the strain localized region seems to be a dominant factor of micromechanism of fatigue crack growth. The maximum principal strain, which is the linear summation of the shear strain and the normal strain perpendicular to the crack path, has been identified as an important parameter in multiaxial fatigue at high temperature. A good correlation was obtained between the principal strain range and the crack growth rate.

Microcrack initiation and growth behavior under creep-fatigue in a plain specimen of degraded CrMoV cast steel.

Microcrack initiation and growth behavior were observed on the surface of nondegraded and degraded CrMoV cast steels under various strain wave shape loadings at high temperature. The degraded steel was taken from the inside of a main steam valve casing after 140 000 hours of service. No signigicant difference in crack growth behavior was observed under low cycle fatigue for either steel. However, the crack growth rate in the degraded steel was higher than that in the nondegraded one under creep-fatigue resulting from slow/fast straining. This was caused mainly by grown and coarsened grain boundary carbides. A residual life assessment method, based on microcrack growth and the statistical properties of microcrack length distribution, closely predicts the experimental results.

Micro-crack initiation and growth behavior under creep-fatigue in plain specimen of a degraded CrMoV cast steel.

Micro-crack initiation and growth behavior were observed on the surface of nondegraded and degraded CrMoV cast steels under various strain wave shape loadings at a high temperature. The degraded steel was taken from the inside of a main steam valve casing after 140 000 h of service. No significant difference in crack growth behavior was observed in low cycle fatigue in both steels ; however, the crack growth rate in the degraded steel was higher than that in the nondegraded one in creep fatigue under S-F straining. This was caused mainly by coarsened grain boundary carbides. A residual life assessment method, based on micro-crack growth and the statistical properties of micro-crack length distribution, gives an estimation close to the experimental results.