Initial Stage Of Crack Growth Research Articles

Simulation of crack nucleation in materials with regularly arranged spherical pores under multiaxial loading conditions

A numerical study of the initial stage of crack growth in a material containing spherical pores under multiaxial compression (constrained conditions) has been carried out using the finite element method. The influence of the pore spacing and the mechanical properties of the material on the fracture localization and the direction of crack growth has been analyzed. Both brittle and ductile materials were studied. It has been established that material properties and the mutual arrangement of defects have a significant influence on the direction of propagation of cracks arising on the stress concentrators (structure defects), up to mutually perpendicular directions (for uniaxial of the characteristic sizes of the region of influence of spherical pores on each other, beyond which the stress fields of the neighboring elements of the structure do not overlap. The dependences of the sizes of this area on the elastic characteristics of the matrix material are shown. The obtained results are relevant for obtaining numerical estimates of the duration of the initial stages of quasi- brittle fracture of ceramic-based heterophase materials, refractories in particular. Ill. 6. Ref. 26. Tab. 1.

Determination of the fracture parameters of concrete with improved wedge-splitting testing

Considering the quasi-brittle mechanical properties of concrete, wedge-splitting tests are employed and improved in this paper to study the fracture behaviour of concrete. A novel wedge-splitting device is designed by fixing ten springs on the force transmission component. The adaptive spring force can be imposed on top of a concrete specimen to retard the brittle fracture process. With the proposed wedge-splitting test design for notched cuboid specimens, the complete load–strain/CMOD curves of concrete can be generated directly. The fracture toughness and the fracture energy can be calculated easily without numerical fitting using the double-K fracture model. The descending branch of the curve is verified by exponential and power function fitting, where the maximum relative error is approximately 15% and the average relative error is less than 6%. Moreover, the specimen strain distribution presents a polynomial deflection curve, with the upper part of the specimen in tension and the lower part of the specimen in compression, and the initial zero point is located at a crack growth ratio of approximately 0.53 relative to the whole fracture path. The concrete fracture process consists of an initial stage of crack growth, a long-term stable propagation stage, and an unstable propagation stage.

Interdependencies between Dynamic Response and Crack Growth in a 3D-Printed Acrylonitrile Butadiene Styrene (ABS) Cantilever Beam under Thermo-Mechanical Loads.

Acrylonitrile butadiene styrene (ABS) is the most commonly used thermoplastic printing material for fused deposition modelling (FDM). FDM ABS can be used in a variety of complex working environments. Notably, the thermo-mechanical coupled loads under complex operating conditions may lead to cracking and ultimately catastrophic structural failure. Therefore, it is crucial to determine the crack depth and location before a structural fracture occurs. As these parameters affect the dynamic response of the structure, in this study, the fundamental frequency and displacement amplitude response of a cracked 3D-printed ABS cantilever beam in a thermal environment were analytically and experimentally investigated. The existing analytical model, specifically the torsional spring model used to calculate the fundamental frequency change to determine the crack depth and location was enhanced by the proposed Khan-He model. The analytical relationship between the displacement amplitude and crack was established in Khan-He model and validated for the first time for FDM ABS. The results show that a reduced crack depth and location farther from the fixed end correspond to a higher fundamental frequency and displacement amplitude. An elevated ambient temperature decreases the global elastic modulus of the cracked beam and results in a lower fundamental frequency. Moreover, a non-monotonic relationship exists between the displacement amplitude and ambient temperature. The displacement amplitude is more sensitive to the crack change than the fundamental frequency in the initial stages of crack growth.

Effect of styrene content on mechanical and rheological behavior of styrene butadiene rubber

Styrene butadiene rubber(SBR) consists of two random polymerization units, styrene and butadiene. As a rigid segment, styrene chain provides strength and rigidity to the rubber. The content of styrene in SBR directly affects its strength and stiffness. In this paper, the effect of styrene content on the fatigue characteristics and rheological properties of SBR have been investigated. The stress relaxation of SBR was reduced and the cross-linking density of butadiene was increased with the increasing styrene content. Especially the dynamic fatigue life of precracked SBR samples was obviously prolonged via slow down crack growth rate when the styrene content increased, which was accompanied by an augment in the value of J-integral and tearing energy in the case of a same gap extension length. At the initial stage of crack growth, the growth of secondary crack played an important role in the growth of main crack, and when the main crack accelerates, the growth of main crack itself dominated. On the other hand, the glass transition temperature obviously increased with the increase of styrene content.

Comparative Research on Fatigue Performance of High-Strength Bainitic Steel with or without Corrosion

High-strength bainitic steels have acquired a promising place among steels due to their wide applications in engineering structures. It is, therefore, worthwhile to investigate their fatigue performance in different environments. When fatigue is atmospheric, lattice strength of weaker zones including granular bainite, phase boundary and MA constituents determines the nucleation life of fatigue. The deflected microstructural short cracks enhance the early propagation life of fatigue. In lath bainite, fine ferrite laths, high-density dislocations within the grains, small grain size and high proportion of HAGBs inhibit the crack growth process to improve propagation life of fatigue. The fracture reveals transgranular mode for propagation of fatigue. The pre-corrosion strategy is adopted before investigating corrosion fatigue process, so that there is an adequate corrosion effect on samples. The pre-corrosion time is determined to be 3 days through the analysis of electrochemical parameters, viz. corrosion potential and current. For overall fatigue life, corrosion grooves have significant deterioration effect. The transgranular and intergranular features are observed in fracture at initial stage of crack growth. Only transgranular features are found at middle and final stages.

Effects of Mixed Mode Loading Conditions on Fatigue Crack Growth Rate

The fatigue crack growth rate is the most important factor in predicting the life of a product when applying the damage tolerance design concept. Studies related to pure mode I for structures under fatigue loading have been actively conducted, while not many studies are conducted on the mixed mode. In this study, therefore, mixed mode fatigue crack growth experiments were designed using the Compact-Tension-Shear (CTS) specimens and the loading devices, proposed by Richard. Furthermore, the finite element analysis was used in determining the stress intensity factors of CTS specimen. As the results, the fatigue crack growth rate using the equivalent stress intensity factors proposed by previous researchers was lower than that of pure mode I at the initial stage of crack growth when the load angle increases.

Atomistic investigation of crack growth resistance in a single-crystal Al-nanoplate

Abstract

Numerical modelling of fretting fatigue

Fretting fatigue is a consequence of small oscillatory movement between two contacting parts. This type of damage may give rise to significant reduction in fatigue life of the components. Many parameters such as contact geometry, contact load, friction, material strength and hardness can influence the resistance of materials against fretting damage. Experimental study of fretting fatigue parameters such as crack length measurement at the initial stages of crack growth is either very difficult or not very accurate. Numerical simulations provide a suitable tool for parametric study of fretting fatigue behavior of materials and in particular crack growth measurement. In this work, the effect of pad geometry on crack propagation is investigated by numerical simulation. The simulations are carried out using the finite element codes ANSYS and FRANC2D/L and are validated by the results of some fretting fatigue tests. It is shown that the simulations can predict the crack growth and the number of cycles from the principals of fracture mechanics.

Three-dimensional modeling of ductile crack growth: Cohesive zone parameters and crack tip triaxiality

For 10 mm thick smooth-sided compact tension specimens made of a pressure vessel steel 20MnMoNi55, the interrelations between the cohesive zone parameters (the cohesive strength, T max, and the separation energy, Γ) and the crack tip triaxiality are investigated. The slant shear-lip fracture near the side-surfaces is modeled as a normal fracture along the symmetry plane of the specimen. The cohesive zone parameters are determined by fitting the simulated crack extensions to the experimental data of a multi-specimen test. It is found that for constant cohesive zone parameters, the simulated crack extension curves show a strong tunneling effect. For a good fit between simulated and experimental crack growth, both the cohesive strength and the separation energy near the side-surface should be considerably lower than near the midsection. When the same cohesive zone parameters are applied to the 3D model and a plane strain model, the stress triaxiality in the midsection of the 3D model is much lower, the von-Mises equivalent stress is distinctly higher, and the crack growth rate is significantly lower than in the plane strain model. Therefore, the specimen must be considered as a thin specimen. The stress triaxiality varies dramatically during the initial stages of crack growth, but varies only smoothly during the subsequent stable crack growth. In the midsection region, the decrease of the cohesive strength results in a decrease of the stress triaxiality, while the decrease of the separation energy results in an increase of the triaxiality.

Mode I DCB testing of composite laminates reinforced with z-direction pins: a simple model for the investigation of data reduction strategies

The applicability of standard composite mode I interlaminar toughness data reduction schemes is investigated for composites reinforced by inserting straight pins in the z-direction. A simple model of a z-pinned mode I test is used to generate load, displacement and crack length data. Data reduction schemes are applied to the model data and the resulting toughness is compared to the true toughness. These schemes are shown to be inaccurate for the initial stage of crack growth, during which the zone bridged by z-pins is developing and an improved scheme is proposed. Beyond this stage, the toughness is most accurately predicted if the ‘developing’ data is omitted.

複合材料 層間高じん化ＣＦＲＰの混合モード（Ｉ＋ＩＩ）破壊じん性およびその負荷速度依存性

Mixed mode (I+II) fracture toughness of the Interlayer-toughened composite laminates, T800H/3900-2 (Toray), which is a newly developed composite material having a tough ‘interlayer’ containing fine polyamide particles, was investigated over a very wide range of loading rate from static to impact. The MMF (Mixed Mode Flexure) specimen and SHPB (Split Hopkinson Pressure Bar) system were employed for measuring the mixed mode fracture toughness under impact loading. The strain rate dependence of fracture behavior was also studied from both macro- and microscopic aspects to clarify the mesoscopic fracture mechanism of the material. The experimental results showed the crack path was inside the toughened interlayer during the initial stage of crack growth, however, it moved from the toughened interlayer to the carbon/epoxy base lamina during the propagation stage of crack growth. The mixed mode fracture toughness was sensitive to loading rate; the impact fracture toughness for the toughened interlayer was about 23% lower than the maximum value under static loading, and the impact fracture toughness for the carbon/epoxy base lamina was about 11% lower than the maximum value under static loading. The effects of mode mixture varied with loading rate for interlayer fracture, though they did not varied for base-lamina fracture. Microscopic observation showed that the fracture morphology was sensitive to loading rate.

Micromechanics of creep fracture: simulation of intergranular crack growth

A computational model is presented to analyze intergranular creep crack growth in a polycrystalline aggregate in a discrete manner and based directly on the underlying physical micromechanisms. A crack tip process zone is used in which grains and their grain boundaries are represented discretely, while the surrounding undamaged material is described as a continuum. The constitutive description of the grain boundaries accounts for the relevant physical mechanisms, i.e. viscous grain boundary sliding, the nucleation and growth of grain boundary cavities, and microcracking by the coalescence of cavities. Discrete propagation of the main crack occurs by linking up of neighbouring facet microcracks. Assuming small-scale damage conditions, the model is used to simulate the initial stages of crack growth under C ∗ controlled, model I loading conditions. Initially sharp or blunted cracks are considered. The emphasis in this study is on the effect of the grain microstructure on crack growth.

高温強度 １Ｃｒ‐Ｍｏ‐Ｖ鋼のクリープき裂成長特性に及ぼす試験片板厚の影響

Creep crack growth tests on very large 5T-CT specimens of 1Cr-Mo-V steel have been conducted in order to clarify the specimen size effect on creep crack growth properties and to contribute to the standardization of test method in VAMAS project. The creep crack growth rate could be correlated with C* parameter except for the initial stage of crack growth. In this stage, crack was considered to propagate under small-scale creep conditions. In the accelerating stage, the crack growth rate was independent of specimen width but was dependent on specimen thickness. As the specimen thickness increased, the creep crack growth rate increased due to the deformation constraint. The estimation method of displacement rate for CT specimens from the creep strain curve of round bar specimens was also discussed.

Rate Dependence of Mode I Interlaminar Fracture Toughness of Interlayer-Toughened Carbon-Fiber/Epoxy Laminates.

Rate dependence of mode I interlaminar fracture toughness of interlayer-toughened carbonfiber/epoxy laminates (T800H / 3900-2, Toray) was studied over a very wide range of loading rate from quasi-static to impact. A SHPB (Split Hopkinson Pressure Bar) system using a ramped incident stress wave was employed to estimate the accurate fracture toughness at high loading rates. Finite element results proved the validity of a simple estimation formula, which is based on static beam theory, for impact fracture toughness. Experimental results showed that the fracture toughness is strongly rate-sensitive. The fracture toughness decreased stepwise interposing a transition region where the fracture toughness dropped rapidly with increasing loading rate. The decrease of fracture toughness in the transition region was larger in the initial stage of crack growth than in the propagation stage. The crack path was inside the interlayer region in the initial stage, but it shifted to near the interface between the interlayer and carbon-fiber/epoxy lamina in the propagation stage. Rough fracture surfaces including microcracks of resin were observed at low loading rates, but relatively smooth fracture surfaces were observed at high loading rates. The transition of fracture characteristics approximately corresponded to the transition of fracture toughness.

CONFOCAL SCANNING LASER MICROSCOPY MEASUREMENTS OF THE GROWTH AND MORPHOLOGY OF MICROSTRUCTURALLY SHORT FATIGUE CRACKS IN A1 2024‐T351 ALLOY

Abstract— A new technique for studying the growth and morphology of microstructurally short fatigue cracks is described. The technique allows short crack growth, crack depth, and crack front configuration to be measured using Confocal Scanning Laser Microscopy (CSLM). Good agreement is shown between CSLM measurements of the initial stage of crack growth (along an inclined shear plane), crack depth and aspect ratio, together with the measurements obtained using a surface layer removal technique. It is also found that non‐propagating short fatigue cracks have approximately the same aspect ratios in different specimen orientations and that the aspect ratio is independent of stress amplitude.

Creep Crack Growth Estimated by Q* Parameter for Hastelloy XR.

Creep crack growth rate of Hastelloy XR showed a dependence on thermally activated energy at very high temperatures. The Q* parameter was then applied to estimation of the creep cracking of the material. It was found that the Q* parameter well described the initial stage of crack growth as well as the steady-state cracking. The coefficients of the Q* parameter derived here for Hastelloy XR were compared with those of SUS 304 stainless steel and Cr-Mo-V steel and each of the coefficients was explained physically by the creep ductilities.

Evaluation of the Minimum Fatigue Crack Length for Application of the Small-Crack Growth Law.

In the present study, the influences of microstructures of material on the characteristics of small-crack growth were investigated and the minimum crack length to which the small-crack growth law (dl/dN ∝ l) is applicable was evaluated using a ferrite-pearlite steel, a ferrite-martensite steel and a quenched and tempered carbon steel. The crack growth is influenced markedly by grain boundary, pearlite structure and martensite structure at the initial stage of crack growth, and the influences decrease with the increase in crack length. Even in the process in which crack growth is influenced by the microstructures, however, the mean value of crack growth rate is estimated by extrapolating the crack growth rate in the region where the crack growth is mainly controlled by the mechanical parameter: that is, the small-crack growth law is applicable in most regions of the crack growth process.

純銅単結晶のＮａＮＯ２溶液中における応力腐食割れ

The Crystallography of transgranular stress corrosion cracking (SCC) in pure copper single crystals was studied by scanning electron microscopy. The regular prismatic specimens oriented for [001], [101], [111], [112], [213] and 0.50 in Schmid factor were prepared, and they were SCC-tested in 1kmol·m-3 NaNO2 solution at room temperature by slow strain rate technique. The fractographic results showed that many cracks nucleated from the specimen surfaces with frequent slip-steps, producing a black turnish film. The susceptibility to SCC estimated from the nominal stress-strain curves increased markedly in the [101]- and [111]-oriented specimens. Most of the fracture surfaces showed the characteristic of river-like pattern, which was formed by the crack growth along ‹110› direction on {110} plane. The appearance of {111}-‹112› type was also observed partly in the initial stage of crack growth. It was found that the crack growth of {110}-‹110› type was strongly controlled by the normal stress to {110} plane which takes the smallest angle with respect to tensile axis.

Subcritical crack growth of glass

The small radial cracks that occur around the microhardness indentation were used to measure the subcritical crack growth response of glass under constant-bend loading in water. The crack velocity versus stress intensity factor curves indicate that the crack velocity depends on the magnitude of the stress and that the velocity which is high decreases at the initial stage of crack growth, thereafter reaches a minimum and increases with the stress intensity factor until unstable fracture occurs.

Fatigue crack propagation in steels with different yield strengths

1. Fatigue crack propagation in low-alloy and low-carbon steels can be described by the relationship d2a/dN=C(ΔK)n. Two sections — lower and upper — are observed on the curves of crack growth, differing from each other in their slope to the ΔK axis. 2. The length of the fatigue crack corresponding to the change from one section to the other increases with increasing yield strength of the steels. 3. On the lower section, with a steeper slope to the ΔK axis, corresponding to the initial stage of crack growth, the average rate of crack propagation is considerably smaller than in the upper section. 4. In the upper section the relationship between exponent n and coefficient C and the yield strength and ultimate strength of the steels is almost linear. 5. The average rate of fatigue crack propagation in low-alloy and low-carbon steels decreases with increasing yield strength, as the result of which the life of the steel increases. The largest values of vav were obtained for steel 15kp and the smallest for steel 10G2S1.