Slow Crack Growth Rate Research Articles

다구찌 기법을 이용한 이종재료 경사 홀 클린칭 접합부 수평 방향 접합강도 예측 및 검증

Abstract Fiber metal laminates (FMLs) are well known for improved fatigue strength, better impact resistance, superior damage tolerance and slow crack growth rate compared to traditional metallic materials. However, defects and loss of strength of a composite material can occur due to the vertical load from the punch during the joining with a dissimilar material using a conventional clinching method. In the current study, tapered-hole clinching was an alternative process used to join Al 5052 and FMLs. The tapered hole was formed in the FML before the joining. For the better understanding of static and dynamic characteristics, a clinched joining followed by a tensile-shear test was numerically simulated using the finite element analysis. The design parameters were also evaluated for the geometry of the tapered hole by the Taguchi method in order to improve and compare the lateral joining strength of the clinched joint. The influence of the neck thickness and the undercut were evaluated and the contribution of each design parameter was determined. Then, actual experiments for the joining and tensile-shear test were conducted to verify the results of the numerical simulations. In conclusion, the appropriate combination of the design parameters can improve the joining strength and the cross-sections of the tapered-hole clinched joint formed in the actual experiments were in good agreement with the results of the numerical simulations.

Experimental and finite element studies of thin bonded and hybrid carbon fibre double lap joints used in aircraft structures

Finite element analysis (FEA) is performed to verify the static and fatigue strength of mechanically fastened, bonded and hybrid double lap joints. These joints are made from thin carbon fibre/epoxy laminates applied in aircraft structures. Several configurations are considered, including variations in rivet array and the addition of bondline defects. Adhesive nonlinear material properties, rivet surface contacts and frictional forces were included in the three-dimensional (3D) Finite Element (FE) models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and the stress state for all specimens, whilst the strain energy release rate (SERR) as a function of crack length for bonded and hybrid specimens are also compared. Results have shown the FE models are able to accurately predict the bonded, riveted and hybrid joint strengths. The position of the first row of fasteners is critical in determining the crack growth rate. As the crack enters the fasteners' clamping zone there is a significant drop in SERR resulting in a much slower crack growth rate, therefore increasing the fatigue resistance of the hybrid joint configuration.

ビーチマーク法による内部疲労き裂伝ぱ速度の評価

Internal fatigue crack growth rate was measured by using beach marks resulting from repeated two-step fatigue tests for JIS-SCM440 low-alloy steel. This material showed internal fractures under stress ratio of R = 0, and the inclusion sizes of the internal fracture origins ranged from 14 to 40 μm. Two types of beach marks were observed: One was small beach marks indicating internal cracks at an initial stage of fatigue. The other was large ones indicating the internal cracks at a final stage. The internal crack shape at the initial stage was a half-ellipse and asymmetric, while that at the final stage were a circle and symmetric. The asymmetric internal crack shape requires three-dimensional modeling, and therefore, an extremely slow crack growth rate, smaller than the lattice length, is expected in two-dimensional modeling. The growth rate of the asymmetric internal crack was evaluated by d√area /dN instead of conventional da/dN. As the result, the asymmetric internal crack revealed the extremely slow crack growth rate, in spite of the conventional crack growth rate of the symmetric internal crack. The border of the two-types internal cracks was corresponding to conventional ΔKth. Moreover, the growth rate of the asymmetric internal crack showed good agreements with that estimated by Omata’s method based on the conventional fatigue test results. These results support validity of Tanaka-Akiniwa model for the internal crack growth. This research thus proved that gigacycle fatigue life of high-strength steel could be predicted by crack growth life based on Tanaka-Akiniwa model.

Failure assessment diagram analysis of high density polyethylene pipes

This paper presents an analysis of failure assessment diagram for PE4710 grade high density polyethylene pipes, which are recently concerned to use in the nuclear industry. Available test data of tensile, single edge notched tension specimens and pressurised pipes are summarised and suitably characterised for use in the failure assessment diagram in R6 procedure. Critical crack lengths and failure mode for the pressurised pipes and single edge notched tension specimens are determined under sustained load and compared with test results. Based on the concept of linear elastic fracture mechanics, a slow crack growth rate of PE4710 at 95°C is obtained excluding crack initiation time. To consider the effect of temperature on slow crack growth rate, a shift method is introduced. Remaining life is calculated against the crack margin and compared with the failure time.

Study of Cu–Zn–Co ternary alloy-coated steel cord in cobalt-free skim compound

In this study, it is shown that the rate of bond degradation due to ageing in hot & humid conditions can be significantly reduced by the use of a Cu–Zn–Co ternary alloy coating, compared to current compounds and normal brass-coated steel cord. To better understand the adhesion mechanism, X-ray photoelectron spectroscopy is used to investigate the adhesion interface upon hot and humid ageing. In addition, the effect of removal of cobalt salts from compound on static and dynamic rubber compound properties is studied: it is found that this leads to improved heat ageing resistance, slower crack growth rate and less hysteresis. Hence, the new Cu–Zn–Co ternary alloy coating for steel cord allows cobalt salts to be removed from tires, which makes tires more durable and eco-friendly.

Fracture of Irradiated Austenitic Stainless Steel with Special Reference to Irradiation Assisted Stress Corrosion Cracking

This paper deals with fracture of neutron irradiated austenitic Ti-stabilized stainless steel 08Ch18N10T. The steel had been tested in air and in water environment (320°C) using several tests representing different stress strain conditions for crack initiation and growth; Slow Strain Rate and Crack Growth Rate tests were performed in the water. Without irradiation the steel did not suffer from stress corrosion cracking in the water, but on irradiated specimens appeared areas of intergranular fracture mixed with transgranular cleavage-like facets and secondary cracks typical for IASCC phenomenon. The differences between fracture of irradiated and non-irradiated specimens in air and in water are documented and discussed.

Experimental and Analytical Studies on the Effect of Excessive Loading on Fatigue Crack Propagation in Piping Materials

The seismic design review guide in Japan was revised in September 2006 to address the occurrence of a large earthquake beyond the design basis. In addition, Japanese nuclear power plants (NPPs) experienced multiple large earthquakes, such as Niigata-ken Chuetsu-Oki Earthquake in 2007 and the Great East Japan Earthquake in 2011. Therefore, it is very important to assess the structural integrity of reactor piping under such a large earthquake when a crack exists in the piping. In this work, crack growth behavior after excessive loading during the large-scale earthquake were experimentally and analytically evaluated for carbon steel and austenitic stainless steel. Some cyclic loading patterns with increasing and decreasing load amplitudes and maximum loads were applied to fatigue crack growth test specimens. From the results, the retardation of crack growth rate was clearly observed after excessive loading. In addition, the applicability to the retardation effect of the modified Wheeler model was confirmed. It is also concluded that the retardation effect has little influence on the failure probability due to seismic loading using probabilistic fracture mechanics (PFM) analyses with the modified Wheeler model.

Crack growth rate and crack path in adhesively bonded joints: Comparison of creep, fatigue and fracture

The quality of an adhesive–adherend combination is often judged by whether the failure surface or crack path is cohesive or interfacial. The relationship between crack path and test method was examined by comparing the performance of individual adhesive–adherend combinations (six) in quasi-static fracture, mixed-mode fatigue to the threshold, and creep crack growth. In mixed-mode loading, crack paths in creep were similar to those in quasi-static fracture even though the crack growth rates in creep were much smaller. This was attributed to similar crack-tip plastic zone sizes in the toughened epoxy adhesive during creep and quasi-static fracture. Both creep and threshold fatigue tests produced interfacial failure when the test environment created condensed moisture at the crack tip and the loading was mixed-mode. Interfacial failure was also evident in room-temperature dry environments in mixed-mode fatigue at slow crack growth rates near the threshold, but was not seen in either creep or quasi-static fracture under mixed-mode loading in dry air. This increased sensitivity of near-threshold, mixed-mode fatigue to surface properties was caused by the proximity of the crack path to the interface as a result of a smaller plastic zone size. Therefore, the interfacial or cohesive failure of an adhesive system can be a function of the test being conducted and may not be an absolute indicator of joint quality.

Characterisation of crack tip fields under non-uniform fatigue loading

The paper analyses previously reported work, which uses digital image correlation to measure fatigue crack closure. As well as determining crack opening loads, the information on crack shape may be used to estimate the stress intensity factor, as well as other parameters in more complex models of crack tip fields. A number of specimens were subjected to single overload cycles, which produced a significant retardation in crack growth rate. The method previously applied to the analysis of constant amplitude loading is here used to analyse the single overload case. The stress intensity factor history is found to be very different in the two cases and the consequences of this observation for analysis of fatigue crack propagation are discussed.

Constraint effect on the slow crack growth in polyethylene

PurposeFrom the practical point of view, most relevant damage to high density polyethylene (HDPE) structures is caused by slow crack growth. Therefore, detailed information about this type of damage is necessary. Experimental results transfer from specimens to real structure can be influenced by structure geometry (constraint). Therefore, the purpose of this paper is to investigate and discuss the effect of the constraint and relation between crack mouth opening displacement (CMOD) and crack length.Design/methodology/approachThe constraint effect is mainly effect of the structure geometry and can be quantified by T‐stress. Two different test specimens with different constraint level (T‐stress) were prepared: single edge notched specimen and modified single edge notch (SEN) specimen. The crack mouth opening displacement, crack tip opening displacement and crack length was measured.FindingsThe main conclusions of this work can be summarized as: the slow crack growth rate in HDPE materials corresponds to velocity of CMOD; the influence of the presented specimen geometry on slow crack growth rate can be considered as negligible; and for transfer of the experimental results from specimens to real structure the influence of the structure geometry (constraint) is not critical.Originality/valueExperimental results obtained from different specimens with different constraint level are rare and can lead to better data transfer from experimental specimens to the real structures.

Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements

Synchrotron-based, high-energy X-ray diffraction measurements are used to study the local strain fields underlying the transient fatigue crack growth rate retardation produced by a single overload cycle known as the overload effect. Specifically, 4140 steel compact tension specimens fatigued for varying levels of crack growth after an overload cycle have been studied with in-situ diffraction under varying external loads. The load responses of the strain at the overload-position, versus at the crack tip, are focused upon in detail. The large compressive residual strain at the overload-point is observed to remain essentially unchanged even after the overload-point is left in the wake of the propagating crack tip. The differential strain-load response at the crack-tip/overload position before and immediately after the overload is seen to be unchanged. Once the overload point is behind the crack tip, a highly nonlinear behavior is observed in which the load response of the strain field transfers from the overload -point to the crack tip when the load exceeds a critical value. The results are discussed in terms of plasticity-induced crack face contact at the overload point as an important local mechanism contributing to the “overload effect” in this specific system.

Stress intensity factor analysis of through thickness effects

The study of crack tip fields in mode I is often conducted assuming a homogeneous behaviour through the thickness. Depending on the specimen thickness, a state of plane stress or plane strain is normally presumed. However, recent studies have shown a more complex behaviour along the thickness. On the one hand, plasticity-induced crack closure effects affect mainly to a small region close to the specimen surface. On the other hand, the plastic zone evolution along the thickness is not as simple as the classic dog-bone shape normally described in Fracture Mechanics textbooks. Unlike what is normally expected, the size of the plastic zone decreases in a very small region close to the surface, as we move from the interior to the surface of the specimen. These two effects can be detected if the mesh used in the finite element model is sufficiently fine. Both effects are probably related to an uneven distribution of the load along the thickness. One of the consequences of these effects on the fatigue crack growth is the curvature of the crack front which can be explained by two mechanisms. The first one is related to the crack closure effect near the surface, it would imply a smaller effective ΔK close to the surface, and therefore a slower crack growth rate. The second one (plastic zone size decrease in a small region close to surface) is probably due to ΔK being smaller near the surface than in the interior. The current work attempts to evaluate numerically both effects in order to separate their individual influence and their magnitude. This is done by evaluating the K distribution along the thickness at different planes on an Al 2024-T35 compact tension specimen under mode I nominal loading. The plastic wake effect is removed from the model in order to distinguish between both effects.

Cohesive zone model for intergranular slow crack growth in ceramics: influence of the process and the microstructure

Ceramic polycrystals are prone to slow crack growth (SCG) which is stress and environmentally assisted, similarly to observations reported for silica glasses. The kinetics of fracture are known to be dependent on the load level, the temperature and the relative humidity. In addition, evidence is available on the influence of the microstructure on the SCG rate with an increase in the crack velocity with decreasing the grain size. Crack propagation takes place beyond a load threshold, which is grain size dependent. We present a cohesive zone model for the intergranular failure process. The methodology accounts for an intrinsic opening that governs the length of the cohesive zone and allows the investigation of grain size effects. A rate and temperature-dependent cohesive model is proposed (Romero de la Osa M, Estevez R et al 2009 J. Mech. Adv. Mater. Struct. 16 623–31) to mimic the reaction–rupture mechanism. The formulation is inspired by Michalske and Freiman's picture (Michalske and Freiman 1983 J. Am. Ceram. Soc. 66 284–8) together with a recent study by Zhu et al (2005 J. Mech. Phys. Solids 53 1597–623) of the reaction–rupture mechanism. The present investigation extends a previous work (Romero de la Osa et al 2009 Int. J. Fracture 158 157–67) in which the problem is formulated. Here, we explore the influence of the microstructure in terms of grain size, their elastic properties and residual thermal stresses originating from the cooling from the sintering temperature down to ambient conditions. Their influence on SCG for static loadings is reported and the predictions compared with experimental trends. We show that the initial stress state is responsible for the grain size dependence reported experimentally for SCG. Furthermore, the account for the initial stresses enables the prediction of a load threshold below which no crack growth is observed: a crack arrest takes place when the crack path meets a region in compression.

The effect of a single tensile overload on stress corrosion cracking growth of stainless steel in a light water reactor environment

It has been found that a single tensile overload applied during constant load amplitude might cause crack growth rate retardation in various crack propagating experiments which include fatigue test and stress corrosion cracking (SCC) test. To understand the affecting mechanism of a single tensile overload on SCC growth rate of stainless steel or nickel base alloy in light water reactor environment, based on elastic-plastic finite element method (EPFEM), the residual plastic strain in both tips of stationary and growing crack of contoured double cantilever beam (CDCB) specimen was simulated and analyzed in this study. The results of this investigation demonstrate that a residual plastic strain in the region immediately ahead of the crack tips will be produced when a single tensile overload is applied, and the residual plastic strain will decrease the plastic strain rate level in the growing crack tip, which will causes crack growth rate retardation in the tip of SCC.

<i>In Situ</i> Observation of Crack Propagation in Epoxy Composite Reinforced with Crushed Silica Particles

In-situ observation of fatigue crack growth of epoxy resin composite reinforced with crushed silica particle was carried out. The test was performed under constant ΔK condition. Based on the results, the crack propagation mechanism was discussed. The in-situ observation revealed that in front of the main crack, a microcrack was nucleated at the interface of matrix/particle and then coalesced with the main crack. At the same time, new microcracking occurred ahead of the crack tip and the crack propagated by repeating these processes. Retardation of crack growth rate was found to result from crack bridging induced by microcracking at silica particles and crack deflection.

Fatigue threshold behavior of adhesive joints

The fatigue threshold and slow crack growth rate behavior of a highly-toughened epoxy adhesive was studied as a function of the starting condition (fatigue precrack and fillet), testing approach and interfacial bond strength. For cohesive failure, the threshold behavior was very similar when starting from a fatigue precrack or an uncracked fillet; however, the threshold was lower when interfacial cracks grew from a fatigue precrack. When compared to the time required for a crack to reach a significant length in a practical joint, the crack initiation from a fillet at fatigue threshold loads occurred relatively quickly. For loading and displacement ratios of 0.1 and 0.5, fatigue testing under force control proved to be equivalent to the faster and more convenient testing under displacement control. Mixed-mode fatigue crack growth at slow rates near the threshold became progressively interfacial as the cyclic crack speed decreased, revealing differences in bond strength that were unapparent in fracture tests and in fatigue tests at higher crack speeds. Slow crack growth fatigue tests were thus found to be exceptionally sensitive to differences in interfacial bond strength caused by changes in adhesive batch, adherend roughness and surface pretreatment.

Temperature and Load Ratio Effects on Crack-Growth Behavior of Austenitic Superalloys

The role of temperature and load ratio (R) on the crack propagation rate (da/dN) of Alloys 276 and 617 under cyclic loading was investigated. The results indicate that the rate of cracking was gradually enhanced with increasing temperature when the R value was kept constant. However, the temperature effect was more pronounced at 100–150°C. Both alloys exhibited maximum da/dN values at a load range of 4.5 kN that corresponds to an R value of 0.1. The number of cycles to failure for Alloy 276 was relatively higher compared with that of Alloy 617, indicating its slower crack-growth rate. Fractographic evaluation of the broken specimen surface revealed combined fatigue and ductile failures.

Cohesive zone model and slow crack growth in ceramic polycrystals

Ceramics polycrystals are subjected to slow crack growth (SCG) and also environmentally assisted failure, similarly to what is observed for glasses. The kinetics of fracture are known to be dependent on the load level, the temperature and also on the Relative Humidity (RH). However, evidences are available on the influence of the microstructure on the SCG rate with a marked increase in the resistance to the crack advance when comparing the response of a single crystal to that of a polycrystal. The clarification of the origin of the latter observation motivates the development of a local approach of fracture. We propose a physically based cohesive zone description that mimics the reaction-rupture process underlying SCG. We present how the parameters involved in the cohesive zone formulation can be determined to capture SCG in a single crystal. We then present simulations of intergranular failure in a 2D plane strain polycrystal. The triple junctions are shown to be key in the resistance to crack growth. The presence of defects at these junctions and their influence on the crack propagation is then investigated. We also show that the description is able to capture the damage induced by the process cooling, thus providing insight of the appropriate temperature for the sintering, for instance. The present numerical tool is then thought valuable to perform virtual tests on ceramics fracture in order to estimate thermal as well as mechanical loads limits for a safe use as well as proper the process conditions.

Mapping and load response of overload strain fields: Synchrotron X-ray measurements

High energy synchrotron X-ray diffraction measurements have been performed to provide quantitative microscopic guidance for modeling of fatigue crack growth. Specifically we report local strain mapping, along with in situ loading strain response, results on 4140 steel fatigue specimens exhibiting the crack growth retardation “overload effect”. Detailed, 2D, ε yy -strain field mapping shows that a single overload (OL) cycle creates a compressive strain field extending millimeters above and below the crack plane. The OL strain field structures are shown to persist after the crack tip has grown well beyond the OL position. The specimen exhibiting the maximal crack growth rate retardation following overload exhibits a tensile residual strain region at the crack tip. Strain field results, on in situ tensile loaded specimens, show a striking critical threshold load, F c, phenomenon in their strain response. At loads below F c the strain response is dominated by a rapid suppression of the compressive OL feature with modest response at the crack tip. At loads above F c the strain response at the OL position terminates and the response at the crack tip becomes large. This threshold load response behavior is shown to exhibit lower F c values, and dramatically enhanced rates of strain change with load as the crack tip propagates farther beyond the OL position. The OL strain feature behind the crack tip also is shown to be suppressed by removing the opposing crack faces via an electron discharge cut passing through the crack tip. Finally unique 2D strain field mapping (imaging) results, through the depth of the specimen, of the fatigue crack front and the OL feature in the wake are also presented.

Effect of units in different sizes on thermal fatigue behavior of 3Cr2W8V die steel with biomimetic non-smooth surface

Aiming to deeply research the thermal fatigue behavior of 3Cr2W8V die steel with biomimetic surface, the specimens with different sizes of concave-shape and striation-shape units were processed by laser technique. By establishing cross-section model of the units and by calculating their volume, the size of the units was quantitatively expressed, and its effect on thermal fatigue behavior of 3Cr2W8V die steel was investigated and compared. The results showed that the units in different sizes have different degrees of effects on resisting the thermal fatigue cracks; especially those in the larger size can play a greater role in preventing the crack initiation and blocking the crack propagation; and further the influence of the size of the units on thermal fatigue behavior is more sensitive to resist the crack propagation. Based on the self-strengthening of the units, the increased resistance of the units in the larger size to crack initiation can be attributed to the crack prevention mechanism owing to the reduced probability of crack nucleation and the enhanced resistance to plastic deformation on the biomimetic surface, while the enhanced resistance to crack propagation is due to the crack blocking mechanism that results in the slow crack growth rate.