Fatigue Crack Growth Retardation Research Articles

Cohesive modeling of fatigue crack retardation in polymers: Crack closure effect

Abstract A numerical model of fatigue crack growth retardation in polymers induced by artificial crack closure is proposed. The approach relies on the combination of cohesive modeling and a contact algorithm incorporated in the wake of the advancing crack to account for the effect of the introduced wedge. Numerical results are compared with existing experimental observations, showing the ability of the cohesive model to capture the key features of wedge-induced crack retardation. A study is conducted to quantify the effects of relevant parameters such as applied load levels, wedge distance to the crack tip and wedge stiffness. The model is also discussed in the context of self-healing polymers [White SR, Sottos NR, Moore J, Geubelle PH, Kessler M, Brown E, et al. Autonomic healing of polymer composites. Nature 2001;409:794–7], where the wedging effect is associated with the polymerization of the healing agent.

A study on spot heating induced fatigue crack growth retardation

ABSTRACTThe propagation of a growing fatigue crack can be effectively retarded by heating a spot near the crack tip (under zero stress condition). Spot heating to a subcritical temperature and at a precise location modifies the crack growth behaviour in a way, more or less, similar to specimens subjected to an overload spike. It is observed that the magnitude of spot heating induced crack growth retardation increases with increase in spot temperature. It is also observed that the crack growth behaviour is influenced by the position of the heating spot and there exists an optimum position of hot spot that produces maximum retardation in fatigue crack growth rate. The plastic zone length due to spot heating has been estimated using experimental data. It is found that the plastic zone length due to spot heating increases exponentially with increase in spot temperature. The Wheeler model for crack growth retardation has been modified by introducing a plastic zone correction factor λ. The values of λ and the shaping exponent, m, in the Wheeler model have been obtained for different spot heating temperatures.

Plasticity induced fatigue crack growth retardation model for steel elements reinforced by composite patch

Fatigue tests on notched steel plates reinforced by composite patch showed that the application of carbon fiber reinforced polymers (CFRP) strips with pretension of the overlays prior to bonding. This resulted in a significant amount of additional fatigue life. In particular, the pre-tension produces a compressive field in the steel plate which reduces the stress ratio that enhances crack growth retardation. The fatigue crack propagation rate is postulated to be a function of the effective strain energy density factor range. Fatigue crack growth data showed that standard crack growth retardation model cannot be used to evaluate the minimum effective stress. Hence, an ad hoc plasticity model is introduced and validated using experimental results. The proposed technique is an extension of the well know Newman’s model. The bridging effect due to the reinforcing strips is analytically modeled in order to estimate the reduction of crack opening displacement and finally the magnification of the crack growth retardation. Numerical and experimental results match well and show a significant influence of the pre-tension level on the expected fatigue crack growth rate of a reinforced steel plate.

The effect of residual thermal stresses on the fatigue crack growth of laser-annealed 304 stainless steels

The fatigue crack growth behavior of AISI 304 stainless steels annealed by a CO 2 laser was investigated in the study. For the laser-annealed specimen prior to introducing a notch, the residual tensile stress was obtained around the center of the laser-annealed zone (LAZ), and the residual stress field changed gradually from tensile into compressive stress with increasing the distance away from the centerline of LAZ. Based on the experimental result, the laser-annealed specimen tested under low Δ K had a higher resistance to fatigue crack growth in the region preceding the LAZ after introducing a notch perpendicular to the LAZ. In contrast, the retardation of fatigue crack growth disappeared for the laser-annealed specimen stress-relieved at 850 °C for one hour. The presence of residual compressive stress field ahead of the crack tip resulted in the decreased fatigue crack growth rate (FCGR). Moreover, residual compressive stresses enhanced the effect of crack closure, and led to the formation of rubbed marks on the fatigue-fractured surface. As the fatigue crack propagated further into the LAZ, the residual tensile stress field ahead of crack tip did not accelerate the crack growth. The results indicated that tensile stress field ahead of the crack tip had little effect on the FCGR of the ductile 304 stainless steel.

A Study on Evaluation of Overload Effects on Fatigue Crack Growth

In the fatigue tests of specimens, sufficiently large tensile overload included in cyclic constant amplitude loading causes the retardation in fatigue crack growth. Crack retardation remains for some period of time after the overloading. The number of cycles in the retarded crack growth has been shown to be related to the plastic zone developed due to the overload. The magnitude of the elastic-plastic zone around the crack tip of DENT(Double Edge Notched Tension) specimen after a overloading was measured by ESPI(Electronic Speckle Pattern Interferometry) system. The fatigue crack growth rate also was measured by a traveling microscope. The relationship between the measured magnitude of plastic zone and the crack growth rate was compared with the equations proposed by Wheeler. Crack growth retardation model that was characterized by crack growth length and the size of plastic zone was proposed and compared with test result. From the research, the validity of proposed model is examined on crack growth retardation, and consequently fatigue life.

Effect of stress ratio on fatigue crack propagation in a functionally graded metal matrix composite

A functionally graded metal matrix composite, gradually distributed SiC particulate reinforced Al matrix composite, was fabricated by powder metallurgy processing. Fatigue crack growth tests of the composite were conducted in crack growth direction of from 5% to 30% SiC volume fraction layers under sinusoidal wave-form with stress ratios of 0.1, 0.3, 0.5 and 0.7, respectively. The fatigue crack growth rates increase with an increase in stress ratio. This is interpreted by crack closure mechanism. The retardation of fatigue crack growth was found when crack propagated from low-volume fraction of SiC layer to high-volume fraction of SiC layer. The crack deflection and branching at interfaces were observed, which decreased crack growth rates.

Estimation of fatigue crack growth retardation due to crack branching

Quantitative analysis is provided to estimate the reduction of fatigue crack growth rate due to overload crack branching. A recent mixed-mode fatigue crack growth model based on the dilatational component of the accumulated strain energy density near the crack tip is modified to quantify the retardation factor of crack growth rate following an overload. It is found that crack branching due to an overload results in considerable reduction of fatigue crack growth rate. The retardation factor estimated by the proposed methodology is correlated with test results for the 2090-T8E41 aluminum–lithium alloy indicating encouraging agreement.

The effects of loading frequency, tensile overload and compressive underload on the fatigue crack propagation behaviour of polymethyl methacrylate

An experimental investigation into the fatigue crack propagation in polymethyl methacrylate (PMMA) was performed to determine the effects of loading frequency, tensile overload and compressive underload in an otherwise constant amplitude loading scenario. It was found that contrary to what was observed for ductile materials, the loading frequency had little or no influence on the fatigue behaviour of PMMA at room temperature in air. A single tensile overload produced a small amount of fatigue crack growth retardation in PMMA. The amount of retardation decreased with higher overload level. In addition, an overload applied at shorter crack length produced more retardation than one applied at longer crack length. Furthermore, there was also more retardation when multiple overloads were applied, compared to a single overload. However, a single compressive overload produced an insignificant amount of fatigue crack growth retardation or acceleration in PMMA that showed no apparent relationship between the underload level and the amount of retardation or acceleration.

링압인을 이용한 피로균열의 성장지연효과

균열주위에 링압인을 부가함으로서 피로균열을 지연하는 수법을 제시하였다. 균열주위의 잔류응력분포는 Bueckner가중함수를 이용한 파괴역학적 수법으로 평가하였다. 본 연구는 균열을 가진 재료에 대한 파단시까지의 피로수명을 향상시키기 위한 단순하면서도 표과적인 수법을 개발하는데 있다. 알루미늄재료에 대한 피로실험결과 본 수법이 피로지연효과를 효율적으로 얻을 수 있는 것으로 나타났다. A method for the retardation of fatigue crack growth using ring indentation at the vicinity of a crack is examined. Residual stresses near crack tip are evaluated using fracture mechanics approach using the Bueckner weight function. The motivation is to develop a simple and effective method for obtaining an increase in fatigue lives to total failure of materials with crack. Fatigue testing of aluminum specimen showed that the retardation effects are observed after the application of the method.

Effect of ring indentation on fatigue crack growth in an aluminum plate

In the present investigation, a more efficient method for retardation of fatigue crack growth is examined. Residual stresses were created around the crack tip by indenting a cracked panel with rigid ring punches. The residual stresses near the crack tip were evaluated using fracture mechanics principles with the Bueckner weight function. It was observed that a state of compressive residual stress is introduced inside the indentation area. Fatigue testing of an aluminum specimen with single edge crack showed that the significant retardation effects were observed after the application of the ring indentation. It is also shown that the ring-indentation process increases the total fatigue life.

Fatigue crack growth in SiC particulates reinforced Al matrix graded composite

The SiC/Al graded composite was fabricated by powder metallurgy processing and its fatigue crack growth behavior was studied. The volume percentage of SiC particulates was distributed from 5 to 30% layer by layer on the cross section. Since the aluminium was dissolved together, there was no evident interface between the two layers with different volume fraction of SiC particulates. Fatigue crack growth was in direction of from 5 to 30% SiC layers under sinusoidal wave-form. The retardation of fatigue crack growth was found when crack propagated from low volume fraction of SiC to high volume fraction of SiC. The crack deflection and branching between two layers were observed, which decreased crack growth rates. In view of crack tip driving force, the plasticity mismatch between the layers shielded crack tip driving force, i.e. decreased the effective J-integral at the tip of the crack as the plastic zone of the crack tip spread from the weaker material into the stronger material.

균열선단의 소성스트레치를 이용한 피로균열성장모델

피로균열성장모델을 유도하고 지연모델을 제안하였다. 피로균열성장모델은 피로균열선단의 소성변형으로 인하여 균열표면에 발생하는 잔류소성스트레치를 고려하고 있다. 균열 성장률은 균열선단 재료요소의 소성변형에너지와 누적피로손상으로부터 계산된다. 유도한 균열성장모델로부터 계산한 균열성장률은 AL6061-T651과 17-4PH 주강의 시험결과와 잘 일치하고 있다. 피로균열성장지연모델은 인장과대하중으로부터 생성된 잔류소성스트레치를 근거로 하고 있으며, 인장과대하중은 다음 하중 사이클의 소성변형률을 감소시킨다. Strip-yield모델을 이용하여 균열선단의 소성역을 계산하였다. 새로 제안된 지연모델은 인장과대하중하의 피로균열선장특성 및 지체지연 현상을 잘 기술하고 있다. The fatigue crack growth model is derived and the retardation model is proposed. The fatigue crack growth model considers the residual plastic stretch on the crack surface which results from the plastic deformation at the tip of fatigue crack. The fatigue crack growth rate is calculated by using the cumulative fatigue damage and plastic strain energy in the material elements at the crack tip. This model gives the crack growth rate in reasonable agreement with test data for aluminum alloy AL6061-T651 and 17-4PH casting steel. The fatigue crack growth retardation model is based on the residual plastic stretch produced from a tensile overload which reduced the plastic strain range of the following load cycles. A strip-yield model of a crack tip plasticity is used for the calculation of a plastic zone size. The proposed retardation model characterized the observed features and delayed retardation of the fatigue crack growth under tensile overload.

Effect of single and multiple overloading on the residual fatigue life of a structural steel

ABSTRACTThis work was carried out to determine the effect of overload cycles on the fatigue life of a structural steel used for offshore applications. Single and multiple overloads were adopted and the corresponding fatigue crack growth retardation was evaluated. Residual stress fields were measured in the vicinity of the crack tip using an X‐ray diffraction technique and their size compared with that of the overload cyclic plastic zone. In regard to crack growth retardation, the results indicated that the extension in fatigue life increases with an increase in overload, as a consequence of the generation of higher compressive residual stress levels over a larger distance ahead of the crack tip. The effect of two equal and consecutive overloads, with the second one applied at different intervals of crack propagation from the first, was also considered. Larger intervals were shown to lead to a longer residual fatigue life.

Fatigue crack growth retardation in spot heated mild steel sheet

A fatigue crack can be effectively retarded by heating a spot near the crack tip under nil remote stress condition. The subcritical spot heating at a proper position modifies the crack growth behaviour in a way, more or less, similar to specimen subjected to overload spike. It is observed that the extent of crack growth retardation increases with increasing level of overload as well as with increasing spot temperature. It is also observed that modification in crack growth behaviour is a function of location of heating spot and maximum retardation is observed at + 5 position.

Spot heating induced fatigue crack growth retardation

Retardation in a growing fatigue crack can be achieved by heating (to a sub-critical temperature) a spot ahead of the crack. The retardation behaviour is comparable to that obtained in a specimen subjected to an overload spike. It is observed that the retardation increases with increase in spot heating temperature. The maximum retardation is obtained in specimens spot heated at a position 5 mm ahead of the crack tip.

735 SiC 粒子強化 Al 基傾斜複合材料の疲労亀裂伝ぱ挙動

The SiC/Al graded composite was fabricated by powder metallurgy processing and its fatigue crack growth behavior was studied. Fatigue crack growth was in direction of from 5% to 30%SiC layers under sinusoidal wave-form. The retardation of fatigue crack growth was found when crack propagated from low volume fraction of SiC to high volume fraction of SiC. The crack deflection and branching at interfaces were observed, which decreased crack growth rates. In view of crack tip driving force, the plasticity mismatch between the lavers shielded crack tip driving force, i.e., decreased the effective J-integral at the tip of the crack as the plastic zone of the crack tip spread across the interface from the weaker material into the stronger material.

An Experimental Study on the Factors that Affect Fatigue Crack Growth Retardation Behavior in S45C Steel

Constant ΔK fatigue crack growth tests were performed by applying an intermediate multiple overload for S45C steel. The purpose of the present study is to investigate effects of specimen thickness at various baseline stress intensity factor range levels (ΔK(sub)b), overload application position (a/W) and overload application frequency (OL(sub)HZ) on fatigue crack growth retardation behavior. The principal results are summarized as follows. The amount of retardation for a given ΔK(sub)b level is increased with increasing the baseline stress intensity factor range level for all specimen thickness. The normalized minimum crack growth rate is increased with increasing the specimen thickness, except for ΔK=45MPa√m. The retardation cycle is decreased with increasing a/W and increased with OL(sub)HZ.

Effects of a single tensile overload on fatigue crack growth in a 316L steel

ABSTRACTThe aim of this study was to investigate the effects of a single tensile overload on subsequent fatigue crack growth in a 316L stainless steel. Fatigue tests were conducted under the plane stress condition, and further supplemented with compliance measurements and field emission scanning electron microscopy (FESEM) observations. Effects of a tensile overload, e.g. initial acceleration and subsequent retardation of fatigue crack growth, were explained and quantified by FESEM and compliance measurements. The FESEM observations suggest that the initial crack growth acceleration stems from void and quasi‐cleavage fracture within the fatigue damage zone in the vicinity of the crack tip. Systematic compliance measurements taken during fatigue crack growth suggest that the overall crack growth retardation is related to strain hardening and residual compressive stress produced by the plastic deformation associated with the tensile overload.

LOAD INTERACTION EFFECTS DURING FATIGUE CRACK GROWTH UNDER VARIABLE AMPLITUDE LOADING—A LITERATURE REVIEW. PART I: EMPIRICAL TRENDS

A summary is given of reported trends in fatigue crack growth observed in variable amplitude fatigue tests on metallic materials, specifically on steels, under both simple and complex load histories. The effects of load variables, specimen geometry, material properties, microstructure and environment are considered. Attention is given to the threshold behaviour and small crack effects. The reviewed data suggest that, depending on a particular combination of load parameters, material, geometry and environment, variable amplitude load sequences of the same type can produce either retardation or acceleration in fatigue crack growth.

FATIGUE CRACK GROWTH BEHAVIOR OF VARIOUS STRUCTURAL STEEL AFTER SINGLE AND PERIODIC OVERLOADS

In order to evaluate accurately fatigue life of weldments under variable amplitude loading, load interaction effect due to load sequence has to be clarified. A simple case is the fatigue crack growth retardation effect resulting from single or periodic overloads. In the present study, fatigue crack growth rates were measured under single and periodic overload conditions on center-cracked tension specimens of structural steel SS400, SM520B and HT780. Effects of several parameters including overload ratio, stress ratio, yield strength of material and interval between overloads on fatigue crack growth retardation were investigated.