Crack Closure Effect Research Articles

The fatigue crack behavior of 7N01-T6 aluminum alloy in different particle environments

An experimental method of evaluating the fatigue behavior of alloys in different particle environments was designed, and the effects of four kinds of particles (i.e., graphite, CaO, Al2O3, and MnO2) on the crack propagating behavior of 7N01-T6 behaviour alloys were investigated. The results show that the particles deposited on the crack surface exert significant influence on the fatigue crack propagation behavior thereof. This influence strongly depends on the elastic moduli of the particles (Ep). As Ep is less than that of aluminium alloy (EAl), the particle accelerates the fatigue-crack-growth rate (FCGR) in the alloys due to the lubrication of the particles on the mating fracture surfaces. When the difference between Ep and EAl is small, the particle effect on the FCGRs of the alloys is small due to the counteraction between the decrease in friction and the promotion on the crack closure of mating fracture surfaces. When Ep is greater than EAl, the particles slow down the FCGRs of the alloys on account of significant crack closure effect. As Ep is much greater than EAl, the particles increase the FCGRs because of the increasing stress concentration at the crack tip.

Evolution of nano-cracks in single-crystal silicon during ultraprecision mechanical polishing

Abstract The evolution of nano-cracks in mono-crystalline silicon during three-body polishing was investigated by three-dimension molecular dynamics (MD) simulations and theoretical calculations. Atomic displacement, coordination numbers (CN), temperature and stress during the polishing process are studied in detail. The results show that cracks will close rather than expand in ultraprecision mechanical polishing process. The simulation results show two different ways for crack closure due to different crack inclinations: the coordinated displacement of atoms and the phase transformation filling of atoms. Crack closure will result in significant changes in CN. Furthermore, the crack closure effect is the best at 600 K. The calculation shows an obvious uneven stress distribution along the crack line in the polishing process. In addition, the distribution of the normal stress and shear stress at the lower tip of cracks are related to the mode of closure, indicating that simulation is related to the calculated stresses in theoretical model.

Effect of pearlite interlamellar spacing on fatigue property of a wheel steel

AbstractDifferent pearlite interlamellar spacings of 0.54 % carbon (C) wheel steel were obtained by heat treatment, and the influence of interlamellar spacing on fatigue behavior of the steel was studied through conducting the staircase method fatigue testing and decreasing/increasing stress intensity factor range (ΔK) fatigue crack growth tests. The results indicate that the fatigue endurance limit and fatigue threshold with smaller interlamellar spacing are higher than those with larger one, which can be well explained by dislocation slipping theory. Furthermore, the fatigue crack growth rates in the near‐threshold and Paris regions were found to reduce with decreasing the interlamellar spacing. The decreased growth rate is attributed to the deflected crack path induced crack closure effect, as evidenced by fatigue steps on the fatigue fracture surface. The present results show how to enhance the fatigue property of wheel steel through refining the pearlite interlamellar spacing.

Accounting for crack closure effects in TMF crack growth tests with extended hold times in gas turbine blade alloys

Crack closure effects are known to have a large impact on crack growth behaviour. In this work, tests were performed on Inconel 792 specimens under TMF loading conditions at 100–850 °C with extended hold times at tensile stress. The effective stress-intensity range was estimated experimentally using a compliance-based method leading to the conclusion that crack closure appears to have a primary impact on the crack growth behaviour for this material under the conditions studied. The crack closure behaviour for the tests was successfully modelled using numerical simulations, including creep.

Quantitative characterization of laser ultrasonic based on energy loss and resonance phenomenon

A novel method is proposed for quantitative characterization of cracks by laser ultrasound. It is found that cracks with different depth will have different feedbacks to sound waves based on crack closure effect, energy loss theory and resonance phenomenon. Therefore, damage sensitive characteristic parameters can be extracted from the acoustic waves and used to quantitatively characterize the crack size, including the waveform duration, the reflection energy coefficient and frequency spectral area. In this paper, cracks with gradient depth are used as experimental objects, and the relevant data is obtained through simulation and experiments. The defects are positioned, and the positioning error is less than 3%. The study found that as the crack depth increases, the waveform duration and the reflection energy coefficient gradually decrease, and the frequency spectral area first increases and then decreases. When the crack depth is larger than 0.9mm, characteristic parameters tend to be fixed values. When the crack depth is 0.5mm-0.7mm, abrupt phenomena occurred in both the reflection energy coefficient and the frequency spectrum area due to resonance phenomenon. By analyzing the corresponding relationship between damage sensitivity characteristic parameters and depth, the ability of laser ultrasonic quantitative measurement can be improved.

Fatigue life prediction model of metallic materials considering crack propagation and closure effect

To consider the effect of crack closure on the mechanical properties of metals, the fatigue life of metal specimens is predicted based on energy dissipation model. The main feature of the model consists in considering the relationship between the total failure energy and the energy density increment. The total failure energy model considers the fatigue crack size and stress amplitude. It is assumed the energy density increment gradually decreases and tends to be stable. The influence of crack closure effect is considered. According to the law of metal fatigue characteristics, a new mathematical model for predicting fatigue life is established.

Near-threshold fatigue crack growth rates of laser powder bed fusion produced Ti-6Al-4V

When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependant on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependant crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependant on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V.

Mechanism of Precipitate Microstructure Affecting Fatigue Behavior of 7020 Aluminum Alloy.

The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 107 cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m1/2.

Numerical determination and experimental verification of the optimum autofrettage pressure for a complex aluminium high‐pressure valve to foster crack closure

AbstractThe determination of the optimum autofrettage pressure enables a clear improvement of the fatigue life for an internally highly pressurized component. The autofrettage process induces residual compressive stress after the release of a single static overload pressure, leading to plastic deformation at the inner wall whereas the outer part is only elastically stressed. This autofrettage pressure is clearly above the subsequent pulsating operating pressure range. Due to the complex geometry of the aluminium valve body, a detailed elastic–plastic finite element analysis is used to determine the critical area and the optimum autofrettage pressure. Based on an experimental stress–strain curve, three important load steps are simulated in a non‐linear way. The FKM guideline is used to assess fatigue life and crack initiation with detailed subsequent experimental verification. Even if small cracks occur, residual compressive stresses prohibit crack growth (nonpropagating crack), which can be analytically verified by fracture mechanical considerations (crack closure effect).

Elevation of the Fatigue Strength of Pump Rods as a Result of Treatment with a Special Medium

We analyze the causes of the in-service loss of integrity of the steel rods of pumps due to the fatigue fracture of their main part as a result of propagation of the surface cracks. To increase the service life of the rods, it is proposed to use a special liquid working medium inhibiting the process of propagation of fatigue cracks as a result of the artificial creation of the effect of crack closure. As a result of scaled-down experiments on the high-cycle fatigue of rods made of 35KhM and 20N2M steels performed after long periods of operation, it was discovered that, on the one hand, a 3% NaCl solution exerts a noticeable negative influence on the fatigue strength and, on the other hand, the working medium exerts a significant positive effect. This effect manifests itself in the increase in the fatigue strength of the rods not only in corrosive media but also in air. For the practical realization of the method under the field conditions, we propose a design of a special chamber, which is fixed on the rod and covers its part.

Effects of Crack Closure on the Fatigue Crack Growth Rates of Ferritic Steels Subjected to Severe Reversing Loads

Abstract Low-alloy steels are extensively used in pressure boundary components of nuclear power plants. The structural integrity of the components made of low-alloy steels can be evaluated by the flaw evaluation procedure provided by Section XI of the ASME Boiler and Pressure Vessel Code. According to the Code, the stress intensity factor range ΔK can be used to determine the fatigue crack growth rates of the material. However, it has been reported that the fatigue crack growth rate under severe reversing loads is also strongly influenced by crack closure behavior. This paper discusses the relation between applied stresses and the fatigue crack growth rate for cracks in low-alloy steels exposed to air. Compressive-tensile cyclic loadings are applied to center-notched plates to obtain the fatigue crack growth curves. The test data demonstrate that effective stress intensity factor range predicted by our closure model described the crack growth property more accurately. A comparison among crack closure models indicates that our crack closure model is suitable to predict the crack growth rates when low constraint conditions are assumed at the crack tip due to severe reversing loads.

Near-threshold fatigue crack propagation without oxide-induced crack closure

An accurate value for the threshold stress intensity factor range, ΔKth, is a key parameter for studying crack-like defects. However, it is difficult to obtain accurate ΔKth values due to oxide-induced crack closure. In this study, we report conditions for minimizing the effects of oxide-induced crack closure near the threshold region, where a concave curve of the effect on the loading frequency on oxide-induced crack closure is achieved. The resulting conditions allow for an accurate determination of ΔKth, which is a key material parameter relating to the pertinent loading ratio.

Study on fatigue crack growth rate of 15CrMo steel based on stress ratio and corrosion environment

In this paper, the fatigue crack growth rate of 15CrMo steel, a typical gas cylinder material, is studied under the condition of simulating the actual working condition of gas cylinder. The low-frequency corrosion fatigue tests with stress ratio of 0.1 and 0.5 are carried out in air and different concentrations of wet hydrogen sulfide environment. The relationships between da/dN and ΔK are expressed by Paris equation. The results show that the crack growth rate in hydrogen sulfide environment with concentration of 200 ppm is 27 times higher that in air. The addition of corrosion environment greatly fasts the crack growth rate of 15CrMo steel. With the increase of H2S concentration, the crack growth rate increases exponentially. With the increase of stress ratio, the average stress increases, the crack closure effect decreases, the crack tip is fully opened, and the contact area with corrosion medium increases, which results in the increase of crack growth rate.

Effects of crack closure and friction for linear crack under normal impact

This paper concerns the fracture mechanics problem for elastic cracked materials under transient dynamic loading. The nonlinear contact problem for a linear crack under normal impact loading is solved by the boundary integral equations in the frequency domain and the components of the solution are presented by the Fourier exponential series. The algorithm convergence is proved and the solution is obtained for different friction coefficients. The dynamic stress intensity factors are computed for different stress pulses and compared with those obtained neglecting the crack closure and friction.

Crack closure in the fatigue delamination of composite multidirectional DCB laminates with large-scale fiber bridging

The stress ratio is commonly known to have an effect on the fatigue delamination growth (FDG) behavior of composite laminates. The crack closure phenomenon during delamination growth may change the value of stress ratio. Whether the stress ratio effect on the FDG behavior is resulted by the crack closure or not is investigated in this study and experimental evidences are presented in order to obtain a reliable conclusion. Using the quasi-static unloading approach, the crack closure loads during fatigue tests are firstly determined. The effect of crack closure on the mode I FDG in multidirectional composite laminates, which are widely used in engineering field, was experimentally studied. Test results show that the crack closure has an obvious effect on the minimum fatigue load and a linear relationship exists between the crack closure load and the delamination length. However, the influence of crack closure on the FDG rate is negligible. Further discussions illustrate that the crack closure does not fully explain the stress ratio effect in composite multidirectional laminates with large-scale bridging fibers.

Modeling of Multi-scale Fatigue Crack Growth in Titanium Alloy TC4

A modified multi-scale fatigue crack growth rate model for titanium alloy TC4 is provided in this investigation. The model gives a unified expression to evaluate the crack growth rate from MSC to LC stage by incorporating the effects of crack closure, large-scale yielding, and microstructural dissimilitude on crack growth behavior. Results show that the present model is able to describe the multi-scale crack growth rate for titanium alloy TC4. The model concurrently considers the grain size distribution and can reflect the large fluctuation of small-crack growth rate to some extent. Additionally, through integrating the multi-scale crack growth rate model, the crack growth life of TC4 can also be evaluated. The present model has a good prediction ability for the crack growth life and the residual fatigue life of TC4.

Finite element modeling of face/core interface fracture in homogenized honeycomb core sandwich SCB specimens

Fracture mechanics analysis of a face/core interface crack in a honeycomb core sandwich under mode I-dominated loading conditions is considered. The main objective of the study is to examine how to represent the homogenized in-plane extensional properties of hexagonal honeycomb core under deformation constraints due to bonding of the core to the face sheets. 2D parametric finite element analysis is conducted over a wide range of homogenized in-plane extensional properties of the core. The results show that plane stress assumption may lead to crack closure effects and highly distorted core elements for core with low in-plane modulus, while plane strain stabilizes the core deformation. Experimental results from SCB testing of a honeycomb core sandwich support the plane strain constrained modulus approach to face/core debond analysis.

Fatigue Life Evaluation Method for Foundry Crane Metal Structure Considering Load Dynamic Response and Crack Closure Effect

To compensate for the shortcomings of quasi-static law in anti-fatigue analysis of foundry crane metal structures, the fatigue life evaluation method of foundry crane metal structure considering load dynamic response and crack closure effect is proposed. In line with the theory of mechanical vibration, a dynamic model of crane structure during the working cycle is constructed, and dynamic coefficients under diverse actions are analysed. Calculation models of the internal force dynamic change process of dangerous cross-sections and a simulation model of first principal stress-time history are established by using the steel structure design criteria, which is utilised to extract the change of first principal stress of danger points over time. Then, the double-parameter stress spectrum is obtained by the rain flow counting method. The fatigue life calculation formula is corrected by introducing a crack closure parameter that can be calculated by the stress ratio and the effective stress ratio. Under the finite element model imported into Msc. Patran, crack propagation analysis is performed by the growth method in the fatigue integration module Msc. Fatigue. Taking the metal structure of a 100/40t-28.5m foundry crane with track offset as an example, the accuracy of calculation results and the feasibility and applicability of the proposed method are verified by theoretical calculation and finite element simulation, which provide a theoretical basis for improvement of the fatigue resistance design of foundry cranes.

Near-Threshold Fatigue Crack Growth Rates of Laser Powder Bed Fusion Produced Ti-6Al-4V

When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependent on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependent crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependent on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V.

Fatigue crack growth behaviour in Ti6Al4V alloy specimens produced by selective laser melting

The current study presents the fatigue crack growth behaviour of titanium alloy Ti6Al4V parts manufactured by selective laser melting (SLM), obtained as standard 6 mm thick compact specimens (CT). Both the crack propagation under constant amplitude loading and the transient crack growth behaviour after the application of overloads were studied. The effect of the mean stress and the transient retardation behaviour were analysed using the crack closure parameter, obtained both by compliance and digital image correlation techniques. A reduced crack closure level for the stress ratio $$\hbox {R}=0$$ was detected and for $$\hbox {R}=0.4$$ no crack closure was observed. The digital image correlation technique showed better results in the Paris regime and during the transient retardation behaviour. The overload application produced crack growth retardation due to the increase of the crack closure effect. The failure surfaces showed a transgranular crack growth in $$\upbeta $$ phase contouring the martensitic $$\upalpha $$ phase.