Fatigue Damage Propagation Research Articles

Reconstruction and prediction of Mode-I cohesive law using artificial neural network

Cohesive zone model (CZM) is a widely used tool for simulating both static and fatigue damage propagation for bonded area in composite materials. Under static loading, the damage propagation behavior is governed by a cohesive law (CL), the shape of which is an important property especially when non-linear damage mechanisms such as fiber bridging is involved. In this work, a CZM driven by artificially neural network (ANN) for simulating mode-I damage propagation is proposed, where the traction-separation relationship is calculated by a multi-layer perceptron (MLP). More importantly, a reconstruction method is proposed to extract the CL through the training of the neural network using a simple load-displacement (P–U) relation of a double cantilever beam (DCB) as the only inputs, without the need to measure the crack opening displacement (COD). The proposed method is first validated using finite element generated virtual experimental data with various CL shapes before being applied to experimental data, where good correlations are obtained, proving the effectiveness of the proposed method.

Mesoscopic Analysis of Fatigue Damage Development in Asphalt Mixture Based on Modified Burgers Contact Algorithm in Discrete Element Modeling.

This study is aimed at examining the mesoscopic mechanical response and crack development characteristics of asphalt mixtures using the three-dimensional discrete element approach via particle flow code (PFC). The material is considered an assembly of three phases of aggregate, mortar, and voids, for which three types of contact are identified and described using a modified Burgers model allowing for bond failure and crack formation at contact. The laboratory splitting test is conducted to determine the contact parameters and to provide the basis for selecting three different load levels used in the indirect tensile fatigue test and simulation. The reliability of the simulation is verified by comparing the fatigue lives and dissipated energies against those from the test. Under cyclic loading, the internal tensile and compressive force chains vary dynamically as a response to the cyclic loading; both are initially concentrated beneath the top loading strip and then extend downward along the loading line. The compressive chains are oriented roughly vertically and develop an elliptic shape as damage grows, while the tensile chains are mostly horizontal and become denser. An analysis based on the histories of the numbers of different contact types indicates that damage mainly originates from bond failures among the aggregate particles and at the aggregate-mortar interfaces. In terms of location, cracking is initiated below the loading point (consistent with observations from the force chains) and propagates downward and laterally, leading to the macrocrack along the vertical diameter. The findings provide a mesoscopic understanding of the fatigue damage initiation and propagation in asphalt mixture.

Spar Cap/Shear Web Debonding Under Fatigue Loading Studied On The DTU 12.6m Wind Turbine Blade

Understanding fatigue damage initiation and propagation of composite wind turbine blades is an essential step towards the design of reliable prognostic and health management systems (PHM) and to improve lifetime predictions. This study presents a numerical investigation of the debonding process in the spar cap/shear web region of a full-scale composite wind turbine blade under fatigue loading and presents the manufacturing approach for creating initial debondings. For this purpose, numerical simulations have been performed to design and specify the size and locations of artificial defects to provoke steady, controllable damage growth in the adhesive bondline between the rear shear web and cap region of the DTU 12.6m wind turbine blade under fatigue loading. Subsequently, these artificial defects are embedded into the manufacturing process of the composite wind turbine blade with the help of slip foils and partially perforated slip foils to create regions with no bonding, 30% and 50% bonding of the contact surfaces of the adhesive joint, respectively.The numerical predictions as well as the manufactured and instrumented blade sets the framework to subsequently execute experimental tests of damage growth in the adhesive joint of the bondline in order to compare numerical and experimental data. The experimental demonstration will allow to better understand the complexity of damage growth of composite structures under fatigue loading and the necessity of accurate numerical prediction tools for reliable prognostic health management systems.

Fatigue damage propagation of FRP under coupling effect of cyclic load and corrosion

AbstractInterlaminar fracture or delamination is one of the major failure modes for fiber reinforced polymer (FRP) laminate. Especially, the synergistic effect of environmental exposure and fatigue loads accelerates interface degradation. The main objective in this work was to investigate the fatigue damage propagation of basalt FRP (BFRP) laminate under coupling of acidic environment and fatigue load. The fatigue cycling of 40,000 and the maximum stress of 30% fu (fu = ultimate tensile strength) were adopted. The fatigue specimens under air condition (air fatigue), immersing in water (water fatigue) and immersing in acid (acid fatigue) were introduced. The double cantilever beam (DCB) test was used to evaluate the interlaminar damage of BFRP laminate after fatigue cycling. The fatigue damage propagation mechanism was explored by means of failure modes, load displacement curves, mode I fracture toughness and crack propagation resistance model. The results show that the fatigue weakened the interlaminar properties and resulted in reduced fiber bridging. Especially for the acid fatigue, the laminate was damaged by the coupling of fatigue and acid corrosion. The linear loading stage, nonlinear slow growing stage, stable crack propagation stage and linear unloading stage for DCB after fatigue were observed in the load–displacement curve. The fatigue cycling has caused fracture toughness decrease of 15.0%, 13.8% and 24.1% on air fatigue, water fatigue and acid fatigue respectively. Compared to air fatigue, additional 9.1% interlaminar toughness decrease was evidenced for acidic corrosion fatigue due to the coupling effect of chemical corrosion and physical swelling. The fluctuating decline of crack resistance was caused by discontinuous interface damage, and the interlayer damage was randomly distributed. Thus the interface damage model was proposed to characterize the damage distribution and the damage size.Highlights Degradation law of BFRP under the coupling effect of acid immersion and fatigue load. Interface damage model for characterization of the damage distribution and damage size. Fatigue damage propagation of BFRP under coupling effect of cyclic load and corrosion.

Damage Propagation and Residual Strength of Simple Block-Loaded CFRP Plates with Circular Holes under Tension–Tension Fatigue Conditions

Holes and their effects on the fatigue behavior and damage propagation of thin-walled structural components remain objects of research. In this paper, the previously untreated effect of round holes in thin plain-woven carbon fiber-reinforced plastic plates subjected to simple block loading is examined, and the implication on both damage propagation and residual tensile strength is investigated. Using three-dimensional digital image correlation, the damage propagation in the performed experimental tests is acquired, and the damage size is quantified. The evaluations reveal a relationship between the damage propagation and applied load level, for which an empirical model has been previously established by the authors. As the number of cycles increases, a saturation behavior is found. Once the increased load is imposed on the plate, damage propagation resumes, leading to further damage propagation that can be described with the same empirical model as the initial damage propagation, including renewed saturation behavior. The subsequent experimental tests to determine the residual tensile strength reveal a positive effect of the existing damage size, as the ultimate load significantly exceeds the ultimate load of the non-damaged plate.

Study on fatigue damage and fatigue crack propagation of rubber concrete

In order to study the crack propagation process and damage mechanism of rubber concrete under fatigue loading, the fatigue test of plain concrete beam and rubber concrete beam with precast cracks was carried out, and the loading process was monitored in real time by acoustic emission. The results show that after the addition of rubber, the maximum deformation of rubber concrete under fatigue load is about twice that of plain concrete, and the fatigue life and fracture energy increase. It is confirmed by AE signals that there is hysteresis between cracks and loads under fatigue load due to the lag of energy transfer. The incorporation of rubber particles also reduces the range of changes in stress strength factors. The b-value of AE reflects the damage state and crack development of the concrete. When the b-value is reduced to 0.8, it can be used as an early warning indicator of structural failure.

Experimental investigation on damage evolution and failure mechanism of pre-corroded AA7075-T6 under multiaxial fatigue loading

To investigate the tension–torsion fatigue failure of pre-corroded AA7075-T6, fatigue damage accumulation, macro-crack formation and propagation are quantitatively characterized through 3D-DIC based strain maps and damage severity curves, micro-cracking characteristics are further examined by SEM, TEM, and EBSD observation. Experimental results show that corrosion pit induces fatigue macro-cracks formation, and the identified nucleation time, amount, and location are diverse for different loading conditions. Four dominated fatigue micro-crack initiation forms and corresponding mechanisms were analyzed, namely, pit bottom, sidewall dis-continuous surface, corrosion micro-feature (micro-pits/jut-ins), and pit subsurface, which depends on multi-factors involving corrosion macro-morphology, corrosion micro-morphology, loading condition and underlying microstructure.

Recyclable, Healable, and Tough Ionogels Insensitive to Crack Propagation.

Most gels and elastomers introduce sacrificial bonds in the covalent network to dissipate energy. However, long-term cyclic loading caused irreversible fatigue damage and crack propagation cannot be prevented. Furthermore, because of the irreversible covalent crosslinked networks, it is a huge challenge to implement reversible mechanical interlocking and reorganize the polymer segments to realize the recycling and reuse of ionogels. Here, covalent crosslinking of host materials is replaced with entanglement. The entangled microdomains are used as physical crosslinking while introducing reversible bond interactions. The interpenetrating, entangled, and elastic microdomains of linear segments and covalent-network microspheres provide mechanical stability, eliminate stress concentration at the crack tip under load, and achieve unprecedented tear and fatigue resistance of ionogels in any load direction. Moreover, reversible entanglements and noncovalent interactions can be disentangled and recombined to achieve recycling and mechanical regeneration, and the recyclability of covalent-network microdomains is realized.

Fatigue damage behaviors of pre-flawed granite containing hole and fissures exposed to variable-frequency multi-level constant amplitude (VFMLCA) cyclic loads

Rock mass is often encountered with complicated stress disturbance caused by human-induced engineering loads and the environmental loads. Cyclic or fatigue loading path with fixed loading frequency are widely performed. To date, however, damage and fracture evolution of the pre-flawed rock under variable-frequency multi-level constant amplitude (VFMLCA) cyclic loads was not well understood. In the present study, granite samples containing a circular hole and two fissures were prepared to conduct VFMLCA cyclic loading tests. The fatigue strength, deformation, energy conversion, damage propagation, and crack coalescence were comprehensively investigated. Testing results show that the fatigue strength increases with increasing fissure angle. Rock volumetric strain and the dissipated energy are the maximum for rock with a 50° fissure angle. The dissipated energy increases with decreasing of loading frequency and an exponential relation is revealed between them. In addition, a damage evolution model that can well describe the two-stage and three-stage damage evolution for each cyclic loading level and the entire loading process is proposed and proved. Moreover, three failure modes of single tensile coalescence, double tensile coalescence, and double shear coalescence were revealed from the reconstructed CT images. It is suggested that the double shear coalescence is prone to forming large crack network.

Effect of voids on fatigue damage propagation in 3D5D braided composites revealed via automated algorithms using X-ray computed tomography

The main objective of this study is to analyze the effect of voids on damage evolution in three-dimensional five-directional (3D5D) braided composites under fatigue loading. This was achieved by the combination of Micro-CT and novel image algorithms during the interruptive fatigue tests. The present methodology enables the automatic capture of the detailed sequence of events during individual voids’ coalescence to cracking. Also, the “two-step” damage classification method was used to analyze the progressive damage mechanism of 3D5D braided composites under fatigue loading. Debonding dominates the fatigue damage propagation in the material within 76.9% of fatigue life and accounts for 82.5% of all fatigue damage. The voids located at the interface of yarns were inferred to be the critical voids under fatigue loading. More significantly, the deflection of the crack front of the voids was quantified via automated algorithms. It is expected that this study can provide unparalleled quantitative data for the development of fatigue life prediction models of 3D5D braided composites with voids.

Fatigue damage propagation and creep behavior on sisal/epoxy composites

The lack of knowledge about the behavior under creep and fatigue limits the use of polymeric composites reinforced with natural fibers. Thus, this work assessed the behavior of epoxy composites reinforced with sisal fibers under tensile, fatigue, and creep tests. Also, thermogravimetry and scanning electron microscopy assessed the sodium hydroxide (NaOH) treatment efficiency in sisal fibers. Further, differential scanning calorimetry determined the degree of cure of the composites, and stereomicroscopy allowed the evaluation of the surface’s fracture. As a result, the tensile strength of the composite was 1.4 times the value of neat epoxy resin after 100,000 cycles in the fatigue test. Moreover, when loaded with 20% of the maximum tensile strength, it is estimated that the composite resists 200,000 h without rupturing by creep. To conclude, the efficient adhesion between sisal fibers and epoxy obtained by NaOH treatment allowed good mechanical behavior to the epoxy composite.

Compressive fatigue response and reliability analysis of thermoplastic composite with low‐velocity impact damage

AbstractThermoplastic composite has attracted more and more attentions for its higher fracture toughness and better damage tolerance than thermosetting composites. AS4/PEEK laminate is a representative thermoplastic composite, which has been widely used in the fabrication of fuselages and aircraft wings. In this paper, low‐velocity impact tests had been carried out on AS4/PEEK laminates. Different impact energy levels were selected for forming impact dent with depth of 1 mm for the three stacking sequences: [0/0/‐45/90/45/0/0/45/90/‐45/0]S, [0/0/45/90/‐45/0/45/90/‐45/0]S, and [45/‐45/0/45/90/‐45/0/45/‐45/0]S, which were 25.5, 21.0, and 20.0 J successively. Then, compression–compression fatigue tests with stress ratio of 10 and frequency of 10 Hz were used to obtain the residual fatigue life and damage propagation. It was found that significant local buckling appeared around the impact dent, which extended perpendicular to the loading direction during the compressive fatigue loading. The increase of the transverse dimension of local buckling could be divided into three stages. The experimental results indicated that a joint Weibull distribution could be used to describe the residual compressive fatigue life of the impacted laminates. This paper assumed an equal rank relationship between the residual strength and fatigue life, and the B‐basis value of fatigue stress was calculated to ensure the reliability of the impacted AS4/PEEK laminates, which were 0.53, 0.50, and 0.53 (normalized by residual compressive stress) at 106 cycles for the three kinds of laminates mentioned earlier.

Experimental and numerical study on debonding damage propagation of brazed titanium alloy honeycomb under fatigue load

In this paper, the fatigue damage propagation of honeycomb structures under three-point bending fatigue load is studied by experiment and numerical simulation. Firstly, the three-point bending fatigue experiment of honeycomb structure with prefabricated damage was carried out, and the length of interfacial debonding with loading cycle was studied. Then, the damage propagation model of honeycomb structure is established based on VCCT theory and Pairs formula. The results show that the numerical results are in good agreement with the experimental results. The numerical model was validated.

Flexural fatigue behaviors and damage evolution analysis of edge-oxidized graphene oxide (EOGO) reinforced concrete composites

Edge-oxidized graphene oxide (EOGO) produced through a low-cost mechanochemical process has a high potential for bulk-scale applications. This study investigates the effect of EOGO on flexural fatigue behaviors of concrete composites. The results demonstrate that the fatigue performances of normal concrete and steel-fiber reinforced concrete (SFRC) are enhanced by EOGO of 0.1% by weight of cement. Single-logarithm fatigue equations are proposed for different mixtures and failure probabilities. The results show that EOGOs reduces the flexural strains over the fatigue life and increases the stiffness. By adding EOGOs, however, larger plastic deformation was observed in the SFRC than in the normal concrete as the fatigue damage is accumulated. For the EOGO-mixed SFRC, the fatigue damage propagation was mainly observed in the second half of fatigue life, whereas that of EOGO-mixed concrete appeared mostly in the first half. Morphological and elemental analyses of the EOGO-cement paste were performed. The analysis results indicated that EOGOs are mainly detected in the areas of calcium-silicate-hydrate (C–S–H) and calcium-aluminum-silicate-hydrate (C-A-S-H) as hydration proceeds. The experimental results support an interfacial bonding mechanism between EOGO and C–S–H/C-A-S-H through oxygen groups, creating the links of COO–Ca, O–Ca, and O–Al. As a result, EOGOs can bridge over microcracks; thus, not only reduce creep deformation but also delay crack initiation by extending the range of behavior before the macrocracks.

Earthquake-induced damage updating for remaining-life assessment of steel frame substructure systems

This paper presents a framework combining theoretical, numerical, and experimental approaches for evaluating the remaining-life of high-rise steel buildings based on earthquake-induced fatigue damage propagation in local beam-to-column connections. A mesh-independent finite element model is developed and implemented using the concept of lumped damage mechanics to estimate the fatigue-induced life loss associated with the extent of crack propagation until complete rupture failure occurred in beam-to-column connection substructure of high-rises. In this framework, a damage state variable is introduced at each end of beam elements and can be linked to the extension of the fatigue fracture in real frame building joints. Then, the damage evolution law is developed by extending the conventional Manson–Coffin law and a new crack-driving parameter to quantify the extent of damage states and predict the remaining life of the associated components in high-rises buildings. The proposed model is validated by a damage index measured from the local wireless sensors in substructure tests of ductile steel beam-to-column connections subjected to ultra-low cycle fatigue loadings. The residual resistance can be quantified by updating the damage curve with the identified damaged conditions.

Reliability-Based Lifetime Fatigue Damage Assessment of Offshore Composite Wind Turbine Blades

This paper presents a method for stochastic deterioration modeling and fatigue damage assessment for composite wind turbine blades operating in offshore environments. The fatigue damage of the composite blades is analyzed and assessed based on the estimates for the applied loads along the blade span, stress analysis, fatigue crack evolution, and lifetime probability of fatigue failure. The complex stress states of the blade are mainly caused by the aerodynamic loads generated by corrected blade element momentum theory, gravity loads, and centrifugal loads. The fatigue of the wind turbine blade is then investigated on the basis of the actual fatigue damage propagation process. The stochastic gamma process is introduced to calculate the probability of fatigue failure of the blade for various critical limits, and these results together with lifecycle cost analysis are employed to determine the optimum maintenance strategy. Finally, a numerical example for a National Renewable Energy Laboratory 5-MW wind turbine blade is adopted to demonstrate the applicability of the proposed method. The numerical results show that the proposed approach can provide a reliable tool for estimating stress states, evaluating fatigue damage, analyzing lifetime fatigue failure probability, and optimizing repair time of the composite wind turbine blade.

Mesh size objective fatigue damage propagation in laminated composites using the multiscale discrete damage theory

A mesh size objective multiscale modeling is developed for fatigue failure prediction of long fiber-reinforced composites based on the multiscale discrete damage theory (MDDT). MDDT tracks the failure processes along discrete failure surfaces at the microscale and concurrently bridges it to continuum-based description of damage at the macroscale. The proposed approach achieves mesh-size objectivity by introducing a length scale operator which effectively adjusts the microstructure size as a function of macroscale element size; and when a non-additive fatigue damage evolution law is used to describe progressive cracking at the microscale. Temporal multiscaling is used to track long-term fatigue damage evolution with high computational efficiency. The performance of the proposed model is demonstrated by the analysis of unnotched and open-hole laminate configurations. The results indicate mesh-size objectivity even in the presence of multiple failure mechanisms including splitting, delamination and transverse matrix cracks. The interaction between splitting and transverse cracks is investigated by a parametric study, which reveals the effects of mode I and mode II dominated degradation on the failure behavior under fatigue loading.

On the use of neural networks for dynamic stress prediction in Francis turbines by means of stationary sensors

Nowadays, one of the major mechanical issues of hydraulic turbines and particularly Francis turbines are the failures produced by fatigue. Due to the massive entrance of new renewable energies such as wind or solar, hydraulic turbines have to withstand off-design conditions and multiple transients, which greatly increase the risk of fatigue failures. Fatigue damage and crack propagation models are based on the static and dynamic stresses on the turbine blades. Therefore, an accurate and realistic determination of these stresses is of paramount importance although it is yet a challenging task. Numerical simulations have still limitations when predicting static and dynamic stresses in the most harmful conditions such as deep part load conditions and transients, which are highly stochastic. The installation of strain gauges on the blades gives accurate stress measurement but it involves long and very expensive measurement campaigns, as the turbine runner is submerged, confined and rotating. In this paper we propose a neural network-based method to determine the magnitude of static and dynamic stresses based on the measurements of stationary sensors, which greatly reduces the complexity and costs of strain gauge testing. Inputs of the neural networks are selected based on previous experience monitoring Francis turbines. The trained network can be implemented in advanced monitoring systems that could continuously evaluate the stress level of the turbine and determine the risk of possible fatigue damage.

Morphology-induced fatigue crack arresting in carbon fibre sheet moulding compounds

Carbon Fibre Sheet Moulding Compounds (CF-SMCs) are tow-based composite materials. Interrupted fatigue tests, combined with computed tomography, were performed here to investigate the damage mechanisms in high in-mould flow CF-SMC. The tow-based microstructure created obstacles for fatigue damage propagation, increasing the CF-SMC’s resistance against cyclic loading. Failure is shown to nucleate inside the tows, but inter-tow crack propagation tends to be hindered by the presence of the other tows. Tows oriented perpendicularly to the initial fatigue crack stop the crack itself, showing an intrinsic crack arrest mechanism. Additionally, pre-existing manufacturing cracks or voids do not propagate at all. As a result, flatter slopes of the SN diagrams were observed for CF-SMC than for other carbon or glass fibre composites with short, long and even continuous fibres.

Fretting Fatigue Damage Nucleation and Propagation Lifetime Using a Central Point Movement of Power Spectral Density

This paper presents a new perception in evaluating fretting fatigue damage nucleation and propagation lifetime under periodically forced circulation. A new approach, which is proposed in this paper, is to measure the change of the central point of power spectral density (CP-PSD) in different structural stiffness degradation stages. A notable aspect of this study lies in the combination between vibration amplitude and forced frequency of the fatigue-causing factors in beam structures. Additionally, it is found that randomization of the first phase from 0 to 2π yields more accurate modelling of the fatigue phenomenon. Results show that the CP-PSD parameter is significantly more sensitive compared to the regularly damage-evaluating parameters such as natural frequency, eigenvalues, or stress value. This reflects different levels of fatigue cycle effect on the structure in the experiment. At the same time, CP-PSD also categorizes the degradation level on different points on the structure under the periodically forced circulation. In addition, this paper also quantifies the relation between the changes of CP-PSD and each fatigue state. Results of this research will be a reference source to evaluate the lifespan of the structure by experimental methods.