Fastran crack closure model

Newmans fastran calculations have been implemented and additional modifications made to include roughness induced crack closure ricc. A fatigue crack growth structural analysis program. Crack growth modeling in order to predict the crack growth life using industry codes such as nasgro, darwin, or afgrow, an adequate data set must be evaluated and fit to a crack growth model, to create a model file compatible with the software intended for use. However, other models, such as the fastran crack growth equation with the analytical crackclosure model, include several model parameters that make calibration challenging. Fastran is a computer program for calculating the rate of fatigue crack growth by combining crack growth equations and a simulation of the plasticity at the crack tip. The plasticityinduced crackclosure model, fastran, is able to predict the fatigue lives based on the size and geometry of initial flaws, as well as the fracture mechanic.

Other afgrow user options include five load interaction retardation models closure, fastran, hsu, wheeler, and generalized willenborg, a strainlife based fatigue crack initiation model, and the ability to perform a crack growth analysis for structures with a bonded repair. In previous work, fatigue crack growth tests were conducted on middle crack tension, mt, and compact, ct, specimens made from the same d16cz clad aluminum alloy sheet. He developed the theory for predicting plasticity induced crack closure and software fastran, which is widely used in the aircraft industry. Crack configuration options in fastran twodimensional crack configurations 14middlecrack tensioncrack tension compact and bend type specimens cracks from an open hole crack in a pressurized cylindercrack in a pressurized cylinder periodic array of cracks at holes user defined crack configuration. Citeseerx mixed gaussian process and statespace approach. A closure model to crack growth under largescale yielding.

This paper presents improvements to the statespace model, through enhancements in the calculations of the constraint. Note that the current fastran model and code are only based on plasticityinduced crack closure behavior and is a tstress 0 model. Fatigue limit is the main parameter used to calculate the eifs. Fatigue crack closure due to surface roughness and plastic. Based on this model, fastran requires the use of a master curve for the fcg rate d a d n versus. Fastran models accelerations and retardation and other variable amplitude loading effects in crack growth using a crack closure model. Fastranii is a lifeprediction code based on the crackclosure concept and is used to predict crack length against cycles from a specified initial crack size to failure for many common crack configurations found in structural components fastranii is an update of the previous fastran. In previous work, fatiguecrackgrowth tests were conducted on middlecrack tension, mt, and compact, ct, specimens made from the same d16cz clad aluminum alloy sheet. Jun 29, 2017 however, other models, such as the fastran crack growth equation with the analytical crack closure model, include several model parameters that make calibration challenging. Application of a stripyield model to predict crack growth. These features are unique to fastran and allow more accurate life predictions. As the load is increased, a critical value will be reached at which time the crack becomes open. Nasa langley research center hampton, virginia usa abstract an assessment of the plasticityinduced crack closure concept is made, in light of some of the questions that have been raised on the validity of the concept, and the. Statespace modeling of fatigue crack growth in ductile alloys.

Mixed gaussian process and statespace approach for. The plasticityinduced crack closure model developed by newman is first extended by using the cyclic crack tip opening displacement as the correlating parameter for fatigue crack growth rates. Validation of the statespace model of fatigue crack. Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. An existing geometric asperity based model for roughnessinduced crack closure under constant amplitude conditions was evaluated and extended. K th, relationship of the threshold stress intensity with stress ratio r, and the equivalent initial flaw size eifs. Statespace modeling of fatigue crack growth in ductile alloysp ravindra patankar, asok ray. However, studies show that the fatigue limit may not exist for metallic materials in the very high cycle fatigue fatigue life longer than 10 7 cycles in general 42, 43. A probabilistic approach to modeling microstructural.

Application of a closure model to predict crack growth in. This new approach also enables a better characterization of the effect of. Newmans fastran ii 17,18 computer program utilizes. Roughnessinduced crack closure in the threshold region. An evaluation of the plasticityinduced crackclosure. Development of an optimization tool for calibrating crack. Modelling combined hcflcf fatigue crack growth under the. Crack closure is an important factor affecting fatigue crack growth in high strength alloy materials. A simple fatigue life prediction algorithm using the modified. Results indicated that in the presence of large voids ie, lack. Key words suggested by authors fatigue crack growth crack closure ti6al4v titanium. It is known that the growth behavior of small cracks is far different from that of large cracks 29, 30, and the smallcrack growth mechanism has not been fully understood yet.

The objective of this paper is to apply the fastran crackclosure model to correlate constantamplitude crackgrowth rate data on 2024t3 aluminum alloys and to predict crack. A widely recognized propagation life prediction code, based upon the crack closure model, is fastran ii developed by newman 4 at the nasa langley research center. Fastran incorporates nonlinear or plastic strain material behavior and load interaction effects acceleration or retardation using a crack closure concept. Quantitative fractography results were also used to infer crack closure levels in some cases. Prediction of crack growth under variableamplitude. Citeseerx modelling combined hcflcf fatigue crack growth. Physically is based on dugdale yield zone model, which formation depends on a stress state in the part and can be expressed by coefficient.

Fastran ii is a lifeprediction code based on the crack closure concept and is used to predict crack length against cycles from a specified initial crack size to failure for many common crack configurations found in structural components fastran ii is an update of the previous fastran. Utilization of partial crack closure for fatigue crack growth. The physics driven part has a state variable formulation with underlying physics based on the crack closure concept used in the fastran ii model. Mixed gaussian process and statespace approach for fatigue. This new approach also enables a better characterization of the effect of largescale yielding on short crack growth. Plasticity is known to be the main driver of closure, but in some materials and at some stages other mechanisms such as fracture surface roughness and debris accumulation are also important. Although the crack growth equation in the statespace model is similar to that in the fastranii model newman, 1992, the two models use different algorithms for calculating so. However, the available experimental growth rates are routinely. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in fastran ii by using a constraint factor on tensile yielding at the crack front to. Dualfrequency prediction using the crack closure concept. Fastran plasticityinduced crackclosure model the plasticityinduced crackclosure model was developed for a central through crack and twosymmetric through cracks emanating from a circular hole in a.

This paper presents improvements to the statespace model, through enhancements in the calculations of the constraint factor. Even when these advanced crack growth models which are based on the crack closure are proposed, it is still a tough task today to calculate the fatigue life by directly integrating the crack propagation model from the initial defect size to the critical crack length. Nasa langley research center hampton, virginia usa abstract an assessment of the plasticityinduced crackclosure concept is made, in light of some of the questions that have been raised on the validity of the concept, and the. Keff, as a function of crack growth rate, dadn, should be obtained from the largecrack data. The impact of using other cracktip parameters, such as the cyclic cracktip displacement, to model crackgrowth rate behavior was studied.

This baseline crack growth model consisted of the crack growth rate curve dadn vs. Analytical models based on the plasticity induced closure concept have been very successful in. In this study, the plasticityinduced closure model does not collapse the largecrackgrowth threshold. Roughnessinduced crack closure in the threshold region for. Strip yield modelling of fatigue crack under variable. An analysis of fatigue crack growth of a notched aircraft. Experimentally, improved local methods to measure crack closure behavior, or crack opening loads, must be used to validate andor improve the crack closure based lifeprediction models. Fastranii is a lifeprediction code based on the crackclosure concept and is used to predict crack length against cycles from a specified initial crack size to failure for many common crack. Fastran fatigue crack growth analysis of structures a closure model, computer software management and information center, university of georgia. The data driven part of the model uses a kernel based gp regression model, which is being considered as the equivalent of an infinite neuron neural network model. The life prediction model imbedded in the fastran code is newmans crack closure model. Note that the current fastran model and code are only based on plasticityinduced crackclosure behavior and is a tstress 0 model. Prediction of crack growth under variableamplitude loading.

The lifeprediction method used in fastranii is built around an analytical crackclosure model. The fastran model was not developed to be used for multiple crack offset cracks cases in which it is necessary to independently calculate crack growth for multiple crack tips. Prediction of fatigue lives in additively manufactured alloys based on the crackgrowth concept aref yadollahi1,2, mohammad j. In this paper, mechanics analyses of growing fatigue cracks that account for plasticityinduced closure are used to study load shed effects on crack opening levels during simulated threshold testing.

The objective of this paper is to apply the fastran crack closure model to correlate constantamplitude crack growth rate data on 2024t3 aluminum alloys and to predict crack. Journal of biomimetics, biomaterials and biomedical engineering materials science. The fastran lifeprediction code was used with either the crackclosure model or lefm procedures. This model attempts to simulate the experimental observation made. The closure model is based on the dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Experimentally, improved local methods to measure crack closure behavior, or crackopening loads, must be used to validate andor improve the crack closurebased lifeprediction models. Formulation of the crack growth model in statespace setting the statespace model of crack growth is formulated based on mechanistic principles of the crackclosure concept and is supported by fatigue test data for variableamplitude cyclic loading e. Validation of the statespace model of fatigue crack growth.

Fatiguelife prediction methodology using a crackclosure. The popular fastran code was developed by newman and uses his wellknown numerical crack closure model, which assumes that the plastic wake behind the crack tip controls crack closure phenomenon. Thus, variableamplitude load histories did not have to be reordered to capture the damaging effects of major and minor load. The attractiveness of the crack closure model for engine applications is its ability to accurately predict. In this paper, the fatigue limit is determined based on the test data in the region where the fatigue life is larger than 10 6 cycles. The program uses a strip yield model of the crack tip that was first proposed by d. These tests were used to generate the effective stress. Basic crack solutions required for closure model crack solutions. Although the crack growth equation in the statespace model is similar to that in the fastran ii model newman, 1992, the two models use different algorithms for calculating so.

Both finite element and fastran strip yield models are employed, and the two approaches are compared. Researchers report on the prediction of low cycle, high cycle and combined cycle fatigue in turbine blades using the crack closure code fastran. The impact of using other crack tip parameters, such as the cyclic crack tip displacement, to model crack growth rate behavior was studied. K effrate for lpbf inconel 718 are shown in figure 3. Prediction of fatigue lives in additively manufactured alloys. The plasticityinduced crack closure model developed by newman is first extended by using the cyclic cracktip opening displacement as the correlating parameter for fatigue crack growth rates. A statespace model of fatigue crack growth in ductile alloys under variableamplitude load was presented by patankar and ray patankar et al. In this paper, a fatigue crack growth prediction code, fastran, based on newmans plasticity induced closure model, is employed to facilitate the estimation of crack growth rates under combinations of lcf and hcf loading sequences with and without the overload effect.

The key finding from the work was that roughness induced crack closure effects can be. These tests were conducted over a wide range of stress ratios r pminpmax 0. To simplify the calibration process and improve the accuracy of crack growth simulations, an automated crack growth model optimization tool was developed. An evaluation of the plasticityinduced crackclosure concept. To perform fatiguelife calculations, effective stress intensity factor. Utilization of partial crack closure for fatigue crack. The physics driven part has a state variable formulation with underlying physics based on the crack closure concept used in the fastranii model. Spectrum loading and surface finish effects in al7075t73. The lifeprediction method used in fastran ii is built around an analytical crack closure model.

Crack closure occurs from the presence of material propping open the crack faces and can arise from. Fastran is a computer program for calculating the rate of fatigue crack growth by combining crack growth equations and a simulation of the plasticity at the crack tip fastran models accelerations and retardation and other variable amplitude loading effects in crack growth using a crack closure model. Fatiguelife prediction methodology using a crackclosure model. In the mid1970s, the fastran ii lifeprediction code was developed by dr. Stress state factor evaluation based on a fractographic. Plasticity is known to be the main driver of closure, but in some materials and at some stages. Pdf fatigue crack closure due to surface roughness and. Fastran ii is a lifeprediction code based on the crack closure concept and is used to predict crack length against cycles from a specified initial crack size to failure for many common crack. Prediction of fatigue lives in additively manufactured. The implications of crack closure on fatigue crack are considered. Generally, fastran is a crack growth prediction approach based on a crack closure concept. Analysis of fatigue crack closure during simulated. The experimental results show that crack closure cannot fully account for the effect of stress ratio, r, on crack growth, and that it cannot be regarded as the sole cause for delay.