A fatigue damage parameter is presented which includes the influence of mean stress in an explicit form. The approach is based on the premise that damage caused by cyclic loading is a function of the stress range and the level of tensile... more
A fatigue damage parameter is presented which includes the influence of mean stress in an explicit form. The approach is based on the premise that damage caused by cyclic loading is a function of the stress range and the level of tensile mean stress. The predictions of the proposed method are compared with the experimental results and the agreement is found to be fairly good. Using the proposed damage parameter, fatigue data for different mean pressures are consolidated into a narrow scatter band.
Research Interests:
Research Interests:
Experimental and numerical results are presented on the transverse stress-strain response of composite systems depending on the applied load direction. Coupon specimens of 6061-T0 aluminum alloy with square array of circular holes (hollow... more
Experimental and numerical results are presented on the transverse stress-strain response of composite systems depending on the applied load direction. Coupon specimens of 6061-T0 aluminum alloy with square array of circular holes (hollow and with steel filament reinforcement) were used to simulate an ideal regular composite system. A different load direction was obtained by a rotation of the pattern of holes with respect to the longitudinal axis of the coupon sample. Numerical results were obtained by FEM analysis on unit cells. A change in the load direction results in the different unit cell to be used. The numerical results show a fair agreement with experimental data. The results of the perforated and reinforced periodic systems indicate the same trend in load direction dependency. This dependency is also affected by the presence of simulated voids. The effect of void pattern has a significant effect on the stress at failure. The variation in the failure stress due to change in load direction was also evaluated by preliminary macroscopic analysis which takes into account the observed failure modes.
Research Interests:
... The value of o-max was calculated from eq. (6a) by setting x = 8*. Based on the analysis in Section 3, the value of Ry for a particular ... [27] HV Staal and JD Elen, Crack closure and influence of cyclic ratio R on fatigue crack... more
... The value of o-max was calculated from eq. (6a) by setting x = 8*. Based on the analysis in Section 3, the value of Ry for a particular ... [27] HV Staal and JD Elen, Crack closure and influence of cyclic ratio R on fatigue crack growth in type 304 stainless steel at room temperature. ...
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
... 0 OS 1.0 1.5 2.0 Aa(mm) Fig. 10. Variation of normalized crack opening load, ^op/^inax' witll the crack extension, Aa =a uo. ... Int. Metals Rev. , 445-476 (1984). (2] NB Leis, AT Hopper and J. Ahmad, Critical review of the... more
... 0 OS 1.0 1.5 2.0 Aa(mm) Fig. 10. Variation of normalized crack opening load, ^op/^inax' witll the crack extension, Aa =a uo. ... Int. Metals Rev. , 445-476 (1984). (2] NB Leis, AT Hopper and J. Ahmad, Critical review of the fatigue growth of short cracks. Engng Fracture Mech. ...
Research Interests:
ABSTRACT
Research Interests:
his paper presents the results of an experimental investigation into transverse cracking of symmetric and unsymmetric glass-fibre/epoxy-resin laminates. The test specimens were fabricated in house and were of two types: symmetric [±... more
his paper presents the results of an experimental investigation into transverse cracking of symmetric and unsymmetric glass-fibre/epoxy-resin laminates. The test specimens were fabricated in house and were of two types: symmetric [± θ903]s with θ = 0 °, 25 °, 45 °, and unsymmetric [± gq/90602]T with θ = 25 ° and 45 °. Through-specimen-width matrix cracks were recorded visually.
Research Interests:
Research Interests:
In this article, we examine the 50 years of observations and interpretations into the subject of fatigue crack closure in lab air and aqueous solutions: disputes and implications. Over this period, several closure concepts have been... more
In this article, we examine the 50 years of observations and interpretations into the subject of fatigue crack closure in lab air and aqueous solutions: disputes and implications. Over this period, several closure concepts have been invoked to explain why the crack propagation threshold ΔKth should reduce with load ratio R. Among these, three types are the most popular and these are closure induced by crack (i) plasticity (PICC), (ii) oxides (OICC), and (iii) roughness (RICC). Therefore, these three and their role in shielding the fatigue crack tip are critically examined. This analysis finds that the reduction in ΔKth with increasing R‐ratio in lab air and aqueous solutions is weakly linked to decreasing crack closure behind the crack tip but strongly related to the access of the environment to the crack tip region and its influence on fatigue damage at and ahead of the crack tip. Supporting evidence for this interpretation comes from the lack of ΔKth dependence on R in a good vacu...
Research Interests:
In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of... more
In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of the fatigue crack tip are reexamined. Supporting evidence for these insights comes from the lack of Kth dependence on R in a high vacuum (with partial pressure of 10-5 Pa or less). The presented new critical chemical-mechanical analyses are based on experimental results reported in the literature that demonstrate the marginal R-ratio effect on Kth of long cracks in vacuum for both planar/wavy slip alloys but show R-dependence in the lab air and in chemical environment. The latter is due to the formation of viscous nature of the oxide, which forms in humid air at the newly expose fresh fracture surfaces. It is demonstrated that the dominant factor related to the experimentally observed R-ratio effects on fatigue crack growth (FCG) behavior (on several alloys) in not related to crack closure but the access of the environment to the crack tip region that affects fatigue damage. In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when crack flanks contact occurs, most of the oxidized metallic species will exist as aquo-complexes, gel, or colloids, that have insufficient shear strength to wedge crack faces during unloading. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. The roughness induced crack closure has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered.
Research Interests:
Most engineering components are subjected to cyclically varying loads with mean stress (or mean strain). Often the loads may result in a multiaxial stress state. It is well known that the fatigue process is sensitive to superposed tensile... more
Most engineering components are subjected to cyclically varying loads with mean stress (or mean strain). Often the loads may result in a multiaxial stress state. It is well known that the fatigue process is sensitive to superposed tensile mean stress in both the high-cycle and low-cycle fatigue regimes. In an elastoplastic regime, a material which experiences a stress-controlled cyclic loading with a non-zero mean stress, will exhibit an accumulated ratcheting strain (cyclic creep strain). This ratcheting strain can cause an additional damage in the material. Thus, a complete fatigue theory should be capable to evaluate the effects of mean stress and ratcheting strain. A large number of multiaxial fatigue theories have been proposed in the past and are reviewed in the literature, for example, see Refs [1–2]. These theories may be divided into four categories, viz: equivalent-stress and -strain, energy-based criteria and the critical plane approaches. The first two approaches usually have difficulty to include the mean stress effect and to account for path dependency of the cyclic plastic deformation. This essential interaction between stress path and strain path is inherently included in the energy approach. In the present paper an energy-based fatigue theory proposed by Ellyin and his collaborators [3] is extended to include the mean stress and ratcheting strain effects on the fatigue life of materials.
Research Interests:
The currently used American Society for Testing and Materials (ASTM) procedure and a modified American Petroleum Institute (API) method for cyclic pressure strength of fibre reinforced plastic (FRP) pipes are critically discussed together... more
The currently used American Society for Testing and Materials (ASTM) procedure and a modified American Petroleum Institute (API) method for cyclic pressure strength of fibre reinforced plastic (FRP) pipes are critically discussed together with the test results generated by Battelle. In addition, two new damage parameters, recently proposed by the authors, are examined using the Battelle tests as a database. An equivalent stress range damage parameter, Δ¯σ, is recommended which combined the loading condition, hoop stress range and normalized mean stress, and FRP pipe material properties. The proposed damage parameter, Δ¯σ, can be used in the engineering evaluation of fatigue performance of pressured fiberglass pipes. The Δ¯σ provides substantial improvement over the ASTM recommended parameter, hoop stress range. Using the Δ¯σ, the fatigue data for different mean pressures are consolidated into a narrow scatter band.
Research Interests:
Research Interests:
Research Interests:
Abstract In this paper a simple analytical method is proposed to estimate an elastic–plastic strain and stress in the plastic zone ahead of the tip of a stable growing fatigue crack. Analytical estimations are compared with experimental... more
Abstract In this paper a simple analytical method is proposed to estimate an elastic–plastic strain and stress in the plastic zone ahead of the tip of a stable growing fatigue crack. Analytical estimations are compared with experimental results from digital image correlations (DIC) and integrated finite element (FE) analyses taken from literature. A fairly good agreement is demonstrated between the proposed analytical method and the DIC and FE data.
Research Interests:
It is well recognized that crack length has significant effects on both the monotonic failure stress and cyclic threshold stress conditions. Existing experimental data indicates that LEFM predicts correctly the failure and threshold... more
It is well recognized that crack length has significant effects on both the monotonic failure stress and cyclic threshold stress conditions. Existing experimental data indicates that LEFM predicts correctly the failure and threshold stresses only for relatively long cracks. For small/short cracks, the LEFM overestimates both the failure and threshold stresses. Usually, an argument has been made that LEFM is violated in terms of a crack length with respect to the crack-tip plastic zone that results in overestimating the failure stress for small cracks. On the other hand, the Kitagawa diagram has been used to experimentally illustrate the connection between the observed threshold stresses for very small cracks and predictions from LEFM analysis.
Research Interests:
Research Interests:
It is well recognized that environment has a significant role on the failure of mechanically loaded structures. In most cases of cyclic loading, fatigue crack growth (FCG) behavior exhibits lower threshold and faster growth rate in air... more
It is well recognized that environment has a significant role on the failure of mechanically loaded structures. In most cases of cyclic loading, fatigue crack growth (FCG) behavior exhibits lower threshold and faster growth rate in air than in vacuum. It is well documented that the effect of loading frequency on FCG behavior can be more pronounced in aggressive environment/material systems. This is seen in the
Research Interests:
Research Interests:
A cloud-based fatigue analysis and life prediction tool is presented. Users around the globe may access it via Internet by means of multiple platforms such as desktop and laptop computers, tablets and/or smart-phones. It is an intuitive... more
A cloud-based fatigue analysis and life prediction tool is presented. Users around the globe may access it via Internet by means of multiple platforms such as desktop and laptop computers, tablets and/or smart-phones. It is an intuitive educational software, aimed also to assist designers in the pre-prototyping stage in fatigue life estimation for smooth or notched parts subjected to constant amplitude, block loading, and spectrum loading histories. For spectrum amplitude loading a dedicated spectrum software package is provided, which is essential for a potential clean-up and/or desired modifications of a raw spectrum data. Subsequently, a rainflow method is utilized and the corresponding hysteresis loops at the notch-root or critical “hot spot” location are determined and plotted. For notched components, an interactive Neuber’s master curve is utilized and discussed. It is shown, that the Neubers’s master curve is only material dependent and is applicable for both monotonic and cy...
Research Interests:
... The value of o-max was calculated from eq. (6a) by setting x = 8*. Based on the analysis in Section 3, the value of Ry for a particular ... [27] HV Staal and JD Elen, Crack closure and influence of cyclic ratio R on fatigue crack... more
... The value of o-max was calculated from eq. (6a) by setting x = 8*. Based on the analysis in Section 3, the value of Ry for a particular ... [27] HV Staal and JD Elen, Crack closure and influence of cyclic ratio R on fatigue crack growth in type 304 stainless steel at room temperature. ...
Research Interests:
In this paper, previously proposed two-parameter (Kmax and ∆K) driving force model is adopted for fatigue crack growth analyses for both positive and negative load ratios. It is based on the premise that the damage at the cracktip process... more
In this paper, previously proposed two-parameter (Kmax and ∆K) driving force model is adopted for fatigue crack growth analyses for both positive and negative load ratios. It is based on the premise that the damage at the cracktip process zone is an interplay of two damage mechanisms, namely a monotonic damage due to Kmax and a cyclic damage due to ∆K. Fatigue crack growth rate, for a constant amplitude loading, is represented by a new three dimensional crack propagation (CP) table in terms of Kmax and ∆K in accordance with the two parameter model. It is shown that the CP table provides a general representation of crack growth data for constant amplitude loading. Experimental data taken from literature for 7055-T7511 aluminum alloy under various load ratios ranging from -1 to 0.7 were used to illustrate the two parameter approach.
Research Interests:
Research Interests:
Research Interests:
A fatigue damage parameter is presented which includes the influence of mean stress in an explicit form. The approach is based on the premise that damage caused by cyclic loading is a function of the stress range and the level of tensile... more
A fatigue damage parameter is presented which includes the influence of mean stress in an explicit form. The approach is based on the premise that damage caused by cyclic loading is a function of the stress range and the level of tensile mean stress. The predictions of the proposed method are compared with the experimental results and the agreement is found to be fairly good. Using the proposed damage parameter, fatigue data for different mean pressures are consolidated into a narrow scatter band.
Research Interests:
A new technique that can determine the elastic-plastic properties of metallic materials using an instrumented indentation testing and iterative finite element (FE) simulations is proposed. This non-destructive technique can be applied to... more
A new technique that can determine the elastic-plastic properties of metallic materials using an instrumented indentation testing and iterative finite element (FE) simulations is proposed. This non-destructive technique can be applied to isotropic, additively manufactured, and/or surface treated metallic components of various scale. Currently, the measurement of material properties using the instrumented microor nanoindentation test is limited to the elastic modulus and surface hardness. A number of experimental and numerical approaches have been suggested for prediction of monotonic properties of metallic materials including yield strength, strain hardening parameters, ultimate strength, and fracture toughness. However, the past efforts to measure the stress-strain behavior using a single instrumented indentation test were not successful because there is no straightforward correlation between forcedisplacement relation and the elastic-plastic relation. In this study, both experimen...
Research Interests:
In this paper, previously proposed two-parameter (Kmax and ∆K) driving force model is adopted for fatigue crack growth analyses for both positive and negative load ratios. It is based on the premise that the damage at the cracktip process... more
In this paper, previously proposed two-parameter (Kmax and ∆K) driving force model is adopted for fatigue crack growth analyses for both positive and negative load ratios. It is based on the premise that the damage at the cracktip process zone is an interplay of two damage mechanisms, namely a monotonic damage due to Kmax and a cyclic damage due to ∆K. Fatigue crack growth rate, for a constant amplitude loading, is represented by a new three dimensional crack propagation (CP) table in terms of Kmax and ∆K in accordance with the two parameter model. It is shown that the CP table provides a general representation of crack growth data for constant amplitude loading. Experimental data taken from literature for 7055-T7511 aluminum alloy under various load ratios ranging from -1 to 0.7 were used to illustrate the two parameter approach.
Research Interests:
Research Interests:
In this paper, the strain energy density approach is used to characterize the fatigue resistance of metals. The results suggest that in the high- and low-life regions the total strain energy density is a consistent damage parameter.... more
In this paper, the strain energy density approach is used to characterize the fatigue resistance of metals. The results suggest that in the high- and low-life regions the total strain energy density is a consistent damage parameter. Predictions of the proposed method are in agreement with the experimental data, and demonstrate the appropiateness to predict the mean stress effect on fatigue life. A number of relations describing the effect of mean stress on fatigue resistance, are shown to be particular cases of the one developed herein.
Research Interests:
This paper presents the results of an experimental investigattion into the tensile monotonic and fatigue behaviour of symmetric fibreglass-epoxy coupons using three ply angle orientations, 45°, -45° and 0° which were interchanged in three... more
This paper presents the results of an experimental investigattion into the tensile monotonic and fatigue behaviour of symmetric fibreglass-epoxy coupons using three ply angle orientations, 45°, -45° and 0° which were interchanged in three different configurations: [45 2 , -45 2 , O 2 ] s , [45 2 , O 2 , -45 2 ] s and [O 2 , 45 2 , -45 2 ] s .The results show that there is significant sequcnce effect on the monotonic strength and the fatigue performance. The monotonic results demonstrate an incrcease of 11% in the strength and 6% in stiffness between the weakest laminate, [O 2 , 45 2 , -45 2 ] s , and the strongest laminate, [45 2 , -45 2 , O 2 ] s . The fatigue performance depends on lay-up and stress level. At higher stresses, the trend in stacking sequence effect on fatigue strength is the same as for monotonic loading. In contrast, at lower stresses the trend is opposite. The sequence effect observed in the monotonic and fatigue loading is discussed subsequently based on the nume...
Research Interests:
Research Interests:
This paper presents a damaging stress intensity function K for analyses of R-ratio effects on fatigue crack growth (FCG) in metals. The proposed formulation is based on the sum of strain and complementary energy and its role in FCG rate... more
This paper presents a damaging stress intensity function K for analyses of R-ratio effects on fatigue crack growth (FCG) in metals. The proposed formulation is based on the sum of strain and complementary energy and its role in FCG rate behavior in threshold and Paris region at R-ratios ranging from -2 to 0.97. It doesn’t invoke a crack closure assumption or fitting parameters for R<0.5-0.6. For a high R>0.7 it utilizes an experimentally determine correction factor, which accounts for excessive plastic dissipation in the monotonic plastic zone (MPZ).
ABSTRACT Following previous work (Krempl, 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room... more
ABSTRACT Following previous work (Krempl, 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room temperature. The test results show that the creep strain accumulated in a given period of time depends strongly on the stress-rate preceding the creep test. In constant stress-rate zero-to-tension loading the creep strain accumulated in a fixed time-period at a given stress level is always higher during loading than during unloading. Continued cycling causes an exhaustion of creep ratchetting which depends on the stress-rate. Periods of creep and relaxation introduced during completely reversed plastic cycling show that the curved portions of the hysteresis loop exhibit most of the inelasticity. In the straight portions, creep and relaxation are small and there exists a region commencing after unloading where the behavior is similar to that at the origin for virgin materials. This region does not extend to zero stress.The results are at variance with creep theory and with viscoplasticity theories which assume that the yield surface expands with the stress. They support the theory of viscoplasticity based on total strain and overstress.
Research Interests:
Byung-Joo Kim and Daniel Kujawski Fatigue Endurance Diagram for Materials with Defects REFERENCE: Kim, BJ. and Kujawski, D.," Fatigue Endurance Diagram for Materials with Defects," Fatigue and Fracture Mechanics: S4'h... more
Byung-Joo Kim and Daniel Kujawski Fatigue Endurance Diagram for Materials with Defects REFERENCE: Kim, BJ. and Kujawski, D.," Fatigue Endurance Diagram for Materials with Defects," Fatigue and Fracture Mechanics: S4'h Volume, ASTM STP 1461, SR Daniewicz, ...
Research Interests:
: The first part of this paper will outline the conception of the testing apparatus (Figure 1), along with its operation and preliminary results. The second part of the paper will discuss a new methodology used to correlate the dependence... more
: The first part of this paper will outline the conception of the testing apparatus (Figure 1), along with its operation and preliminary results. The second part of the paper will discuss a new methodology used to correlate the dependence of crack growth rate for strain crystallizing natural rubbers in terms of tearing energy. The tearing energy which depends on the type of elastomer, geometry and stress strain behavior of a particular specimen demonstrates a direct correlation with the crack growth rate at different R-ratios (= min tearing energy/max tearing energy).
Research Interests:
Abstract This work addresses FEM-based numerical simulations of fatigue crack propagation in a cruciform specimen under biaxial loading and small scale yielding (SSY) conditions. Three non-linear models are used as fatigue crack... more
Abstract This work addresses FEM-based numerical simulations of fatigue crack propagation in a cruciform specimen under biaxial loading and small scale yielding (SSY) conditions. Three non-linear models are used as fatigue crack propagation laws. The cruciform specimens are made of aluminium alloy D16T and modelled with non-linear material properties. In the experimental tests, starting from a surface flow mechanically created in the centre of the cruciform specimen, biaxial loading conditions are applied to create various pre-cracked configurations that correspond to different values of initial crack aspect ratios. In the numerical simulations, the material behaviour is modelled with a low hardening bilinear law. Each simulation is carried out with one of the established non-linear laws. At the end of the fatigue crack propagation simulations, several crack aspect ratios are obtained. Finally, these aspect ratios are compared with the corresponding experimental tests available in the literature.