Fatigue cracking simulation based on crack closure effects in Al-based sheet materials subjected to biaxial cyclic loads

Fatigue crack growth experiments have been carried out on cruciform specimens in the range of thickness 1.2–10 mm of Al-based alloys, loaded under constant (regular) and variable (irregular) amplitudes of uniaxial and biaxial loads, including sequences of various overloads. Different cases for crack closure effects are considered because of shear lips development, crack-growth direction re-orientation after multiparameters change of cyclic loads, by examining plastic blunting effect at a crack tip during an overload and interaction effects analyzing the crack retardation length and associated parameters together with their relationships. Crack closure effect because of rotation instability of material mesovolumes under biaxial compression–tension has suggested to analyse semi-elliptical cracks. Under biaxial cyclic loads in the range of load ratio-1.4 < λ < +1.5, and R-ratios from 0.05 to 0.8, for frequency variations ?, fatigue striation formation takes place beyond a crack-growth rate close to 4 × 10⁻⁸ m/cycle. The striation spacing and the crack-growth rate increase as the ?-angle of the out-of-phase biaxial loads increases (in the range of ? from 0° to 180°). Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method that involves an equivalent stress intensity factor K_e=K_IF(λ,R,?,?). The values of F(λ,R,?,?) are determined. The calculated crack growth period (predicted using F(λ,R,?,?)) in regular and irregular cases of cyclic loads, including material cracking after overloads, is correlated with the experimental data, and the error is of the order of 15%.     

Fatigue behaviour of a box-welded joint under biaxial cyclic loads

Multiaxial fatigue behaviour of box-welded (wrap-around) joints in a JIS SM400B steel (12-mm-thick plate) was examined using a biaxial fatigue test facility. For the specimen, two stiffeners were attached to a main plate by a CO₂ semi-automatic welding procedure. Residual stress measurements and finite element (FE) analyses were also performed. Fatigue tests were performed under both uniaxial and biaxial (mainly out-of-phase) cyclic loads, and both results were compared and examined. It was found that fatigue cracks in the biaxial fatigue test specimens were initiated at the boxing-weld toes and propagated almost in the direction of the lateral loads. This is considered to be due to the dominant direction of tensile residual stresses from welding and the stress concentration in the vicinity of the boxing-weld toe. From the relation between the strain range near a weld toe, Δε₅ , and the fatigue lives, it was found that crack initiation life, N_c , was almost equivalent in the biaxial and uniaxial fatigue tests, while the failure life, N_f , was slightly longer in the biaxial tests. However, when the fatigue lives are put in order using the stress range near a weld toe, Δσ₅ , the crack initiation life, N_c , in the out-of-phase biaxial tests (phase difference of π) is ~30% lower than in the in-phase biaxial and uniaxial tests, while the failure life, N_f , was almost equivalent in the biaxial and uniaxial tests. From these results, it is concluded that an increase in Δσ₅ (lowering of the minimum value of σ₅ ), induced by the out-of-phase lateral loads, leads to an increase in fatigue damage where the high tensile welding residual stresses exist in the vicinity of the boxing-weld toe. Finally, a simple life estimation for the biaxial fatigue tests was made using FE analyses and the results of the uniaxial fatigue tests, proving that the effects of the lateral loads should be taken into consideration.     

Fatigue life prediction of a woven fabric composite subjected to biaxial cyclic loads

The purpose of this paper is to apply the Bridging micromechanics model to simulate the fatigue strength and S–N curve of a plain-woven fabric reinforced composite subjected to multiaxial cycling loads. Only the in situ constituent fiber and matrix properties, including the S–N data under the same cycling condition as that applied to the composite and the fiber volume fraction, are required. A unit cell of the woven composite is subdivided into small slices, each of which can be considered as a unidirectional (UD) composite. The load shared by each UD composite can be determined based on an assemblage scheme, as long as the overall fatigue load has been given. The Bridging model is adopted to explicitly relate the internal stresses in the constituent fiber and matrix materials with the load shared by the UD composite, and further, with the overall fatigue load on the woven composite. These stresses are then detected using the maximum normal stress criterion of isotropic materials against the constituent fatigue strengths, and the composite fatigue failure (strength) is attained (defined) when a constituent fails. A plain-woven glass fiber fabric reinforced polyester matrix composite subjected to uniaxial and biaxial static and fatigue loads has been analyzed. Agreement between the predicted and available experimental results is reasonable.     

Fatigue behaviour of a box-welded joint under biaxial cyclic loading: effects of biaxial load range ratio and cyclic compressive loads in the lateral direction

ABSTRACT The biaxial fatigue of a steel plate (JIS SM400B) having a box‐welded (wrap‐around) joint was experimentally studied. Special concerns were focused on the effects of the biaxial load range ratio and compressive cyclic loading in the lateral direction. The direction of fatigue crack propagation under biaxial cyclic tensile loading, which has a phase difference of π, changed according to the biaxial load range ratio, R_xy = ΔP_x/ΔP_y. When R_xy was less than 0.56, fatigue cracks propagated along the toe of the weld in the x‐direction because the principal tensile stress range Δσ_y at that location exceeded the orthogonal value Δσ_x at the box‐weld toe. The fatigue lives in biaxial tests related well to the data from uniaxial tests when invoking the Δσ₅ criterion. However, the location and direction of Δσ₅ should be chosen according to the R_xy value and the failure crack direction. An increase in Δσ₅, as induced by the Poisson's ratio effect from either the out‐of‐phase tensile loading or the in‐phase compressive loading in the y‐direction, leads to an increase in fatigue damage (decrease in fatigue resistance or specifically a faster crack propagation rate), and this effect can be successfully estimated from uniaxial fatigue test data.     

Simulation of disruption loads on first-wall high-temperature materials

The behaviour of carbon materials under thermal load in fusion reactors has been simulated by laser-pulse irradiation in a scanning electron microscope (SEM). In this way material damage, such as thermal shock crack formation and propagation, and erosion behaviour, can be studied in situ in the SEM with high lateral resolution. The dependence of damage initiation and propagation on the laser-beam parameters (pulse number, energy, spot size, spot duration and energy density), is of special interest. The damage behaviour is strongly determined by special materials structures. Because of its fibre reinforcement, the investigated CFC composite materials proved to be more stable to erosion and crack formation than homogeneous finegrained graphites. High-temperature damage may be diminished by the use of carbon materials with creep-resistant components.     

Strength of composite laminates under biaxial loads

Five well known failure criteria and one simple progressive model have been used in conjunction with laminate theory, which allows for nonlinear lamina shear behaviour, to predict the initial and final failure strengths of filament wound composite tubes. The predictions have been compared with experimental leakage and fracture stresses for ±75°, ±55° and ±45° filament wound GRP tubes subjected to a wide range of biaxial stress systems including biaxial compression. In some cases the fracture strengths were a factor of 10 higher than the initial failure predictions. The simple progressive failure theory predictions gave the best agreement with the experimental results.©British Crown Copyright 1996, Defence Evaluation and Research Agency published by Kluwer Academic Publishers with permission.     

Behaviour of steel-fibre-reinforced concretes under biaxial compression loads

In the case of metal fibre concretes under biaxial compression, the gain of strength yielded by metal fibres can be considerable. Using a biaxial press with brush bearing platens (to avoid lateral confinement), we verified this result and studied the behaviour of metal fibre concrete subjected to biaxial loadings. Two different types of fibres were used. The results obtained demonstrate the following points: the addition of fibres makes the material much more ductile; there is an influence of the type of fibre on mode of failure, a gain of strength and an influence of the orientation of the fibres.     

Failure surfaces for brittle honeycombs with Plateau borders under in-plane biaxial loads

Brittle honeycombs without any macrocrack under in-plane biaxial loads fail by elastic buckling and brittle rupturing of cell edges. The failure surfaces of brittle honeycombs with non-uniform thickness cell edges are affected by the cell-edge modulus of rupture and the distribution of solid between three cell edges and a vertex. In the paper, the non-uniform cross-section of cell edges is taken to be a Plateau border. Meanwhile, the variability of cell-edge modulus of rupture is taken into account by assuming that it follows a Weibull distribution. As a result of those, the failure surfaces of brittle honeycombs with Plateau borders caused by elastic buckling and brittle rupturing for a prescribed survival probability can be generated. The effects of the solid distribution in cell edges, the Weibull modulus and the prescribed survival probability on the failure surfaces of brittle honeycombs are also evaluated.     

Fatigue behavior of aluminum alloys under biaxial loading

The biaxiality effect, especially the effect of non-singular stress cycling, on the fatigue behavior was studied, employing cruciform specimens of aluminum alloys 1100-H14 and 7075-T651. The specimens, containing a transverse or a 45^o inclined center notch, were subjected to in-phase (IP) or 100% out-of-phase (hereinafter referred to as “out-of-phase or OP”) loading of stress ratio 0.1 in air. The biaxiality ratio λ ranged from 0 to 1.5, and 3 levels of stress were applied. It was observed that: (1) at a given λ, a lower longitudinal stress induced a longer fatigue life under IP and OP loading, and the fatigue life was longer under IP loading, (2) the fatigue crack path profile was influenced by λ, phase angle (0^o or 180^o), and initial center notch (transverse or 45^o inclined); (3) the fatigue crack path profiles, predicted analytically and determined experimentally, had similar features for the specimens with a transverse center notch under IP loading; and (4) the fatigue crack growth rate was lower and the fatigue life longer for a greater λ under IP loading, whereas it changed little with change in λ under OP loading. These results demonstrate that non-singular stress cycling affects the biaxial fatigue behavior of aluminum alloys 1100-H14 and 7065-T651under IP and OP loading.     

Yield surfaces for hexagonal honeycombs with plateau borders under in-plane biaxial loads

Summary In this paper, we model three different yielding mechanisms of hexagonal honeycombs with nonuniform cell-edge thickness under in-plane biaxial loads to develop equations describing their yield surfaces. The nonuniform cell-edge thickness of hexagonal honeycombs is taken into account in determining the locations of plastic hinges and corresponding yield surfaces. Results indicate that the yield surfaces for hexagonal honeycombs with nonuniform cell-edge thickness depend on their relative density and the distribution of solid between three cell edges and a vertex. The effects of solid distribution on the yield surfaces, hydrostatic and deviatoric yield strengths of hexagonal honeycombs under in-plane biaxial loads are also investigated.     

Fracture loads for ceramic samples with rounded notches

Fracture loads of ceramic components with rounded notches cannot be computed by linear elastic fracture mechanics techniques because no stress singularity exists. We propose a procedure to estimate such fracture loads, which is based on the cohesive zone model and supported by experimental evidence with alumina, zirconia and silicon ceramics. Data from 18 ceramic materials and different notched geometries were used. The only material parameters needed were the tensile strength and the fracture toughness.     

Fatigue crack development rate with random changes in cyclic loads

On the basis of an analysis of the change in residual plastic deformations occurring in growth of fatigue cracks a model has been developed for calculation of the change in crack development rate in the zone of retarding (acceleration) of their development with a random change in loads and overloads. The proposed model was experimentally verified on VSt3sp-5 steel at temperatures of +20 and –50°C. Good agreement of the calculated and experimental results was obtained.Translated from Problemy Prochnosti, No. 4, pp. 25–30, April, 1990.     

Fatigue behavior of concrete subjected to biaxial loading in the compression region

Concrete hollow cylinders subjected to combined compression and torsion were used to simulate concrete airport pavements subjected to biaxial fatigue loading in the compression region. It was found that the increase in the compliance in the post-peak period is due to the damage evolution of the specimen. The static failure mechanisms was explained by fracture mechanics. Similar failure was observed in fatigue loading. It was found that with the crack growth as a parameter, the static response acts as an envelope for the fatigue failure response. The rate of the crack growth under fatigue loading follows a two-stage process: a deceleration stage followed by an acceleration stage up to failure. In the deceleration stage, the growth is governed by the R-curve of the specimen. In the acceleration stage, it is governed by the Paris Law. The previously proposed model in the biaxial tension region was extended to the biaxial compression region. In the biaxial compression region, static and fatigue behaviors under pure compressive loading were modelled in terms of inelastic displacement, rather than crack length.