Critical Plane Parameter Research Articles

Multiaxial fatigue evaluation using discriminating strain paths

Fatigue life and available cycle counting methodologies based on the critical plane approach are examined under discriminating axial-torsion strain paths with random and incremental changes in straining direction. Fatigue lives for quenched and tempered 1050 steel with no non-proportional hardening were found to be more sensitive to non-proportionality of loadings as compared to 304L stainless steel with significant non-proportional hardening. Proportional or in-phase axial-torsion cycles with different axial to shear strain ratios within an equivalent strain circle when applied in a random sequence resulted in significant additional hardening for 304L stainless steel, similar to the non-proportional cyclic hardening observed in 90° out-of-phase loading. In contrast, when such cycles are applied with a gradual increment of the axial to shear strain ratio, the stress response of 304L stainless steel is closer to that observed for in-phase loading. However, the sequence of loading did not significantly affect fatigue life for either material. Experimentally observed failure planes for all strain paths were in very good agreements with predicted failure planes based on the Fatemi–Socie critical plane parameter. Finally, fatigue lives for both materials under various strain paths were predicted satisfactorily employing Fatemi–Socie parameter, Palmgren–Miner linear damage rule, and either Bannantine–Socie or Wang–Brown cycle counting method.

Multiaxial fatigue of titanium including step loading and load path alteration and sequence effects

Load sequence effects on low cycle multiaxial fatigue behavior of titanium are investigated in this study. Constant and variable amplitude axial, torsion, as well as in-phase and out-of-phase axial–torsion strain-controlled tests were conducted on tubular specimens made from pure titanium and titanium alloy BT9. Load non-proportionality had no effect on deformation behavior and cyclic stress–strain data were correlated by von Mises equivalent criterion. However, non-proportional loading resulted in significantly shorter lives and fatigue data could not be correlated by the von Mises criterion. The Fatemi–Socie (FS) critical plane parameter was found to correlate constant amplitude data of both titanium grades very well. von Mises criterion and the critical plane parameter in combination with the linear cumulative damage rule were also employed for fatigue life prediction of both materials under various block loadings with different combinations of axial, torsion, and 90° out-of-phase strain paths. Predicted low cycle fatigue lives based on linear damage rule and the FS critical plane approach were in good agreements with the experimental observations. Predicted failure planes by the critical plane parameter were also found to be consistent with experimentally observed failure planes for all the loading blocks used in the experimental study.

Prediction of low-cycle contact fatigue life of sleeve–pin–shaft connections under axial and torsional cyclic loading

This study experimentally analyzes the contact fatigue life of the sleeve–pin–shaft connection specimens under cyclic axial and pure torsional loading. Based on the stress/strain results along the pin–hole edge of the shaft obtained using the finite element method, four stress-based and three strain-based critical plane parameters were considered to predict the locations of crack initiation and crack initiation life. This work describes in detail the contact finite element analysis and the interfacial stress/strain behavior of the connections under push–pull and torsional loading can be obtained. In the present paper, a comparison of predicted results with experimental data indicates that the Fatemi–Socie parameter expressed as the combination of the shear strain amplitude and the normal stress on the critical plane estimates crack initiation life and damage locations more accurately than other theories. The application of fretting wear parameters in the evaluation of crack initiation life of the studied connection specimens is also discussed.

Fatigue Life Evaluation of Press-Fitted Specimens by Using Multiaxial Fatigue Theory at Contact Edge

In the shrink or press-fitted shafts such as railway axles, fretting can occur by cyclic stress and micro-slippage due to local movement between shaft and boss. When the fretting occurs in the press-fitted shaft, the fatigue strength remarkably decreases compared with that of without fretting. In this paper fretting fatigue life of press-fitted specimens was evaluated using multiaxial fatigue criteria based on critical plane approaches. An elastic-plastic analysis of contact stresses in a press-fitted shaft in contact with a boss was conducted by finite element method and micro-slip due to the bending load was analyzed. The number of cycles of fretting fatigue and the crack orientation were compared with the experimental results obtained by rotating bending tests. It is found that the crack initiation of fretting fatigue between shaft and boss occurs at the contact edge and the normal stress on the critical plane of contact interface was an important parameter for fretting fatigue crack initiation. Furthermore, the results indicated that a critical plane parameter could predict the orientation of crack initiation in the press-fitted shaft.

Multiaxial low-cycle fatigue of 63Sn-37Pb solder

A series of low-cycle fatigue experiments of tubular specimens under uniaxial, torsional, and proportional multiaxial loading was performed on 63Sn-37Pb solder. Several criteria of multiaxial low-cycle fatigue life were evaluated based on the fatigue life data of 63Sn-37Pb solder. It was shown that the maximum normal strain approach gives life prediction on the unconservative side, but the maximum shear strain approach presents quite good predictions. The Lee-Kim-Nam (LKN) parameter gave an acceptable prediction, but the prediction for uniaxial fatigue life was on the conservative side. The shear-based critical plane parameters, Kandil-Brown-Miller (KBM) parameter and Chen-Xu-Huang (CXH) parameter, presented good predictions of multiaxial fatigue life.

Critical Plane Fatigue Modeling and Characterization of Single Crystal Nickel Superalloys

Single crystal nickel-base superalloys deform by shearing along 〈111〉 planes, sometimes referred to as “octahedral” slip planes. Under fatigue loading, cyclic stress produces alternating slip reversals on the critical slip systems which eventually results in fatigue crack initiation along the “critical” octahedral planes. A “critical plane” fatigue modeling approach was developed in the present study to analyze high cycle fatigue (HCF) failures in single crystal materials. This approach accounted for the effects of crystal orientation and the micromechanics of the deformation and slip mechanisms observed in single crystal materials. Three-dimensional stress and strain transformation equations were developed to determine stresses and strains along the crystallographic octahedral planes and corresponding slip systems. These stresses and strains were then used to calculate several multiaxial critical plane parameters to determine the amount of fatigue damage and also the “critical planes” along which HCF failures would initiate. The computed fatigue damage parameters were used along with experimentally measured fatigue lives, at 1100°F, to correlate the data for different loading orientations. Microscopic observations of the fracture surfaces were used to determine the actual octahedral plane (or facet) on which fatigue initiation occurred. X-ray diffraction measurements were then used to uniquely identify this damage initiation facet with respect to the crystal orientation in each specimen. These experimentally determined HCF initiation planes were compared with the analytically predicted “critical planes.”

A simple approach for multiaxial fatigue damage prediction based on FEM post-processing

Abstract A simple methodology to predict crack initiation life is described in the fatigue damage assessment of metallic structures typically used in ground vehicle industry. A phenomenological constitutive model is integrated with a notch stress–strain analysis method and local loads under general multiaxial fatigue loads are modeled with linear elastic FE analyses. The computed stress–strain response is used to predict the fatigue crack initiation life using effective strain range parameters and two critical plane parameters. The proposed methodology is employed in the fatigue test cycle prediction of the biaxial cornering tests of light-alloy wheels. Numerical simulations indicate that estimates using critical plane models provide better correlations between the cornering test cycles and predicted cycles. Also, comparisons in terms test failure locations and estimated crack initiation sites are given.

Investigation of high cycle and low cycle fatigue interaction on fretting behavior

Fretting-fatigue behavior and damage accumulation under a variable-amplitude cycling load is investigated in a configuration involving a cylindrical indenter in contact with finite width plate. Relative magnitudes of cyclic tangential and bulk loads not only affect the contact conditions, but also their relative positions with respect to each other. Several stick–slip conditions on the contact surface may develop during the application of variable-amplitude fatigue load, and these are secondary and tertiary slips as well as shake-down. Further, residual shear traction develops during the application of cyclic load. The appropriate characterization of fretting-fatigue behavior or life should, therefore, include the complete history of applied cyclic tangential and bulk loads. Furthermore, experiments from a previous study conducted under a variable-amplitude fatigue loading condition are analyzed to characterize the damage accrual from its individual components involving constant-amplitude fatigue load by incorporating the contact mechanics and a multi-axial fatigue critical plane parameter. This analysis shows that there is nonlinear damage accumulation during variable-amplitude fretting-fatigue load.

Combined experimental–numerical investigation of fretting fatigue crack initiation

This study investigated the fretting fatigue crack initiation behavior of titanium alloy, Ti–6Al–4V. Tests were conducted to generate fretting fatigue failures from 2×10 4 to 5×10 7 cycles at 200 Hz. Fractography was employed to determine number of cycles to crack initiation, crack location and angle of crack orientation. Finite element analysis was conducted based on the experimental information in order to assess the ability of two critical plane approaches to predict fretting fatigue crack initiation behavior; the Smith–Watson–Topper critical plane parameter and the maximum shear stress range critical plane parameter. When properly formulated, these parameters predicted number of cycles to crack initiation and location of crack initiation which were in agreement with the experimental counterparts. However, these two parameters predicted different orientation angles of crack initiation at the contact surface. Based on the observations of orientation angles, the combined experimental–numerical approach showed that the mechanism for fretting fatigue crack initiation was governed by the maximum shear stress range on the critical plane.

Closure‐free biaxial fatigue crack growth rate and life prediction under various biaxiality ratios in SAE 1045 steel

Biaxial fatigue tests were performed on thin‐walled tubular 1045 steel specimens in a test fixture that applied internal and external pressure and axial load. There were two test series, one in which constant amplitude fully reversed strains (CAS) were applied and another in which large periodic compressive overstrain (PCO) cycles causing strains normal to the crack plane were inserted in a constant amplitude history of smaller strain cycles. Ratios of hoop strain to axial strain of λ = −1, −0.625, −ν and +1 were used in each test series. Fatigue crack growth behaviours under CAS and PCO histories were compared, and revealed that the morphology of the fracture surface near the crack tip and the crack growth rate changed dramatically with the application of the compressive overstrains. When the magnitude of the compressive overstrains was increased, the height of the fracture surface irregularities was reduced as the increasing overstrain progressively flattened the fracture surface asperities near the crack tip. The reduced asperity height was accompanied by drastic increases in crack growth rate and decreases in fatigue life.Using a pressurizing device attached to the confocal scanning laser microscope (CSLM), crack opening measurements were obtained. Crack opening measurements showed that the biaxial cracks were fully open at zero internal pressure for block strain histories containing in‐phase PCO cycles of yield stress magnitude. Therefore, for the shear‐strained samples, there was no crack face interference and the strain intensity range was fully effective. For PCO tests (with biaxial strain ratios of −0.625 and +1), effective strain intensity data were obtained from tests with positive stress ratios for which cracks did not close. A number of strain intensity parameters derived from well‐known fatigue life parameters were used to correlate fatigue crack growth rates for the various strain ratios investigated. Predicted fatigue lifetimes based on a fatigue crack growth rate prediction program using critical shear plane parameters showed good agreement with the experimental fatigue life data.