A Study On The Residual Stresses Around The Crack Tip Based On The Depth-sensing Indentation Technique During Fatigue Crack Growth

Damage tolerance analysis and life assessment are necessary for fatigue-critical parts and structures (such as the aircraft, turbine engine) in services, which are generally subjected to complex variable-amplitude loadings. Load variations in magnitude or order lead to the changes in the residual stresses, plastic zone and damage process zone around the crack tip, which are responsible for accelerations and/or retardations in crack growth rate, and thus seriously influence the structural damage tolerance behavior and crack propagation life. Therefore, adequate evaluation and analysis of the residual stress and damage behavior near the crack tip caused by plastic deformation due to the typical overload spectrums (such as a single peak tensile/compressive overload and its combinations) is of great value for a better understanding of damage behavior under variable-amplitude loading, and thus for the development and improvement of damage tolerance design and fatigue life prediction model.In this paper current depth-sensing indentation methods to determine residual stresses have been reviewed. And the deficiencies and limitations of residual stress evaluation models were analyzed based on summarizing these methods. Four representative models for residual stress evaluation such as Suresh’s model, Carlsson’s model, Lee’s model and Wang’s model have been highlighted.Based on theoretical analysis and finite element simulation, the influences of residual stresses on the indentation response (F-h) curve, indentation morphology and so on were systematically studied. The correlation between indentation characteristic parameters (such as HV, S, hr/hmax and Wp/Wt,) and residual stresses was analyzed on focus. Results have shown that residual stresses have significant effects on the indentation characteristic parameters which present remarkable nonlinear relationship with residual stresses. It was found that the main factors affecting the nonlinearity are the intrinsic material properties such as the strain hardening exponent (n) and yield strain (σy/E). Based on the nonlinear effect of residual stresses on the indentation parameters, a unified model for calculating the residual stress using the sharp instrumented indentation technique has been established by analyzing and comparing the four representative models (Suresh et al’s model, Lee et al’s model, Carlsson et al’s model and Wang et al’s model), and it is found that all the four models can be expressed as the unified model. Numerical simulations are used to investigate the applicability of these four models for materials with different mechanical properties, and the results show that the accuracy of these models is dependent on both strain hardening exponent (n) and yield strain (σy/E). It is also indicated that Suresh et al’s model is more suitable for materials with a low strain hardening exponent and yield strain; Lee et al’s model seems more appropriate to materials with a medium strain hardening exponent and yield strain; Wang et al’s model has relatively good accuracy for materials with a high strain hardening exponent and yield strain; Carlsson et al’s model is approximately similar to Suresh et al’s model in the case of a low strain hardening exponent and yield strain.For the first time, elasto-plastic properties around the crack tip on M(T) specimens of aluminum alloy2524-T3were studied by depth-sensing indentation technique under a single tensile/compressive overload during crack propagation. First, indentation characteristic parameters around the crack tip before/after overloads were determined from the experimental load-penetration depth (F-h) curves, and the distribution of the above parameters around the crack tip was obtained and compared. The elasto-plastic properties (E,σy,n) of aluminum alloy2524-T3around the crack tip before/after overloads were estimated using a well-developed analysis algorithm proposed by Dao et al. with the above indentation parameters. The distribution around the crack tip of these mechanical properties was further discussed under a single tensile/compressive overload. The analysis showed that the combined effects of the changes of the crack tip plastic hardening, residual stress and damage seem to be the key factor to account for the gradient distribution of the elasto-plastic properties around the crack tip zone after overloads.Using the unified model based on depth-sensing indentation technique, the changing characteristics of residual stresses around the crack tip on M(T) specimens of aluminum alloy2524-T3were systematically studied under a single tensile overload or compressive underload. The load transient effects were analyzed and discussed according to the residual stress distribution around the crack tip before/after overloads. Results have shown that after single peak tensile overloads, an enlarged range of compressive residual stresses existed both in front of crack tip and in the wake of crack tip. The peak compressive residual stress value at the crack tip and the region of compressive residual stresses both increased with increasing Rol. It is thought that the combined effects of the changes in the residual-stress state both before and after the crack tip seem to be a key factor to account for the observed retardation phenomena. After single compressive underloads, the tensile residual stresses around the crack tip lead to the accelerations in crack growth rate. In addition, the crack tip shapes also affect the crack growth to a certain extent. In addition, the changing characteristics of the crack tip shape and fracture morphology were used to explain the fatigue crack growth retardation and acceleration behavior.In order to verify the applicability of the finite element simulation of residual stresses around the fatigue crack tip, the residual stress distribution near the crack tip was also analyzed using the elasto-plastic finite element method under the same fatigue loads in3single peak overloads corresponding to experiment. Comparisons of the results from FE simulation and IIT showed that the simulated residual stress fields around the crack tip agreed well with the measured data. It suggested that the estimation of crack-tip residual stress using FE simulation is feasible. It would lay a solid foundation for quantitative expression of the crack tip residual stress field and the follow-up improvement of crack propagation model.