Crack Tip Deformation Fields Research Articles

Multi-scale characterization of fatigue crack tip deformation and propagation in QP steel

A novel measurement method for multi-scale characterization of fatigue crack tip deformation and propagation behaviors in high strength and plasticity, multi-phase, fine-grain Quenching and Partitioning (QP) steel was proposed. The fatigue crack propagation (FCP) test system with the macro/micro images acquisition devices was built to measure the FCP rate and crack tip deformation simultaneously. First, at the mesoscale, the evolution characteristics of the crack tip deformation fields, plasticity induced crack closure (PICC) behaviors and plastic zone during the FCP process were analyzed using the sub-image stitching and matching digital image correlation (DIC) method. Then, the FCP models were established with special emphasis on the model considering the PICC effect based on the obtained crack opening load at the mesoscale. Moreover, the corresponding microscale surface crack and fracture morphologies, microstructure phase transformation and deformations were observed and analyzed by electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) techniques.

A study on J-integral and J-integral range ΔJ applicable to finite deformation elastic-plastic problems

This paper summarizes the formulations and some of the applications of the three-dimensional J-integral and J-integral range ΔJ that were proposed by the authors. The proposed crack parameters are evaluated by the domain integral method. Their values are guaranteed to be unconditionally independent of size and shape of integral domain by their formulation. The numerical examples demonstrate that the proposed crack parameters can characterize the severity of the crack tip deformation field. Hence, they can characterize the crack propagation behavior.

Finite deformation near a crack tip terminated at an interface between two neo-Hookean sheets

A crack terminating at an arbitrary angle to the interface between two neo-Hookean sheets is investigated under plane stress conditions using finite deformation theory. The asymptotic crack-tip deformation and stress fields are analyzed as a function of the ratio of the moduli and the angle of the crack relative to the interface. Full-field numerical calculations and experimental studies validate the analytical results. A stretch-based crack growth criterion is developed using crack-tip field solutions. Such criterion can predict the delay of crack growth through the bi-material interface observed in experiments and can be extended to any heterogeneity and material.

Effect of creep-fatigue loading condition on crack tip fields of grade 91 steel at crack initiation and growth

In this paper, the effect of creep-fatigue loading condition on crack tip deformation and stress fields of Grade 91 steel at 600 °C at crack initiation and growth is studied. By using finite element debond analysis method, creep-fatigue crack growth tests using C(T) specimens under various load ratios and hold times are simulated, and the effect of creep-fatigue load ratio and hold time on the crack opening profile, equivalent stress, crack-opening stress and triaxiality at crack initiation and growth is presented. It is found that the crack tip deformation and stress fields under tension–compression creep-fatigue loading are overall different from those under tension-tension creep-fatigue and pure creep loading, due to re-sharpening of the crack tip.

Evaluation of mixed-mode stress intensity factor and T-stress in continuous epoxy glass functionally graded beam using digital image correlation

In the present study, the fracture parameters and crack tip deformation field in continuous functionally graded (FG) glass-filled epoxy composite were experimentally investigated employing digital image correlation. Beams with monotonic variation Young’s modulus and cracks along the variation were studied. Two specimens with single edge cracks were examined in different crack positions in relation to the increase or decrease in Young’s modulus. The mechanical properties of the FG composite were extracted by performing a tensile test of samples machined according to ASTM D638. The stress intensity factors and T- stresses (high order term) were evaluated from the displacement field by adopting William’s series. This is the first research which calculated the mixed-mode stress intensity factors, as well as the T-stress in beam made of continuous epoxy glass functionally graded beam using DIC technique. The crack tip was elastically protected on the compliant side of the specimen unlike the crack on the stiffer side.

Crack tip fields at crack initiation and growth under monotonic and large amplitude cyclic loading: Experimental and FE analyses

In this paper, experiment and FE analyses are performed to investigate crack tip deformation and stress fields under monotonic and large amplitude cyclic loading. Fracture tests using C(T) specimens made of SA508 Gr.1a and TP316 are performed under monotonic and cyclic loading. Using the debonding analysis, it is found that the isotropic hardening model is relevant for monotonic loading, the kinematic hardening for SA508 Gr.1a under cyclic loading, and the Chaboche combined hardening for TP316 under cyclic loading. Using the relevant hardening model, crack tip deformation and stress fields at crack initiation and growth are presented.

一种针对裂尖变形场测量的正则化全局DIC方法

一种针对裂尖变形场测量的正则化全局DIC方法

Real-time dual-sensitive shearography for simultaneous in-plane and out-of-plane strain measurements.

A real-time, dual-sensitive shearography system using a single-wavelength laser was developed for simultaneous and dynamic in-plane and out-of-plane strain measurements. The shearography system is capable of measuring crack-tip deformation fields quantitatively. A spatial multiplexing technique based on Fourier transform is employed for simultaneous and dynamic multi-component phase retrieval. Two slit spatial filters and a common-path shearing interferometer are used to obtain an improved phase quality for crack-tip deformation measurements. Mode-I fracture experiments under three-point bending were conducted to validate the feasibility and the capability of this method.

Mapping the nonlinear crack tip deformation field in soft elastomer with a particle tracking method

Stretchable and fracture-resistant elastomers or gels are highly desirable in a wide range of existing and emerging applications. Fracture of soft materials, especially the ones with significantly enhanced toughness, typically involves severe deformation near the crack tip. However, experimental characterization of the crack tip deformation field in soft materials has been challenging due to its highly non-uniform and nonlinear nature. This is an important limitation towards a complete understanding of the intricate mechanics involved in soft material fracture. In this paper we present experimental data of the nonlinear, multiaxial crack tip deformation field obtained by tracking randomly distributed tracer particles. Specifically, we measured the fields of displacement and deformation gradient around the tip of a Mode-I crack in a soft silicone elastomer. We experimentally observed, for the first time, a region of validity for the asymptotic solution of crack tip deformation field in hyperelastic solids with strong strain stiffening. Furthermore, the measured crack tip deformation field enables local evaluation of energy release rate through the J-integral, which can remove the requirement of specific experimental geometries, such as the pure shear fracture test, to measure the fracture toughness. It also enables determination of the true crack extension length defined in the reference configuration. Based on these two capabilities, we were able to measure the crack growth resistance behavior and the rate-dependence of fracture toughness for soft materials with highly blunted cracks.

DIC-based studies of the overloading effects on the fatigue crack propagation behavior of Ti-6Al-4V ELI alloy

In this paper experimental correlations between da/dN and ΔK and between a vs. N of an extra-low-interstitial titanium (Ti-6Al-4V ELI) alloy under four typical overload conditions (OL, UL, OL-UL and UL-OL) in an otherwise constant amplitude loading were obtained. By means of the digital image correlation (DIC) technique, the crack-tip deformation fields as well as the crack opening process corresponding to the four overloading patterns were measured. The results shown that both the crack propagation behaviors and the crack-tip strain distributions of the Ti-alloy were greatly influenced by the overloading conditions applied. The effect of the overloading pattern (sequence) on the fatigue crack propagation behavior of the Ti-alloy was then analyzed by combining the crack-tip deformation measurements with the fatigue crack propagation results from three possible mechanisms, i.e., the crack-tip blunting effect, the crack closure effect and crack-tip plastic zone effect. It was found that for the present investigated Ti-6Al-4V ELI alloy, the above three mechanisms made contributions in different extents to the fatigue crack growth retardation behavior.

Finite deformation analysis of crack tip fields in plastically compressible hardening–softening–hardening solids

Crack tip fields are calculated under plane strain small scale yielding conditions. The material is characterized by a finite strain elastic–viscoplastic constitutive relation with various hardening–softening–hardening hardness functions. Both plastically compressible and plastically incompressible solids are considered. Displacements corresponding to the isotropic linear elastic mode I crack field are prescribed on a remote boundary. The initial crack is taken to be a semi-circular notch and symmetry about the crack plane is imposed. Plastic compressibility is found to give an increased crack opening displacement for a given value of the applied loading. The plastic zone size and shape are found to depend on the plastic compressibility, but not much on whether material softening occurs near the crack tip. On the other hand, the near crack tip stress and deformation fields depend sensitively on whether or not material softening occurs. The combination of plastic compressibility and softening (or softening–hardening) has a particularly strong effect on the near crack tip stress and deformation fields.

The Research on Oblique Crack Tip Deformation Field of Airship Envelope Composites Based on the Digital Speckle Correlation Method

The tearing mechanical properties and the deformation field distribution of 45 degree oblique direction of the crack tip was studied on the stratospheric airship envelope reinforced by plain woven Kevlar-fiber composite. Here, the material samples were prefabricated with the initial length of 20mm unilateral incision, and the digital speckle correlation experimental method was used to obtain the perpendicular crack tip displacement and strain fields with artificial spots under mono-axial tear test. The results show the deformation field changes with the characteristic of discrete step and the strain concentration phenomenon, and the strain concentration zone expands as increasing tensile load. The warp and weft step maximum displacement reaches 6.00mm and 1.7mm, and deformation in every step exhibits a diminishing step law, the warp deformation in the rear area of the crack tip are evenly, while the shear and weft strain volatile in a wide range

A constraint-equivalent approach for assessing fracture toughness of RPV steels under neutron irradiation

Neutron irradiation damages reactor pressure vessel (RPV) steels. As a result of material degradation, the mechanical properties of the material are changed and the crack tip stress and deformation fields are altered. The reduction of fracture toughness due to neutron irradiation is shown to be equivalent to a “constraint shift” exhibited by the ductile–brittle transition curve. Using the J–A2 two-parameter fracture theory to quantify the crack tip stress field and a critical crack opening stress as the fracture criterion, the fracture toughness of an irradiated RPV steel can be predicted from its nonirradiated value. The procedure and examples are given in the paper.

Speed-dependent tip fields of a moving crack in a hyperelastic material

This paper gives an asymptotic analysis of plane strain deformation and stress fields near the tip of a steady moving crack in a compressible hyperelastic material. The semi-infinite crack is assumed to be in an infinite homogeneous harmonic material under a constant remote mode-I loading. The crack tip deformation and stress fields are derived up to the third order, which ensures the strict positivity of the Jacobian determinant in the vicinity of the moving crack tip. The derived solutions are applied to a specific hyperelastic material for which some recent experimental data are available in the literature. The comparison with the experimental data showed that the crack-face profile and the energy release rate predicted by the present model are in reasonable agreement with experiments and several recent finite-strain elastic models. In addition, the crack branching angle predicted by the present model also agrees well with some known experimental data. All of these results suggest that the harmonic material model has the potential to capture the main features of finite deformation phenomena in the dynamic fracture of hyperelastic materials.

Mode I Fracture at Spot Welds in Dual-Phase Steel: An Application of Reverse Digital Image Correlation

Strain fields in 600 grade dual-phase steel V-notch tensile specimens, both with and without a spot weld, were measured after mode I fracture initiation. Starting with the final image of a fully developed crack, a novel reverse digital image correlation (DIC) analysis was used to determine the path that the crack followed at the onset of fracture as well as the crack tip deformation field. This gave the pixel coordinates of grid points on both sides (i.e. fracture surfaces) of the crack path in the undeformed image. Strain fields that develop in the base material regions surrounding the two fracture surfaces were subsequently measured with forward DIC analysis. Steady state values of the crack tip opening displacement (CTOD) and crack tip opening angle (CTOA), which are important fracture parameters, were measured for the base DP600 metal. Notch tip opening displacement (NTOD) and notch tip opening angle were also measured. It was found that steady state values of the CTOD and CTOA are reached within 2 mm or so of crack growth following completion of the flat-to-slant transition of the fracture surface and stabilization of the crack tunneling effect.

Suppressed plastic deformation at blunt crack-tips due to strain gradient effects

Large deformation gradients occur near a crack-tip and strain gradient dependent crack-tip deformation and stress fields are expected. Nevertheless, for material length scales much smaller than the scale of the deformation gradients, a conventional elastic–plastic solution is obtained. On the other hand, for significant large material length scales, a conventional elastic solution is obtained. This transition in behaviour is investigated based on a finite strain version of the Fleck–Hutchinson strain gradient plasticity model from 2001. The predictions show that for a wide range of material parameters, the transition from the conventional elastic–plastic to the elastic solution occurs for length scales ranging from 0.001 times the size of the plastic zone to a length scale of the same order of magnitude as the plastic zone.

Crack tip deformation fields and fatigue crack growth rates in Ti–6Al–4V

In this paper we present an overview of experimental and modelling studies of fatigue crack growth rates in aerospace titanium alloy Ti–6Al–4V. We review work done on the subject since the 1980s to the present day, identifying test programmes and procedures and their results, as well as predictive approaches developed over this period. We then present the results of some of our recent experiments and simulations. Fatigue crack growth rates (FCGRs) under constant applied load were evaluated as a function of crack length, and the effect of overload (retardation) was considered. Crack opening was measured during cycling using digital image correlation, and residual stress intensity factor was determined using synchrotron X-ray diffraction mapping. Modelling techniques used for the prediction of FCGRs are then reviewed, and an approach based on the analysis of energy dissipation at the crack tip is proposed. Finally, directions for further research are identified.

Full-Field Measurements of Fracture Initiation and Crack Growth in Superelastic Nitinol

Abstract We present results for full-field measurements of crack tip deformation fields in Nitinol using high sensitivity moiré interferometry. Three fatigue loading scenarios are examined illustrating the effect of stress induced martensite transformations at the tip of the crack. Understanding the behavior of fatigue crack growth is paramount to the successful use of this highly complex material response, especially for such application as implantable medical devices. The results demonstrate the sensitivity of heterogeneous phase transformations on the crack growth behavior.

Extension of the Coherent Gradient Sensor (CGS) to the Combined Measurement of In-Plane and Out-of-Plane Displacement Field Gradients

The Coherent Gradient Sensor (CGS) is extended to the optical differentiation of specular, diffracted wave fronts leading to the combined measurement of in- and out-of-plane displacement field gradients. A derivation of the underlying optical interference principles is presented along with an analysis of the effective instrument sensitivity. In order to demonstrate the capabilities of the technique, experimental measurements of crack-tip deformation fields were conducted under various loading conditions corresponding to mode-I, mode-II, and mixed mode near-tip crack fields. The experimental procedures and results of these tests are presented as validation of the technique.

Fracture investigation of PMMA specimen using coherent gradient sensing (CGS) technology

In this paper, the principle of coherent gradient sensing (CGS) technology in the transmission mode is briefly introduced. Fracture experiments of the PMMA specimens with offset crack under three-point-bending are performed using the CGS optical method. A series of CGS fringe patterns around the crack tip is used to study the fracture characteristics of the cracked specimens, such as the evolution of the crack tip stresses, deformation field and crack initiation, etc. Based on the K-dominance condition, the stress intensity factors at the crack tip are extracted by means of the distribution of CGS fringes, which show good agreement compared with the results from finite element analysis. The influences of the offset distance on fracture characterization are discussed. These results will be useful for evaluating the strength and lifespan of PMMA materials and structures.