Giancarlo L.G. Gonzáles

Determining SIFs Using DIC Considering Crack Closure and Blunting

Three analytical-experimental hybrid approaches for determining the range of stress intensity factors (SIF) and the pseudo-SIFs of fatigue cracks on the presence of crack closure, crack tip plasticity and blunting are presented and evaluated. These approaches use the Digital Image Correlation (DIC) technique to measure cyclic-varying displacement fields near the crack tip. The first uses displacement data of points located on the cracked component surface near and along the crack faces, finding the parallel and orthogonal displacements of symmetrical points with relation to the crack faces to determine the crack opening displacements (COD), which are placed in the crack displacement field equations to obtain the SIF values. The second determines coefficients of generalized Westergaard displacement field functions by an over-deterministic nonlinear Least Square scheme that allows the accurate localization of the crack tip coordinates. The third computes the J-integral along an arbitrary but elastic contour path placed around the crack tip to determine the SIF, using experimentally determined displacement gradients, the calculated stresses and the calculated energy densities for points along the path. The three approaches are applied to the case of cracks propagating in disk-shaped compact-tension DC(T) specimens subjected to mode I cyclic loads, considering the non-linear effects mentioned above.

Strain field measurements around notches using SIFT features and meshless methods

This work proposes a hybrid experimental-numerical technique with the aim to improve strain measurements at stress concentration regions. The novel technique is performed employing the computer vision scale invariant feature transform (SIFT) algorithm and meshless methods, here termed SIFT-meshless. The SIFT is applied to perform feature points matching in two images of the specimen surface at different stages of mechanical deformation. The output data are provided as a set of displacement measurements by tracking matched feature points. This information is then used to model displacement and strain field on the surface by means of a meshless formulation based on the moving least squares approximation. By applying the proposed SIFT-meshless method, the strain distribution around a semicircular notch in a plate under bending load was investigated. The experimental results were compared with those obtained by a digital image correlation technique based on a subset approach and to simulations from finite element analysis software. The experimental results demonstrated that the present method is capable of performing reliable strain measurements at distances close to the notch where the peak strain value is expected, even in the presence of high strain gradients.

Detecting Fatigue Crack Closure and Crack Growth Delays After an Overload Using DIC Measurements

The closure and crack growth behaviors of a fatigue cracked DC(T) 4340 steel specimen before and after a single 100% overload on the stress intensity factor (SIF) peak, applied over an otherwise mode I loading with quasi-constant SIF range and load ratio, was studied using Digital Image Correlation (DIC) and strain gage techniques. A significant retardation in the post-overload fatigue crack growth rate was observed, and it recovered its pre-overload value only after the crack grew a distance much larger than the Irwin plastic zone induced by the overload. This behavior is attributed to discontinuous closure, since high values of crack opening loads were observed even outside the overload plastic zone. Full-field displacements and strains determined ahead of the crack tip and crack flank opening displacements measured at points along the crack faces from DIC analysis, as well as redundant back-face strain-gage measurements, support this claim.

DIC Analysis for Crack Closure Investigations During Fatigue Crack Growth Following Overloads

The DIC technique was used to determine displacement and strain fields surrounding the tip of a fatigue crack in DC(T) (disk compact tension) specimen of normalized AISI 4340 steel, before and after an 120% overload was applied over otherwise fixed loading conditions in mode I ΔK = 31 MP√m and R = Kmin/Kmax = 0.1. The specimen has thickness of 4.6 mm, crack length to thickness ratio of 1.3 and width of 55.55 mm. From the DIC analysis, important parameters could be extracted, such as stress intensity factors, crack opening displacements, J-integrals, crack tip blunting, crack opening loads, and plastic zone sizes. These parameters were used to investigate the crack closure behavior before and after the application of the overload event, which produced a significant retardation in the subsequent crack growth rate. The role of the non-linearities observed in the experimental results during the crack closure phase, which are most important to correctly measure the above mentioned parameters, are analyzed and discussed.

Accurate Measurements of High Strain Gradients near Notches Using a Feature-Based DIC Algorithm

In this work, the efficiency of a feature-based DIC (digital image correlation) algorithm for measuring high strain gradients was investigated by means of numerical and actual experiments. The so-called SIFT-Meshless method consisted of a novel formulation involving the SIFT (scale-invariant feature transform) feature detector with a self-adaptive meshless formulation. Whereas the numerical experiments aimed to evaluate the accuracy and the spatial resolution, the actual experiments aimed to demonstrate in practice the above findings. A stereoscopic system and a micro-stereoscopic system were used to perform high strain gradient measurements in notched specimens of different materials and notch sizes. This paper concludes that the feature-based algorithm is able to provide accurate strain measurements at high strain gradient regions, even under conditions of plasticity. Moreover, the algorithm showed its efficiency to capture the peak strain near the notch boundary. Lastly, a spatial resolution study proposes a link between the desired accuracy and the pixel resolution required to perform accurate measurements of high strain gradients.

Variable mode-mixity during fatigue cycles – crack tip parameters determined from displacement fields measured by digital image correlation

This paper focusses on discussing equivalent stress intensity factors and kink angles after a change of mode-mixity from one cycle to the next and when the mode-mixity changes continuously during the fatigue cycles. Thin-walled tubes with through-wall cracks have been loaded by proportional and non-proportional tension and torsion. In the experimental investigation, the region of fatigue crack growth was observed by applying the digital image correlation technique. Data on the variations of the displacement and strain fields during the cycles were acquired and used to determine mixed-mode variations of stress intensity factors associated with opening modes I, II and III. For each specific specimen the crack path was observed in order to relate its curvature – both kinks and continuously developing warped cracks – with the variations of the displacement field and associated stress intensity factors.

On DIC measurements of DeltaKeff to verify if it is the FCG driving force

Redundant data obtained under quasi-constant {DeltaK, Kmax} loading conditions is used to verify if the effective stress intensity factor (SIF) range DeltaKeff DeltaKmax DeltaKop is indeed the fatigue crack driving force. The crack opening SIF Kop is measured along the entire crack path on DC(T) low carbon steel specimens by a series of strain gages bonded along the crack paths, by a strain gage bonded on their back faces, and by a digital image correlation technique. All such measurements showed a significant Kop decrease as the crack sizes increased, while the fatigue crack growth rates remained essentially constant both in the thin and thick specimens, a behavior that cannot be explained by DeltaKeff arguments.