Crack Length Prediction Research Articles

The influence of low‐strain thermo‐mechanical processing on grain boundary network characteristics in type 304 austenitic stainless steel

Grain boundary engineering of austenitic stainless steel, through the introduction of plastic strain and thermal annealing, can be used to develop microstructures with improved resistance to inter-granular degradation. The influence of low-strain thermo-mechanical processing on grain boundary network development, with systematic variations of annealing treatments, has been investigated. Three stages of the microstructure development during grain boundary engineering in low-strain processing conditions are identified, and correlated with changes in grain boundary character and deviation distributions. Low-energy connected length segments at triple junctions, which have been proposed to be responsible for crack bridging during inter-granular stress corrosion cracking, can be influenced by the choice of the annealing treatment parameters. The development of individual grain boundary length segments of different character showed consistent trends with increasing grain size. Crack length predictions are consistent with the beneficial effect of designing microstructures with high fractions of twin grain boundaries and smaller grain size.

Prediction of crack length and crack growth rate of adhesive joints by a piezoelectric method

As adhesive joints have been widely used for fastening thin adherends, the damage tolerance design of adhesive joints has become important, and the estimation of initiation and propagation of a fatigue crack in the adhesive has become necessary. However, the measurement of crack length of tubular joints has been difficult because the observation of crack initiation and growth in the adhesive layer by conventional methods is not easy. In this work, a prediction method for the fatigue crack length in the adhesive layer of tubular single-lap adhesive joints was developed by the piezoelectric method. In order to obtain the relationship between the fatigue crack length and the piezoelectric signal, finite element analysis was conducted and verified by experiments. The damage of the adhesive joints was monitored by the piezoelectric method during torsional fatigue tests on tubular single-lap adhesive joints. Using the damage monitoring signals and the relationship between the fatigue crack length and the piezoelectric signal, a method for predicting fatigue crack growth in the adhesive layer of tubular single-lap adhesive joints was developed.

A Full Field DC Potential Drop Calibration for an Asymmetrically Cracked M(T) Specimen

Abstract A direct current (DC) potential drop (PD) calibration tool, in the form of a computer program, is provided that can determine the PD response at any point in an M(T) specimen containing either a symmetric or asymmetric crack. Results from the program were verified using data in the literature and results from an analog simulation. If a standard PD calibration is applied to the PD response for an asymmetric crack, the total crack length will be underestimated which leads to significant stress intensity factor errors for even small amounts of asymmetry. Using the PD response of an asymmetric crack from an analog simulation, three methods to predict individual crack lengths are evaluated. The methods use a two-dimensional (2-D) iterative procedure to solve for crack length given voltages from at least two sets of probes located at different positions on the specimen. Any of the different methods, which use different probe locations as the calibration inputs, yield good crack length predictions for crack length to specimen width ratios a/W greater than 0.25. However, the most numerically robust method resulting in the least error over a large crack length range uses the PD from a centerline probe and the PD difference from two offset probes to predict the crack length. These predictions result in a crack length error that does not exceed ±0.004 W and consequently result in insignificant stress intensity factor errors.

Determining the Potential Drop Calibration of a Fatigue Crack Growth Specimen Subject to Limited Experimental Observations

Abstract Six separate calibration techniques are applied to potential drop (PD) and crack length data from 23 experimental datasets. Five of the techniques use limited experimental input gained during precracking and after test completion to derive the coefficients associated with the calibrations. The datasets include SE(B) and M(T) specimens, fatigue crack growth (FCG) tests and foil analog simulations as well as various aluminum alloys and steel. A comparison of the calibration techniques is undertaken in terms of crack length and the subsequent effect on stress intensity factor errors. The best calibration techniques using two PD and crack length data pairs are the two-point modified and post-test corrected Johnson's equation. Using these methods, 21 of the 23 tests satisfy a criterion based on an acceptable ±2% mean variation in stress intensity factor. If a statistical assessment of the data is made, only 30 to 35% of the tests satisfy this criterion. Finally, the crack length prediction errors that result from the different calibrations can typically cause a 15 to 30% variation in FCG rate da/dN at a given ΔK level. This difference is found to be primarily due to errors in the ΔK calculation.

Evaluation of the Tentative JI-R Curve Testing Procedure by Round Robin Tests of HY130 Steel

Abstract A round robin test program of the tentative test procedure for the determination of the plane strain J-integral crack growth resistance curve was conducted under the auspices of ASTM Committee Section E24.08.03. This test program used HY130 steel and involved 19 participating laboratories in seven countries. Tests were carried out with both 25.4 mm (1-in.) thick compact specimens and three-point bend bar specimens. The results showed that the tentative procedure is an adequate document to guide JI-R curve testing. The range of scatter in crack length and crack extension measurements was well within prescribed limits. Analysis of compact specimen test data showed that various classifications of test data based upon agreement of crack length prediction and measurement did not significantly change the mean JI-R curves, and the range of scatter. The tentative procedure is seen to result in accurate JI-R curves when testing this specimen geometry. Results of three-point bend tests displayed more scatter than seen with compact specimens. Mean JI-R curves of selected bend bar specimens did not coincide with data from compact specimens. Specific experimental and analytical requirements to enhance the tentative JI-R curve procedure were identified.

Fatigue Crack Growth from Surface Planar Flaws (Part I)

Surface defects in structural members are apt to be origins of fatigue crack growth, which may cause serious failure of whole structures. In real structures, defects are often found rather in multiple clustered configuration at some geometrically discontinuous parts of the structures, such as welded joints, corner parts, etc. Analysis on crack coalescence and growth from these multiple defects, therefore is one of the most important subjects for the safe-life estimation of structures.The authors have performed an experimental analysis on fatigue crack propagation from two colinear surface defects. Mutual interaction effects of cracks are discussed using a simplified formula on crack propagation rate, together with a trial analysis on two-dimensional cracks. Changes in crack shape during propagation are well characterized by analogy with a single surface crack model. A set of formulas is presented on the prediction of crack length and life at the point of crack penetration of plate thickness.