Crack Closure Measurements Research Articles

Effect of super absorbent polymers on the self-healing capability of macrocracked ultra-high performance concrete under highly aggressive environments

In this study, the performance of superabsorbent polymers (SAPs) as self-healing agents in macrocracked ultra-high performance concrete (UHPC) was extensively evaluated, with a focus on compressive strength behavior in different aggressive environments. Three UHPC mixtures were designed: a control mixture, a UHPC with 0.3 % sodium polyacrylate (poly(AA)), and a UHPC with 0.3 % polyacrylate-co-acrylamide (poly(AA-co-AM)). Samples with macrocrack widths of 0.3 mm, 0.5 mm, and 1 mm, as well as uncracked samples, were prepared. The samples underwent immersion in deionized water, chloride saltwater, and compound saltwater. The performance of self-healing was evaluated by measuring crack closure rates, recovered compressive strength, and stereomicroscopic inspections. Further, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDX) was performed to monitor mineral formation and the healing process. The results indicate that both poly(AA) and poly(AA-co-AM) SAPs significantly enhanced the self-healing capabilities of UHPC, with poly(AA) demonstrating superior performance. Self-healing was more pronounced in samples with cracks width of 0.3 mm, whereas samples with cracks widths of 0.5 mm and 1.0 mm exhibited incomplete and negligible healing, respectively. The healed samples recovered a substantial portion of their compressive strength, regardless of the crack width. However, the presence of chloride and/or sulfate ions was found to impede the self-healing process. As observed from the SEM/EDX results, in addition to CaCO3 and C-S-H gel, undesirable healing products like Friedel’s salt and ettringite were also formed in chloride and compound saltwater environments which significantly affected self-healing durability.

Effect of hemp fibers and crystalline admixtures on the properties and self-healing efficiency of lime and clay-based mortars

This study investigates the impact of hemp fibers on two fiber-reinforced compositions, clay-based and lime-pozzolan mortar. Three compositions were prepared for each mortar, reference samples without fiber incorporation, samples with untreated hemp fibers, and samples with hydrothermally treated hemp fibers with crystallines. Crystalline admixtures (CAs) were incorporated into the hydrothermal pretreatment of fibers to promote the self-healing properties of the mortar when needed. Physico-mechanical and thermal properties took place to address the overall effect of hemp on the mortars. Adding hemp fibers enhanced the mechanical strength and reduced the mortars' shrinkage. The hydrothermally treated fibers have better cohesion with mortars, preserving the best results. To evaluate self-healing efficiency, pre-damaged cubic samples from each composition were subjected to tap water immersion for lime-pozzolan mortars and a controlled humid chamber for clay mortars after a curing period. The healing rate was assessed by measuring crack closure, compressive strength recovery, and the dynamic modulus of elasticity recovery. The results indicated significant strength recovery in lime-pozzolan mortars with hemp fibers. Clay-based mortars showed moderate strength increases and limited self-healing.

Cycle-by-cycle crack closure measurements and fatigue crack growth in CT specimen made of 7075-T7351 aluminium

In this study, an advanced piezoelectric strain sensor is applied to measure the crack opening loads of a compact tension specimen with a fatigue crack under variable amplitude block loading. The crack opening loads are subsequently utilised to modify the stress intensity factor range by taking into account the proportion of each load cycle for which the fatigue crack remained closed. This effective stress intensity factor is applied to collapse the growth rate curve, which initially displays a strong dependence on stress ratio, into a single master curve where the only variable governing crack extension is the effective stress intensity factor. The findings of this study provide strong evidence for the crack closure concept, which is important for the development of algorithms which can compress/truncate large spectrums. These algorithms rely on a detailed understanding of how much the crack is open or closed during each load cycle. Furthermore, the results indicate that the piezoelectric sensor could be used to study crack closure under realistic spectrum loading conditions, which will be the subject of future investigations.

Application of an advanced piezoelectric strain sensor for crack closure measurement

It is well known that fatigue crack growth in airframe structures is governed by sequence effects in aircraft load spectrums, which typically include millions of load cycles of varying amplitude. In predicting the fatigue life of airframe structures, one advanced method used to account for these effects considers the concept of crack closure. This concept requires the measurement or prediction of the load required to open the crack tip, otherwise known as the crack opening load. However, traditional methods used to directly measure the level of crack closure for each cycle tend to be prohibitive for large load sequences. This is mainly due to sensor limitations and large scatter using traditional measurement techniques. The current study proposes a new piezoelectric strain sensor for detection of the crack opening load to addresses these issues. In this work, a comparison is conducted using three sensors on a compact fatigue test specimen: an extensometer (clip gauge), a conventional back face strain gauge, and the piezoelectric strain sensor. It is shown that the piezoelectric strain sensor exhibits far superior performance in measuring the crack opening load relative to the back face strain gauge and the extensometer. At minimum, for all cases considered the piezoelectric strain sensor displayed 100% or better reduction in measurement scatter than the next best sensing method considered. In some cases where the stress ratio is high and the crack length is small, the improvement seen using the piezoelectric strain sensor approached 200%. Crack tip opening loads were detected in almost all cycles across a wide range of crack sizes and load ratios; while the efficiency of the traditional sensors reduced significantly for relatively short cracks and high load ratios. The outcomes of this work will help to better understand the load sequence effects on crack growth rates, improve fatigue life predictive methods and develop effective procedures of truncation/compression of load spectrums for accelerated testing of aircraft components.

Measuring crack growth and related opening and closing stresses using continuous potential drop recording

To improve resolution of in-situ measurement of crack closure and opening in fatigue, potential drop (PD) measurement technique has been further developed to measure thousands of times through each load cycle with high precision. The results are interpreted with physical phenomena like strain, Poisson’s effect, piezo-resistivity, plasticity, and crack growth. Application of the technique to fatigue crack growth tests on aluminum 2024-T3 CCT specimens at different maximum stresses and stress ratios, demonstrate that indeed variations in PD can be associated to development of plasticity and crack opening and closure. Hence, the technique allows to measure timing and magnitude of crack opening and closure stresses in-situ in fatigue crack growth experiments.

Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.

Full-field experimental and numerical characterisation of a growing fatigue crack in a stainless steel

Full-field mechanical characterisation has been carried out for growing cracks under cyclic loading conditions using both Digital Image Correlation (DIC) and integrated finite element (FE) analysis. Compact tension specimens of a model material, stainless steel 316L, were used in the experiments, where the position of the crack tip and the evolution of the near-tip displacements and strains were tracked at selected observation points along the crack path. “Critical” strains at the advancing crack tip were captured at selected locations along the crack path. The influence of DIC data processing strategies and the choice of a characteristic length on the critical strain were examined. Potential attenuation effects behind the crack tip, commonly referred to as crack closure, were also investigated by evaluating the crack opening displacement (COD) at selected instances to the crack tip during the crack growth, where the experiment was interrupted to allow DIC measurements taken during a complete cycle. The impact of measured crack closure on the near-tip strains and crack driving force in terms of J-integral was assessed using both standard and integrated FE analysis.

Identification of Relationships between Heat Treatment and Fatigue Crack Growth of αβ Titanium Alloys

This study deals with the influence of microstructure on the fatigue crack growth resistance of αβ titanium alloys: Ti-6Al-4V ELI (Extra Low Interstitial) that may compete with the conventional Ti-6Al-4V alloy in the manufacture of high performance aircraft. Six different microstructures have been considered: the as-received bimodal microstructures and five distinct fully lamellar microstructures. The characteristic parameters of these microstructures were determined and crack growth tests were performed with crack closure measurements in order to evaluate the shielding effect induced by closure. A comparison of crack growth rates, fracture surfaces, and crack path was carried out for the different microstructures. The results outline a transition between two propagation regimes from a microstructure-sensitive to microstructure-insensitive propagation.

Fatigue-crack growth in two aluminum alloys and crack-closure analyses under constant-amplitude and spectrum loading

Fatigue-crack-growth tests were conducted on compact, C(T), and middle-crack tension, M(T), specimens made of 7075-T6 and 7249-T76511 aluminum alloys. The 7075 tests were conducted on thin-sheet (2-mm thick) material; whereas, the 7249 tests were conducted on three thicknesses (2, 3.2 and 6.35-mm). The thin-sheet 7075 alloy did not have residual stresses, but the 7249 alloy (B = 2 mm) had low-level (<10 MPa) residual stresses from measurements. However, the other two 7249 alloy thicknesses were residual-stress free based on measurements. These tests were conducted to generate fatigue-crack-growth-rate data from threshold to near fracture at two load ratios (R = 0.1 and 0.7). Two methods were used to generate near threshold data on C(T) specimens: (1) compression pre-cracking constant-amplitude (CPCA) and (2) compression pre-cracking load-reduction (CPLR). Constant-amplitude tests were conducted on the M(T) specimens at the same two load ratios. Crack-closure measurements using a compliance method were made on some of the low R tests and a crack-closure model were used to develop an effective stress-intensity factor range against rate relation using a constant constraint factor (α = 1.85). The constraint factor accounts for three-dimensional stress-state effects on plastic yielding at crack tips and crack-closure behavior.Simulated aircraft wing spectrum tests were conducted on the M(T) specimens using two modified full-scale-fatigue-test (FSFT) spectra used on the P-3C aircraft. Tensile pre-cracking was used on all of the M(T) specimens, except one 3.2-mm thick 7249 specimen, before the spectra were applied. The same spectrum sequence was applied, but one had tension-compression loads, whereas, the other only tension-tension loads. The spectrum tests were used to calibrate the constraint-loss regime (plane-strain to plane-stress; α = 1.85–1.0) behavior for the FASTRAN strip-yield model. Comparisons were made between the spectrum tests and calculations made with the FASTRAN life-prediction code; and the calculated crack-growth lives were generally within ±20% of the test results.

Physically short and long-crack growth behavior of MIG welded Al-5.8%Mg alloy

In this study, we investigated the physically short and long-crack growth behavior of Al-5.8%Mg alloy, a special hardened alloy used for welding of Al-5083 plates. Microstructural analysis on both longitudinal and cross-sectional plane of welded plates showed no crystallographic texture with nearly equi-axed grains of size 85 μm and gas porosities. Fatigue tests were done at 50 Hz using CT-specimens at different R-ratios (0.1,0.5 and 0.8) to characterize the long-crack growth behavior. Compliance method was used for crack-closure measurements. An increase in crack growth rates and reduction in threshold ΔK was observed with increasing R-ratio. This R-ratio effect could be rationalized after compensating for crack-closure. Special specimens for short-crack growth fatigue tests were cut from pre-cracked CT-specimens. Constant load-range fatigue tests at 50 Hz were conducted at three different R-ratios (-1,0.1 and 0.5) to characterize the effect of mean-stress on short-crack growth behavior. It appeared that the physically short-cracks initially propagated in closure-free environment and then deviated towards their long-crack growth rates. The microstructural effects on short-cracks were more pronounced at lower mean-stresses or R-ratios. The physically short-crack growth effect could not be removed completely using the crack-closure concept alone, suggesting the role of other growth-retarding mechanisms. A modified empirical model including the effect of other possible growth-retarding mechanisms was proposed to capture the behavior of physically short-cracks in this material.

A comparison of DIC-based techniques to measure crack closure in LCF

Crack closure is one of the most important phenomenon for the comprehension of fatigue behavior of metallic alloys. The effect of mean stress, overloads and variable amplitude loadings can be predicted modeling the crack closure in terms of opening and closing stress levels. Experimentally, the characterization of the crack closure in high and low cycle fatigue has gained large attention in the last fifty years. In fact, the proper detection of the opening and closing levels enhances the definition of a proper crack driving force and its modeling. This work primarily focuses on the measurement of the opening and closing stresses for cracks propagating in low cycle fatigue conditions. High-resolution full-field digital image correlation technique was adopted to track the crack profiles during cyclic loading and different approaches were adopted to analyze the displacement fields extracted with the virtual extensometers. The closure measurements were performed for three metallic alloys, different strain ranges (from elastic to dominant-plastic behaviors) and strain ratio to enlarge the field of applicability of the techniques presented.

Determination of crack growth for 6082-T6 aluminium subjected to periodic single and block overloads and underloads using a two dimensional finite element model

The estimation of crack growth under variable amplitude loading is complex due to interaction effects such as plasticity, crack tip blunting, residual stresses, crack tip closure and crack tip branching. Crack closure has been identified to be one of the main interaction effects. In order to study the effect of crack closure the authors have previously carried out experimental testing to obtain more accurate measurements of crack opening and closure (Aguilar-Espinosa, 2009, Aguilar-Espinosa et al., 2013). They have also developed two dimensional plane stress Finite Element models utilising high mesh density whilst maintaining the ability to measure crack growth over long crack lengths (Aguilar Espinosa et al. 2017). This initial work has been extended in this paper to examine the effects of single and block overloads and random spectrum loading on crack growth. The crack length distance that is affected by overloads and underloads measured experimentally and predicted numerically are shown to be very close when using cyclic hardening material properties and kinematic hardening. In addition the comparison of experimental and numerical crack growth versus crack length graphs shows good correlation of the crack growth acceleration and retardation after the applied overload which has not been seen previously. These comparisons seem to be a very useful tool to validate numerical models.

In situ measurement of fatigue crack opening stresses by nonlinear vibro-acoustic modulation testing

The difficulty of accurately measuring crack closure regions has hindered measurements of opening or closure stresses, and ultrasonic methods have shown great potentialities in evaluating closure area. Many recent researches have confirmed the advantages of nonclassical nonlinear acoustic approaches in detecting short cracks and even micro- or meso-cracks. However, there are few reports on using nonclassical nonlinear acoustic approaches to measure crack closure. The aim of this article is to verify the applicability of vibro-acoustic modulation approach in estimating opening stresses of fatigue cracks. A procedure of approximating the opening stress by the applied tensile load of the inflection point of the modulation intensity curve is proposed. To study the performance of this measuring method experimentally, in situ vibro-acoustic modulation tests were carried out on a central cracked 5052-H32 aluminum alloy fatigue sample. The experimental estimations match well with other researches. Analysis of the experimental results suggests that the vibro-acoustic modulation technology is not only capable of measuring the fatigue cracks opening stresses, but also capable of analyzing the crack closure behavior, even for physically short cracks.

Stress Intensity Factor calculation from displacement fields

In the last two decades, visual image techniques such as Digital Image Correlation (DIC) enabled to experimentally determine the crack tip displacement and strain fields at small scales. The displacements are tracked during loading, and parameters as the Stress Intensity Factor (SIF), opening and closing loads, T-stress can be readily measured. In particular, the SIFs and the T-stress can be obtained by fitting the analytical equation of the Williamstype expansion with the experimentally-determined displacement fields. The results in terms of fracture mechanics parameters strictly depend on the dimension of the area considered around the crack tip in conjunction with the crack length, the maximum SIF (and thus the plastic tip radius), and the number of terms to be considered in the Williams-type expansion. This work focuses in understanding the accuracy of the SIF calculation based on these factors. The study is based on Finite Element Analysis simulations where purely elastic material behavior is considered. The accuracy of the estimation of the SIF is investigated and a guide-line is provided to properly set the DIC measurements. The analysis is then experimentally validated for crack closure measurements adopting the SENT specimen geometry.

Crack-Closure Measurements in Low-Cycle Fatigue With Digital Image Correlation

Abstract In this work, an experimental campaign devoted to measure closure levels onto specimens subjected to low-cycle fatigue (LCF) conditions is described. Two different straining conditions were investigated (i.e., R = −1 and R = 0.5). The measurements of σop, crack-opening stresses, were carried out by adopting the concept of “crack-tip opening displacements” and “local strain cycles.” However, instead of the original strain gauges measurements above the plane of the propagating crack, the strain cycles were measured by DIC in proximity of the crack tip. Experimental results were compared to those provided by the analytical model proposed by Newman.

Measuring fatigue crack growth and closure in Polyether Ether Ketone (PEEK)

Throughout industry, polymers are being used to replace parts that were previously fabricated from metals. Thus, the fatigue resistance of polymers needs to be well understood. Polyether Ether Ketone (PEEK) is often chosen for its good mechanical properties and high glass transition temperature. In this paper, the experiments and methods to study fatigue crack growth and subsequent crack closure are presented. Compact specimens made of PEEK were used to experimentally monitor fatigue crack growth at R=0.1 and R=0.7. Some unique aspects of the experiments include the use of compression pre-cracking and a back-face strain gauge to measure crack length. While measuring crack closure was the primary focus of the experiments, crack growth rate versus stress intensity factor range results were also generated. Using remote methods of measuring crack closure show that PEEK experiences a small level of crack closure, though the effects are minimal. However, indirect methods of measuring crack closure show that the phenomenon is more prevalent in the material than what was measured. Further research for continuing the study of fatigue crack closure in PEEK are discussed.

Effect of Plastic Deformation on Compliance Curve Based Crack Closure Measurement

Fatigue crack growth depends heavily on near tip stress-strain behavior controlled by many micromechanical and microstructural factors. Crack closure is widely used to rationalize crack growth behaviour under complex loading conditions. Reliable crack closure measurement is essential for enhanced damage tolerance design and remains a challenge to the industry. This paper focuses on the effect of plastic deformation ahead of a notch/crack on the non-linearity of compliance curves of 6082-T651 aluminium alloy specimens to highlight a potential issue in the conventional compliance curve based crack closure measurement technique. Experimental and numerical simulation results demonstrate that plastic deformation ahead of the notch will introduce non-linear stress-strain behavior in the absence of crack closure. It is proposed that the effect of crack tip plasticity on the non-linearity of the compliance curve be separated to obtain reliable crack closure measurement.

Crack closure effects during low cycle fatigue propagation in line pipe steel: An analysis with digital image correlation

Fatigue crack growth was investigated in a line pipe steel. Severe straining conditions, like those experienced by pipelines, were considered during the experiments. Crack closure effects were investigated during the tests by adopting an innovative technique based on digital image correlation. Experimentally measured crack closure levels were implemented in a crack propagation model based on elastic–plastic fracture mechanics. A modified formulation of ΔJeff, necessary to accurately describe crack propagation driving forces, is presented and employed to assess fatigue life in presence of high plastic strains.

Multi-scale crack closure measurements with digital image correlation on Haynes 230

An experimental campaign was developed to study fatigue crack growth in Haynes 230, a Ni-based superalloy. The effects of crack closure were investigated with digital image correlation, by applying two different approaches. Initially, full field regression algorithms were applied to extract the effective stress intensity factor ranges from the singular displacement field measured at crack tips. Local closure measurements were then performed by considering crack flanks relative displacements. Two points virtual extensometers were applied in this phase. Experimental results were then compared to the reference da/dN –?Keff curve: it was found that the correct estimation of crack opening levels shifts all the experimental points on the reference curve, showing that DIC can be successfully applied to measure crack closure effects.

Effect of load ratio on fatigue crack growth in the near-threshold regime: A literature review, and a combined crack closure and driving force approach

The current state of influence of load ratio, R, on fatigue crack growth (FCG) behavior is thoroughly reviewed. Two main approaches, including crack closure concept and driving force parameter method, when used independently, are found to be short of accuracy during load-shedding procedure in the near-threshold regime. By combining an equivalent driving force model and crack closure approach, a new crack opening equation and a general FCG law is established for the load-shedding technique. This merging method does not involve crack closure measurement, and is capable to predict R dependence of FCG curves and fatigue thresholds fairly well.