Sizing the Depth and Width of Narrow Cracks in Real Parts by Laser-Spot Lock-In Thermography

We present a complete characterization of the width and depth of a very narrow fatigue crack developed in an Al-alloy dog bone plate using laser-spot lock-in thermography. Unlike visible micrographs, which show many surface scratches, the thermographic image clearly identifies the presence of a single crack about 1.5 mm long. Once detected, we focus a modulated laser beam close to the crack and we record the temperature amplitude. By fitting the numerical model to the temperature profile across the crack, we obtain both the width and depth simultaneously, at the location of the laser spot. Repeating the process for different positions of the laser spot along the crack length, we obtain the distribution of the crack width and depth. We show that the crack has an almost constant depth (0.7 mm) and width (1.5 µm) along 0.7 mm and features a fast reduction in both quantities until the crack vanishes. The results prove the ability of laser-spot lock-in thermography to fully characterize quantitatively narrow cracks, even below 1 µm.

Effect of Raw Sugarcane Bagasse Ash as Sand Replacement on the Fiber-Bridging Properties of Engineered Cementitious Composites

The use of raw sugarcane bagasse ash (SCBA) as sand replacement in the production of engineered cementitious composites (ECCs) can improve its cost-effectiveness and practicality. A recent study by the authors showed that the use of raw SCBA as a replacement to sand in ECC mixtures substantially enhances the tensile ductility and provides mild improvements in tensile strength; however, it also indicated a need to further elucidate the mechanisms producing such improvements. Therefore, the present study examined the effects of raw SCBA as a sand replacement in ECC’s fundamental fiber-bridging relationship, [Formula: see text], through single crack tensile test (SCTT) using 1% polyvinyl alcohol (PVA) fiber volume fraction. The PVA fiber volume fraction was reduced from 1.5% in the previous study to 1% in this study to ensure that a single crack was produced, which is a necessary condition to obtain the fundamental [Formula: see text] relationship. A total of five mixtures were evaluated at different replacement levels of sand with raw SCBA (i.e., 0%, 25%, 50%, 75%, and 100%). SCTT results revealed that raw SCBA produced minor effects on the fiber-bridging capacity but significantly increased the complementary energy ( [Formula: see text]). A positive correlation was observed between the pseudo strain-hardening (PSH) strength index and raw SCBA content. Since the PSH strength index was higher than the recommended value (i.e., 1.3) for robust PSH behavior, it was concluded that the main factor contributing to tensile ductility enhancements was the increase in the PSH energy index resulting from the notable increase of [Formula: see text] and potential decrease in matrix fracture toughness.

The Analysis of a Cracked Material under Combined Unsymmetric Thermal Flux and Symmetric Linear Mechanical Loading

An improved partially permeable crack model is put forward to deal with the problem of a single crack embedded in an orthotropic or isotropic material under combined unsymmetric thermal flux and symmetric linear mechanical loading. With the application of the Fourier transform technique (FTT), the thermoelastic field is given in a closed form. Numerical results show combined unsymmetric linear thermal flux, symmetric linear mechanical loading, and dimensionless thermal conductivity, and the coefficient has influences on fracture parameters. For the improved partially permeable crack, the mode II stress intensity factor and the energy release rate might be zero or positive under combined unsymmetric thermal flux and symmetric linear mechanical loading. Therefore, closure of the crack tip region need not be considered under combined unsymmetric thermal flux and symmetric linear mechanical loading when making use of fracture parameters as a criterion.

Electrical Characterization of a Double-Layered Conductive Pattern with Different Crack Configurations for Durable E-Textiles

For the conductive patterns of electronic textiles (e-textiles), it is still challenging to maintain low electrical resistance, even under large or cyclic tensile deformation. This study investigated a double-layered pattern with different crack configurations as a possible solution. Patterns with single crack growth exhibit a low initial resistance and resistance change rate. In contrast, patterns with multiple crack growth maintain their conductivity under deformation, where electrical failure occurs in those with single crack growth. We considered that a double-layered structure could combine the electrical characteristics of patterns with single and multiple crack growths. In this study, each layer was theoretically designed to control the crack configuration. Then, meandering copper patterns, silver ink patterns, and their double layers were fabricated on textiles as patterns with single and multiple crack growths and double-layered patterns, respectively. Their resistance changes under the single (large) and cyclic tensile deformations were characterized. The results confirmed that the double-layered patterns maintained the lowest resistance at the high elongation rate and cycle. The resistance change rates of the meandering copper and silver ink patterns were constant, and changed monotonically against the elongation rate/cycle, respectively. In contrast, the change rate of the double-layered patterns varied considerably when electrical failure occurred in the copper layer. The change rate after the failure was much higher than that before the failure, and on the same order as that of the silver ink patterns.

Burst Pressure Prediction of Pipes With SCC Colonies: Development of Intelligent Flaw Interaction Rules

Stress Corrosion Cracking (SCC) often occurs in clusters or colonies containing anywhere from a few cracks to hundreds of individual cracks. Multiple closely spaced cracks may interact, resulting in a burst pressure lower than what might be expected from a single crack. Most existing flaw interaction rules account for these interactions by using a single interacting crack to represent multiple cracks when the separations between them are less than a critical spacing. The length of this interacting crack is usually the sum of the individual crack length plus the spacing between them. Using this interacting length and the maximum depth in the colony could produce overly conservative burst pressure predictions which can lead to unnecessary hydrotests and/or other remediation actions. This two-paper series covers the PRCI-funded work aimed at the development of intelligent flaw interaction rules (termed PRCI-CRES SIA-1-5 rules) that can account more accurately the impact of multiple cracks without being overly conservative. This paper focuses on the development of the rules using numerical analyses. A companion paper covers the evaluation of the rules through full-scale burst tests. The PRCI-CRES SIA-1-5 rules use the principles of equivalent impact among multiple interacting cracks and represent the magnitude of the impact by a single virtual crack. The new rules do not rely on a critical spacing to determine whether there is an interaction. The magnitude of the interaction is a continuous function of the size of adjacent cracks and the spacing between them. A large number of finite element analyses (FEA) were conducted to examine the interaction among cracks for many crack configurations, including coplanar and noncoplanar cracks with different sizes and spacings. An analysis process was then developed to use the sizes and spacings of all cracks in an SCC colony to predict the equivalent virtual crack size and burst pressure.

Review of Technical Basis for ASME Code Section XI Appendix L

Equivalent Single Crack (ESC) sizes are provided in ASME Code, Section XI, Nonmandatory Appendix L, Tables L-3210-1 (for ferritic piping) and L-3210-2 (for austenitic piping). These two tables define initial flaw aspect ratios for use in fatigue flaw tolerance evaluations. These ESC sizes were based on the results of probabilistic fracture mechanics (PFM) evaluations that determined the equivalent single crack size that resulted in the same probability of through-wall leakage as the case when multiple cracks are initiated and grown around the inner circumference of a pipe. The PFM software, pc-PRAISE, used for the evaluation of ESC sizes had fracture mechanics models based on available data and models in the early 2000s. The stress intensity factor solutions used in pc-PRAISE were generated for a pipe radius-to-thickness ratio, Ri/t, of 5, and used a root-mean-square (RMS) averaged methodology. And the crack growth model was based on NUREG/CR-2189, Volume 5. This paper presents the results of evaluations to calculate a limited number of ESC sizes using updated fracture mechanics models for stress intensity factor and fatigue crack growth rates. The effect of crack growth due to stress corrosion cracking (SCC) in determining the ESCs is also discussed. The impact of the revised ESCs by performing two sample fatigue flaw tolerance problems and the associated results are also presented and discussed in this paper.