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NASA NTRS1984

Effect of microcracks on the thermal expansion of composite laminates

A finite element analysis was used to quantitatively predict the effect of matrix microcracks in the 90 deg plies of graphite/epoxy composites on the coefficient of thermal expansion in the 0 deg direction, alpha (y) (perpendicular to the cracks). Results were generated for (0m/90n)s, (0/+ or 45/90)s and (0/90/ + or - 45)s graphite/epoxy laminate configurations. Analytical predictions were compared with experimental results for the two quasi-isotropic laminate configurations. Both analytical and experimental results showed that microcracks reduced the effective stiffness of the 90 deg plies, thus causing the laminates, thermal response to be more like that of a (0) laminate. The change in alpha(y) was a function of lamina material properties, microcrack density, fiber orientation, and stacking sequence. A combination of classical lamination theory and finite element analysis was used to predict the effect of microcracks in both the 90 deg and 0 deg plies. Analytical results showed that the addition of microcracks in the 0 deg plies do affect alpha(y), but to a lesser extent than those in the 90 deg plies.

Bowles, D. E.↗

NASA NTRS1990

Fracture mechanics analysis of composite microcracking - Experimental results in fatigue

The Nairn (1989) variational mechanics analysis, which yields the energy release rate of a microcrack's formation between two existing microcracks, has proven useful in the fracture mechanics interpretation of cross-ply laminates' microcracking. Attention is presently given to the application of this energy release rate analysis to a fracture mechanics-based interpretation of microcrack formation during fatigue loading, for the case of fatigue experiments on three layups of Avimid K/IM6 laminates and four layups of Fiberite 934/T300 laminates. The single master Paris-law plot onto which the data from all layups of a given material system fall is claimed to offer a complete characterization of that system's microcrack-formation resistance during fatigue loading.

Nairn, J. A.↗

NASA NTRS1993

Master plot analysis of microcracking in graphite/epoxy and graphite/PEEK laminates

We used a variational stress analysis and an energy release rate failure criterion to construct a master plot analysis of matrix microcracking. In the master plot, the results for all laminates of a single material are predicted to fall on a single line whose slope gives the microcracking toughness of the material. Experimental results from 18 different layups of AS4/3501-6 laminates show that the master plot analysis can explain all observations. In particular, it can explain the differences between microcracking of central 90 deg plies and of free-surface 90 deg plies. Experimental results from two different AS4/PEEK laminates tested at different temperatures can be explained by a modified master plot that accounts for changes in the residual thermal stresses. Finally, we constructed similar master plot analyses for previous literature microcracking models. All microcracking theories that ignore the thickness dependence of the stresses gave poor results.

Nairn, John A.↗

NASA NTRS1995

The Dependence of the Change in the Coefficient of Thermal Expansion of Graphite Fiber Reinforced Polyimide IM7-K3B on Microcracking due to Thermal Cycling

Composite IM7-K3B was subjected to a simulated high speed aircraft thermal environment to determine the effects of microcracking on the change in CTE. IM7-K3B is a graphite fiber reinforced polyimide laminate, manufactured by Dupont. The lay-up for the material was (0.90((Sub 3)(Sub s))). The specimens were placed in a laser-interferometric dilatometer to obtain thermal expansion measurements and were then repeatedly cycled between -65 F and 350 F up to 1000 cycles. After cycling they were scanned for microcracks at a magnification of 400x. The material was expected not to crack and to have a near zero CTE. Some microcracking did occur in all specimens and extensive microcracking occurred in one specimen. Further testing is required to determine how closely the CTE and microcracking are related.

Stewart, Melissa C.↗

DOE OSTI2021

Automatic identification and quantification of dense microcracks in high-performance fiber-reinforced cementitious composites through deep learning-based computer vision

Highlights: • A method is presented to detect, locate, quantify, and visualize dense microcracks in HPFRCC. • Quantification of dense microcracks is realized using deep learning method for the first time. • The presented method uses deep learning models that are trained using a realistic dataset size. • The presented method has a high computation efficiency for identifying and quantifying cracks. • The presented method provides crack width with errors up to 50 μm and a R{sup 2} value of 0.984. High-performance fiber-reinforced cementitious composites (HPFRCCs) feature high mechanical strengths, crack resistance, and durability. Under excessive loading, HPFRCCs demonstrate dense microcracks that are difficult to identify using existing methods. This study presents a computer vision method for identification, quantification, and visualization of microcracks in HPFRCCs based on deep learning. The presented method integrates multiple deep learning models and computer vision techniques in a hierarchical architecture. The crack pattern (e.g., number, width, and spacing of cracks) are automatically determined from pictures without human intervention. This study shows that the presented method achieves an accuracy of 0.992 for crack detection and an accuracy finer than 50 μm (R{sup 2} > 0.984) for quantification of crack width when deep learning models are trained using only 200 pictures of HPFRCCs and 200 pictures of conventional concrete with incorporation of data augmentation. The presented method is expected to be also applicable to other materials featuring complex cracks.

36 MATERIALS SCIENCE↗

NASA NTRS1978

Characteristics of microcracks in samples from the drill hole Noerdlingen 1973 in the Ries crater, Germany

Samples from the Noerdlingen 1973 drill core contain abundant shock-induced microfractures which exhibit varying amounts of healing and sealing. Many of the microcracks resemble morphologically the open microcracks present in returned lunar samples. Data derived from petrography, scanning electron microscopy, and differential strain analysis indicate that fewer microcracks were formed at greater depths and crack sealing processes are more effective for cracks associated with planar elements. Although the microcracks in the Ries core are now sealed, they are valid analogues of the open shock-induced cracks of lunar rocks and demonstrate that open cracks do form in rocks at depth during a naturally-occurring shock event. The healing of the shock-induced cracks in the Ries core also precludes their use for laboratory measurements of physical properties intended to be used as analogue measurements of lunar samples in situ.

Padovani, E. R.↗

NASA NTRS1987

Elastic interaction of a crack with a microcrack array. I - Formulation of the problem and general form of the solution. II - Elastic solution for two crack configurations (piecewise constant and linear approximations)

The elastic interactions of a two-dimensional configuration consisting of a crack with an array of microcracks located near the tip are studied. The general form of the solution is based on the potential representations and approximations of tractions on the microcracks by polynomials. In the second part, the technique is applied to two simple two-dimensional configurations involving one and two microcracks. The problems of stress shielding and stress amplification (the reduction or increase of the effective stress intensity factor due to the presence of microcracks) are discussed, and the refinements introduced by higher order polynomial approximations are illustrated.

Chudnovsky, A.↗

NASA NTRS1991

The initiation, propagation, and effect of matrix microcracks in cross-ply and related laminates

Recently, a variational mechanics approach was used to determine the thermoelastic stress state in cracked laminates. Described here is a generalization of the variational mechanics techniques to handle other cross-ply laminates, related laminates, and to account for delaminations emanating from microcrack tips. Microcracking experiments on Hercules 3501-6/AS4 carbon fiber/epoxy laminates show a staggered cracking pattern. These results can be explained by the variational mechanics analysis. The analysis of delaminations emanating from microcrack tips has resulted in predictions about the structural and material variables controlling competition between microcracking and delamination failure modes.

Nairn, John A.↗

NASA NTRS1993

A critical evaluation of theories for predicting microcracking in composite laminates

We present experimental results on 21 different layups of Hercules AS4 carbon fiber/3501-6 epoxy laminates. All laminates had 90 deg plies; some had them in the middle, while some had them on a free surface. During tensile loading, the first form of damage in all laminates was microcracking of the 90 deg plies. For each laminate, we recorded both the crack density and the complete distribution of crack spacings as a function of the applied load. By rearranging various microcracking theories, we developed a master-curve approach that permitted plotting the results from all laminates on a single plot. By comparing master-curve plots for different theories, it was possible to critically evaluate the quality of those theories. We found that a critical-energy-release-rate criterion calculated using a 2D variational stress analysis gave the best results. All microcracking theories based on a strength-failure criteria gave poor results. All microcracking theories using 1D stress analyses, regardless of the failure criterion, also gave poor results.

Nairn, John A.↗

NASA NTRS1992

Evaluation of microcrack thermal shock damage in ceramics: Modeling and experiment

In this paper we present an experimental and theoretical study of the effect of microcrack damage on ceramic properties. For the experimental investigation, ceramic samples of aluminum oxide and reaction bonded silicon nitride (RBSN) are used. Thermal shock treatment from different temperatures up to 1000 C is applied to produce the microcracks. Both surface and bulk ultrasonic wave methods are used to correlate the change of elastic constants to microstructural degradation and to determine the change in elastic anisotropy induced by microcrack damage. For the theoretical investigation, damage mechanics, which relates microstructural damage to material service life and mechanical failure, is used. The change in elastic properties due to microcrack damage calculated from the theoretical model is compared with the experimental results for determination of the applicability of damage theory. It is shown that two independent experimental methods (bulk wave and surface wave) give the same results for shear moduli of damaged ceramics. The experimental results aagree reasonably well with the moduli predicted from the cracked solid model.

Chu, Y. C.↗

NASA NTRS2001

Evaluation of Microcracking in Two Carbon-Fiber/Epoxy-Matrix Composite Cryogenic Tanks

Two graphite/epoxy cryogenic pressure vessels were evaluated for microcracking. The X-33 LH2 tank lobe skins were extensively examined for microcracks. Specimens were removed from the inner skin of the X-33 tank for tensile testing. The data obtained from these tests were used to model expected microcrack density as a function of stress. Additionally, the laminate used in the Marshall Space Flight Center (MSFC) Composite Conformal, Cryogenic, Common Bulkhead, Aerogel-Insulated Tank (CBAT) was evaluated. Testing was performed in an attempt to predict potential microcracking during testing of the CBAT.

Hodge, A. J.↗

NASA NTRS2012

Finite Element Modeling of Thermal Cycling Induced Microcracking in Carbon/Epoxy Triaxial Braided Composites

The microcrack distribution and mass change in PR520/T700s and 3502/T700s carbon/epoxy braided composites exposed to thermal cycling was evaluated experimentally. Acoustic emission was utilized to record the crack initiation and propagation under cyclic thermal loading between -55 C and 120 C. Transverse microcrack morphology was investigated using X-ray Computed Tomography. Different performance of two kinds of composites was discovered and analyzed. Based on the observations of microcrack formation, a meso-mechanical finite element model was developed to obtain the resultant mechanical properties. The simulation results exhibited a decrease in strength and stiffness with increasing crack density. Strength and stiffness reduction versus crack densities in different orientations were compared. The changes of global mechanical behavior in both axial and transverse loading conditions were studied. Keywords: Thermal cycles; Microcrack; Finite Element Model; Braided Composite

Zhang, Chao↗

NASA NTRS2012

Microcracking in Composite Laminates: Simulation of Crack-Induced Ultrasound Attenuation

Microcracking in composite laminates is a known precursor to the growth of inter-ply delaminations and larger scale damage. Microcracking can lead to the attenuation of ultrasonic waves due to the crack-induced scattering. 3D elastodynamic finite integration technique (EFIT) has been implemented to explore the scattering of ultrasonic waves due to microcracks in anisotropic composite laminates. X-ray microfocus computed tomography data was directly input into the EFIT simulation for these purposes. The validated anisotropic 3D EFIT code is shown to be a useful tool for exploring the complex multiple-scattering which arises from extensive microcracking.

Leckey, C. A. C.↗

NASA NTRS1976

Microcracks, micropores, and their petrologic interpretation for 72415 and 15418

Lunar samples 72415 and 15418 have complex microstructures that indicate a series of fracturing and healing events. Both samples contain relatively few open microcracks but many sealed and healed microcracks. Dunite 72415 contains abundant healed cracks that formed tectonically, symplectic intergrowths spatially and probably genetically related to microcracks, and a cataclastic matrix that has been extensively sintered. Metamorphosed breccia 15418 contains many post-metamorphic healed cracks, large shock induced cracks that have been sealed with glass, and a few younger, thin, open shock induced cracks.

Richter, D.↗

NASA NTRS1979

Transverse microcracking in Celion 6000/PMR-15 graphite-polyimide

The effects of room temperature tensile loading and five thermal loadings, in the range -320 F (-196C) to 625F (330CC), upon the development of transverse microcracks (TVM) in Celion 6000/PMR-15 graphite-polyimide laminates were investigated. Microcracks were observed using a replicating technique, microscopy and X-ray. The mechanical or thermal load at which microcracking initiates and the ply residual stresses were predicted using laminate analysis with stress- and temperature-dependent material properties.

Mills, J. S.↗

NASA NTRS1985

Fundamentals of microcrack nucleation mechanics

A foundation for ultrasonic evaluation of microcrack nucleation mechanics is identified in order to establish a basis for correlations between plane strain fracture toughness and ultrasonic factors through the interaction of elastic waves with material microstructures. Since microcracking is the origin of (brittle) fracture, it is appropriate to consider the role of stress waves in the dynamics of microcracking. Therefore, the following topics are discussed: (1) microstress distributions with typical microstructural defects located in the stress field; (2) elastic wave scattering from various idealized defects; and (3) dynamic effective-properties of media with randomly distributed inhomogeneities.

Fu, L. S.↗

NASA NTRS1987

Macrocrack interaction with transverse array of microcracks

General formulation of a problem involving a macrocrack propagating through an area with microcracks is considered. The analysis is based on the simultaneous solution of a system of singular integral equations. Various methods described in the literature are discussed in detail and compared. The specific problem considered was a macrocrack approaching an infinite transverse array of microcracks. Results illustrate the effects of different loading types and can be used for interpretation of the microcrack toughening mechanisms. Numerical comparisons are made with data recently appearing in literature which demonstrates the importance of numerical accuracy. Reported results differ by a factor in certain cases as compared with data given in other literature.

Rubinstein, A. A.↗

NASA NTRS1988

Macrocrack interaction with transverse array of microcracks

Rubinstein, A. A.↗