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Short fatigue cracks in polycrystalline materials are very important from both practical and basic aspects, yet they are very difficult to observe. Therefore, it is suggested to emulate some properties of short cracks with long fatigue cracks in monocrystals. Indeed, such experiments in pure nickel crystals prove that the three peculiar properties of short fatigue cracks in polycrystalline metals are observed in long cracks in monocrystals, i.e., a lower threshold of the stress intensity factor (ΔKI)th, a higher crack propagation rate at low ΔKI regimes, and the fact that different cracks exhibit different growth behaviors. The fatigue experiments in monocrystals reveal interesting details of the slip activity at the front of fatigue cracks, including a selection rule for the active slip systems above and below the crack, the slip behavior under conditions of steep strain gradients, and the activation of a new slip system.

Pultrusion is a continuous process of forming constant cross-sections of unidirectional composites with a significant long length. This unique process is implemented widely in the composites industry due to its continuous, automated, and highly productive nature. The current research focused on mechanical response characterization at three modes of loading: tensile, compression, and shear loading of coupons made from a graphite/epoxy 1 mm sheet. In addition, the effects of holes and notches were examined in terms of mechanical properties. The mechanical behavior was assessed through stress–strain curves with careful attention on the curve profile, macroscopic fracture modes observations, and optical microscopic tracking with continuous video records. The mechanical tests follow standards with some critiques on the shear test. Finite element analysis (FEA) was used to accurately determine the shear modulus, and for other mechanical investigations. By nature, under tension, the unidirectional fiber composite at 0° orientation exhibits high strength (2800 MPa), with very low strength at 90° orientation (40 MPa). Both orientations display linear mechanical behavior. Under compression, 0° orientation exhibits low strength (1175 MPa), as compared to tension due to the kinking phenomena, which is the origin in the deviation from linear behavior. Under shear, both orientations exhibit approximately the same shear strength (45 MPa for 0° and 47 MPa for 90°), which is mainly related to the mechanical properties of the epoxy resin. In general, in the presence of holes, the remote fracture stress in the various modes of loading did not change significantly, as compared to uniform coupons; however, some localized delamination crack growth occurred at the vicinity of the holes, manifested by load drops up to the final fracture. This behavior is also attributed to the tension of notched coupons. FEA shows that the shear values were unaffected by manufacturing imperfections, coupon thickness, and by asymmetrical gripping up to 3 mm, with minor effect in the case of a small deviation from the load line. Selected experimental tests support the FEA tendencies.

Short fatigue cracks in polycrystalline materials are very important from both practical and basic aspects, yet they are very difficult to observe. Therefore, it is suggested to emulate some properties of short cracks with long fatigue cracks in monocrystals. Indeed, such experiments in pure nickel crystals prove that the three peculiar properties of short fatigue cracks in polycrystalline metals are observed in long cracks in monocrystals, i.e., a lower threshold of the stress intensity factor (ΔKI)th, a higher crack propagation rate at low ΔKI regimes, and the fact that different cracks exhibit different growth behaviors. The fatigue experiments in monocrystals reveal interesting details of the slip activity at the front of fatigue cracks, including a selection rule for the active slip systems above and below the crack, the slip behavior under conditions of steep strain gradients, and the activation of a new slip system. [ABSTRACT FROM AUTHOR]

Pultrusion is a continuous process of forming constant cross-sections of unidirectional composites with a significant long length. This unique process is implemented widely in the composites industry due to its continuous, automated, and highly productive nature. The current research focused on mechanical response characterization at three modes of loading: tensile, compression, and shear loading of coupons made from a graphite/epoxy 1 mm sheet. In addition, the effects of holes and notches were examined in terms of mechanical properties. The mechanical behavior was assessed through stress–strain curves with careful attention on the curve profile, macroscopic fracture modes observations, and optical microscopic tracking with continuous video records. The mechanical tests follow standards with some critiques on the shear test. Finite element analysis (FEA) was used to accurately determine the shear modulus, and for other mechanical investigations. By nature, under tension, the unidirectional fiber composite at 0° orientation exhibits high strength (2800 MPa), with very low strength at 90° orientation (40 MPa). Both orientations display linear mechanical behavior. Under compression, 0° orientation exhibits low strength (1175 MPa), as compared to tension due to the kinking phenomena, which is the origin in the deviation from linear behavior. Under shear, both orientations exhibit approximately the same shear strength (45 MPa for 0° and 47 MPa for 90°), which is mainly related to the mechanical properties of the epoxy resin. In general, in the presence of holes, the remote fracture stress in the various modes of loading did not change significantly, as compared to uniform coupons; however, some localized delamination crack growth occurred at the vicinity of the holes, manifested by load drops up to the final fracture. This behavior is also attributed to the tension of notched coupons. FEA shows that the shear values were unaffected by manufacturing imperfections, coupon thickness, and by asymmetrical gripping up to 3 mm, with minor effect in the case of a small deviation from the load line. Selected experimental tests support the FEA tendencies. [ABSTRACT FROM AUTHOR]

In today's additive manufacturing sector, one of the most popular areas is selective laser melting (SLM) due to its capability of producing geometrically complex metal parts directly from CAD model in a few short steps. Many studies have been reported on static mechanical properties of SLM components; however, dynamic properties of SLM components of different materials have not been thoroughly investigated. Only few papers have been published on the dynamic mechanical behavior, especially in the crack resistance of selective laser melted AlSi10Mg alloy. In the present study, the effect of loading rate, dynamic versus quasi static, on the fracture toughness of the as-built alloy (X and Z orientations) has been investigated. The experimental results revealed the inherently anisotropic behavior for loading rates where the Z orientation exhibited lower toughness compared to the x orientation. The dynamic loading by impact, resulted in a significant decrease of the toughness values up to about 50% compared to the quasi-static loading. This mechanical response was attributed to the increase in the yield stress which alters the state stress at the crack tip from the plane-stress to some extent of the mix-mode plane stress/strain. [ABSTRACT FROM AUTHOR]

The capability to predict parameters and to develop quantitative predictive models in sub-cooled boiling flow is considerable importance to nuclear reactor safety and in many other industrial processes. Such predictions are based on heat transfer mechanisms derived from theoretical models and from experimental data on bubble dynamics from growth and motion aspects. In practice, it is essential to predict the complete boiling curve in system with specific conditions, which performs an accurate description of the boiling process, and to point out the transition between the regions. The prediction of the boiling curve is based on temperature measurements on the surface of the heating element using thermocouples or thermographic and high speed cameras [1, 2]. The accuracy of these measurements could be improved by applying acoustic emission (AE) method especially in complicated geometries. The current study adopts the AE techniques in order to examine the potential of the AE in detection of bubbles during boiling process and to point out the transition zones. Preliminary AE work has been conducted in order to determine the appropriate AE parameters, and to eliminate mechanical and electromagnetic interferences. The AE findings in terms of counts, amplitude, peak frequency, etc., as a function of the applied current, revealed the early stages of bubble formation during boiling process. The AE data also indicates the transition between the different regions of the boiling curve. The AE results were analyzed by spectrum analysis and wavelet analysis in order to determine characteristic frequencies and sequences of the water boiling processes. The AE results show a great potential to detect in situ the early stages of boiling process and can be used as an efficient tool for monitoring continuously possible boiling in industrial pipes where the boiling might be critical and damage process. [ABSTRACT FROM AUTHOR]

Carbon-carbon (C/C) composites with different densities (1.8, 1.35, 0.8 g/cm³), produced by chemical vapor infiltration (CVI) were tested mechanically under quasi-static loading in bending mode of uniform and notched beams. Acoustic emission (AE) technique was used to track the mechanical threshold parameters as well as to characterize the damage build-up profile to fracture. In both states of stress (uniform and tri-axial), threshold values detected by AE activity indicated the damage onset. The sensitivity of the AE method to the density changes was apparent by variations of the threshold values. Decreasing the density from 1.8 to 0.8 g/cm³ decreases the thresholds values (σth, KIth) from 25 to 2 MPa and from 0.8 to 0.1 MPa·m1/2, respectively. Three stages in damage evolution to fracture were observed: Stage I, with no AE activity, Stage II, gradual/linear growth in AE counts up to an abrupt jump and Stage III with exponential increase in AE counts. Similarity in profile and threshold value were found between the cumulative AE counts vs. strain data and the crack density vs. strain predicted by micro mechanical model, indicating the importance of using AE in monitoring the damage evolution in composites with regard to structural integrity aspect. Wave analysis using fast Fourier transform (FFT) and short-time fast Fourier transform (ST-FFT) points out four possible failure micro-mechanisms: multilayer cracking, breaking of fiber bundles, interfacial matrix de-bonding and micro-crack growth. Breaking of fiber bundles was found to be the major damage mechanism for the low density C/C composite. [ABSTRACT FROM AUTHOR]