Size effect on the contact state between fracture specimen and supports in Hopkinson bar loaded fracture test

To thoroughly understand the dynamic behavior of a fracture specimen under stress wave loading, dynamic fracture test with various three-point bend (3PB) specimens are performed on the Hopkinson bar loaded experimental apparatus. The contact state between the fracture specimen and supports during the loading process is examined via stress wave propagation analysis. The experimental results show that the fracture specimen with usual dimensions does not keep contact with supports in the initial loading stage, i.e. a loss of contact phenomenon occurred. The specimen dimensions and the span of the loading apparatus are important factors affecting specimen’s contact state. The loss of contact is more obvious with increasing span under the same specimen dimensions. Conversely, the loss of contact gradually disappears with increasing specimen length or increasing width under a fixed span. Based on experimental investigations, a criterion is established to ensure the fracture specimen keep in contact with supports during dynamic fracture test.     

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material under thermo-mechanical loading

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material (FGM) is investigated using a three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus in conjunction with induction coil heating system is used during elevated temperature dynamic loading experiments. A simple and accurate technique has been developed to identify the time corresponding to the load at which the fracture initiates. A series of experiments are conducted at different temperatures ranging from room temperature to 800 °C, and the effect of temperature and loading rate on the fracture initiation toughness is investigated. The material fracture toughness is found to be sensitive to temperature and the fracture initiation toughness increases as the temperature increases. Furthermore, the fracture initiation toughness is strain rate sensitive and is higher for dynamic loading as compared to quasi-static loading.     

Evaluation on Fracture Toughness at Dynamic Loading for Welded Joint Based on the Local Approach

The changes in mechanical properties and fracture toughness by dynamic loading were investigated with experiments. The parameter R, which can reflect the effect of the loading rate and the temperature rising during the high loading rate, could be employed to describe the constituent relation for the typical structure steel and its weld metal. The dynamic loading effect on the stress/strain fields and the temperature variation in the vicinity of the crack tip was analyzed by the finite element method, the dynamic fracture behavior was evaluated based on the local approach. It has been found that the Weibull stress is an effective fracture parameter, independent of the temperature and the loading rate.     

Effect of bending moment on the dynamic fracture of a beam or plate under tensile loading

The dynamic fracture response of a long beam of brittle elastic material under tensile loading is studied. If the magnitude of the applied loading is increased to a critical value, a crack is assumed to propagate across the beam cross section. As an extension of previous work, an induced bending moment generated during fracture is incorporated into the analysis and this improved formulation is presented. The crack length, crack tip speed, axial force and bending moment on the fracturing section are determined as functions of time after crack initiation. It is found that the bending moment has a significant effect on the fracture process in that it tends to retard fracture and causes a drastic change in the slope of the loading curve for large crack depths. Finally, by appropriate change of the elastic modulus, the results may be applied to plane strain fracture of a plate in pure tensile loading.     

Experimental studies on mixed‐mode dynamic fracture behaviour of aluminium alloy plates with narrow U‐notch

Mixed‐mode dynamic fracture behaviour of cast aluminium alloy ZL205A thin plates with narrow U‐notch was studied by split Hopkinson tensile bar apparatus. Specimens with different loading angles were designed to realize different fracture modes. The same loading condition was maintained during the tests. Recovery specimens show that crack propagates along the notch direction. Force–elongation relations show that with the loading angle increasing, the fracture force increases while the final elongation decreases. Deformation and fracture process was observed by a high‐speed camera. Displacement distribution around the crack was calculated through digital image correlation technique. Based on the photos and displacement results, initiation time of the crack was derived. Besides, two stress components (normal stress and shear stress) applied on the fracture surface were investigated. Results show that crack initiation stresses at different loading angles satisfy the ellipse equation. Pure mode I and II fracture stresses are 425.3 and 236.7 MPa, respectively. Furthermore, specific fracture energy of different specimens was calculated. The energy data vary with loading angle and located on an approximate upward parabolic curve. From the curve, the minimum specific fracture energy of the thin plate specimen is 42.0 kJ/m² under loading angle of 76.3°.     

Monotonic vis-à-vis cyclic fracture behaviour of AISI 304LN stainless steel

This investigation is aimed to examine the monotonic and cyclic fracture behaviour of AISI 304LN stainless steel and its weldments, in order to assess their integrity under seismic loading conditions. The monotonic fracture resistance of the steel has been determined using standard J-integral technique; whereas the cyclic fracture resistance has been evaluated using periodic unloading to different extents fixed by pre-determined R-ratio. Comparison of the fracture toughness values of the steel estimated under monotonic and cyclic loading indicates that the latter could be as low as one-fifth of the former. The observed degradation in cyclic fracture resistance has been attributed to crack tip re-sharpening during cyclic loading.     

Effect of substrate roughness on the contact damage of thin brittle films on brittle substrates

The effect of substrate and surface roughness on the contact fracture of diamond-like carbon coatings on brittle soda-lime glass substrates has been investigated. The average surface roughness (R_a) of the examined samples ranged from 15 nm to 571 nm. Contact damage was simulated by means of spherical nanoindentation, and fracture was subsequently assessed by focused ion beam microscopy. It was found that, in the absence of sub-surface damage in the substrate, fracture occurs in the coating in the form of radial, and ring/cone cracks during loading, and lateral cracks during unloading. Increasing the surface roughness results in a decrease in the critical load for crack initiation during loading, and in the suppression of fracture modes during unloading from high loads. When sub-surface damage (lateral cracks) is present in the substrate, severe spalling takes place during loading, causing a large discontinuity in the load-displacement curve. The results have implications concerning the design of damage-tolerant coated systems consisting of a brittle film on a brittle substrate.     

Dynamic fracture behaviour of Fe78Si9B13 metallic glass ribbon under laser shock loading

The fracture behaviour of Fe₇₈Si₉B₁₃ metallic glass under laser shock loading was investigated. Morphologies of the fracture surface and laser irradiated surface were characterized using scanning electron microscope. The results show that the fracture surface consists of sliding region and final fracture region with crack propagation. Liquid droplets and melted belts are scattered on the fracture surface as the notable features compared with fracture surface morphology under quasistatic loading, indicating the significant temperature increase in shear bands during dynamic loading. The primary and secondary shear bands are distributed on the specimen surface resulting from the simultaneous operation of multiple shear bands at high strain rates. Ripples with the characteristic spacing of about 1 µm are generated on the laser irradiated surface because of the interaction of laser pulse with solid surface.     

Study on stress evolution in SiC particles during crack propagation in cast hybrid metal matrix composites using Raman spectroscopy

The evolution of stress in the SiC particles during crack propagation under monotonic loading in a cast hybrid MMC was investigated by micro Raman spectroscopy. The experiment was carried out in situ in the Raman spectroscopy. Experimental results showed that cracks due to monotonic loading propagated by the debonding of the particle/matrix interface and particle fracture. Secondary cracks those formed in front of the main crack tip coalesced with the main crack in subsequent loading and final failure occurred. A high decrease in stress (several hundreds in MPa) was observed with the interfacial debonding at the interface and with the particle fracture on the particle. Moreover, the critical tensile stresses for particle–matrix interface debonding and particle fracture developed in hybrid MMC were also estimated during the crack propagation.     

The role of microdamage in tensile failure of graphite/epoxy laminates

Damage development during quasistatic tensile loading of several laminates of graphite/epoxy material is examined and compared to damage development in laminates of a similar graphite/epoxy material subjected to tension-tension fatigue loading. Emphasis is placed upon following damage development at the microstructural level. Evidence of the important role of off-axis ply cracks in localizing and controlling fiber fracture in adjacent load-bearing plies for both loading modes is resented. The relationship between fiber fracture density and static load level is presented for tensile loading of unidirectional and cross-ply laminates by direct observation of fiber fracture in situ. The frequencies of occurrence of multiple adjacent fiber fractures are also reported. The cross-ply laminate results are compared with those from fatigue testing. Significant differences are described and discussed.     

Rate effects in the fracture of glass fibre chopped strand mat composites

The effect of varying loading rate on the fracture properties of composites based on glass fibre chopped strand mats and epoxy-based vinyl ester resins has been studied. Initially, the fracture behaviour of the pure resins was studied by using simple flexure and fracture mechanics test techniques. Following this, a series of perforation tests was undertaken on small plate-like samples in order to assess the rate sensitivity of the perforation threshold and to determine if this value could be related to the fracture properties of the composite.

It was found that the work of fracture, W_f, increased with increasing loading rate over the wide range of conditions examined. It is believed that this increase is related to the rate-dependent fracture properties of the glass fibres. Increasing the loading rate results in an increase in fibre failure stress and, as a result, the stored elastic energy. This in turn results in increased crack bifurcation and the formation of a larger damage zone during fracture. Similar trends were observed in the plate impact data, with the threshold increasing rapidly with loading rate. It was found that the work of fracture determined on single-edge-notch bend samples correlated well with the drop-weight impact perforation data.     

Experimental investigation of fracture damage of notched granite beams under cyclic loading using DIC and AE techniques

Fracture experiments of three‐point bending notched granite beams were performed under cyclic loading using digital image correlation (DIC) and acoustic emission (AE) techniques. The damage evolution process of the specimen under cyclic loading was analysed on the basis of AE ring count and b value. The strain and displacement fields and the fracture process zone (FPZ) ahead of the crack tip were revealed by DIC. The results showed that the AE characteristics of rock fracture indicated a noticeable Kaiser effect in the stage of cyclic loading and unloading. Moreover, when the loading force reached 70% of its peak value, the AE characteristics showed the Felicity effect. The damage produced during the loading‐unloading process contributed to the development of the cracks leading to the catastrophic fracture. Besides, a relatively high loading rate was found to help to suppress the development of the FPZ at the crack tip.     

Material deformation and fracture under impulsive loading conditions

Engineering structures experience impulsive loads during the time of natural disasters like earthquakes, cyclones and collisions. The design of structures resistant to such natural disasters requires an understanding of the deformation and fracture behaviour of the materials constituting the structure under impulsive loading conditions. In this paper the various aspects of dynamic plastic deformation and fracture of common engineering materials are reviewed and contrasted with their behaviour under static loading conditions.     

Fracture energy of UHP-FRC under direct tensile loading applied at low strain rates

This research investigates the fracture energy of ultra-high performance fiber reinforced concretes (UHP-FRC) under direct tensile loading applied at relatively low strain rates. Nine UHP-FRC series incorporating three types of steel fibers (straight, end-hooked, and twisted fibers), each in three different fiber volume fractions, are tested under uniaxial tensile loading at four different strain rates, ranging from 0.0001 s⁻¹ to 0.1 s⁻¹. Particular attention is given to clearly distinguish between the dissipated energy during the strain hardening and softening portions of the loading regime. The test results show that: 1) the fracture energy is mainly influenced by a parameter, termed fiber factor, which is a function of the fiber volume fraction and slenderness, and 2) all three types of UHP-FRCs exhibit increases in fracture energy with increasing strain rates. The observed strain rate sensitivity of the fracture energy suggests it is likely associated with the strain sensitive micro-cracking that occurs during fiber pull-out.     

Synchrotron radiation small-angle X-ray scattering study on fracture process of carbon nanotube/poly(ethylene terephthalate) composite films

Time-resolved small-angle X-ray scattering (SAXS) measurements have been conducted during tensile deformation of carbon nanotube (CNT)/amorphous poly(ethylene terephthalate) (PET) composite films using synchrotron radiation in order to investigate the fracture process. The observed SAXS patterns consisted of the streaks parallel to the loading direction caused by the total reflection at craze/polymer interfaces, the streaks perpendicular to the loading direction caused by the fibril/void structure of crazes and the scattering from CNTs. The formation, widening and fracture processes of the crazes were investigated based on the changes of SAXS patterns during deformation and the fracture toughness of the composite films determined with essential work of fracture method. The influences of CNT addition on the mechanical properties of PET varied depending on the specimen geometries used for the mechanical tests and marked influences were obtained with surface-notched specimens. The CNT addition increased the energy needed to widen the crazes and retarded the growth and fracture of the crazes during deformation. This lead to the increases in the plastic work of fracture and the fracture toughness of PET. The CNT aggregates formed at the CNT fraction beyond 3 wt%, however, caused reduction of the fracture toughness.     

Bone fracture characterization using the end notched flexure test

A miniaturized version of the end notch flexure test was used in the context of pure mode II fracture characterization of bovine cortical bone. To overcome the difficulties intrinsic to crack length monitoring during its propagation an equivalent crack method was employed as data reduction scheme. The proposed method was validated numerically by means of a finite element analysis including a cohesive zone modeling and subsequently applied to experimental results to determine the fracture energy of bone under pure mode II loading. Finally, a cohesive law representative of fracture behavior of each specimen was determined employing an inverse method, considering a trapezoidal shape for the softening law. The consistency of the obtained results leads to the conclusion that the trapezoidal law is adequate to simulate fracture behavior of bone under mode II loading. The proposed testing setup and the employed data reduction scheme constitute powerful tools in which concerns fracture characterization of bone under pure mode II loading and can be viewed as the main outcomes of this work.     

On the symmetric dynamic fracture of a beam under tensile loading

The dynamic fracture response of a long beam of brittle elastic material under tensile loading is studied by means of two different one-dimensional models. If the magnitude of the applied loading is increased quasi-statically to a critical value, two coplanar edge cracks are assumed to propagate across the beam's cross section. The first model parallels that of [6] with the crack length, crack speed and the loading on the fracturing section being determined as functions of time after fracture initiation. The second model is derived by means of energy considerations in the vicinity of the fracturing section. The results obtained from both models are similar except during the final phase of the fracture process.     

CREEP-FATIGUE CRACK GROWTH AND FRACTOGRAPHY OF A TYPE 304 STAINLESS STEEL AT ELEVATED TEMPERATURE

Abstract— Creep-fatigue crack growth behaviour of a Type 304 stainless steel under four types of reversed loading patterns (P-P, P-C, C-P and C-C) was investigated and the results are discussed in the light of fracture mechanics and fractography. The crack growth rate for all of the four types of loading was successfully correlated in terms of the cyclic integral range λ J. It was unnecessary, for practical purpose, to divide Ay into a fatigue component, λ J _f, and a creep component, λ J _c, as has been done elsewhere. The transition of the correlating fracture mechanics parameter from fatigue to creep was not necessarily associated with the fracture morphology. This was related to the longer transition hold time in morphology in C-C type loading compared to C-P type loading, and was attributed to recovery of grain boundary sliding during the compression hold in the C-C type loading.     

Fracture of polymethyl methacrylate during the explosion of a gas in a cavity

Results are presented from an experimental study of the fracture of bulk specimens of polymethylmethacrylate during the explosion of an acetylene-air mixture in a cylindrical cavity with disk-shaped and radial cracks about its circumference. It is shown that the character of fracture depends not only on the rate of loading of the walls of the cavity and the form of the initial crack, but also on the interval of load application. Crack velocity under experimental conditions is intermediate between the velocity of slow cracks associated with quasistatic hydraulic rupture and the velocity of fast cracks formed in impulsive fracture. The duration of action of the critical level of stresses during fracture is discussed. Critical fracture surfaces are constructed in the space of the loading parameters for the criterion of the maximum stress-intensity factor for disk-shaped and radial cracks.Translated from Problemy Prochnosti, No. 7, pp. 84–89, July, 1995.     

Eine Bruchmechanikprobe und Belastungsvorrichtung zur Bestimmung von Rißzähigkeiten bei überlagerter Normal- und Schubbeanspruchung

A fracture mechanics specimen and loading device for the determination of fracture toughness under superimposed normal and shear modes of loading A simple as well as compact fracture mechanics specimen and a surprisingly uncomplicated loading device is suggested for the determination of fracture toughnesses under loading mode I, loading mode II and mixed loading mode. With this loading device the different loading modes can be applied to the specimen by means of a simple tensile test.