Dynamic fracture behavior of syntactic epoxy foams: optical measurements using coherent gradient sensing

Dynamic fracture behavior of syntactic foams made of thin-walled microballoons dispersed in epoxy matrix is studied. Monotonically decreasing dynamic Young's modulus with increasing volume fraction of microballoons is observed using ultrasonic pulse-echo and density measurements. The results are also in good agreement with the Hashin–Shtrikman lower-bound predictions for elastic porous solids. Dynamic crack initiation toughness and crack growth behaviors are examined using instrumented drop-tower tests and optical measurements. Crack initiation toughness shows a linear relationship with Young's modulus over the entire range of volume fraction of microballoons studied. A proposed model based on simple extension of micromechanics prediction agrees well with the measurements. The optical method of coherent gradient sensing (CGS) has been used along with high-speed photography to record crack tip deformation histories in syntactic foam samples subjected to impact loading. Pre- and post-crack initiation events have been successfully captured and apparent dynamic stress intensity factor histories are extracted from the interferograms. Results suggest increasing crack speeds with volume fraction of microballoons. No significant dependence of dynamic fracture toughness on crack speed in any of the volume fractions is observed.     

Dynamic instability in intergranular fracture

A molecular-dynamics model for crack propagation under steady-state conditions is used to study dynamic instabilities along a grain boundary in aluminum that occur when the crack speed approaches 1/3 of the material's Rayleigh wave speed. Instead of crack branching, as is characteristic for a crack propagating in a homogeneous environment, the instability of an intergranular crack results in a periodic series of dislocation bursts. These bursts limit the crack speed and produce velocity oscillations with a large increase in energy dissipation that increases the grain boundary toughness.     

The fracture mechanics of glassy polymers

The application of fracture mechanics to glassy polymers, in particular crack growth in PMMA, is discussed. Particular attention is paid to two processes which modulate the energy supply to the crack tip: viscoelastic dissipation at slow crack speeds and specimen inertia at large crack speeds. The relation between fracture energy and crack speed is reviewed, and, where possible, fracture surface observations are correlated with dynamic behavior.     

Mode II brittle fracture assessment using an energy based criterion

In this paper the minimum strain energy density criterion is modified to predict the values of mode II fracture toughness reported in the literature for several brittle and quasi-brittle materials. The experimental results are all related to mode II fracture tests performed on the semicircular bend specimen. The modified mode II fracture criterion takes into account the effect of T-stress (in addition to the singular terms of stresses/strains) when calculating the strain energy density factor at a very small critical distance from the crack tip. It is shown that the proposed criterion provides significantly better predictions for mode II fracture toughness compared with the classical minimum strain energy density criterion.     

Crack propagation in amorphous polymers and their composites

The fracture energy of a polymer depends strongly on the viscoelastic responses of the material, and therefore is a function of temperature and crack velocity. The toughness of a composite is determined by the way in which the reinforcing filler modifies the energy dissipating mechanisms of the polymeric matrix.

The fracture toughness of a variety of polymeric glasses and their composites with glass beads, glass fibers, and rubber particles was measured. The velocity of rapidly moving cracks and the crack propagation rates under controlled loading conditions were also measured.

It was found that the crack propagation velocities in unfilled and glass bead filled materials were controlled by the longitudinal stress waves in the matrix and that the only effects of the glass beads were to blunt the crack tip and limit the viscous deformation. The effect on fracture toughness was relatively small and either positive or negative, depending on which of the above two factors dominated.

The presence of rubber particles as a second phase lowered terminal crack propagation velocities and greatly increased the fracture toughness, indicating a crack retarding effect of the rubber particles. This is related to the induction of crazes in the matrix by the rubber phase.

Glass fibers had a tendency to bridge the tip of a propagating crack, thereby greatly increasing the fracture toughness. In this case the work of fracture comes from a combination of the elastic strain energy stored in the fibers, the energy dissipated in debonding the fibers from the matrix, and the fracture energy of the matrix itself.     

Photoelastic investigation of interfacial fracture between orthotropic and isotropic materials

Static and dynamic fracture of interfaces between orthotropic and isotropic materials were studied using photoelasticity. In this study, a bi-material specimen made of PSM-1^® and Scotchply^® 1002, a unidirectional glass fiber reinforced epoxy composite, was used. Two fiber orientations, fibers parallel to the interface (α=0°) and fibers perpendicular to the interface (α=90°) were considered. Center crack bi-material specimens having different crack lengths were loaded quasi-statically and the full-field isochromatics were recorded using a digital camera. The complex stress intensity factor corresponding to each crack length was calculated from the isochromatics and the values were compared to that obtained from boundary collocation method. Dynamic interfacial fracture was studied with an edge crack bi-material geometry for the two different fiber orientations. The isochromatics around the propagating crack were recorded using a digital high-speed camera. The fracture parameters such as crack speed, complex stress intensity factor and energy release rate were extracted from the isochromatics using the asymptotic stress field equations. The complex stress intensity factor obtained from the static experiments was in close agreement with that calculated using the boundary collocation method. The results also indicated that the fiber orientation with respect to the interface influences the fracture parameters for stationary and propagating cracks.     

Mixed mode I/II fracture investigation of Perspex based on the averaged strain energy density criterion

In this work, some recent mixed mode I/II fracture toughness results obtained from Perspex (or polymethylmethacrylate (PMMA)) with four simple cracked specimens subjected to the conventional three-point bend loading are reanalysed based on local energy concept. Although all the mentioned samples have been tested under the same and similar mode mixities, different fracture toughness envelopes were obtained for mixed mode I/II fracture of PMMA. The averaged strain energy density (SED) criterion has been applied in the past for different types of notched specimens (including U, V, O and keyhole notches). It is shown that the mixed mode tensile-in plane shear fracture toughness data obtained from the semicircular and triangular crack type specimens are successfully predicted for sharp cracked PMMA samples using the SED criterion.     

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

Fracture energy of gels

To clarify effects of crack speed and cross-link density on the fracture energy of acrylamide gels, we evaluated the roughness of the fracture surface and measured the fracture energy taking into account the roughness. The fracture energy increases linearly with crack speed Vin a fast crack speed region, and the increasing rate of fracture energy with V decreases with increasing cross-link density in the gels. In a slow crack speed region the fracture energy depends on crack speed more strongly than in the fast crack speed region. This indicates that a qualitative change exists in the fracture process of the gels. Received 8 March 2000 and Received in final form 7 July 2000     

Caustics method in dynamic fracture problem of orthotropic materials

Caustics method is a powerful optical technique in fracture mechanics because of its high sensitivity to stress gradients. In this paper, it is applied to resolve dynamic fracture problems in orthotropic composites. Considering most orthotropic materials are opaque, reflective caustics method is derived here by combining the fundamental principle of caustics method with the mechanical properties of orthotropic materials. Meanwhile, corresponding experiments are carried out for typical glass fiber-reinforced composites, where mode I and mixed-mode fracture states are taken into account. By recording and analyzing shadow spot patterns during the crack propagation process carefully, crack onset time, dynamic fracture toughness and crack growth velocity of orthotropic composite are determined. These results will be useful to evaluate the dynamic fracture properties of composites and further to optimize their designs.     

The influence of interface reaction zone on interfacial fracture toughness of SiC fiber reinforced titanium matrix composites

A fiber-reaction zone-matrix three-phase model is developed to evaluate the interfacial fracture toughness of titanium alloys reinforced by SiC monofilaments. Based on fracture mechanics, theoretical equations of G_IIc are presented, and the effects of several key factors such as crack length and the interface reaction zone thickness on the critical applied stress necessary for crack growth and interfacial fracture toughness are discussed. Finally, the interfacial fracture toughness of typical composites including Sigma1240/Ti-6Al-4V, SCS-6/Ti-6Al-4V, SCS-6/Timetal 834, SCS-6/Timetal 21s, SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model. The results show that the model can reliably predict the interfacial fracture toughness of the titanium matrix composites.     

金属穿晶脆性断裂统计理论

讨论了金属穿晶脆性断裂统计理论.根据穿晶裂纹和晶界作用的界面能模型及断裂非平衡统计理论框架,推导出了裂纹扩展速率、断裂强度、断裂韧性、脆性—韧性转变温度及其统计分布函数随晶粒尺度和界面能变化的公式. 关键词：     

Nanoscale periodic morphologies on the fracture surface of brittle metallic glasses

Out-of-plane, nanoscale periodic corrugations are observed in the dynamic fracture surface of brittle bulk metallic glasses with fracture toughness approaching that of silica glasses. A model based on the meniscus instability and plastic zone theory is used to explain such dynamic crack instability. The results indicate that the local softening mechanism in the fracture is an essential ingredient for controlling the formation of the unique corrugations, and might provide a new insight into the origin of fracture surface roughening in brittle materials.     

Impact toughness of EK-181 ferritic-martensitic chromium (12%) steel under loading by concentrated bending

The low-temperature fracture of a high-temperature low-activated ferritic-martensitic EK-181 chromium (12%) steel (RUSFER-EK-181: Fe-12Cr-2W-V-Ta-B) is studied using impact and static concentrated bending tests as a function of the specimen dimensions (standard, small), the type of stress concentrator (V-shaped notch, fatigue crack), and the temperature (from −196 to +100°C). The ductile-brittle transition temperature falls in the range from −85 to +35°C. The temperature dependences of stress-intensity factor K _Ic and fracture toughness J _Ic are determined. The severest type of impact toughness tests is represented by tests of V-notched specimens with an additional fatigue crack and two lateral V-shaped notches (three-sided V-shaped notch with a central fatigue crack). The fracture energy of the steel depends on the type of stress concentrator and the specimen dimensions and is determined by the elastic energy and the plastic deformation conditions in the near-surface layers of a specimen, which are controlled by the lateral notches. At the same test temperature, the impact toughness and the fracture toughness are interrelated. Irrespective of the type of specimen (including notches and a fatigue crack), the ferritic-martensitic steel exhibits the same fracture mechanism.     

Unstable Fracture Behavior of Rubber Toughened Poly(Styrene-co-Acrylonitrile)

A linear elastic fracture mechanics (LEFM) approach was used to study fracture characteristics of ABS materials. The effects of crack (ligament) length and rubber content on the microscopic deformations taking place at the front of crack tip and in the bulk of the specimens were investigated. The results of fractography studies showed that, in addition to rubber content, the microscopic deformations are influenced by crack length. For some materials this manifests itself as a change in macroscopic response. The ligament length dependent behavior was increased for the samples with higher rubber contents. The results also showed that, although the elastic behavior with unstable crack growth is the dominant micromechanism of deformation, stable crack propagation still occurred in some compositions. All the fracture parameters, including fracture toughness, fracture energy, plastic zone size, and crack tip opening, increased with rubber content. The changes in microscopic and, as a consequence, in the macroscopic deformation behavior of a given specimen with ligament length were attributed to changes in yield stress of the sample and maximum stress on the ligament.     

Predicting Interfacial Strengthening Behaviour of Particulate-Reinforced MMC — A Micro-mechanistic Approach

The fracture properties of particulate-reinforced metal matrix composites (MMCs) are influenced by several factors, such as particle size, inter-particle spacing and volume fraction of the reinforcement. In addition, complex microstructural mechanisms, such as precipitation hardening induced by heat treatment processing, affect the fracture toughness of MMCs. Precipitates that are formed at the particle/matrix interface region, lead to improvement of the interfacial strength, and hence enhancement of the macroscopic strength properties of the composite material. In this paper, a micro-mechanics model, based on thermodynamics principles, is proposed to determine the fracture strength of the interface at a segregated state in MMCs. This model uses energy considerations to express the fracture toughness of the interface in terms of interfacial critical strain energy release rate and elastic modulus. The interfacial fracture toughness is further expressed as a function of the macroscopic fracture toughness and mechanical properties of the composite, using a toughening mechanism model based on crack deflection and interface cracking. Mechanical testing is also performed to obtain macroscopic data, such as the fracture strength, elastic modulus and fracture toughness of the composite, which are used as input to the model. Based on the experimental data and the analysis, the interfacial strength is determined for SiC particle-reinforced aluminium matrix composites subjected to different heat treatment processing conditions.     

The measurement of cohesive and interfacial toughness for bonded metal joints with epoxy adhesives

The types of crack growth in adhesive joints are reviewed and three are identified, namely central cohesive, asymmetric cohesive and interfacial. Test methods for measuring fracture toughness associated with these cracks are then outlined and include a Tapered Double Cantilever Beam (TDCB) test for a central cohesive crack and peel tests on flexible laminates for the other types of crack. In particular, fixed arm and mandrel peel tests are used. Two aerospace adhesives are used to prepare test specimens in order to conduct these tests. For one of these adhesives, all three types of crack growth were recorded and this provided an opportunity to make detailed comparisons of the three associated fracture toughness values. Of particular interest was the use of the mandrel peel method because it enabled a fracture transition (asymmetric cohesive to interfacial fracture) to be observed during the test. The fracture toughness value associated with a central cohesive crack was similar in magnitude to that for an asymmetric cohesive crack. However, the fracture toughness for interfacial fracture was much lower, but similar in magnitude to the expected value of half the fracture toughness from a TDCB test.     

Atomistic scale fracture behaviours in hierarchically nanotwinned metals

In the present study, a series of large-scale molecular dynamics simulations have been performed to investigate the atomistic scale fracture behaviours along the boundaries of primary twins in Cu with hierarchically nanotwinned structures (HTS), and compare their fracture behaviours with those in monolithic twins. The results indicate that crack propagation along [1?1?2] on the twin plane in monolithic nanotwins is brittle cleavage and fracture, resulting in low crack resistance and fracture toughness. However, the crack resistance along the boundaries of primary twins in HTS is much higher, and a smaller spacing of secondary twins (λ ₂) leads to even higher fracture toughness. With large λ ₂, the crack growth is achieved by void nucleation, growth and coalescence. However, considerable plastic deformation and enhanced fracture toughness in HTS could be achieved by the crack blunting and by the extensive dislocation accommodation ahead of the crack tip when λ ₂ is small.     

声发射特性与材料断裂韧性相关性研究

以高强度铝合金的断裂过程为研究对象，利用先进的声发射试验系统，考察了拉伸试样和带有预制疲劳裂纹的DCB试样在断裂时的声发射事件特性，揭示了材料断裂韧性和声发射特性 间的关系.试验结果表明，材料稳态裂纹扩展起始于K_1C并发射出许多幅值超过 阈值35 dB的信号，材料裂纹失稳扩展时爆发出强烈的声发射信号，材料断裂的累积声发射能与宏 观断裂能呈线性关系. 关键词： 声发射 断裂韧性 铝合金     

Dynamic caustics and its applications

For the study of elastodynamic problems of propagating cracks it is necessary to evaluate the dynamic stress intensity factor K^d_I which depends on the form of expressions for the stress components existing at the running crack tip at any instant of the propagation of the crack and the corresponding dynamic mechanical and optical properties of the material of the specimen under identical loading conditions. In this paper the distortion of the form of the corresponding reflected caustic from the lateral faces of a dynamically loaded transparent and optically inert specimen containing a transverse crack running under constant velocity was studied on the basis of complex potential elasticity theory and the influence of this form on the value of the dynamic stress intensity factor was given. The method was applied to the study of a propagating Mode I crack in a PMMA specimen under various propagation velocities and the corresponding dynamic stress intensity factor K^d_I evaluated. Also, crack propagation behaviour of notched composites in dynamic loading modes are reviewed and evaluated. A relatively large data base using metal-epoxy particulates, rubber-toughened poly(methyl methacrylate), and Sandwich plates are given. In all cases, a combination of high-speed photography and the optical method of dynamic caustics has been used. Results on the dynamic crack propagation mode, fracture toughness and crack propagation velocities of several rubber-modified composite models are presented. The composite models studied include specimens with one and/or two ‘complex’ two-stage inclusions, i.e. PMMA round inclusions surrounded by concentric rubber rings, one and/or press-fifting inclusions without rubber interface, all under dynamic loading. In all cases both qualitative and quantitative results were obtained. Also, results on crack propagation mode, crack propagation velocity, stress intensity factors and on the influence of the sandwich phases on crack propagation mode are presented.