Subsonic and Intersonic Failure of Composites: High-Speed Optical and Thermographic Measurements and Numerical Simulations

2000 ◽  
Author(s):  
A. J. Rosakis ◽  
D. Coker ◽  
C. Yu ◽  
M. Ortiz
Keyword(s):  
Crack Growth ◽  
High Speed ◽  
Large Scale ◽  
Wave Speed ◽  
Epoxy Composite ◽  
Shear Crack ◽  
Crack Tip ◽  
Composite Plates ◽  
Crack Face ◽  
Shear Wave Speed

Abstract In this paper dynamic fracture behavior of unidirectional graphite-epoxy composite plates is investigated experimentally and numerically. Crack propagation experiments are conducted on thick unidirectional graphite-epoxy composite plates subjected to in-plane, symmetric and asymmetric, impact loading. The coherent gradient sensing technique (CGS) is used in conjunction with high-speed photography to visualize the crack growth events. Cracks are found to propagate at subsonic speeds in the Mode-I case, whereas in both mixed mode and Mode-II the crack tip speed clearly exceeds the shear wave speed of the laminate. In the case of symmetric loading (Mode-I), the crack tip speeds approach the Rayleigh wave speed of the composite (1500 m/s), however it never exceeds it as predicted by asymptotic analysis. The situation is found to be entirely different for growing shear (Mode-II) cracks. A shock wave emanating from the crack tip is observed in the optical patterns. This provides direct evidence that the crack propagates faster than the shear wave speed of the composite. The crack tip speed is then observed to jump to a level close to the axial longitudinal wave speed along the fibers (7500 m/s) and then to stabilize to a lower level of approximately 6500 m/s. This speed corresponds to the speed at which the energy release rate required for shear crack growth is non-zero as determined from asymptotic analysis. The CGS interferograms also reveal the existence of large-scale frictional contact of the crack faces behind the moving shear cracks. In addition high speed thermographic measurements are conducted that show concentrated hot spots behind the crack tip indicating crack face frictional contact. Finally, these experiments are modeled by a detailed dynamic finite element calculation involving cohesive elements, newly developed adaptive remeshing using subdivision and edge collapse, composites element, and penalty contact. The numerical calculations are calibrated on the basis of fundamental material properties measured in the laboratory. The numerical methodology is subsequently validated by direct comparison to optical experimental measurements (crack speed record and near tip deformation field structure). For shear crack growth the numerics also reveal the experimentally observed shock wave structure and confirm the optical observation of large-scale crack face contact.

Development of a dynamic decohesion criterion for subsonic fracture of the interface between two dissimilar materials

1995 ◽  
Vol 451 (1943) ◽  
pp. 711-736 ◽  
Keyword(s):  
Crack Growth ◽  
High Speed ◽  
Crack Tip ◽  
Dissimilar Materials ◽  
Complex Stress ◽  
High Speed Photography ◽  
Dynamic Crack ◽  
Shear Wave Speed ◽  
Loading Rates ◽  
Lateral Shearing Interferometer

We present findings of an experimental study of dynamic decohesion of bimaterial systems composed of constituents with a large material property mismatch. Poly-methylmethacrylate (PMMA)-steel and PMMA-aluminium bimaterial fracture specimens were used. Dynamic one-point bend loading was accomplished with a drop-weight tower device (for low and intermediate loading rates) or a high-speed gas gun (for high loading rates). High-speed interferometric measurements were made using the lateral shearing interferometer of coherent gradient sensing in conjunction with high-speed photography. Very high crack propagation speeds (terminal crack-tip speeds up to 1.5 c s PMMA , where c s PMMA is the shear wave speed of PMMA) and high accelerations (of about 10 7 g , where g is the acceleration of gravity) were observed and are reported. Issues regarding data analysis of the high-speed interferograms are discussed. The effects of near-tip three-dimensionality are also analysed. Dynamic complex stress factor histories are obtained by fitting the experimental data to available asymptotic crack-tip fields. A dynamic crack growth criterion for crack growth along bimaterial interfaces is proposed. In the subsonic regime of crack growth it is seen that the opening and shearing displacements behind the propagating crack tip remain constant and equal to their value at initiation, i.e. the crack retains a self-similar profile during crack growth at any speed. This forms the basis of the proposed dynamic interfacial fracture criterion.

Dynamic Fracture Criteria for Crack Growth Along Bimaterial Interfaces

1998 ◽  
Vol 65 (2) ◽  
pp. 293-299 ◽  
Author(s):  
M. Kavaturu ◽  
A. Shukla
Keyword(s):  
Crack Propagation ◽  
Crack Growth ◽  
Shear Wave ◽  
Wave Speed ◽  
Crack Tip ◽  
Fracture Criteria ◽  
Shear Wave Speed ◽  
Bimaterial Interfaces ◽  
Opening Mode ◽  
Tip Velocity

Dynamic fracture criteria based on experimental observations are proposed for subsonic crack growth along bimaterial interfaces. These criteria are based on the premise that the crack-face displacements at a point behind the crack tip increase exponentially with the instantaneous crack-tip velocity. This assumption establishes a generalized relationship between the dynamic energy release rate and the instanta-neous crack-tip velocity. Experiments are performed on PSM-1/aluminum bimaterial systems for both shear dominated and opening-mode dominated crack growth to verify the proposed criteria. Two different bimaterial specimen geometries are employed to obtain the complete range of crack-tip speeds in the subsonic regime. The dynamic loading is achieved either by detonating two explosive charges on the specimen or by impacting the specimen in one-point bend configuration. Dynamic photoelasticity in conjunction with high-speed photography is used to analyze the fracture event. Explosive loading of the interface crack results in crack propagation speeds on the order of 65 percent of the shear wave speed of PSM-1 and the crack growth is observed to be stable and opening-mode dominated. In contrast, the impact loading results in very high crack propagation speeds on the order of shear wave speed of PSM-1 and the crack growth is observed to be shear dominated.

Subsonic and Intersonic Dynamic Crack Growth in Unidirectional Composites

1999 ◽  
Author(s):  
Demirkan Coker ◽  
Ares J. Rosakis ◽  
Yonggang Y. Huang
Keyword(s):  
Crack Growth ◽  
Wave Speed ◽  
Composite Plates ◽  
Mode I ◽  
Mode Ii ◽  
Fiber Direction ◽  
High Speed Photography ◽  
Dynamic Crack ◽  
Dynamic Crack Growth ◽  
Shear Wave Speed

Abstract Some recent experimental observations of highly dynamic crack growth events in thick unidirectional graphite fiber-reinforced epoxy matrix composite plates are presented. The composite plates were symmetrically (mode-I) and asymmetrically (mode-II) loaded in a one-point bend configuration with an edge pre-notch machined in the fiber direction. The lateral shearing interferometric technique of coherent gradient sensing (CGS) was used in conjunction with high-speed photography. Symmetric, mode-I cracks initiated at 1300 m/s and subsequently accelerated up to the Rayleigh wave speed but never exceeded it. For asymmetric, Mode-II types of loading, the results reveal highly unstable and intersonic, shear-dominated crack growth along the fibers. The intersonic cracks propagated with unprecedented speeds reaching 7400 m/s, more than three times the shear wave speed of the composite, and featured a shock wave structure typical of disturbances travelling with speeds higher than one of the characteristic wave speeds in the solid.

Cohesive Release Loads in a General Finite Element Model of a Propagating Crack

2009 ◽  
Vol 417-418 ◽  
pp. 517-520 ◽  
Author(s):  
A. Fontana ◽  
M. Minotti ◽  
Pietro Salvini
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Crack Tip ◽  
Reference Value ◽  
Element Model ◽  
Cohesive Model ◽  
Plastic Materials ◽  
T Stress ◽  
Speed Mode ◽  
Elastic Plastic Materials

High speed MODE I crack growth in elastic-plastic materials, involving large scale plasticity and dynamic effects connected to rapid propagation, is faced through a cohesive model to tune force nodal release. The stress resisting to the opening of the edges in the cohesive zone should account of effective stress field ahead crack tip. In this paper a reference value is accounted: it represents the maximum closing stress measured at the crack tip, where the cohesive effects begin. A bi-parametric analytical formulation of stress distribution ahead the crack tip is suggested. The bi-parametric formulation is able to extrapolate the stress at the tip whatever is the T-stress (i.e. the stress acting in the direction of fracture propagation), thus completely defining the cohesive loads.

Dynamical Fracture Instabilities Due to Local Hyperelasticity at Crack Tips

2006 ◽  
Vol 929 ◽  
Author(s):  
Markus J. Buehler ◽  
Huajian Gao
Keyword(s):  
Large Scale ◽  
Brittle Materials ◽  
Crack Tip ◽  
Local Stress ◽  
Nonlinear Material ◽  
Physical Reason ◽  
Shear Wave Speed ◽  
Linear Elastic ◽  
Strongly Nonlinear ◽  
Wide Range

ABSTRACTWhen materials break and cracks propagate, bonds between atoms are broken generating two new material surfaces. Most existing theories of fracture assume a linear elastic stress-strain law. However, the relation between stress and strain in real solids is strongly nonlinear due to large deformation near a moving crack tip, a phenomenon referred to as hyperelasticity or nonlinear elasticity. Cracks moving at low speeds create atomically flat mirror-like surfaces, whereas cracks at higher speeds leave misty and hackly fracture surfaces. This change in fracture surface morphology is a universal phenomenon found in a wide range of different brittle materials, but the underlying physical reason has been debated over an extensive period. Using massively parallel large-scale atomistic simulations employing a new, simple atomistic material model allowing a systematic transition from linear elastic to strongly nonlinear material behaviors, we show that hyperelasticity can play a governing role in dynamical crack tip instabilities in fracture of brittle materials. We report a generalized model that treats the instability problem as a competition between different mechanisms controlled by local stress field and local energy flow near the crack tip. Our results indicate that the fracture instabilities do not only appear in defected materials, but instead are an intrinsic phenomenon of dynamical fracture. Our findings help to explain controversial experimental and computational results, including experimental observation of crack propagation at speeds beyond the shear wave speed in rubber-like materials.

Intersonic Crack Propagation—Part I: The Fundamental Solution

2000 ◽  
Vol 68 (2) ◽  
pp. 169-175 ◽  
Author(s):  
Y. Huang ◽  
H. Gao
Keyword(s):  
Fundamental Solution ◽  
Crack Propagation ◽  
Cohesive Zone Model ◽  
Shear Crack ◽  
Crack Tip ◽  
Zone Model ◽  
Shear Wave Speed ◽  
Numerical Studies ◽  
Intersonic Crack Propagation ◽  
Tip Velocity

Recent experiments of Rosakis et al. have clearly shown that the crack-tip velocity can exceed the shear wave speed for a crack tip propagating between two weakly bonded, identical and isotropic solids under shear-dominated loading. This has motivated recent theoretical and numerical studies on intersonic crack propagation. We have obtained analytically the fundamental solution for mode-II intersonic crack propagation in this paper. This fundamental solution can provide the general solutions for intersonic crack propagation under arbitrarily initial equilibrium fields. We have also developed a cohesive zone model to determine the crack-tip energy release for an intersonic shear crack.

Numerical Analyses of Ductile Fracture Behavior in 2D Plane Strain and Axisymmetric Models Using the Complete Gurson Model

2009 ◽  
Author(s):  
Jie Xu ◽  
Zhiliang Zhang ◽  
Erling O̸stby ◽  
Ba˚rd Nyhus ◽  
Dongbai Sun
Keyword(s):  
Crack Growth ◽  
Fracture Behavior ◽  
Large Scale ◽  
Crack Tip ◽  
Ductile Crack ◽  
Geometry Dependence ◽  
Ductile Crack Growth ◽  
Scale Yielding ◽  
Resistance Curves ◽  
Crack Tip Constraint

Ductile crack growth plays an important role in the analyses of fracture behavior of structures. A strong geometry dependence of ductile crack growth resistance emerges under large scale yielding conditions. This geometry dependence is associated with different levels of crack tip constraint. However, an independent relationship between the fracture resistance and crack tip constraint has also been observed in experimental studies for selected specimen geometries. To verify these results, crack growth resistance curves for plane strain, mode I crack growth under large scale yielding have been computed using the complete Gurson model. Single edge notched bending (SENB) and tension (SENT) specimens with three different crack geometries have been selected for the numerical analyses. Specimen size effect on ductile crack growth behavior has also been studied. In addition, the SENT specimen appears as an alternative to conventional fracture specimens to characterize fracture toughness of circumferentially cracked pipes due to its similar geometry constraint ahead of the crack tip with that of cracks in pipes. 2D axisymmetric models have been carried out to investigate the effect of biaxial loading (axial tension combined with internal pressure) on the resistance curves for pipes with long internal circumferential cracks under large scale yielding conditions.

scholarly journals Discontinuous Growth Mechanisms of Mode II Crack under High-Speed Impact Conditions

2008 ◽  
Vol 41-42 ◽  
pp. 169-173
Author(s):  
Wei Ma
Keyword(s):  
Crack Growth ◽  
Main Crack ◽  
Wave Speed ◽  
Impact Testing ◽  
Growth Speed ◽  
Mode Ii ◽  
Growth Mechanisms ◽  
Shear Wave Speed ◽  
Mode Ii Crack ◽  
Crack Growth Model

A recoverable plate impact testing technology has been used for studying the growth mechanisms of mode II crack. The results show that interactions of microcracks ahead of a crack tip cause the crack growth unsteadily. Failure mode transitions of materials were observed. Based on the observations, a discontinuous crack growth model was established. Analysis shows that the shear crack grows unsteady as the growth speed is between the Rayleigh wave speed cR and the shear wave speed cs; however, when the growth speed approaches 2cs, the crack grows steadily. The transient microcrack growth makes the main crack speed to jump from subsonic to intersonic and the steady growth of all the sub-cracks leads the main crack to grow stably at an intersonic speed.

Mean Stress Effect on the Shear Fatigue Crack Model—Part 2

1969 ◽  
Vol 36 (4) ◽  
pp. 731-735 ◽  
Author(s):  
D. O. Swenson
Keyword(s):  
Crack Growth ◽  
Shear Crack ◽  
Crack Tip ◽  
Shear Loading ◽  
Stress Effect ◽  
Mean Stress ◽  
Stress Range ◽  
Cyclic Shear ◽  
The Mean ◽  
Tip Displacement

An expanded version of Swenson’s shear crack fatigue model is developed to include the effect of the mean stress on cyclic shear crack growth. Five results are concluded for this model. 1 As the mean shear stress is increased for a specified stress range, the cyclic shear crack tip displacement decreases. 2 The shear displacement at the center of the crack appears to be independent of the mean stress and remains constant for a given stress range. 3 An inherent blunting process for crack growth predicts retardation of tip displacement and crack growth for small values of stress range and all mean stress values. 4 The overall plactic zone length for this model does not change during cyclic shear loading although plastic deformation is occurring. 5 The model’s crack tip displacement per cycle is a linear function of the half crack length for prescribed stress range, mean stress, and material properties.

Sign in / Sign up

Export Citation Format

Share Document