Subsonic interfacial fracture using strain gages in isotropic–orthotropic bimaterial

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Abstract

An experimental study has been conducted in which strain fields were used to investigate the behavior of subsonic crack propagation along the interface of an isotropic–orthotropic bimaterial system. Strain field equations were developed from available field equations and critically evaluated in a parametric study to identify optimum strain gage location and orientation. Bimaterial specimens were prepared with PSM-1 polycarbonate and Scotchply® 1002 unidirectional, glass-fiber-reinforced, epoxy composite. Dynamic experiments were conducted using these specimens with strain gages mounted on the composite half to obtain values of the dynamic complex stress intensity factor, K=K1+iK2, in the region of the crack tip while photoelasticity was used on the PSM-1 half. Results show that the trend and magnitude of K obtained using strain gages compare favorably with those obtained using photoelasticity.

Introduction

Advances in material performance, driven by military and industrial applications, depends on the ability to develop new multi-phase materials. Their performance is controlled not only by the properties of individual phases but also by the strength of their interfaces. Example bimaterials include heavy and light composite armors and fiber and whisker reinforced composites. Applications include metal/composite and metal/polymer bimaterial combinations. Because of their low cost and ease of use, strain gages are the predominant testing device in industry. Thus, strain gage techniques are developed and demonstrated to facilitate the analysis of interface problems in practical applications.

Strain gage methods have been used to conduct fracture research on homogeneous isotropic materials [1], [2], and orthotropic composite materials [3], [4]. Additionally, an analytical study has provided the stress and displacement fields for subsonic crack propagation in homogeneous orthotropic material [5].

Substantial progress has been made in the study of dynamic interfacial fracture. The asymptotic structure of the most singular term of the steady-state elastodynamic interfacial crack-tip fields have been developed, and it has been shown that it takes only a finite amount of energy to maintain the crack propagation at the Rayleigh wave velocity of the more compliant material [6]. A complete series solution for the stress field around a crack-tip for steady-state interface crack propagation has also been developed [7], [8]. A more general higher-order asymptotic analysis for unsteady interface crack propagation that accounted for transient effects was subsequently obtained [9]. This was followed by the earliest experimental study on phenomenon of dynamic crack initiation and growth in bimaterials [10].

Additional work in dynamic crack propagation along interfaces used a special specimen geometry and proposed an experimental technique to measure dynamic initiation, propagation, and arrest in a single experiment [11], and further investigated the effect of inclined interfaces on opening-mode dominated crack propagation [12].

Recent works have shown that crack tip behavior in non-homogeneous systems tends to be multi-scale in character. The stress intensity factor (SIF), being based on the asymptotic solution where the limit goes to zero, is not adequate. In fact, it cannot explain some of the experimental results observed in piezoelectric materials [13], [14].

To date, work on isotropic–orthotropic bimaterials and interface fracture using strain gages is limited at best. The field equations for an orthotropic bimaterial [5] have been used to subsequently develop the field equations for an isotropic–orthotropic bimaterial [15]. Also, experimental study with strain gages and photoelastic techniques has been conducted to evaluate interface fracture parameters in bimaterials under quasi-static loads [16]. Given the wide availability of strain gages in industry and academia, there is a need to expand the body of knowledge relative to bimaterial interface fracture and to develop strain gage techniques as a viable alternative to the relatively more sophisticated and expensive experimental techniques used in previous studies. Thus, this study focuses on developing strain field equations from existing theoretical relations and critically examining them via experimentation to demonstrate the feasibility of the strain gage method.

Section snippets

Strain fields around an interfacially propagating crack tip

Crack growth along a bimaterial interface is generally referenced with respect to the material properties of the more compliant material. Crack propagation is considered subsonic for crack-tip velocities, v, below the shear wave velocity, cs, of the more compliant material. Fig. 1 illustrates the moving coordinate system at the tip of a crack propagating subsonically along an interface between two elastic materials: Material 1, the more compliant material, and Material 2, the stiffer material.

Parametric investigation

A parametric study was conducted to understand the development of the strain field close to the crack tip. Subsequently, this information was used to optimize the location and orientation of the strain gages for the development of strain gage techniques used in this study to obtain the dynamic complex SIF K from a propagating interfacial crack.

Experimental procedure

The parametric study was followed by experimentation, in which strain gage techniques were used to obtain values of K from a propagating crack as it passed a series of strain gages. Results from the strain gage data were then compared to results obtained from photoelastic data conducted as part of these experiments.

Results and discussion

Experiments were conducted on the PSM-1/Scotchply bimaterial specimen described above. Photos of the fracture surfaces (i.e. the interface sides of the materials) taken following the experiment are shown in Fig. 8. Fig. 8(a) and (b) depict views of the interface edges on the PSM-1 and Scotchply halves, respectively, showing pure interface failure. This is particularly clear in Fig. 8(c), which is a close up of the PSM-1 interface. It shows pieces of Teflon tape from the starter crack and 100%

Conclusion

An experimental study was conducted in which strain fields were developed and used to investigate the behavior of cracks propagating along the interface of an isotropic–orthotropic bimaterial. Analytical work focused on the influence of the dynamic CSIF, K, on the strain field surrounding a crack tip, which yielded the optimum strain gage orientation. In the experimentation that followed, K for a PSM-1/Scotchply bimaterial was determined using strain gages. The trend and magnitude of K obtained

Acknowledgements

The support of the Naval Undersea Warfare Center ILIR Program, the National Science Foundation (grant no. CMS 9870947), and the Korean Science and Engineering Foundation (grant no. 971-1003-017-2) are greatly appreciated.

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