An investigation of brittle failure in ductile, notch‐sensitive thermoplastics

Abstract

Brittle failure, a significant design issue for plastic components subject to impact loads, is especially catastrophic when the material is normally ductile. Such behavior is not adequately understood relative to the micromechanisms, controlling parameters, and design consequences in plastics. Previous work has identified the process of crazing as being relevant to these failures in thermoplastics. The relationship between crazes generated through mechanical loading and subsequent brittle failure of amorphous thermoplastics is discussed and the hypothesis that the craze event is a necessary but insufficient condition for brittle failure is employed. Emphasis is focused upon the engineering prediction of craze formation and its use as a conservative brittle failure criteria for defining geometric details to prevent brittle failure. First, a series of experiments using one geometry is applied to study the concept of crazing as a precursor to brittle fracture in the two amorphous polymers polycarbonate and polyetherimide. Second, three‐dimensional finite element analyses are used to assess the effects of changes in geometric detail upon the continuum stress state and eventual failure of the specimen for these two materials.

Notch-sensitive fracture of polycarbonate

“…Ishikawa et al, 1977;Zuber, 1985;Nimmer and Woods, 1992;Narisawa and Yee, 1993). Experimental observations on notched specimens of polycarbonate tested in monotonic bending at moderately slow rates at room temperature show that (a) if the notch has a reasonably large root radius, then failure is initiated by ductile tearing at the notch root, while (b) if the notch has a relatively small root radius, then the failure initiates in the form of a crack-like feature at the tip of the plastic zone surrounding the notch, in the region of high hydrostatic tension (e.g., see Fig.…”

“…Ishikawa et al, 1977;Nimmer and Woods, 1992). For example, Narisawa and his colleagues have estimated r c for slow-cooled polycarbonate to be approximately 87-89 MPa; to obtain such an estimated they used classical slip-line field analysis, coupled with a measurement of the craze location below the notch-tip in three-point bending experiments.…”

“…For example, Narisawa and his colleagues have estimated r c for slow-cooled polycarbonate to be approximately 87-89 MPa; to obtain such an estimated they used classical slip-line field analysis, coupled with a measurement of the craze location below the notch-tip in three-point bending experiments. 2 A similar estimate of r c % 90-100 MPa for polycarbonate has been obtained by Nimmer and Woods (1992); they performed a finite element analysis, using a non-hardening J 2 -flow theory of plasticity, of a three-point bending experiment to determine the hydrostatic stress field at the instant when the corresponding physical experiment showed a load drop due to the initiation of an internal crack.…”

Mechanics of Taylor impact testing of polycarbonate

“…We note that under moderate strain rate loading events, isotropic PC cavitates and fails in a brittle manner when experiencing hydrostatic stresses of $80 MPa(Nimmer and Woods, 1992;Socrate and Boyce, 2000;Johnson, 2001). Cavitation and failure are not observed in this experiment, perhaps due to the small time duration of negative pressure or due to the correspondingly increased modulus at this strain rate decreasing the volumetric strain associated with this negative pressure.…”

Micromechanics of toughening in ductile/brittle polymeric microlaminates: Effect of volume fraction

“…We note that the conditions for brittle failure in PC are reached at large hydrostatic stresses %85 MPa and negligible plastic strain; brittle failure of PC is observed in the presence of notches or cracks (e.g, Nimmer and Woods, 1992;Narisawa and Yee, 1993;Johnson, 2001;Gearing and Anand, 2004a). This condition is not met in the uni-axial deformation of PC/PMMA laminates.…”