Material PropertiesImpact tensile fracture testing of a brittle polymer
Introduction
The fracture behavior of brittle polymers has been investigated under various loading conditions to determine their mechanical reliability. For example, under a static load, the stress intensity factor, K, or energy-release rate, G, is usually determined based on linear elastic fracture mechanics by measuring crack length and the load or external work applied to a tensile or bending specimen. The concept of K or G based on linear fracture mechanics reportedly plays an important role in understanding fracture initiation. K or G is also measured under impact loading to estimate their dynamic values using different experimental techniques [1], [2], [3]. Most polymers generally exhibit some non-elastic effect due to viscoelastic and plastic deformation [4]. Brittle fracture also causes inertia or a dynamic effect, since a crack propagates dynamically within the specimen [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Hence, these two effects should be determined and considered when evaluating K or G. However, quantitative discussion of these two effects on K or G has been limited.
To study this problem under static loading, Arakawa and Mada [22], [23] measured the brittle fracture of polymethyl methacrylate (PMMA) using a high-speed camera, and a high-speed extensometer consisting of an optical fiber and a position-sensing detector (PSD). The dynamic and non-elastic effects were then estimated from the dynamic response and residual deformation of the specimen after fracture. They suggested that G is overestimated if the dynamic and non-elastic energies are included.
In this study, we examined these two effects under impact tensile loading using single-edge-cracked tensile specimens of methylmethacrylate–butadiene–styrene (MBS) resin. The impact load and displacement of the specimens were measured using a Piezo sensor and high-speed extensometer, respectively, to evaluate the external work, Uex, applied to the specimen. The elastic energy, Ee, and non-elastic energy, En, were estimated from the oscillation of the split specimen just after fracture. The fracture energy, Ef, was then determined and correlated with the fracture load, Pc. For the fracture surface, As, the energy-release rate was evaluated using Gt=Uex/As or Gf=Ef/As. The mean crack velocity, vm, was also estimated as a function of Pc, and their correlation is discussed.
Section snippets
Experimental methods
Experiments were performed on single-edge-cracked tensile specimens of 4 mm-thick MBS resin plates. The specimen geometry is shown in Fig. 1. To change the fracture initiation load of the specimens, sharp pre-cracks of different lengths of 2–4 mm were generated by forcing a razor blade into a pre-machined saw-cut on the specimen edge.
An impact tensile load was introduced using a special loading device, which utilizes the free fall of a weight. As illustrated in Fig. 2, this device consists of an
Definition of fracture energy
Fig. 4 plots the load, P, versus displacement, δ, for the dead weight of the frame (see Fig. 2). In this figure, Po and δo indicate the initial static values due to the dead weight, and Pc and δc denote the critical dynamic values at the onset of fracture. Although P–δ relationships generally differ between static and dynamic loads, they were assumed to be identical to simplify the analysis. The external work, Uex, applied to half of the specimen is then given asEq. (1) is valid when
Results and discussion
The dynamic load, P′, applied to the specimen was measured using the Piezo sensor (see Fig. 2). Fig. 5 plots the value of P′ as a function of time, t. P′ increased with t, and then fell abruptly from as the crack started to propagate. The fall time from to P=0 was about 38 μs, suggesting that the mean crack velocity in the specimen was about 190 m/s.
Fig. 6 shows the dynamic displacement, δ′, measured using the optical fiber and PSD (see Fig. 3). As shown in the figure, δ′ increased with
Conclusions
The brittle fracture of MBS resin was studied using an impact tensile device and an optical high-speed extensometer. The impact load and displacement of single-edge-cracked specimens were measured to determine the external work, Uex, applied to the specimen. The elastic energy, Ee, and non-elastic energy, En, just after fracture were estimated from the oscillation of the split specimen. The fracture energy, Ef, was then determined and correlated with the fracture load, Pc. With the fracture
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