Characterizing Dynamic Fracture Behavior of Adhesive Joints under Quasi-Static and Impact Loading

Abstract

An experimental evaluation of the dynamic fracture properties of an automotive epoxy is presented. Pronounced stick-slip behavior was observed in both quasi-static and impact tests of aluminum and composite adherends bonded with this adhesive. An experimental technique for conducting low speed impact of adhesively bonded automotive composite joints is presented. Based on the use of a modified drop tower, mode I, II, and mixed mode values for critical energy release rate were determined to create a fracture envelope for the composite/epoxy system. Because load measurements are erratic and unreliable at higher test rates, displacement-based relationships were used to quantify these energy release rates. Displacement data were collected with an imaging system that utilizes edge detection to determine displacement profiles, end displacements, and opening displacements where applicable. Because of the resolution of the image-based approach being used, determining crack length experimentally is difficult. As a result, numerical methods based on edge detection algorithms were developed to objectively determine the crack length based on the available experimental data in mode I, II, and mixed mode I/II configurations.

On the Use of a Driven Wedge Test to Acquire Dynamic Fracture Energies of Bonded Beam Specimens

“…In this test, 1 the wedges apply load via the end blocks rather than by direct contact with the fracture faces. Extensions of this drop tower technique to end load split (ELS) tests for mode II characterization and the single leg bend (SLB) specimen for mixed mode I=II conditions have also been made for composite bonds [19].…”

“…For this reason, some have argued that the true arrest value of the material is invariably greater than the apparent arrest values that are observed in practice [8]. A simple approach can be used to calculate the average fracture energy, defined as the change in stored energy divided by the increment of crack growth during a rapid fracture event [19]. This quantity represents an upper bound on the fracture energy averaged over the debond area, as some energy may remain as kinetic energy after crack arrest or be dissipated during the fracture event.…”

Ductile–brittle transitions in the fracture of plastically-deforming, adhesively-bonded structures. Part I: Experimental studies

“…The crack is then arrested, and the process repeats in a cyclic fashion. Experimental observations interpreted in these terms (Blackman et al, 1995(Blackman et al, , 1996Simon et al, 2005) seem to indicate that the crack arrest is relatively insensitive to loading rate, but disappears at low loading rates.…”

Higher Rate and Impact Tests

“…Both specimens have also been applied to high-rate fracture testing of adhesive joints [15][16][17]. For mixed mode I/II (see Section 3.6), the mixed mode flexure (MMF) also known as the single leg bend (SLB) test and the fixed-ratio mixed-mode (FRMM) are again popular for slow rates and can be used for faster rates also.…”