Date of Award

Fall 2019

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Addis Kidane

Abstract

Fracture mechanics has been a subject of great interest in the engineering community for decades. During this period, fracture parameters such as Stress Intensity Factor (SIF), J-integral, and Crack-Tip Opening Displacement (CTOD) have been developed and used to characterize the fracture properties of most engineering materials under quasi-static loading condition. Usually, these properties are obtained experimentally by using standard methods such as ASTM E399, E1820 or E1920 to evaluate the stress intensity factor 𝐾𝐼𝑐 π‘ π‘‘π‘Žπ‘‘π‘–π‘, elastic-plastic toughness 𝐽𝐼𝑐 π‘ π‘‘π‘Žπ‘‘π‘–π‘ and crack tip opening displacement (CTOD) respectively. Conversely, most critical engineering applications are subjected to a sudden or high strain rate which could cause a dynamic fracture event. In this case, the quasi-static methods are insufficient in accurately determining the dynamic fracture parameters in materials. In light of this, three projects related to develop a new experiment method to measure dynamic initiation toughness are presented in this document.

First, a novel experimental and numerical approach is proposed to determine the dynamic fracture initiation toughness of materials based on cylindrical specimen subjected to the nondispersive torsional wave. Cylindrical tubular specimens with a full spiral crack on the surface are subjected to dynamic torsion loading using a torsional Hopkinson bar apparatus. The torsion load creates predominantly a tensile stress perpendicular to the spiral v-groove of the specimen that causes mode I fracture. The torque applied to the specimen and the time of fracture are measured.

Stereo Digital Image Correlation is used to measure the time at which the crack propagation initiated. A 3D format of the dynamic interaction integral method is utilized to calculate three component of dynamic stress intensity factors by using auxiliary and actually fields. Using the torque and the time of fracture as input, commercial FE package, ABAQUS, is applied to analyze an entire model of the spiral crack body and extract the dynamic fracture parameters. The result shows that the spiral crack-torsional loading configuration indeed generates a mode I fracture. Three alumni alloys; Al 7050-T6, Al 2024-T3, and Al 6061-T6, were considered, and the results were consistent, repeatable and in good agreement with the results in the literature. For the three materials, the dynamic fracture initiation toughness KId was higher than the corresponding quasi-static fracture toughness KIc . Following are The advantages of this method: avoid the axial inertia load effects; avoid friction force effect; and reduce the wave dispersion phenomena effect.

Secondly, A solution is proposed to obtain a geometry factor for a Mode I stress intensity factor of a cylindrical specimen with spiral crack subjected to torsion. Cylindrical torsion specimens, solid and tubular, with a spiral crack on the surface, were subjected to pure torsion. The torque at fracture was measured and used as input for finite element analysis to extract the stress intensity factor at the corresponding fracture load by using a numerical solution of interaction integral method. From the fracture intensity factor obtained from the FE, and the geometry of the specimen the geometry factor for different crack depth was calculated inversely. Finally, following Benthem’s asymptotic solution approach, the geometry factor for cylindrical samples with a spiral crack on the surface is presented in a standard form. The proposed model was verified by testing a polycarbonate cylindrical specimen and comparing the existing fracture intensity value of different vi materials in the open literature. The proposed formulas are in good agreement with the standard methods with a maximum difference of about 1.7%.

In overall, the results show that the spiral crack torsional loading configuration at the inclined angle, 45, indeed generates a pure mode I fracture, and the results are consistent, repeatable and in good agreement with the results in the literature. For the dynamic fracture initiation toughness KId was higher than the corresponding quasi-static fracture toughness.

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