Golam M. Newaz

Influence of matrix material on flexural fatigue performance of unidirectional composites

Abstract A deflection-controlled flexural fatigue study of unidirectional glass fiber reinforced epoxy and vinyl ester composites was undertaken. Damage initiation and growth for various deflection levels were evaluated. Also, quantitative assessment of damage was made by monitoring stiffness loss in the composites as a function of fatigue cycles. Results show that the glass/epoxy composite has better performance compared with the glass/vinyl ester composite, especially at low deflection amplitudes. Fatigue behavior of the composites at low deflection amplitudes is found to be primarily influenced by matric and fiber-matrix interfacial damage in the form of longitudinal splitting.

On interfacial failure in notched unidirectional glass/epoxy composites

The splitting mechanism in notched unidirectional glass/epoxy composites under tensile loading has been studied to gain insight into this failure process. An already- available mathematical expression based on the J-integral technique proposed by Tirosh [1] has been used for this purpose. This expression relates the length of the split crack to the in-plane elastic constants of the composite, original notch length, inter facial shear yield stress, and the applied tensile stress. This model has been used to distinguish between the quality of fiber-matrix bonding in two E-glass fiber-reinforced epoxy composites by evaluating their interfacial shear yield stress using transversely edge-notched unidirectional composites. The experimental results demonstrate that an important requirement to control the length of the split crack is higher fiber-matrix in terfacial shear yield stress. Also, the Tirosh model is found to effectively predict the progression of splitting in notched unidirectional composites.

A Quantitative Assessment of Debonding in Unidirectional Composites Under Long-Term Loading

Transverse tensile fatigue behavior of unidirectional E-glass/epoxy and E-glass/ vinyl ester composites with 48-50 volume percent glass fiber was evaluated using a strain-controlled test condition. Analysis based on Weibull statistics shows that there is a significant difference in performance between E-glass/epoxy and E-glass/vinyl ester composites. At all strain amplitudes, the moduli of the composites were retained until final failure occurred. This is related to the development of a single dominant crack and the absence of multiple cracking which leads to the final separation in the test specimen. This behavior is similar to homogeneous materials such as metals. Fatigue life in these composites is found to be initiation dominated and, correspondingly, the propagation life is negligible. This study provides an overall insight into the fatigue behavior and damage in unidirectional composites subjected to long term transverse tensile loading. Also, it is useful in materials selection for controlling longitudinal split ting in unidirectional composite structures in fatigue loading.

Apparent Interlaminar Shear Strength of Epoxy and Vinyl Ester Composites

The interlaminar shear strength characteristics of unidirectional E-glass fiber/epoxy and vinyl ester composites were investigated. The short-beam flexure test (ASTM D-2344) was utilized for comparing the performance characteristics of the two systems. It was found that E-glass fiber/epoxy composite has higher interlaminar shear strength compared to the vinyl ester composite in dry condition. However, for short-term hygrothermal condition, the retained shear strength values were nearly the same. Optical photomicrography revealed that interlaminar shear damage primarily consisted of fiber-matrix interfacial debonding and matrix cracking. Voids were found to have adverse effect on interlaminar shear strength. The influence of voids was studied in relation to mode II crack or damage propagation.

An expression for the optimum case thickness of notched cylindrical carburized fatigue members

Abstract A simple expression is proposed for the quantitative prediction of the optimum case thickness of notched carburized cylindrical members under both axial and bending fatigue loading. The development utilizes Sines method of multiaxial fatigue. The expression incorporates the fatigue properties of the case and the core, the residual stresses generated due to carburization, the geometry of the member and the imposed loading.

Fatigue Life Prediction Scheme for Carburized Members Subjected to ln-Phase Multiaxial Stresses

A dimensionless fatigue life parameter is developed for the correlation of fatigue data of carburized and core material. It is shown that the Smith-Watson-Topper parameter can be generalized to account for the fatigue strength differentials of dissimilar materials such as the core and case-core interface. Fatigue data generated using circumferentially notched specimens was successfully correlated to the core material fatigue data using this parameter at low stresses. From these observations, a scheme is proposed to predict low-stress fatigue life of a carburized member experiencing multiaxial stresses based on the core material fatigue response only.