Chong Soo Lee

Failure of carbon/epoxy composite tubes under combined axial and torsional loading 1. Experimental results and prediction of biaxial strength by the use of neural networks

Abstract Biaxial tests have been conducted on cross-ply carbon/epoxy composite tube under combined torsion and axial tension/compression up to failure. Strength properties and distributions were evaluated with reference to the biaxial loading ratio. The scatter of the biaxial strength data was analyzed by using a Weibull distribution function. Artificial neural networks were introduced to predict failure strength by means of the error back-propagation algorithm for learning, providing a different and new approach to the representation of complicated behavior of composite materials. Further prediction is made from experimental data by the use of Tsai–Wu theory and a combined optimized tensor polynomial theory. Comparison shows that the artificial neural network has the smallest root-mean-square error of the three prediction methods.

Fabrication and mechanical properties of aluminum matrix composite materials

Aluminum matrix composite materials containing SiC whisker and Saffil alumina short fiber are fabricated by the direct squeeze infiltration method. Optimum processing conditions for preforms and squeeze casting are suggested. For minimum damage of the reinforcements, the relatively low pressure of 25 MPa is applied.

Effect of interlamellar spacing on the delamination of pearlitic steel wires

Abstract The increased interlamellar spacing causes the larger sized globular cementite particles in colonies initially aligned transverse to the drawing axis, and results in the occurrence of more frequent and larger sized void formation during drawing. The array of those voids would act as one of the origins for delamination or offer the preferential site for delamination during torsion. Thus, the occurrence of delamination in coarse pearlite is much easier than in fine pearlite.

Microstructural influence on fatigue properties of a high-strength spring steel

Abstract A study has been made to investigate the fatigue properties of a high-strength spring steel in relation to the microstructural variation via different heat treatments. Rotating–bending fatigue and fatigue crack growth (FCG) tests were conducted to evaluate the fatigue properties, and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) unit was used to characterize the tempered microstructure. The results indicate that the fatigue endurance σf increases with increasing tempering temperature, reaching a maximum at 450°C, then decreases. The increase of σf is mainly attributed to the refined distribution of precipitation, together with the structural uniformity of tempered martensite. The softening of tempered martensite due to excessive precipitation accounts for the decrease of σf. By contrast, the FCG results show an insensitivity of the stage-II growth behavior to the microstructural changes for the whole range of tempering temperature tested. The insensitivity is interpreted in terms of the counterbalancing microstructure-dependent contributions to the FCG behavior.

Failure of carbon/epoxy composite tubes under combined axial and torsional loading. 2. Fracture morphology and failure mechanism

Failure mechanisms of cross-ply composite tubes made by the lapped moulding technique were investigated following biaxial testing, as reported in an earlier study (Part 1). Before mechanical testing the undamaged specimens were inspected to characterize their microstructure, and the source of first material damage was also inspected. From phenomenological failure analysis three types of failure mode were exhibited, depending on the biaxial ratio, and the corresponding failure mechanisms are suggested. By means of fractographic observations of the fracture surface, microscopic failure was investigated as a function of biaxial ratio, and it is suggested how the performance of fiber reinforced composite materials tube for engineering applications might be improved. The main factors involved at low biaxial ratio are matrix strength, the bond strength of the seam, and uniform distribution of fiber and matrix, while at a high biaxial ratio the fiber strength is the main factor.

Precise determination of fatigue crack closure in Al alloys

Abstract The crack closure phenomenon is important when evaluating the effective driving force for crack growth. Hence, the precise measurement of the crack closure load is an essential preprequisite for the determination of the effective stress intensity factor range (ΔKeff). In the present investigation, an acoustic emission (AE) technique was employed to measure crack closure during the fatigue cycling of Al alloys and the results are compared with those obtained by the crack opening displacement (COD) method, the back face strain (BFS) gauge method and the surface strain (SS) gauge method. Results show that the intrinsic fatigue crack growth rates (da/dN vs. ΔKeff) obtained by the AE technique gave the best fit for the high stress ratio (R = 0.8) test, which implies that the AE technique should be considered as a reliable crack closure measurement method.

Microstructural evolution during superplastic bulge forming of Ti-6Al-4V alloy

Abstract An analytical model reflecting the microstructural evolution was made on the basis of Dutta and Mukherjees work, and applied to the superplastic bulge forming of Ti–6Al–4V alloy. Bulge forming of fine grain (2.5 μm) Ti–6Al–4V alloy sheets was conducted at 900°C, at three strain rates of 2.5×10 −4 , 5×10 −4 and 1×10 −3 s −1 . It was found that the grain growth rate of biaxially bulge formed samples was quite different from that of uniaxially deformed specimens and that the grain growth rate was independent of the strain rate imposed at a fixed temperature. After incorporating the grain growth rate of biaxially deformed material into the analytical model, the bulge forming process was successfully controlled at different strain rates. Prediction of the thickness distribution by the modified model was in agreement with the experimental results.

Microstructural influence on the intrinsic fatigue properties of AlLi 8090 alloy

Abstract A study has been made to investigate the influence of microstructure on the extrinsic and intrinsic fatigue properties of the Alue5f8Li alloy, 8090. Two types of microstructure have been produced to compare the relative fatigue properties, one with a δ′ phase dominant microstructure and the other with a S′ + δ′ microstructure. Crack closure loads measured by the crack-opening displacement method have been used to obtain intrinsic fatigue resistance of the δ′ and S′ + δ′ microstructures. Results have shown that the extrinsic fatigue resistance of the δ′ microstructure was considerably higher than that of the S′ + δ′ microstructure, especially at lower growth rate, which was mainly due to the more severe crack path tortuosity and associated high levels of crack closure. In addition, the intrinsic fatigue resistance of the δ′ microstructure was also observed to be higher than that of the S′ + δ′ microstructure, presumably due to greater slip reversibility in the δ′ microstructure.

Dislocation structure associated with deformation behavior of Fe3Al alloys

Abstract The aim of the present investigation is to correlate the change of tensile yield strength with the dislocation structure of Fe-23Al and Fe-30Al alloys tested at room and high temperatures. The results showed that the yield strength of Fe-23Al alloy increased with increasing the quench and test temperatures. This was mainly attributed to the increase in the partition of imperfect superdislocations, leaving NNN and NN type APB trails behind during the deformation. In Fe-30Al alloy, the yield strength increased with increasing the test temperature, but decreased with increasing the quench temperature. The lower yield strength of the specimen quenched at 500 ° C was thought to be due to relatively lower NNN APB energy. It is considered that the yield strength of Fe-30Al alloy is governed by two competing factors; the partition of perfect and imperfect variants of dislocations and the shear stress required for the dislocation movement.