M. Manoharan

Crack initiation and growth toughness of an aluminum metal-matrix composite

Abstract The effects of systematic changes in matrix microstructure on crack initiation and growth toughnesses were determined on an AlZnMgCu alloy containing 0, 15, 20% by volume of SiC particulates. Materials were heat treated to underaged (UA) and overaged (OA) conditions of equivalent matrix microhardness and flow stress. Although both the fracture initiation and growth toughnesses, as measured by J Ic and tearing modulus, were similar for the unreinforced materials in the UA and OA conditions, significant effects of microstructure on both J Ic and tearing modulus were observed in the composites. SEM and TEM observations of fracture paths in the two conditions are utilized to rationalize these observations in light of existing theories of ductile fracture propagation.

Effect of reinforcement size and matrix microstructure on the fracture properties of an aluminum metal matrix composite

Abstract The effects of systematic changes in reinforcement size and matrix microstructure on the crack initiation and growth toughness of a 7091 aluminum alloy reinforced with SiC particulates were studied. It is shown that changes in matrix microstructure have a significant effect on both initiation and growth toughness. The effect of reinforcement size on these properties is far less marked. These observations have been related to local microstructural parameters and the nature of the distribution of the reinforcement.

Effects of microstructure of the behavior of an aluminum alloy and an aluminum matrix composite tested under low levels of superimposed hydrostatic pressure

Experiments have been conducted on an aluminum alloy and an aluminum matrix composite tested in tension under the influence of superimposed hydrostatic pressure. Monolithic alloys heat-treated to underaged (UA) and overaged (OA) conditions exhibited significant differences in their responses to the superimposition of hydrostatic pressure during tension testing. Significant increases in ductility were obtained with moderate increases in confining pressure for the OA alloy, while the UA alloy exhibited little effect of pressure. In contrast, significant increases in ductility were obtained for the composites, regardless of the matrix aging condition. The effects of pressure on frature are determined in light of the micromechanisms of fracture in these materials.

Effects of aging condition on the fracture toughness of 2XXX and 7XXX series aluminum alloy composites

Results are presented on the effects of matrix aging condition (i.e., matrix temper) on the fracture toughness of 2XXX and 7XXX Al matrix alloys reinforced with SiC particulates, and the results are compared with the mechanical behavior. Fracture toughness testing was conducted on fatigue precracked bend specimens, and fracture surfaces were examined using SEM. Results revealed dramatic differences in the effect of matrix microstructure on the fracture properties of the two composite series. In the 7XXX material, the toughness values decreased from the underaged (UA) condition to the overaged (OA) condition by approximately 40 percent, while in the 2XXX series composite, the effect of matrix microstructure was marginal. In the 7XXX series composites, a transition in fracture mode from particle cracking (in UA) to matrix and linear-interface failure (in OA) was observed, while the 2XXX series composite failed predominantly by particle cracking.

Combined mode I-mode III fracture toughness of a spherodized 1090 steel

Abstract The aim of this investigation was to determine the fracture behavior of a spherodized 1090 steel under combined mode I-mode III loading conditions. Suitably defined formulations of the J integral denoted Jic and Jiiic were used to characterize the elastic-plastic fracture of this steel. As the mode III component in the system is increased, the resolved mode I J integral at initiation decreases, its mode III counterpart increases and the total J value remains nearly a constant. This implies a constant energy requirement for fracture initiation under mixed mode loading. As the crack plane becomes less inclined to the load line, the slopes of the mode I and total J resistance curves increase from their pure mode I values until a crack inclination angle of about 65° is reached. Somewhere in the region of 65-55°, a maximum in these values is reached and they fall off rapidly for larger mode III components. This drop is accompanied by the breakup of the crack front into mode I and mode III steps, which is shown to be an energetically more favorable process for this steel.

Laminated composites with improved toughness

The composite studied in the present investigation was a powder metallurgy 2XXX series alloy, designated MB-85 containing in wt. % : 3.5 Cu, 1.5 Mg, 0.4 Zr, 0.21 Mn, Bal A1 reinforced with 15 volume percent silicon carbide particulate (average size 13 microns). Heat treatments consisted of solution treatment at 495 C/4 hours, followed by a cold water quench and artificial aging at 190 C/3 hours to produce a composite in the underaged condition. The backing material used in the laminated specimen was nominally a 6061 aluminum alloy. The initiation and growth toughness of the composite and the monolithic 6061 alloy were tested using a compact tension specimen geometry. The effect of the ductile backing on retarding crack propagation was studied. The proportion of the 6061 alloy thickness in the laminated specimen was about 25%. Enhancement in the crack growth toughness is achieved because more energy is required to initiate and propagate a crack in the 6061 backing in comparison to the composite as originally proposed. Thus one can take advantage of the enhancement in stiffness and strength offered by the incorporation of a very strong reinforcement without overly sacrificing the flaw tolerant nature of aluminum alloys.

Combined mode I - mode III fracture toughness of a high carbon steel

Abstract 1. 1. Brittle materials exhibit minimum energy dissipation under pure mode I loading 2. 2. In these materials, the mode IIIcomponent essentially provides redundant plastic work and increases J totalc . 3. 3. This behavior contrasts with more ductile materials where combined mode I - mode III loading produces The minimum energy dissipation.

Effect of microstructure and notch root radius on fracture toughness of an aluminum metal matrix composite

Recent results on the effects of matrix aging condition (matrix temper) and notch root radius on the measured fracture toughness of a SiC particulate reinforced aluminum alloy are reviewed. Stress intensity factors at catastrophic fracture were obtained for both underaged and overaged composites reveal. The linear relation found between apparent fracture toughness and the square root of the notch root radius implies a linear dependence of the crack opening displacement on the notch root radius. The results suggest a strain controlled fracture process, and indicate that there are differences in the fracture micromechanisms of the two aging conditions.

In-situ deformation studies of an aluminum metal-matrix composite in a scanning electron microscope

In the fracture toughness tests reported elsewhere, crack propagation was monitored optically using a travelling microscope fitted to a stage equipped with a LVDT. For this purpose, the external surfaces of the specimens were polished metallographically, and the details of crack propagation were studied in-situ with the use of a high power optical microscope (magnification 500 X) connected to a video recording system. The authors describe in-situ measurements similarly made on specimens designed to permit controlled propagation of a crack, as reported elsewhere. These observations suggest that microcracking occurred ahead of the crack-tip, which could also contribute to energy absorption during fracture. Tensile specimens were tested in-situ in a scanning electron microscope equipped with a deformation stage to directly observe these phenomena.

Fracture characteristics of an Al-Si-Mg model composite system

Abstract The fracture toughness characteristics of an Al–Si–Mg model composite system were examined. Through manipulating composition and processing variables, controlled variations in particle volume fraction and matrix flow properties were produced and tested using JIC tests. Metallographic analysis shows that the fracture process proceeds by the cracking of Si particles ahead of the crack tip in a very localized region. This is compared with results from smooth uniaxial tensile specimens which, while exhibiting the same particle cracking mode of fracture, have damage distributed over a wider area of the spicimen. The results for the Al–Si–Mg model alloys are compared with JIC results for commercial composite systems, and the differences observed are discussed in terms of the characteristics of the reinforcement particles.