M. T. Milan

Tensile and fracture toughness properties of SiCp reinforced Al alloys: Effects of particle size, particle volume fraction, and matrix strength

The goal of this work was to evaluate the effects of particle size, particle volume fraction, and matrix strength on the monotonic fracture properties of two different Al alloys, namely T1-Al2124 and T1-Al6061, reinforced with silicon carbide particles (SiCp). From the tensile tests, an increase in particle volume fraction and/or matrix strength increased strength and decreased ductility. On the other hand, an increase in particle size reduced strength and increased the composite ductility. In fracture toughness tests, an increase in particle volume fraction reduced the toughness of the composites. An increase in matrix strength reduced both Kcrit and δcrit values. However, in terms of KQ (5%) values, the Al6061 composite showed a value similar to the corresponding Al2124 composite. This was mainly attributed to premature yielding caused by the high ductility/low strength of the Al6061 matrix and the testpiece dimensions. The effect of particle size on the fracture toughness depends on the type of matrix and toughness parameter used. In general, an increase in particle size decreased the KQ (5%) value, but simultaneously increased the amount of plastic strain that the matrix is capable of accommodating, increasing both δcrit and Kcrit values.

Fatigue and monotonic properties of an interstitial free steel sheet (FMPIF)

Abstract In the present work, the monotonic and low cycle fatigue properties of a thin sheet produced from an interstitial free steel containing titanium additions were investigated. A special grip alignment fixture permitted the fatigue tests be conducted under deformation control and completely reversed cyclic strain. The material presented different fatigue behavior for the transverse and longitudinal directions, related to the rolling direction. A comparison between the monotonic and cyclic stress—strain curves in the transverse direction showed that the material exhibits cyclic hardening at all tested strain amplitudes. The strain—life relationship was obtained for the transverse direction; buckling of longitudinal specimens made it impossible to obtain such a relationship.

Mechanical performance of carbon-epoxy laminates Part I: quasi-static and impact bending properties

In Part I of this study, quasi-static and impact bending properties of four aeronautical grade carbon-epoxy laminates have been determined and compared. Materials tested were unidirectional cross-ply (tape) and bidirectional woven textile (fabric) carbon fiber lay-up architectures, impregnated with standard and rubber-toughened resins, respectively, giving rise to 1.5 mm-thick laminates. Quasi-static mechanical properties assessed in transversal mode loading were modulus of elasticity, flexural strength and tenacity at the maximum load, whereas the net absorbed energy was determined under translaminar impact conditions. Two-dimensional woven carbon fiber reinforcements embedded in a rubber-toughened matrix presented the best mechanical performance under static loading. Under dynamic loading conditions, woven fiber fabric pre-forms were favorably sensitive to increasing impact energies regardless the nature of the employed epoxy resin. However, it was concluded that great care should be taken with this material within the low energy impact regimen.

Effect of surface rolling on fatigue behavior of a pearlitic ductile cast iron

Surface rolling is a mechanical treatment usually used in parts fabricated with steel and ductile cast iron, specifically in stress concentration regions, to improve fatigue properties. This process hardens and introduces compressive residual stresses to the surface of the material through the application of controlled strains, thus provoking a reduction of resulting tensile stress at its surface under cyclic loading. This work deals with the effect of surface rolling on high cycle fatigue behavior of a pearlitic ductile cast iron used in crankshafts by the automotive industry. Rotating bending fatigue tests were performed in both smooth and notched specimens, the latter either with or without a surface rolling treatment. Compressive residual stresses and heavy plastic deformation imposed on the surface grains due to cold work made difficult the nucleation and propagation of the crack at the rolled surface of the notch. As a consequence, surface-rolled notch testpieces presented a higher endurance limit (478 MPa) than both smooth (299 MPa) and notched (166 MPa) testpieces did. The surface rolling apparatus developed for this work proved to be very efficient and simple, providing good control of parameters involved in the process (i.e., rolling load, frequency, and number of revolutions).

Mechanical performance of carbon-epoxy laminates. Part II: quasi-static and fatigue tensile properties

In Part II of this work, quasi-static tensile properties of four aeronautical grade carbon-epoxy composite laminates, in both the as-received and pre-fatigued states, have been determined and compared. Quasi-static mechanical properties assessed were tensile strength and stiffness, tenacity (toughness) at the maximum load and for a 50% load drop-off. In general, as-molded unidirectional cross-ply carbon fiber (tape) reinforcements impregnated with either standard or rubber-toughened epoxy resin exhibited the maximum performance. The materials also displayed a significant tenacification (toughening) after exposed to cyclic loading, resulting from the increased stress (the so-called wear-in phenomenon) and/or strain at the maximum load capacity of the specimens. With no exceptions, two-dimensional woven textile (fabric) pre-forms fractured catastrophically under identical cyclic loading conditions imposed to the fiber tape architecture, thus preventing their residual properties from being determined.

Fatigue crack growth resistance of SiCp reinforced Al alloys: Effects of particle size, particle volume fraction, and matrix strength

The main aim of this work was to study the effects of particle size, particle volume fraction, and matrix strength on the long fatigue crack growth resistance of two different grades of Al alloys (Al2124-T1 and Al6061-T1) reinforced with SiC particles. Basically, it was found that an increase in particle volume fraction and particle size increases the fatigue crack growth resistance at near threshold and Paris regimen, with matrix strength having a smaller effect. Near final failure, the stronger and more brittle composites are affected more by static modes of failure as the applied maximum stress intensity factor (Kmax) approaches mode I plane strain fracture toughness (KIC).

SLOT MACHINING EFFECTS ON RESIDUAL STRESS MEASUREMENTS USING THE CRACK COMPLIANCE METHOD

The main aim of this paper was to evaluate the practical aspects and compare two of the most common machining techniques employed in the crack compliance (incremental slitting) method for the determination of residual stresses, namely wire electrodischarge machining (WEDM) and circular abrasive saws. For the circular saws, the effect of the rotational speed and blade thickness was also evaluated. Results show that the associated level of errors introduced during thin saw machining can be as low as the results obtained by WEDM machining. However, for practical reasons, WEDM machining offers a better control of cut increment length than sawing techniques. Additionally, higher rotational saw speeds are likely to introduce larger errors in strain readings probably due to higher frictional heat and plasticity generation ahead of the slot tip.

Residual analysis applied to S-N data of a surface rolled cast iron

Surface rolling is a process extensively employed in the manufacture of ductile cast iron crankshafts, specifically in regions containing stress concentrators with the main aim to enhance fatigue strength. Such process hardens and introduces compressive residual stresses to the surface as a result of controlled strains, reducing cyclic tensile stresses near the surface of the part. The main purpose of this work was to apply the residual analysis to check the suitability of the S-N approach to describe the fatigue properties of a surface rolled cast iron. The analysis procedure proved to be very efficient and easy to implement and it can be applied in the verification of any other statistical model used to describe fatigue behavior. Results show that the conventional S-N methodology is able to model the high cycle fatigue behavior of surface rolled notch testpieces of a pearlitic ductile cast iron submitted to rotating bending fatigue tests.

Fatigue Crack Growth Behavior of Friction Stir Welded 2024-T3 Aluminum Alloy Tested under Accelerated Salt Fog Exposure

Fatigue crack growth properties of friction stir welded joints of 2024-T3 aluminum alloy were studied via constant amplitude load (increasing ΔK) testing under open air and accelerated salt fog exposure conditions. For low ΔK values, longitudinal crack growth in the weld line center was slower under corrosion fatigue than by air because of the prevalence of corrosion-induced crack closure. On the other hand, growth rates of transverse crack crossing the weld nugget were not affected by the environment aggressiveness because of a balance between hydrogen embrittlement and corrosion inducing crack closure along with welding compressive residual stresses. For intermediate ΔK values, propagation rates 5 times faster were observed for both longitudinal and transverse cracks under salt fog ambient as a result of hydrogen-induced embrittlement, which is favored by larger crack tip openings allowing local corrosive medium access. As the final catastrophic fracture approaches, longitudinal and transversal crack propagation rates under corrosion fatigue approximated the values observed in air testing, as time, under the 30-Hz loading frequency applied, was scarce for an effective interaction between crack tip fresh material and the hostile environment. For longitudinal cracks, precracking in air resulted in fatigue-corrosion limit factor ΔKth slightly lower than that obtained in salt fog precracked specimens, as the corrosive role played by the salt fog environment along the precrack introduction contributed to crack-closure effect, so increasing the apparent value of ΔKth. On the other hand, the environment in which a transverse precrack was created had a negligible effect on ΔKth because of the establishment of an equilibrium condition between corrosion-induced precrack closure and hydrogen embrittlement.

Evaluating the Berkovitz Method to Predict Fatigue Loads in Mechanical Failure Investigations

This article evaluates a proposed analytical-experimental methodology by which the fatigue load levels leading to failure of structural components is inferred. The so-called Berkovitz method is recognized to depend fundamentally on a 1:1 relationship of micro- and macroscopic crack propagation rates. Compact tensile specimens of a high-strength aluminum alloy were fatigue tested at room temperature according to ASTM-E647, in plane-stress and plane-strain conditions, respectively. Unloading elastic compliance and low-magnification visual techniques monitored crack propagation rates. Topographical survey of fractured surfaces was carried out in a scanning electron microscope to measure striation spacing at constant-ΔK locations. By inputting these values in the Berkovitz model, the load spectrum applied during the fatigue testing could be derived. Research results have shown that, if correctly and carefully used, the assessed procedure provides accurate estimation of fatigue loads, so constituting a powerful tool during failure analysis of mechanical components operating in constant amplitude loading conditions.