T.C. Lindley

Electron backscattering diffraction study of acicular ferrite, bainite, and martensite steel microstructures

Abstract This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In conclusion, it is shown that EBSD is a powerful tool for studying phase transformation and fracture mechanisms in steels on a microscopic scale.

Aspects of equivalence between contact mechanics and fracture mechanics: theoretical connections and a life-prediction methodology for fretting-fatigue

Abstract We identify aspects of quantitative equivalence between contact mechanics and fracture mechanics via asymptotic matching. An analogy is invoked between the geometry of the near-tip regions of cracked specimens and that of the sharp-edged contact region between two contacting surfaces. We then demonstrate that the asymptotic elastic stress and strain fields around the rim of the contact region, as derived from classical contact mechanics analyses, are identical to those extracted from linear-elastic fracture mechanics solutions for analogous geometries. Conditions of validity of this model for contact mechanics are established by recourse to the singular fields and small-scale-yielding concepts of fracture mechanics. With this method, the geometry of the contact-pad/substrate system naturally introduces a fictitious crack length, thereby providing a physical basis to analyze contact-fatigue fracture initiation and growth. Possible extensions of this crack analogue model are then suggested, for situations where static or cyclic mechanical loads are superimposed parallel to the direction of interfacial friction, using the two-parameter fracture characterization that involves the stress intensity factor K and the non-singular T-stress, or alternatively, the J integral and the triaxiality parameter, Q. The predictions of the crack analogue model are then compared with fretting-fatigue experiments and are shown to be in agreement with a variety of independent experimental observations. A new life-prediction methodology for fretting fatigue is also proposed on the basis of the present approach.

Fretting wear behaviour of polymethylmethacrylate under linear motions and torsional contact conditions

The fretting wear behaviour of PMMA against a rigid counterface has been investigated under various contact zone kinematic conditions. A specific device has been used in order to achieve load axis spin or stationary rolling motions in a contact between a PMMA flat and a steel ball. Wear processes under such conditions have been investigated by means of laser profilometry and in-situ optical observations of the contact area during tests. Very different wear patterns were produced depending on the contact kinematics. For stationary rolling conditions, the progressive accumulation and compaction of debris induced the formation of a single ripple located in the middle of the contact. Very little debris was found to be eliminated from the contact and the resulting wear was quite low. On the other hand, little accumulation of debris was observed for torsional contact conditions and the wear was drastically enhanced. These results are analysed by considering the effects of contact zone kinematics on particle detachment and third body elimination.

Contact damage of poly(methylmethacrylate) during complex microdisplacements

Abstract The wear behaviour of a PMMA substrate contacting against a steel ball has been investigated under small amplitude oscillating motions. A friction device has been developed, which allows the combination, to various extents, of linear sliding and torsional contact conditions. Using laser profilometry, the amounts of wear debris trapped within the contact or displaced from the contact have been quantified for the various loading configurations. The contact zone kinematics has been found to have a major influence upon the wear resistance of the PMMA. When the contact conditions progressed from torsional to linear sliding, a rapid decrease in the wear volumes was observed, which was associated with the progressive accumulation and compaction of an increased part of the detached PMMA particles within a central roll. These results have been interpreted by considering both the shape and the magnitude of the sliding paths that were generated within the contact under the various loading configurations. Using nano-indentation tests, it was also observed that the consolidated third body resulting from debris compaction within the rolls exhibits mechanical properties (hardness, modulus) similar to those of the original PMMA substrate. The achievement of a high level of compaction within the roll formation was explained by considering the effects of third body accumulation on the load carrying capacity of the contact.

Microstructure-based fatigue life prediction for cast A356-T6 aluminum-silicon alloys

Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356 alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment. The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment.

The role of adhesion in contact fatigue

Abstract By incorporating the effects of interfacial adhesion in the mechanics of rounded contact between two bodies, a new approach is proposed for the quantitative analysis of a wide variety of contact fatigue situations involving cyclic normal, tangential or torsional loading. In this method, conditions of “strong” and “weak” adhesion are identified by relating contact mechanics and fracture mechanics theories. Invoking the notion that for strong and weak adhesive contact, a square-root stress singularity exists at the rounded contact edge or at the stick–slip boundary, respectively, mode I, II or III stress intensity factors are obtained for normal, sliding and torsional contact loading, accordingly. A comparison of the cyclic variations in local stress intensity factors with the threshold stress intensity factor range for the onset of fatigue crack growth then provides critical conditions for crack initiation in contact fatigue. It is shown that the location of crack initiation within the contact area and the initial direction of crack growth from the contact surface into the substrate can be quantitatively determined by this approach. This method obviates the need for the assumption of an artifical length scale, i.e. the initial crack size, in the use of known fracture mechanics concepts for the analyses of complex contact fatigue situations involving rounded contact edges. Predictions of the present approach are compared with a wide variety of experimental observations.