characterized by crack growth rates existing within a narrow range of :~K, and this is because of the lower fracture toughness and relatively high threshold values in composites compared with those in monolithic alloys. An enhanced Paris region slope attributed to the monotonic fracture contribution is reported and the extent of this contribution is found to depend on particle size. The effects of the ageing condition on crack growth rates and particle size dependence of threshold values are also treated.
Effect of microstructure on the cyclic response and fatigue behaviour of an XD aluminium metal matrix composite. Vylete/, G.M., Van Aken,
D.C. and Allison, J.E. Scr. Metal/. Mater. Oct. 1991 25 (10) 2405-2410 The effect of microstrueture on the fatigue life, cyclic response, and strain localization of an AI-Cu (2219) based XD particulate-reinforced metal-matrix composite is investigated. The presence of shearable precipitates produced by natural ageing leads to a cyclically unstable microstructure and strain localization, This cyclic response consists of a rapid hardening region followed by a region where the hardening rate decreases and ultimately a region of cyclic softening. The presence of non-shearabte precipitates produced by artificial ageing leads to a cyclically stable structure with no evidence of strain localization. Although precipitate morphology was observed to influence the cyclic response of these composites significantly, it generally has no influence on the fatigue life under either plastic strain-controlled or under stress-controlled fatigue testing.
Fatigue crack growth
ARALL-4 at ambient temperature.
Davidson, D.L. and Austin, L.K. Fatigue Fract. Eng. Mater. Struct. 1991 14 (10) 939-951 Fatigue cracks were grown in the five layer AI alloy (e.g. 2024-TS)-Aramid fibre laminate composite ARALL-4 over the range of cyclic stress intensity factors (:IK) from 3.5 to 91 Mpa~/m. Near the threshold, crack growth rate was about the same as for unreinforced AI alloys, but at high AK, crack growth rates were significantly lower. Crack closure was measured over this range of growth rates and found to be different than for unreinforced AI alloys, The magnitude of closure was also dependent on crack length. Cracks opened progressively towards the tip with increasing load in much the same way as for unreinforced AI alloys. Removal of the AI outer layer and some of the epoxy revealed that fibres were intact close to the crack tip, but heavily damaged further away. By adjusting the fatigue crack growth curve of an unreinforced AI ahoy for the closure exhibited by the composite, it was possible to approximate the crack-growth rate for the composite over the lower to mid range of ~,K, but at higher values of ,',K, this model seriously overestimated measured crack-growth rates. Therefore, fibre bridging affects both closure and maximum stress intensity factor at the crack tip. Standard fracture mechanics cannot be appged to describe these effects.
Joining Growth and coalescence of fatigue cracks at weld toes in steel. Otegui,
J.L., Burns, D.J., Kerr, H.W. and Mohaupt, U.H. Int. J. Pressure Vessels Piping Nov. 1991 48 (2) 129-165 The early fatigue growth of cracks starting from defects at the toe of good-quality manual and automatic steel (e.g. HSLA) T-plate welds was investigated. Evolution of aspect ratios and growth rates was monitored, and linear elastic fracture mechanics models of fatigue propagation were developed. It was found that smallcrack effects caused by crack closure, weld metal microstructure and notch plasticity, together with the uncertainty associated with random initiation and early coalescence of microcracks, made any deterministic linear elastic fracture mechanics modelling of cracks less than ai - 0.3 mm impossible. Fatigue cracks on automatic welds developed much smaller aspect ratios, and grew much faster, than cracks on manual welds. Models of the evolution of aspect ratios on both manual and automatic welds are presented and discussed, and life predictions are compared with experimental results.
A streingauge technique for monitoring small fatigue cracks in welds.
Microstructural and mechanical characterisation of 43/43/14 tin/lead/bismuth. Marshal/, J.L., Ca/deron, J. and Sees, J.
Soldering & Surface Mount Technol. Oct. 1991 (9) 25-27, 33 A mechanical and microstructural study was performed of 43-43-14 Sn-Pb-Bi solder. This alloy melts lower than the commonly used Sn-Pb solders and therefore holds promise as a useful material in two-step soldering processes or in processes with thermally sensitive components. Mechanical testing of 43-43-14 Sn-Pb-Bi showed a strength comparable to that of Sn-Pb solders but increased creep rate. Microstructural analysis (scanning electron microscopy/energy dispersive X-ray) exhibited the same mechanism of fatigue as for Sn-Pb solders, i.e. heterogeneous coarsening. Thermocyclic fatigue demonstrated that the long-term reliability of 43-43-14 Sn-Pb-Bi is comparable to that of Sn-Pb solders.
Reliability prediction tools for SMD solder joints. Wha//ey, D.C. Soldering & Surface Mount Techno/. Oct. 1991 (9) 8-9 There is continued concern over the ability of SMD solder joints to survive in the harsh operating environments endured, for example, by automotive and aerospace products. The techniques available for analysing solder joint fatigue behaviour are reviewed. A software tool designed to allow estimation and comparison of the thermal fatigue life of solder joints exposed to a complex operating profile is described. The modelling technique takes account of the large changes with temperature of the solder alloys stress strain-rate behaviour and also the complex geometry of the component/solder,'PCS assembly, while avoiding the very high cost of nonlinear finite element analysis. This is achieved by first performing a linear stress analysis of the assembly to determine its compfiance, and then using this compliance estimate in the solution of a nonlinear differential equation describing the relationship between temperature, stress and strain-rate in the joint. The technique has been implemented as a software package known as ECLIPS-electronic connection life prediction system. This software package will run on a workstation or PC and has been shown to give results very close to those from nonlinear finite element analysis, but at approx 1/20 the cost.
A statistical study on fatigue fracture of spot-welded joints in cross
tension type. Fujisawa, Y. and Kikuchi, 7-,
Prec. 34th Japan Congress on Materials Research, Kyoto, Japan, Sept. 1990, pp 79-86 Fatigue tests were conducted in cross tension on spot-welded joints in commercial thin steel plate for automobile structural use (KBCF45). The load L and fatigue life N characteristics were examined along with fatigue life distributions at two different levels of applied ioad. Monotonic fracture tests were also performed to investigate the distribution characteristics of static strength using the same loading type. SEM observations were made on the fracture surfaces. Particular attention was paid to the crack initiation site and the crack propagation path. Based on these experimental results, the fatigue properties are discussed from the probabilistic viewpoint. The main conclusions included the following observations: distributions of static strength and fatigue life are well represented by three parameter Weibull distribution; scatter of fatigue life and static strength data is not larger than that of the parent material; there are four primary sites around the weld nugget where the feasibility of crack initiation is the same; fatigue fracture is caused by propagation of a second crack which occurs at the side opposite the initial crack; and a significant correlation exists between fatigue life and the crack refracted angle.
Mechanical Properties Fatigue and fracture mechanics evaluation for structural PM parts.
Jhansa/e, H.R. Met. Powder Rep. Sept. 1991 46 (9) 52-56 PM technology has developed to the stage that materials suitable for structural applications are being produced and increasingly used by industry. Fatigue is one of the most common modes of failure in load-carrying parts. Since such failures occur suddenly without warning, their consequence is usually very serious and expensive. There is a need for the development and usage of modern fatigue and fracture mechanics techniques for PM structural materials to assure reliable performance and total success. Steels used include Fe-Cu-Ni and Fe-Ni-Mo-Cu low alloy steels.
Otegui, J.L., Mohaupt, U.H, and Burns, D.J. Eng. Fract. Mech. 1991 40 (3) 549-569 A technique has been developed to monitor growth and closure levels of mildly irregular single and multiple cracks growing at the toe of we4ds. It is based on measurements of strain at several positions on the plate surface, near the weld toe. Calibration procedures for crack shape measurements are given, and numerical models are compared with experimental results. Finally, some experimental determinations of crack closure levels are presented. Surface cracks as small as 0.2 mm deep can be confidently detected. Accurate measurements of crack depth, surface length, and closure levels can be obtained for cracks deeper than approx 0.4 ram,
Fatigue characteristics of pulsed MIG-welded A l - Z n - M g alloy. Ghosh,
P.K., Gupta, S.R., Gupta, P,C. and Rathi, R. J. Mat. ScL Nov. 1991 26 (22) 6161-6170 Pulse-currant MIG welding of AI-Zn-Mg alloy (AI-4.5Zn-1.25Mg-0.45Fe-0.3Si) was carried out using an extruded section of base material and AI-Mg (5183) filler wire. During welding the pulse parameters such as the mean current and pulse frequency were varied; their effect on the geometry and porosity content of weld deposit, and the fatigue life of the weldment was studied. The pulse parameters were found to affect significantly the geometry and porosity content of weld deposit and, consequently, the fatigue life of the weldment. For a comparative study, weldments were also prepared by using conventional continuous current MIG-welding process, where welding currents equivalent to the mean currents of the pulsed process were used. The fatigue life of the weldment was correlated with the geometry and porosity content of weld deposit.
Effect of variable single cycle peak overload on fatigue life. Kumar, R. int. J. Pressure Vessels Piping Dec. 1991 48 (3) 293-303 Crack propagation experiments were conducted on 6061-T6 AI-alloy, for various overload ratios (1.75, 2.00, 2.15, 2.25, and 2.5). On the basis of these experiments a power law is developed to predict the delay period. The delay period after the application of a single overload increases as the magnitude of overload increases. Crack growth decreases after the application of an overload cycle and after a certain number of cycles it tends to attain its CAL crack propagation rate.
A two-year clinical study of light-cured composite and amalgam restorations in primary molars. Barr-Agholme, M., eden, A., Dahllof, G.
and Modeer, 7. Dental Mater. Oct. 1991 7 (4) 230-233 The clinical behaviour of composite and amalgam restorations with respect to anatomical form, marginal adaptation, caries adjacent to margin, and post-operative sensitivity is compared. Children (n = 43) with an average age of 6.4 years exhibiting proximal caries lesions in primary molars distributed on both left and right sides were selected, In each child, one amalgam and one composite side were randomly chosen, resulting in 64 fillings for composite and 55 for amalgam. After a two-year period, significantly (p < 0.05) more composite fillings (88%) were clinically classified as satisfactory compared with amalgam fillings (68%). No significant relationship was found between the success rate of proximal fillings and the caries activity of the individuals. The results indicate that composite can be used successfully as a class II f/fling material in primary molars in children.
Int J Fatigue September 1992