Chaosuan Kanchanomai

Low cycle fatigue test for solders using non-contact digital image measurement system

In a strain controlled low cycle fatigue test it is customary to use a contacting extensometer. However the extensometer can cause premature fracture at the contact points by inducing localized plastic deformation, particularly in a soft metal such as a lead– tin solder. In order to avoid this problem, a non-contacting, digital-image measurement system has been developed to measure the displacement of the specimen gage length during a cyclic loading. The capability of this system for strain-controlled fatigue testing was studied in preliminary experiments, and the effect of variables in the digital image measurement system on the accuracy and the reliability were established. The results confirmed that this system was capable of measuring the displacements in straincontrolled fatigue tests. Low cycle fatigue tests on three solder materials at 20°C were then carried out using this system. The solders were: Ag–Sn eutectic solder (3.5Ag/96.5Sn), Sn–Pb eutectic solder (63Sn/37Pb) and Sn–Pb solid solution solder (5Sn/956Pb). It was found that at a given strain range the Sn–Pb eutectic solder (63Sn/37Pb) had the lowest low cycle fatigue resistance. The lead free eutectic alloy (3.5Ag/96.5Sn) has the highest low cycle fatigue resistance in low strain range regime (p 1%). At higher strain range regime (p 1%), the 5Sn/95Pb alloy has better low cycle fatigue resistance. For all three types of solder, the initial failure mode at the surface of specimens was intergranular. For the 63Sn/37Pb alloy the subsequent fracture path was also intergranular, but for the 3.5Ag/96.5Sn and the 5Sn/95Pb alloys, the subsequent crack paths were generally transgranular.  2001 Elsevier Science Ltd. All rights reserved.

Low cycle fatigue behavior and mechanisms of a eutectic Sn-Pb solder 63Sn/37Pb

Abstract Low cycle fatigue tests of as-casted Sn–Pb eutectic solder (63Sn/37Pb) were carried out using the non-contact strain controlled system at 20°C. The fatigue behavior followed the Coffin–Manson equation with the fatigue ductility exponent of 0.63. Without local deformation and stress concentration at contact points between the extensometer and the specimen surface in strain-controlled fatigue tests, the crack initiation and propagation were studied by SEM examination of specimen surface with replication technique, longitudinal cross section and fracture surface. Colony boundary sliding and microcracks along colony boundaries were detected in the early stage of fatigue life. Cavities on colony boundaries, especially on the interphases between Sn–Pb phases, and wedge cracks at the triple-point colony coners were the initiation sites of microcracks. Stage II crack propagated in intergranular manner along the eutectic colony boundaries. This propagation of stage II cracks could be expressed by the relation of dac/dN=1.6×10−10 [ΔJ]1.2, where ac is average crack length and ΔJ is J-integral range. After fatigue tests, small grains were observed in Sn-rich phases near fracture surface.

Strain-rate effects on low cycle fatigue mechanism of eutectic Sn–Pb solder

Abstract Low cycle fatigue tests of as-casted Sn–Pb eutectic solder (63Sn/37Pb) were carried out using the non-contact strain-controlled system at 20°C in order to avoid local deformation and stress concentration at contact points between the extensometer and the specimen surface. The fatigue mechanisms were studied by SEM examination of polished surface of specimens and fracture surfaces. Wedge cracking due to grain boundary sliding was the dominant mechanism in the low strain rate regime, while extensive cavitation was observed on the colony boundaries at high strain rate regime. An estimation of the transition strain rate for grain boundary cracking due to sliding ( e ∗ w ) yielded a value lowers than that of the experimental result by 2–3 orders of magnitude. Relationship between time to failure and strain rate for the present results and the reported results could be expressed by a double-linear curve with the transition strain rate of approximately 10 −3 s −1 .

Low cycle fatigue and fatigue crack growth behaviour of Sn–Ag eutectic solder

Low cycle fatigue tests on as‐cast Sn–Ag eutectic solder (96.5Sn–3.5Ag) were carried out using a non‐contact strain controlled system at 20°C with different frequencies (10−3–1 Hz). Steps at the boundaries of Sn‐dendrites were found to be the initiation sites for microcracks in the case of low frequency fatigue tests, while for high frequency tests, cracks predominantly initiated at the boundaries of subgrains formed within Sn‐dendrites. The link up of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through Sn–Ag eutectic phases, and intergranularly along Sn‐dendrite boundaries and/or subgrain boundaries. Propagation of stage II cracks for various frequencies could be characterized by the C*‐parameter.

Fatigue crack growth behaviour of lead‐containing and lead‐free solders

Fatigue crack growth (FCG) tests on lead‐containing solders and lead‐free solders have been carried out at frequencies ranging from 0.01 to 10 Hz and stress ratios in the range 0.1–0.7. The FCG resistance of lead‐free solders was found to be superior to that of lead‐containing solders. For both types of solder, cycle dependent behaviour is dominant for the tests at low stress ratios and high frequencies, while time‐dependent effects become important at high stress ratios and low frequencies. For cycle dependent testing conditions, cracks primarily propagated in a transgranular manner, while a mixed trans/intergranular mode of crack propagation was observed for testing conditions where time dependent effects were dominant. The propagation path of intergranular cracks depended on the test materials, and along interfaces. After the FCG tests, the formation of small grains was observed.

Effects of time and stress state on fracture of closed-cell polyvinyl chloride foam

The effects of time and stress state on fracture behaviors and mechanisms of polyvinyl chloride foam have been evaluated by varying displacement rates (0.1 to 1000 mm/min) and specimen thicknesses (5 to 37.5 mm). For thin specimens, the crack tips were under plane-stress condition at all displacement rates. The excessive deformation under plane-stress condition induced severe damages of cell walls around the crack tip, which resulted in low fracture toughness. For thick specimens at low displacement rate, the crack tips were under plane-stress condition. The marginal inelastic deformation under plane-stress condition was high enough to cause crack-tip blunting. Thus, relatively high fracture toughnesses were observed. For thick specimens at high displacement rate, the crack tips were under plane-strain condition. Brittle fracture of cell walls around crack tip was observed. The fracture toughnesses were lower than those of thick specimens at low displacement rate.

Time-dependent deformation of closed-cell PVC foam

Time-dependent deformation behaviors of closed-cell polyvinyl chloride (PVC) foams (113 and 176 kg/m 3 foam densities) under tension were studied at various strain rates (10—5—10—1 s —1). It was found that the tensile properties including modulus, yield strength, ultimate tensile strength, and fracture strain were time dependent. Modulus and strength increased with increasing strain rate, while fracture strain decreased with increasing strain rate. Strain recoveries after unloading were also found to be time dependent. At low strain rates, more permanent deformations were observed, and the strain recoveries were less than those at high strain rates. Cell-wall buckling and crazing were dominant mechanisms at low strain rates, which enhanced the time-dependent strain and reduced the foam strength. However, the time-dependent strains decreased, and the failure mechanism became brittle fracture at higher strain rates.

Interaction of plastic zone, pores, and stress ratio with fatigue crack growth of sintered stainless steel

The relationships between plastic zone, pores, fatigue crack growth rate (da/dN), and fracture mechanics parameters of metal injection molding (MIM) 316L stainless steel at various stress ratios (R = 0.1 and 0.4) are investigated in the present work. Using two-dimensional (2D) plane strain finite element analysis (FEA), the stress intensity factor and cyclic-plastic zone at crack tip are evaluated. The slow-growth da/dN at near-threshold regime is in brittle manner, and can be characterized by maximum stress intensity factor (Kmax). While, the mid-growth da/dN at Paris regime is in ductile manner, and can be characterized by stress intensity factor range (ΔK). As a fracture mechanics parameter that combines Kmax and ΔK, the K* successfully characterizes the stress ratio effect on FCG rate at near-threshold and Paris regimes. The transition from slow-growth da/dN to mid-growth da/dN occurs when the cyclic-plastic zone at crack tip coalesces with pore. The appearances of fracture surfaces at near-threshold and Paris regimes are in good agreement with the FEA results.

C*-parameter approach to low cycle fatigue crack growth of solders

Time-dependent fracture parameter (C*) and elasto-plastic fracture parameter (J-integral) have been used to correlate low cycle fatigue (LCF) crack growth behavior of Pb-free solder (96.5Sn/3.5Ag) and Pb-containing solder (63Sn/37Pb) under various frequencies (10/sup -3/ - 10/sup -1/ Hz) and temperatures (20, 85, 120/spl deg/C). The results showed that J-integral would not characterize the LCF crack growth of 96.5Sn/3.5Ag under various frequencies and temperatures. However, C* was successfully correlated with the LCF crack growth rate of 96.5Sn/3.5Ag and 63Sn/37Pb. The plots of da/sub c//dt and C* for various frequencies and temperatures were located on a common straight line and in a narrow scatter band. It was confirmed that the LCF process of both solders is dominated by time-dependent mechanism.

Fretting Fatigue with Cylindrical-On-Flat Contact: Crack Nucleation, Crack Path and Fatigue Life

Fretting fatigue experiments and finite element analysis were carried out to investigate the influence of cylindrical-on-flat contact on crack nucleation, crack path and fatigue life of medium-carbon steel. The location of crack nucleation was predicted using the maximum shear stress range criterion and the maximum relative slip amplitude criterion. The prediction using the maximum relative slip amplitude criterion gave the better agreement with the experimental result, and should be used for the prediction of the location of crack nucleation. Crack openings under compressive bulk stresses were found in the fretting fatigues with flat-on-flat contact and cylindrical-on-flat contacts, i.e., fretting-contact-induced crack openings. The crack opening stress of specimen with flat-on-flat contact was lower than those of specimens with cylindrical-on-flat contacts, while that of specimen with 60-mm radius contact pad was lower than that of specimen with 15-mm radius contact pad. The fretting fatigue lives were estimated by integrating the fatigue crack growth curve from an initial propagating crack length to a critical crack length. The predictions of fretting fatigue life with consideration of crack opening were in good agreement with the experimental results.