Characterizing Mode I Fracture Behaviors of Wood Using Compact Tension in Selected System Crack Propagation

The fracture behaviors of four wood species commonly used in wood products were characterized when subjected to compact tension (CT) load in radial-longitudinal (RL) system crack propagation. Meanwhile, the failure modes of evaluated CT samples were compared and analyzed using the fractal dimension method. The results showed that wood species had a significant effect on fracture characteristic values, including maximum fracture load, critical stress intensity factor and fracture energy. These characteristic values changed in the same way, i.e., beech wood CT samples obtained the maximum characteristic values, followed by ash, okoume, and poplar in descending order. The fracture behaviors of all wood species evaluated can be described by combining linear and exponential fitting equations at the crack initial stage and evolution stage, respectively. Linear positive proportional relationships were observed between fracture characteristic values and fractal dimensions calculated using cracks in front and left views of CT samples. However, the relationships between fracture characteristic values and fractal dimensions calculated using fracture surfaces were negative. The fractal dimensions of cracks in front view of CT samples could be a better indicator used to predict critical stress intensity factor and fracture energy, which had greater correlation coefficients beyond 0.95.

Failure Analysis of SAC305 Ball Grid Array Solder Joint at Extremely Cryogenic Temperature

To verify the reliability of a typical Pb-free circuit board applied for space exploration, five circuits were put into low temperature and shock test. However, after the test, memories on all five circuits were out of function. To investigate the cause of the failure, a series of methods for failure analysis was carried out, including X-ray detection, cross-section analysis, Scanning Electron Microscope (SEM) analysis, and contrast test. Through failure analysis, the failure was located in the Pb-free (Sn-3.0Ag-0.5Cu) solder joint, and we confirmed that the failure occurred because of the low temperature and change of fracture characteristic of Sn-3.0Ag-0.5Cu (SAC305). A verification test was conducted to verify the failure mechanism. Through analyzing data and fracture surface morphology, the cause of failure was ascertained. At low temperature, the fracture characteristic of SAC305 changed from ductileness to brittleness. The crack occurred at solder joints because of stress loaded by shock test. When the crack reached a specific length, the failure occurred. The temperature of the material’s characteristic change was −70–−80 °C. It could be a reference for Pb-free circuit board use in a space environment.