摘 要 本研究主旨在探討負荷應變比與張應變持時此二種效應對於Sn-3.5Ag-0.5Cu無鉛銲錫低週疲勞行為的影響。為了避免傳統接觸式延伸計傷害試棒表面而產生提早破壞，本研究發產出一套非接觸式應變量測系統，並將之應用於低週疲勞試驗。此外，亦利用掃描式電子顯微鏡(SEM)觀察表面裂縫與破斷面，以了解此款無鉛銲錫之疲勞破裂機制。 實驗結果顯示，在各種應變振幅、應變比和張應變持時的試驗條件組合中，所得的疲勞壽命皆可以不同的Coffin-Manson關係式個別描述之。當應變比從-1增加至0和0.5時，疲勞壽命明顯降低，此結果乃是受平均應變而非平均應力的影響。而當張應變持時從0秒增加至100秒時，疲勞壽命也會明顯降低，這是因為引入潛變破壞機制所致。為描述平均應變與張應變持時此二種效應對此款無鉛銲錫低週疲勞行為的影響，本研究分別提出兩種修正型的Coffin-Manson關係式。基於這兩種修正型Coffin-Manson關係式的成功應用，本研究更提出了一統合型的Coffin-Manson修正關係式，並成功地應用於描述本研究各式不同組合試驗條件下的所有低週疲勞壽命。 由SEM觀察得知，微小的表面裂縫起始於富錫樹枝狀結構與共晶組織兩相的交界處，主要原因是兩相強度不同，循環受力後容易產生晶界差階造成應力集中所導致。裂縫的成長和連結，主要為繼續沿著富錫樹枝狀結構與共晶組織兩相的交界處，並伴隨穿過共晶組織相的穿沿晶混合模式。隨著受力循環數增加，微小裂縫連結成更大裂縫並向試棒內部成長，直到試棒完全斷裂。 ABSTRACT The purpose of this study is to investigate the influence of strain ratio and tensile hold time on low-cycle fatigue (LCF) behavior of a lead-free Sn-3.5Ag-0.5Cu solder alloy. A non-contact strain measurement system was developed and applied to the LCF tests to avoid premature failure caused by traditional contact type of extensometers. Fractography analysis with scanning electron microscopy (SEM) was conducted to determine the LCF fracture mechanism for the given solder alloy. Results showed that LCF life of the given Sn-3.5Ag-0.5Cu, under various combinations of strain amplitude, strain ratio and tensile hold time, could be individually described by a Coffin-Manson relationship for each given testing condition. An increase of strain ratio from R = –1 to 0 and 0.5 would cause a significant reduction of LCF life due to the influence of mean strain instead of mean stress. LCF life was markedly reduced when the hold time at tensile peak strain was increased from 0 to 100 sec as a result of additional creep damages generated during LCF loading. Several modified Coffin-Manson models were proposed to describe the LCF behavior with a consideration of the effects of strain ratio and tensile hold time separately. Based on the success of these modified models, a unified LCF lifetime model was then proposed and did an excellent work in describing the LCF lives for all the given testing conditions with various combinations of strain amplitude, strain ratio and tensile hold time in the current study. From SEM observations, it could be found that fatigue cracks were initiated at the interphases between b-Sn dendrites and eutectic phases. The fatigue cracks propagated and linked up in a mixed mode of intergranular manner along boundaries between b-Sn dendrites and eutectic phases and transgranular manner through eutectic phases.
