Background

In-situ spot weld failure analysis under complex loading conditions is significantly restricted by the enclosed space around the weld nugget. Failure can be only observed at the post-failure stage which provides only limited information of the manner in which damage progresses and leads to crack propagation.

Objective

Novel testing and analysis techniques were developed to observe the through thickness evolution of cracks and failure in real-time.

Methods

Two new double-half weld (DHW) specimens for normal and shear load testing, compatible with digital image correlation (DIC) strain measurement, were developed that can be used to directly observe and record damage processes and cracking within spot welded samples. The test samples were analyzed with a speckle pattern to create strain maps and with a macro-etched surface to observe in-situ crack propagation. This novel testing method was used to determine complex failure mechanisms/modes for spot welds in hot stamped, ultra-high strength Al-Si coated 22MnB5 steel.

Results

Interfacial and pull-out failure mechanisms in normal and shear loading were observed though the thickness of spot weld and were recorded for the first time. Interfacial failure is initiated by strain localization at the weld notch followed by corona debonding and crack propagation towards the weld nugget. In pull-out failure, the crack follows the shape of a transient softened zone at the fusion boundary, while, interfacial failure is dominated by ductile shearing at the faying surface of the sheets.

Conclusions

The results for the DHW test geometries reveal complex failure mechanisms/modes in spot welds that can be quantified using DIC analysis.

中文翻译：

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使用数字图像相关技术进行损伤进展和失效分析的新型双半点焊测试技术

背景

复杂载荷条件下的原位点焊失效分析受到焊核周围封闭空间的显着限制。失效只能在失效后阶段观察到，这只能提供有限的关于损伤进展和导致裂纹扩展的方式的信息。

客观的

开发了新的测试和分析技术来实时观察裂纹和故障的整个厚度演变。

方法

开发了两种新的双半焊缝 (DHW) 试样，用于法向和剪切载荷测试，与数字图像相关 (DIC) 应变测量兼容，可用于直接观察和记录点焊样品中的损坏过程和裂纹。用散斑图案分析测试样品以创建应变图，并用宏观蚀刻表面观察原位裂纹扩展。这种新颖的测试方法用于确定热冲压超高强度 Al-Si 涂层 22MnB5 钢中点焊的复杂失效机制/模式。

结果

通过点焊的厚度观察并首次记录了正常和剪切载荷下的界面和拉出破坏机制。界面失效是由焊接缺口处的应变局部化引发的，然后是电晕脱粘和裂纹向焊接熔核扩展。在拉出破坏中，裂纹遵循熔合边界处的瞬态软化区的形状，而界面破坏则以板材接合面的韧性剪切为主。

结论

DHW 测试几何结构的结果揭示了点焊中复杂的失效机制/模式，可以使用 DIC 分析对其进行量化。