A Comparative Analysis of Non-Destructive Testing (NDT) Methods for Weld Quality Assessment: A Case Study of Ultrasonic and Liquid Penetrant Testing

Abstract

Non-destructive testing (NDT) constitutes an indispensable component of quality assurance programs across industries where welded structures must maintain structural integrity under demanding service conditions. This study presents a comprehensive comparative analysis of ultrasonic testing (UT) and liquid penetrant testing (PT) for weld quality assessment in steel structures, based on detailed inspection data from multiple weldments examined under ASME V and VIII codes. The inspection results reveal a significant disparity: UT examinations identified numerous rejectable defects including lack of fusion (LOF), porosity, and slag inclusions with indication lengths ranging from 7mm to 800mm, while PT evaluations predominantly reported "No Indication" and accepted most welds. This discrepancy is analyzed through the lens of each method's fundamental physical principles, detection capabilities, and the application of ASME acceptance criteria. The findings demonstrate that the apparent contradiction is not a methodological conflict but rather reflects the complementary nature of these NDT methods—UT detects internal volumetric discontinuities while PT reveals only surface-breaking defects. The study provides detailed technical explanations for the observed disparities, discusses the implications for quality assurance programs, and offers evidence-based recommendations for optimizing NDT method selection. The research concludes that for critical structural welds, a hybrid approach incorporating both volumetric (UT) and surface (PT/MT) methods provides the highest level of quality assurance, and that method selection should be guided by risk assessment, defect type distribution, and operational considerations.