Determination of notch class-dependent limit values for internal imperfections in welded joints – Limits of internal imperfections - LIMPER

Project description

Market Demand and Societal Challenge
Internal irregularities in welds—porosity, inclusions, and bonding defects—cannot be completely avoided even with the most advanced welding processes. Current standards (ISO 5817, IIW Guideline, Eurocode 3) classify these defects solely by type and size. The location—particularly the distance from the surface—is not taken into account, even though research findings clearly show that defects located in the center are significantly less critical than those near the surface. At the same time, rising material and energy prices are driving the industry toward higher strength classes (FAT), which can be achieved through post-weld treatment methods such as shot blasting or high-frequency hammering (HFH), making internal defects the limiting factor. The result: time-consuming non-destructive testing, costly rework (repair rates exceeding 10%), and in some cases the dismantling of entire structures—even though many of the detected defects are actually inconsequential for the service life. Small and medium-sized enterprises (SMEs) in the steel construction, wind energy, and offshore sectors are particularly affected, as they lack both their own quality standards and the research capabilities to conduct individual assessments.
 

Project Approach
LIMPER combines extensive experimental fatigue testing (approx. 180 specimens) with numerical parameter studies (approx. 3,000 FEM calculations) to determine limit values for internal irregularities based on notch type class. Three industrially relevant materials (S355, S690, X6CrNiTi18-10) are being investigated. Service lives are experimentally determined on welded specimens with deliberately introduced defects of defined size, position, and orientation, and are numerically reproduced using a validated two-phase model (crack initiation + crack propagation). In parallel, a non-destructive testing (NDT) interlaboratory test involving 7 testing companies quantifies the detectability and measurement accuracy of modern ultrasonic methods. The results are converted into position-dependent threshold tables that can be directly integrated into existing standards.

Contributing to Overcoming the Challenges
For the first time, the project provides the industry with scientifically sound, position-dependent limit values that prevent unnecessary rework on non-critical internal defects—while simultaneously ensuring fatigue strength. The results are directly incorporated into ISO 5817, ISO 10675-1, and the IIW recommendations and are thus immediately applicable to the entire German and international steel and welding industries. For SMEs, this means significant cost savings in manufacturing and inspection, fewer production delays, and better utilization of the actual performance capabilities of welded joints. In the long term, this supports lightweight construction and resource efficiency in key sectors such as wind energy, offshore, bridge construction, and plant engineering.

Fraunhofer IWM’s work packages in the project:

Material properties for weld metal materials:

• A comprehensive experimental database comprising at least 125 tests: damage parameter Wöhler lines (PRAM), cyclic stress-strain curves, as well as fracture toughness, threshold values, and crack propagation curves at the microstructural level
• Comprehensive material characterization that covers both local damage mechanisms and engineering-relevant properties—from the micro- to the macro-scale

Mathematically validated limit values for internal weld irregularities:

• Calibrated FE models of internal irregularities based on real CT and fracture surface data
• Approximately 3,000 systematic life-span calculations covering the complete parameter space of defect size, edge distance, defect cluster spacing, and sheet thickness (10–100 mm) for three defect types and eight load levels
• A robust mapping of calculated service lives to notch failure classes ranging from FAT63 to FAT160—serving as a direct basis for design recommendations

International regulatory recommendation:

• A tabular recommendation supplementing ISO 5817 and the IIW recommendations, which consolidates all experimental and computational project results in accordance with standards
• Through active participation in the IIW Commissions C-X and C-XIII, Fraunhofer IWM ensures that these recommendations are incorporated into international standards