Fraunhofer Institute for Mechanics of Materials IWMFlow-hole-forming screws (FLS) are the key technology for profile-intensive mixed construction in modern vehicle bodies—yet for the increasingly common three-panel joints, there are currently no validated simulation methods for crash scenarios. Previous surrogate models were developed exclusively for 2-sheet joints and cannot reliably reproduce the complex failure mechanisms in three-layer FLS joints. For the first time, this project systematically identifies the cause-and-effect relationships between the arrangement of the joined parts, the pre-drilled hole configuration, the combination of sheet thicknesses, and the failure behavior of FLS-joined 3-BV. On this basis, practical FE surrogate models and empirical prediction formulas for crash simulation are being developed and validated on component-like structures. For SMEs in the automotive industry, engineering firms, and joining technology, this means: reliable design of three-panel assemblies already in the virtual development phase—without costly iterative loops.
Development of Methods for Predicting the Failure Behavior of FLS-Joined 3-Sheet Joints in Crash Simulation
Ongoing research project
Project description
Market Needs and Challenges
To meet strict CO₂ regulations and increase the range of electric vehicles, the automotive industry is increasingly turning to lightweight construction using a multi-material approach. In this process, different types of materials—high-strength steels and aluminum profiles—are joined using mechanical fastening methods. Flow-hole-forming screws (FLS) have established themselves as a cost-effective, mass-production-ready, and removable joining technology for applications with access from one side, particularly for profile-intensive mixed-material constructions and fluid-tight battery enclosure cover joints. In modern automotive body construction, 3-sheet joints (3-BV) are increasingly used—for example, in the sill area, on A-pillar reinforcements, or on add-on parts—to reduce the number of joints and utilize material in a manner better suited to the applied stresses. However, no validated simulation methods exist for the virtual crash design of these 3-BV joints. The existing surrogate models were developed for 2-sheet joints and do not account for the influence of the third sheet layer. In practice, this leads to unreliable predictions, over- or under-dimensioning, and costly experimental iteration loops—a serious disadvantage, particularly for SMEs such as engineering firms, suppliers, and simulation service providers.
Project Approach
The project pursues an integrated experimental-numerical approach: In extensive experiments, the significant factors influencing the load-bearing and failure behavior of FLS-joined 3-BV are systematically identified—including load application direction, joint configuration, sheet thickness combination, material yield strength, pre-drill configuration, and loading rate. In parallel, Fraunhofer IWM is developing detailed numerical models to simulate the failure mechanisms and is verifying the transferability of the behavior from 2-BV to 3-BV. On this basis, existing FE surrogate models (including *CONSTRAINED_SPR3 and *MAT_100_DA) for 3-BV are being further developed, and an empirical prediction model for estimating the joint load-carrying capacity is being created. Validation is performed on component-like T-joint structures under quasi-static and impact loading.
Contribution to Addressing the Challenges
For the first time, the project results provide SMEs and large companies with a validated method for the safe design of FLS-joined 3-BV in crash simulation. This enables significant reductions in development times and costs, minimizes physical prototype testing, and allows the full lightweighting potential of multi-sheet joints to be realized. The results are directly incorporated into existing technical guidelines (DVS/EFB® 3440-4 and 3445-1) and are available via the widely used FE solver LS-DYNA. This contributes to increasing the efficiency of modern electric vehicles, reducing CO₂ emissions, and strengthening the competitiveness of small and medium-sized enterprises in the sheet metal processing industry in particular.
Fraunhofer IWM’s work packages in the project:
Systematic influence analysis and numerical failure model for 3-sheet metal joints
Fraunhofer IWM is developing a comprehensive, quantitatively validated picture of all relevant factors influencing the load-bearing capacity of FLS-joined 3-sheet joints—based on high-resolution FE models derived directly from the actual joint geometry. The result is a systematic influence matrix for the parameters joining plane, support effect, pilot hole position, sheet thickness combination, and load angle, supplemented by the transferability to 2-sheet and 4-sheet joints.
Practical calculation model for predicting the load-bearing capacity of 3-sheet joints
Fraunhofer IWM is developing an empirical prediction model that enables the joint load-carrying capacity of FLS-joined 3-sheet joints to be calculated using a formula based on tensile strength, sheet thickness, load application, and number of pilot holes. This results in a directly applicable engineering tool that synthesizes experimental evidence and numerical analysis into a compact, practical design formula.
Validated surrogate models for industrial crash simulation of 3-sheet joints
Fraunhofer IWM provides improved, 3-BV-optimized surrogate models (*CONSTRAINED_SPR3, *MAT_100_DA, and *MAT_240) in which the developed predictive model is implemented. These models have been validated using component-like T-joint tests under complex mixed loading and reliably predict force-displacement behavior, failure mechanisms, and failure sequences.
Based on the project results, Fraunhofer IWM can offer industrial companies the following research and development services:
1. Crash simulation of FLS-joined three-panel joints using validated equivalent models
Fraunhofer IWM simulates the failure behavior of multi-section FLS joints using validated, industrially applicable equivalent models and predicts force-displacement behavior, failure mechanisms, and failure sequences under real crash loads.
2. Empirical load-carrying capacity prediction for customer-specific FLS three-panel joint configurations
Based on its validated prediction model, Fraunhofer IWM calculates the expected load-carrying capacity and the dominant failure mechanism for any FLS-3 sheet metal joint configuration—quickly, using formula-based methods, and without the need for time-consuming detailed simulations.
3. Numerical detailed simulation to determine the causes of damage and analyze novel joints
Fraunhofer IWM identifies and quantifies local failure mechanisms in FLS joints using high-resolution detailed FE models—both in cases of unexpected damage and when evaluating novel configurations.
4. Assessment of the transferability of 2-BV parameter databases to applications involving 3-sheet joints
Fraunhofer IWM systematically analyzes the conditions under which existing 2-BV joint parameter values can be transferred to 3-BV configurations—and identifies where corrections or additions are necessary.
5. Simulation-Based Optimization of Joint Arrangement and Pilot Hole Strategy
Fraunhofer IWM uses simulation to determine which sheet metal arrangement and pre-punch configuration delivers the highest energy absorption, load-bearing capacity, and crash performance for a specific component configuration.
6. Parameterization and calibration of FE surrogate models for mechanically joined connections
Fraunhofer IWM calibrates and validates surrogate model parameters for FLS joints based on a small number of experimental data points—and delivers simulation models that are immediately ready for use in the customer’s environment.
7. Consulting on crash-safe design of FLS-joined battery housing connections
Fraunhofer IWM supports manufacturers of electric vehicles and battery systems in the crash-safe design of fluid-tight FLS multi-sheet joints in battery housings—taking into account the specific requirements for safety, weight, and disassembly.
