Material solution for increasing crash stability through selective reinforcement along principal stress trajectories

Abstract

Structural component (1) for absorbing mechanical loads (2) in particular impact or crash loads (2), characterized by that the component consists at least partially of a composite material or locally modified material, where a targeted, direction-dependent strengthening (4,5) is provided in areas along the principal stress trajectories (4,5) determined by means of stress analysis, wherein the strengthening (4,5) is realized by at least one of the following measures: (a) anisotropic fiber orientation along the principal stress directions (4,5), (b) local increase in material density or stiffness, (c) Functionalization through material gradients (4,5) (e.g. with respect to microstructure, composition or porosity), (d) targeted micromechanical or crystalline structure modification (4,5), (e) Insertion of mechanically effective inserts (4,5), inlays or reinforcing inserts (4,5), (f) structural optimization by topology optimized, load-path-oriented lattice or hollow structures (4,5), wherein the determination of the principal stress trajectories (4,5) is based on a computational stress analysis using the finite element method (FEM) or by experimental stress optics, so that a load path-appropriate material distribution (4,5) is achieved, which allows a reduction in material design in low-stress areas.

Description

[0001] To optimize the crash safety of structural components, it is crucial to understand and specifically influence the mechanical stress distributions within the material under load. In particular, the analysis of principal stress trajectories—that is, the lines along which the maximum normal stresses (principal stresses) run—offers a promising basis for material-related optimization of components in highly stressed areas.

[0002] At every point in a loaded solid, a three-dimensional stress state can exist, described by the stress tensor field. The principal stresses (σ1, σ2, σ3) are the eigenvalues ​​of this tensor and represent the normal stresses in those directions where the shear stress is zero. The corresponding directions (eigenvectors) define the principal stress directions.

[0003] The principal stress trajectories are curves that are tangential to the respective principal stress direction (e.g., σ1) at all points. They form a network of directional fields along which the load preferentially propagates within the material. These lines indicate where the material is subjected to the greatest stress and where reinforcement through material modification (e.g., fiber reinforcement) is technically advantageous.

[0004] Shear stress, as a measure of the distortion in the material, occurs at its maximum in planes inclined at 45° to the principal stress directions. Knowledge of both – principal stresses and shear stresses – is therefore essential for a complete understanding of the stress state.

[0005] The currently common method for determining principal stress trajectories in complex component geometries is computational, using the finite element method (FEM). Here, the component is digitally discretized into a mesh, the boundary conditions (forces, supports) are defined, and the material behavior (e.g., elastic, plastic, anisotropic) is modeled.

[0006] The results of the FEM analysis allow the stresses in each element to be calculated during post-processing. From these stresses, the local principal stress directions and their associated trajectories can be derived by vector field integration. Modern FEM software allows the direct visualization of these lines. Well-known programs such as ABAQUS, ANSYS, Altair OptiStruct, and COMSOL Multiphysics offer corresponding functions.

[0007] This calculation is highly precise, but depends on: • the quality of the network (mesh), • the accuracy of the material modeling, • and the realistic representation of the boundary conditions.

[0008] One experimental method for analyzing stress distribution is photoelasticity. In this method, a transparent, optically anisotropic model of the component is produced, which exhibits stress birefringence under load in a polarizing light field.

[0009] The resulting interference patterns (isoclines and isochromes) provide information about: • Principal stress directions (isoclines), • Principal voltage difference (isochromes), • and the local stress concentrations.

[0010] The trajectories of the principal stresses can be visually detected and mapped from the isoclines. This method offers the advantage of a rapid qualitative assessment of even complex geometries, but is limited to models and requires calibration for quantification.

[0011] In current technology, materials are primarily optimized for crash performance geometrically (e.g., through ribs, crumple zones) or through homogeneous material selection. Targeted material reinforcement along the principal stress trajectories is currently only implemented in very specialized applications—for example, in high-performance fiber composite structures in the aerospace or motorsport sectors. Here, the orientation of the fibers (e.g., unidirectional fibers, tapes) is selected manually or algorithmically to align with the principal stress lines (e.g., through tailored fiber placement). However, these methods are not yet widely established in mass production (e.g., the automotive industry) because they are complex, expensive, and difficult to automate.A comprehensive approach that translates numerically or experimentally determined principal stress trajectories into a material reinforcement strategy, while also incorporating manufacturing technologies such as additive manufacturing, functional grading, or targeted microlattice structuring, is not yet adequately described in the current state of the art. To increase crash stability, predominantly constructive measures such as crumple zones or homogeneous material reinforcements are currently employed. Modern topology optimization approaches utilize numerical methods to identify highly stressed zones, but a direct transfer of these analysis results into targeted material-based reinforcements along the principal stress trajectories is currently only possible in highly specialized applications, such as in the field of high-performance fiber composites.The integration of such stress distributions into the material design of components, especially in complex geometries and series production, has not yet been sufficiently resolved.

[0012] It is therefore the object of the present invention to provide a material-based solution that enables targeted reinforcement of structural components along mechanically highly stressed zones - in particular along the principal stress trajectories - in order to increase crash stability while using less material.

[0013] The present invention provides a novel material engineering solution that enables the targeted reinforcement of structural components along mechanically highly stressed zones – in particular along the principal stress trajectories. This selective reinforcement is based on the analysis of the local stress state within the component and leads to a significant improvement in crash stability while simultaneously reducing material usage.

[0014] A key feature of the invention is that the material design is not homogeneous across the entire component, but rather stress-guided, i.e., along the actual force paths within the material. These force paths are described by the principal stress trajectories – that is, the lines along which the maximum principal stresses propagate. In these areas, the highest mechanical stress occurs under load, while only low stresses are present in other zones.

[0015] The invention uses this stress distribution to intelligently distribute the material: • Areas subjected to high stress are specifically reinforced - for example by additional fibers, higher density, locally strengthened structure or adapted grid structures. • Low-stress areas, on the other hand, can be designed to save material, for example by using material cutouts, reduced wall thicknesses or less expensive materials.

[0016] This results in a functionally optimized component that, at the same or lower weight, exhibits significantly higher energy absorption capacity in the event of a crash. At the same time, material consumption is reduced, which is advantageous both economically and ecologically.

[0017] To identify the highly stressed zones, a stress analysis is performed. This can be done in either of the following ways: • numerically, e.g. using the finite element method (FEM), or • experimentally, e.g. by means of photoelasticity techniques.

[0018] The analysis yields principal stress fields, from which trajectory calculations are used to derive the lines representing the preferred load paths. These serve as the basis for material reinforcement. • Fiber-reinforced plastics with aligned fibers along the trajectories (e.g., through Tailored Fiber Placement) • Metallic materials with locally adapted heat treatment or strength (e.g., through laser beam strengthening) • Functionally graded materials (FGW), whose material composition varies along the stress lines • Additively manufactured structures in which the internal lattice geometry is adapted along the trajectories • Integrated inserts or reinforcing elements in matrix materials that are aligned with the principal stress paths

[0019] A key advantage of the invention is that the entire component does not need to be designed for maximum load, as is often the case in the prior art. Instead, a targeted understanding of the stress distribution allows for a differentiation in material selection: • In low-stress zones, material can be reduced or made lighter (e.g., through ribs, cavities, lower density). • In heavily congested areas, targeted reinforcement is carried out - but only where it is actually necessary.

[0020] This leads to a massive increase in efficiency, as material, weight and costs are saved without compromising mechanical performance - on the contrary, the targeted reinforcement even leads to a significantly improved crash performance.

[0021] The invention thus provides a materially feasible method with which components can be designed in such a way that they: • Identify and understand energy pathways, • specifically reinforce these, • and thereby save material where it is not needed.

[0022] This innovative combination of stress-based analysis and material engineering implementation has not been realized in the state of the art to date and enables a new generation of lightweight, function-optimized and crash-resistant components.

[0023] Possible applications • Automotive engineering: door reinforcements, bumper supports, crash boxes • Aviation: load-bearing lightweight structures • Mechanical engineering: Impact zones, protective elements • Defense technology: ballistic protection systems with optimized energy distribution • Medical technology: Prostheses with targeted force application Reference symbol list 1 structural component 2 individual forces 3 Force introduction 4 Principal stress trajectory σ1 5 Principal stress trajectory σ2

Claims

[1] Structural component (1) for absorbing mechanical loads (2) in particular impact or crash loads (2), characterized by , that the component consists at least partially of a composite material or locally modified material, where a targeted, direction-dependent strengthening (4,5) is provided in areas along the principal stress trajectories (4,5) determined by means of stress analysis, wherein the strengthening (4,5) is realized by at least one of the following measures: (a) anisotropic fiber orientation along the principal stress directions (4,5), (b) local increase in material density or stiffness, (c) Functionalization through material gradients (4,5) (e.g. with respect to microstructure, composition or porosity), (d) targeted micromechanical or crystalline structure modification (4,5), (e) Insertion of mechanically effective inserts (4,5), inlays or reinforcing inserts (4,5), (f) structural optimization by topology optimized, load-path-oriented lattice or hollow structures (4,5), wherein the determination of the principal stress trajectories (4,5) is based on a computational stress analysis using the finite element method (FEM) or by experimental stress optics, so that a load path-appropriate material distribution (4,5) is achieved, which allows a reduction in material design in low-stress areas. [2] Structural component (1) according to claim 1 characterized by , that the material is a metallic material (1)l in which local Heat or cold forming processes produced a targeted strengthening of the microstructure such as martensite along the principal stress trajectories (4,5). [3] Structural component (1) according to claim 2 characterized by, that the material is a functionally graded material (FGW) whose material composition or density varies along the principal stress trajectories (4,5). [4] Structural component (1) according to claim 3 characterized by , that carbon, glass, aramid or basalt fibers are used as reinforcing fibers (4,5). [5] Structural component (1) according to claim 4 characterized by , that the reinforcement (4,5) is achieved by Tailored Fiber Placement (TFP), whereby the fibers are placed along the principal stress trajectories. [6] Structural component (1) according to claim 5 characterized by , that the principal stress trajectories (4,5) are determined by a finite element simulation under quasi-static or dynamic loads (2). [7] Structural component (1) according to claim 6 characterized by, that the principal stress directions (4,5) are determined experimentally using photoelasticity, speckle interferometry or digital image correlation. [8] Structural component (1) according to claim 7 characterized by , that the orientation of the fibers (4,5) or reinforcement structure (4,5) is automatically derived from the stress fields (4,5) and transferred to a manufacturing system. [9] Structural component (1) according to claim 8 characterized by , that the enhancement areas (4,5) are generated by laser beam strengthening or structuring (4,5). [10] Structural component (1) according to claim 9 characterized by that it is used as a crash structure in automotive engineering, especially as a longitudinal or transverse beam, sill, battery tray or door impact protection.

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