Newly Released VDI Guideline
In September 2025, the first version of VDI Guideline 2016 on the strength assessment of thermoplastic components has been published. The main objective of the guideline is to define a standardized procedure that leads to the same results for different users performing the same task. The procedures described refer to the published state of the art and are of an estimative nature. Existing gaps and limitations in applicability cannot be eliminated in all cases.
With regard to local stresses in the component, the guideline assumes a previous structural mechanical simulation that provides the corresponding stress and strain tensors at each point of the component.
It deals with the (static) strength assessment for components made of unreinforced and short fiber-reinforced thermoplastics. This includes components manufactured using primary forming processes such as injection molding or from semi-finished products manufactured using primary forming processes. It is not suitable for verifying stability in terms of buckling or denting, for evaluating elastomers or thermosets, for components manufactured using sintering processes, or for weld seams.
In its published form, the directive is divided into two sheets. Sheet 1 provides the basis for the guideline, defines the scope of application, and explains the conventions and terminology used. In particular, it distinguishes between the different types of load: single, repeated, short-term, long-term, and cyclic. The failure mechanisms acting on the material and the associated material properties determined in tests are explained. Furthermore, Sheet 1 briefly discusses the influencing factors taken into account with regard to component strength and names the strength hypotheses used.
Sheet 2 describes the static strength verification. The verification is formally divided into nine individual steps, which are processed in one of three possible procedures (A, B, and C) (see Figure 1).
After selecting the appropriate verification method, the local stress is determined. To do this, in step 2, the stresses or strains calculated in the FE simulation are converted into a suitable comparative value (e.g. von-Mises). Steps 3 to 5 provide a value of the strength limit adapted to the material used and the current loading conditions, which determines the local stress limit taking into account the safety factor (steps 6 and 7). The ratio of the stress characteristic value and the permissible stress ultimately results in the local utilization factor. The assessment is considered to be satisfied if the utilization factor in the entire component is less than or equal to one.
Conversely, however, exceeding this limit locally in terms of the utilization factor does not necessarily mean that a component will fail. This question can only be answered by means of a global load-bearing capacity check (optional step 9), which is not part of the current version of the guideline.
The choice of the appropriate assessment method (step 1) depends mainly on the availability of the necessary material data. For method A, the guideline assumes the availability of load-analogous material properties. This means that all stiffness- and strength-related properties of the material have been determined in tests that correspond to the operating conditions of the component. This applies in particular to temperature, time, and media influence. See also Figure 2. Method A provides a stress-related utilization factor.
In method B, the results from standard tests (e.g., DIN ISO 527) are used instead of load-analogous measurement values. The effect of the various influencing factors on component strength is considered using reduction factors. The procedure is similar to the method according to Oberbach [OBE1981], but differs in the way in which the applicable reduction factors are determined. Unlike the Oberbach method, however, the guideline only allows a static strength assessment. A strain-related utilization factor is provided.
Finally, method C only uses representative design limits for material classes, thus completely refraining from the evaluation of stress/strain curves or similar. The method is intended for the approximate evaluation of an initial component design. In method C, a strain-related utilization factor is also calculated.
For the treatment of anisotropic, short fiber-reinforced plastics, the guideline generally assumes that the calculation of stresses and strains in the structural simulation is performed using an isotropic material model. The influence of the local fiber orientation and the degree of orientation is therefore taken into account generalized in all methods.
Implementiation in S-Life
With its simplified strength assessment method, S-Life Plastics has been offering a powerful method for evaluating static and cyclic strength under short-term and long-term loads for many years. In this method, long-term and cyclic stress limits are estimated on the basis of static characteristic values and flat, material-specific reduction factors. The method is described in detail in [SSK2025] and is based on the work of [OBE1981].
With the fall release of S-Life, users now also have access to methods A and B from VDI 2016 (Figure 3). Method C, described in the guideline, has not yet been implemented in the upcoming version of S-Life.
For reasons of clarity, the verification method is selected at the application level, i.e., all isotropic components of the FE model are treated using the selected method. For components for which the stresses were calculated using Converse with an anisotropic material model, the familiar comprehensive anisotropic verification is also available.
The actual implementation of the verification has changed little for the user. After importing the results file from the respective FE solver, the material is assigned to the components to be assessed and the relevant calculation increments are selected as load cases. If necessary, loads can be scaled or superimposed within S-Life. After specifying the load type (single, multiple, long-term, etc.), different verification parameters must be specified depending on the selected method, for example, a seam weld or a safety factor. S-Life or MatScape provide useful defaults for all required values, which can be adjusted by the user if corresponding values are available.
Material Description in MatScape
The assessment for the entire component is performed at the touch of a button, and the calculated utilization factor is displayed as a contour plot. A detailed assessment report can be generated for each node of the model upon request.
Ultimately, the individual assessment methods differ mainly in terms of strength, i.e., in relation to the determined stress limits. The differences are particularly evident in the MatScape software module, which is inseparably linked to every S-Life version.
MatScape provides both the material cards required for structural simulation of a material and the stress limits, depending on the type of load and external boundary conditions. Additional influencing factors, such as weld seam factors or strain reserves to be taken into account when the glass temperature is exceeded (strain shift), can also be specified or estimated in MatScape. The introduction of the VDI 2016 guideline has resulted in a number of innovations, particularly with regard to stress limits.
As usual, MatScape displays the allowable stresses versus the operating temperature for the established PART Oberbach method, differentiated according to load type (Figure 4). Within the individual ranges, the exact value of the stress limit is adjusted according to the stress condition. As mentioned above, the reduction factors used for each load type depend on the material type (amorphous, semi-crystalline, fiber-reinforced). Stress limits are always determined for room temperature first and then transferred to other temperatures if necessary.
For method B, the reduction factors to be applied are not determined by the material type, but by the specific stress/strain curve (per temperature). In the double-standardized stress/strain diagram (see Figure 5), the shape of the curve and its deviation from the linear elastic curve are evaluated. The normalized stress limits can be read directly for each load type. In addition to the graphical method, the guideline also provides an analytical formula for their calculation.
In addition to the standardized values per temperature, MatScape also displays the values of stress limits over temperature, similar to the PART Oberbach method (Figure 6).
In method A, no reduction factors relating to temperature or time are used, as the underlying stress/strain curves must be load-analogous. The failure characteristics (fracture or yield stress) read from the measurement curves correspond directly to the strengths to be expected in the component. However, for long-term loading (creep) in particular, the time-dependent stress limit in method A is not estimated from short-term data, but is read from a creep curve that must be provided. The relevant data can be entered in MatScape (Figure 7).
Literature:
[OBE81] Karl Oberbach, „Nachdruck: Berechnung von Kunststoff-Bauteilen, Berechnungsmethoden und zulässige Werkstoffanstrengungen“. DE. In: Proceedings Konstruieren mit Kunststoffen, 11. Konstruktions-Symposium der DECHEMA. Vol 91. Frankfurt/Main, 1981, pp181-196. URL: https://www.partengineering.com/de/benutzerbereich/dokumente/artikel.
[SSK25] Markus Stommel, Marcus Stojek, Wolfgang Korte. FEM für die Berechnung von Kunststoffbauteilen, 3. Auflage, Carl Hanser Verlag, München, 2025. Available only in German language here. An edition in English language is in preparation.
Author: Dr. Marcus Stojek is Managing Director at PART Engineering GmbH, Bergisch Gladbach
