Strength Verification of Plastic Components According to VDI 2016

Strength Verification in the Proof-of-Concept Phase

To illustrate early-stage strength verification, a pipe coupling element for domestic indoor water piping is considered. The goal is a cost-efficient design, motivating the use of a thermoplastic material. In this early development phase, unreinforced polypropylene (PP) is assumed; the specific grade has not yet been defined. Manufacturing is planned via injection molding.

Operating conditions are defined as a system pressure of 3 bar, a maximum operating temperature of 80 °C, and a design lifetime of 30 years. The component is exposed only to ambient air and is not in direct contact with water.

Pipe coupling element for indoor drinking water systems | © CADFEM / ID: 44CBPX

Strength verification is carried out in the proof-of-concept phase using Method C of VDI 2016. The objective is a fast feasibility and dimensioning assessment based on a simplified simulation—before selecting a specific material or detailed material data.

Method C represents the short- and long-term behavior of the polypropylene polymer class using analytical approaches and does not require material-specific stress–strain or creep data. Influences such as temperature, service time, and weld lines are considered via standardized factors.

Overview: What Does VDI 2016 Include?

The guideline VDI 2016 “Strength verification of plastic components” provides a standardized methodology for assessing structural integrity and long-term strength of thermoplastic components. It closes a gap left by classical metal-centered engineering methods, which are only partially suitable for plastics due to their pronounced time- and temperature-dependent behavior.

VDI 2016 distinguishes three calculation methods (A, B, and C), tailored to different phases of product development.

• Method A: detailed nonlinear finite element analysis for final design validation
• Method B: simplified simulation approach

• Method C: deliberately conservative, highly simplified method for early proof-of-concept phases

A key feature of Methods B and C is the strain-based verification instead of a purely stress-based approach. This is crucial for plastics, as properties such as Young’s modulus change significantly with temperature and load duration due to creep. By comparing calculated equivalent strain with allowable material strain, VDI 2016 enables a more realistic long-term assessment and helps identify creep failure or excessive deformation early.

Overview of strength verification methods according to VDI 2016 | © Matthias De Monte, 2025 / ID: 6XEB19

Calculation of Equivalent Strain in the Component

For the strain-based verification according to VDI 2016, Method C, the governing strain in the component must be determined. Strains in the pipe coupling element are obtained via structural FEM simulation in Ansys Mechanical.

The model considers a screw preload preventing component opening as well as the internal pressure of 3 bar. Temperature is set constant at the maximum operating temperature of 80 °C, representing a conservative long-term scenario.

The Young’s modulus used in the simulation follows the workflow defined in VDI 2016, Method C, applying a time- and temperature-reduced modulus.

eper: permissible strain; sy: yield stress; ey: yield strain
Workflow for determining Young’s modulus (Method C) | © Matthias De Monte, 2025 / ID: V17UGK

The simulation uses a linear-elastic material model with a tangential modulus adapted for 30 years of service at 80 °C. The FEM parameters are E_FEA = 140 MPa and ν_FEA = 0,50. The governing quantity in Method C is the maximum positive principal strain. Under an internal pressure of 3 bar at 80 °C, a maximum principal strain of 1.94% occurs in the hinge opening area. Due to the design, a weld line is conservatively assumed at this critical location in the early development phase.

Maximum elastic principal strains under 3 bar and 80 °C | © CADFEM / ID: DI5V5K

Calculation of Allowable Strain and Utilization

For strength verification according to VDI 2016, Method C, the allowable strain is the decisive comparison value. The starting point is the material-specific limit strain describing the onset of micro-damage. According to VDI 2016 Part II, it can be determined from tables or derived from Young’s modulus at 23 °C. For polypropylene, a limit strain of 2.33% applies. This limit strain is reduced by relevant influence factors, particularly service time, operating temperature, and weld lines. The time- and temperature-dependent interaction is represented by the material-specific interaction factor m = 2.3 for PP. A reduction factor of C_C,nb = 0,85 is applied for the weld line; media influence is neglected due to the absence of water contact. With a safety factor S_C = 1,2, the allowable strain results in εₚₑᵣ = 1,66%.

The utilization a_C is defined as the ratio of equivalent strain to allowable strain at the critical location. The safety factor is already included; a_C < 100% indicates sufficient strength. For this load case, Method C yields a maximum utilization of 117%.

This exceedance is not a numerical inaccuracy but reflects the deliberately conservative nature of Method C and provides a clear decision metric in the early concept phase.

In later development stages, optimized gate design can avoid weld lines in highly stressed areas. Moreover, Method C does not consider material-specific creep strength. Applying Method A, which includes real creep data for a specific PP grade, reduces utilization to 94%, thus fulfilling the strength requirement.

Utilization according to VDI 2016 using Method C and Method A | © CADFEM / ID: AW1MP2

From Pipe Coupling Example to Safe Everyday Design

The polypropylene pipe coupling example clearly illustrates how VDI 2016 enables robust engineering decisions already in early development phases. Method C deliberately serves as a conservative filter to identify critical regions at an early stage—such as the hinge opening with a potential weld line in this case. The subsequent transition to Method A then allows for a targeted, material‑specific assessment without unnecessarily over‑dimensioning the component. This approach can be directly transferred to a wide range of thermoplastic component applications where temperature, service time, and creep behavior are decisive design factors.

The author of this article is Dr. Matthias De Monte, Senior Expert and responsible for Design for Reliability in the Bosch business sector Cross Domain Computing Solutions. In this role, he is responsible for the design and validation of plastic‑based components for driver assistance and control systems subjected to thermal, mechanical, and time‑dependent loads. His technical focus lies in the fatigue, creep, and aging behavior of polymer materials. As a member of the Bosch‑wide Center of Competence for Plastics, he regularly contributes this industrial perspective to CADFEM training programs—both in the training on the strain-based strength verification according to VDI 2016 described here, and through complementary courses: Assess the fatigue life of plastics.

This article cannot and should not replace a training, but is intended to provide a practical introduction to strain-based strength verification according to VDI2016. Engineers who want to apply this methodology with confidence, avoid typical modeling and evaluation pitfalls, and systematically transfer the presented approaches to their own components can deepen their expertise in the CADFEM seminar Strength assessment for plastic parts with Ansys according to VDI 2016, which is taught by Dr. Matthias De Monte himself.