This article provides a comprehensive technical analysis of simple supported beam impact testing, commonly known as the Charpy impact test, specifically applied to 3D printed sheet materials. Understanding the impact resistance and fracture toughness of additive manufactured parts is crucial for engineers seeking to ensure reliability in functional prototypes and end-use components.
Technical Overview
Simple supported beam impact testing, widely recognized as the Charpy impact test, is a standardized method used to determine the energy absorbed by a material during fracture. For 3D printed panels and sheets, this test is essential for characterizing the material's behavior under sudden dynamic loads. Unlike isotropic materials created via traditional injection molding, 3D printed parts possess unique microstructures, including layer lines and potential voids, which significantly influence their impact resistance. This test quantifies the difference in energy absorption between various printing parameters and material types.
Testing Samples
Sample Geometry and Dimensions
The specimens for simple supported beam impact testing are typically rectangular bars. Standard dimensions often follow ISO 179 or ASTM D6110 protocols. However, for 3D printed panels, the thickness may correspond to the specific sheet thickness used in the application. The critical feature of the sample is the notch, which is machined or printed into the specimen to serve as a stress concentrator, ensuring the fracture occurs at a specific location.
Printing Orientation Considerations
The orientation in which the sample is printed is the most significant variable in this test. Because Fused Deposition Modeling (FDM) and similar technologies create parts layer by layer, the inter-layer adhesion is often the weak point. Samples must be tested in multiple orientations, such as flat, on-edge, and upright, to evaluate anisotropic properties. An upright print, where layers run perpendicular to the impact force, typically shows significantly lower impact strength compared to a flat print.
Testing Items
- Notched Impact Strength: This is the primary metric, calculated as the impact energy absorbed divided by the cross-sectional area of the specimen at the notch. It measures the material's sensitivity to stress concentration.
- Unnotched Impact Strength: Some 3D printing materials are so brittle or have such strong inter-layer bonding that they are tested without a notch to understand the bulk material toughness.
- Fracture Mode: The analysis involves observing the fracture surface to classify the failure as brittle, ductile, or mixed-mode. For 3D printed materials, delamination between layers is a common failure mode to document.
Testing Methods
Equipment Setup
The test utilizes a pendulum impact testing machine. The 3D printed beam is placed horizontally on two supports, creating a simple support condition. The pendulum is released from a specific height to strike the sample at the center. The machine measures the difference between the potential energy of the pendulum before impact and the remaining energy after breaking the sample.
Test Procedure
First, the dimensions of the sample, particularly the width and thickness at the notch, are measured precisely. The sample is positioned so the pendulum strikes the face opposite the notch. After the impact, the energy value displayed by the machine is recorded. This value is used to calculate the impact strength. Multiple specimens, usually at least five per group, are tested to account for statistical variance common in additive manufacturing.
Applications
This testing methodology is vital for several industries utilizing 3D printing for functional parts. In the automotive sector, it validates the durability of protective covers and brackets. For consumer goods, it ensures that drop impacts will not cause catastrophic failure. In material development, this test compares new polymer formulations or composite filaments, providing data on how fillers like carbon fiber or glass fiber influence toughness versus weight.
FAQ
Why is simple supported beam testing important for 3D printing?
It is crucial because 3D printed parts have anisotropic properties. This test reveals the weakest direction of a printed part, helping designers orient prints correctly to withstand impact loads during actual use.
How does infill density affect the results?
Infill density has a direct correlation with impact strength. A 100% solid infill provides the highest resistance to impact. Lower infill percentages reduce the effective cross-sectional area absorbing energy, leading to lower impact values and potential structural collapse upon fracture.
What is the difference between Charpy and Izod tests?
While both measure impact resistance, the Charpy test uses a simple supported beam configuration where the sample is supported at both ends. The Izod test uses a cantilever configuration where the sample is clamped at one end. The Charpy method is more common for testing standardized 3D printed panels.