Test principle
The drop hammer impact testing machine lifts a certain mass of heavy hammer to the specified height and releases it through free fall, so that it can impact the surface of the specimen in a vertical direction. This process simulates low-speed impact events that composite materials may encounter in actual service, such as tool falls, debris impacts, etc. The core of the test is to measure parameters such as energy absorption, peak load, and failure form during impact to quantify the material's ability to resist impact loads. For laminated composites, low-speed impact often leads to invisible damage such as internal interlayer delamination and matrix cracking, which will significantly reduce the residual mechanical properties of the material. Therefore, the drop weight impact test is an important method to evaluate the impact toughness and damage tolerance of composite materials, and the results have direct reference value for structural design and material selection.
Equipment composition
The drop hammer impact testing machine is mainly composed of a guide system, an impact hammer head, a specimen fixture, a speed measuring device and a force sensor. The guide system should ensure that the hammer head falls vertically, and the frictional resistance should be as small as possible. The quality of the hammer head can be adjusted within the standard range according to the test requirements, and its end shape is divided into hemispherical shape, pyramid shape, etc., and the hemispherical shape is commonly used (such as diameter 16 mm or 20 mm). The specimen gripper is clamped pneumatically or mechanically to ensure that the specimen does not produce overall displacement during impact. The force sensor is mounted on the hammer head or base and is used to record the impact force-time curve in real time. Velocity measurements typically employ photoelectric gates that accurately calculate the instantaneous velocity of impact on the hammer head fall path. The complete equipment needs to meet the accuracy requirements of standards such as ISO 6603-2 or ASTM D7136.
Test method
The impact energy level should be determined before the test. Impact energy E is calculated by the following formula:
E = mgh
where m is the mass of the hammer head, g is the gravitational acceleration, and h is the release height. In practical applications, different energies are often achieved by adjusting the height of the fixed mass or adjusting the mass with a fixed height. Specimen sizes are typically 100 mm × 100 mm or 150 mm × 100 mm, depending on the material specification. After the impact is complete, the following parameters need to be recorded:
| Peak load (N) | Impact force-The maximum force value on the time curve |
| Absorb Energy (J) | The total energy absorbed by the material during the impact process can be obtained by integrating the area under the curve |
| Damage area (mm²) | Post-impact stratification area measured by ultrasound C-scan or visual method |
| Energy Absorption Rate (%) | The ratio of absorbed energy to total impact energy |
In addition, the damage tolerance can be further evaluated by the residual compressive strength test (CAI), which compresses the specimen after impact to observe the degree of strength decay.
Composite material characterization analysis
The response of composites under the impact of drop weights has anisotropic and multi-mode failure characteristics. Matrix cracking first appeared in the impact contact area, and then extended to the thickness. When the stress reaches the limit of strength between layers, delamination begins to form and expand. Fiber breakage typically occurs at higher energy levels and manifests as hammer penetration or backside fiber tensile breakage. Different layering sequences have a significant impact on impact response, for example, increasing the proportion of ± 45° layer can improve energy absorption capacity. The test results are often presented as force-displacement curves, and the area under the curve represents the total absorbed energy. By comparing the damage patterns under different energy levels, the evolution map of material impact damage can be established, which provides a basis for structural impact resistance design.
Evaluation of results
In actual evaluation, not a single index can fully reflect the impact resistance of materials. For example, high peak loads do not necessarily mean good toughness, and some brittle materials may have high peaks and low energy absorption. Therefore, a comprehensive analysis of damage area and energy absorption rate is usually combined. The following table lists examples of impact performance comparisons of common composites:
| Carbon fiber/epoxy | The peak load is about 4500 N, the absorbed energy is 25 J, and the damage area is about 700 mm² |
| Fiberglass/polyester | The peak load is about 3200 N, the absorbed energy is 40 J, and the damage area is about 1200 mm² |
| Aramid fiber/phenolic | The peak load is about 2800 N, the absorbed energy is 50 J, and the damage area is about 950 mm² |
This data can be used for material selection and comparison. In industry applications, drop weight impact testing is widely used in aerospace, automotive lightweighting, sports equipment, and other fields to evaluate the resistance of structural components to low-speed impacts and guide material improvement or protective layer design.
Standard reference
This method refers to the following standards:
- ISO 6603-2 "Plastics - Determination of impact properties of rigid plastics - Part 2: Instrumented impact tests";
—— ASTM D7136 "Impact Test Method for Drop Weights of Polymer Matrix Composites";
—— ASTM D7137 "Test Method for Compressive Strength After Impact".
During the test, attention should be paid to the influence of ambient temperature and humidity on the properties of composite materials, and the state adjustment of the specimen should be strictly implemented in accordance with the standard. At the same time, ensure that the center of the impact hammer head is aligned with the geometric center of the specimen to avoid data deviation caused by eccentric impact. For high-toughness materials, it is recommended to use a larger mass hammer head to avoid rebound interference. Check the status of the equipment guidance system and sensors after each test to ensure data accuracy.