Guojing Li

Low Strain Rate Testing Based on Drop Weight Impact Tester

As the use of fiber composites extends to automotive applications, there is a need to characterize the composite properties at low strain rates, such as below 100/s. In performing such low strain rate tests, the commonly used split Hopkinson’s pressure bar for high strain rate investigations should become inadequate. A high-speed hydraulic testing machine may be suitable for testing at the low strain rates; however, it is not necessarily available to the general engineers because of its high cost. On the basis of a drop weight impact tester, this study presents an affordable testing technique for characterizing a carbon composite at low strain rates. Both compressive and tensile tests are demonstrated in this study.

An Innovative Instrumented Projectile for Measuring Impact-Induced Force History

Measuring impact-induced contact force history is critically important to understanding the nature of impact events and the associated damage processes of materials/structures involved in the impact. In order to investigate the impact-induced contact force history involved in free impacts, such as ballistic impact and high-speed crash, this study developed an innovative instrumented projectile. Both instrumented projectiles with and without an innovative geometry were constructed, tested, and compared. An independent optical method was also used to justify the measurements from the innovative instrumented projectile. Experimental results confirmed the effectiveness and the accuracy of the innovative instrumented projectile in measuring impact-induced force history.

Instrumented Projectiles for Dynamic Testing

Drop weight impact testers (DWIT) are commonly used in experiments for characterizing material properties and structural performance. With large mass, DWIT can provide large impact energy. However, the impact velocity involved in DWIT is usually low due to the fact that the impact velocity is proportional to the square-root of dropping height, i.e. v = (2gh)1/2. Even if the impact velocity can be increased, the wave propagation involved in impact may become entangled, resulting in distorted wave for dynamic analysis. This study investigates a mechanical technique to reduce the effect due to wave propagation and its application to dynamic impact measurements.

Characterization of Fiber Composites at Lower Strain Rates

Materials and structures are commonly subjected to dynamic loading. The behavior of materials and structures under dynamic loading, however, can be significantly different from that under static loading. This is especially true for so-called strain rate sensitive materials, such as polymeric materials and fiber reinforced polymer-matrix composite materials. Based on a previous study, this paper continues on the characterization of low strain rate effects, such as those up to 100/s, on glass-fiber-reinforced polymer-matrix composite materials. Testing techniques based on slip Hopkinson’s pressure bar have been constantly used for investigation of strain rate effects. However, because of the low strain rate range, the commonly used slip Hopkinson’s pressure bar should become over-qualified. On the other hand, the commonly used drop-weight impact testers have been found to be useful for low strain rate characterizations. The constitutive relations of the composite material at low strain rates are presented in this study based on the testing technique for lower strain rates. Besides, the effect of fiber orientation on the strain rate effects is also discussed.

An Innovative Instrumented Projectile for Dynamic Testing and Material Characterization

Measuring impact-induced contact force history is critically important to understanding the nature of impact events and the associated damage processes of materials/structures involved in the impact. In order to investigate the impact-induced contact force history involved in free impacts, such as ballistic impact and high-speed crash, this study developed an innovative instrumented projectile. Both instrumented projectiles with and without an innovative geometry were constructed, tested and compared. An independent optical method was also used to justify the measurements from the innovative instrumented projectile. Experimental results confirmed the effectiveness and accuracy of the innovative instrumented projectile in measuring impact-induced force history.

Response of Material Under Blast Loading

A shock tube based laboratory testing facility was used for simulating blast loading to characterize the response of circular metallic specimens. Measuring techniques including electrical resistance strain gages, fringe projection and others were used for recording and comparing the critical strains and deformation histories during and after blast loading.

A Testing Technique for Characterizing Composite at Strain Rates up to 100/s

As the use of fiber composites extends to automotive applications, there is a need to characterize the composite properties at low strain rates, such as below 100/s. In performing such low strain rate tests, the commonly used split Hopkinson’s pressure bar for high strain rate investigations should become inadequate. A high-speed hydraulic testing machine may be suitable for the low strain rate testing, however, it is not necessary within the reach of general engineers due to its high cost. Based on a drop weight impact tester, this study presents an affordable testing technique for characterizing a carbon composite at low strain rates up to 125/s. Experimental results from static testing and slip Hopkinson’s bar testing were also performed for comparisons.

Laboratory Blast Simulator for Composite Materials Characterization

Blasts and explosives have raised serious concerns in recent years due to the fatal injury and catastrophic damage they have caused in the combat zones and due to industrial accidents. Owing to their lightweight and complex damage process, fiber-reinforced composite materials have been found to have higher energy absorption capability and to be able to generate less lethal debris than conventional metals when subjected to impact loading. In order to characterize the blast resistance of composite materials, a piston-assisted shock tube has been modified for simulating blast tests in the laboratory due to its high safety, repeatability, accessibility and low cost. Although real blasts can be simulated relatively easily by using TNT or other chemicals, they, however, cannot be performed in general laboratories like many materials and structures testing due to their potential danger and restriction, hence hindering the design of new materials with high blast resistance. By carefully adjusting the individual components, piston-assisted shock tube has been shown to be able to produce blast waves for characterizing composite materials.