Research Background

Improving quantitative understanding of material deformation and failure under complex loads is a key challenge in the development of next-generation near-space vehicles. Currently, materials candidates for next-generation aerospace missions undergo expensive development and certification cycles, but inadequate assessment of loading conditions under real-world conditions leads to an incomplete understanding of the damage mechanisms that ultimately induce structural failure.   

In 2022, the journal Composite Science and Technology published the work of researchers from the Department of Mechanical Engineering of the University of Utah using X-ray tomography technology to study the elastic-plastic deformation and progressive damage behavior of composite materials under biaxial stress.  

As a proof of concept of the μCT technology, the biaxial tensile test of two advanced composites was designed. The deformation and failure behavior of tear resistant nylon fabric (Nylon 66) used for parachute retarder and the damage evolution behavior of carbon/epoxy tape laminates filled with rocket fuel and oxidizer were investigated. The increment of fabric porosity and the change of individual fiber straightenness were measured after 1:1 and 1:2 loads were applied to nylon 66. The initiation and thickness evolution of cracks between adjacent layers were recorded by X-ray μCT images of biaxial stress lamination samples. The results show that the new biaxial test method can observe the deformation and damage evolution of the material from the microscopic scale.

In-situ biaxial loading system

Fig. 1 (a) in situ biaxial loading frame  (b) clamping arm  (c) vertical carbon/epoxy frame column

Table 1 In situ biaxial loading system specifications

In-situ X-ray scan parameters and beamlines

The 3D tomography images in this study were generated from X-ray plates collected when the test instrument was rotated 180°. The CT scan time of the test piece was about 5 minutes. All tomographic scans were performed in white light mode, and the ALS light source released an X-ray energy spectrum through the test piece. A scintillator is used to convert the X-rays into visible light, which is captured by an industrial camera. The raw data for each set of scans was 2,625 X-ray radiographs with an exposure time of 100ms.

Fig.2 In-situ biaxial loading system utilizing ALS 8.3.2 light source

Material One:Deformation and failure behavior  (Nylon 66)

Fig. 3 Geometric dimensions of nylon 66 test piece (x-y coordinates correspond to the global loading direction, and 1-2 coordinates correspond to the local material direction in the central region）

The specimen is tested at 1:1 (x: y) pull-pull load ratio. The researchers found that the strain ratio in the center of the measurement area was about 1.2:1 before the failure load of 450N. Based on this, X-ray imaging at 150N and 300N loading intervals is designed.

Fig. 4 Evolution process of adjacent braid intervoids when 1:1 biaxial load is applied

The researchers found that in terms of interfiber porosity, the gap openings varied from 2.2% under a slight preload (baseline scan) to 3.0% and 5.0% under biaxial loads of 150 N and 300 N.

Fig. 5 Results of the segmentation of a single bidirectional fiber image under different load ratios

The researchers segmented the image of a single fiber and compared the local curvature of the fiber, the bending of the fiber in the plane (1-2 planes) and the bending of the fiber out of the plane (2-3 planes) under three load ratios. It was found that the application of biaxial load would lead to the reduction of fiber curvature and the bending of the fiber out of the plane.

Material two:Crack formation  (carbon/epoxy tape laminates)

Fig. 6 Geometric dimensions of the carbon/epoxy tape laminate test piece

In order to make the thickness region of the composite tape laminate sample gradually evolve into full thickness damage during the loading process of the biaxial cross test piece of this configuration, and to better observe the progressive damage process of the port, the researchers designed the layup sequence and prefabricated cracks. The layup sequence in the central area of the test piece is [45/60/90/60/0]s, and the vertical crack is initiated at the 45° surface layer.

Fig. 7 Evolution process of fracture surface under different load ratios

Fig. 8 3D damage view considering fracture length, spacing and fracture displacement

By comprehensively analyzing the evolution process of the fracture surface, the researchers obtained the following experimental results and analyses:

    1. The crack of 45° paving surface is accompanied by the crack growth of 60° paving surface. Once the 45° is disconnected, because the transverse fracture toughness of the 60° paving is greater than the fracture toughness of the entire laminate, it will lead to the formation of a large number of pin shaped cracks.

      2.the damage of the 90° layer is more stable, and the influence of machining notch and vertical crack in the surface layer will rapidly attenuate, slowing down the crack expansion speed in the internal laminate.

      3. All 20 seam cracks in the 90° layer are evolved from 9 different cracks in the 60° layer, and the crack thickening can be visually seen.

     4. Considering the 45°, 60° and 90° paving comprehensively, due to the existence of prefabricated cracks, many cracks will be formed in the surface layer of the laminate, but only a few dominant cracks will expand to the middle layer of the laminate.

Summary

1.when the load is increased from 20N preload to 300N uniform biaxial tension, the gap of nylon 66 material can be more than doubled. With the increase of biaxial load, the curvature and out-of-plane bending of the fiber are significantly reduced. Due to friction and fiber entanglement, individual fibers do not exhibit significant in-plane motion under biaxial load.

2. When the interface Angle between the adjacent layers is small, the cracks between the adjacent layers can evolve at the same time, and no layered expansion occurs. The damage does not evolve independently, nor does it break away from the existing damage. They evolve due to the stress concentration caused by the damage of adjacent layers.

3.the new X-ray tomography technology biaxial test method can observe the deformation and damage evolution process of materials from the microscopic scale, which provides a reference for the synchronous visualization of mechanical testing under multi-axis complex stress.

Original text:：

French J , Dahlkamp C , Befus E ,et al.In-situ imaging of advanced materials subjected to in-plane biaxial loading using X-ray micro-computed tomography[J].Composites science and technology, 2022.