Study of Allowable Interlaminar Normal Stress Based on the Time–Temperature Equivalence Principle in Automated Fiber Placement Process

Automatic fiber placement (AFP) is a type of labor-saving automatic technology for forming composite materials that are widely used in aviation and other fields. In this process, concave surface delamination is a common defect, as existing research on the conditions for this defect to occur is insufficient. To predict the occurrence of this defect, the concept of allowable interlaminar normal stress is proposed to define its occurrence conditions, and based on this concept, probe tests are carried out using the principle of time–temperature equivalence. Through the laying speed/allowable normal stress curve measured in the probe experiment, the physical meaning of allowable normal stress is discussed. At the same time, the measured curve is quantitatively analyzed, combined with viscoelastic theory and the molecular diffusion reptation model, and the dominating effect in the formation of a metal/prepreg layer and prepreg/prepreg layer is determined. Finally, the experimental data are used to guide the parameter selection in an automatic placement engineering case and prove its correctness.

Numerical and experimental investigation of delaminations in a unidirectional composite plate using NDT and SHM techniques

In this paper, the non-destructive testing (NDT), structural health monitoring (SHM), and scanning laser Doppler vibrometer (SLDV) techniques were presented to quantify three simulated delaminations inserted at different depths of a unidirectional composite plate. First, the RollerFORM and Omniscan equipment were sufficiently used to identify the delaminations. Second, in conjunction with guided waves, the developed imaging method was successfully used to detect and quantify the interested delaminations. The tuning curves were determined experimentally to define the dominant Lamb wave modes of incident waves. Third, multi-physics three-dimensional finite element simulations of propagating and interacting Lamb waves with delaminations were implemented to extract the wavefield data for wavenumber analysis. The experimental part was conducted to validate the numerical results using SLDV. The effect of the delamination depth on the trapped waves generated over the delamination region was studied numerically and experimentally. The results showed that trapped waves could be affected by the delamination depth. Both numerical and experimental results demonstrated that the near surface delamination has strong trapped waves over the delamination region while the far surface delamination has weak trapped waves. The energy distribution maps of numerical and experimental wavefields data sufficiently quantified the interested delaminations. A good agreement was achieved between the numerical and experimental results.

Conservation of Subfossil Bones from a Lacustrine Setting: Uncontrolled and Controlled Drying of Late Quaternary Vertebrate Remains from Cold Lake, Western Canada

Water-saturated vertebrate remains (e.g., bone, antler, and ivory) are particularly challenging to stabilize for long-term storage, research, and analysis. These types of specimens are sensitive to damage caused by water loss, which frequently results in delamination, twisting, and cracking. The recovery of late Quaternary vertebrate remains from underwater areas of Cold Lake, western Canada, prompted us to conduct a series of analyses to better understand the preservation of the remains and their susceptibility to damage, and to test different conservation techniques to achieve successful drying. X-ray fluorescence analysis of a sample of specimens revealed that the remains have particularly high iron concentrations, a condition that might have contributed to weaken their structure, further compromising their integrity when drying. Additionally, we found that certain bone elements, such as long bones, are more susceptible to severe surface delamination than others, and as a result these specimens should be monitored more closely during their treatment. Of the four drying methods we tested, controlled air-drying produced the best results, followed by solvent-drying. In contrast, vacuum freeze-drying and vacuum freeze-drying with 20% v/v Acrysol WS-24 in water, an acrylic dispersion, while rapid, resulted in differing degrees of delamination and cracking.

Study on bending fretting fatigue damage in 17CrNiMo6 steel

Bending fretting fatigue behavior of 17CrNiMo6 alloy structural steel at room temperature was investigated under different bending and contact loads; and the [Formula: see text]–[Formula: see text] curve also was built up. The results showed that the [Formula: see text]–[Formula: see text] curve had a “C” shape. The bending fretting fatigue life was mainly dependent on the bending fatigue stress and fretting displacement. The limit of the specimens and the fretting fatigue life were dramatically decreased by fretting actions. The bending fretting fatigue damage changed under varied bending fatigue stress levels. When the wear first occurred, there is a lower bending fatigue stress; and with a higher bending fatigue load, microcracks were generated. However, some serious wear and surface delamination were observed under the highest fatigue load.

Use of Direct Active Thermal Imaging Technique for Sub Surface Delamination Detection in Aviation Composites

Various non-destructive testing and evaluation (NDTE) techniques are in use by the aviation industry. Thermal imaging as an NDTE tool for composite material is becoming an effective methodology. In this research, a direct energy method approach to active thermal imaging is applied with test specimen placed between the heating source and the camera to take only one diagnostics snap per unit area of region of interest (ROI). Purpose of this study is to assess the utility of direct method as a reliable NDTE technique during aerospace inspections as a quick ‘Go / No Go’ tool for the detection of sub-surface delamination in multilayered composite sheets. The research presents a quantitative comparison of temperature profiles as well as qualitative analysis of 2D active infrared thermo graphic testing of glass fiber epoxy to detect sub-surface delamination. The experimental results are in close agreement with the actual locations of delamination in test samples. The technique may serve as a reliable tool to quickly categorize the component under inspection. However, the size of delamination could not be ascertained with acceptable accuracy in this study, possibly due to the spread of epoxy to the delaminated regions during preparation of defective samples.