Combining Passive Thermography and Acoustic Emission for Large Area Fatigue Damage Growth Assessment of a Composite Structure

Passive thermography and acoustic emission data were obtained for improved real time damage detection during fatigue loading. A strong positive correlation was demonstrated between acoustic energy event location and thermal heating, especially if the structure under load was nearing ultimate failure. An image processing routine was developed to map the acoustic emission data onto the thermal imagery. This required removing optical barrel distortion and angular rotation from the thermal data. The acoustic emission data were then mapped onto thermal data, revealing the cluster of acoustic emission event locations around the thermal signatures of interest. By combining both techniques, progression of damage growth is confirmed and areas of failure are identified. This technology provides improved real time inspections of advanced composite structures during fatigue testing.Keywords: Thermal nondestructive evaluation, fatigue damage detection, aerospace composite inspection, acoustic emission, passive thermograph

Design of an Automated Ultrasonic Scanning System for In-Situ Composite Cure Monitoring and Defect Detection

The preliminary design and development of an automated ultrasonic scanning system for in-situ composite cure monitoring and defect detection in the high temperature environment of an oven was completed. This preliminary design is a stepping stone to deployment in the high temperature and high pressure environment of an autoclave, the primary cure method of aerospace grade thermoset composites. Cure monitoring with real-time defect detection during the process could determine when defects form and how they move. In addition, real-time defect detection during cure could assist validating physics-based process models for predicting defects at all stages of the cure cycle. A physics-based process model for predicting porosity and fiber waviness originating during cure is currently under development by the NASA Advanced Composites Project (ACP). For the design, an ultrasonic contact scanner is enclosed in an insulating box that is placed inside an oven during cure. Throughout the cure cycle, the box is nitrogen-cooled to approximately room temperature to maintain a standard operating environment for the scanner. The composite part is mounted on the outside of the box in a vacuum bag on the build/tool plate. The build plate is attached to the bottom surface of the box. The scanner inspects the composite panel through the build plate, tracking the movement of defects introduced during layup and searching for new defects that may form during cure. The focus of this paper is the evaluation and selection of the build plate material and thickness. The selection was based on the required operating temperature of the scanner, the cure temperature of the composite material, thermal conductivity models of the candidate build plates, and a series of ultrasonic attenuation tests. This analysis led to the determination that a 63.5 mm thick build plate of borosilicate glass would be utilized for the system. The borosilicate glass plate was selected as the build plate material due to the low ultrasonic attenuation it demonstrated, its ability to efficiently insulate the scanner while supporting an elevated temperature on the part side of the plate, and the availability of a 63.5 mm thick plate without the need for lamination

Real Time Fatigue Damage Growth Assessment of a Composite Three-Stringer Panel Using Passive Thermography

Fatigue testing of advanced composite structures is critical to validate both structural designs and damage prediction models. In-situ inspection methods are necessary to track damage onset and growth as a function of load cycles. Passive thermography is a large area, noncontact inspection technique that is used to detect composite damage onset and growth in real time as a function of fatigue cycles. The thermal images are acquired in synchronicity to the applied compressive load using a dual infrared camera acquisition system for full (front and back) coverage. Image processing algorithms are investigated to increase defect contrast areas. The thermal results are compared to non-immersion ultrasound inspections and acoustic emission data

Principal Component Analysis of Thermographic Data

Principal Component Analysis (PCA) has been shown effective for reducing thermographic NDE data. While a reliable technique for enhancing the visibility of defects in thermal data, PCA can be computationally intense and time consuming when applied to the large data sets typical in thermography. Additionally, PCA can experience problems when very large defects are present (defects that dominate the field-of-view), since the calculation of the eigenvectors is now governed by the presence of the defect, not the "good" material. To increase the processing speed and to minimize the negative effects of large defects, an alternative method of PCA is being pursued where a fixed set of eigenvectors, generated from an analytic model of the thermal response of the material under examination, is used to process the thermal data from composite materials. This method has been applied for characterization of flaws

Thermography Inspection for Early Detection of Composite Damage in Structures During Fatigue Loading

Advanced composite structures are commonly tested under controlled loading. Understanding the initiation and progression of composite damage under load is critical for validating design concepts and structural analysis tools. Thermal nondestructive evaluation (NDE) is used to detect and characterize damage in composite structures during fatigue loading. A difference image processing algorithm is demonstrated to enhance damage detection and characterization by removing thermal variations not associated with defects. In addition, a one-dimensional multilayered thermal model is used to characterize damage. Lastly, the thermography results are compared to other inspections such as non-immersion ultrasonic inspections and computed tomography X-ray

Nondestructive Evaluation of Additive Manufacturing State-of-the-Discipline Report

This report summarizes the National Aeronautics and Space Administrations (NASA) state of the art of nondestructive evaluation (NDE) for additive manufacturing (AM), or "3-D printed", hardware. NASA's unique need for highly customized spacecraft and instrumentation is suited for AM, which offers a compelling alternative to traditional subtractive manufacturing approaches. The Agency has an opportunity to push the envelope on how this technology is used in zero gravity, an enable in-space manufacturing of flight spares and replacement hardware crucial for long-duration, manned missions to Mars. The Agency is leveraging AM technology developed internally and by industry, academia, and other government agencies for its unique needs. Recent technical interchange meetings and workshops attended by NASA have identified NDE as a universal need for all aspects of additive manufacturing. The impact of NDE on AM is cross cutting and spans materials, processing quality assurance, testing and modeling disciplines. Appropriate NDE methods are needed before, during, and after the AM production process

Elevated Temperature, Residual Compressive Strength of Impact-Damaged Sandwich Structure Manufactured Out-of-Autoclave

Several 1/16th-scale curved sandwich composite panel sections of a 10 m diameter barrel were fabricated to demonstrate the manufacturability of large-scale curved sections using minimum gauge, [+60/-60/0]s, toughened epoxy composite facesheets co-cured with low density (50 kilograms per cubic meters) aluminum honeycomb core. One of these panels was fabricated out of autoclave (OoA) by the vacuum bag oven (VBO) process using Cycom(Registered Trademark) T40-800b/5320-1 prepreg system while another panel with the same lay-up and dimensions was fabricated using the autoclave-cure, toughened epoxy prepreg system Cycom(Registered Trademark) IM7/977-3. The resulting 2.44 m x 2 m curved panels were investigated by non-destructive evaluation (NDE) at NASA Langley Research Center (NASA LaRC) to determine initial fabrication quality and then cut into smaller coupons for elevated temperature wet (ETW) mechanical property characterization. Mechanical property characterization of the sandwich coupons was conducted including edge-wise compression (EWC), and compression-after-impact (CAI) at conditions ranging from 25 C/dry to 150 C/wet. The details and results of this characterization effort are presented in this paper

In-Process Thermal Imaging of the Electron Beam Freeform Fabrication Process

Researchers at NASA Langley Research Center have been developing the Electron Beam Freeform Fabrication (EBF3) metal additive manufacturing process for the past 15 years. In this process, an electron beam is used as a heat source to create a small molten pool on a substrate into which wire is fed. The electron beam and wire feed assembly are translated with respect to the substrate to follow a predetermined tool path. This process is repeated in a layer-wise fashion to fabricate metal structural components. In-process imaging has been integrated into the EBF3 system using a near-infrared (NIR) camera. The images are processed to provide thermal and spatial measurements that have been incorporated into a closed-loop control system to maintain consistent thermal conditions throughout the build. Other information in the thermal images is being used to assess quality in real time by detecting flaws in prior layers of the deposit. NIR camera incorporation into the system has improved the consistency of the deposited material and provides the potential for real-time flaw detection which, ultimately, could lead to the manufacture of better, more reliable components using this additive manufacturing process

Ballistic Puncture Self-Healing Polymeric Materials

Space exploration launch costs on the order of $10,000 per pound provide an incentive to seek ways to reduce structural mass while maintaining structural function to assure safety and reliability. Damage-tolerant structural systems provide a route to avoiding weight penalty while enhancing vehicle safety and reliability. Self-healing polymers capable of spontaneous puncture repair show promise to mitigate potentially catastrophic damage from events such as micrometeoroid penetration. Effective self-repair requires these materials to quickly heal following projectile penetration while retaining some structural function during the healing processes. Although there are materials known to possess this capability, they are typically not considered for structural applications. Current efforts use inexpensive experimental methods to inflict damage, after which analytical procedures are identified to verify that function is restored. Two candidate self-healing polymer materials for structural engineering systems are used to test these experimental methods

Parents\u27 Goals: An Analysis of Therapist Reasoning

Purpose: Illustrate the use of DDDM to develop parent-identified goals for occupational therapy and to identify underlying sensory integration factors hypothesized to be impacting participation