Effect of Defect

Thomas Menacher, summer semester 2017

The Effect of Defect (EoD) is the evaluation of defects in mainly composite materials and semi-manufactured composite forms. The measured results of a None Destructive Testing (NDT) method are analysed, to compare the data to already existing data of defects. The Effect of Defect analysis is an interaction between none destructive testing, statics and construction.^[1]

Effect of Defect for what reason?

For component designing three different strategies of construction are possible:

• Safe life: No critical damages are allowed during lifetime. Components have to be designed well-spaced

• Fail Safe: In case of a damage, other structural elements replace function

• Damage tolerance: Component has to withstand damage within two inspections intervals ^[2]

Safe life and fail safe strategies are inefficient with regard to lightweight design. Therefore, most components made out of composite materials are designed to damage tolerance if possible. Manufactured parts (e.g. composite parts) are often evaluated by none destructive testing methods. In professional practice most NDT methods and critical defects are predefined for a specific component. It’s possible that irrelevant defects are detected and relevant defects aren’t detected with this approach. A better way to assure the quality of a component is to create a database to know the appearance of a defect and how critical this specific defect is, before it is detected. ^[3]

If a defect has been detected while testing, it’s important to know whether the defect is relevant or not. The testing technician should be aware of the relevance of specific defects to predict a component failure or examine a part in case of doubt more precisely. A database can also be combined with an automatized inspection.

Defects

Deliberately caused defects

Defects in composite materials aren’t inspected with NDT methods for each case. Deliberately drilled holes in CFK laminates are standard for example in aeronautical applications. The hole is for a bolt conjunction between same or different materials. Position, location and dimensions of the defect are known. For these defects characteristic values exist for standard laminates. The effect of defect can be calculated with formulas or with FEM for simple laminate buildups. Additional to the drilled holes, defects like fringes and delamination can emerge as a result of processing steps. ^[3][4]

Important questions after a defect has been detected

• Which defect was detected?

• Where is the location of the defect?

• How big is the defect?

• What is the Effect of Defect on the specimen? ^[1]

The first three questions should be answered by none destructive testing. The fourth question can be answered by empirical data.

^{[2] [5]}

Characterisation of defects

One method creating database is, to characterise systematically a defect (e.g. porosity) with an NDT method (e.g. ultrasound). A second NDT method is needed as a reference. Specimen with the defect (in figure 2: porosity) are scanned with two different NDT methods (in figure 2: ultrasound,CT ). In combination with the reference scan (from CT) the pattern of damage of the defect can be reconstructed. By change systematically the quantity of one defect, a characterisation of the NDT method is gained. The specimen now can be tested for the mechanical properties (e.g. mechanical tensile test to destruction). Mechanical tests have to be adapted to real intended purpose. There is a big difference in the material behaviour of a static load compared to a cyclic load. In figure 2 the approach is visualized. ^[6]

Defects can be characterized also, by causing specific defects (e.g. drilled holes, delamination) to a laminate. The defects inside the laminate, in which all parameters are known, are verified with an NDT method. With this step, a characterisation of a specific defect is gained. To characterise the material properties (with defect), mechanical test have to be performed. It’s important to be aware that synthetic imperfections don’t exactly have the same properties like manufacturing induced fatigue induced or impact induced defects. ^[6]

^[7]

Characterisation of defects

• Defect can’t be detected

• Defect can’t be identified

• Defect can’t be quantified

• Defect can’t be evaluated sufficient

• An evaluation isn’t possible (part is unusable) ^[1]

Difficulties of defects in composite materials

In contrast to usual materials in mechanical engineering, like steel and aluminium, composite materials are inhomogeneous. This inhomogeneity exacerbates to test with none destructive methods, but also exacerbates databases for particular defects and NDT methods. Because of many different types of composite prepregs , the integral design and many design criteria, almost each part made of composite materials has a different fibre-layup. With different fibre-layups it could be necessary to characterise every new part for defects.

Interaction between several departments

NDT is a part of a quality assurance system. After calibrating , the NDT engineer knows about which imperfections can be detected by applying a specific NDT method. Because of uncertainties, it’s not possible to detect all defects which are theoretically detectable. The goal is, to detect a certain percentage of the detectable defects (compare Probability of Detection (PoD)). With this information and characterisation data stress analysts and statisticians calculate a risk for a manufactured part. Depending on how safety-critical a part is, the construction department has to reconstruct it.

Literature

1. Oster, R. Schuller, H., Redenbacher T.: ZfP an Hubrschrauberbauteilen aus Hochleistungs-Faserverbundwerkstoffen im Spannungsfeld von Fertigung, Konstruktion, Statik und Versuch. DGZfP-Berichtsband 94-CD. Rostock (2005).
2. Flemming, M.; Roth, S.: Faserverbundbauweisen, Eigenschaften. Mechanische, konstruktive, thermische, elektrische, ökologische, wirtschaftliche Aspekte. Springer- Verlag. Heidelberg (2003)
3. Große, C.: Qualitätssicherung von Faserverbundwerkstoffen in der Fertigung mit Zerstörungsfreien Prüfverfahren. In: Werkstoffe in der Fertigung, Ausgabe 2/ April 2017.HW-Verlag. Mering (2017). p.30 - 32
4. Wilmes, H.; Hermann, A.S.; Kolesnikov, B.; Kröber, I.: Festigkeitsanalysen von Bolzenverbindungen für CFK-Bauteile mit dem Ziel der Erstellung von Dimensionierungsrichtlinien. DGLR Jahrestagung 1999. Berlin (1999).
5. Fraunhofer IPA. Prüfung von Faserverbundteilen aus CFK/GFK. Stuttgart (2017). Online: https://www.ipa.fraunhofer.de/de/Kompetenzen/bild--und-signalverarbeitung/qualitaetssicherung-mit-thermographie/pruefung-von-faserverbundbauteilen-aus-cfk-gfk.html
6. Oster, R.: Herausforderungen an die ZfP bei Ihrer Anwendung an Faserverbundbauteilen. DACH-Jahrestagung. Graz (2012).
7. Kochan, A.: Untersuchungen zur zerstörungsfreien Prüfung von CFK-Bauteilen für die fertigungsbegleitende Qualitätssicherung im Automobilbau. TU Dresden. Dissertation. Dresden (2011). p.61 by Busse, G.: Kunststoffe zerstörungsfrei prüfen: Rechtzeitige Erkennung von Schäden in Bauteilen. In: Kunststoffe 90 (2000) AIF: Zerstörungsfreie Prüfung von Klebverbindungen mittels der Ultraschallangeregten Thermographie. In: AIF Abschlussbericht zum Projekt AiF 13.249 N, Lehr- und Forschungsgebiet Klebtechnik und Institut für Füge- und Schweißtechnik Braunschweig (2004). Erb, T.: Methodik zur Bewertung von Fehlern in Strukturbauteilen aus Faser- Kunststoff-Verbunden im Automobilbau, Universität Darmstadt, Diss., 2003.