<figure><img src="image_1.png" /></figure>

# Non-destructive Testing of novel Joints and Composites of renewable raw Materials

Ulrike SIEMER¹, Ralf FÖRSTER², Andreas LOTH³, Astrid HAIBEL⁴

1,2 Department of Mechanical Engineering, Berliner Hochschule für Technik (BHT), Berlin, Germany;

¹Ulrike.Siemer@bht-berlin.de, ²Ralf.Foerster@bht-berlin.de

³ Department of Mechatronics, Berliner Hochschule für Technik (BHT), Berlin, Germany;

Andreas.Loth@bht-berlin.de

⁴ Department of Mathematics, Physics, Chemistry, Berliner Hochschule für Technik (BHT), Berlin, Germany;

Astrid.Haibel@bht-berlin.de

## Abstract

The emerging challenges brought about by sustainability requirements are leading to new applications of renewable resources. Unlike most conventional materials, renewable raw materials lower their carbon footprint by storing CO₂ while they grow, making them highly attractive for industrial use. New composite concepts and joints incorporating these materials are being analysed with a view to develop non-destructive testing (NDT) solutions for quality control for subsequent production processes. NDT methods with potential to control specifically determined inspection tasks, such as computed tomography, ultrasonic, microwave or terahertz testing and active thermography, are being evaluated in terms of their respective strengths and limitations. Assessments of testability are discussed on the basis of both application-related examples and experimental investigation. The focus of this work is adapting active thermography to the considered renewable raw materials (timber and bamboo), their novel joints (resistance butt welds), and sustainable composites (COMBOO material). New approaches for thermal excitation provide initial information on detectable material and structural inhomogeneities. Analyses of the resistance butt welds with computer tomography reveal details of the inner structure and possible defects in the adhesion zone, making important information for further NDT development available.

**Keywords:** Non-destructive testing (NDT), renewable materials (timber, bamboo), sustainable composites, resistance butt welds, materials characterisation, quality control, active thermography, computed tomography

## 1. Introduction

Renewable raw materials even have a negative carbon footprint while they grow. They can make a significant contribution to achieving the increasing sustainability targets. New composite and joining concepts with renewable materials timber and bamboo as alternatives to conventional composite materials like CLT (cross laminated timber) structures, for example for lightweight construction or not least as design elements in architecture, are the focus of extensive research [1, 2]. While research into application properties is already advanced, supporting processes like non-destructive testing (NDT) solutions for subsequent manufacturing are not yet available. NDT is an attractive prospect for industry as a means of improving quality or reducing costs in production processes. Therefore, recent progress in the development of novel composites and joints of timber and bamboo requires ongoing investigation of NDT opportunities. NDT should be provided for quality control for overall efficient and sustainable production processes. This research continues previous investigation of non-destructive testing of renewable materials [3, 4], deepening the findings for testing its novel composite and joint applications. Previous studies into the suitability of NDT methods for defect detection determined different promising approaches including active thermography as a method with particular capabilities for such applications. The results of first positive trials for so called COMBOO composites should be intensified with more in-depth investigations of already used excitation sources as well as they should be further explored by analysing more promising active thermography options, including both varying the test setup configurations and using additional thermal excitation sources. Another focus of this work is the non-destructive characterisation of adhesion properties and failures of resistance butt welds in renewable materials as timber and bamboo. As thermography is not supposed to be a suitable method for this application, other approaches are needed.
**2. Materials and methods**

The considered raw materials and corresponding composites and joints will be briefly described so far as the properties or structure is relevant for NDT, especially for active thermography methods. Typical defect appearances are shown, the objected testing tasks are derived and classified. The presented NDT method preselection gives insides into the specific potentials and clarifies why which methods are used in the experiments.

**2.1 Sustainable material and joining concepts**

**2.1.1 Raw materials**

Timber (as renewable material) is an organic material consisting of cells and chemical compounds such as cellulose, hemicellulose and lignin. The cell wall is composed of many layers: Microfibrils in a lignin matrix glued together by polyoses. The microfibrils make up approx. 40-50 % of the timber mass and are oriented roughly in the direction of the fibres. The lignin content is approx. 25-30 % and fills the cavities between the fibrils [5]. Fig. 1 shows the inner structure of frequently used timber species softwood spruce and hardwood oak. The illustration exemplifies the large differences in structure between different types of timber material. Additionally, annual rings as exemplary shown for a pine tree slice, complement the foregoing statement. The layer thicknesses differ depending on the growth conditions from year to year so that the samples can vary a lot in its distribution. Cross sections cut in different orientations demonstrate again the high anisotropy that occur in timber components.

<figure><img src="image_2.png" /><figcaption>Figure 1. Inner structure of softwood (a) and hardwood (b) [5], tree slice with annual rings (c), annual ring size variation (d), anisotropic timber structure in different directions (e)</figcaption></figure>

Timber is a highly heterogeneous construction material, exhibiting significant variations in its properties. It has a pronounced anisotropy, which can vary considerably depending on the species and origin. Physical properties such as thermal conductivity and mechanical strength depend heavily on direction. It conducts heat slowly, shows high mechanical damping, and is nearly an insulator for electric current. The properties are also affected by humidity content.

Bamboo (as renewable material) is a fast-growing plant of the grass family with timber-like properties. Over 1250 species exist worldwide, all with widely varying properties. The anatomical structure of bamboo appears relatively uniform compared to timber. The bamboo culm consists of nodes and internodes, the intermediate sections. The thin partitions in the nodes are called diaphragms. The lengths of the internodes as well as the thickness of the culm walls differ considerably between the different bamboo species. Fig. 2 shows the wide variety of different bamboo kinds, the culm arrangement with nodes and internodes and an intersection visualized by a computer tomography illustrates the inner structure in an internode section. The bamboo culm consists of parenchyma cells, fibres and vascular bundles. The cross-section shows the dark vascular bundles with their fibre agglomerates in the parenchyma tissue. All bamboo species show an analogous pattern in the distribution of their cells. Bamboo structures have a comparable characteristic, forms and sizes may vary from species to species. The advantage of bamboo is its great hardness. The origin, moisture content, age and diameter of the cane have a great influence on the material properties [6, 7]. Very similar to wood, bamboo
is highly inhomogeneous, owning characteristic patterns and variable structures. Humidity significantly affects its properties, and moisture loss can lead to cracking.

<figure><img src="image_3.png" /><figcaption>Figure 2. Bamboo variety (a) and culm structure (b), variations of inner hulm structure (c) [7]</figcaption></figure>

**Glas fibre reinforced polymer (GFRP)** is a composite material and consists of a polymer matrix (polyester or epoxy resin) and glass fibres of different sizes, i. e. approx. 1 mm up to more than 50 mm length. The glass fibres serve as reinforcement and give the material high strength and stiffness combined with low weight. The material is also characterised by its corrosion resistance and good thermal insulation. Due to its mouldability it can be brought into different shapes and enables complex designs. Generally, GFRP have similar heat conduction velocities compared with timber. The material behaves relatively isotropically. Other important aspects for analysing GFRP material are its transparency properties.

**Adhesives** (as auxiliary material) glue components together in a materially bonded manner. They are classified into physically setting and chemically reacting adhesives. The interaction of surface adhesion and internal cohesion of adhesives results in a force-transmitting effect. The structure of the parts to be joined remains unchanged. Different types of adhesives are used depending on the timber material and its intended use. Accordingly, the mechanical properties and thicknesses of the adhesive layers vary. The adhesive used to join the considered composite components was the one-component polyurethane-based glue OTTOCOLL® P84.

### 2.1.2 Novel composite and joining concepts

**Composites** of renewable materials are already widely used as for example building material. Cross laminated timber (CLT) is a typical material for load-bearing constructions. It is a layered structure of solid timber panels in different orientations. The size and thickness of the panels vary widely depending on the application. COMBOO composites are a new type of sandwich structures built up with a bamboo-based core material consisting of a honeycomb-like arrangement of bamboo ring sections covered with various top and bottom layers.

<figure><img src="image_4.png" /><figcaption>Figure 3. COMBOO structure (a), central bamboo layer in COMBOO composites (b), section of COMBOO material with cover layers of timber or GFRP (c, d), round structures with GFRP “bamboo design” (e)</figcaption></figure>

Fig. 3 shows the structure of a COMBOO composite: the inner bamboo comb layer and the section of a 3-layered COMBOO composite with two cover layers of timber or GFRP. The use of different face layers thus results in stiffer or more ductile composites. Results of previous tests with the novel bamboo honeycomb arrangement in combination with timber show that very good mechanical properties can be achieved compared to conventional materials. Since
GFRP cover layers make transparent structures possible, this material is gaining a high level of attractiveness for decorative purposes. [1, 2]

Joints are important to be able to produce components to build up structures, etc. In this paper novel friction welded samples are considered as shown in Fig. 4 for timber and bamboo. For such samples initially determined toughness showed good results compared to ordinary connection types like glued joints. Currently, up to 60% of the final strength of timber-based materials can be achieved. The advantage of friction-welded joints is the absence of chemical, possibly toxic substances making sustainable use difficult or impossible. Due to the lignin present in both bamboo and timber, they can be joined together without adhesives. [8]

<figure><img src="image_5.png" /><figcaption>Figure 4. Friction butt welding of exemplary timber bars: samples (a), clamped in lathe (b), bonding (c)</figcaption></figure>

## 2.2 Defect characteristics and NDT inspection task definition

### 2.2.1 Typical defect appearances

For the later use of the materials, it is important that no defects impair the properties. Defects must be monitored in production processes or during life cycles. For this purpose, particular NDT applications are essential. Specific defect patterns that influence mechanical properties, e. g. reduce strength, are shown in Fig. 5. For the different composites there are horizontal delamination from the cover layers to bamboo core material and cracks inside the base material of the cover layers. There can be also cracks in the bamboo core material, which occur as a result of shrinkage when the moisture content in the base material falls below a critical level.

<figure><img src="image_6.png" /><figcaption>Figure 5. Failure patterns in COMBOO composites: cracks in base material timber and delamination between bamboo and timber (a), delamination between bamboo and GFRP (b), cracks in bamboo core material (c)</figcaption></figure>

Fig. 6 shows cross-section surfaces of considered timber and bamboo (or in combination) friction welded samples after tensile testing. Possible adhesion defects arise in the fracture plane area, where the connection is partially interrupted.

<figure><img src="image_7.png" /><figcaption>Figure 6. Friction welded bars timber-timber / bamboo-bamboo / bamboo / timber (a), cross-section surface after tensile break of two timber species and bamboo (b, c) and cracks in bamboo (d)</figcaption></figure>

### 2.2.2 Inspection task definition

Controlling the identified irregularities, combined with the specific material properties of the renewable materials (e.g. low thermal conductivity and high anisotropy) and their composite structures (e.g. restricted accessibility, difficult thickness ratios), is mostly unfamiliar and poses new challenges. To find appropriate NDT solutions for later quality control processes, the
intended NDT inspection tasks are derived from previous considerations and illustrated schematically in Tab. 1. The dimensions of the defect patterns as well as the composite structure proportions can be scaled up or down.

<table><caption>Table 1. "Objected" variants of inspection tasks</caption><thead><tr><th></th><th>Geometry</th><th>Material(-combination)</th><th>Defects</th></tr></thead><tbody><tr><td>1</td><td></td><td>COMBOO composite<br>timber / bamboo + adhesive</td><td>- cracks<br>- delamination</td></tr><tr><td>2</td><td></td><td>COMBOO composite<br>GFRP / bamboo</td><td>- cracks<br>- delamination</td></tr><tr><td>3</td><td></td><td>resistance butt weld<br>timber / timber</td><td>- no (loss in) adhesion<br>- cracks</td></tr><tr><td>4</td><td></td><td>resistance butt weld<br>bamboo / bamboo</td><td>- no (loss in) adhesion<br>- cracks</td></tr><tr><td>5</td><td></td><td>resistance butt weld<br>bamboo / timber</td><td>- no (loss in) adhesion<br>- cracks</td></tr></tbody></table>

This paper will focus on exploring options for detecting cracks in timber and in bamboo, both as it is used in COMBOO composite material and as base material for resistance butt welding. Another key aspect is to characterise the bonding quality of the resistance butt welds, not least to receive required information for further development.

## 2.3 NDT methods & Experiments

### 2.3.1 NDT method preselection

There is a variety of non-destructive testing methods, each with its own specific capabilities and limitations. Considering the testing principles shows which methods are fundamentally unsuitable for the materials and defects in question, and which methods should be examined further, as they have a potential to solve the listed inspection tasks. A preliminary evaluation of the capabilities of NDT methods, as presented in Tab. 2, taking also recent research findings into account, leads to the preselection of NDT methods for further investigation.

<table><caption>Table 2. Evaluation of NDT methods for considered renewable raw materials</caption><thead><tr><th>NDT Method</th><th>Principle suitable?</th><th>Remarks</th></tr></thead><tbody><tr><td>Visual Testing</td><td>-</td><td>no inner failures detectable</td></tr><tr><td>Ultrasonic Testing</td><td>(+)</td><td>previously used for humidity or decay detection</td></tr><tr><td>Acoustic Testing</td><td>(+)</td><td>conceivable for crack monitoring</td></tr><tr><td>Penetration Testing</td><td>-</td><td>not for highly absorbing materials</td></tr><tr><td>Magnetic Particle Testing</td><td>-</td><td>only for permeable materials</td></tr><tr><td>Eddy Current Testing</td><td>-</td><td>no electric current inducible</td></tr><tr><td>X-Ray Testing</td><td>+/-</td><td>mostly limited to detect volume failures</td></tr><tr><td>Computed tomography</td><td>+</td><td>high resolution, cost intensive, long test durations</td></tr><tr><td>Thermography</td><td>+</td><td>high variety of testing modifications</td></tr><tr><td>Terahertz Testing</td><td>(+)</td><td>decreasing geometric resolution</td></tr><tr><td>Microwave Testing</td><td>(+)</td><td>even less precision, but higher penetration depth</td></tr></tbody></table>

More insights into why ultrasonic, terahertz and microwave testing are assessed as potential test methods are given in a previous paper [3]. Due to the characteristic noise development of timber and bamboo while cracking, acoustic testing can become an attractive testing method for e. g. health monitoring. The assumed opportunities especially of ultrasound, terahertz- or
microwave testing for e. g. finding defects in adhesion after friction welding will be subject of follow-up investigation. To gain the basic information, needed for a targeted NDT development, the bonding and defect characteristics will firstly be clarified here with computed tomography. Computer tomography often plays an important role, when a high precision is needed or no other NDT is available. Because of its high costs and long test duration it is less attractive for production control and is used more as reference method. With its high variability of heat excitation, thermography methods are expected to show manifold testability for renewable materials, their composites and joints. Abilities of active thermography have already been demonstrated for delamination of thin glued surface layers on timber [9]. Although findings in the literature do not refer directly to the materials and structures considered here, results with comparable materials can provide assistance in a certain way. The usability of passive thermography for online testing during the production process, which has become apparent, will be evaluated further in other approaches.

### 2.3.2 Experimental Setup

The test setup for active thermography includes the infrared bolometer camera a T1020® microbolometer from FLIR Systems to record the surface's infrared radiation. Excitation sources base on manifold physical effects to generate heat flows in the specimens. The following optical, convective and ultrasonic heat sources are used:

<figure><img src="image_8.png" /></figure>

halogen lamp Hensel C-Light 1000, max. 1.000 W

<figure><img src="image_9.png" /></figure>

flash light Hensel EH Pro 6000, max. 3.000 W

<figure><img src="image_10.png" /></figure>

laser Edevis LTvis 200 FL, 1070 nm, max. 200 W

<figure><img src="image_11.png" /></figure>

hot air dryer Bosch GHG 23-66, max. 2.300 W

<figure><img src="image_12.png" /></figure>

ultrasound generator Sonikks Sonobonder US 6015, max. 15.000 W

The computer tomography setup for analysing the resistance butt welds is shown in Fig. 8, using a phoenix nanotom® m from GE Sensing & Inspection Technologies GmbH.

<figure><img src="image_13.png" /><figcaption>Figure 8. Friction welded sample in CT setup with X-ray source and a digital detector</figcaption></figure>

## 3. Results and discussion

### 3.1 Active thermography

The exploration of various thermographic excitation sources to detect cracks in timber or bamboo led to new promising approaches for thermographic testing. Potentials are illustrated in the following by instantaneous recordings of corresponding infrared sequences, serving as a basis for the assessment which methods should be pursued further continuing with the optimisation of source modulation and evaluation methods. The mechanical excitation with ultrasound can indicate cracks in timber base material (Fig. 9): Cracks can be heated selectively and be identified by resulting hot spots. The limits of recognisability cannot be specified yet and must be referred to very specific circumstances, e. g. outer and inner timber structure. Homogenous heating of larger areas in timber generates a (more or less) homogenous heat flow, while its deviations can make cracks visible. Minimum detectable defect sizes are dependent
on the anisotropy of the timber species. Based on the present findings a limit of 5-10 mm defect size in an area of approx. 50x50 mm is estimated.

<figure><img src="image_14.png" /><figcaption>Figure 9. Crack detection in timber with ultrasound excitation: selective heating (a), heat flow deviation (b)</figcaption></figure>

Cracks in bamboo can be seen after optical laser line heating. They induce deviations in the regular heat flow without cracks. Fig. 10 shows indications in infrared images after a rotated excitation. The investigation will be continued with different defect sizes to determine the detection sensitivity.

<figure><img src="image_15.png" /><figcaption>Figure 10. Crack detection in bamboo with optical excitation: heat flow deviation after laser scan heating</figcaption></figure>

To detect cracks in bamboo as core material in COMBOO composites optical heating has emerged as promising excitation method. Sensitivities to detect structure properties in general for COMBOO composites with 2 mm thick timber outer layers can be estimated from Fig. 11. The presence of the bamboo rings is clearly visible. Bamboo ring cracks with openings from approx. more than 3 mm can be determined. The areas of delamination are not recognisable. They depend strongly on the present gap distances, which need to be investigated in more detail.

<figure><img src="image_16.png" /><figcaption>Figure 11. Backside of COMBOO with timber (a), thermal image after flashlight heating in reflection mode (b)</figcaption></figure>

The analysis of COMBOO composite with GFRP layers was carried out with homogenous convective hot air, optical flashlight and halogen light excitation, in transmission or/ and reflection mode, show at least equivalent contrasts to map the inner core material structure. Supplementary FEM simulations pointed out, that grades of transparencies have a significant influence to the results. [4] As the effects in the measurement are suspected to be caused by partial transparencies, analyses with different cover layers and flashlight excitation in reflection mode were added to get a better understanding of heating effects and heat propagation (Fig. 12). The results clarify effects depending on the transparency of the outer GFRP layers. Partially direct heating of the bamboo rings beneath the GFRP surface layer while optical excitation, e. g. after 0,5 s, can overlap in such a way, that contrasts can even be cancelled out for moments, e. g. after approx. 2 s. The ring in the infrared images, caused by bamboo material, turns from hot into cold. The other two only show cold rings, i. e. a heat removal, at different times depending on different heat conduction velocities.
<figure><img src="image_17.png" /><figcaption>Figure 12. Infrared images after flashlight heating in reflection mode with different top layers: 1. CFRP (non-transparent), 2. GFRP (partial-transparent), 3. Aramid (non-transparent) - after 0,5 s (a) and 2 s (b) and 6 s (c)</figcaption></figure>

## 3.2 *Computed tomography*

The recorded CT images in Fig. 12 allow precise insights into the internal structure of the butt welded samples of timber, bamboo and the combination of both. The timber-timber sample shows the strongly oriented timber structure with microfibrils and fibres. Thicknesses and orientations of year rings become clearly visible. In a similar precision the different components of bamboo can be figured out in the bamboo-bamboo sample, that is demonstrating its high anisotropic properties as it is the case for timber. In each of the joints, the compressed lignin is responsible for the bonding quality. Defects are visible in these adhesion layers. The analysis of the timber-timber adhesion layer shows relatively even, rectangular shape of the bonding zone. While the bonding of bamboo-bamboo material shows more rounded shapes. The combination of both timber-bamboo results in an intermediate stage. Cracks are also clearly visible in all material combinations. These properties can be important and must be kept in mind for ongoing development with other NDT methods.

<figure><img src="image_18.png" /><figcaption>Figure 16. CT images of resistance butt welds a) timber-timber, b) bamboo-bamboo, c) bamboo-timber samples</figcaption></figure>
## 4. Conclusion and outlook

Exploring different excitation sources for active thermography in the NDT inspection tasks opens up new opportunities for quality control solutions, particularly for COMBOO composites that include renewable materials as timber and bamboo. The positive results obtained by using ultrasound or laser excitation for crack detection will be further investigated to determine the specific detection sensitivities. Also, the use of halogen and flashlight excitation for crack testing of the bamboo core material of COMBOO composites will be optimised, identifying the most suitable heat excitation modulation and infrared signal evaluation modes. Combining the practical results with initial FEM simulations from parallel research has already led to a deeper understanding of the thermal effects in COMBOO layers, that are taking place while heat generation for active thermography testing.

As thermography is not supposed to be a suitable method for testing adhesion properties and failures in resistance butt welds, computed tomography remains the best NDT testing option for the moment. The application of CT for timber or bamboo materials enables high penetration depths into the interior of the material and excellent resolution capabilities down to the nanometre range. Detailed information about the resistance butt weld properties and failures, e. g. small detachments in the adhesion zones, allow predictions about the bonding quality. Due to long testing durations, depending strongly on the desired resolution, as well as its cost-intensive character caused by the need for extensive testing technology and peripherals, CT is less attractive for series quality control. Thus, research for finding a more efficient NDT solution is still needed. However, the CT results provide valuable insights that can support and shorten the development of alternative NDT solutions. CT can be used as a reference method, that can describe the current properties non-destructively. To confirm their relationship with mechanical or bonding strengths further test series in comparison with tensile tests are needed.

Other alternative NDT methods, expected to have promising testing abilities, will be further evaluated. Approaches with terahertz or microwave testing are to be carried out according to [10, 11] or with guided ultrasound waves according to [12] for resistance butt welds or also for the COMBOO composites. The potential of acoustic testing for timber and bamboo material will be tested due to the expected opportunities for health monitoring applications.

## 5. Acknowledgements

We thank the ‘Institut für angewandte Forschung Berlin (IFAF)‘, the ‘Bundesministerium für Wirtschaft und Klimaschutz (BMWK)‘ and the ‘Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke e. V.“ (AIF)‘ for funding the research projects ‘MaiVe‘, ‘CCLT‘ and for supporting the development of the friction welding machine. We also thank edevis GmbH for supporting the laser line excited active thermography measurements.

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