Jan Ivens

KU Leuven | ku leuven · Department of Metallurgy and Materials Engineering

Natural fibre-based materials offer various advantages compared to synthetic fibres, however their applications are limited mainly due to their hygroscopic properties, which are affected by their chemical composition, microstructure and the porosity of the plant cells of which the fibre is composed. Therefore, this work investigates the hygroscopic...

The translaminar fracture toughness is a key property that governs the damage tolerance and notch sensitivity of fibre-reinforced composites. Compact tension tests were performed to investigate the translaminar fracture toughness of composites reinforced with three types of high-performance polymer fibres: polyarylate (PAR), polybenzobisoxazole (PB...

Despite the higher specific mechanical properties and the lower density of polymeric foams, these materials present cumulative damage behaviour that implies in the second and successive impacts, their mechanical properties decrease drastically. However, cork and cork agglomerates have the ability to absorb multiple impacts so they could be a more s...

Cork and cork agglomerates could be suitable replacements for petroleum-based polymeric foams due to their similar internal structure of cells and grains. Additionally, cork products have a renewable origin and are recyclable. Despite these notable properties, few studies have analysed the mechanical properties, especially the specific properties,...

Expanded polystyrene (EPS) is a material that is widely used in energy absorbing applications, especially in helmets, despite its non-renewable origin. Cork and its derivatives, however, are proposed as a substitute for polystyrene foam (EPS) due to their renewable origin and their easy recyclability. In spite of the low-environmental footprint of...

The paper investigates the split-disk test methodology for measuring mechanical properties of a filament wound composite. Hoop strain is measured by three-dimensional (3D) Digital Image Correlation (DIC) on the outer surface of ring specimens. A finite element model (FEM) is employed to investigate the strain distribution along the circumference an...

Objective Medical data has lead to the common understanding that bicycle helmets need to be improved to better protect against brain injuries resulting from rotational acceleration. Although many different technologies exist for reducing rotational acceleration during impacts, the lack of an official testing standard means that their evaluation is...

Expanded polystyrene (EPS) foam is a material, frequently used in a range of applications for its good energy absorption, thermal insulation, durability, acoustic absorption and low thermal conductivity. In this wide range of applications, the material undergoes a variety of loads that can extend from static to dynamic. When it comes to crush appli...

The application of natural fibers as reinforcement material in composites can make an important contribution toward a more sustainable bio-based economy. To use these natural fibers, they need to be released from the stem. Traditionally, this is realized by dew retting. However, there are some disadvantages to this retting procedure. Due to changin...

The effect of treatment of flax with strategic enzyme combinations on the ease of fiber extraction and the chemical fiber composition is reported in this study. To contribute to the increasing demand for bio-based and sustainable materials, it is of great importance to develop optimal enzyme formulations which can replace the yet poorly controlled...

The application of enzymes as alternative to dew retting of flax was studied in correlation to the characteristics of composites reinforced with these natural fibers. Fiber fineness and mechanical properties of biocomposites were evaluated. Furthermore, moisture absorption by biocomposites was studied and fracture surfaces were investigated using S...

The influence of moisture on the mechanical properties of bamboo fibre reinforced composites was assessed both for static and hygroscopic cyclic conditioning. Static conditioning revealed that bamboo fibre reinforced composites show a smaller decrease in Young's modulus with moisture content than other natural fibre reinforced composites such as fo...

Four Phyllostachys species grown in Europe were analysed with the aim of extracting long bamboo fibres for composite applications. The morphological characteristics of the plant were determined and compared to the species grown in their native region. Though the species tend to grow smaller, the Phyllostachys vivax showed similar Young’s modulus as...

Enzymatic treatment of flax is gaining more interest as a promising alternative for dew retting, which is known for its dependence on weather and climate. Therefore, the effect of enzymatic treatments of flax on the effectiveness of fiber separation from each other and chemical fiber composition was investigated in this study. Chemical composition...

More than 1200 bamboo species are known; therefore, the question arises which species are suited for composites and which microstructural characteristics of the plant are decisive. Three different bamboo species are considered, each of two ages. Technical fibre tensile tests and impregnated fibre bundle tests were performed to characterize the fibr...

For industrial applications such as construction, sports or aviation the interest in novel 3D woven structures grows. Current production routes have typical downsides such as low production speed, need for dedicated looms for weaving one specific fabric type or time consuming modifications to the weaving loom. In contrast, the present research show...

The strain-rate sensitivity and stress relaxation of self-reinforced polypropylene (SRPP) and hybrid carbon fibre/SRPP composites were analysed under bending loads. For this purpose, three point bending static tests were performed at room temperature and displacement rates of 200, 20, 2, 0.2 and 0.02 mm/min. Tests of stress relaxation were also car...

Enzymes are highly advantageous compared to dew retting to reach fibers of high and consistent quality. However, no unambiguous insights have been retained from the research, i.e. lacking a clear directive of which enzyme activities are strictly needed. Methods for evaluating enzymatic retting should be standardized, with characterization of chemic...

A new type of fiber reinforced filament materials for Fused Deposition Modeling applications has been developed. Polylactic acid (PLA), compounded with two types of plasticizer, is reinforced with bamboo and flax fibers. The fiber fractions are characterized by measuring their length (l) over diameter (d) before and after compounding so that the ef...

The paper presents an overview of cases in which the analysis of the internal structure and mechanical properties of fibre reinforced composites is performed based on the micro-computed X-ray tomography (micro-CT) reconstruction of the composite reinforcement geometry. In all the cases, the analysis relies on structure tensor-based algorithms for q...

The mechanical properties of bamboo technical fibre, from the species Guadua angustifolia, have been studied showing values of strength up to 800 MPa and E-modulus up to 43 GPa, proving their adequate tensile properties that make this natural fibre suitable as reinforcement in composite materials. To fully explore the good mechanical properties and...

Expanded Polystyrene (EPS) foam material is widely used as an energy absorption engineering material. Its compression behaviour, both quasi-statically and dynamically, has been studied widely. However, its mechanical behaviour under combined compression-shear loading is poorly understood due to the difficulty of performing such tests. A novel test...

In Part I, we performed quasi-static compression-shear tests on expanded polystyrene (EPS) foam. In the present study, dynamic combined compression-shear loading tests were performed on different densities of EPS foam by integrating the previously developed loading rig with a lab-built drop weight tower. Two impact loading wedge angles (45° and 60°...

Introduction This paper describes the methodology to obtain and analyse data in composite reinforcement fabric compaction tests in dry and wet condition on a standard electromechanical test machine using a non-direct thickness measurement method. Diverse examples of dry fabrics compression tests using non-direct thickness measurement methods can be...

The compression behavior of a material cannot be accurately determined if the machine compliance is not accounted prior to the measurements. This work discusses the machine compliance during a compressibility test with fiberglass fabrics. The thickness variation was measured during loading and unloading cycles with a relaxation stage of 30 minutes...

The scalp plays a crucial role in head impact biomechanics, being the first tissue involved in the impact and providing a sliding interface between the impactor and/or helmet and the skull. It is important to understand both the scalp-skull and the scalp-helmet sliding in order to determine the head response due to an impact. However, experimental...

Interlayer hybrid carbon fiber/self-reinforced polypropylene composites possess a rare combination of lightness, stiffness and ductility, but they do suffer from a catastrophic stress drop when the carbon fiber layer fractures. To promote a gradual failure of the hybrids, we introduced discontinuities in the carbon fiber layer by partially cutting...

Oblique impact is the most common accident situation that occupants in traffic accidents or athletes in professional sports experience. During oblique impact, the human head is subjected to a combination of linear and rotational accelerations. Rotational movement is known to be responsible for traumatic brain injuries. In this article, composite fo...

Anisotropy in foams generally originates from cell elongation in a certain direction. In this study, a composite concept is utilized to create anisotropy in foams at macro level. For this, layered composite foam is proposed by combining discrete layers of expanded polystyrene foam foam with different densities. The layers are positioned in parallel...

This study evaluates the efficiency of the extraction of enzymatically treated flax fibers for application in composite materials. Three extraction parameters were introduced. Fiber Efficiency (FE) represents the yield of long fibers extracted after enzymatic treatment. Time Efficiency (TE) is introduced in order to evaluate the ease of extraction....

Expanded Polystyrene (EPS) foam material is widely used as an energy absorption engineering material. Its compression behaviour, both quasi-statically and dynamically, has been studied widely. However, its mechanical behaviour under combined compression-shear loading is poorly understood due to the difficulty of performing such tests. A novel test...

The viscoelastic behaviour of self-reinforced polypropylene (SRPP) was investigated under bending loads. For this purpose, an overfed Twill 2/2 weave pattern was used, which in the hot compacted form is known under the trade name of Curv®. The areal density of the SRPP fabric is 130 g/cm³ and the density of this grade polypropylene is 0.92 g/cm³. T...

This paper performs a systematic study of the effect of various unidirectional fibre patterns, ranging from different overlapping lengths of adjacent fibre bundles to a complete randomization of the position of individual fibre ends. The study is benchmarked with the mechanical behaviour of a fully continuous unidirectional bamboo fibre-epoxy compo...

Rotational acceleration of the head is known to be the cause of traumatic brain injuries. It was hypothesized that by introducing anisotropy in a foam liner in head protection applications, for example, protective helmets, rotational acceleration transmitted to the head can be further mitigated. Therefore, composite foam with a cylinder/matrix conf...

Three different bamboo species of two different ages are investigated in this research. The bamboos were sourced from South America, Europe and Southeastern Asia. Fibre tensile tests and impregnated fibre bundle tests were performed to characterise the mechanical properties. The results obtained from both tests were in agreement. Fibre stiffness va...

Polymeric foams are extensively used in applications such as packaging, sports goods and sandwich structures. Since in-service loading conditions are often multi-axial, characterisation of foams under multi-axial loading is essential. In this article, quasi-static combined shear-compression behaviour of isotropic expanded polystyrene foam and aniso...

Although current standard bicycle helmets protect cyclists against linear acceleration, they still lack sufficient protection against rotational acceleration during oblique impact events. Rotational acceleration is correlated with serious traumatic brain injuries such as acute subdural haematoma and thus should be minimized. This study proposes usi...

Rotational acceleration experienced by the head during oblique impacts is known to cause traumatic brain injuries. It is hypothesized that shear properties of a foam layer, used for head protection (e.g., protective helmet liners, headliners in cars) can be related to the extent of rotational acceleration transmitted to the head. Furthermore, it is...

A method is presented to test fibres in tension using direct strain measurement. This eliminates the need to test the fibres at multiple gauge lengths to correct for machine compliance, reducing the number of samples. Additionally, fibre slippage can contribute to the underestimation of the stiffness since this is not considered in the correction p...

Acute subdural hematoma (ASDH) is a type of intracranial haemorrhage following head impact, with high mortality rates. Bridging vein (BV) rupture is a major cause of ASDH, which is why a biofidelic representation of BVs in finite element (FE) head models is essential for the successful prediction of ASDH. We investigated the mechanical behavior of...

A problem that arises when performing stereo digital image correlation in applications with large out-of-plane displacements is that the images may become unfocused. This unfocusing could result in correlation instabilities or inaccuracies. When performing DIC measurements and expecting large out-of-plane displacements researchers either trust on t...

This paper proposes a methodology to determine wall thickness distributions in thermoformed products derived from in-situ surface strain measurements obtained with stereo digital image correlation (DIC), under the assumption of material incompressibility. Wall thickness equations are derived for the Green-Lagrange, Hencky, Biot, logarithmic Euler-A...

The presented work investigates the tensile properties as well as the sorption behaviour and chemical composition of European bamboo fibres. Single fibre test results show that there might be an influence of the time of harvesting on the tensile properties after fibre extraction. The sorption behaviour is in line with other natural fibres, taking t...

The final position of carbon nanotubes (CNTs) in a fiber-reinforced composite and its mechanical properties can be influenced by the dispersion quality of CNTs in the resin. This topic is investigated here on the example of a woven glass fiber/epoxy composite. Two different localization states of CNTs in the composite are achieved by choosing matri...

In this paper, a new anisotropic material concept namely composite foam is proposed as an alternative to next generation helmet liners which can potentially reduce head rotational accelerations. The Layered Composite foam concept comprises of foam layers with different densities which are glued to each other in a parallel or series configuration. I...

Numerical simulation tools for the thermoforming of unfilled thermoplastic polymers already exist for a while, but are seldom used to full extent in industry. When it is used, it is mostly only for comparative studies and prediction of relative wall thickness. One of the major reasons is the difficulty to correctly calibrate and integrate all neces...

Cyclists during bicycle traffic accidents, are prone to oblique impact which leads to rotational accelerations. Rotational acceleration is known to cause significant brain injuries, and should be minimized. Foam materials inside bicycle helmets undergo a combination of shear and compression loads during oblique impact. Therefore, developing an appa...

Bamboo culms consist of a nodal structure in which the fibres are as long as the internode length (between 15 and 30 cm), limiting the length of the reinforcing fibres. The implementation of these fibres to reinforce polymers in large-scale applications, leads to the production of discontinuous composites in unidirectional configuration. The influe...

The specific stiffness of flax fibres is comparable to that of glass fibres. However, the application of flax fibres in composites still poses two important problems. The orientation of the fibres in the composite is not impeccable and the fibre-matrix adhesion is too weak. This research focuses on the former. To produce a continuous yarn for the t...

Bamboo culms consist of a nodal structure in which the fibres are as long as the internode length (between 15 and 30 cm), limiting the length of the reinforcing fibres. The implementation of these fibres to reinforce polymers in large-scale applications, leads to the production of discontinuous composites in unidirectional configuration. The influe...

Cyclists during bicycle traffic accidents, are prone to oblique impact which leads to rotational accelerations. Rotational acceleration is known to cause significant brain injuries, and should be minimized. Foam materials inside bicycle helmets undergo a combination of shear and compression loads during oblique impact. Therefore, developing an appa...

Although current standard bicycle helmets provide protection against so-called linear acceleration, they cannot deliver sufficient protection against rotational acceleration during oblique impact events. These events can cause severe head injuries and the rotational accelerations need to be minimized. We pursue this goal by developing anisotropic f...

Heavy gauge thermoforming on an industrial scale often involves small production series. The cost of the mold then constitutes a considerable part of the production cost. Besides this financial aspect, thermoforming is primarily an experience and trial and error based technology, resulting in suboptimal processes and lengthy optimization procedures...

The homogeneity of the wall thickness (distribution) is considered to be the most critical parameter in the quality assessment of a thermoformed product. Numerous previous studies have characterized the thickness distribution by means of manual discrete tactile measurements. Such approach is slow, operator dependent and only gives results on specif...

In the current study, the effect of the thickness and the foam density in three-dimensional integrated woven sandwich composites on quasi-static mechanical properties under three-point bending was investigated. Bending modulus and core shear modulus were determined by subjecting the samples, which were cut with varying span lengths according to the...

In the current study, the effect of the thickness and the foam density in three-dimensional integrated woven sandwich composites on quasi-static properties was investigated. For this purpose, produced samples were subjected to uniaxial flatwise compression tests and their compression strength and moduli were determined. Obtained results were optimi...

Current mold technology, used in thermoforming processes such as vacuum forming, often lack in material and energy efficiency during their lifecycle. These molds, mostly milled out of solid blocks of al-uminium, take long to produce and come at a high cost. Moreover the tempering of the vast mass of these molds requires more energy than that of she...

Polymeric matrix composites are susceptible to degradation and material properties changes if subjected to low-temperature environmental conditions. This paper attempts to present a study on effective coefficient of thermal expansion for various hybrid carbon fibers/glass fibers polymeric composite structures previously subjected to low-temperature...