ABSTRACT

This work is aimed at impact damage characterisation by transient thermography on ®Twintex, an E-glass/polypropylene commingled laminate, used mainly in automotive applications. Low velocity impact damage was monitored both on Twintex specimens and on a Twintex automotive component (side intrusion beam). The results allowed the comparison between impact properties and damage mode of three different laminates. On the automotive component, a laminate configuration for the production of a side intrusion beam was selected, that presented the best impact performance.

INTRODUCTION

The interest of commingled laminates, reinforced with E-glass and polypropylene fibres, lies in particular in the possibility of introducing high volume of reinforcement (up to 75% in weight) in the polymer matrix, so to achieve a high resistance to impact damage with simple manufacturing process, such as compression moulding [1-2]. When moulding parameters are optimised, void content on the material can be very low (up to approximately 0.3%), so that impact properties can be higher than for other E-glass fibre woven composites [3]. However, the cost of Twintex composites is still quite high, compared to other E-glass fibre reinforced thermoplastic matrix composites, such as GMT (glass mat thermoplastics). However, in automotive applications use of Twintex may represent an asset, because of its low weight, coupled with high toughness and impact properties.

In this case, the use of NDE methods is of particular interest to measure and characterise impact-damaged regions. Pulse thermography has been used for a long time for damage characterisation in composite materials, in cases where a fast, reliable and non-contact method is required [4-7]. In the specific case of impact damage on composites, pulse thermography can offer the possibility of damage monitoring, detecting defects up to around 1.5 below the surface of the laminate, a limitation which is less critical in case of low velocity impact, where an accurate description of damage may be possible [8-9]. On the other side, low velocity impact damage i.e., not leading to the complete failure of the structure, is important for a range of automotive composites, including bumpers and side intrusion beams, where it indicates damage tolerance of the component.

EXPERIMENTAL

Materials

Three different commingled E-glass fibre/ polypropylene laminates, all with 60% wt. fibre content, have been tested (details in Table 1) These are: a balanced laminate with the same fibre content in the two planar directions (Twintex 1:1), a directional laminate for preferential use in the direction, where the fibre content is four times higher than in the other planar direction (Twintex 4:1) and a balanced laminate with addition of 1-2% through-thickness fibres, able to confer a higher impact resistance (3-D Twintex).

Laminate	Weave structure	Fabric weight (g/m²)	Thickness per layer (mm)	Counts/cm
Twintex 1:1	Balanced twill	1870	1.25	2
Twintex 4:1	Directional twill	935	0.63	2
3-D Twintex	Balanced plain	1300	0.8	1.3
Table 1 Materials characteristics

All the laminates have been compression moulded applying a 3 MPa pressure at 200°C for 70 seconds with an oven-press transfer time not exceeding 10 seconds [10-11]. In contrast, the components (Figure 1) have been obtained from laminate sheets of 2.2 m^², preconsolidated at 220ºC for an hour, applying a pressure of 0.11 MPa. The number of layers preconsolidated in a sheet was selected so to give a thickness of around 3.5 mm, so that the component thickness (6.5-7 mm) was obtained by moulding together two preconsolidated sheets. The component material was preheated for 4 minutes to reach a temperature around 190°C, and then consolidated under a 6 MPa pressure in a mould heated at 60ºC. The oven-press transfer time was around 20 seconds. The component has been produced in four configurations: 4:1 Twintex 4:1, 3-D Twintex, 1:1 Twintex 0°/90° and 1:1 Twintex ±45°.

Fig 1: Prospect of the component

Impact

Fig 2: IR thermography set-up

Square plates of the three laminates, dimensions 80x80x3.5 mm, have been impacted with energies between 15 J and their penetration energy, using a Rosand IFW5 falling weight tower. The samples were clamped with a pressure of 0.15 MPa under a 40 mm diameter anvil, and were impacted with a 12.7 mm diameter striker, with a global mass of 25.65 kg. In impact tests on components, these were simply supported between two supports at a mutual distance of 750 mm, and hit from a height of 3.25 m with a hemispherical striker of diameter 75 mm (total mass 19.9 kg, corresponding to an impact energy of 637 J).

IR thermography

Impact-damaged areas were measured using an Agema Thermovision 900 SW thermal camera, with spectral response 2-5.4 micron e sensitivity 0.1ºC. The camera was using a 25 mm lens, allowing a minimal distance of observation of 480 mm. Thermograms were acquired during the first 30 seconds transient following a three-seconds pulse, obtained with a 500 W infrared lamp (Figure 2).

RESULTS

Impact on the laminates

The advantages of transient thermography with respect to the visual inspection of impact damage are evident in Figures 3a and 3b, referred respectively to a Twintex 1:1 and a 3-D Twintex laminate. In particular, thermograms allow one to discern and quantify regions at different level of damage in the impact area. The accuracy of the measurement is limited by the pixel resolution (one pixel = 0.4 mm).

Fig 3a: Impacted area photo and corresponding thermogram (1:1 Twintex)

Fig 3a: Impacted area photo and corresponding thermogram (3-D Twintex)

The three laminates have shown different characteristics for impact damage absorption, as is indicated by Figure 4, where laminates all impacted at 20 J are represented. In particular, 1:1 Twintex shows a quasi-circular damage area, while 3-D Twintex, having a coarser weave, has a delamination area oriented in the higher resistance direction. The coarser weave of 3-D Twintex has contributed to stop the progression of delamination, while the through-thickness fibres are capable of effectively containing the damage under the surface of the laminate. The effect of impact on the laminates is summarised in Table 2.

Fig 4: Damaged areas for impact at 20 J

Material	3-D Twintex	4:1 Twintex	1:1 Twintex
Through thickness damage	Low	High	Medium
Surface delamination	High	Medium	Low
Table 2 Effect of impact on the different laminates

On 4:1 Twintex, the damage is increasingly oriented in the prevalent fibre direction with growing impact energy, as shown in Figure 5 (35 J impact). The region where the delamination initiates corresponds often to zone with a larger presence of interlaminar voids [12]: as a consequence, the directional laminate is more affected than the balanced ones by sub-surface defects. Variation of impact areas dimension with growing impact energy is shown in Figure 6, where it can be observed as for 1:1 Twintex delamination area has a quasi-circular shape, even for energy values close to penetration (values of penetration energies in Table 3). This is true also for 3-D Twintex, where in addition preferential propagation of delamination in presence of weave crossovers can be noted. This can indicate the non-perfect moulding of this laminate, due most probably to non-uniform material preheating, as shown also by the larger scattering of penetration energy values in Table 3.

Fig 5. Thermography of a 4:1 Twintex laminate impacted at 35 J and fibre orientation in the impacted region	Fig 6: Delamination areas for impact energies between 15 and 45 JDamage areas measured from thermograms (average on five samples per laminate and per impact energy)
	Laminate Penetration energy (laminate thickness 3.5 mm.) (J) 1:1 Twintex 49.9±9.8 4:1 Twintex 62.8±10.7 3-D Twintex 51.1±12.6 Table 3 Penetration energy of the laminates (avg. and standard deviation on 20 samples)

This was explained with the coarser weave of 3-D Twintex, which improves impact properties, albeit making it more sensitive to localised defects or to fibre bundle torsion, necessary to introduce 3-D fibres in the weave structure [13]. Transient thermography allowed also the measurement of the total dimension of delamination areas with growing impact energy. From the results in Figure 7, delamination area grows proportionally to the impact energy for balanced laminates. 1:1 Twintex shows a delamination area smaller with respect to 3-D Twintex. In contrast, in 4:1 Twintex, which shows higher resistance to penetration, the extension of delamination area depends not only on impact energy, but also on the local characteristics of the impacted region (fibre content, presence of crossovers).

Fig 7: Damage areas measured from thermograms (average on five samples per laminate and per impact energy)

Impact on automotive components

Balanced configurations 1:1 and 3-D Twintex offered for the same energy the higher maximum load, therefore the maximum resistance to impact force, whilst the performance of 4:1 Twintex component was somehow deceptive (Table 4). Curves in Figure 8 represent energy accumulation during impact, and allow one to note that with equal energy corresponds a larger total displacement for 3-D Twintex with respect to 1:1 Twintex, ad therefore a slightly lower dynamic stiffness. This indicates that 3-D moulding problems are worsened by the higher thickness of the component.

Component	Avg. thickness (mm.)	Weight (kg)	Max. load (kN)	Avg. load (kN)	Total displacement (mm)
Twintex 3-D	6.7	1.654	11.69	2.60	184.48
Twintex 4:1	6.2	1.636	8.36	3.38	163.03
Twintex 1:1	(45º) 6.6	1.598	6.96	3.28	152.34
Twintex 1:1	6.9	1.722	11.25	3.62	144.01
Table 4 Impact on the components (energy = 637 J)

Fig 8: Impact energy vs. displacement curves (average on five samples per laminate and per impact energy)

Fig 9: Thermal images of the impacted surface of the four configurations of the automotive component impacted at 637 J

IR thermography, in spite of non-simple geometry of the component, allowed an accurate observation of impact areas (Figure 9). Components in 3-D and 1:1 Twintex presented damage limited to a long crack on the flange, with two delamination zones of the curved surfaces slightly larger in 3-D component. In contrast, in the 4:1 Twintex component, the area surrounding the crack appears also damaged. From measurement in Table 5, where delaminated areas are divided in A, B and C in function of the value of thermal gradient, and hence of damage severity, 4:1 Twintex appear to be of lower quality, although it should be noted that no component was close to penetration at this impact energy, 637 Joules. In conclusion, although the 4:1 Twintex component presents a higher resistance to perforation, damage variability, as observed by means of transient thermography, 1:1 Twintex is probably preferable for use in components for the better predictability of its damage patterns. However, 4:1 Twintex can be useful to confer an additional reinforcement, especially on the curved surfaces. In addition, the difficult moulding of 3-D Twintex did probably not justify its large use in components, even considering the slight improvement in penetration energy it shows with respect to 1:1 Twintex.

Component laminate	A	B	C	Total area (mm²)
	(black)	(blue)	(violet)
2-D 1:1 (0º/90º)	35	30	40	105
2-D 1:1 (+45º/-45º)	25	150	130	305
2-D 4:1	20	110	135	265
3-D	10	35	125	170
Table 5 Impact damage levels in the component as measured by thermal images

CONCLUSIONS

In this work, transient thermography has been used to measure and characterise impact damage in E-glass/polypropylene commingled automotive laminates (®Twintex). With this aim, post-impact thermographic observations have been carried out both on samples and on a component prototype. NDE has allowed clarifying the respective properties of the three laminates, with the objective to select the most suitable component configuration. Transient thermography, a simple and versatile technique, has been useful to contribute to laminate selection for use in components, enabling to visualise surface and sub-surface damage. Further work will concern correlation of thermographic images with micrographs, with the aim of fully characterise mode and severity of impact damage.

ACKNOWLEDGEMENTS

The author acknowledges the support of: Ford Motor Company Limited, Jaguar Cars Limited, MIRA, BMW Group, Borealis, ESI (Engineering Systems International), DOW Automotive, Magna Interior Systems, Mapleline, Park Hill Textiles, Polynorm Plastics, Security Composites, Symalit, Vetrotex International S.A, Warwick University, DTI (Department of Trade and Industry), EPRSC (Engineering and Physical Sciences Research Council, UK) and all the personnel of the Mechanical Engineering Dept. - Nottingham University.

REFERENCES

1. Brooks R, Composites in automotive applications: design, 6.16 in Comprehensive Composite Materials, Elsevier 2001, ISBN: 0-080-43725-7.
2. Price CB, Automotive Composites Workshop, Brands Hatch, Kent, UK, 1998, pp. 47-56.
3. Osten S, St. John C, Guillon D, Zanella G, Renault T, Compression molding of TwintexTM and random fiber thermoplastic molding materials, Annual Technical Conference - ANTEC, Conference Proceedings, v 2, 1997, pp. 2432-2436.
4. Maldague X, Applications of Infrared Thermography in NonDestructive Evaluation, Trends in Optical Nondestructive Testing (invited chapter), Pramod Rastogi ed., 2000, pp. 591- 609.
5. Cawley P, The rapid non-destructive inspection of large composite structures, Composites 25, 1994, pp. 351-357.
6. Golfman Y, Nondestructive evaluation of aerospace components using ultrasound and thermography technologies, Journal of Advanced Materials v.33, 2001, pp.21-25.
7. Grinzato E, Stato dell'arte sulle tecniche termografiche per il controllo non distruttivo e principali applicazioni, 9° congresso AIPnD, Padova September 1997.
8. Ball RJ, Almond DP, The detection and measurement of impact damage in thick carbon fibre reinforced laminates by transient thermography, NDT & E International 31, 1998, pp. 165-173.
9. Vavilov VP, Almond DP, Busse G, Grinzato E, Krapez JC, Maldague X, Marinetti S, Peng W, Shirayev V, Wu. D, Infrared thermographic detection and characterization of impact damage in carbon fibre composites: results of the round robin test, Proceedings QIRT, Naples, 1998.
10. Santulli C, Brooks R, Rudd CD, Long AC, Influence of microstructural voids on the mechanical and impact properties in commingled E-glass/polypropylene thermoplastic composites, Proceedings of the Institution of Mechanical Engineers -Journal of materials-Design and Applications Part L 216, 2002, pp. 85-100.
11. Santulli C, Brooks R, Long AC, Rudd CD, Impact properties of compression moulded commingled E-glass/polypropylene composites, Plastics, Rubber and Composites 31, 2002, pp. 270-277.
12. Santulli C, Study of impact hysteresis curves on E-glass reinforced polypropylene laminates, In press, Journal of Materials Science Letters, June 2003.
13. Santulli C, Fibre 3-D e prestazioni ad impatto in compositi tessuti fibra di vetro/polipropilene (Twintex), 6º Congresso AIMAT, Modena, September 2002.

© NDT.net

|Top|