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. 2019 Nov 1;12(21):3598.
doi: 10.3390/ma12213598.

Creep-Induced Screw Preload Loss of Carbon-Fiber Sheet Molding Compound at Elevated Temperature

David Finck  1 , Christian Seidel  2 , Joachim Hausmann  3 , Thomas Rief  4
Affiliations

Creep-Induced Screw Preload Loss of Carbon-Fiber Sheet Molding Compound at Elevated Temperature

David Finck et al. Materials (Basel). .

Abstract

The application of chopped-fiber reinforced polymers in screwed connections at high temperatures raises the question of creep under long-term loading. While up to now thermoplastic materials have mainly been the focus of attention when it comes to creep, this paper shows that thermoset carbon-fiber SMCs (sheet mold compounds) can also be affected by this phenomenon. Screwed connections were investigated regarding their loss of preload force at 120 °C ambient temperature. Additionally, strain-time diagrams were recorded at different stress levels at 120 °C in a creep test setup of a universal testing machine by using optical strain tracking of SMC coupons. The transverse modulus under compression in thickness direction was determined in the same test setup. For data application within a FEA (finite element analysis) software power law curves according to Norton-Bailey creep law were fitted in the strain-time graphs. The applicability of the obtained creep law was crosschecked with a test carried out on the loss of preload force of a screwed connection. The developed simulative methodology offers the possibility to simulate various mounting situations of the bolted connection and to investigate measures against the loss of preload force easily. A promising possibility to limit the loss of preload force due to creep was simulatively evaluated.

Keywords: composites; creep; numerical analysis; preload loss; relaxation; screw; sheet mold compound.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
As-delivered (a) and pressed (b). C-SMC (carbon-fiber sheet molding compound) sheet: carbon-fiber sheet mold compound “Polynt SMCarbon 80 CF60-3K/2” [16].
Figure 2
Figure 2
Preparation of SMC specimen out of the molded plate.
Figure 3
Figure 3
(a) C-SMC stackup between pressure discs in the universal testing machine; (b) C-SMC stackup in close-up view.
Figure 4
Figure 4
Alternating strains for determination of transverse modulus in z direction.
Figure 5
Figure 5
Measured, simulated (Section 4.1) and curve fitted creep strain–time graphs at different stress levels in z direction for “Polynt SMCarbon 80 CF60-3K/2” at 120 °C.
Figure 6
Figure 6
Screw assemblies for measurement of pretension loss; screw assembly (a) with 15 C-SMC specimens; screw assembly (b) with only steel clamped.
Figure 7
Figure 7
Loss of screw force over time.
Figure 8
Figure 8
Simulation of screw assembly A in SIEMENS NX Simcenter 12 at 0.01 s (highest bolt pretension) and 300,000 s simulation time. (a) Stress in z direction at 0.01 s (MPa); (b) Stress in z direction at 0.01 s (MPa); (c) Creep strain at 0.01 s (no dim.); (d) Stress in z direction at 300,000 s (MPa); (e) Creep strain at 300,000 s (no dim).
Figure 9
Figure 9
Application-oriented screw assemblies. (a) without conical spring washer. (assembly view); (b) with conical spring washer (assembly and detail view).
Figure 10
Figure 10
Simulated screw preload force over time of application-oriented screw assemblies.
See this image and copyright information in PMC

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