Crush tests

Definition (What it is?)

Crush tests are mechanical tests in which a material, component, or assembly is subjected to controlled compressive loading—quasi-static or dynamic—until a specified deformation, collapse mode, or failure criterion is reached. They are used to quantify resistance to deformation, structural stability (buckling/folding), energy absorption, and safety margins. In practice, crush tests span coupon-level materials (foams, honeycombs, laminates), subcomponents (crash boxes, battery enclosures, pillars), and even vehicle-level roof-strength evaluations (e.g., FMVSS 216/216a roof crush in the U.S.).

Function and purpose (what do they evaluate?)

  • Load-bearing and collapse behavior:
    • Crush strength (peak load), compressive stiffness/modulus, yield/plateau stress, densification strain (for cellular materials).
    • Stability and failure modes: progressive folding, buckling, splaying, cracking, delamination or fragmentation.
  • Energy absorption and crashworthiness:
    • Energy absorbed (and specific energy absorption, SEA), mean crush load, crush force efficiency (CFE, mean-to-peak load ratio), and stroke efficiency.
    • Evaluation of load–displacement shape to promote stable, controlled crushing and avoid injurious load spikes.
  • Safety and integrity:
    • Component/system integrity under axial, lateral, or oblique crush to limit intrusion into occupant or battery spaces.
    • For electrochemical cells/modules: onset of internal short, venting, temperature rise, gas release, and thermal runaway thresholds.
  • Design validation and simulation correlation:
    • High-quality force–displacement and deformation data to calibrate and validate finite-element models (metals, foams, composites, and battery structures).

Test modes and parameters

  • Loading rate:
    • Quasi-static compression (low strain rates, typically ~10^−4 to 10^−1 s^−1) for full stress–strain curves, stiffness, yield/plateau, densification.
    • Dynamic/impact crush (higher strain rates via drop-weight, servo-hydraulic, sled, or pendulum) to capture rate sensitivity and failure kinetics.
  • Direction and contact:
    • Axial, lateral/radial, flat-plate, hemispherical or wedge indenters, edge crush, and oblique loading.
  • Boundary conditions and fixtures:
    • Platen geometry, end constraints, support fixtures, and friction control (e.g., PTFE sheets/lubricants).
  • Scale:
    • Coupons (materials), subcomponents (tubes, boxes, enclosures), assemblies (modules/packs), and vehicle-level roof segments or complete bodies.
  • Environment and state:
    • Temperature/humidity conditioning; for batteries, state of charge and thermal preconditioning are critical.
  • Instrumentation:
    • Load cells, displacement transducers, strain gauges, high-speed imaging and digital image correlation (DIC).
    • For batteries: thermocouples/IR, voltage taps, current/impedance, gas or smoke detection, and containment enclosures.

Outputs and data reduction

  • Force–displacement and stress–strain curves; absorbed energy (area under curve).
  • Peak and mean crush loads; CFE (mean/peak), SEA (energy per unit mass), stroke efficiency.
  • Characterization of collapse mode and post-crush integrity (often via CT, sectioning, or microscopy).
  • For battery tests: onset load/strain for venting or internal short, temperature/time to event, and propagation metrics.

Relevance and applications (including EV design)

  • Crashworthiness and lightweighting:
    • Validates energy-absorbing structures (crash cans, bumper beams, composite crush tubes, sandwich cores) to manage impact pulses, reduce peak loads, and enable mass reduction.
    • Guides design of crush initiators/triggers that stabilize progressive buckling and improve CFE.
  • Battery safety and structural integration in EVs:
    • Verifies enclosure stiffness, intrusion rails, and underbody protection to prevent cell/module damage and thermal propagation during crashes or road strikes.
    • Supports compliance with safety requirements and OEM/NCAP protocols for mechanical abuse and intrusion.
  • Packaging and logistics:
    • Confirms compression and edge-crush resistance of containers, pallets, and protective foams.
  • Virtual validation:
    • Provides calibration data for material and component models to improve predictiveness of CAE for regulatory and consumer crash tests.

Standards and typical procedures (illustrative, not exhaustive)

  • Materials/components: ASTM D695 (compressive properties of rigid plastics), ISO 604 (compressive properties of plastics), ASTM D642 (compression resistance of shipping containers). Edge-crush tests are common in packaging (corrugated board ECT).
  • Vehicle structures: FMVSS 216/216a (roof strength, quasi-static roof crush).
  • Batteries and battery systems: UL 1642 (lithium cell safety), UL 2580 (EV battery systems), SAE J2464 (abuse testing), UN 38.3 (transport tests including impact or crush for certain formats), IEC 62660 series for road-vehicle Li-ion cells (reliability/safety, including mechanical tests), plus OEM- or region-specific procedures.

Typical materials and manufacturing features that affect crushing

  • Metals: Mild steels, AHSS, aluminum/magnesium alloys; extrusions, roll-formed and hydroformed sections; tailored blanks/welds; heat treatments to tune collapse modes.
  • Composites: CFRP/GFRP laminates and tubes (autoclave, RTM/VARTM, pultrusion, filament winding); crush caps, chamfers, frangible initiators, interleaves, and toughened resins for stable progressive crushing.
  • Cellular structures: Polymer foams (EPP/EPS/EPE/TPU), metallic foams, honeycombs; often used in sandwich panels with bonded skins.
  • Battery structures: Cell cans and pouches, module frames, pack trays/enclosures (stamped, cast, or composite), structural adhesives, bead foams, and thermal barriers.

Test considerations and pitfalls

  • End constraints, alignment, and friction substantially influence peak loads and collapse modes.
  • Geometric imperfections and size effects govern buckling; machined triggers may be required for repeatable folding.
  • Strain-rate and temperature sensitivity can be large (especially for polymers, foams, and composites).
  • For batteries: rigorous safety protocols, containment, and post-test handling are mandatory; results depend strongly on state of charge and thermal conditions.
  • Machine compliance and data filtering must be accounted for when interpreting stiffness and energy.

Synonyms and related terms

  • Synonyms/variants: compression test (context-dependent), axial crush, lateral/radial crush, wedge or hemispherical indentation, edge crush.
  • Related: energy absorption test, crashworthiness test, drop-weight impact test, quasi-static indentation, three-/four-point bending (related but not pure crush), nail penetration or pinch/shear intrusion tests for batteries.

Notes

  • While all crush tests involve compression, the term usually implies loading into the nonlinear regime to induce controlled collapse and quantify energy absorption and failure modes, rather than only small-strain compressive properties.