3. Load path architecture

Before a crash sled fixture can be analyzed, it must be understood as a load-carrying system. A crash pulse applied at the sled interface does not simply "shake" the fixture; it travels through a specific sequence of members, joints, and attachment points before it is finally reacted into the ground or into the deceleration device. If any link in that chain is weak, compliant, or poorly defined, the fixture will deform, the specimen will experience a distorted pulse, and the test will lose validity. Load path architecture is therefore the discipline of tracing that chain explicitly, member by member, and confirming that every element is capable of carrying its share of the representative crash loading.

What "Load Path" Means in a Sled Fixture

A load path is the continuous route by which force is transmitted from its point of application to its point of reaction. In a crash sled fixture, the origin is the specimen attachment interface — the brackets, bolts, and plates that hold the test article. The reaction is the sled carriage, the deceleration trolley, or the reaction mass. Between these two ends, force passes through primary structural members (the main beams, plates, and frames that define the fixture's shape), through joints (welds, bolted connections, bonded interfaces), and through attachment points (the discrete locations where members connect to each other or to the sled).

Three terms must be kept distinct throughout this chapter:

• Primary structural member — a component whose primary function is to carry crash load. Removing it would break the load path. Examples: a longitudinal main beam, a vertical reaction post, a base plate.
• Joint — the interface between two members. A joint may be welded, bolted, pinned, or bonded. Joints are usually the weakest and least predictable part of the path.
• Attachment point — a discrete, localized connection where load enters or leaves a member, such as a bolt group, a clevis, or a mounting pad. Attachment points concentrate stress and are common initiation sites for cracks.

Establishing the Representative Load Case First

You cannot identify a load path without a load case. For a frontal sled test, the representative loading is typically defined by a deceleration pulse and the specimen's mass. A common starting assumption is a peak deceleration of 40 g with a 50 kg specimen, giving a peak inertial force of approximately:

F_peak = m * a = 50 kg * 40 * 9.81 m/s^2 ≈ 19,620 N ≈ 19.6 kN

This 19.6 kN is the design-level force that must be traced through the fixture. In practice, a dynamic amplification factor (DAF) of 1.3 to 1.5 is applied to account for pulse shape and resonance, giving a design force of roughly 25 to 30 kN. The direction of this force — typically longitudinal, but often with vertical and lateral components during a real pulse — defines the orientation of the primary load path.

Tracing the Path: A Worked Example

Consider a simple frontal sled fixture: a specimen mounted on a horizontal mounting plate, which is bolted to two longitudinal main beams, which are in turn bolted to the sled carriage. Under a longitudinal deceleration pulse, the specimen's inertia pushes it forward (relative to the sled), so the reaction force travels backward through the fixture.

The path can be traced as follows:

1. Specimen interface — the specimen's mounting feet transfer load into the mounting plate through four M12 bolts. Each bolt carries approximately 25 kN / 4 ≈ 6.25 kN in shear.
2. Mounting plate — the plate carries the load in bending and shear to its own bolt pattern. Plate thickness and bolt spacing determine local stress.
3. Plate-to-beam joint — a bolted connection transfers load from the plate into the two main beams. This joint is loaded in shear and must be checked for bolt slip and bearing.
4. Main beams — the beams carry the full 25 kN in bending and axial compression toward the sled interface.
5. Beam-to-carriage attachment — the final attachment point reacts the load into the sled carriage.

Each of these five links must be verified. If any one is under-designed, the path is broken and the fixture will not deliver the intended pulse to the specimen.

Classifying Members by Load Direction

Once the path is traced, each primary member should be classified by the dominant internal force it carries. This classification drives the verification method:

Joint and Attachment Point Detailing

Joints deserve disproportionate attention because they are where load paths most often fail. For a bolted joint carrying 25 kN in shear through four M12 bolts, the nominal shear stress per bolt is:

τ = F / (n * A_bolt) = 25,000 N / (4 * 113 mm^2) ≈ 55 MPa

For a property class 8.8 bolt with a shear strength of roughly 0.6 × 640 MPa = 384 MPa, the safety factor is approximately 7. This looks comfortable, but the real risk is not bolt shear — it is joint slip. If the bolts are not preloaded sufficiently, the joint will slip, the fixture will shift, and the specimen will see a distorted pulse. Slip resistance depends on clamp force, friction coefficient, and hole clearance. A slip-resistant joint with preloaded bolts is therefore preferred over a bearing-type joint for crash fixtures.

Attachment Points and Stress Concentration

Attachment points are the discrete locations where load enters or leaves a member. They are characterized by geometric discontinuity — bolt holes, fillets, weld toes — and therefore by stress concentration. A hole in a plate loaded in tension can raise local stress by a factor of 2 to 3 compared with the nominal section. For a plate with a nominal stress of 100 MPa, the local stress at the hole edge could reach 250 to 300 MPa, which may exceed the yield strength of mild steel.

Practical measures include increasing edge distance, using larger washers or backing plates, adding fillets at weld toes, and avoiding abrupt section changes. In crash fixtures, it is also common to add gussets or ribs at attachment points to spread load over a larger area and reduce local stress concentration.

Verifying the Path: A Simple Check

After the path is traced and each link is sized, a simple verification check can be performed by comparing the capacity of each link against the design force. The weakest link determines the fixture's overall capacity. If the weakest link has a safety factor below the target (typically 1.5 for dynamic crash loading), it must be redesigned. This check should be repeated for each load direction — longitudinal, lateral, and vertical — because the primary path may change with direction.

A useful habit is to sketch the load path on the fixture drawing itself, labeling each member, joint, and attachment point with its calculated capacity. This makes the architecture visible and ensures that no link is overlooked during design review or structural verification.